6. Aluminium

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ASM Handbook, Volume 4E, Heat Treating of Nonferrous Alloys
G.E. Totten and D.S. MacKenzie, editors
Quenching of Aluminum Alloys
D. Scott MacKenzie, Houghton International
Niels Bogh, Bogh Industries
Tom Croucher, Tom Croucher and Associates
QUENCHING refers to the rapid cooling of
metal from the solution treating temperature,
typically between 465 and 565 C (870 and
1050 F) for aluminum alloys. The fundamental
objective of quenching is to preserve, as nearly
as possible, a metastable solid solution formed
at the solution heat treating temperature, by
rapidly cooling to some lower temperature, usually near room temperature. When quenching
rates from the solution temperature are sufficiently rapid to retain a solid solution, solute
atoms are available to form zones of homogeneous (coherent or semicoherent) precipitation
for strengthening by age hardening at room
temperature or moderately elevated temperatures. In addition, another purpose of quenching
is to maintain a certain minimum number of
vacant lattice sites to assist in promoting the
low-temperature diffusion during the aging
stage of precipitation hardening.
Precipitation during quenching can lead to
localized overaging, a loss of grain-boundary
corrosion resistance, and, in extreme cases,
poor response during the age hardening treatment. When quenching rates from the solution
temperature are not sufficiently rapid, the solute
atoms that diffuse to grain boundaries, as well
as the vacancies that migrate (with extreme
rapidity) to disordered regions, are irretrievably
lost for practical purposes and fail to contribute
to the process of age hardening.
The highest attainable strengths from aging
hardening are generally associated with the
most rapid quenching rates. Nonetheless, a
maximum quenching rate is not necessarily a
one-sided argument, because both distortion
and residual stresses also develop with an
increase in the rate of cooling. A balance must
be obtained between the need to quench sufficiently fast to retain most of the hardening
elements and compounds in solution and the
need to minimize residual stress and distortion
in the parts being quenched. Obtaining properties and low distortion is usually a balancing
act. Often, optimal properties are obtained at
the expense of high residual stresses or high
distortion. Low distortion or residual stresses
are usually obtained at a sacrifice in properties.
Therefore, the optimum rate of cooling is one
where properties are just met, so that distortion
is reduced.
Aluminum also is extremely prone to distortion due to both solution heat treatment and
quenching. The coefficient of linear expansion
of aluminum is twice that of steel (2.38 105 mm/mm C for aluminum versus 1.12 105 mm/mm C for steel), and large thermal
strains can develop within and across the surface of parts due to thermal expansion during
solution heat treatment. In addition, temperatures during solution treatment approach the
liquidus temperature and result in low strength
and high plasticity.
The most troublesome changes in dimensions
and shape can occur during quenching, due to
the extent of nonuniform cooling, particularly
in thin sections of parts that contain variations
in thickness. If distortion is to be minimized,
the temperature differences between different
areas of a part must be minimized consistent
with cooling all regions of the part sufficiently
fast to avoid excessive precipitation during
quenching. Aluminum alloys have a relatively
high thermal conductivity, between 1.4 and
2.38 W/cm K (975 and 1650 Btu in./h ft2 F),
compared to a conductivity of approximately
0.14 to 0.29 W/cm K (100 to 200 Btu in./h ft2 F) for austenite in most carbon and lowalloy steels. The high conductivity of aluminum
can be both a benefit and a problem. If heat is
being rapidly extracted at the part surface by
the quenchant, the high conductivity results in
rapid temperature losses in thin sections and
large temperature differences between thick
and thin sections. If heat is being extracted
more slowly, the high metal conductivity aids
in maintaining temperature uniformity within
the part.
This article provides an overview on the factors used to determine a suitable cooling rate
and the appropriate quenching process to
develop a suitable cooling rate. The practical
difficulty lies in establishing just how fast a part
of a particular alloy must be quenched, while
also minimizing, as much as possible, thermal
gradients within the part that may cause plastic
deformation and residual stress. To retain a sufficient amount of alloying elements in solid
solution for subsequent age hardening, it is necessary to determine a suitable cooling rate for a
given alloy and part by considering the following questions (Ref 1):
Are there some temperatures during the
quench when diffusion-controlled reactions
will occur faster?
What is the critical quench rate for how fast
the part must be quenched to effectively
retard diffusion of solute atoms in the solid
solution?
What happens when cooling rates during a
quench are slower than the critical quench
rate?
When a suitable cooling rate is determined, the
appropriate quench process is determined by
several operational factors, such as:
The heat-extracting potential of the quench-
ing fluid in the quiescent state at normal
fluid temperatures and pressures (standard
conditions)
Changes in the heat-extracting potential of
the fluid brought about by nonstandard conditions of agitation, temperature, or pressure
Product thickness and internal conditions of
the part that affect heat flow to the surface
Surface and other external conditions that
affect the removal of heat
Many different methods and quench media
have been used in quenching aluminum alloys,
depending on the alloy and product thickness
and configuration. Hot or cold water and polyalkylene glycol (PAG) quenchants are used
almost exclusively in the quenching of aluminum alloys. Water is the fluid most frequently
employed for quenching aluminum, but there
are a variety of quenchants and methods that
have been used (Ref 1):
Cold water immersion
Hot water immersion
Boiling water
Water spray
Quenching of Aluminum Alloys / 149
Quench Sensitivity of Alloys
Polyalkylene glycol solutions
Air blast
Still air
Liquid nitrogen
Fast quenching oils
Brine solutions
Hot or cold water and PAG quenchants are used
almost exclusively in the quenching of aluminum alloys, and these quenchants are discussed
in separate sections of this article. Water is the
fluid most frequently employed for quenching
aluminum, and highly agitated cold water is an
excellent quenchant in terms of obtaining high
cooling rates. However, quenching in cold water
can produce large differences in temperature
between thick and thin sections, resulting in localized plastic flow and distortion observed after
quenching or during machining. Usually, distortion is controlled in aluminum parts by adding
polymers to water quenchants to reduce the convective or film coefficient between the part and
the water, as discussed further in this article.
Fig. 1
Effect of temperature on the extent of
supersaturation and diffusion rate, which drive
the rate of precipitation
amounts of precipitation follow a C-shaped pattern (Fig.1).
Using isothermal quenching techniques, Fink
and Willey pioneered the attempts to describe
the effects of quench rates with the use of
C-curves (Ref 2). Using isothermal quenching
techniques, they developed C-curves for
strength of 7075-T6 and corrosion behavior of
2024-T4. The C-curves were plots of the time
required at different temperatures to precipitate
a sufficient amount of solute to either reduce
strength (Fig. 2a), cause a change in the corrosion behavior (Fig. 2b), or relate to other properties, such as electrical conductivity, fracture
toughness, or isothermal quench conditions.
The nose of the C-curves identifies the region
of highest precipitation rates, which Fink and
Willey called the critical temperature range.
For alloy 7075 (Fig. 2a), this range was determined to be 400 to 290 C (750 to 550 F).
Investigators have used isothermal timetemperature-transformation diagrams to compare
the quench sensitivity of aluminum alloys.
Figures 3 and 4 are examples of isothermal
transformation diagrams that compare the
quench sensitivity of various aluminum alloys.
The critical temperature range is roughly in the
same range of 400 to 290 C (750 to 550 F),
which is the approximate critical temperature
range typical of many age-hardenable aluminum
alloys. Some sources quote this range (or a
slightly different range) as the most critical
range for quenching of many aluminum alloys,
but the range can be alloy-dependent.
Quench sensitivity of aluminum alloys also
has been evaluated from the effects of average
quenching rates through the critical range from
400 to 290 C (750 to 550 F). In Fig. 5(a), for
example, the effects of quenching on the yield
strength of five alloys are compared in terms
of average quenching rates through the critical
range from 400 to 290 C (750 to 550 F).
For alloys relatively high in sensitivity to
quenching rate, such as 7075, rates of approximately 300 C/s (540 F/s) or higher are
The most important metallurgical factor affecting the properties of age-hardenable aluminum
alloys is solute loss, referring to the solutes that
are chemically bonded with other elements and
thus unavailable for precipitation hardening.
There are several factors that can lead to solute
loss and loss of properties. The rate of cooling in
the part during quenching is one key factor, but
solute loss also is affected by metallurgical factors
such as the type of dispersoids in an alloy, solvus
temperatures, and casting homogenization temperatures (see the article “Metallurgy of Heat
Treatable Aluminum Alloys” in this Volume).
During quenching, the objective is to prevent
the precipitation of solutes during the cooling
of high-temperature solid solution. If appreciable precipitation during cooling is to be
avoided, two requirements must be satisfied.
First, the time required for transfer of the load
from the furnace to the quenching medium
must be short enough to preclude slow precooling into the temperature range where very rapid
precipitation takes place. The second requirement for avoidance of appreciable precipitation
during quenching is that the volume, heatabsorption capacity, and rate of flow of the
quenching medium be such that little or no precipitation occurs during cooling. Any interruption of the quench that may allow reheating
into a temperature range where rapid precipitation can occur must be prohibited.
The rate of precipitation, as a function of
temperature, depends on two factors: the degree
of supersaturation and the rate of diffusion
(Fig. 1). Diffusion rates increase at higher temperatures, but the nucleation rate of precipitates
is low because the degree of supersaturation is
low at higher temperatures. Conversely, the
precipitation rate also is low at low temperatures, where the degree of supersaturation is
high but with low rates of diffusion. At intermediate temperatures, the precipitation rate is
highest. Consequently, times to produce equal
900
900
800
800
600
100%
98%
80%
90%
300
500
400
200
300
200
10–1
Temperature, °F
700
Temperature, °C
Temperature, °F
400
10
102
Fig. 2
300
500
200
300
200
10–1
103
Time, s
(a)
600
400
400
Tensile strength
Yield strength
1
Type of corrosion attack
Pitting
Pitting plus slight intergranular
Pitting plus intergranular
intergranular
700
Temperature, °C
(b)
1
10
102
103
Time, s
Time-temperature C-curve indicating the effect of time and temperature of an interrupted quench on (a) strength of alloy 7075 as a percentage of maximum from an
uninterrupted quench and (b) type of corrosion attack in 2024-T4 sheet. Source: Ref 2
150 / Heat Treating of Aluminum and Its Alloys
500
700
800
930
7075
600
T6 temper
500
400
750
T6 temper
Aged at 173 °C
(340 °F) for 8 h
300
570
200
390
Temperature, °F
600
Temperature, °C
7050
Temperature, K
Temperature, °F
700
500
400
300
.1
1
10
(a)
Fig. 3
1
102
10
(b)
103
104
Critical time, s
1110
500
930
400
750
A
B
C
D
800
425
700
370
6205
6063
600
315
300
570
200
390
500
260
212
400
20
A: 7075
B: 2017
C: 6061
D: 6063
100
32
0
1
(a)
Fig. 4
6061
Temperature, °C
600
Temperature, °F
Time-temperature C-curve indicating time and temperature of an interrupted quench on (a) yield strength of alloys 7075 and 7050 and (b) 99.5% maximum yield strength of
6351-T6 extrusion. Source: Ref 3–5
Temperature, °F
Temperature, °C
100
Time, s
102
10
Time, s
103
1
(b)
0
102
103
Time of isothermal hold, s
Examples of isothermal C-curves in comparing quench sensitivity of various aluminum alloys. (a) 95% of maximum tensile stress for various alloys. (b) 90% of maximum
yield strength of 6xxx alloys in T6 temper. Source: Ref 6, 7
required to obtain near-maximum strength after
precipitation heat treatment. The other alloys in
Fig. 5(a) maintain their strengths at cooling
rates as low as approximately 100 C/s (180 F/s).
Figure 6 provides a similar comparison of agehardened tensile strength from different quench
rates, including the effect of quench rate on stress
corrosion of 2024.
The effects of quench rates on tensile
strength do not necessarily serve as the criterion
to determine quench sensitivity. For example,
the detrimental effects on resistance to corrosion of 2024-T4 occur at quenching rates considerably higher than those that mark the
initial decrease in tensile strength (Fig. 7). Consequently, tensile properties do not serve as a
criterion to determine whether quenching was
sufficiently rapid to provide optimum corrosion
resistance. However, in the case of 7075, the
most rapid decrease in tensile properties occurs
at cooling rates somewhat higher than those
that have the greatest effects on corrosion
(Fig. 7). To avoid completely the intergranular
type of attack, rates in excess of approximately
165 C/s (300 F/s) for 7075-T6 and 555 C/s
(1000 F/s) for 2024-T4 are needed. Such rates
are not attainable with thick sections. Therefore, when thick-section parts are required to
endure service conditions conducive to stress
corrosion, artificially aged tempers of 2xxxseries alloy, in which precipitation is general
within the grains, and the T73 special stresscorrosion-resistant temper of 7075 are preferred
(Ref 11).
Average quench rates through a critical temperature range can provide reasonable comparison of property in different alloys under similar
product-process conditions (Fig. 8) and reasonable property predictions if cooling rates are
fairly uniform. However, average quench rates
do not reflect actual conditions under continuous cooling in a typical quench or with parts
of variable section thickness. It is not necessarily sufficient just to ensure that the cooling
curve misses the nose of the C-curve, because
precipitation occurs throughout the cooling
cycling—even when cooling curves involve
holding times either above or below the critical
temperature range. Under conditions of variable
cooling rates over the entire quenching cycle, a
more quantitative procedure, known as quenchfactor analysis, uses information from the entire
C-curve to predict how a variable quench curve
affects properties. See the section “QuenchFactor Analysis” in this article for more information. Quench-factor analysis is useful in
evaluating the property effects when cooling
rates are nonuniform, such as when cooling
curves involve holding times either above or
below the critical temperature range. Another
useful method in evaluating quench sensitivity
of alloys is the development of continuous
cooling precipitation diagrams (see the article
Quenching of Aluminum Alloys / 151
50 °C/s
100
Stress, ksi
80
Tensile
strength
60
Yield strength
40
20
7178-T6
7075-T6
0
(a)
2
10
103
1
10
Quenching rate between 750 and 550 °F, °F/s
(a)
500 °C/s
90
Strengths, ksi, or % loss by corrosion
120
80
70 Tensile
strength
60
50
40
Yield strength
30
Stress corrosion
20
10
10
102
103
Average quenching rate, °F/s
1
(b)
102
Fig. 6
(b)
Fig. 5
Quench sensitivity of various aluminum alloys
as a function of average quench rates in the
critical temperature range between 400 and 290 C (750
and 550 F). (a) Yield strength after aging of five
wrought alloys. (b) Tensile strength after aging of eight
wrought alloys
Type
Corrosion: I = intergranular, P = pitting, SI = slight intergranular
2024
I
P+I
P
7075
I
P
I or I+P
Por P+SI
Max depth,
mils
Attack in NaCl-H2O2
Quench sensitivity of various aluminum alloys as a function of average quench rates in the critical
temperature range between 400 and 290 C (750 and 550 F). (a) Tensile and yield strengths of 7178-T6
and 7075-T6. (b) Tensile strength and corrosion properties of 2024-T4 sheet. Corrosion specimens were stressed to
75% of yield strength and exposed 48 h by alternate immersion in salt-peroxide solution. Corrosion losses are based
on tensile strength. Source: Ref 8, 9
10
5
2024
7075
0
50 °C/s
500 °C/s
Tensile
strength,
ksi
100
“Quench Sensitivity of Aluminum Alloys”
in this Volume). More details on the effects
of quench rates also are given in the article
“Metallurgy of Heat Treatable Aluminum
Alloys” in this Volume.
7075
80
2024
60
80
Quench Mechanisms
2024
Stressed to 75% of yield strength
60
Loss in
tensile
strength, %
There are normally three distinct stages of
quenching when a hot component is immersed
in a liquid quenchant (Fig. 9, Ref 13):
Vapor stage: Stage A or vapor blanket stage
Boiling stage: Stage B or nucleate boiling
Fig. 7
Unstressed
7075
20
0
10
stage
Convection stage: Stage C
These three general standard stages of quench
mechanisms have very distinct effects on cooling rates (Fig. 9), although the different
mechanisms may not be completely uniform
over the surface of a given quenched part,
depending on details of the quench method
and the workpiece. For immersion quenching,
quench rate commonly is defined conservatively as the rate of cooling when parts are fully
immersed in the quenchant. Other nonstandard
factors that can affect quench rate include time
delay prior to full immersion and the degree
of fluid agitation, which reduces the extent of
40
102
103
Average cooling rate (from 750 to 550 °F), °F/s
104
Effects of quenching rate on tensile properties and resistance to corrosion of 2024-T4 and 7075-T6. Source:
Ref 10
vapor-phase formation, thus promoting more
rapid onset of nucleate boiling (B stage) for a
rapid cooling rate (Fig. 9).
The first stage of the quench process is the
vapor stage, which is encountered when the
hot surface of the heated component first comes
in contact with the liquid quenchant. The component becomes surrounded with a blanket of
vapor. In this stage, heat transfer is very slow
and occurs primarily by radiation through the
vapor blanket. Some conduction also occurs
through the vapor phase. This blanket is very
stable, and its removal can only be enhanced
by agitation or speed-improving additives.
This stage is responsible for many of the
surface soft spots encountered in quenching.
152 / Heat Treating of Aluminum and Its Alloys
110
7050 forgings
Cold
ST 475 °C (890°F) 4 h, quenched,
water
aged 120 °C (250 °F) 24 h
quench
150 °F water
quench
Stress, ksi
90
Boiling water
quench
600
80
500
70
Stress, MPa
100
7050
60
7075
7175
50
400
Fty
L
7049
1
10
102
Average colling rate, °F/s
103
Fig. 8
Effect of average cooling rate after solution
treatment on maximum yield sterngth of
various aluminum alloys after aging. Source: Ref 12
High-pressure sprays and strong agitation eliminate this stage. If they are allowed to persist,
undesirable microconstituents can form.
The second stage encountered in quenching
is the boiling stage. This is where the vapor
stage starts to collapse and all liquid in contact
with the component surface erupts into boiling
bubbles. This is the fastest stage of quenching.
The high heat-extraction rates are due to carrying away heat from the hot surface and transferring it further into the liquid quenchant, which
allows cooled liquid to replace it at the surface.
