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Understanding Austempered Ductile Iron
Process, Production, Properties and Applications – Part III
S. Gowri1
and K. Hayrynen2
1
General Manager – Hightemp Furnaces Limited, Bangalore, E-mail : gowri@hightemp-furnaces.com
2
Director of Research & Development – Applied Process Inc., USA
TECHNICAL NOTE
INTRODUCTION
ADI is an acronym for austempered ductile iron. It is produced by
austemperingaductileironmaterialtoformapredominantlyausferritic
matrix. Ausferrite consists of a combination of high carbon-stabilised
austenite and acicular (needle-shaped) ferrite. It is this unique
microstructurethatisresponsiblefortheremarkablecombinationsof
strength, ductility, toughness and wear resistance that are exhibited
by ADI.
This is the third part of the series on Understanding Austempered
Ductile Iron. It will focus on the properties of ADI and its applications.
WorldwidemarketsectorsforADIinclude: lightvehicle,heavyvehicle,
agriculture,railroad,construction,miningandmiscellaneousindustrial
applications
PROPERTIES OF ADI
ADI refers to a family of heat-treated ductile iron. According to ASTM
A897/897M-06 (2011), there are six different grades of ADI. The
minimum properties to meet each grade are listed in Table-1. The
rangeofpropertiesavailableforADIisdependentonthechoiceofheat
treatment parameters which will, in turn, determine the
microstructural scale of the ausferrite as well as the relative amounts
of austenite and ferrite within the ausferrite. This range in
microstructures is shown in Fig. 1. Note that the first grade of ADI
TTTTTablablablablable-1: Gre-1: Gre-1: Gre-1: Gre-1: Graaaaadededededes and Prs and Prs and Prs and Prs and Propopopopopererererertietietietieties of ADI ps of ADI ps of ADI ps of ADI ps of ADI per ASer ASer ASer ASer ASTM A897/897M-06 (rTM A897/897M-06 (rTM A897/897M-06 (rTM A897/897M-06 (rTM A897/897M-06 (reeeeeaaaaapprpprpprpprpprooooovvvvveeeeed 2011)d 2011)d 2011)d 2011)d 2011)[1][1][1][1][1]
GradeGradeGradeGradeGrade FFFFFormerormerormerormerormer TTTTTensilensilensilensilensileeeee YieldYieldYieldYieldYield Elong.Elong.Elong.Elong.Elong. ImpactImpactImpactImpactImpact TTTTTypicalypicalypicalypicalypical
DesignationDesignationDesignationDesignationDesignation StrengthStrengthStrengthStrengthStrength StrengthStrengthStrengthStrengthStrength (%)(%)(%)(%)(%) EnergyEnergyEnergyEnergyEnergy HardnessHardnessHardnessHardnessHardness
(MPa/Ksi)(MPa/Ksi)(MPa/Ksi)(MPa/Ksi)(MPa/Ksi) (MPa/Ksi)(MPa/Ksi)(MPa/Ksi)(MPa/Ksi)(MPa/Ksi) (J/lb-ft)(J/lb-ft)(J/lb-ft)(J/lb-ft)(J/lb-ft) (HBW)(HBW)(HBW)(HBW)(HBW)
750-500-11750-500-11750-500-11750-500-11750-500-11
750 / 110 500 / 70 11 110 / 80 241 - 302
(110-70-11)(110-70-11)(110-70-11)(110-70-11)(110-70-11)
900-650-09900-650-09900-650-09900-650-09900-650-09 Grade 1
900 / 130 650 / 90 9 100 / 75 269 – 341
(130-90-09)(130-90-09)(130-90-09)(130-90-09)(130-90-09)
1050-750-071050-750-071050-750-071050-750-071050-750-07 Grade 2
1050 / 150 750 / 110 7 80 / 60 302 – 375
(150-110-07)(150-110-07)(150-110-07)(150-110-07)(150-110-07)
1200-850-041200-850-041200-850-041200-850-041200-850-04 Grade 3
1200 / 175 850 / 125 4 60 / 45 341 – 444
(175-125-04)(175-125-04)(175-125-04)(175-125-04)(175-125-04)
1400-1100-021400-1100-021400-1100-021400-1100-021400-1100-02 Grade 4
1400 / 200 1100 / 155 2 35 / 25 388 – 477
(200-155-02)(200-155-02)(200-155-02)(200-155-02)(200-155-02)
1600-1300-011600-1300-011600-1300-011600-1300-011600-1300-01 Grade 5
1600 / 230 1300 / 185 1 20 / 15 402 - 512
(230-185-01)(230-185-01)(230-185-01)(230-185-01)(230-185-01)

