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Understanding Austempered Ductile IronProcess, Production, Properties and Applications – Part III

S. Gowri1 and K. Hayrynen2

1General Manager – Hightemp Furnaces Limited, Bangalore, E-mail : gowri@hightemp-furnaces.com2Director of Research & Development – Applied Process Inc., USA

TECHNICAL NOTE

INTRODUCTION

ADI is an acronym for austempered ductile iron. It is produced byaustempering a ductile iron material to form a predominantly ausferriticmatrix. Ausferrite consists of a combination of high carbon-stabilisedaustenite and acicular (needle-shaped) ferrite. It is this uniquemicrostructure that is responsible for the remarkable combinations ofstrength, ductility, toughness and wear resistance that are exhibitedby ADI.

This is the third part of the series on Understanding AustemperedDuctile Iron. It will focus on the properties of ADI and its applications.Worldwide market sectors for ADI include: light vehicle, heavy vehicle,

agriculture, railroad, construction, mining and miscellaneous industrialapplications

PROPERTIES OF ADI

ADI refers to a family of heat-treated ductile iron. According to ASTMA897/897M-06 (2011), there are six different grades of ADI. Theminimum properties to meet each grade are listed in Table-1. Therange of properties available for ADI is dependent on the choice of heattreatment parameters which will, in turn, determine themicrostructural scale of the ausferrite as well as the relative amountsof austenite and ferrite within the ausferrite. This range inmicrostructures 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/Ks i )(MPa/Ks i )(MPa/Ks i )(MPa/Ks i )(MPa/Ks i ) (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)

7 5 0 - 5 0 0 - 1 17 5 0 - 5 0 0 - 1 17 5 0 - 5 0 0 - 1 17 5 0 - 5 0 0 - 1 17 5 0 - 5 0 0 - 1 1750 / 110 500 / 70 11 110 / 80 241 - 302

( 1 1 0 - 7 0 - 1 1 )( 1 1 0 - 7 0 - 1 1 )( 1 1 0 - 7 0 - 1 1 )( 1 1 0 - 7 0 - 1 1 )( 1 1 0 - 7 0 - 1 1 )

9 0 0 - 6 5 0 - 0 99 0 0 - 6 5 0 - 0 99 0 0 - 6 5 0 - 0 99 0 0 - 6 5 0 - 0 99 0 0 - 6 5 0 - 0 9 Grade 1900 / 130 650 / 90 9 100 / 75 269 – 341

( 1 3 0 - 9 0 - 0 9 )( 1 3 0 - 9 0 - 0 9 )( 1 3 0 - 9 0 - 0 9 )( 1 3 0 - 9 0 - 0 9 )( 1 3 0 - 9 0 - 0 9 )

1 0 5 0 - 7 5 0 - 0 71 0 5 0 - 7 5 0 - 0 71 0 5 0 - 7 5 0 - 0 71 0 5 0 - 7 5 0 - 0 71 0 5 0 - 7 5 0 - 0 7 Grade 21050 / 150 750 / 110 7 80 / 60 302 – 375

( 1 5 0 - 1 1 0 - 0 7 )( 1 5 0 - 1 1 0 - 0 7 )( 1 5 0 - 1 1 0 - 0 7 )( 1 5 0 - 1 1 0 - 0 7 )( 1 5 0 - 1 1 0 - 0 7 )

1 2 0 0 - 8 5 0 - 0 41 2 0 0 - 8 5 0 - 0 41 2 0 0 - 8 5 0 - 0 41 2 0 0 - 8 5 0 - 0 41 2 0 0 - 8 5 0 - 0 4 Grade 31200 / 175 850 / 125 4 60 / 45 341 – 444

(175-125-04)(175-125-04)(175-125-04)(175-125-04)(175-125-04)

1 4 0 0 - 1 1 0 0 - 0 21 4 0 0 - 1 1 0 0 - 0 21 4 0 0 - 1 1 0 0 - 0 21 4 0 0 - 1 1 0 0 - 0 21 4 0 0 - 1 1 0 0 - 0 2 Grade 41400 / 200 1100 / 155 2 35 / 25 388 – 477

( 2 0 0 - 1 5 5 - 0 2 )( 2 0 0 - 1 5 5 - 0 2 )( 2 0 0 - 1 5 5 - 0 2 )( 2 0 0 - 1 5 5 - 0 2 )( 2 0 0 - 1 5 5 - 0 2 )