In many quenchants, additives have been added to
enhance the maximum cooling rates obtained by a
given fluid. The boiling stage stops when the temperature of the component surface reaches a temperature below the boiling point of the liquid.
For many distortion-prone components, elevatedtemperature or polymer quenchants are used if
the medium is fast enough to maintain supersaturated solid solution in quenched aluminum.
The final stage of quenching is the convection stage. This occurs when the component
has reached a point below that of the quenchant boiling temperature. Heat is removed by
convection and is controlled by the quenchant
specific heat and thermal conductivity and the
temperature differential between the component temperature and that of the quenchant.
The cooling rate during the convection stage
is usually the slowest of the three stages.
Typically, it is this stage where most distortion occurs.
Standards Tests for Cooling Curves. The
most useful way of accurately describing
the complex mechanism of quenching is to
develop a cooling curve for the quenching
medium under controlled conditions. Cooling
curves are developed by quenching, from an
elevated temperature, a test probe in a sample
of the quenching medium. Sometimes an
austenitic stainless steel specimen is used to
avoid scaling or the necessity for a protective
atmosphere. A high-speed recorder or digital
data-acquisition unit is used for plotting
Fig. 9
Cooling stages and mechanisms during immersion quenching. Source: Ref 13
temperature changes, as measured by one or
more thermocouples embedded in the probe.
The resulting time-temperature curve indicates
the heat-transfer characteristics of the quenching fluid.
In general practice, the use of an Inconel
probe such as that described by ASTM
D6200 is used. This precludes the issues of
oxidation and latent heats of transformation.
This type of probe is the most commonly used
probe. However, other probes manufactured
from silver or other materials can be used.
The probe material should always be specified.
It is generally not possible to compare probes
of different materials and tested to different
methods. A comparison of the different cooling curve methods and probe designs is shown
in Table 1.
Quench Severity and Cooling Rates
Several material and quenchant characteristics influence the rate of heat removal from
the part being quenched. An infinite quench is
one that instantly decreases the skin of the part
to the bath temperature. In practice, quenchants
never provide an idealized infinite quench, but
with a very rapid rate of cooling at the surface,
the rate of cooling in the part is then a function
only of the diffusivity of the metal, that is, its
ability to diffuse heat from the interior to the
surface.
The rate of cooling in production operations,
which is not always easily measured, depends
on several factors:
Type of quench fluid (water, air, polymer,
liquid nitrogen)
Quench method (immersion, spray, still air,
air blast)
Thickness, configuration, and surface condi-
tion of the parts
Internal conditions of the workpiece that
affect the supply of heat to the surface
Surface and other external conditions that
affect the removal of heat
Heat-extracting potential of the quenching
fluid in the quiescent state at normal fluid
temperatures and pressures (measured under
standard conditions with various probes,
e.g., see the section “Standards Tests for
Cooling Curves” in this article)
Changes in the heat-extracting potential of
the fluid brought about by nonstandard conditions of agitation, temperature, or pressure
The severity of the quench at the surface
depends, in practice, on the vapor blanket formation, boiling characteristics, and the velocity,
temperature, specific heat, heat of vaporization,
conductivity, density, viscosity, and wetting
characteristics of the quenching fluid. Practically, the cooling rates are controlled by the
fluid selected, the type and amount of any polymer put in water-based quenchants, and the
bath temperature and velocity.
Quench rate also depends on several product
factors, such as part thickness and surface conditions. As the thickness is increased, a slower
cooling rate occurs at the interior and midplane
of the part. If the quench rate is slow enough, or
the product thickness is large, then significant
precipitation can occur. For example, cooling
rates determined experimentally for 1.6 mm to
20 cm (0.06 to 8 in.) thick sections that were
quenched by immersion in water at five different
temperatures and by cooling in still air are
shown in Fig. 10. The dashed line at the extreme
right in Fig. 10 delineates the maximum theoretical cooling rate at the midplane of the plate.
This assumes an infinite cooling rate and a thermal diffusivity of 1400 cm2/s. This cooling rate
assumes that the surface of the plate has instantaneously changed to the temperature of the
quenchant. This is often used as a limiting case
Quenching of Aluminum Alloys / 153
Table 1 Cooling curve test methods
Method
JIS K2242
Z8 E 45003
ASTM D6200
International
Inconel 600
12.5 60 (0.5 2.4)
Various
115 ± 5 (4.5 ± 0.2) diam
2000 (68)
40 ± 2 (105 ± 4)
850 ± 5 (1560 ± 9)
France
99.999% Ag
16 48 (0.6 1.9)
Various
138 diam 99 high (5.4 3.9)
800 (27)
50 ± 2 (120 ± 4)
800 ± 5 (1470 ± 9)
Japan
99.999% Ag
10 30 (0.4 1.2)
Dioctyl phthalate
300 mL (10 oz) beaker
250 (8.5)
80, 120, 160 (175, 250, 320)
810 ± 5 (1490 ± 9)
China
99.999% Ag
10 30 (0.4 1.2)
Dioctyl phthalate
300 mL (10 oz) beaker
250 (8.5)
80 ± 2 (175 ± 4)
810 ± 5 (1490 ± 9)
United States
Inconel 600
12.5 60 (0.5 2.4)
Various
115 ± 5 (4.5 ± 0.2) diam
2000 (68)
40 ± 2 (105 ± 4)
850 ± 5 (1560 ± 9)
Average colling rate at 400–290 °C, °C/s
1
10
102
10–1
103
25
1
Thickness, in.
Thickness, mm
10
Computed maximum
(assumes instantaneous
cooling of surface from
875–210 °F or 470–100 °C)
75
7.5
Air cool
212 °F
180 °F
(100 °C)
150 °F
(80 °C)
200 °F
(65 °C)
(95 °C)
Immersion in water at
indicated temperature
0.75
1
10
102
Average colling rate at 750–550 °F, °F/s
103
Q ¼ h AðTS T1 Þ
(Eq 1)
where Q is the amount of heat transferred, A is
the surface area of the part in contact with the
fluid, TS is the surface temperature, T1 is the
fluid temperature away from the surface, and
h is the interfacial or film coefficient. If this
equation is rearranged, h, the film coefficient,
can be defined in terms of the part area, difference in temperature between the part and the
quenchant, and the heat being transferred. The
rate of heat transfer at the surface is thus:
200
6
150
Square
4
100
Round
2
50
2
4
6
8
10
Section size, in.
12
14
Experimentally
determined
correlation
between the average cooling rates of 400 to
290 C/s (750 to 550 F/s) of rod and square bars to
plates. Rates were measured at the centers of sections.
Source: Ref 14
75 °F
(25 °C)
10–2
104
Effects of thickness and quenching medium on average cooling rates at midplane of aluminum alloy sheet
and plate quenched from solution temperatures. The dashed line delineates the maximum cooling rates
theoretically obtainable at the midplane of plate, assuming an infinite heat-transfer coefficient and a diffusivity factor
2
of 1400 cm /s. Source: Ref 14
dQ
¼ hðTS T1 Þ
dt S
Section size, mm
100 150 200 250 300 350
250
Fig. 11
Fig. 10
for quenching. No rates higher than this can be
achieved, although rates approaching the theoretical limit have been observed with impinging
spray quenching. Experimentally determined
relationships between the thickness of plates
and either the diameter of rounds or the dimensions of square bars having equal cooling rates
are shown in Fig. 11.
Quench Severity at the Surface. Mathematically, heat transfer at the surface of the part can
be described using Newton’s law of cooling:
50
8
0
0
10–1
2.5
0.25
10–1
10
Thickness of plate having
equal cooling rate, mm
250
AFNOR NFT 6077B
Thickness of plate having
equal cooling rate, in.
Country
Probe alloy
Probe dimensions, mm (in.)
Reference fluid
Vessel dimensions, mm (in.)
Oil volume, mL (oz)
Oil temperature, C ( F)
Probe temperature, C ( F)
ISO 9950
(Eq 2)
such that TS is time-dependent. The greatest
conceivable quench severity would be when
the surface is cooled instantaneously to that of
the quenchant. A less severe quench leads to
less rapid temperature reduction.
An analytical determination of the interface
coefficient, h, requires that the properties of
the fluid moving past the part be examined.
The properties of the quenchant, including boiling temperature, viscosity, density, thermal
conductivity, and specific heat, combine to
make the quenchant an important, if not the
most important, variable affecting quench
severity. Increases in quenchant velocity generally increase the quench severity. Increasing the
bath temperature puts the quenchant nearer its
boiling point, decreases the temperature difference between the part and bath, and decreases
the quench severity.
Quench severity and cooling rates also are
very sensitive to the surface condition of the
parts. Lowest rates are observed with products
having freshly machined or bright-etched, clean
surfaces, or products that have been coated with
materials that decrease heat transfer. The presence of oxide films or stains increases cooling
rates. Further marked changes can be effected
through the application of nonreflective coatings, which also accelerate heating (Fig. 12).
Surface roughness exerts a similar effect; this
appears to be related to vapor film stability.
In addition, the manner in which parts enter
the quenching medium can significantly alter
the relative cooling rates at various points,
thereby affecting mechanical properties and
residual stresses established during quenching.
In batch heat treating operations, placement
and spacing of parts on the racks can be a major
factor in determining the quenching rates. In
immersion quenching, adequate volumes of the
quenching medium must be provided to prevent
an excessive temperature rise in the medium.
When jet agitation is used to induce water flow
between parts, jets should not impinge directly
and cause rapid localized cooling
Heat-Transfer and Cooling Rates within
the Part. Cooling rates in the part depend on
the size and configuration of the part. Similarly,
quenching complex shapes (such as engineered
castings, die forgings, or extruded shapes
whose wall thicknesses differ widely) poses
special problems if distortion and stresses are
to be minimized. If all workpieces were symmetrical and alike in shape (no odd
154 / Heat Treating of Aluminum and Its Alloys
of the part, and dT/dx is the thermal gradient
in the part.
To obtain the temperature distribution and
gradients within a part over time requires the
use of Fourier’s second law of heat conduction,
which, in simplified form for one dimension, is:
2 dT
d T
¼a
dt
dx2
(Eq 8)
where a is the thermal diffusivity of the metal
part, which is related to density (r), specific
heat (Cp), and thermal conductivity as k = a
r Cp. Solution of the differential equations
for Fourier’s second law under appropriate
boundary conditions (e.g., surface temperature, part shape, part size) requires numerical
integration of the differential equations for
heat transfer. In the case of heat transfer from
the interior of a bar, while neglecting axial
flow, the relation is:
d2 T 1 dT 1 dT
þ
¼
dr 2 r dr dt
(Eq 9)
where r is the bar radius, a is the thermal diffusivity, and dT/dr is the thermal gradient. The
thermal conductivity (k) of the part can be
related to the film coefficient (h) of the bath
using Biot’s number (Bi) in the equation:
Biot’s numberðBi Þ ¼ hX=k
(Eq 10)
where X is the characteristic length of the part. A
similar ratio, more widely used in quenching, is
the Grossman number, defined by the equation:
Fig. 12
Effect of surface conditions on the midplane cooling of a 13 mm (0.5 in.) thick plate of 7075 from quenching
in (a) 20 C (70 F) water and (b) boiling water
configurations) and were of the same size and/
or weight, obtaining the desired properties
would be simple. In practice, however, such
conditions rarely exist, and the cooling rate
depends on the internal conditions of the workpiece that affect the supply of heat to the
surface.
Because heat transfer during quenching basically is limited by resistance at the surface in
contact with the quenching medium, the average
rate of cooling can be estimated as a function of
the ratio of surface area to volume. This ratio
may vary considerably, depending on the shape
of the product. For sheet and plate, as well as
other products of similar shape, average cooling
rates (through the critical temperature range
measured at a center or midplane location) vary
with thickness in a relatively simple manner.
The relation can be approximated by:
logðR1 Þ ¼ logðR2 Þ C logðtÞ
More generally, the flux of heat and temperature gradient within a part are related by Fourier’s
equation:
dQ
dT
¼k
dt
dx
(Eq 4)
where k is the thermal conductivity of steel. At
the surface, the temperature gradient is:
dQ
dT
¼k
dt
dx S
(Eq 5)
So that at the surface:
dT
h
¼ ðTS T1 Þ
dx S k
(Eq 6)
The actual heat flow over a surface area, A, from
the interior to the surface of a part being quenched
can be described with Fourier’s equation:
(Eq 3)
where R1 is the average cooling rate at thickness t, R2 is the average cooling rate at 1 cm
(0.4 in.) thickness, and C is a constant.
Q ¼ k A dT=dx
(Eq 7)
where Q is the amount of heat transferred, k is
thermal conductivity of the alloy, A is the area
H ¼ h=2k
(Eq 11)
The Grossman number has been reported to
equal approximately 1 for 25 mm (1 in.) sections quenched in still water. As an example
in the following sections (from Ref 15), a
finite-difference heat-transfer program was used
to solve Eq 9 for various experimental conditions with cylindrical (bar) test probes.
Grossman numbers (H) and the film coefficient (h) provide useful information about the
rate of heat removal from the surface of a part.
From experimental data of cooling curves
obtained from quenching solution-treated 7075
probes under controlled conditions, Grossman
numbers (H) and film (heat-transfer) coefficients were calculated for water quenches under
a range of velocity and temperature conditions
and with additions of polymers under selected
conditions (Table 2). The 7075 aluminum probe
had a length of 280 mm (11 in.) and a diameter
of 75 mm (3 in.), with thermocouples inserted
in the centerline and 6.35 mm (0.25 in.) from
the surface. Probes were solution treated at
465 C (870 F), and experimental data of the
cooling curves are given in Fig. 13. The temperature difference between the two thermocouples is also plotted.
With the thermal conductivity of 7075 aluminum at approximately 1.70 W/cm K (1150
Quenching of Aluminum Alloys / 155
Btu in./ft2 h F), Grossman numbers (H) and
film (or heat-transfer) coefficients were calculated. Unagitated water at 25 C (75 F) has a
Grossman number of approximately 1, as
reported by Grossman, and a film coefficient
of approximately 3.55 W/cm2 K (2460 Btu in./ft2 h F). The film coefficient increased to
4.78 and 5.14 W/cm2 K (3105 and 3565 Btu in./ft2 h F) at velocities of 0.25 and 0.50 m/s
(50 and 100 ft/min), respectively. These high
film coefficient values can therefore cause cold
water quenching to create high thermal
Table 2 Grossmann numbers and heat-transfer coefficients (C) of quenchant-to-part films
Quenchant
F
m/s
ft/min
Grossmann
number
(H = C/2k)
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0.00
0.25
0.50
0
50
100
0
50
100
0
50
100
0
50
100
0
50
100
0
50
100
0
50
100
0
50
100
0
50
100
0
50
100
1.07
1.35
1.55
0.99
1.21
1.48
1.10
1.29
1.60
0.86
1.09
1.33
0.21
0.57
0.79
0.11
0.21
0.27
0.06
0.08
0.09
0.04
0.04
0.04
0.19
0.21
0.23
0.44
0.40
0.42
Temperature
Type
C
Water
27
80
Water
38
100
Water
49
120
Water
60
140
Water
71
160
Water
82
180
Water
93
200
Water
100
212
Polyalkylene glycol
(UCON A)(a)
30
85
Polyvinyl pyrrolidone
(PVP 90)(a)
30
85
Velocity
Effective heat-transfer coefficient (C)
W/cm2 K
Btu/ft2 h F
3.55
4.78
5.14
3.28
4.01
4.91
3.65
4.29
5.31
2.85
3.62
4.41
0.70
1.89
2.62
0.36
0.69
0.89
0.20
0.27
0.30
0.13
0.13
0.13
0.63
0.70
0.77
1.49
1.34
1.41
2460
3105
3565
2275
2785
3400
2530
2970
3680
1980
2510
3060
485
1310
1815
255
485
620
138
184
207
92
92
92
429
475
529
1012
912
966
(a) Polymer quenchants with concentrations of 25%. k is equal to the thermal conductivity of the aluminum alloy (7075). Source: Ref 15
550
1000
550
1000
500
900
500
900
450
800
800
250
200
400
Temperature, °C
300
ΔT in 60 °C (140 °F)
water
700
Temperature, °F
Temperature, °C
400
350
ΔT in 32 °C (90 °F)
water
600
500
400
Cooling curves
in 60 °C (140 °F) water
350
300
250
200
500
400
300
200
100
200
50
100
50
100
0
0
0
0
300
100
0
(a)
20
40
60
80
100
ΔT in 32 °C (90 °F)
water at 0.25 m/s
(50 ft/min)
600
150
150
ΔT in 25% PQ90,
0.25 m/s (50 ft/min) 32 °C (90 °F)
700
Temperature, °F
450
gradients from surface to center of a part and
high temperature differences between thick
and thin sections. The film (heat-transfer) coefficients generally decreased with increasing
water temperature until, at temperatures of 90
to 100 C (195 to 212 F), the film coefficients
were in the range of 0.13 to 0.30 W/cm2 K
(92 to 207 Btu in./ft2 h F).
Table 2 provides similar data on two polymer
solutions under selected conditions. The 25%
solution of polyalkalene glycol (UCON A) produced Grossman numbers of 0.19 to 0.23 and
film coefficients of 0.63 to 0.77 W/cm2 K
(429 to 529 Btu in./ft2 h F) when the bath
was operated at 30 C (85 F) and with velocities from 0 to 0.5 m/s (0 to 100 ft/min). Polyvinyl pyrrolidone (PVP) 90 produced Grossman
numbers of 0.40 to 0.44 and film coefficients
of 1.34 to 1.49 W/cm2 K (912 to 1012
Btu in./ft2 h F) under these conditions. The
higher film coefficient of PVP 90 suggests that
it can be used to quench heavier sectioned parts.
If the Grossman number, H, or the effective
interface heat-transfer coefficient, h, between
the part and the quenchant is established, the
quench factor in commercial shapes can be calculated using finite-element or finite-difference
heat-transfer programs. The results of calculations on sheets and plates made using constant
film (or heat-transfer) coefficients are illustrated in Fig. 14. These figures illustrate the
interrelationship between aluminum sheet or
plate thickness, film coefficient, and quench
factor. The calculations for these graphs were
made using film coefficients indicated at the
end of each diagonal line. The diagonal lines
represent lines of constant film coefficient.