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TECHNICAL NOTE
listed in Table-1 is an exception to this general rule as it is produced by
inter-critical austenitising which results in a final microstructure that
containsproeutectoidferriteincombinationwithausferrite. Thisgrade
of ADI (Grade 750 ADI) will be discussed later in this article.
If a component is alloyed correctly, it is possible to produce any of the
gradesofADIinTable-1(exceptforGR750ADI)bytheproperselection
of heat treatment parameters i.e. temperatures and times. In general,
lower austempering temperatures will lead to the production of the
higher strength, higher hardness grades of ADI.
Typically, ADI exhibits twice the strength of as-cast ductile iron for a
given ductility. This is shown in Fig. 2 which illustrates the relationship
of yield strength to ductility for various metallic materials. Prior to the
introduction of ADI, the material of choice for yield strength above
600 MPa was largely limited to steel.
When high normal forces are applied to an ADI component in service,
alocalisedstrain-inducedtransformationoftheaustenitecomponent
in the Ausferrite microstructure will harden the contact surface to a
depth of approximately 5μm. This is illustrated in Fig. 3. As a result,
whenADIisusedinahighstressabrasionenvironment,ittendstohave
improved wear resistance over conventionally hardened steels.
One of the earliest high volume applications of ADI was that of hypoid
gear sets in light vehicles. Throughout the last three decades, ADI has
also been used for timing gears, worm gears, helical gears and spur
gears. Figures 4 and 5 compare the allowable contact stress and
allowablegeartoothrootbendingfatigueasafunctionofhardnessfor
ADI vs. competitive steel alternatives. If one considers that ADI is 10%
lower in density, can be cast nearer to net shape (minimising metal
Fig.1:Fig.1:Fig.1:Fig.1:Fig.1: Photomicrographs of the Ausferrite microstructure in ADI.
Grade 900-650-09 ADI is shown in (a) while Grade 1600-1300-
01 ADI is shown in (b). The Grade 900 ADI was produced by
austempering at 371°C while Grade 1600 ADI was produced by
austempering at 260°C. Etched with 5% Nital.
(a)
(b)
Fig. 2:Fig. 2:Fig. 2:Fig. 2:Fig. 2: This chart compares the yield (proof) strength vs.
ductility of various metallic materials.
Fig. 3:Fig. 3:Fig. 3:Fig. 3:Fig. 3: Vickers microhardness profile vs. the depth below the
surface for Grade 1050-750-07 ADI. [2]

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TECHNICAL NOTE
removal) and is typically lower in cost, it can be concluded that ADI is
a suitable alternative material for the manufacture of gears.
Fig. 4:Fig. 4:Fig. 4:Fig. 4:Fig. 4: A comparison of the allowable contact stress vs.
hardness for ADI and various steel-based material/process
combinations. [3]
Fig. 5:Fig. 5:Fig. 5:Fig. 5:Fig. 5: A comparison of the allowable Gear Tooth Root Bending
Fatigue vs. Hardness for ADI and various steel-based material/
process combinations. [3]
Fig. 6:Fig. 6:Fig. 6:Fig. 6:Fig. 6: Typical 10MM cycle allowable bending stress (MPa) for various materials. ADI is material 20.[4]
Figures 6 and 7 contain rotating bending fatigue strength and fracture
toughnessasafunctionofyieldstrength,respectively,forADIinaddition
toothermaterial/processcombinations. Examinationofbothofthese
figures would indicate that ADI is competitive with many steel
alternativesandimprovedoveraluminiumalternatives. (ADIismaterial
20 in both figures.)
AllowableBendingStress(N/mm2
)
AllowableContactStress(N/mm2
)

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ADI Hybrids
Twodifferent“hybrids”ofADIhavebeendevelopedtoaddressspecific
challenges with conventional ADI. Grade 750 ADI from Table-1 was
developedtoenablemanufacturerstomorereadilymachineADIafter
austempering. It is produced by austenitising in an intercritical range
resulting in a final microstructure that consists of a combination of
proeutectoid ferrite and Ausferrite. Because a lower austenitising
temperature is used to produce GR 750 ADI, the carbon content of
the austenite is lower than that for conventional ADI resulting in
increased alloy requirements for the base ductile iron for GR 750 ADI.
Increased alloy requirements mean an increased cost in the base
material which has to be justified by cost savings in machining. The
other ADI hybrid of interest is not included in any ADI standards. It is
produced by austempering ductile iron with carbide volumes of upto
60%. ThismaterialiscalledcarbidicADIorCADI. CADIwasdeveloped
to increase the wear resistance of GR 1600 ADI. Representative
microstructuresofthesehybridsofADIareshowninFigs.8(a)and(b).
(a)(a)(a)(a)(a)
TECHNICAL NOTE
Fig. 7:Fig. 7:Fig. 7:Fig. 7:Fig. 7: Room temperature fracture toughness of ADI compared to several material/process combinations. ADI is material 20. [4]

