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Materials Today: Proceedings 5 (2018) 25615–25624
www.materialstoday.com/proceedings
IConAMMA_2017
Manufacturing of Engineering components with Austempered Ductile
Iron – A Review
a
a
a
a
S.Samaddar , T.Das , A.K.Chowdhury , M.Singh *
CSIR-CMERI,Durgapur,713209, India.
Abstract
The Austempered Ductile Iron (ADI) is a ferrous cast material that provides best combination of low cost, design flexibility, good
machinability, high strength to weight ratio, good toughness, wear resistance and fatigue strength. It is being manufactured from high
quality ductile iron through Austempering process. This Austempering process gives equivalent or superior quality ADI engineering
component compared to components manufactured from other ferrous as well as non-ferrous material. It’s unique property gives
engineers a scope to design in a more versatile and economically viable way. In this review paper the salient considerations to produce
Austempered ductile iron and the different grades of ADI has been discussed. The various factors related to Austempering of S.G. Iron
that effects on final microstructures and the mechanical properties of this material, has been highlighted. The process technology is
mapped against various possible applications in developing new product. Some pre-developed products have been reviewed. An
approach on developing mining component by ADI process route has also been discussed.
© 2018 Elsevier Ltd. All rights reserved.
Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA
2017].
Keywords:Ductile Iron; Austempering;Ausferrite; Tensile Strength;Wear resistance; Hardness; Fatigue strength.
1. Introduction
Gray cast iron is a special type of cast iron with graphitic (continuous flakes) microstructure with carbon (C%) of about
2.5 to 4.0 % and silicon content of about 1-3%. Ductile iron or Spheroidal Graphite (SG) cast iron is produced by
treating the molten cast iron with magnesium before casting. Magnesium treatment promotes the precipitation of
graphite in the form of discrete nodules in ductile iron. SG cast iron with a Ausferrite( Retained Austenite with ferrite)
microstructure is called Austempered ductile iron(ADI). Austempered ductile iron is produced from ductile iron by heat
treatment process called Austempering. Austempering is a isothermal heat treatment process where ductile iron is heated
0
0
up to its Austenizing temperature (840 C- 900 C) and held for certain time period for complete transformation of the
matrix, viz. Austenizing.
Nomenclature
ADI
Austempered Ductile Iron.
SG
Spheroidal Graphite Cast Iron.
CE
Carbon Equivalent.
UTS
Ultimate Tensile Strength.
YS
Yield Strength.
MRR
Material Removal Rate.
* Corresponding author. Tel.:+919474882554,Fax No:+91-343 2546745
* E-mail :manju@cmeri.res.in
2214-7853© 2018 Elsevier Ltd. All rights reserved.
Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Applications [IConAMMA
2017].
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M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
o
o
This is followed by salt bath/oil quenching at temperature of about (250 C-370 C) [1]. Fig. 1.0 shows flow chart for
manufacturing components with ADI.
Fig. 1.0 Flow chart for manufacturing components from ADI
Austempered Ductile iron, as material possesses very good strength to weight ratio, dynamic wear resistance properties,
high toughness and fatigue, manufacturability and cast compatibility. Fig.1.1 (a)shows the comparison of ADI with
different material w.r.t. relative weight per unit strength.Net shape manufacturing with Austempered Ductile Iron has
been a trending technology that has been induced in the conventional manufacturing processes. It is driven by innovative
product design and improved process technology, providing a competitive price. Fig.1.1 (b) shows the comparison of
ADI with different material w.r.t. relative cost per unit weight.ADI material can be used to achieve design flexibility,
maximum material utilization, minimum energy requirement, hence providing a better cost to benefit ratio with low cost
production. This will be beneficial to wide spectra of user industry that includes automotive sector, mining sector,
agricultural sector, Railway, etc.
Fig.1.1.(a)Comparison of ADI with different material w.r.t. relative weight per unit strength[2]; .(b) comparison of ADI with different material
w.r.t. relative cost per unit weight.[2]
ADI is in the western market since 1972 and has been utilized to develop several components in the sector discussed
above. Table 1 shows the ADI market share in North American and European professions. But this grade of ductile iron
is relatively new and highly growing in the Indian market. In India CSIR-CMERI started research on developing ADI
components since 2000.Other universities and laboratories, viz. NML, NIFFT, MSRIT Bangalore, JNUTH college of
Engineering are also doing research work on process route development with ADI at laboratory scale.