The important feature of these figures is that
(with data on film coefficient such as that
Cooling curves in
25% PQ90, 32 °C (90 °F)
Cooling curves
in 32 °C (90 °F) water
0
20
40
60
80
100
150
200
250
Time, s
Time, s
0
50
100
ΔT, °F
0
(b)
Fig. 13
20
40
60
80
100
120
140
ΔT, °C
Cooling curves and temperature difference across a 76.2 mm (3 in.) diameter 7075 alloy probe with quenchant flow of 0.25 m/s (50 ft/min). (a) Water quenchant at
60 and 32 C (140 and 90 F). (b) Water quenchant and 25% PQ90 quenchant at 32 C (90 F). Source: Ref 15
156 / Heat Treating of Aluminum and Its Alloys
presented in Table 2) estimates can be made
about the ability of specific quenchants and
operating conditions to provide cooling rates
sufficiently high to meet minimum mechanical
properties in parts of various thicknesses. (See
the section “Quench-Factor Analysis” in this
article for more details.)
specific heat of vaporization and high specific
heat capacity. The thermal conductivity is very
small compared to most metals. Therefore,
water is used wherever it is practical, that is,
where the drastic quench afforded by water
does not result in excessive distortion or warping of the workpiece. Other advantages of water
include:
Water as a Quenchant
C
20
15
10
5
1.25
(a)
4
.14
=0
16
0.2
C = 0.288
C=
32
C = 0.4
20
C = 0.7
C = 1.44
2.5
3.75
5
6.25
7.5
Sheet thickness, mm
=
1.
15
C=
C=
0.7
2
0.4
3
.216
44
1.
16
8
C 2.8
=
C 4.32
= 0
C
.2
=7
C
25
=
20
15
10
2.
5
0
0
12.5
25
37.5
50
Plate thickness, mm
(b)
62.5
75
Plots of quench factors derived from finite-element analysis with given (a) sheet and (b) plate product sizes
and film (heat-transfer) coefficients (C). Heat-transfer coefficients between the quenchant and part are
expressed in W/cm2 K. Source: Ref 15
1000
1800
100
Cooling rate, °F/s
150
1800
1600
1400
800
Temperature, °C
1200
800
1400
1200
600
1000
800
400
Water temperature:
40 °C
50 °C
60 °C
70 °C
80 °C
90 °C
600
400
200
200
0
0
(a)
Fig. 15
10
20
30
Time, s
40
50
250
1600
1000
200
200
1000
600
400
50
Temperature, °F
Water temperature:
40 °C
50 °C
60 °C
70 °C
80 °C
90 °C
800
Temperature, °C
C=0
30
0
60
(b)
0
20
40
60
80
100
120
140
Temperature, °F
25
=
35
=
C
40
3
C
8
10
0.
45
2.5
C
30
50
Sheet thickness, in.
1
1.5
2
0.5
0
C = 0.072
2
07
0.
C
0.0
35
=
36
40
Fig. 14
0.3
•K
W/
45
0
0
0.25
One disadvantage of plain water as a
quenchant is that its rapid cooling rate persists
throughout the lower temperature range, contributing to large thermal stress and distortion
Quench factor for 99.5% of
attainable yield strength for alloy 7075-T73
Sheet thickness, in.
0.1
0.15
0.2
0.05
cm 2
0
50
C=
Quench factor for 99.5% of
attainable yield strength for alloy 7075-T73
Water is the fluid most frequently employed
for quenching aluminum, and it is widely used
for quenching all aluminum alloys. Water
quenching is done either by total immersion or
by spray. As a quenching medium, plain water
approaches the maximum cooling rate attainable in a liquid. Water has a high quenching
power (heat-transfer coefficient) due to the high
Lack of flammability
Low cost
No health hazards
Easy scale removal by filtration
No environmental hazards associated with
water
in quenched parts. Another disadvantage of
using plain water is that its vapor blanket stage
may be prolonged. This prolongation, which
varies with the degree to which the complexity
of the part being quenched encourages vapor
entrapment and with the temperature of the
quench water, results in uneven hardness,
unfavorable distribution of stress, and
distortion.
To obtain reproducible results by water
quenching, the temperature, agitation, and contamination must be controlled. Water temperature is the largest primary variable controlling
the cooling rate. Surface cooling power of
water decreases rapidly as water temperature
increases Fig. 15. As the temperature of water
is raised, the stability of the vapor phase
increases, and the onset of nucleate boiling in
a stagnant fluid is suppressed.
Agitation helps dispel the prolonged vapor
phase and increases the cooling rate (Fig. 16).
Agitation is especially important in water
quenching because it provides for more
uniform heat transfer around parts. If the
water is not well agitated, then highly variable heat transfer can occur. This is especially
true of nonsymmetrical parts, where large differences in heat transfer can result in distortion or cracking. Very large differences in
heat transfer also can occur when the vapor
phase persists in holes, cavities, and on surfaces with adjacent parts.
Water Contamination. An early study using
cooling curves (Ref 16) showed that quenching
into still water caused rapid heat transfer. This
study showed that heat transfer at the surface
of the part was very turbulent at the metal/water
interface. This study also showed that there was
a marked difference between hard water and
distilled water. Distilled water showed an
extensive vapor blanket that extended to very
low temperatures (Fig. 17).
If a water quenchant contains foreign substances, such as emulsions, dissolved salts,
600
400
200
160
Cooling rate, °C/s
Effect of bath temperature on heat removal in an ASTM D6200 probe. (a) Cooling curves. (b) Cooling rate curves. Quenchant is water having 0.25 m/s (50 ft/min) velocity.
Quenching of Aluminum Alloys / 157
The first groups of contaminants are those
that are poorly soluble in water. Solids such as
soot and liquids that contain soaps, fats, and
oils form suspensions or emulsions that are
prone to surface reactions that promote vaporphase stability. This results in an increased
duration of the vapor phase, with a lower temperature of nucleate boiling initiation. Oils,
soaps, and fats are the most damaging. As a
result, this group increases nonuniform heat
transfer. This nonuniformity manifests itself as
spotty hardness or increased distortion. Dissolved gases behave in a similar fashion, with
increased dissolved gases coming out of solution during the quenching process. This is one
reason why compressed air is not recommended
for agitating water quenchants.
Salts, acids, and alkalis readily dissolve in
water. These act to reduce the stability of the
vapor phase during quenching. If the concentration is high, then the vapor phase will not form
at all. This type of contamination can be taken
advantage of to create quenchants with very
fast quench rates.
Brine Quenching. Addition of a low percentage of salt (up to 10%) increases the speed
of quenching over that of cold water. The saltwater solution tends to break up the vapor
phases, thus increasing cooling power. It is only
rarely used when water is not sufficient in
obtaining properties in thicker sections.
Cooling curves and cooling rate curves for a 25 mm (1 in.) diameter stainless steel probe quenched in 55 C
(130 F) water that is flowing at selected velocities from 0 to 0.75 m/s (0 to 150 ft/min)
1650
900
Temperature, °C
800
Hard water
20 °C (70 °F)
40 °C (105 °F)
60 °C (140 °F)
Distilled water
20 °C (70 °F)
40 °C (105 °F)
60 °C (140 °F)
1470
700
1290
600
1110
500
930
400
750
300
570
200
390
100
212
Temperature, °F
Fig. 16
32
0
0
10
20
30
40
50
60
Time, s
Fig. 17
Comparison of hard and distilled water cooling curves at different temperatures
or gases, the quenching characteristics can be
drastically altered. Local ground water or tap
water can have large differences in soluble
gases, salts, or solids. This results in different
localities having different quenching characteristics. As was noted previously, the most
important factor of water quenching is the
persistent vapor phase. Based on this, contaminants may be divided according to their
effect on the vapor phase, that is, either
increasing or decreasing the stability of the
vapor phase.
Immersion Water Quenching
For most aluminum applications, either cold
or hot water immersion quenching is performed. Maximum properties are obtained with
quenching in cold water, while distortion control and residual stresses are minimized with
quenching by hot water at 70 C (160 F) or
more. In quenching some products, water at
below 38 C (100 F) provides the required
quench rate for optimum properties of the alloy
being heat treated. In others, the water may be
purposely heated to the boiling point to control
distortion and residual stresses.
Some normally accepted water quenching
practices for common aluminum alloys are summarized in Table 3. Quench rates (defined as the
cooling rates when the parts are fully immersed
in the quenchant) depend primarily on water
temperature and agitation. In addition, two other
factors influence the extent of unintended precipitation (solute loss) after solution treatment:
Quench delay: the total time from opening
the furnace until the parts are submerged in
the quenchant (or otherwise fully subjected
to a quench)
Immersion rate: the rate at which parts enter
the quenchant (or quench system)
The cooling rate depends primarily on water
temperature and agitation (Ref 17). The cooling
158 / Heat Treating of Aluminum and Its Alloys
rate of water quenching is independent of material properties such as thermal conductivity and
specific heat. Water temperature is the largest
primary variable controlling the cooling rate.
Surface cooling power of water decreases rapidly as water temperature increases (Fig. 15).
Hot water has a low cooling power because,
as the boiling point is approached, the vapor
phase becomes prolonged. Agitation is important, because it provides more uniform heat
transfer around parts by disrupting the prolonged formation of vapor phase. As agitation
is increased, the variation in hardness decreases
and distortion decreases.
Water at a temperature of 15 to 25 C (60 to
75 F) can provide uniform quenching speed
and reproducible results. Quenching into water
at less than 50 to 60 C (120 to 140 F) often
produces nonuniform quenching. This nonuniformity manifests itself as spotty hardness, distortion, and cracking. This nonuniformity is
caused by relatively unstable vapor blanket formation. Because of this difficulty, immersion
quenching in cold water usually is restricted to
the quenching of simple, symmetrical parts.
Polyalkylene glycol quenchants also are used
to provide a quench rate in between that of
water and oil. By control of agitation, temperature, and concentration, quench rates similar to
water can be achieved.
Because rapid cooling rates are achieved with
water at lower temperatures, water near room
temperature (15 to 25 C, or 60 to 75 F) is used
for many aluminum quenching operations. The
temperature at the onset of quenching is usually
in the range of 15 to 30 C (60 to 90 F). Most
specifications limit the temperature of the water
to below 30 C (90 F), with the maximum rise
of no more than 5 C (10 F). This requirement
governs the design of most quench tanks regarding the total volume in an immersion quench
tank. (See the section “Quench Tank Systems”
in this article for more details.)
Effect of Water Temperature. When water
quenchant temperature is increased, two things
occur. First, the vapor phase becomes much
more pronounced and stable. Second, the maximum cooling rate during nucleate boiling
decreases. In addition, the temperature of maximum cooling also decreases as the temperature
of the water is increased (Table 4). These
effects also are illustrated in the cooling curves
of Fig. 15. In general, as the temperature of
water is raised, the stability of the vapor phase
increases, and the onset of nucleate boiling in
a stagnant fluid is suppressed.
Table 5 summarizes the effects of water temperature on cooling rates for aluminum alloys
of different thicknesses. When water temperature is raised above 70 C (160 F), the quenching rates are drastically reduced (Fig. 18a). This
can result in a significant loss of strength when
heat treating the more quench-sensitive 2xxx and
7xxx alloys, such as 7075, 7049, and 7178. For
example, Fig. 19 illustrates the effect of quench
rate on the strength of 7075 plate with a thickness of 13 mm (0.5 in.). Significant losses in
water lowers strength even further but has the
beneficial effect of lowering residual stresses
below that of cold or hot water quench. The
compromise is to reduce attainable strength in
favor of a more dimensional stability. However,
depending on the agitation within the tank,
quench speeds in boiling water are sometimes
strength of thicker products also can occur with
a water temperature of 50 C (120 F).
Because of the rapid change in quenching
characteristics of water above 70 C (160 F),
close control of agitation is required to ensure
better control of quenching rates at various
locations in the tank. Quenching in boiling
Table 3 Water quenching practices for common aluminum alloys
Cold water
Form
Alloy/temper
20–32 C
(70–90 F)
Sheet
2014
2024
2219
6061
7075
7049
7050
7175
2024
2219
6061
7075
7049
7050
7175
2014-T6
2014-T61
2024
2219
6061
7075
7049
7050
7175
C355
A356
A356 premium
A357 premium
A201
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
...
...
...
...
...
...
...
X
...
...
X
X
X
Plate
Forgings
Castings
Hot water
Boiling water
55–65 C
(130–150 F)
60–70 C
(140–160 F)
65–100 C
(150–212 F)
95–100 C
(202–212 F)
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
X
...
...
X
X
X
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
X
...
X
X
X
X
...
X
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
X
...
...
...
...
...
...
...
...
...
...
...
...
Source: Ref 1
Table 4 Effect of water temperature on cooling rates
Water temperature
C
40
50
60
70
80
90
Maximum cooling rate
F
105
120
140
160
175
195
C/s
153
137
115
99
79
48
Maximum cooling-rate temperature
F/s
275
247
207
178
142
86
C
535
542
482
448
369
270
F
995
1008
900
838
696
518
Cooling rate at specified temperature, C/s ( F/s)
704 C (1299 F)
60
32
20
17
15
12
(108)
(58)
(36)
(31)
(27)
(22)
343 C (649 F)
97
94
87
84
77
26
(175)
(169)
(157)
(151)
(139)
(47)
232 C (450 F)
51
51
46
47
47
42
(92)
(92)
(83)
(85)
(85)
(76)
Source: Ref 18
Table 5 Effects of water temperature on cooling rates for aluminum alloys of different
thicknesses
Quenching rate for specified thickness, C/s ( F/s)
Water quenchant
C
32
55
70
88
100
Source: Ref 1
F
90
130
160
190
212
0.75 mm (0.030 in.)
5,600
1,700
500
140
50
(10,000)
(3000)
(900)
(250)
(90)
1.75 mm (0.070 in.)
3,300
560
170
60
17
(6000)
(1000)
(300)
(105)
(30)
25 mm (1.00 in.)
75 mm (3.00 in.)
120 (220)
90 (160)
35 (65)
4.5 (8)
2 (4)
16 (28)
12 (22)
8 (15)
1.7 (3)
1 (2)
Quenching of Aluminum Alloys / 159
900
Table 6 Typical quench delay times for
aluminum alloys of various thicknesses
400
200
700
600
300
500
400
75
300
200
0
130
4
200
160
8
(a)
12
16
Minimum
thickness
Temperature, °C
Temperature, °F
800
mm
Up to 0.41, inclusive
Over 0.41 to 0.79,
inclusive
Over 0.79 to 2.29,
inclusive
Over 2.29
20
Time, s
in.
Up to 0.016, inclusive
Over 0.016 to 0.031,
inclusive
Over 0.031 to 0.090,
inclusive
Over 0.090
Maximum
time, s
5
7
10
15
Source: Ref 19
900
800
60%
600
300
500
400
200
40%
300 H2O
200
0
(b)
Temperature, °C
Temperature, °F
400
700
30%
12%
4
20%
8
12
16
20
Time, s
Fig. 18
Comparison cooling rates for 12.5 mm (0.5
in.) thick wrought 7075 aluminum plate
when (a) quenching with water at different temperatures
and (b) quenching with different concentrations of AMS
type I polymer quenchant
uneven, resulting in uneven residual stresses.
When quenched in boiling water, the part is
actually being quenched in a continuous vapor
pocket, especially during the early stages of
the quench (Ref 1). The alternative is to consider PAG quenchants, which can achieve
higher strength and lower residual stress compared to a boiling water quench.
In practice, aluminum parts prone to distortion
(especially castings and forgings) are quenched
in either hot or boiling water. In the case of other
wrought products, such as plates and extrusions,
only the less quench-sensitive alloys (such as
6061) are quenched in boiling water. In some
other specific applications, boiling water may
be used to achieve a high level of dimensional
stability. For example, alloy 2014 has a specific
temper (2014-T61) of a boiling water quench
for dimensional stability (Ref 1).
There is relatively little impact of increasing
water temperature on C-stage cooling rates.
Because the objective of raising the water temperature is to decrease cracking and distortion,
and because the tendency for cracking to occur
is often proportional to C-stage cooling rates,
these figures show that raising the water temperature is a relatively ineffective procedure.
These data explain why alternative quenchants
such as aqueous polymers are used. Another
reason for the use of these quenchants as alternatives to hot water is that these media produce
much more uniform wetting of the metal surface during the quenching. This is critically
important if localized surface cracking and distortion are to be avoided.
The interaction of temperature and agitation
on the vapor phase is much stronger for water
Fig. 19
Effect of water temperature on the strength of
7075 plate with a thickness of 13 mm (0.5 in.)
than in other quenchants. This is why water
quenching is generally not used for complex
shapes or blind holes. These geometrical
obstructions result in a nonuniform heat transfer
around the surface of the part. In areas of persistent vapor phase, poor quenching and solute
precipitation can occur.
Quench Delay. Whether the transfer of
parts from the furnace to the quench is performed manually or mechanically, it must be
completed in less than the specified maximum
time. The maximum allowable transfer time,
or quench delay, varies with the temperature
and velocity of the ambient air and the mass
and emissivity of the parts. The maximum
quench delays (Table 6) can be determined that
will ensure complete immersion before the
parts cool below 400 C (750 F). Aerospace
Materials Specification (AMS) 2770 specifies
maximum quench delays for high-strength
alloys of 5, 7, 10, and 15 s for thickness ranges
of up to 0.41 mm (0.016 in.), 0.41 to 0.79 mm
(0.016 to 0.031 in.), 0.79 to 2.29 mm (0.031
to 0.090 in.), and over 2.29 mm (0.090 in.),
respectively.
Quench delay is conservatively defined as
commencing “when the furnace door begins to
open or the first corner of a load emerges from
a salt bath” and ending “when the last corner of
the load is immersed in the water quench tank.”
Recommended maximum quench delay times
are listed in Table 6. However, exceeding the
maximum delay time is permitted if temperature measurements of the load prove that all
parts are above 415 C (775 F) when
quenched. The C-curves used in quench-factor
analysis can also assist in determining a maximum allowable delay.
It is relatively easy to control quench delay
in day-to-day operations by using a stopwatch
or, if necessary, by attaching thermocouples to
parts. However, although the cooling rate
between 400 and 260 C (750 and 500 F) is
most critical and must be extremely high for
many high-strength alloys, it cannot be directly
measured in production operations. It is usual to
rely on standardized practices, augmented by
results of tension tests and tests of susceptibility
to intergranular corrosion.