1 6 0 0 - 1 3 0 0 - 0 11 6 0 0 - 1 3 0 0 - 0 11 6 0 0 - 1 3 0 0 - 0 11 6 0 0 - 1 3 0 0 - 0 11 6 0 0 - 1 3 0 0 - 0 1 Grade 51600 / 230 1300 / 185 1 20 / 15 402 - 512

( 2 3 0 - 1 8 5 - 0 1 )( 2 3 0 - 1 8 5 - 0 1 )( 2 3 0 - 1 8 5 - 0 1 )( 2 3 0 - 1 8 5 - 0 1 )( 2 3 0 - 1 8 5 - 0 1 )

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TECHNICAL NOTE

listed in Table-1 is an exception to this general rule as it is produced byinter-critical austenitising which results in a final microstructure thatcontains proeutectoid ferrite in combination with ausferrite. This gradeof ADI (Grade 750 ADI) will be discussed later in this article.

If a component is alloyed correctly, it is possible to produce any of thegrades of ADI in Table-1 (except for GR 750 ADI) by the proper selectionof heat treatment parameters i.e. temperatures and times. In general,lower austempering temperatures will lead to the production of thehigher strength, higher hardness grades of ADI.

Typically, ADI exhibits twice the strength of as-cast ductile iron for agiven ductility. This is shown in Fig. 2 which illustrates the relationshipof yield strength to ductility for various metallic materials. Prior to theintroduction of ADI, the material of choice for yield strength above600 MPa was largely limited to steel.

When high normal forces are applied to an ADI component in service,a localised strain-induced transformation of the austenite componentin the Ausferrite microstructure will harden the contact surface to adepth of approximately 5μm. This is illustrated in Fig. 3. As a result,when ADI is used in a high stress abrasion environment, it tends to haveimproved wear resistance over conventionally hardened steels.

One of the earliest high volume applications of ADI was that of hypoidgear sets in light vehicles. Throughout the last three decades, ADI hasalso been used for timing gears, worm gears, helical gears and spurgears. Figures 4 and 5 compare the allowable contact stress andallowable gear tooth root bending fatigue as a function of hardness forADI 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 byaustempering 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 thesurface for Grade 1050-750-07 ADI. [2]

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removal) and is typically lower in cost, it can be concluded that ADI isa 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 BendingFatigue 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 fracturetoughness as a function of yield strength, respectively, for ADI in additionto other material/process combinations. Examination of both of these

figures would indicate that ADI is competitive with many steelalternatives and improved over aluminium alternatives. (ADI is material20 in both figures.)

Allow

able

Bend

ing S

tress

(N/m

m2 )

Allow

able

Conta

ct St

ress

(N/m

m2 )

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ADI Hybrids

Two different “hybrids” of ADI have been developed to address specificchallenges with conventional ADI. Grade 750 ADI from Table-1 wasdeveloped to enable manufacturers to more readily machine ADI afteraustempering. It is produced by austenitising in an intercritical rangeresulting in a final microstructure that consists of a combination ofproeutectoid ferrite and Ausferrite. Because a lower austenitisingtemperature is used to produce GR 750 ADI, the carbon content ofthe austenite is lower than that for conventional ADI resulting inincreased alloy requirements for the base ductile iron for GR 750 ADI.Increased alloy requirements mean an increased cost in the basematerial which has to be justified by cost savings in machining. Theother ADI hybrid of interest is not included in any ADI standards. It isproduced by austempering ductile iron with carbide volumes of upto60%. This material is called carbidic ADI or CADI. CADI was developedto increase the wear resistance of GR 1600 ADI. Representativemicrostructures of these hybrids of ADI are shown in Figs. 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 whichhave conflicting results/conclusions. The reasons for the confusionarise from studies that are concerned with either trying new types oftooling or simply cutting metal without understanding the uniqueattributes of ADI.

Perhaps the most important thing to remember when setting up tomachine ADI is that consideration of hardness only will lead to lowthroughput and poor tool life. The essential knowledge related to thesuccessful 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 toolmoments to minimise vibration during machining.

When acted upon with a high normal force, ADI undergoes astrain-induced surface transformation that results in the austenitein the ausferrite transforming to hard martensitic particles thatare present in an acicular ferrite nest.

A thin chip may harden through its entire section while athicker chip may harden only at the tool interface allowingthe discontinuous chip to peel off presenting a new, moremachinable, ADI surface to the tool

This phenomenon makes thread rolling, perhaps, the mostdifficult machining operation with ADI.