Table 1.The ADI market share in North American and European professions [3]
Industry
ADI ratio
of the total
production
volume
(%)
Medium and heavy duty
truck
2007
26
year
North
America
2010
25
year
Europe (2007)
34
Light
vehicle
Building
&Mining
Agricultural
machinery
General
machinery
Railway
25
8
14
11
16
20
11
17
15
12
14
22
4
5
3
Energy
&source
18
M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
25617
In this paper, the scope of suitability and sustainability of ADI in various engineering fields has been discussed briefly.
This review analysis will help the designer to accumulate all data regarding manufacturing with ADI. Therefore, this
paper aims at lucidification of the aspects of designing and manufacturing engineering components with ADI.
2. Process Technology to develop ADI
2.1. The Ductile Iron process
Ductile iron is an iron-based alloy which contains a carbon content that is high enough to exceed its solubility in
iron; resulting in the presence of pure carbon or graphite dispersed within an iron matrix. In the case of ductile iron, the
shape of the graphite is spheroidal or round and is described as having graphite nodules.
To get ADI, high quality ductile iron is required to be Austempered . For the purpose of Austempering, high quality
2
can be defined as: Minimum nodule count (100 per mm ); minimum nodularity of 80%;Porosity, carbide, inclusions
and Micro shrinkage <1.5%; Throughout same chemical composition.
High nodule count is important to minimize the segregation of alloy element which can promote the presence of
carbides as well as delay the rate of Ausferrite transformation. Additionally, higher nodule count will prevent in the
formation of porosity on micro shrinkage as well as promote the formation of small, round graphite nodule. [2]
2.1.1.
Standard Grades of SG Iron.
According to the tensile strength and elongation of the SG Iron , it can be classified in following grades shown in
Table 2. These grades of SG Iron can ideally be transformed into Austempered Ductile Iron .
Table 2. Grades of Ductile iron as per ASTM A536-84(2009) Standard Specification For Ductile Iron[4]
Grade
60-40-18
65-45-12
80-55-06
100-70-3
UTS
Yield Strength
psi/ MPa
psi/ MPa
60000/414
65000/448
80000/552
100000/689
40000/276
45000/310
55000/379
70000/483
%Elongation
18
12
6
3
2.2. The ADI Process
ADI is produced from ductile iron through a heat treat process called Austempering. Austempering process is an
isothermal heat treatment process. It generally consists of heating ductile iron to a temperature, called Austenising
temperature, to produce Austenite. After Austenising, heat treated ductile iron is subjected to rapid quenching to avoid
formation of Pearlite and other micro-constituents at a temperature above Martensite start(Ms) .
This quenching temperature is called Austempering temperature. Fig 2.1shows the (a) Schematic representation of
the Austempering process in ductile iron (b) Effect of Austenising temperature on carbon concentration in austenite.
Ductile iron is quenched at the austempering temperature indicated in fig.2.1(a) for a sufficient time for complete
transformation of the SG matrix into the Ausferrite structure.(Bainitic for steel and Ausferrite for Ductile iron). The
quenching time is solely dependent on the cross section of the sample to be heat treated.
Heat treatment temperature and time can be varied along with cross section of the sample. As the section size
increases the quenching time also increases. Austempering technique include nitrate salt quench, hot oil quench.To
avoid high temperature reaction products, salt bath quench severities can be increased with water addition or with
alloying elements (such as Copper, Nickel, Magnasium or Molybdenum) that enhance Pearlite hardenability.
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M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
a.
b.
Fig 2.1 (a) Schematic representation of the Austempering process in ductile iron (b) Effect of Austenising temperature on carbon concentration
in Austenite .[5]
Table 3.depicts the different grades of ADI that can be produced by heat treating SG Iron.It is clear from the
table that an Austempering treatment applied to the SG grade of Ductile Iron can improve its UTS ,YS and
Elongation.
By variation of Austempering Tempering temperatures and Quenching durations, Six standard grades of ADI
can be produced given in Table 3. So,the designer can choose the required property as per the component to be
developed.