Immersion Rate Control. Control of immersion rates is very important to minimize warpage when quenching sheet or thin-gage
products. A faster immersion rate results in less
distortion, and more modern furnaces are
designed with controls to select immersion
rates. Immersion rates are normally in the range
of 0.15 to 3.0 m/s (0.5 to 10 ft/s) (Ref 1).
Spray or Fog Quenching
Spray techniques vary from the application
of a heavy spray to that of a light mist or fog.
Spray quenching is confined almost entirely to
products having one long dimension, including
extrusions, rolled structural shapes, sheet, and
plate. Because spray quenching avoids the hazard of steam pocketing, it can result in a vigorous quench. However, it requires judicious
design and application to avoid nonuniform
cooling that would cause undesirable distortion.
Spray quenching generally is accomplished by
passing the product through a chamber containing a multiplicity of spray nozzles, arranged to
provide thorough, complete coverage of all
exposed surfaces. The entry sprays are most
important. Unless the entire periphery of the
metal is contacted uniformly, distortion and
bowing occur. The nozzles for these sprays
are selected and arranged to deliver a flat pattern; if sections of variable height are to be
processed, the nozzles should be adjustable.
Flooding-type or square-pattern nozzles usually
follow the entry sprays. The spray chamber
should be long enough to permit a rate of metal
movement into the sprays that avoids excessive
precooling of the rear of the charge before that
part reaches the lead spray. The terminal sprays
at the discharge end frequently are directed into
the quench chamber to minimize carry-out of
quench medium; they may be followed by airblast nozzles to reduce this possibility further.
If possible, cooling performance should be
verified by cooling curves in the production
chamber.
For spray quenching, the quench rate is controlled by the velocity of the water and by volume of water per unit area per unit time of
impingement of the water on the workpiece.
Spray quenching rates are much slower than
even high-concentration polymer quenchants. If
160 / Heat Treating of Aluminum and Its Alloys
spray quenching is applied on quench-sensitive
alloys, a heavy volume spray is needed to
almost flood the part. Large flow rates require
an abundant water supply. Cost generally dictates the use of a reservoir and recirculating
system rather than drawing water from normal
supply lines and discharging it after one use.
Location of the quench tank or spray chamber
with respect to the heat treating furnace is
important. To ensure the quench speed generally required, these facilities should be adjacent
to the furnace that they service. In elevator-type
furnaces, such as the bottom-door or verticaltower, placing the quench tank directly beneath
the heating chamber is ideal. Quench tanks
should have a ready means of draining and
cleaning, for removing contaminants that may
discolor the product.
Rate of travel of the workpiece through the
sprays is an important variable. Local increases
in temperature that occur within the first few
seconds of quenching, caused by a phenomenon
such as plugged spray nozzles, are particularly
deleterious. The remaining internal heat may be
sufficient to reheat the surface region. When this
happens, a large loss in strength occurs at the
previously quenched surface. The loss of
strength in the affected area of a heavy part is
much more severe than that caused by an inadequate quenching rate alone. This is illustrated for
75 mm (3 in.) thick 7075-T62 plate in Fig. 20,
which compares, at various depths, the properties of a plate for which quenching was
100
0
0.5
1.0
Depth, in.
1.5
2.0
2.5
3.0
80
70
Longitudinal yield strength, MPa
Longitudinal tensile strength, MPa
50
0
Side A
0
600
Control specimen
Quenched from side A only
Quenched from side B, interrupted after 3 s
10
20
0.5
40
50
30
Depth, mm
Depth, in.
1.0
1.5
2.0
60
70
2.5
80
Side B
3.0
80
500
70
60
400
50
300
0
Side A
550
0
10
20
0.5
30
1.0
40
50
Depth, mm
Depth, in.
1.5
2.0
60
70
2.5
80
Side B
3.0
70
450
60
350
50
40
250
30
150
0
Side A
Fig. 20
10
20
30
40
50
Depth, mm
60
70
Longitudinal tensile strength, ksi
60
Longitudinal yield strength, ksi
Hardness, HRB
90
80
Side B
Through-thickness property variations due to
quench rate and temperature-rise effects in
75 mm (3 in.) thick 7075-T62 plate
interrupted on one side after 3 s with those of a
plate that was quenched from one side only.
Polymer Quenchants
Polymer quenchants are widely used in the
quenching of aluminum to control distortion
and residual stresses. Polymer quenchants retard
cooling rates by the formation of films around
the part. The effective film coefficient is essentially the heat-transfer coefficient (C), which is
related to the Grossmann number (H) as compared with water in Table 2. The application of
polymer quenchants is covered in AMS specifications 3025 and 2770, although many aluminum
and aerospace companies have developed internal specifications that differ from AMS 2770.
Typical parameters for quenching wrought products (other than forgings) in glycol-water solutions are presented in Table 7.
For all polymer quenchants, the primary
manufacturing variables to achieve a desired
quenching rate include:
Concentration
Agitation
Temperature
As the concentration of the polymer is
increased, the effective quench rate is reduced.
As the concentration is increased, a limit will
be reached where additions of polymer will
not significantly reduce the cooling rate. This
concentration is dependant of the molecular
weight of the polymer and the type of polymer
chosen.
Benefits of using aqueous-based polymer
quenchant solutions are many fold. Concentrations can be changed quickly and tailored to
specific products. There are no fire hazards (in
contrast to oil), and an economic recovery system can be put in place to cut the water and
chemical costs of operating the systems. Based
on experience in the aerospace industry,
straightening costs can be reduced by up to
60% compared to using a water quench.
Polymer quenchants tend to be more sensitive to agitation. Increasing the agitation
Table 7 Limits for quenching in glycolwater solutions
Data are for wrought aluminum alloy products other than
forgings.
Maximum
thickness
Glycol
concentration,
vol%
Alloys
12–16
17–22
23–28
29–34
35–40
2014,
7075,
2014,
7075,
2014,
7075,
7075,
2014, 2117, 2024,
7075,
2017, 2117, 2024,
7079, 7175, 7178,
2017, 2117, 2024,
7079, 7175, 7178,
2017, 2117, 2024,
7079, 7175, 7178,
7079, 7175, 7178,
mm
2219 2.03
7175 25.4
2219 1.80
6061 12.7
2219 1.60
6061 9.53
2219 1.02
6061 6.35
6061 2.03
in.
0.080
1.000
0.071
0.500
0.063
0.375
0.040
0.250
0.080
increases the cooling rate and reduces the polymer film thickness. However, decreasing the
agitation can produce nonuniform quenching
because of nonuniform film thickness. It also
limits the transport of polymer to the part surface.
As in every quenching operation, the magnitude
and uniformity of agitation is extremely important. Racking of parts is more critical in polymer
quenchants because of the strong effects of temperature. Agitation tends to minimize these thermal gradients within the quenchant.
The effective quench rate of polymer solutions is affected by temperature. As temperature
is increased, the quench rate is reduced.
Increasing temperature also increases the oxidation and reduces thermal stability of the polymer, effectively shortening the life of the
polymer. The amount of degradation is dependent on the amount of polymer used and the
application temperature. Depending on the
polymer used, there is also a limit on the bulk
quench temperature of the quenchant, because
some quenchants will tend to separate or precipitate from solution. This is true of PAG-type
polymer solutions. In general, the typical
operating temperature range of polymer
quenchants is 20 to 40 C (70 to 105 F). Many
specifications limit the temperature rise during
quenching to 5.5 C (10 F), with a maximum
temperature of 43 C (110 F).
Polyalkylene Glycol Quenchants
Polyalkylene glycol quenchants are the most
commonly used polymer quenchants in the heat
treating market today (2016). Polyalkylene glycols, or polyalkylene glycol ethers, were first
introduced as a family of commercial products
in the early 1940s. These materials are formulated by the random polymerization of ethylene
and propylene oxides (although higher-alkylene
oxides and/or aryl oxides may be used also).
Although block polymerizations of these same
oxides are possible, these derivatives are less
attractive as quenchants.
The PAG quenchants are an example of a
copolymer. This quenchant is derived from
two monomeric units, ethylene oxide and propylene oxide (Fig. 21). By varying the molecular weights and the ratio of oxides, polymers
having broad applicability may be produced.
Certain of the higher-molecular-weight products were shown to have utility as metal
quenchants when used in aqueous solution
(U.S. Patent 3,230,893). Proper selection of the
polymer composition, and its molecular weight,
provides a PAG product that is completely soluble in water at room temperature. However,
the selected PAG molecules exhibit the
unique behavior of inverse solubility in water,
that is, water insolubility at elevated temperatures. This phenomenon provides the unique
mechanism for cooling hot metal by surrounding
the metal piece with a polymer-rich coating that
serves to govern the rate of heat extraction into
the surrounding aqueous solution. As the temperature of the metal part approaches the
Quenching of Aluminum Alloys / 161
temperature of the quenchant itself (stage C), the
PAG polymer coating dissolves to again provide
a uniform concentration in the quenchant bath.
This is shown in Fig. 22.
This mechanism of inverse solubility is limited to two polymer quenchant classes: polyalkylene glycol and polyethyloxazoline. In these
systems, as the temperature of the solution is
raised, the thermal energy of the system
becomes greater than the energy of the hydrogen bond interactions with water. When this
occurs, a two-phase system develops, with
one layer being water-rich and the other a
polymer-rich layer. This is not a clean separation because both phases have some of the other
component. The temperature at which this separation occurs is called the cloud point. In PAG
quenchants, the ratio of the monomers used to
produce PAG quenchants controls the cloud
point. In this case, the cloud temperature
decreases as the propylene oxide monomer proportion increases. The cloud point also is the
basis for the practice of purifying the quenchant
bath (see the section “Polyalkylene Glycol
Quenchant Reclamation” in this article).
In the section describing the cooling characteristics of water, one of the disadvantages cited
for plain water is that the vapor blanket stage
(stage A) may be prolonged. This prolongation
encourages vapor entrapment that may result
in uneven hardness and unfavorable distribution
of stress, which, in turn, may cause cracking
and/or distortion. By using PAG quenchants,
uniform wetting of the metal surface results,
O
thereby avoiding unevenness and the accompanying soft spotting. In fact, selection of the
proper PAG quenchant can provide accelerated
wetting so that the cooling rates achieved are
faster than water and approach those achieved
by brines. Thus, brine quenching is possible
without the hazards and corrosiveness attendant
with the use of salts or caustic solutions.
Whereas rusting can be a drawback when
quenching with water alone, particularly where
recirculation of treated water is not employed,
solutions of PAG quenchants may be inhibited
to provide corrosion protection of the quench
system components. Corrosion inhibition of
quenched parts will be of short duration, so that
specific protection should be provided following the tempering operation.
In AMS 2770, the Aerospace Materials
Engineering Committee of SAE has determined the recommended concentrations to be
used when quenching aluminum. These concentrations are extremely conservative and
are based on a zero-delta strength difference
between water and PAG quenchants. They
have further categorized the PAG quenchant
types into two categories: type I and type II
(from SAE International AMS 3025). The
physical property differences of these two
types of PAG quenchants are shown in Table 8.
Allowable concentrations of polymer quenchants for aluminum in accordance with AMS
2770 are shown in Table 9.
The influence of polymer concentration on
cooling rates is illustrated by the cooling curves
O
nCH2 – CH2 + mCH2 – CH
[(CH2CH2O)n(CH2CHO)m]–
CH3
Ethylene
oxide
Propylene
oxide
CH3
PAO
shown in Fig. 18(b). Cooling curves as a function of PAG concentration are similar to
quenching with water at different temperatures
(Fig. 18a). The slower rates of cooling achieved
at the higher concentrations reflect the thickness
of the polymer layer that surrounds the heated
part during quenching. The PAG quenchants
also are less sensitive to minor changes in polymer concentration, which is a recognized deficiency of polyvinyl alcohol and the other filmforming polymer quenchants.
Just as water exhibits a marked decrease in
cooling capability as its temperature is elevated (Fig. 15, 18a), this same loss is translated to the aqueous solutions of PAG
quenchants. The curves shown in Fig. 23 are
illustrative of the general trends that would
occur with changes in bath temperature; more
detailed data would require specific identification of the particular PAG quenchant
employed. In general, low to moderate agitation is essential to ensure that adequate replenishment of polymer occurs at the hot metal
surface and to provide uniform heat transfer
from the hot part to the surrounding reservoir
of cooler quenchant. Figure 24 clearly illustrates that, as agitation is increased, the cooling curves shift to more rapid rates.
Polyalkylene glycol quenchants have shown
remarkable growth and utility in reducing
residual stresses and distortion in aluminum
sheet metal, forgings, and castings. The degree
of distortion control that can be achieved using
PAG quenchants is illustrated in Table 10.
Early work (Ref 21) at Boeing using AMS
3025 type I polymer quenchants showed that
the use of polymer quenchants substantially
reduced the distortion of quenched sheet metal
parts. The data (Fig. 25) showed that when
quenching 1 mm (0.040 in.) thick aluminum
sheet, quench rates exceeding 2700 C/s
(4860 F/s) were obtained when quenching
in ambient-temperature water. Using a 40%
concentration of a type I quenchant, a maximum quenching rate of 1000 C/s (1800 F/s)
was observed. This quench rate is
Fig. 21
Synthesis of polyalkylene glycol quenchants
Fig. 22
Sequence of quenching in a polyalkylene-glycol-type polymer. (a) Moment of immersion; polymer film deposits on component surface. (b) After 15 s, film becomes active.
(c) After 25 s, boiling occurs over entire surface. (d) After 35 s, boiling ceases and convection phase begins. (e) After 60 s, polymer starts to redissolve into solution. (f) After
75 s, film has completely redissolved and heat removal is entirely by convection. Courtesy of Houghton International
162 / Heat Treating of Aluminum and Its Alloys
approximately 10 times the rate necessary to
achieve full properties in aluminum sheet.
Further work (Fig. 26) showed significant
reductions in distortion as the concentration
of type I quenchant was increased.
Additional work at Northrup (Ref 22) showed
that sheet metal products could be quenched in
polymer concentrations up to 40%. When
quenching similar sheet metal parts in ambient
water, quenching rates through the critical range
of 400 to 300 C (750 to 570 F) exceeded
2200 C/s (3960 F/s), significantly higher than
the 100 C/s (180 F) necessary to achieve full
quenching. The results of this work are shown in
Fig. 27. A dramatic visual demonstrating the benefits of PAG quenchants in controlling distortion
is shown in Fig. 28.
Often, large castings and forgings are
quenched in hot water to achieve residual-stress
control. Often, these quench rates are slow and
properties are diminished; however, the residualstress and distortion concerns outweigh property
considerations. It is apparent that the cooling
rates of PAG quenchants are similar at high concentrations to that of elevated-temperature water
(Fig. 18). Concentrations of up to 60% type
I quenchant achieved similar quench rates as a
93 C (200 F) water quench (Ref 23, 24). These
data (Fig. 29) provide for proper selection of PAG
concentrations for large forgings and castings,
provided that reference data on alloy and properties can be determined for elevated-temperature
water quenching.
Cooling curve analysis of polymer quenchants is more dependent on agitation than are oil
quenchants. To overcome this, two standards have
been developed to accurately measure the cooling
curves of polymer quenchants: ASTM D6482
(Tensi method) and ASTM D6549 (Drayton
method). Each method provides for a different
method of agitation of the quenchant. The results
of the tests cannot be compared to each other.
Many auditing agencies (National Aerospace and
Defense Contractors Accreditation Program,
CQI-9 Heat Treatment System Assessment, etc.)
are requiring cooling curve analysis of polymer
quenchants on a monthly or quarterly basis.
Table 8 Physical properties of polyalkylene glycol quenchants in accordance with
AMS 3025
Neat
Diluted to 20%
Property
Type I
Type II
Water content, neat %
Specific gravity
Refractive index
Viscosity at 38 C (100 F), cSt
Viscosity at 38 C (100 F), cSt
Cloud point, C ( F)
45–48
1.094 ± 0.005
1.4140 ± 0.005
535 ± 70
5.5 ± 0.5
165 ± 5 (330 ± 9)
57–63
1.080 ± 0.025
1.3910 ± 0.005
300 ± 20
4.4 ± 0.5
165 ± 5 (330 ± 9)
Table 9 Limits for quenching in polymer solutions in accordance with AMS 2770H
Maximum thickness(b)
Polymer type(a)
Alloy
I
2024
Sheet, extrusions
2219
6061
Sheet, extrusions
Sheet, plate, bar,
extrusions
7049
7050
7075
Sheet, plate, bar
6061
7075
Forgings
7049
7149
Forgings
7050
Forgings
7049
7050
7075
2024
Extrusions
II
mm
in.
Polymer concentration
(a)(c), %
1.02
1.60
1.80
2.03
1.85
6.35
9.52
25.4
2.03
6.35
9.52
12.70
25.4
25.4
50.8
63.5
25.4
50.8
76.2
25.4
50.8
76.2
101.5
6.35
9.52
0.040
0.063
0.071
0.080
0.073
0.250
0.375
1.00
0.080
0.250
0.375
0.500
1.00
1.00
2.00
2.50
1.00
2.00
3.00
1.00
2.00
3.00
4.00
0.250
0.375
34 max
28 max
22 max
16 max
22 max
40 max
32 max
22 max
40 max
34 max
28 max
22 max
16 max
18–22
11–15
8–12
18–22
11–15
8–12
28–32
24–28
18–22
13–17
28 max
22 max
(d)
(d)
(d)
(d)
(d)
...
1.02
1.60
2.03
1.02
4.83
6.35
0.040
0.063
0.080
0.040
0.190
0.250
34
22
16
34
20
18
(d)
(d)
(d)
...
1.0
2.0
11– 15
8–12
Form
Sheet, extrusions
Sheet, plate, bar
6061
7049
7050
7075
6061
7075
Forgings
25.4
50.8
Notes
...
...
(e)
(e)
...
...
...
max
max
max
max
max
max
...
