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Austempered ductile iron (ADI) is stronger perunit weight than aluminium, highly wearresistant and easier to machine than freemachining steel - and it has the potential forup to 50% cost savings.

Austempering is a special isothermal heattreatment that can be applied to ferrousmaterials for increased strength and toughness.Fig. 1 shows a schematic isothermal diagramcomparing the austempering (green line) andthe quench and tempering (red line) processes.Austempering consists of austenitisingfollowed by rapidly quenching to a temperaturerange (260-385˚C) where the materialtransforms isothermally to form eitherausferrite in cast iron or bainite in steel.

The quench and temper process consistsof austenitising and then rapidly quenchingbelow the martensite start line. The martensiteformed is a very hard, brittle phase and requiresone or more tempering processes to obtainstrength and toughness.

As an isothermal process, austemperingoffers quality advantages versus quench andtemper. Formation of ausferrite and bainiteoccurs over minutes or hours at a singletemperature; dimensional tolerances are morereadily maintained and cracking avoided. Incontrast the formation of martensite occurswith shear immediately as the metaltemperature reaches the martensite start line.Since cooling rates vary according to section,the transformation is not homogeneous,significantly increasing the risks of distortionand cracking.

Austempering services are now availablefrom contractors such as ADI Treatments Ltd(West Bromwich, UK). Working with theEuropean foundry industry and castings users,the company operates large scale, specialistfurnaces and assists in design and developmentof ADI components.

ADI grades, properties and benefitsADI materials are versatile; table 1 listing thegrades in common use. The designer can selectcast composition and heat treatments toprovide specific properties required for theapplication. Fig. 2 illustrates the ausferritemicrostructure, a mix of acicular ferrite andcarbon stabilised austenite that gives ADI itsunique properties.

Cost advantagesThe price of ADI material is lower per kilo thansteel or aluminium, but this accounts for onlya fraction of the potential savings as an ADIdesigned component can save cost at eachstage of manufacture. ADI equivalents can thenbe produced for less than a steel forging or athalf the cost of aluminium parts. Severalfactors favour ADI in value engineering:

• Excellent castability: Readily cast intocomplex shapes, ductile iron has a very highyield rate i.e. the proportion of metalpoured versus metal shipped.

• Lower machining cost: Well suited to nearnet shape casting, ADI requires less startingmaterial and less metal removal. Prior toaustempering, ductile iron exhibits bettermachinability than free machining steels.

Both ductile iron and ADI produce dense,discontinuous chips that are easily handled,further reducing cost per kilo.

• Heat treatment savings: Austemperinggenerally costs less than carburising orinduction hardening, and produces a higherdegree of uniformity and predictabledimensional changes.

• Low energy content: Producing a typicalADI casting consumes 50% less energythan a steel casting, and 80% less energythan a steel forging.

• Best value: When comparing relative costper unit of yield strength, ADI is generallythe best buy.

Performance advantagesDue to superior performance ADI castings arerapidly displacing steel forgings, weldedfabrications, carburised steel, and aluminium inkey applications:

• Strength comparable to steel: Because of itsequivalent strength, nearly 80% of all castand forged steels can be replaced with somegrade of ductile iron or ADI.

• Lower density than steel: The relativeweight per unit of strength of ADI allowseconomy in design without loss ofperformance. For a given shape, an ADIcomponent will be 10% lighter than steel.

• ‘Lighter’ than aluminium: ADI is threetimes stronger than the best cast or forgedaluminium and weighs only 2.5 times asmuch. Because it is twice as stiff, a properlydesigned ADI part can replace aluminium ata weight saving.

• Excellent fatigue strength: ADI’s dynamic

When ductile iron is subjected to anaustempering process, the material undergoesa remarkable transformation - ideal for manyautomotive applications.

ADI solutions aid vehicle design

Fig. 6 Ford Mustang Cobra suspension arm wins casting award (courtesy Intermet and BentelerCorporations)

Grade Min tensile stress Min yield stress Elongation HardnessMPa MPa %

EN-GJS-800-8 800 500 8 260/320

EN-GJS-1000-5 1000 700 5 300/360

EN-GJS-1200-2 1200 850 2 340/440

EN-GJS-1400-1 1400 1100 1 380/480

Table 1. EN1564: 1997 ADI European grades

Fig. 1 Schematic isothermal diagram illustrating theaustempering (green line) plus quench and tempering (redline) processes

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materials, TVR created curves in the body thatcould not be accomplished with regular steelstampings. Less visible, the ADI crankshaft isanother significant innovation.

Forged steel was originally chosen as thecrankshaft material for use in the inline sixcylinder engine. Due to high cost ofmanufacture, steel was later deselected andTVR turned to 800/2 ductile iron for the designprototype. Tests were carried out on a benchdynamometer and in vehicles; however theparts failed, in some cases with a fatigue crackat a fillet radius on the flywheel end.

ADI (fig. 4) became the next choice. TVRwas already using the material in V8 enginesbut had initial reservations about distortion inthe manufacture of the new crank. Tocompensate for distortion in the design phase,the crankshaft was rough machined, heattreated and then finish machined. Thissignificantly reduced the concern, while finalmachining actually increased the strength ofthe component.

The ADI crank out performed the ductileiron version, showing no signs of fatiguecracking during bench testing. ADI’s internaldamping characteristics also gave the enginesuperior noise properties compared to engineswith steel or ductile iron crankshafts.Mechanical results performed on all threematerials are shown in table 2.