Table 3. Standard Grades of ADI as designated by ASTM A897-A897M-06 [6]
Grade
UTS
Mpa/ksi
Yield Strength
%Elongation
Impact Energy
500/70
11
110
241-302
Mpa/ksi
J
Hardness
BHN
750-500-11
750/110
900-650-9
900/130
650/90
9
100
269-341
1050-750-7
1050/150
750/110
7
80
302-375
1200-850-4
1200/175
850/125
4
60
341-444
1400-1100-2
1400/200
1100/155
2
35
388-477
1600-1300-1
1600/230
1300/185
1
20
402-512
3.Properties of ADI Material .
The property of ADI is dependent on the heat treatment temperature and quenching time. As a result varying the
temperature and time ADI material exihibit wide range of properties shown in Table 4. It shows the comparison of
yield strength(σy), fracture toughness(KIC), flaw tolerance between ADI, Conventional and austenitic Ductile Irons,
Quenched and Tempered (Q&T) steels.
3.1. Fatigue strength
0
A high Austempering temperature (400 C) produces ADI with high ductility, yield strength in the range of
0
500Mpa with good fatigue and impact strength. A lower transformation temperature (260 C) results ADI with high
yield strength (1400Mpa), high Hardness, Excellent wear resistance and contact fatigue strength. The modulus of
elasticity for ADI lies in the range of 155-163Gpa. Fig3.1(a). Shows comparison of fatigue strength of ADI and other
steel. Fig.3.1(b) shows Fatigue strength at different Austempering temperature.
M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
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Table 4. Comparison of ADI alloy (A-2,B-5,C-1,C-3,C-5) ,Ductile Iron and Steel(Heat Treated) properties. [7][8]
Alloy
Heat
A--2
850oC,
1 hr Salt
Quench
B--5
o
850 C,
1 hr Salt
Quench
Treatment
260oC*
300oC
350oC
400oC
430oC
260oC
300oC
350oC
400oC
1205.4
1107.4
989.8
744.8
744.6
1029.0
980.1
793.7
756.0
KIC
(MPam1/2)
73.49
68.62
72.10
72.91
74.52
75.18
75.40
73.68
76.01
1151.5
86.00
C--1
850oC,
1 hr Salt
Quench
300oC
C--3
o
850 C, 1 hr
salt/quench
o
C--5
850oC, 1 hr
salt/quench
300 C
350oC
400oC
300oC
Ductile Iron, Ferritic, 1.55% Si, 1.5%
Ni, 1.2% Ni
Ductile Iron, Ferritic, 3.6% C, 2.5% Si,
0.38% Ni, 0.35% Mo
Ductile Iron Pearlitic, 0.5% Mo
Ductile Iron, 80--60--03
Ductile Iron, D7003
Ductile Iron, Ni--Resist D-5B
a.
y
(MPa)
(KIC/sy)2
(mm)
5.58
3.72
3.84
5.30
9.58
10.00
5.34
5.92
8.62
10.08
1199.5
900.3
908.8
118.2
78.20
61.60
59.40
85.74
4.25
4.68
4.27
5.88
269
42.8
25.3
331
48.3
21.3
483
432
717
324
48.3
27.1
51.7
64.1
10.0
3.9
5.2
39.1
Alloy
(KIC/sy)2
(mm)
1449
1587
1518
KIC
(MPam1/2)
43.80
55.00
55.60
1380
1449
65.05
57.25
2.22
1.56
204oC
246oC
323oC
348oC
200oC
280oC
350oC
400oC
1380
1449
1414.5
1393.8
1345
1504.2
1497.3
1449.0
89.12
72.64
53.30
58.46
65.38
66.81
87.69
100.22
4.18
2.52
1.42
1.76
2.36
1.97
3.43
4.78
246oC
280oC
1380.0
1393.8
90.55
69.01
4.31
2.45
Heat
Treatment
AISI
4140
870oC,
1 hr Oil
Quench
204oC
280oC
396oC
1100oC,
1 hr., Oil
Quench
1200oC,
1 hr., Oil
Quench
204oC
246oC
870oC,
1 hr Oil
Quench
AISI
4340
y
(MPa)
1200oC, 1
hr., Oil
Quench
843oC, 1
hr., OQ
Tempered
260oC
427oC
1642.2
1421.4
48.79
84.47
.92
1.2
1.34
.88
* Isothermal Transformation Time: 1 hour.
b.