(e)
(a) Types I and II polymer solutions and concentrations shall conform to AMS 3025. Concentrations are percentages by volume of the undiluted
polymer as furnished by the producer. (b) Thickness is the minimum dimension of the largest section at the time of heat treatment. (c) Where only
maximum concentration is shown, any 4% range may be used, except the maximum shown shall not be exceeded. When concentration is specified on
a drawing or purchase order without tolerance or range, the tolerance shall be ±2%. (d) Applicable when final temper is T4 or T42. When final temper is T6 or T62, sheet and plate up to 6.35 mm (0.250 in.), inclusive, may be quenched in types I or II polymer solution at 22% max. (e) Prohibited
for 7075 alloy over 25 mm (1 in.) when final temper is T6
35
70
110
Cooling rate, °F/s
145 180 215 250
325
360
35
900
1470
800
1290
700
3
2
1
600
1110
500
930
400
750
Curve 1: 20 °C
Curve 2: 40 °C
Curve 3: 60 °C
300
20
Fig. 23
40
60
80
100 120 140
Cooling rate, °C/s
290
325
360
1650
1470
2
1
1290
3 4
1110
930
Flow rate m/s
Curve 1: Nil
Curve 2: 0.8
Curve 3: 1.6
Curve 4: 2.4
300
100
200
Influence of temperature on cooling curves of a polyalkylene glycol
quenchant. Courtesy of Houghton International
Cooling rate, °F/s
145 180 215 250
400
Concentration: 25%
212
Agitation: Vigorous
180
110
500
200
160
70
600
390
200
100
570
Temperature, °C
1650
800
Temperature, °F
900
700
Temperature, °C
290
570
390
Concentration: 25%
Temperature: 40 °C
20
Fig. 24
750
40
60
80
100 120 140
Cooling rate, °C/s
160
180
Temperature, °F
Condition
212
200
Influence of agitation on the cooling curves of a polyalkylene glycol
quenchant. Courtesy of Houghton International
Quenching of Aluminum Alloys / 163
Polyalkylene Glycol Bath Maintenance
industrial environment will eventually be contaminated with dirt and debris from the parts
that are being heat treated. This, of course,
includes scale. Quench performance over time
will be affected by alterations of the total chemistry of the bath by these impurities. Filtering
the solutions with cartridge or bag filters using
5 to 10 mm filter media has proven sufficient to
keep the bath in a condition where concentration
measurements are accurate. Bath conditioning
also consists of controlling the bioburden in
the bath.
Concentration control is accomplished
using densitometers, refractometers, and viscosity meters. Several of the instruments require
frequent calibration, which adds to the maintenance burden in the factory. The refractive
index monitor with remote sensing and optional
connection to a PLC has proven to be very stable if the solution is conditioned and filtered.
Obtainable accuracy levels over time are within
±0.5%.
The refractive index of PAG polymer solutions (in the range employed for quenching) is
essentially linear with concentration (Fig. 30).
Thus, the refractive index of a PAG quenchant
solution serves as a measure of product concentration. Industrial model optical refractometers
that employ an arbitrary scale may be cali-
To be effective and economical, aqueous
polymer solutions require proper maintenance.
The ability to obtain better and more controlled
quench rates is a major factor in deciding to
implement a PAG quench into a production
process. Proper control and performance of
these solutions involves concentration control
and bath conditioning. Reclamation also is an
important economic consideration.
The concentration of the polymer in the
quench bath has one of the most significant
influences on the finished product. The cleanliness of the bath directly influences the accuracy
of the measurements. With the use of a programmable logic controller (PLC) and operator
interfaces, concentration changes tailored to
the product can be carried out accurately and
quickly. The use of fully automatic systems
has proven somewhat impractical because troubleshooting becomes difficult. For example, the
status of the filling and draining operations is
hard to monitor. Semiautomatic systems, where
each phase is initiated by an operator, have
proven more robust and less troublesome.
Bath conditioning consists of filtering and
tending to the control of any biological impurities in the solutions. Any bath that is used in an
Table 10 Degree of distortion control achieved using polyalkylene glycol (PAG)
quenchants in aluminum
Distortion in PAG
quenchants
Distortion in water
Part type
Die forging
Die forging
Machined bar
Dip-brazed chassis
Alloy
mm
7075-T6
2014-T6
7075-T6
6061-T6
2.5–3.8
in.
mm
0.10–0.15
0.05
in.
0.002
Unknown
10
None
0.40
Unknown
0.075
0.08
0.003
0.003
Source: Ref 20
1800
3600
Cooling rate, °F/s
5400
7200
9000
10,800
12,600
Air
LN2
Other
Other
0
40
20
LN2
40
PAG in water at 26 °C, %
14
11
8
6
4
Water
temperature,
°C
Air
17
2
56
30
20
11
6
2
25
10
0
1000
2000
3000
4000
Cooling rate, °C/s
5000
6000
7000
Effect of type I polyalkylene glycol (PAG) concentration on the maximum
cooling rate when quenching 1 mm (0.04 in.) thick 2024 aluminum sheet.
Source: Ref 21
Water
Quenchant
Type I PAG 26 °C, %
22
Fig. 25
brated. Whereas such instruments prove invaluable for day-to-day monitoring of the quenchant
concentration, the refractometer also will register other water-soluble components that are
introduced to the used quenchant. When the
indicated refractometer reading begins to provide erroneous numbers, some other analytical
test is required to define the effective quenchant
concentration. With PAG quenchants, kinematic viscosity measurements (which are correlated with concentration) have proven to be
most useful.
These plots also are available from quenchant manufacturers and are similar to those for
the refractive index, with one exception: The
viscosity usually has a nonlinear relationship
to the polymer concentration. Therefore, firstorder linear regression analysis should not be
used as a line-fitting procedure. Examples of
kinematic viscosity versus concentration are
shown in Fig. 31 and 32 for types I and II PAG
quenchants.
Biological Contamination and Control.
Because PAG quenchants are, for the most part,
resistant to bacteria and fungi, the addition of a
bactericide to the as-supplied quenchant is not
required. However, growth of microorganisms
can occur in use when contaminants (such as
oils) act as a nutrient. Cleanliness and oxygen
content are the keys. Solutions should remain
agitated to minimize available sources of nutrients; rust and other solids are typical food
sources.
Anaerobic bacteria and fungi thrive in oxygendepleted environments. Stagnant solutions contribute to localized oxygen depletion; thus,
quenchants should be kept moving to prevent
oxygen depletion. Additional bubbling of air can
be used, but it is generally most effective to
continue agitating the quenchants during shutdowns. Anaerobic bacteria and fungi can breed
in typical polymer quench tanks. These bacteria
and fungi are not usually a health hazard but an
26 °C
0
Fig. 26
2
4
6
8
Distortion, %
10
12
14
Effect of type I polyalkylene glycol (PAG) concentration on the distortion of
1 mm (0.04 in.) thick 2024 aluminum sheet. Source: Ref 21
164 / Heat Treating of Aluminum and Its Alloys
odor issue. These bacteria contribute to “Monday
morning smells” when equipment is first turned
on after a weekend of sitting stagnant.
Aqueous solutions will experience bacteria
and algae growth if there are no biocides
present. Bacteria growth can cause corrosion
of parts (microbiologically induced corrosion)
and can detrimentally affect membranes used
for separation in reclamation systems. The bacteria can also reduce the sodium nitrate in the
Cooling rate (400–300 °C), °C/s
0.51
10
0.81
3
1800
1.02
1.80
3.18
Cooling rate (750–570 °F), °F/s
18,000
104
180
102
0
5
10
15
20
25
30
35
40
Type I quenchant, %
Fig. 27
Effect of type I polyalkylene glycol concentration on quenching rates through the critical range of 400 to
300 C (750 to 570 F) on different thicknesses of sheet metal (0.50 to 3.2 mm, or 0.02 to 0.13 in.)
Fig. 28
Sheet metal quenched in ambient water and 20% type I polyalkylene glycol (PAG). (a) Water-quenched
sheet metal. (b) Identical sheet metal panels quenched in 20% type I PAG. Courtesy of Houghton
International
103
bath if they are anhydrous bacteria. Algae will
coat the insides of the tanks and piping and
result in incorrect concentration data. Biocides
are used with various successes.
A properly designed filtration system, with
adequate turnover, can help minimize bacteria
and fungi problems. Sand (swimming pool) filters are very effective and are capable of filtering to 6 to 8 mm. Bag and cartridge filters can
be a breeding ground for fungi and bacteria
because of low fluid flows and a high concentration of food sources. Should fungi and bacteria become a problem, then effective treatment
is necessary by the application of the appropriate biocide. Microbiological treatment, such as
is employed with other aqueous metal working
fluids, generally can keep biological activity
under control.
The polymer quenchant supplier can offer
assistance in the selection of the proper biocide
and the amount to be added. Usually, a large
kill dose is recommended. Constant application
of biocides can contribute to resistant bacteria.
In the event that this occurs, large kill doses
and switching biocides on a routine basis are
recommended. Because biocides are designed
to kill living organisms, it is absolutely critical
that proper personal protective gear be used.
Refer to the biocide suppler for proper recommendation regarding dosage and protective
equipment.
Biocides with glutealdehyde are the most
commonly used. They last from 10 to 21 days
in the bath and must be replenished periodically
to remain effective. Shop test procedures that
check for bacteria and fungi will tell the operators of the need to treat the bath. Small paddle
sticks are used for this testing, with satisfactory
results. An occasional change of biocide will
keep the bacteria from becoming resistant to
the product.
Contamination is a common occurrence in
the heat treating shop. Hydraulic fluids and
solids such as soot and rust are common. Sediment such as scale and soot can hinder concentration control by making the refractive index
hard to measure. These contaminants change
45
1800
180
25.4
50.8
10
18
76.2
1.8
35
30
Concentration, %
10
12.7
2
Cooling rate (750–570 °F), °F/s
Cooling rate (400–300 °C), °C/s
40
25
20
15
10
5
1
0
5
10
15
20
25
Type 1 PAG concentration, %
30
35
40
0
0
5
10
15
Fig. 29
Interrelationship of part thickness and type I polyalkylene glycol (PAG)
concentration for forgings and castings from 12.5 to 75 mm (0.5 to 3.0
in.) thickness. Source: Ref 23
20
25
Brix, degrees
Type I
Fig. 30
Type II
Relationship of refractive index, measured by Brix refractometers, on the
concentration of types I and II polyalkylene glycol quenchants
45
45
40
40
35
35
30
30
Concentration, %
Concentration, %
Quenching of Aluminum Alloys / 165
25
20
15
25
20
15
10
10
5
5
0
0
Fig. 31
5
10
15
Kinematic viscosity, cSt
20
25
Kinematic viscosity versus concentration for type I polyalkylene glycol
quenchants
the cooling curve behavior. The effect of each
of these contaminants is site and process specific. In general, excessive contamination can
be eliminated through proper filtration and
skimming of oils and other contaminants. In
the case of PAG quenchants, thermal separation
and the readdition of appropriate corrosion inhibitors can clean the polymer solution.
Chemical Control. The basic chemistry of
new PAG changes very little over time when
used in the quench bath. The pH level can
change and must be maintained by adding buffers in accordance with the manufacturers’
recommendations. The use of reverse osmosis
(RO) membranes requires special modifications
to the PAG. Mainly, the pH value is lowered
slightly to increase membrane life. However,
the pH must not be lowered too much, because
the PAG becomes unstable at pH values less
than 6 to 6.5. A low pH value can cause corrosion of aluminum parts during quenching.
The corrosion inhibitor used in PAG is commonly sodium nitrate. This salt will be depleted
over time and must be replenished to protect piping, pumps, and other equipment. Sodium nitrate
is also one of the first products that migrates
through a worn RO membrane and creates high
electrical conductivity in the permeate water.
This can be used as an indicator for tracking
the condition of the membranes over time.
Polyalkylene Glycol Quenchant
Reclamation
With the development of fully closed loop
systems with variable concentration control
and conditioning, the costs of PAG replacement have drastically decreased in comparison
to previous practice when rinse water was
flushed to a drain, causing dragout from the
quench tank to be lost. The capital cost of
installing these systems must be compared to
the savings in PAG replacement cost. Wastewater reduction also can be a major factor in
some regions.
0
0
Fig. 32
2
4
6
8
10
12
Kinematic viscosity, cSt
14
16
18
Kinematic viscosity versus concentration for type II polyalkylene glycol
quenchants
There are basically three ways of separating
PAG from the water after it has been diluted
to the concentration needed by the user:
Heat separation
Membrane separation
using micro- or
nanofiltration
Membrane separation using RO technology
The cost of maintaining the systems has
decreased. This is particularly true with RO
systems, because third-generation control and
hardware are now in place. These have proven
to be reliable over time.
Heat Separation. As noted, the cloud point
phenomenon can be used as a way to purify
the quenchant bath. The temperature of the bath
is increased until separation occurs. The tank is
heated to approximately 75 to 85 C (165 to
185 F), and the PAG will settle out to the bottom, unless there are considerable amounts of
salt present, in which case it settles to the top.
After separation and the polymer settling on
the bottom of the tank, water is pumped or
siphoned from the tank. Fresh water is then
added to the desired concentration. Inorganic
rust inhibitors are usually present as part of
the water phase and also must be added to the
system to retain the proper corrosion inhibition.
One additional consideration is that the increased
temperature will tend to oxidize the polymer,
resulting in shorter life.
Membrane Separation Using Micro- or
Nanofiltration. This method uses membranes
that allow the water to pass but not the PAG
and salts, which stay on the process side of
the membranes. However, small amounts of
PAG will pass through the membranes with
the water, resulting in some waste of PAG.
The PAG also will break down over time to
smaller molecules as a result of mechanical
and thermal action on the polymer.
Membrane Separation Using RO Technology. Figure 33(a) illustrates a typical closedloop RO system. With this method, PAG is separated from the water using membranes that allow
the water to pass but reject the PAG and salt,
which stay on the process side of the membranes. The water (permeate) is stored in a water
tank for later use or sent to drain. This technology does not work well in conjunction with salt
baths or steel heat treating because the salt concentration in the PAG will increase during the
concentration cycle. Salt is not desired in the
quench bath because it can cause corrosion on
the parts. Steel scale and free iron will damage
the membranes and must be removed from the
solution before it reaches the RO machine.
A new separation method has been developed that uses the heat separation concept,
and it requires only a single pass-through.
Figure 33(b) shows the schematic for this system. Note that the process tank is optional compared to the RO system shown in Fig. 33(a).
The heat separation method does not use membranes and is not sensitive to salt or iron in the
bath. Production testing concentrated a 1%
PAG solution into a 60% solution and clean
water in one pass at a rate of 3.8 L/min (1
gal/min). Other concentrations included 22%
PAG, where the recovery rate also proved to
be up to 60% PAG on the product side of the
stream and clean water on the other side. The
system is very compact, robust, and less costly
than RO separation. At this time (2016), the
method is implemented in several locations.
Other Quenching Media
Air Quenching. For maximum dimensional
stability, some forgings and castings are fan
cooled or still-air cooled. In such instances, precipitation-hardening response is limited, but
satisfactory values of strength and hardness
are obtained. Alloys that are relatively dilute,
such as 6063 and 7005, are particularly well
suited to air quenching, and their mechanical
properties are not greatly affected by its low
cooling rate. Lower quenching rates are also
employed for forgings, castings, and complex
166 / Heat Treating of Aluminum and Its Alloys
Fig. 33
Membrane separation of polyalkylene glycol (PAG) and water with (a) closed-loop reverse osmosis system and (b) one-pass heat separation. Courtesy of Bogh Industries
shapes to minimize warpage or other distortion
and the magnitude of residual stresses developed as a consequence of temperature nonuniformity from surface to interior.
Certain alloys that are relatively insensitive
to cooling rate during quenching can be either
air cooled or water quenched directly from a
final hot-working operation. In either condition,
these alloys respond strongly to precipitation
heat treatment. This practice is widely used in
producing thin extruded shapes of alloys 6061,
6063, 6463, and 7005. Upon precipitation heat
treating after quenching at the extrusion press,
these alloys develop strengths nearly equal to
those obtained by adding a separate solution
heat treating operation.
Quenching in Nitrogen. There has been
limited interest in quenching with nitrogen.
The motivation was that the extremely cold
temperatures of liquid nitrogen could provide
a rapid quench rate. However, with vaporization of liquid nitrogen in contact with the hot
part, the quenchant medium is actually nitrogen
vapor around the part, and the vapor pocket
does not collapse even with reasonable agitation (Ref 1). Thus, nitrogen is very limited to
quench-insensitive alloys, such as thin gages
of 6061 sheet or very thin (<0.75 mm, or
0.030 in.) gages of 2024.
Fast Quenching Oils. With the advent of
polymer quenchants, quenching oils are rarely
used. Prior to polymer quenchants, oil quenchants were only used in rare instances to control
distortion for castings of certain thicknesses.
Fast quenching oils were required for aluminum
alloy quenching.
Fluidized-Bed Quenching. Recently, there
has been interest in using fluidized-bed technology for the solution heat treatment of aluminum
alloys (Ref 25, 26). A fluidized bed consists of
a medium of fine, hard particles (i.e., sand) that
is partially suspended by a fluidizing gas. The
partial suspension of the medium allows the
particles to easily slide past each other,
resulting in the fluidizing bed acting very similar to a fluid. The fluidlike nature of the fluidized bed allows for easy insertion,
conveyance, and extraction of parts for heat
treating. Fluidized-bed technology has found
wide use in the heat treatment of steels, but its
use for heat treating other metals has been
limited.
In the heat treatment of aluminum alloys,
fluidized-bed quenching is an attractive alternative to liquid quenching processes because the
part does not develop a vapor barrier during
quenching. This lack of a vapor barrier can significantly reduce residual stresses and part distortion. Compared to quenching in water, it
was shown that quenching in the fluidized bed
reduced residual stresses by nearly 70% in an
A356.2 casting (Ref 27). The heat-transfer rate
of fluidized-bed quenching is intermediate
between that of water and air quenching
(Fig. 34). The lower heat-transfer rate limits
fluidized-bed quenching to thinner sections or
alloys with a low quench sensitivity, such as
319 casting alloy.
Fig. 34
Comparison of heat-transfer coefficient for
water, forced-air, and fluidized-bed quenching
precipitation that occurs during cooling, and
the method of quench-factor analysis also provides a method of correlating cooling curves
with quantitative prediction of properties.