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MACHINABILITY OF ADI
There are numerous publications on machining ADI, many of which
have conflicting results/conclusions. The reasons for the confusion
arise from studies that are concerned with either trying new types of
tooling or simply cutting metal without understanding the unique
attributes of ADI.
Perhaps the most important thing to remember when setting up to
machine ADI is that consideration of hardness only will lead to low
throughput and poor tool life. The essential knowledge related to the
successful machining of ADI includes:
 ADI has a high yield (proof) strength yet a Young’s Modulus (E)
that is 20% lower than that of steel.
 This requires a very stiff work holding set-up and short tool
momentstominimisevibrationduringmachining.
 When acted upon with a high normal force, ADI undergoes a
strain-inducedsurfacetransformationthatresultsintheaustenite
in the ausferrite transforming to hard martensitic particles that
are present in an acicular ferrite nest.
 A thin chip may harden through its entire section while a
thicker chip may harden only at the tool interface allowing
the discontinuous chip to peel off presenting a new, more
machinable, ADI surface to the tool
 This phenomenon makes thread rolling, perhaps, the most
difficultmachiningoperationwithADI.
 ADI has lower thermal conductivity than either ductile iron or
plain carbon steels resulting in a high workpiece/tool interface
temperature
 To maximise metal removal rate, the selected tools must
have good toughness and be able to withstand high
temperatures at the cutting face.
APPLICATIONS OF ADI
Many successful applications of ADI have occurred since the first
commercial applications in the early 1970’s. In general, those
components that need good dynamic properties like fatigue strength
or fracture toughness are most suited to GR 900 and GR 1050 ADI.
When wear properties are of concern, grades 1400 and 1600 are
used. When a good compromise between dynamic properties and
(a)(a)(a)(a)(a)
Fig. 9:Fig. 9:Fig. 9:Fig. 9:Fig. 9: Successful high volume automotive applications of ADI
include (a) constant velocity joints and (b) tow hooks. [5]
(b)(b)(b)(b)(b)
TECHNICAL NOTE
(b)(b)(b)(b)(b)
Fig. 8:Fig. 8:Fig. 8:Fig. 8:Fig. 8: Photomicrographs of (a) Grade 750 ADI and (b) CADI.
The GR 750 ADI has been heat tinted to highlight the presence of
the proeutectoid ferrite (light phase). The light phase in the CADI
photomicrograph is carbide – etched with 5% Nital.

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wear is needed, GR 1200 ADI is chosen. For the most part, GR 750
ADI is most suitable for components with relatively thin section sizes
that absolutely must be machined after heat treatment.
TECHNICAL NOTE
Fig. 10:Fig. 10:Fig. 10:Fig. 10:Fig. 10: An ADI suspension beam that replaced a steel forging
and increased durability.
Fig. 11:Fig. 11:Fig. 11:Fig. 11:Fig. 11: A sway bar bushing for a heavy truck axle. [5]
Fig. 12:Fig. 12:Fig. 12:Fig. 12:Fig. 12: An ADI hub along with the competitive aluminium
hub that it replaces. [5]
Fig. 14:Fig. 14:Fig. 14:Fig. 14:Fig. 14: A drive wheel for the track system of construction
equipment. This one-piece casting replaced an 84-piece
weldment. [6]
(a)(a)(a)(a)(a)
(b)(b)(b)(b)(b)
Fig.13:Fig.13:Fig.13:Fig.13:Fig.13: Ground engaging applications of ADI include (a) a seed
boot and (b) a plow point. [6]