Fig.3.1 (a) Comparison of fatigue strength of ADI and other steel [9]; (b) Fatigue strength at different Austempering temperature [9]
ADI is heat treated to produce high strength has a static fracture toughness of 55-70Mpa, which is greater than that of
Ductile iron matrix of tempered Martensite or Pearlite. ADI heat treated to lower strength grade has fracture toughness
in excess of 100Mpa [10].
ADI component at 30 to 40 RC will wear comparably lower to a quenched and tempered steel component.
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M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
3.2. Strength
The strength of Austempered ductile iron is solely dependent on the relative amount of Ferritic and Pearlitic
structure present in the ADI matrix. The Ferritic volume in the matrix is inversely proportional to the strength i.e. as the
Ferrite volume in S.G microstructure increases the strength decreases. The Pearlite provides hardness to the matrix and
contribute in Ausferrite structure formation after heat treatment. Now the ferrite structure being soft provide higher
hardness to the Ausferrite structure along with nodular graphite, as shown in Fig.3.2 provide unique tensile strength &
elongation % , that is equivalent to steel, employed to design components above 500Mpa yield strength.
Fig. 3.2. Microstructure of ADI showing nodular
Graphite and ausferrite structure[10]
Fig. 3.3 Tensile Strength- Toughness curve
for ADI versus ASTM standards.[11]
Since ADI provided a greater strength to toughness value as shown in Fig 3.3. It can be considered as an alternative
material for application in areas seeking such criteria, viz. Steering knuckles, Crank shafts, Gudgeon pin. etc
considering high strength, ADI has very good toughness. Toughness of ADI is much greater than that of conventional
ductile iron and equivalent or superior to competitive cast and forged steel as shown in Fig. 3.3.Like other properties of
ADI, its toughness is strongly dependent on microstructure.
3.3. Dynamic Performance
Another salient feature of the material that a mechanical designer must consider in order to design a component
is the fatigue life & toughness of the material and its behavior under dynamic loading. Owing to the unique Ausferrite
structure the ADI material provides, high strength to toughness combination. Therefore it provide itself superior to
ductile iron with Martensitic structure. Table 5.shows comparison between Forged steel, Pearlitic Ductile Iron and ADI
grade 1050-750-7.
Table 5. Comparison of Forged steel
Mechanical
Properties
Pearlitic Ductile Iron & ADI[12]
MATERIAL
Forged steel
Yield Strength; ksi (MPa)
75(520)
Tensile strength; ksi (MPa)
115(790)
Pealitic Ductile Iron
70(480)
100(690)
Grade 150/100/7 ADI
120(830)
160(1100)
Elongation %
10
3
10
Hardness ;BHN
262
262
286
Impact Strength; (J)
175
55
165
3.4.Abrasion Resistance
The application that requires longer life of the component when subjected to abrasive forces, ADI suits the
best. As compared to heat treated and surface hardened steel, Austempered ductile iron provides a unique work
hardening characteristics that allows the design engineer to deploy the material in highly abrasive environment. From
Fig 3.4 (a) & (b), it is found that the ADI material with higher hardness grades have less volume loss and relatively high
abrasion resistance compared to steel. This property is due to, strain induced transformation of stabilized Austenite
which occur when the surface of an ADI component is subjected to deformation.
M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
25621
With increasing the Austempering temp, the bulk hardness increases hence the stabilized Austenite increases.
o
o
This increases the abrasion resistance. So with optimum Austempering temperature of about 330 C-380 C a unique
combination of high strength and abrasion resistance is produced that suits various engineering application.
a.
AUSTEMPERED STEEL
Q & T STEEL
Q&T DUCTILE IRON
ADI
b.
.