Quench-factor analysis is based on the principle of additivity when a precipitation process
is isokinetic, where transformation rates only
differ by a rate constant according to the
Avrami equation:
hti
d ¼ 1 exp
k
Quench-Factor Analysis
Although average quench rates through a
critical temperature range can provide reasonable property predictions if cooling rates are
fairly uniform, cooling rates typically vary considerably during the quench. Isothermal timetemperature-transformation (TTT) diagrams,
although useful to define critical times for a
specified amount of precipitation at a given
temperature, do not quantify the overall extent
of precipitation during continuous cooling.
Quench-factor analysis is a method of quantifying the extent of precipitation after continuous cooling from the isothermal C-curve and
either an experimental or analytical cooling
curve. The properties of aluminum alloys also
are dependent on the amount of alloy
where d is the fraction of precipitation that
occurs in time t during the quench, and k is a
temperature-independent constant. The value
of k depends on the degree of supersaturation
and the rate of diffusion from the study of
transformation during continuous cooling processes using isothermal kinetics by Avrami
(Ref 28, 29). Subsequently, Cahn demonstrated
that the precipitation process during continuous
(nonisothermal) cooling is additive for isokinetic transformation, such that (Ref 30):
Z
tf
t¼
ti
dt
CT
(Eq 12)
where t is the measured amount of transformed
product (also designated as Q in quench-factor
Quenching of Aluminum Alloys / 167
analysis), t is the time from the cooling curve, ti
is the time at the start of the quench, tf is the
time at the end of the quench, and CT is the critical time from the TTT C-curve.
When t (or Q) = 1, the fraction transformed
equals the fraction represented by the isothermal C-curve. Therefore, the continually cooled
material will exhibit the same behavior as one
quenched instantaneously to an intermediate
temperature, held for the critical time at that
temperature, and quenched instantaneously to
room temperature.
From these basic relationships, Evancho and
Staley (Ref 3) developed a method of quenchfactor analysis that quantitatively calculated
the extent of precipitation and the properties
from a cooling curve and the critical times
(CT) along the isothermal C-curve. They
defined the constant, k, in the Avrami equation
relative to the critical time, such that:
a nucleus, k4 is a constant related to the solvus
temperature, k5 is a constant related to the activation energy for diffusion, R is the gas constant
equal to 8.3143 J K1 mol1, and T is the temperature in Kelvin.
Numerical values for each of these constants
define the C-curve for the particular alloy composition and temper condition, and published
coefficients for the CT values of various alloys
are listed in Table 11.
With a given cooling curve and the isothermal C-curve of a given alloy, the quench-factor
calculation basically involves the summation of
incremental quench factors over a sequence of
time intervals, as illustrated in Fig. 35. The
incremental quench factor (qi) over a small time
interval (△ti) is:
k ¼ CT =k1
where CTi is the critical time defined as the
time difference between the cooling curve
where k1 is a constant that equals the natural
logarithm of the fraction untransformed (i.e.,
1 minus the fraction defined by the C-curve);
if 0.5% is untransformed, then k1 = ln[0.995]
= 0.0050. Moreover, because the properties
of aluminum alloys also are dependent on the
amount of alloy precipitation that occurs during
cooling, the k1 constant based on the specified
fraction of untransformed product also provides
a specified fraction of a property (such as
99.5% of attainable yield strength after aging
with k1 = 0.0050). In their 1974 paper, Evancho and Staley showed that from the Avrami
equation, the attainable strength is a function of
the amount of solute remaining in solution after
the quench and is related to k1:
k1 ¼ ln
sx smin
smax smin
qi ¼
Dti
CTi
of the quench process and the isothermal Ccurve (Fig. 36). Over the entire cooling
curve, the total quench factor (Q) is thus
the summation of incremental quench factor
as follows:
Q¼
Dt
final
X
The cumulative quench factor (Q, or t in
Eq 12) reflects not only the precipitation kinetics of the alloy but also the cooling rates (which
are a function of quenchant fluid, quenchant
velocity, temperature, part thickness, and other
variables). An alloy with a low rate of precipitation will produce a lower Q value at a given
cooling rate compared to an alloy with a high
precipitation rate. The overall extent of precipitation (and thus property development) also can
be defined by quench factors for a given alloy
and temper. For example, the quench factors
for alloy 7075-T63 in Table 12 list the levels
Table 11 Coefficients for calculating quench factors at 99.5% of attainable yield strength
Calculated range
k1(a)
Alloy
7010-T76
7050-T76
7075-T6
7075-T73
7175-T73
2017-T4
2024-T6
2024-T851
2219-T87
6061-T6
356-T6
357-T6
Al-2.7Cu-1.6Li-T8
0.00501
0.00501
0.00501
0.00501
0.00501
0.00501
0.00501
0.00501
0.00501
0.00501
0.0066
0.0062
0.0050
k2 , s
5.6
2.2
4.1
1.37
1.8
6.8
2.38
1.72
0.28
5.1
3.0
1.1
1.8
1020
1019
1013
1013
109
1021
1012
1011
107
108
104
1010
108
k3, J/mol
k4, K
5780
5190
1050
1069
526
978
1310
45
200
412
61
154
1520
897
850
780
737
750
822
840
750
900
750
764
750
870
k5, J/mol
1.90
1.8
1.4
1.37
1.017
2.068
1.47
3.2
2.5
9.418
1.3
1.31
1.02
105
105
105
105
105
105
105
104
104
104
105
105
105
CT ¼ k1 k2 exp
R T ðk4 T Þ2
!
C
425–150
425–150
425–150
425–150
425–150
425–150
425–150
425–150
425–150
425–150
425–150
425–150
425–150
F
Ref
800–300
800–300
800–300
800–300
800–300
800–300
800–300
800–300
800–300
800–300
800–300
800–300
800–300
31
3
3
32
33
32
34
35
36
32
37
37
37
(a) k1 is a unitless value that corresponds to the unprecipitated fraction. For this analysis, it is usually 0.995 and ln[0.995] = 0.00501.
Source: Ref 38
with k1 = 0.0050, then sx = 0.995 smax.
With these physical principles relating the
extent of precipitation (and also correlation
with properties associated with the extent of
precipitation during continuous cooling), Evancho and Staley determined the best fit equation
for critical time (CT) values from multiple linear regression analysis of empirical C-curves
for 7050 and 7075 aluminum alloys. The resulting
equation for critical time (CT) along an isothermal
C-curve developed by Evancho and Staley is:
k3 k42
qi
Dt1
k5
exp
RT
where CT is the critical time required to precipitate a constant amount of solute (the locus of the
critical times is the C-curve), k1 is a constant that
equals the natural logarithm of the fraction
untransformed during quenching (i.e., the fraction defined by the C-curve, typically ln[0.99]
or ln[0.995]), k2 is a constant related to the reciprocal of the number of nucleation sites, k3 is a
constant related to the energy required to form
Fig. 35
Method of determining quench factor Q using a cooling curve and a C-curve
168 / Heat Treating of Aluminum and Its Alloys
Table 12 Relationship between quench
factor and yield strength in 7075-T73
Quench
factor (Q)
Fig. 36
Illustration of incremental quench-factor coefficients with cooling curve superimposed on the C-curve of an
isothermal transformation diagram. Source: Ref 15
of obtainable yield strength after aging. These
quench factors provide a quantitative way of
evaluating the necessary cooling curve to obtain
the required yield strength, or the quench factors
could be used to consider suitable properties
with lower quench rates.
Yield strength prediction from quench-factor
analysis is much more accurate than predictions
from average cooling rates (Table 13). Yield
strengths predicted from quench factor agree
very well with measured yield strengths for all
specimens, the maximum error being 19.3 MPa
(2.8 ksi). Yield strengths predicted from average quenching rates, however, differ from
measured values by as much as 226 MPa
(32.8 ksi). The advantage of using the quench
factor for predicting yield strength from cooling curves is apparent. In addition, quenchfactor analysis is particularly advantageous
in evaluating the property effects of cooling
curves that have holding times either above
or below the critical temperature range.
With an upper limit quench factor defined,
the appropriate quenching process and cooling
rate in the section of interest can be determined.
This method of describing quench severity is
different from that used with Grossman numbers (H), which are related solely to the ability
of a quenchant to extract heat and not to the
transformation kinetics of the alloy being heat
treated. However, an underlying assumption
of both quench-factor analysis and averagecooling-rate estimation is that the only effect
of temperature is on the kinetics of precipitation. This assumption is not valid, however,
when portions of the metal are quenched locally
but reheated significantly before quenching is
complete. More information on the application
of quench-factor analysis is in the article
“Quench Sensitivity of Aluminum Alloys” in
this Volume.
Residual Stress and Distortion. Aluminum
is extremely prone to distortion during heat
treatment and quenching. Residual stresses
and distortion originate from temperature gradients, and large thermal strains are developed
within and across the surface of parts during
quenching or solution heat treatment of aluminum alloys. This can contribute to large-part
distortion and residual stresses (Fig. 37).
Of the three basic steps involved in the heat
treating of aluminum alloys (solution heat treating, quenching, and aging), the most severe distortion is usually observed after quenching from
the solution heat treating. In most cases, distortion occurs because the quench is too severe.
However, another important factor is racking
practice. Proper racking of parts, as described
in the next section of this article, is a very
important factor for controlling distortion, both
in the furnace and during the quench.
During the quench, residual stresses and distortion originate from temperature gradients.
The gradient induces plastic deformation from
contraction or expansion in the part. Because
the surface of the part cools first, it tends to contract, thereby imposing a state of compressive
stress on the interior. The reaction places the surface in tension. The surface layer deforms plastically when the tensile stress exceeds the flow
stress of the material. Then, as the interior of
the part cools, it is restrained from contracting
by the cold surface material. The resulting reaction places the surface in a state of compressive
stress and the center in a state of tensile stress.
When the part is completely cooled, it remains
in a state of equilibrium, with the surface under
high compression stresses balanced by tensile
stresses in the interior. Generally, the compressive stresses in the surface layers of a solid cylinder are two-dimensional (longitudinal and
tangential), and the tensile stresses in the core
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
34.0
36.0
38.0
40.0
42.0
44.0
46.0
48.0
50.0
Predicted yield
strength
Attainable yield
strength, %
MPa
ksi
100.0
99.0
98.0
97.0
96.1
95.1
94.2
93.2
92.3
91.4
90.5
89.6
88.7
87.8
86.9
86.0
85.2
84.3
83.5
82.7
81.8
81.0
80.2
79.4
78.6
77.8
475.1
470.2
465.4
461.3
456.5
451.6
447.5
442.7
438.5
434.4
429.6
425.4
421.3
417.2
413.0
408.9
404.7
400.8
396.5
393.0
388.9
384.7
381.3
377.2
373.7
369.6
68.9
68.2
67.5
66.9
66.2
65.5
64.9
64.2
63.6
63.0
62.3
61.7
61.1
60.5
60.0
59.3
58.7
58.1
57.5
57.0
56.4
55.8
55.3
54.7
54.2
53.6
are triaxial (longitudinal, tangential, and radial),
as illustrated in Fig. 38.
The magnitude of the residual stresses is
directly related to the temperature gradients
generated during quenching. Conditions that
decrease the temperature gradient reduce the
residual-stress ranges (Ref 39). Quenching
variables that affect the temperature gradient
include the temperature at which quenching
begins, cooling rate, section size, and variation
in section size for nonflat products. For a part
of a specific shape or thickness, lowering the
temperature from which the part is quenched
or decreasing the cooling rate reduces the magnitude of residual stress by reducing the temperature gradient. Figures 39 and 40 illustrate
the effect of quenching temperature and cooling
rate, respectively. With a specific cooling rate,
the temperature gradient is greater in a section
of large diameter or thickness than it is in a
smaller section. Therefore, the residual stresses
in the larger section are higher (Fig. 41). In products having differences in cross section, large
temperature gradients can be minimized by
covering or coating the thinner sections with a
material that decreases the quench rate, so that
it more closely matches that of the thicker
sections.
The range of residual stresses generated during quenching varies considerably for different
alloys. Those properties related to alloy composition that specifically affect the thermal gradient
and the degree of plastic deformation that occur
during quenching are involved. High residual
stresses are promoted by high values of properties such as Young’s modulus of elasticity,
proportional limit at room and elevated
Quenching of Aluminum Alloys / 169
Table 13 Yield strength values for 7075-T6 sheet predicted from cooling curves using
average quench rate and quench factor
Average quench
rate from 400 to
290 C (750 to
550 F)
Measured yield
strength
Yield strength
predicted from
average quench rate
Yield strength
predicted from
quench factor
Quench
F/s
Quench
factor, t
(or Q)
MPa
ksi
MPa
ksi
MPa
ksi
Cold water
Denatured alcohol to 290 C
(550 F), then cold water
Boiling water to 315 C
(600 F), then cold water
Still air to 370 C (700 F),
then cold water
935
50
1680
90
0.464
8.539
506
476
73.4
69.1
499
463
72.4
67.2
498
478
72.3
69.4
30
55
15.327
458
66.4
443
64.2
463
67.1
5
9
21.334
468
67.9
242
35.1
449
65.1
C/s
Fig. 39
Fig. 38
Residual-stress diagram for 2014 alloy
quenched in cold water from 500 C (935 F)
temperature, the coefficient of thermal expansion, and by a low value of thermal diffusivity.
These property factors affect the magnitude of
residual stresses to different degrees. The
Fig. 37
Distorted 7050 wing spar improperly racked,
resulting in excessive distortion and scrap
Effect of quenching temperature on residual stress in 5056 alloy cylinders 76 by 229 mm (3 by 9 in.)
quenched in water at 24 C (75 F)
influences of the coefficient of thermal expansion
and elevated-temperature yield strength are especially significant. For example, a low coefficient
of thermal expansion can counteract a high proportional limit. The net effect is a low residualstress level, or an alloy such as 2014 can develop
a high residual-stress range because of its very
high elevated-temperature strength, despite average values for coefficient of thermal expansion,
modulus of elasticity, and thermal diffusivity.
The effects of residual stresses from quenching require consideration in the application of
heat treated parts. Where the parts are not
machined, the residual compressive stresses at
the surface may be favorable by lessening the
possibility of stress corrosion or initiation of
fatigue. However, heat treated parts are most
often machined. Where the quenching stresses
are unrelieved, they can result in undesirable
distortion or dimensional change during
machining. Metal removal upsets the balance
of the residual stresses, and the new system of
stresses that restores balance generally results
in warpage of the part. Further, in the final balanced stress system, the machined surfaces of
the finished part can be under tensile stress with
attendant higher risk of stress corrosion or
fatigue. Because of the practical significance
of residual stress in the application of heat treated parts, various methods have been developed
either to minimize the residual stresses generated during quenching or to relieve them after
quenching.
The methods commonly used for stress
relieving heat treated parts include mechanical
and thermal. The methods used to avert the
development of high residual stresses during
quenching rely on a reduced cooling rate to
minimize the internal or external temperature
gradients. Using quenching media that provide
less rapid cooling during quenching of irregular-shaped parts is common practice. For this
reason, the quenching of large die forgings
and castings in hot (60 to 80 C, or 140 to
175 F) or boiling water is a common practice.
However, quench-rate-sensitive alloys may
170 / Heat Treating of Aluminum and Its Alloys
Fig. 40
Effect of quenching rate on residual stresses in 2014 and 355 alloy cylinders 75 mm by 230 mm (3 by 9 in.)
quenched from 500 and 525 C (930 and 975 F), respectively
Fig. 41
Effect of section size on residual stress in 2014 cylinders quenched from 505 C (940 F) in water at 20 C
(70 F)
suffer a loss in mechanical and corrosion properties due to solute loss. Intergranular corrosion
resistance may also be impaired by these
reduced cooling rates. Polyalkylene glycol (10
to 40% in water) is effective as a quenchant in
minimizing residual stresses and distortion with
little loss in properties. Due to inverse solubility,
a film of the liquid organic polymer is immediately deposited on the surface of the hot part
when the part is immersed in the quenchant.
By reducing the rate of heat transfer, the deposited film reduces thermal gradients.
One approach to reducing the cooling rate
differential between surface and center is the
use of a milder quenching medium, such as
water that is hotter than that normally used or
a water-PAG polymer solution. Boiling water,
which is the slowest quenching medium used
for thick sections, is sometimes employed for
quenching wrought products, even though it
lowers mechanical properties and corrosion
resistance. Quenching of castings in boiling
water, however, is standard practice and is
reflected in design-allowable properties.
Another approach to minimizing residual
stresses that is generally successful consists of
rough machining to within 3.2 mm (0.125 in.)
or less of finish dimensions, heat treating, and
then finish machining. This procedure is
intended to reduce the cooling-rate differential
between surface and center by reducing thickness; other benefits that accrue if this technique
is used to reduce or reverse surface tension stresses in finished parts are improvements in
strength, fatigue life, corrosion resistance, and
reduced probability of stress-corrosion cracking.
Several factors (especially quenching warpage) sometimes preclude general use of this
procedure. The thinner and less symmetrical a
section, the more it will warp during quenching,
and the residual stresses resulting from straightening of warped parts (plus straightening costs)
often are less desirable than the quenching
stresses. Holding fixtures and die quenching
may be helpful, but precautions must be taken
to ensure that they do not excessively retard
quenching rates. Other factors that must be considered are the availability of heat treating
facilities and whether or not the advantages of
such a manufacturing sequence offset the delay
and cost entailed in a double-machining setup.
Warpage of thin sections during quenching
is also a problem. Even in the same load, symmetry of cooling usually varies significantly
among identical parts, and the resultant inconsistent warpage usually requires costly hand
straightening. Consequently, a significant
amount of effort has been devoted to reducing
or eliminating warpage by changing racking
position to achieve symmetry of cooling.
For sheet metal parts, one manufacturer uses
a double-screen floor in the quenching rack to
reduce the force of initial contact between
water and parts. Others allow parts to free fall
from rack to quench tank. Spacing and positioning on the rack are carefully controlled so that
parts will enter the water with minimum
impact. With this technique, water turbulence
must be avoided, because it will often cause
parts to float for a few seconds, greatly reducing their cooling rate.
