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INTRODUCTION

ADI is an acronym for Austempered Ductile Iron. ADI is a high strengthmaterial with superior ductility, toughness, hardness, fatigue and wearproperties. ADI is 10% lighter than steel and can be manufactured at20% less cost of steel. Because ADI is lighter and cheaper than steel,several steel parts (forgings, castings, assemblies) have beensuccessfully replaced by ADI cast parts. While a majority of ADIapplication is in the automotive sector, other market distribution inrailways, agriculture, mining and construction is quite significant.

Austempering is a special heat treatment process. Initially,austempering heat treatment was applied to steel (AS). Resultingmicrostructure in steel is known as “Bainite” named after Edgar Bain,an eminent metallurgist who discovered the structure. Three decadeslater, austempering heat treatment was extended to ductile iron (ADI).Resulting microstructure in ductile iron is known as “Ausferrite”. Bainitestructure in steel and ausferrite structure in ADI exhibit remarkabletoughness even at high hardness levels.

Properties obtained with austempering heat treatment are muchsuperior to properties that are obtained by conventional quench andtemper heat treatment of steels. A common heat treatment forobtaining high strength in steel is quench and temper (Q&T) heattreatment. ADI parts have replaced several heat-treated steel partssuccessfully in many applications. Thus, ADI properties andperformances are always compared and referenced against Q&T steels.

MICROSTRUCTURE OF AUSTEMPERED STEEL

Microstructure of austempered steel is bainite. Bainite structureconsists of ferrite dispersed with minute precipitates of carbide. Carbideprecipitates are very small and can be seen only under very highmagnification and resolution like in electron microscope. Upper bainite,which forms at higher temperatures (450 ºC -500 ºC) has featheryappearance and is relatively coarse. It exhibits good impact properties

Understanding Austempered Ductile Iron Process,Production, Properties and Applications – Part II

S. GowriGeneral Manager – Hightemp Furnaces Limited, Bangalore,

E-mail : gowri@hightemp-furnaces.com

but with low strength. Lower bainitie, which forms at lower temperatures(350 ºC to 450 ºC) has acicular appearance and is very strong. Figure1 shows the acicular appearance of lower bainite.

AUSTEMPERED DUCTILE IRON - ADICast iron is an alloy of iron and carbon with carbon above 2%. Incommon steels, carbon and silicon are present in low percentages;typically carbon in the range of 0.1 - 0.8% and silicon less than 0.35%.In cast iron, carbon and silicon are present in high percentages; typicallycarbon in the range of 3-4% and silicon in the range of 2-3%, thusmaking it a ternary system. Figure 2 shows the phase diagram of ironcarbon alloy containing 2% silicon. As seen from the phase diagram,silicon exerts significant effect on the phase diagrams and solidificationprocess.

Silicon in cast iron, promotes carbon to precipitate as graphite. Siliconalso lowers the carbon content of the eutectic from 4.3 %, decreases

Fig. 1 Fig. 1 Fig. 1 Fig. 1 Fig. 1 : Acicular Ferrite in Lower Bainite.

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the austenite field area andbroadens the eutectoidtransformation range. As a resultof this broadening, austenitetransformation to stable roomtemperature phase occursthrough an intermediate step.

Figure 3 shows the IsothermalTransformation (IT) diagram of atypical ductile iron composition.IT diagram for ductile iron clearlyshows that austempering reactiongoes to completion through twostages of transformation. In thefirst stage, austenite transformsto a structure of acicular ferriteand high carbon-stabilisedaustenite. This is the desiredmicrostructure in ADI. This is thestructure that providesremarkable properties to ADI. Thismixture of acicular ferrite and highcarbon-stabilised austenite iscalled Ausferrite.

Stage 1 – austenite ferrite +high carbon-stabilised austenite Fig. 2 :Fig. 2 :Fig. 2 :Fig. 2 :Fig. 2 : Fe -C- Si ternary phase diagram at 2% silicon.