Fig 3.4 (a)Volume loss of different material at different hardness (Pin on Disc Test) ;(b) Relative Abrasion resistance of ADI vs. Steel.[13]
3.5.Machining properties
Due to high strength and ductility of ADI, cutting tool often suffer flank wear and crater wear during
machining of ADI. Therefore cutting tools need to have high wear resistance but require the use of cutting fluids. Pgrade tools can be used in dry cutting.Al2O3 ceramics are successful for continuous cut process.Si3N4 and PCBN
cutting tools are not suggested for machining ADI. Fig. 3.5 shows the effect on (a) Tool Life,(b)MRR By different
Hardness grade of ADI[14]. The following points should be considered while machining ADI material: (i) Machining
ADI at 50% less speed than that of material with similar hardness, with a 50% deeper cut.(ii)For ADI type 900-650-9
(302BHN) cutting speed must be around 300feet/min.(iii)Al2O3 ceramics and K –grade carbide tool with cutting fluid
for continuous machining process.(iv)MRR will be similar to that of RC30 steel.
a
b.
Fig. 3.5.Effect On (a)Tool Life ;(b)MRR By different Hardness Grade Of ADI.[14]
4. A few case studies of components developed at CSIR-CMERI.
Austempered Ductile Iron can be developed in five available grades with standardized process technology discussed
previously. Since standardized process technology can improve the manufacturability that includes machinability, near
net shape production process reliability and dimensional repeatability. ADI open up a huge area for the designer to
consider this new material for application in various Engineering fields. The basic consideration that every designer
should follow in the decision making process includes: Strength (UTS, YS); Dynamic Performance; Wear Resistance;
Application oriented properties (Damping, Corrosion resistance); Ease of manufacturability; Cost efficiency. The
above stated properties of ADI find it suitable to compete with other available materials like Steel and in some case it
has proved itself either superior or equivalent to the available materials.
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M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
Till date CSIR-CMERI already developed following ADI components, shown in Fig.4.1.These
components are successfully installed and tested in various industrial machineries. The components are listed
below:
• Crankshaft
Fig.4.1(a) shows crankshaft developed at CSIR-CMERI. Crankshaft has intrigue design and requires higher degree
of precision for manufacturing. A crankshaft made of ADI has been deployed in 35HP Sonalika Tractor Engine and
have successfully passed 500hrs of Engine test. This proved that proper process control of the ADI component can
replace forged steel crankshaft yielding very high profit margin along with desirable mechanical properties like
fatigue strength, endurance towards bending, torsion & shear.
• Rotavator Blade
Fig.4.1(b) shows Rotavator blade developed at CSIR-CMERI. Rotavator blade is an agriculture component that is
used to till the field, and it is subjected to high amount of abrasion force. The Rotavator blade made from ADI have
successfully replaced steel blades proving itself better with respect to life,that has been incresed upto 200hrs
compared to previously used steel blades. As well as it can be proved that ADI Rotavator blade price is cheaper
than steel blade.
• Gudgeon Pin
Fig.4.1(c) shows Gudgeon pin developed at CSIR-CMERI. Gudgeon Pin of compressor which is made of steel has
been replaced by ADI gudgeon pin. The ADI gudgeon pin has successfully run 600 hrs of test. Hence ADI can
endure high amount of shearing and bending load equivalent to steel components.
• Ore crushing Hammer
Fig.4.1(c) shows ore crushing hammer developed at CSIR-CMERI. The giant ore crushing hammer are subjected to
high amount of impact strength and abrasion. Owing to its huge size, defect free casting is a challenge. As ADI
having unique matrix that reduces shrinkage and also due to its high wear resistance, it can withstand the abrasive
environment better than hardened steel. Hammer, which is made from ADI have successfully completed 158 hrs of
field trial at Mcnally Bharat Pvt. Ltd. proving itself more long lasting and tough than steel.
a.
b.
c
d.
Fig.4.1 Components made with ADI (a) crankshaft,(b) Rotavator blade, (c) Gudgeon pin,(d) Ore crushing hammer[15]
5. Experimental details for manufacturing of Mining Component (Excavator tooth point) with ADI.
In mining industry sharp digger teeth are essential for ground penetration. At present digger teeth are
manufactured from cast steel material. In this experiment efforts have been made to replace the digger teeth
material with Austempered ductile iron. Manufacturing process of ADI digger teeth as per Fig. 1.0. Considering
end application of this component, composition of the melt, process parameter of casting and Austempering process
were appropriately designed as given below.