Because of the difficulties encountered with
quenching in cold water, milder quenchants have
been employed. Indiscriminate use of milder
quenchants can have catastrophic effects; however, when their use is based on sound engineering judgment and metallurgical knowledge of
the effect on the specific alloy, significant
cost-savings or performance improvement can
Quenching of Aluminum Alloys / 171
be realized. The most frequent advantage is the
reduction in costly straightening operations and
in resultant uncontrolled residual stresses. For
example, one aircraft manufacturer uses waterspray and air-blast quenching for weldments
and complete formed parts made from 6061, an
alloy whose corrosion resistance is relatively
insensitive to quenching rate. Straightening
requirements are negligible, and through careful
control of racking and quenchant flow, the
decrease in mechanical properties is minimized,
as shown by the data in Fig. 42. Another
development for reducing straightening cost is
quenching in water-polymer solutions. Quenching of formed sheet metal parts in aqueous solutions of PAG has significantly reduced the cost
of straightening parts after quenching.
Because of the trade-offs of tensile properties
with residual stress, researchers have been
developing methods of analysis that combine
prediction of properties by quench-factor analysis and prediction of stresses from heat-transfer
analyses and other considerations. One of these
methods predicts that a cooling rate that is slow
at the beginning, but continuously accelerates,
can significantly reduce residual stresses while
maintaining the same mechanical properties as
those obtained by quenching in cold water
(Ref 40).
Racking Practices (Ref 1)
Proper racking is a key factor in controlling
distortion and the level of residual stresses
induced during the quench. Racking involves
proper orientation and spacing of parts in a
manner to permit free access of the heating
and quenching media to all surfaces of parts
in all portions of the load. If warpage and
residual-stress levels are to be minimized, parts
must be supported in the furnace correctly
and enter the quenching fluid properly at the
optimum rate. Improper racking and spacing
can lead to low properties, spotty hardness, part
melting, poor quenching response, excessive
distortion, parts not reaching the proper temperature, and high residual stresses.
Although proper racking to minimize distortion and residual stresses is a very complex subject that takes time and experience to master,
proper racking of parts prior to the solution heat
treating operation is probably the single most
effective tool at the disposal of an aluminum
heat treater for controlling distortion, both in
the furnace and during the quench. The critical
factors to consider when racking parts for solution heat treating include:
Elevated-temperature strength characteris-
tics: When aluminum alloys are heated to
high temperatures, the elevated-temperature
strength is extremely low and the material
becomes very soft and pliable.
Thermal expansion: Aluminum alloys have
a high coefficient of expansion and will
grow as the part is heated to the solution
heat treat temperature.
Part support: The part must be supported
properly and not allowed to sag or bow during the heat and cooling operations.
Part spacing: Parts must be spaced correctly, considering the thickness of the part
and allowing sufficient air flow and quenchant to pass over the part to ensure even heating and cooling.
Part configuration: Consideration must be
given to the variation of thickness within a
part that can lead to large differences in
cooling rates, which cause excessive distortion or high levels of residual stress.
Furnace design in relation to part orientation: Parts must be oriented in the furnace
so that the air flow passes evenly over all
surfaces of the part.
Weight/configuration distribution: The size
and weight distribution for the part and its
relationship to the basket, rack, or fixture in
which it is placed will have an effect on
warpage.
Condition of the baskets, racks, or fixture:
Distorted baskets, racks, and fixtures make
distorted parts.
When considering how to rack parts in a
particular furnace or oven, the operator must
understand how the air flow travels so that in
loading the parts, the air flow is not restricted,
which can cause cold spots in the furnace.
For example, if the air flow is from side to
side, sheets of a large planer area must be
positioned lengthwise with the flow so that
the air is not restricted as it passes over all
surfaces.
Softness of Aluminum at Solution Temperatures. At solution heat treating temperature, the aluminum alloy is soft and will not
hold its own weight. If hanging parts, especially
where strength is important, steel wire should
be used, not aluminum wire. For positioning
parts, aluminum wire should be used, because
it will allow the part to expand.
Thermal Expansion. Aluminum alloys have
a high coefficient of expansion and will grow
as the part is heated to solution heat treat temperature. The degree of expansion and contraction during heating and cooling of aluminum
parts is of critical importance. Table 14 illustrates the calculated expansion of a number of
aluminum parts during heating from room temperature to a furnace temperature of 475 C
(890 F). It can be seen that a 4.88 m (16 ft)
long part is going to grow almost 5 cm (2 in.)
in the furnace while it is at temperature.
At room temperature, it may appear that
there is sufficient spacing between the parts.
However, as the temperature of the load is
raised in the furnace, the expanding parts (if
spaced too closely) will exert pressure on each
other as they attempt to expand. Because the
yield strength of the aluminum alloy is
extremely low at elevated temperature, the parts
Fig. 42
Effect of quenching medium on strength of
6061-T6 sheet. Water-immersion quench
equals 100%. Control of coolant flow will minimize the
decrease in mechanical properties.
Table 14 Typical expansion of aluminum
during solution treatment
Part length at room
temperature,
20 C (70 F)
Expansion at
475 C (890 F)
m
ft
mm
in.
2.4
3.65
4.88
8
12
16
24
36
48
0.96
1.42
1.89
relieve the contact pressure by moving or bending to conform to the space available. When the
parts are quenched, the distortion that has
occurred in the furnace will not change; in
many instances, it is erroneously concluded that
the parts have warped during the quench.
Unequal Expansion and Contraction.
During the racking of a part, the problem of
differential contraction between the part and
the supporting baskets and fixtures must be
considered. If a 2.4 m (8 ft) long part were to
be quenched, a sudden contraction back to its
original dimension would occur, and the part
would shrink 25 mm (1 in.). If the part is not
172 / Heat Treating of Aluminum and Its Alloys
allowed to expand or contract uniformly, without restriction, significant residual stresses or
distortion will result during the quenching procedure. Because of these expansion characteristics, the use of fixtures for elevated-temperature
processing operations (forming, welding, heat
treating) must be carefully planned so that
large differentials between the expansion characteristics of the part and its fixtures do not
impart high levels of residual stress to the part,
leading to dimensional instability.
When quenching thin-gaged aluminum
parts, the entire cooling process is completed
in a matter of seconds or less, so the contraction of the part is almost instantaneous.
Restricting the parts movement during the
contraction is a major cause of distortion. If
the parts are positioned by pins or bolts in a
fixture, slotted holes are an absolute necessity.
Also, the length and position of the slots
should be computed, allowing for differential
expansion of the part and the fixture. Centering the pin in the slot also may cause problems, because the direction of the expansion
and contraction will not be based on the central location. Clamping the part too tightly
may even cause dings in the part as the part
expands into the clamp during heating.
Proper racking of aluminum alloy parts to
minimize distortion and residual stresses in the
heat treating process is a very complex subject
and takes time and experience to master. Not
all of the various intricacies of racking can be
covered in these few pages. However, by discussing a few of the basic principles and the
effects racking can have on distortion, the reader
may be convinced of the importance of learning
and practicing good racking techniques.
Effect of Racking Orientation on Part
Warpage. The level and type of warpage is
particularly pertinent when one considers different racking orientations, such as vertical,
horizontal, or even angle racking. Parts that
are laid flat in a basket usually result in the part
bowing, while the same parts racked in a vertical position will normally twist if they have sufficient mass. Thin-gage sheet parts may bow,
with the greater distortion being on the top of
the part that was the last to enter the quenchant.
Check and straightening time and costs also can
be considerably different.
Using basic principles, much quenching distortion can be eliminated or at least minimized.
However, in spite of all possible efforts, many
times, check and straightening operations are
required to bring quenched aluminum parts
back to within dimensional tolerance. If this situation occurs, if every part warps or distorts in
the same manner, straightening costs can be
significantly reduced. Thus, achieving the goal
of what is called controlled warpage becomes
important. This involves the principle that if
all parts are spaced, racked, and quenched in
the same manner, they will warp the same
way. Controlled warpage makes it easier to
straighten, because every part is the same.
Allowance for Part Expansion and Contraction. If parts are restricted in any manner
and not allowed to grow freely, the restriction
will cause the part to distort. This distortion
is frequently encountered when parts are tied
too tightly to steel baskets or racks that are
used to position the parts in the furnace. It also
occurs when parts are racked too close
together. At room temperature, it may appear
that there is sufficient spacing between the
parts. As the temperature of the load is raised
in the furnace, the expanding parts (if spaced
too closely) will exert pressure on each other
as they attempt to expand. Because the yield
strength of the aluminum alloy is extremely
low at elevated temperature, the parts relieve
the contact pressure by moving or bending to
conform to the space available. When the parts
are quenched, the distortion that has occurred
in the furnace will not change; in many
instances, it is erroneously concluded that the
parts have warped during the quench. Restriction by racks, bars, or wires used to position
the parts during the heat treating operation
may cause a similar effect if the holding
device is not sufficiently flexible to allow for
the part to expand and contract freely. Any
device used for positioning parts must be sufficiently loose to allow for thermal expansion
and contraction.
Parts Do not Move during the Quench.
Parts should be tied lightly at the bottom or
positioned appropriately with screen or other
acceptable material at the top to ensure correct
entry into the quenchant and to make sure they
are secured and do not sail or move during the
quench. The selection of a proper immersion
rate is dependent on the part configuration; for
many configurations, a slower immersion rate
is appropriate. For some part configurations,
Fig. 43
no matter how the part is racked, there will
always be a large surface that offers resistance
to the quenchant. In these cases, a slower
immersion rate should be used.
Thickness Variations and Heat Sinks.
Thickness variations, such as encountered in
forged and machined parts, may cause distortion during the quench because thin sections
cool faster than thick sections. When racking
parts with large thickness differences, the operator can compensate for thickness variations to
some degree by his racking technique. A typical
example is a machined part made from a
25 mm (1 in.) thick hand forging with
machined pockets and a rib through the center
with a section, such as shown in Fig. 43. The
base of the pockets is machined down to
3.2 mm (1/8 in.) (before heat treating) with a
13 mm (½ in.) machined rib between the pockets. The part would be racked in the heat treat
basket using small 6.4 to 9.5 mm (¼ to 3/8 in.)
diameter steel rods to support the part vertically. Smooth, round rods made from alloy
4130 or 304 stainless are normally used. Several support rods placed along the thinner sections of the part will compensate for the
variations in thickness (Fig. 43). The presence
of these rods will reduce the cooling rate in
the thin sections, keeping it closer to the rate
in the thicker sections and thereby eliminating
the canning effect that was experienced before
the rods were used.
Use of Heat Sinks. Many warpage problems
that result from large thickness variations can be
solved by the use of heat sinks. The term heat
sink, although commonly used in the heat treating industry, is really a misnomer because these
components, when added to the part, are not
really sinks; they are intended to be heat sources.
The purpose of the heat sink is to increase the
Thickness variations of machined part being quenched, with location of racking bars along the thin bottom
to act as a heat sink. Source: Ref 1
Quenching of Aluminum Alloys / 173
mass of the part in a specific area so that more
equalized cooling is achieved throughout the
entire part. Increasing the thickness or mass in
a thin area allows that area to cool at the same
rate as thicker areas, thereby reducing the warpage during the cooling process.
The use of heat sinks must be done carefully.
It must be performed in a manner so that the
area of the part is not cooled so much that it
affects the final heat treat properties. Inspection
of the affected area must be rigidly controlled.
In some instances, application of quench-sensitivity data can be helpful in determining the
type and size of the method used. Also, careful
adherence to specification requirements must be
practiced. Some prime contractors do not allow
the use of heat sinks without prior approval.
Heat sinks take on different forms. It is common practice to use aluminum screen, sometimes bunched in a ball or wrapped around an
aluminum block, to act as a heat sink. This
technique has proven to be effective in
controlling differential quenching rates in many
cases.
To compensate for severe canning distortion
in the machined pockets of a large wing spar,
another example involved the use of cast aluminum waffle plates, which were positioned in the
bottom of each pocket to equalize the cooling
rates with the surrounding ribs. The waffle plates
were fastened to the part using bolt through holes
in each pocket, and they completely eliminated
the canning of the pocket areas.
Spacing of Parts. As previously noted, the
fundamental requirement of proper racking is
that “all parts be racked or supported in a manner to permit free access of the heating and
quenching media to all surfaces of parts in all
portions of the load.” This requirement establishes that parts must be spaced properly to
ensure the free access of the heating and quenching media. Most specifications have this requirement in some way or another. Unfortunately,
there is a wide difference of opinion among most
experts regarding the exact spacing that is needed
to achieve this goal. Specification requirements
vary significantly regarding the exact spacing that
is needed. Some companies leave the spacing
requirement up to the heat treater as long as the
primary requirement is met.
The first requirement of racking for the
quench is to allow sufficient space around each
part so the quench medium has free access to
all surfaces. This spacing is essential so that
the fluid can effectively extract heat from all
surfaces uniformly throughout the entire
quench. Allowing the quenchant to become
too hot in any location near the part because
of improper spacing or inadequate agitation
can lead to distortion and lower mechanical
properties.
Problems often occur while quenching some
of the bulkier forgings and castings. At first
appearance, because of their size and thickness,
distortion would not appear to be a problem. To
obtain the most production from the available
equipment, the heat treater may stack or simply
dump parts into the basket. When the parts are
quenched, the lack of proper spacing does not
allow free access of the quenching fluid. When
only small amounts of fluid (usually water)
reach between parts, local steam or vapor pockets develop. The presence of these vapor pockets slows down the cooling rate at these
locations. Slower cooling rates allow the alloying elements to come out of solid solution, and
the loss of these alloying constituents can result
in a significant loss of hardness and strength
properties after aging. The reduced properties
are usually observed in the parts during hardness testing and normally appear as soft spots.
In some cases, entire parts will be below the
minimum hardness and strength requirements
for that alloy and temper.
Distortion or the inducement of unwanted
residual stresses can easily occur in heavier
parts when inadequate or uneven spacing is
used. Spacing should increase with the part
thickness, because it will require a larger volume of quenchant between parts to extract the
greater amount of heat contained in the thicker
part. When quenching in water, the spacing
between parts should be increased as the water
temperature is increased. The quenching power
of water decreases as it is heated. Water temperatures above 70 C (160 F) exhibit very
slow cooling rates and should not be used for
most of the thicker parts unless allowances are
made for reduced properties.
Quench Tank Systems
Immersion in tanks, frequently of unlined
waterproof concrete, particularly if they are
below floor level, is the most usual method.
Small tanks and those above floor level usually
are metal. Aluminum or stainless steel is preferred for metal tanks. If carbon steel is used,
suitable surface coatings must be applied to
suppress corrosion, which can cause staining
of the product. Water used for quenching also
may contain a small percentage (usually less
than 1 to 2%) of a rust inhibitor to minimize
rusting of the quench tank and racks.
Quench tanks must be large enough to permit
the furnace charge to be immersed completely,
preferably below the excessively heated top
layer of water. For quenches specified at room
temperature, water volume should be sufficient
to maintain the bath below 40 C (100 F) during the quenching cycle. If this is not practical,
bath temperature must be kept below this maximum through vigorous agitation by such
means as high-velocity introduction of makeup water or recirculation with pumps or propeller-type mixers. When slower quench rates are
required, the quenching medium may be kept
above normal ambient temperature by adding
suitable heating coils or steam injection facilities
to the tank. Uniform quench bath temperature is
essential to optimum product characteristics.
The practical aspects and design considerations of a quench system involve a number of
variables that include alloy type, product type,
load configuration, quenchant fluid, agitation
rate, total load weight, and the density of the
load on the racks. The focus of this section is
on design considerations for immersion tank
systems in terms of:
Batch or continuous process
Materials selection
Heat load
Quench tank and rinse tank agitation
Part tacking and baskets
Cooling/heating
Fluid maintenance
Concentration control and separation methods
Some practical aspects of quench system
design depend on product type. For example,
quench systems are typically divided according
to product gage: one for sheet metal parts and
parts up to 6 mm (¼ in.) thickness, or a system
for heavier-gage parts that have cross sections
more than 6 mm (¼ in.) thick. The main difference between sheet metal quenching and a
heavy-gage load is the fact that sheet metal will
normally be cooled by the time the parts reach
the bottom of the quench bath. The hoist or elevator provides the main means of agitation for
the cooling cycle, while the agitator system only
needs to provide proper mixing and uniformity
of the tank before and during the quench.
Batch or Continuous Process. Quench
tanks will support either a batch process or a
continuous process. Batch quenching is used
for furnaces that use an indexing system to
present a load for quenching in a tank sized
with enough volume of quench that the temperature rise in the tank is within acceptable limits
for each quench cycle. Other types of furnaces
are horizontal or vertical quench furnaces
(drop-bottom furnaces).
Continuous quench tanks are sized with
enough volume of liquid to allow for proper
agitation and heat removal from the work as it
is quenched. This, in connection with a properly sized cooling system, ensures the process
can continue without exceeding the upper limit
of quench temperature. Several different methods are used to provide proper flow and cooling
for the parts. There are chute quench systems,
spray quenching, and simple tanks with a belt.
Continuous quench tanks are mostly employed
in connection with belt furnaces. Other types
of furnaces are walking beam furnaces or screw
furnaces. The continuous quench tanks accommodate both bulk products and single-piece
product quenching.
Tank Materials Selection. Aluminum can be
affected by free iron (rust) in the quench bath
during the quenching. Surface corrosion is particularly troublesome with sheet metal parts.
The corrosion will show up as dark splotches
that, with closer examination, reveal a black spot
in the center (iron particle). Free iron is not the
174 / Heat Treating of Aluminum and Its Alloys
only cause for surface corrosion but can be a contributor. Other contributors can be contamination
of the parts by oil, cutting fluids, and poor materials handling before entry to the heat treat.
For forging and castings, there are normally
no problems because secondary machining or
surface treatment removes the condition. The
main source for the free iron is the tank wall
and agitation system if the tank is made from
mild steel. Secondary contributors can be heat
treating racks and fixtures and finally the piping
materials used for agitation and pumping.
With the aforementioned in mind, the tanks for
heavy castings and forgings are normally made
from mild steel, with stainless baffles, agitators,
and elevators. The tanks for sheet metal parts
are mostly made from stainless steel, with all
internal components made from stainless steel.