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TwosuccessfulhighvolumeautomotiveapplicationsofADIarepictured
in Fig. 9. Constant velocity joints are machined fully out of ferritic
ductile iron before austempering to a final hardness of 450 HBW.
Many different shapes and sizes of tow hooks for trucks and sport
utility vehicles like those pictured in Fig. 9(b) have been austempered
to grades 900 and 1050 ADI.
Heavy truck applications of ADI have included many different
suspension components like the beam pictured in a heavy truck air
suspension system. This ADI beam shown in Fig. 10 replaced a steel
forging, increasing durability by over 350% at a lower cost. Another
heavy truck application, a sway bar bushing, is shown in Fig. 11. This
component is machined fully prior to austempering to GR 1200 ADI.
It includes a precision spline and an acme threaded ID.
Whenweightreductionisconsidered,mostdesignengineersaretrapped
intheparadigmofusinglowerdensitymaterialslikealuminium. Figure
12 shows an ADI heavy truck trailer hub and the aluminium hub that
it replaces at a 4% weight savings. This occurs because ADI is stronger
and2.3timesstifferthanaluminiumwhichallowsfordesignsthattake
TECHNICAL NOTE
(a)(a)(a)(a)(a)
(b)(b)(b)(b)(b)
advantage of reduced section thicknesses.
Agricultural applications of ADI have been one of the fastest growing
sectors in recent years. Figure 13 shows examples of conversions to
ADI for ground engaging applications that take advantage of the wear
properties of ADI. Figure 13 (a) is a seed boot planter that delivers
seed into the soil. This seed boot replaced a steel weldment at a 15%
reduction in weight and a 65% reduction in cost. In addition, it has
marked improved wear performance. Figure 13 (b) is an example of
anADIplowpoint. Suchgroundengagingapplicationsarewell-suited
to grades 1400 and 1600 ADI. Many other ground engaging
applications employ the use of carbidic ADI (CADI).
Construction and mining vehicles use many of the same types of
components that heavy trucks do such as brackets, control arms,
steering knuckles, etc. Figure 14 shows an example of a drive wheel
for a track system for piece of construction equipment. This one-
piece casting replaces an 84-piece weldment.
Gears of many shapes and sizes can be made from ADI. Examples
are shown in Fig. 15.
(d)(d)(d)(d)(d)
(c)(c)(c)(c)(c)
Fig. 15:Fig. 15:Fig. 15:Fig. 15:Fig. 15: Examples of ADI gears: (a) a diesel engine timing gear, (b) hypoid differential gears and pinions, (c) a one-piece gear and axle for a
commercial lawn mower and (d) a large mill gear produced in segments and then assembled after Austempering. [7]

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SUMMARY
ADI refers to a family of heat-treated ductile iron with combinations of
good strength, ductility and wear resistance. It can replace steel
castings and forgings, weldments and aluminium at a cost savings.
The lower strength or dynamic grades of ADI like GR 900 or GR 1050
can compete favourably with steel forgings for both cost and weight
savings. However, attention must be paid to the lower stiffness of ADI
and be accounted for in early design stages.
The higher strength grades of ADI like GR 1400 or GR 1600 can
compete favourably with carburised and hardened steels for wear
resistance at a lower manufacturing cost.
Replacing aluminium with ADI at equal weight or at a small weight
savings is a new paradigm that design engineers are just learning to
embrace. Ascastingtechnologiescontinuetoimprove,itisanticipated
that even more elegant, thin section ADI castings will be designed.
ADI is not a new technology. It has been produced commercially
sincetheearly1970’s. It’sonlybarrierappearstobelackofknowledge
onthepartofdesignengineers. Thissummaryarticlewasintendedto
aid those in learning about ADI who are in need of knowledge about
this versatile material.
Acknowledgments
The authors gratefully acknowledge the assistance of John Keough
and the Applied Process Companies for assistance in preparing this
manuscript.
Selected References
1. ASTMA897/A897M-06,StandardSpecificationforAustempered
Ductile Iron Castings, ASTM International, West Conshohocken,
PA, www.astm.org
2. Ductile Iron Data for Design Engineers, Section IV Austempered
Ductile Iron, www.ductile.org/didata.
3. AGMA 939-A07, Austempered Ductile Iron for Gears, American
Gear Manufacturers Association, Alexandria, VA, www.agma.org
4. Keough, J. R., Hayrynen, K. L. and Pioszak, G. L., Designing with
Austempered Ductile Iron, AFS Transactions, Vol. 118 (2010), p.
503-517.
5. Keough, J. R. and Hayrynen, K. L., Automotive Applications of
Austempered Ductile (ADI): A Critical Review, Paper No. 2000-
01-0764, Society of Automotive Engineers, www.sae.org
6. Keough, J. R., Dorn, T., Hayrynen, K. L., Popovski, V., Sumner, S.
and Rimmer, A., Agricultural Applications of Austempered Iron,
Metal Casting Design & Purchasing, Sept/Oct 2009, p. 28-31.
7. Lefevre, J. and Hayrynen, K., Austempered Materials for
Powertrain Applications, Proceedings of the 26th
ASM Heat
Treating Society Conference, ASM International, Oct 2011.