ADI has lower thermal conductivity than either ductile iron orplain carbon steels resulting in a high workpiece/tool interfacetemperature

To maximise metal removal rate, the selected tools musthave good toughness and be able to withstand hightemperatures at the cutting face.

APPLICATIONS OF ADI

Many successful applications of ADI have occurred since the firstcommercial applications in the early 1970’s. In general, thosecomponents that need good dynamic properties like fatigue strengthor fracture toughness are most suited to GR 900 and GR 1050 ADI.When wear properties are of concern, grades 1400 and 1600 areused. 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 ADIinclude (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 ofthe 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 750ADI is most suitable for components with relatively thin section sizesthat 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 forgingand 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 aluminiumhub that it replaces. [5]

Fig. 14: Fig. 14: Fig. 14: Fig. 14: Fig. 14: A drive wheel for the track system of constructionequipment. 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 seedboot and (b) a plow point. [6]

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Two successful high volume automotive applications of ADI are picturedin Fig. 9. Constant velocity joints are machined fully out of ferriticductile iron before austempering to a final hardness of 450 HBW.Many different shapes and sizes of tow hooks for trucks and sportutility vehicles like those pictured in Fig. 9(b) have been austemperedto grades 900 and 1050 ADI.

Heavy truck applications of ADI have included many differentsuspension components like the beam pictured in a heavy truck airsuspension system. This ADI beam shown in Fig. 10 replaced a steelforging, increasing durability by over 350% at a lower cost. Anotherheavy truck application, a sway bar bushing, is shown in Fig. 11. Thiscomponent is machined fully prior to austempering to GR 1200 ADI.It includes a precision spline and an acme threaded ID.

When weight reduction is considered, most design engineers are trappedin the paradigm of using lower density materials like aluminium. Figure12 shows an ADI heavy truck trailer hub and the aluminium hub thatit replaces at a 4% weight savings. This occurs because ADI is strongerand 2.3 times stiffer than aluminium which allows for designs that take

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 growingsectors in recent years. Figure 13 shows examples of conversions toADI for ground engaging applications that take advantage of the wearproperties of ADI. Figure 13 (a) is a seed boot planter that deliversseed 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 hasmarked improved wear performance. Figure 13 (b) is an example ofan ADI plow point. Such ground engaging applications are well-suitedto grades 1400 and 1600 ADI. Many other ground engagingapplications employ the use of carbidic ADI (CADI).

Construction and mining vehicles use many of the same types ofcomponents that heavy trucks do such as brackets, control arms,steering knuckles, etc. Figure 14 shows an example of a drive wheelfor 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. Examplesare 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 acommercial 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 ofgood strength, ductility and wear resistance. It can replace steelcastings and forgings, weldments and aluminium at a cost savings.

The lower strength or dynamic grades of ADI like GR 900 or GR 1050can compete favourably with steel forgings for both cost and weightsavings. However, attention must be paid to the lower stiffness of ADIand be accounted for in early design stages.

The higher strength grades of ADI like GR 1400 or GR 1600 cancompete favourably with carburised and hardened steels for wearresistance at a lower manufacturing cost.

Replacing aluminium with ADI at equal weight or at a small weightsavings is a new paradigm that design engineers are just learning toembrace. As casting technologies continue to improve, it is anticipatedthat even more elegant, thin section ADI castings will be designed.

ADI is not a new technology. It has been produced commerciallysince the early 1970’s. It’s only barrier appears to be lack of knowledgeon the part of design engineers. This summary article was intended toaid those in learning about ADI who are in need of knowledge aboutthis versatile material.

Acknowledgments

The authors gratefully acknowledge the assistance of John Keoughand the Applied Process Companies for assistance in preparing thismanuscript.

Selected References

1. ASTM A897/A897M-06, Standard Specification for AustemperedDuctile Iron Castings, ASTM International, West Conshohocken,PA, www.astm.org

2. Ductile Iron Data for Design Engineers, Section IV AustemperedDuctile Iron, www.ductile.org/didata.

3 . AGMA 939-A07, Austempered Ductile Iron for Gears, AmericanGear Manufacturers Association, Alexandria, VA, www.agma.org

4. Keough, J. R., Hayrynen, K. L. and Pioszak, G. L., Designing withAustempered Ductile Iron, AFS Transactions, Vol. 118 (2010), p.503-517.

5 . Keough, J. R. and Hayrynen, K. L., Automotive Applications ofAustempered 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 forPowertrain Applications, Proceedings of the 26th ASM HeatTreating Society Conference, ASM International, Oct 2011.