Fig. 3Fig. 3Fig. 3Fig. 3Fig. 3 : Isothermal Transformation diagram of a ductile iron composition.

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In the second step when casting is held for a long time, high carbonaustenite decomposes to a mixture of ferrite and carbide. This resultsin a brittle material and so stage 2 transformation is not preferred inADI and should be avoided.

Stage 2 – high carbon-stabilised austenite ferrite + carbide (Bainite)

Maximum properties are achieved on completion of stage 1transformation. Therefore, controlling austempering time is critical.

Austempering Cycle - ADI

Basic austempering cycle is the same for steel and ductile iron.

ADI heat treatment consists of austenitising ductile iron part followedby quenching to austempering temperature and holding at thattemperature for a controlled time and then cooling to roomtemperature.

During austempering, ductile undergoes two-stage transformationsunlike in steel where austenite transforms to bainite directly by single-stage transformation.

Figure 4 shows schematic diagram of typical austempering cycle forADI. Typical cycle times are marked at the bottom of the diagram.Processing steps are:

Preheat - To decrease furnace time

Heat to austenitising temperature - Temperature is a function ofchemistry

Austenitising time – Function of chemistry, maximum section thickness

Quench into molten salt bath at a cooling rate sufficient to avoid pearliteformation

Austempering Temperature – Function of desired grade and properties,maximum section thickness

Austempering time - Function of chemistry, austempering temperature

Wash to remove the salt

Salt is readily soluble in water and is easily removed. Salt can bereclaimed to over 90% from the washed water and reused.

Austempering is normally carried out between 200 ºC and 400 ºC ina molten salt bath. Different grades of ADI can be obtained by varyingonly the austempering temperature. For high strength and wearcombination (ADI Grade 3-5), lower austempering temperature issuggested while for toughness and impact combination (Grade 1-2),higher austempering temperature is recommended. Since the processwindow of temperature from Grade 1 to Grade 5 is small, it is imperativethat temperature be controlled within few degrees. Otherwise, propertiesobtained will not be in range.

Production of ADI

Successful production of ADI is not very easy. It requires good qualityductile iron castings with consistent chemistry and microstructure,addition of suitable alloying elements to thorough harden the casting,knowledge on the correct cycle for austempering and special heattreatment furnace with precise control of time, temperature and chargetransfer.

Starting material for ADI is a good quality ductile iron. Each foundryhas its own guidelines for producing ductile iron casting. Here, goodquality requirement is defined in terms of consistent chemistry, nodulecount, microstructure, low levels of casting defects such as shrinkage,porosity, oxide, slag and inclusions.

Chemistry determines the ADI cycle parameters. In section onAustempering cycle-ADI, time and temperature parameters are listedas a function of chemistry. Chemistry should be controlled within limitsfor every batch for reproducible results.

Nodularity should be good and above 85% and nodule count shouldbe over 100/ mm2. High nodule counts minimise micro-segregationand micro-shrinkage.

Microstructure should be uniform and consistent; amount of ferriteand pearlite in the as-cast microstructure should be maintained at aconstant ratio. This is very crucial for holding dimensional tolerance inmachined castings. There is a certain amount of growth duringaustempering and the degree of growth depends on the ratio of ferriteto pearlite. As long as the ratio is maintained, growth will be consistent.Microstructure is governed by chemistry, processing parameters andshake out practice.

Casting defects should be kept to very low percentages. They affectthe strength, ductility, machinability and performance of the castings.

Consistent chemistry and consistent casting practices should befollowed each and every time. This will result in good quality castingswith good nodularity, nodule count, consistent microstructure withminimal casting defects. Consistency is the keyword.

Section thickness is very important in determining the amount of alloyingelements needed to thorough harden. ADI is a thorough hardeningheat treatment and microstructure is uniform throughout the sectionthickness. In order to thorough harden the entire section thickness,

Fig. 4Fig. 4Fig. 4Fig. 4Fig. 4 : Schematic diagram of ADI processing steps.