• The raw material should be charged initially bi-layer with MS scrap and carburizer (graphite) one upon another.
• The heating rate should be low until the scrap melts. So that the carbon diffuses in the ferrous matrix uniformly
• The CE value (4.3-4.5) should be fixed as per the required section thickness to the cast.
• Alloying is done as per requirement to stabilize the iron-carbon pearlitic phase i.e. Ni, Mo and Cu may be added.
• The percentage of S (<0.025), Mn ( <0.4), & P (<0.1) must be closely controlled to achieve S.G iron
properties. The inoculation should be done to achieve spherical structure of Carbon (Graphitization).
M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
25623
6. Results and Discussion
Firstly SG Iron test blocks were casted as per ASTM A897 standards and the chosen chemical composition of the
S.G. Iron is shown in Table 6. The microstructure of the S.G Iron test block is shown in Fig.5.1 (a) before etch (b) after
etch with 2% nital. The Figure shows spherical structure of that will neutralize the shrinkage during solidification. The
Figure also indicates sufficiently high nodule count resulting in minimization of porosity and carbide. Microstructure
of S.G.Iron showing Nodule count is shown in Fig.5.2.
Table 6. Chemical composition of S.G Iron.
%C
3.15
a.
%Mn
%Si
0.30
1.65
%S
0.07
%P
0.26
b.
Nodular
Graphite
Fig 5.1. S.G Iron Microstructure (a) before etch ; (b) after etch with 2% nital.
Fig.5.2. Nodule Count of S.G iron Microstructure
shown in Fig. 5.1.
Tensile samples were developed from S.G. Iron test blocks as per ASTM A897 standard. The test sample were
subjected to heat treatment cycle as depicted in Fig 5.3. Excavator Tooth point shown in Fig.5.4 was chosen for
development through the ADI Process route.
a.
b.
Fig.5.3. (a)Cycle 1;(b)Cycle 2 of the Austempering process done on samples(Dimension: 130mmx130mmx130mm)
The Fig. 5.5 shows the complete transformation of S.G. matrix into Ausferrite structure. Table 7. and Table 8. show the
various result of Tensile test and Hardness of ADI sample respectively. The result of tensile test conducted on four
ADI test samples gives average value of UTS 720 MPa and Hardness of 300 BHN respectively.
Fig.5.4.Tooth point of Excavator being
developed with ADI at CSIR-CMERI
Fig.5.5 SEM Microstructure of ADI
test Sample Captured at CSIRCMERI
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M.Singh / Materials Today: Proceedings 5 (2018) 25615–25624
Table 7. Tensile Properties of the ADI samples developed at CMERI
Sample no
UTS
YS
%El
A
721.6
430.2
10.8
B
C
720.5
719.4
441.5
440.7
10.5
11.2
Table 8. Hardness of the ADI test samples developed at CMERI
Sample no
A
B
C
Hardness(BHN)
319
304
297
7. Conclusion
The process route of manufacturing of engineering component with ADI material has been elaborately discussed. The
salient features regarding heat treating ductile iron to get desirable mechanical properties has been reviewed. The case
studies of components manufactured with ADI at CSIR-CMERI, Durgapur have been compared with conventionally
used steel components. In this review paper it has been observed that ADI is suitable contender for mining applications
with indigenous resources and the developed material can satisfy the acceptance criteria of International specifications
(EN/ASTM). It is inferred from the study that ADI is better (or equivalent in some case) considering suitability and
sustainability at particular applied field. Hence from the review it can be concluded that further development can be
made in the manufacturing field to replace ferrous engineering conventional component with this new, efficient, cost
effective and characteristically better Austempered ductile Iron.
Acknowledgements
The authors thankfully acknowledge the technical support provided by members of NNMT Group and AdMac
Group of CMERI for their technical support and providing infrastructure for completion of this review work. The
authors would also like to extend their acknowledgement to the Director,CMERI for encouragement and support to
conduct various experiments.
References
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th
2009 was issued on Dec 30 , 2009.
[4]ASTM Hanbook ASTM A536-84(2009) standard specification for Ductile iron.
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