Most of the piping is made from chlorinated
polyvinyl chloride (CPVC) or stainless steel for
water and hot water quench tanks. For PAG
tanks, the shell and components can be made
from mild steel. This is due to the PAG corrosion
inhibitor (sodium nitrate), which will protect the
tanks and piping and thus the parts.
Several coatings have been tried over the
years, with various successes. The most successful are the two-component epoxy coatings.
However, PAG and hot water have a tendency
to lift any coating from the metal, especially
where mechanical damage has occurred during
operation. Coatings are normally used as a
cost-saving measure compared to the use of
stainless steel. The author does not recommend
this practice because the cost of replacements
and repairs to the coatings will exceed the initial cost of the stainless steel lining.
Polyvinyl chloride and CPVC piping can be
used if care is taken to protect the piping from
the hot load and direct heat from the open
Fig. 44
furnace. It must always be remembered that
Murphy’s law will ensure the tank is exposed
to the full heat from the furnace when it is stuck
under a quick quench or when a hot basket
becomes stuck on an elevator.
Heat Load and Tank Sizing. In accordance
with most of the aluminum specifications, the
tank is sized so the temperature rise does not
exceed 5.5 C (10 F) for parts processed in
accordance with AMS 2770, “Heat Treatment
of Wrought Aluminum Alloy Parts,” and AMS
2771, “Heat Treatment of Aluminum Alloy
Castings.” Other industries allow a higher temperature rise for castings and forgings; typically,
a temperature rise of 11 C (20 F) is allowed.
As an example, Fig. 44 illustrates the
heat load calculations for quenching 2300 kg
(5000 lb) of aluminum parts placed in a
680 kg (1500 lb) steel rack. The temperature
of the water quench is 70 C (160 F). The parts
and rack are heated to 540 C (1000 F) in
the furnace. As shown in the example, the tank
volume must be a minimum of 50,515 L
(13,345 gal) of water to ensure that the quench
temperature does not rise more than the specified 5 C (10 F). Standard practices do not
include the heat requirement for heating the
tank shell and other components that come into
direct contact with the quench. This provides an
additional safety factor for temperature rise.
This calculation should always be done to
determine the minimum volume of the tank.
In addition to the volume, the size of the tank
must also accommodate the parts and rack
being processed. Sufficient clearance is needed
for the instrumentation, agitation, and maintenance access to the components. The next step
in the process is to consider the required agitation rate for the type of product processed in the
quench facility.
Heat load calculations for sizing of a quench tank in terms of British thermal units (Btu), where 1 Btu = 1054 J.
This example is based on quenching 2300 kg (5000 lb) of aluminum parts placed in a 680 kg (1500 lb) steel
rack. The parts and rack are heated to 540 C (1000 F) in a furnace. The temperature of the water quench is 70 C
(160 F), and the allowable temperature rise is 5 C (10 F). See text for additional discussion.
Quench Tank Agitation. There have been
various ways of specifying agitation in a
design, such as:
Changeover of tank volumes (gallons per
hour)
Description of surface movement (babbling
brook)
Measured flow past the parts (feet/seconds)
The best way to specify the quench flow is a
calculated or measured flow past the parts. The
maximum flow that should be specified for aluminum batch quenching with water or PAG
solution is on the order of 24 to 36 cm/s (0.8
to 1.2 ft/s) past the parts. Any flow higher than
this will not add to the cooling of the parts
unless spray quenching is used. This maximum
rate of fluid may not be practical in large tanks
and would mean the complete tank volume
must be changed over every 1 to 3 min.
Many tanks successfully produce heavy-gage
parts with measured flows on the order of 7 to
12 cm/s (0.25 to 0.4 ft/s). The following are
basic guidelines for flows in quench tanks that
have proven acceptable:
Part thickness
Product
Thin sheet metal
Heavy sheet metal
Plate and
machined parts
mm
in.
Flow rate
cm/s
ft/s
<2.3
<0.090
3–9 0.1–0.3
2.3–6
0.090–0.25 9–24 0.3–0.8
Up to 75
Up to 3
15–30 0.5–1.0
Agitation for flow rates more than 36 cm/s
(1.2 ft/s) past the parts will not improve the
cooling rate for water and polymer quench.
As previously noted, the main difference
between quenching sheet metal (up to 6 mm,
or ¼ in., thick) and a load of heavy-gage parts
(more than 6 mm, or ¼ in., thick) is the fact that
sheet metal will normally be cooled by the time
the part reaches the bottom of the quench bath.
The hoist or elevator provides the main means
of agitation for the cooling cycle. As a general
rule for sheet metal parts, the hoist speed
should be as slow as possible to avoid high
hydraulic forces on the soft metal. The quench
must still be completed within the allowable
quench delay for the type of metal and furnace
used. A slower speed will reduce distortion of
the part. To obtain a slow elevator speed, the
travel distance from furnace to quench must
be as short as possible. New drop-bottom furnaces with moveable quench carts are superior
designs compared to older pit-type drop-bottom
furnaces in this regard.
For heavy parts, agitation has a much more
important role in removing heat from the parts.
Practical experience and research by others
has shown that a quench flow of approximately
0.3 m/s (0.8 to 1.2 ft/s) is adequate to generate
the required cooling on heavy-gage parts. Faster water flow does not increase the cooling rate
significantly, and high flow rates increase the
difficulty of design criteria and require higher
Quenching of Aluminum Alloys / 175
power consumption for the agitation. See the
section “Agitation Systems” in this article for
more information.
When dealing with quenchants, it must be
noted that direct high-velocity impingement of
the fluid against the part must be avoided to
ensure that spot cooling does not occur. Spot
cooling can cause severe distortion and uneven
properties in the finished product. It also is
important to realize that quench agitation is different than mixing of chemicals. Heat treat
facilities are specifically looking for the linear
flow with some turbulence past the part that
gives the best and most efficient cooling of
the part in a predictable manner across the
whole section of the product rack or part each
time a quench is performed. Flow patterns and
the extent of flow depend on how the tanks
are configured with the agitation. Tanks can
be divided in halves or thirds, depending on
how the agitation is designed, to optimize the
flow pattern effect (Fig. 45). Tank designs must
take these natural flow patterns into account to
optimize agitation in the load with minimum
power and energy consumption. However, in
many cases, space constraints at the facility create a need to optimize the assigned space and
load size while still generating sufficient flow
around the parts to ensure good properties after
the quench-and-age cycle.
Water Heating and Cooling. The quench
tank must be equipped with a means of initial
heatup if the tank is used for quenching in hot
water. For temperatures above 70 C (160 F),
a precaution is to insulate tanks and piping to
protect personnel. In addition, the insulation
reduces heat losses during slow production.
Some areas of the country have very hard
water, and calcium deposits on heating elements can cause damage.
The heating can be done with steam, natural
gas, or electric. The most commonly used heating medium is a submerged burner tube fired
by natural gas or electric heating elements
submerged directly in the tank. Flow-through
electric heaters are also used. A heat-up time
of 6 to 8 h is normally used. During production,
the parts that are quenched provide the heat.
The control for the heating is an on/off system.
There are no requirements for proportionalintegral-derivative control due to the very slow
response time of the tanks. The agitation must
be a well-integrated design with the heaters to
ensure that there is good flow across the heaters
and that temperature uniformity is achieved in
the tank during heatup.
Cooling of the tanks is done by the use of
heat exchangers or a chiller. The heat exchangers can be water/water or water/air. Water/air
exchangers are placed either inside or outside
of the buildings. The sizing of the cooling systems will depend on how fast the tank is
required to recover to the start temperature.
Part Racking and Baskets. The rack must be
fabricated of materials that can endure repeated
heating and cooling cycles without any detrimental effect on the rack, as described in the
section “Racking Practices” in this article. The
racks must be pinned and bolted together to
allow the rack to expand and contract without
restriction during heating and cooling. Welding
must be eliminated as much as possible,
because racks have a high tendency to crack.
The use of tubing, especially 4130 steel tubing,
has been very successful throughout the aluminum aerospace industry with racks that heat to a
maximum of 565 C (1050 F). These production racks have thousands of cycles without
any repairs or distortion. For temperatures
above 565 C (1050 F), other material is
required. The round tube or rod shape is preferred to structural shapes such as I-beams or
C-channels, which will not cool evenly during
the quench and will distort severely after a
few quenches.
Load configuration is probably the most
important aspect of heat treating heavy-gage
loads. The load must be configured to allow
the air to heat the parts during the heat cycle,
and the parts must be spaced so the quench
has access to all surfaces and can remove the
heat quickly. The tendency to pack same-sized
and -configured parts tightly on the racks can
have a very detrimental effect on the process.
The tightly packed parts can have significantly
different properties after heat treating than the
same parts spaced properly on the fixture. In
fact, this links back to the problem with tank
modeling and parts testing for the design of
quench tanks. If single parts or a small load
are used for this testing, the results can be different than the actual production loads.
The rule of thumb is that there must be a
minimum of 25 mm (1 in.) plus the thickest part
of the material between each part to achieve
good heat transfer. Care must be taken when
racking the parts. As shown in Fig. 46, it can
significantly change the process when different
approaches are used for hanging the same part.
The main concern in the example shown in
Fig. 46 is the fact that the steel rod has a different cool-down rate than the aluminum, and the
part may have a soft spot where the rod is in
contact with the part due to slow cooling and
slow heatup during the solution heat treat cycle.
The rod prevents proper access and cooling for
the quench. The use of thin-walled tubing/pipe
for hanging the parts is preferred compared to
solid rod.
Fluid Maintenance and Control. In addition to tank design and the proper degree of agitation, the cleanliness of the quenchant is an
important factor in the quench system. A dirty,
contaminated quench bath can have significantly different quench qualities and cooling
capabilities than a clean bath. Contamination
can be categorized as particle contamination,
chemical contamination, and biological
contamination.
Particle contamination can come from
several sources: tank and rack scale, sand and
dirt from the factory environment, or from
manufacturing of the part itself. One of the
release agents used in the forging process is
graphite, which adheres to the parts and is
washed off in the quench. Sand from the casting mold is another example.
Filters are employed to remove dirt from the
tank to maintain a reasonably clean quench.
The filters must be sized to allow for maintenance. If they are too small, the changing/cleaning will be too big a burden; if they are too
large, equipment cost is a factor. It is important
to obtain an estimate of the dirt loading in the
tank before the design is decided for the filtering system. Bag filters or cartridge filters are
the most commonly used filter types. Filter
sizes are in the 5 to 10 mm range. For sand
removal, centrifuge-type filters are used along
with conveyor systems for the heavier loadings.
Chemical Composition Control. The use of
PAG polymers and additives involves some
Fig. 45
Basic types of flow patterns in quench tanks.
Load baskets are outlined, and shaded areas
are regions of almost zero flow. (a) Tank is divided into
three flow areas, with central location of flow from
sparger pipes. (b) Flow pattern of tank with side agitator
(draft tube)
Fig. 46
Use of tube for hanging parts. The use of thinwalled tubing/pipe for hanging the parts is
preferred, because solid rod prevents proper access and
cooling for the quench.
176 / Heat Treating of Aluminum and Its Alloys
additional requirements on composition control
(see also the section “Polymer Quenchants” in
this article). The PAG-water solution quenchants consist of polymers and several different
additives. The polymer molecule does not
change much during the life of the bath, which
can be several years in a properly maintained
system. However, some of the components
can disappear over time. The corrosion inhibitor (sodium nitrate) can be diluted and removed
with some concentration methods, and the pH
level can change. Low pH levels can damage
parts by an etching effect. The heat treating
facility must implement a regimented qualityassurance program that will detect problems in
the quench before they become detrimental to
the process. The supplier of the polymers can
assist in testing and replenishments of the chemicals as needed to maintain the bath.
Concentration measurement for PAG quenchants includes densitometers, refractometers, and
viscosity meters. The cleanliness of the bath
directly influences the accuracy of the measurements. Several of the instruments require
frequent calibration. The electronic refractive
index monitor with remote sensing and optional
connection to a PLC has proven to be very stable
if the solution is conditioned and filtered. Accuracy levels can be within ±0.5% over time, with
only very limited maintenance requirements.
Bioburden Control. Bacteria and algae
growth can occur in aqueous solutions if there
are no biocides present. Biocides are used with
various successes to control the problem. Biocides with glutealdehyde are the most commonly used. They last from 10 to 21 days in
the bath and must be replenished periodically
to remain effective. The amount varies according to contamination levels. Approximately
150 to 250 parts per million added every two
weeks can normally keep the bath in complete
control. It is highly recommended to use an
automatic injection system to limit worker
exposure to the very toxic materials used in biocides. Shop test procedures check for bacteria
and fungi. An occasional change of biocide will
keep the bacteria from becoming resistant to
the product.
Agitation Systems
Flow is generated by using several methods.
Quench tanks used in the aluminum industry
can contain very large quantities of water
(38,000 to 227,000 L, or 10,000 to 60,000
gal), and to move this amount of water at an
average speed of 0.3 m/s (1 ft/s) requires significant pumping. There are mainly two options in
these cases: pumping with water-jet eductors at
the sparging pipes or agitators in draft tubes.
Pumping and the use of different types of propeller agitation provide the most common
method. Parts or basket movement are used in
rare occasions.
Pumping is versatile and does not take up
much space in the tank, because sparger pipes,
eductors, and nozzles can be tucked close to
Table 15 Comparison of power
the sidewall or bottom of the tank. Pumping
requirements for draft tubes and pumping
has a low efficiency per gallon of quench
systems
moved compared to other types of agitation
Pumping systems
devices, especially draft tube designs. The use
Draft tubes
(end suction pumps)
of an eductor can significantly increase the Flow rate,
rpm
Propeller type
hp
psi
hp
amount of quench moved inside the tank. gal/min
5600
810
13.5
in.
airfoil
5.5
20
75.5
The volume increases by a factor of 4 and
520 13.5 in. airfoil 2.0
20
42.6
the velocity decreases by the same factor. 3200
2950
426 13.5 in. airfoil 1.0
20
39.0
However, the overall flow generated will be
sufficient to make a good quench. Compared
to nozzles, an eductor provides a better distribution of the flow and does not generate point- Top entering
Elevator
cooling of parts by hitting the part with a very agitator
or
basket
high velocity of fluid at a concentrated spot.
guides
Propeller agitation is divided between open
placement and agitation tube placement. In
Fluid level
addition, there are marine-type propellers and
airfoil-type propellers used for agitation purOverflow
weir
poses. The open-type propellers are most commonly used in side-mounted systems, for
example, integral quench furnaces. These proPropeller
pellers are typically marine-type propellers,
which are slow spinning compared to airfoilGrating or
Draft
type propellers. The swirling action of the
perforated plate
tube
quench when it leaves the propeller tips generates a good nonlinear flow. However, the flow
Directional flow baffles
is very uneven and can affect properties in the
parts. The horsepower requirements are large
compared to airfoil-type systems; however, it Fig. 47 Quench tank with single draft tube agitator
is less than pumping. Table 15 shows a comparison of required power between pumping and
for quench tank and furnace design is used to
draft tubes.
The draft tube is widely used in the larger verify and predict the mechanical design and
open-tank systems. Draft tube design is covered process variables. With the new programs, it
in detail in Ref 13. Basically, the draft tube is possible to provide easy-to-understand graconsists of a propeller (airfoil or marine type) phics that can be used in the design decisions
placed inside a tube (Fig. 47). The placement for different quench tanks. With modeling proof the propeller inside the tube increases the grams, it is possible to review and change paraefficiency of the propeller and gives the meters and then observe the calculated results.
Parts testing is typically employed when
designer the ability to direct the quench flow
in a more controlled and predictable manner. existing equipment is used for new products
The distance from the water to the edge of the or for improvements of existing product proflared tube must be large enough to prevent cesses. A proper test plan must be developed
air from being pulled down into the tube and that addresses the areas that can affect the part:
thus creating bubbles in the quench. The bub- placement of the part in the tank, orientation of
bles can create an insulating layer on the the part in the tank, and areas of high and low
parts and must be avoided in the quench tank. flow in the tank. It is very important to find
Several methods are available to prevent the out where the flow is in the tank by mapping
vortex from being started. One is to place a flat it with a flow meter. An open-type flow meter
plate 5 cm (2 in.) under the surface and force is preferred to a closed-type flow meter, as
the water to enter the agitator in a more shown in Fig. 48.
The measurements of the flow will normally be
horizontal manner. This will create a slight
restriction in the inlet, but normally this will taken without parts in the tank. When the parts
not reduce the volume significantly. The other displace space and volume in the tank, the speed
method is to place the propeller and the flared of the quench around the parts increases. In addicone deep enough to prevent the inlet vortex tion, the thermal action of the rising heated
quench from contact with the parts adds to the
from forming.
Flow modeling and parts tests are useful velocity of the quench past the parts. With this in
tools in evaluating the proper method for gener- mind, it is understandable that tanks with lessating the flow and placing parts baskets in the than-desired flows that empty as described in this
area of maximum flow. The use of mechanical article can, in fact, produce satisfactory parts.
Example 1: Correction of Uneven Flow.
tank modeling can help design an agitation system, but building of models can be time-con- A 57,000 L (15,000 gal) quench tank was agisuming, and scaling from the model to the tated by three large side-mounted marine-type
finished tank size and product may not be a propellers. The quench area for the parts was
straightforward process. Computer modeling in the top 41 cm (16 in.) of the tank, because
Quenching of Aluminum Alloys / 177
parts were quenched one at a time every 20 to
30 s. The flow was very strong but uneven,
as shown in Fig. 49. Several methods were
used to solve the problem. Baffling and flowdirection vanes did very little to even out the
flow. The final fix was to install a perforated
plate under the parts. The perforated plate/plenum
created a very even and desirable flow.
Figure 50 shows the surface of the tank after
installation of the plenum. A crown of approximately 50 mm (2 in.) can be seen in the middle
of the tank where the quench is forced up and
then returns to each side of the tank. The use
of perforated plenums in conjunction with
tube or open-type agitators is very successful in
generating controlled, even flows.
Fig. 48
Fig. 49
ACKNOWLEDGMENT
Tom Croucher, who was widely known in
the U.S. aerospace industry as an expert in heat
treatment, passed away while this article was
being developed. With sadness in the loss of
Tom, the editors want to recognize his contributions and expertise in quenching, distortion
control, polymer quenching, and the application
of uphill quenching process. Tom is missed.
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