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alloying elements like Mn, Cu, Ni and Mo must be added. For a givensection thickness, if the amount of these alloying elements is low,parts will exhibit a mixed structure (with pearlite) resulting in inferiorproperties. There is no gain in adding excess amount of alloys beyondwhat is necessary to thorough harden. Excess amount will only add tothe cost of ADI as alloying elements are generally expensive.

Care should be taken in choosing the alloying elements and maximumamount of addition. During solidification, some of the alloying elementssegregate to cell boundaries. This results in lower ductility andtoughness. Silicon and Cu tend to be located near the nodules. Ni ispresent rather uniformly around the nodules and cell boundaries andtherefore can be added safely upto several per cent. Elements like Mn,Mo, V, Cr, and Ti all other carbide formers segregate significantly atthe cell boundaries. They reduce ductility and create problems duringmachining. For this reason, amount of Mn, Mo and other elementsshould be kept below 0.3%.

Having good quality castings, consistent chemistry and microstructurealone does not automatically guarantee successful production ofADI. Heat treatment plays a major and critical role in the productionof ADI. One must have a good understanding of the exact relationshipbetween chemistry, transformation kinetics, temperature and timeparameters, microstructure and property requirements. Trial and errormethod of heat treat processing will not work for ADI.

Furnace used for heat treatment also contributes to the successfulproduction of ADI. Furnace should have a mechanism to quench theparts from austenite temperature within few seconds; otherwise pearlitewill form in the structure. As the temperature window of processingfor various grades of ADI are very narrow (200 ºC-400 ºC),temperature controls must be within very narrow range of ± 6 ºC. Saltbath must be agitated all the time to keep the bath temperatureuniform and to increase severity of quench. Salt bath can be dangerousand explosive and needs careful handling and safe operation.

It should be understood that ADI is a special heat treatment processand requires careful attention to several crucial factors.

Microstructure of ADI

Matrix structure in ADI is “Ausferrite” - a mixture of high carbon-stabilised austenite and acicular ferrite (Fig. 5). At high austemperingtemperature, amount of carbon-stabilised austenite could be as highas 50%. By varying the proportions of the mixture, wide range ofproperties can be achieved. As the austempering temperature isdecreased, structure becomes finer in scale and refined.

Ausferrite structure is stable to very low temperature. However, highcarbon austenite work hardens and transforms to martensite whensubjected to normal force resulting in a very hard wear-resistant surfacewith a tough ausferrite matrix.

Advantages of ADIAs mentioned earlier, because transformation occurs over manyminutes, there is no stress and there is no cracking during quenching.

Advantages of ADI are twice the strength of ductile iron, comparableproperties to steel but lighter and cheaper than steel, outstandingfatigue and wear resitance, ability to work harden providing a hardwear- resistant surface with tough core.

SUMMARY

ADI is produced by austempering ductile iron. ADI Austempering is anisothermal heat treatment involving quenching from austenite phasefield into molten salt bath maintained at a temperature above themartensite start temperature and held isothermally at that temperatureuntil stage 1 of transformation is complete. Microstructure of ADI is amixture of high carbon-stabilised austenite and acicular ferrite. Byvarying the proportion of mixture, wide range of properties can beobtained.

In the next article in the series, grades of ADI, properties and applicationsof ADI in various fields will be elaborated.

SELECTED REFERENCES

1. Gray and Ductile Iron Castings Handbook.

2 . ASM Handbook, Volume 4, Heat Treating.

3 . Ductile Iron Data for Design Engineers, Ductile Iron Society.

4. Papers published by Applied Process Inc Team, API TechnicalLibrary, USA.

Fig. 5 :Fig. 5 :Fig. 5 :Fig. 5 :Fig. 5 : Microstructure of Ausferrite - Mixture of Acicular Ferriteand High C Stabilised Austenite.

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