Independent truck stabiliser armTrucking in the Australian Outback is achallenging experience. The terrain is rough andisolated and distances may be exceptionallylong between service stops. When making the3,500km trip from Sydney to Perth, the priorityis to complete the journey safely while utilisingthe effective space in the trailer.

Originally, the independent suspensiondesign was a fabrication made from low carbonsteel. The first iteration was a 50mm thick, V-shaped swing arm. It was to be road testedover the route the truck would take in service -from Sydney to Perth and back. However, at thefirst trial of these components, the wheelssplayed under the truck during loading; thefully loaded trailer weighing 22.5 tonnes.

The test was run as planned but thewelded components failed after approximately1,200km. One of the suspension bracketsbegan to crack at its weld points; concern wasalso raised because these brackets would flexso heavily that the negative camber induceduneven tyre wear. A second set of welded steelbrackets was tested, travelling approximately4,000km before failure.

The suspension was re-designed as asingle-piece ductile iron casting; trial batcheswere cast and then austempered to achieveGrade 2 ADI (fig. 5). These parts weresubjected to the same Perth to Sydney trip asthe fabricated steel brackets. At the time ofwriting the ADI brackets had completed over322,000km without problems. As well asproviding an additional 20m3 storage spaceinside the truck, tyre life has been extended byover 80,000km.

Duralite truck hubWalther EMC has developed an ADI trucktrailer hub that is 2% lighter than itsaluminium counterpart and over 30% lower incost. This is a rather high visibility example ofADI replacing aluminium ‘kilo for kilo’, possiblybecause of ADI’s superior strength to weightratio.

Ford Mustang control armGM has demonstrated the feasibility of ADIsuspension control arms on Cadillaclimousines since 1995. More recently BentlerCorporation was contracted by Ford to producea lightweight, cost effective, independentsuspension system for its high performanceMustang Cobra sports car. ADI was chosen forthe upper control arms (fig. 6) for itscombination of low weight (approximately 3kg), noise damping and low manufacturingcost.

The components were FEA modelled totake full advantage of the strength and stiffnessof ADI. An aluminium design was consideredbut it was impossible to fit the much largerand thicker component in the given space. ADIallowed the manufacturers to meet their space,weight, safety critical, and cost objectives. Thesuccess of this application was recognised bythe 2003 AFS Casting Congress - at its annualCastings Contest, five of the top awards were

properties exceed those of forged, cast andmicroalloyed steels. Unlike aluminium,ADI’s endurance limit remains nearlyconstant after tens of millions of cycles.

• Improved noise damping: The presence ofgraphite in the ADI matrix improves noisedamping, for quieter, smoother runningcomponents.

• Superior wear and abrasion resistance:ADI’s abrasion resistance exceeds that ofconventionally processed steels and irons ata lower ‘bulk’ hardness level. Unlikecarburised steel, which loses wearresistance as the carburised layer isremoved, ADI improves in service. Wearresistance is superior to steel at any givenhardness level, making it ideal for earthmoving and other high abrasionapplications.

Case studies in transportTVR crankshaftThe TVR Tuscan Speed Six (fig. 3) made itsdebut in 1999 for sale to the UK and Japan asa right hand drive only vehicle. Striking inappearance and power, the Tuscan Sixaccelerates from 0-96.5km/h (0-60 mph) in 4.2seconds and reaches a top speed of 289.6km/h(180 mph). To achieve performance, thedesigners combined aerodynamic styling and ahigh power to weight ratio. Using composite

Fig. 2 The ADI ausferrite micro-structure - a mix of acicularferrite and carbon stabilised austenite

Fig. 3 Tuscan Speed Six (courtesy TVR Engineering Ltd)

Fig. 4 ADI crankshaft for the Tuscan Speed Six - 29.5kg

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given to ADI parts, including Best in Class forthe new Mustang suspension arm.

Another Best in Class was awarded for adrive wheel, part of a construction and

landscape utility loader. Originally an84 piece steel assembly, the redesignedcomponent is a one piece casting at a15% weight reduction, 55% lower incost. Thirty minutes of assembly timehave been eliminated while the partexhibits superior wear, durability andappearance compared to the original.

Looking aheadADI is being applied increasingly by theautomotive industries as the cost andperformance benefits are recognised.Following historical growth rates,annual world production of the materialis expected to reach 300,000 tonnes by2010, with approximately two thirds inNorth America. Carbidic ADI gradeshave recently been introduced and areadding to the interest worldwide.

Companies like ADI TreatmentsLtd actively help to develop the marketsand technology in partnership withfoundries and their customers. Some ofthe case studies illustrated have been

the result of such co-operations, frequentlystarting with workshops organised on thecustomer’s site. European manufacturers, oftenstimulated by USA connections, are now

Fig. 5 Pair of trailer suspension arms (courtesy Steele andLincoln Foundry)

Steel Ductile iron ADI ASTM ADIGrade 1

Yieldstrength 738 538 827 550MPa

Tensilestrength 910 903 1083 850Mpa

Fatiguestrength 400 324 427 N/AMpa

Impactenergy 325 75 141 100Joules

Elong (%) 23.2 10.8 13.7 10

Hardness 226-266 262-277 300 269-321BHN

Table 2. Test results for the steel, ductile iron, and ADI crankshafts, andASTM 897-90 Standard ADI Grade 1 specifications

implementing their own volume applications:Contract austempering services are set toexpand to meet the demand. The future for ADIis limited only by an ability to fully exploit thisunique material.

ADI Treatments Ltd, Doranda Way,West Bromwich B71 4LE.Tel: +44 (0)121 525 0303. Fax: +44 (0)121 525 0404.e-mail: arronrimmer@adit-uk.com