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Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalVol 59  No. 2  February 2013
INTRODUCTION
ADIisanacronymforAustemperedDuctileIron.ADIisahighstrength
material with superior ductility, toughness, hardness, fatigue and wear
properties. ADI is 10% lighter than steel and can be manufactured at
20% less cost of steel. Because ADI is lighter and cheaper than steel,
several steel parts (forgings, castings, assemblies) have been
successfully replaced by ADI cast parts. While a majority of ADI
application is in the automotive sector, other market distribution in
railways, agriculture, mining and construction is quite significant.
Austempering is a special heat treatment process. Initially,
austempering heat treatment was applied to steel (AS). Resulting
microstructure in steel is known as “Bainite” named after Edgar Bain,
an eminent metallurgist who discovered the structure. Three decades
later,austemperingheattreatmentwasextendedtoductileiron(ADI).
Resultingmicrostructureinductileironisknownas“Ausferrite”.Bainite
structure in steel and ausferrite structure in ADI exhibit remarkable
toughness even at high hardness levels.
Properties obtained with austempering heat treatment are much
superior to properties that are obtained by conventional quench and
temper heat treatment of steels. A common heat treatment for
obtaining high strength in steel is quench and temper (Q&T) heat
treatment. ADI parts have replaced several heat-treated steel parts
successfully in many applications. Thus, ADI properties and
performancesarealwayscomparedandreferencedagainstQ&Tsteels.
MICROSTRUCTURE OF AUSTEMPERED STEEL
Microstructure of austempered steel is bainite. Bainite structure
consistsofferritedispersedwithminuteprecipitatesofcarbide.Carbide
precipitates are very small and can be seen only under very high
magnificationandresolutionlikeinelectronmicroscope.Upperbainite,
which forms at higher temperatures (450 ºC -500 ºC) has feathery
appearance and is relatively coarse. It exhibits good impact properties
Understanding Austempered Ductile Iron Process,
Production, Properties and Applications – Part II
S. Gowri
General Manager – Hightemp Furnaces Limited, Bangalore,
E-mail : gowri@hightemp-furnaces.com
butwithlowstrength.Lowerbainitie,whichformsatlowertemperatures
(350 ºC to 450 ºC) has acicular appearance and is very strong. Figure
1 shows the acicular appearance of lower bainite.
AUSTEMPERED DUCTILE IRON - ADI
Cast iron is an alloy of iron and carbon with carbon above 2%. In
common 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%.
Incastiron,carbonandsiliconarepresentinhighpercentages;typically
carbon in the range of 3-4% and silicon in the range of 2-3%, thus
making it a ternary system. Figure 2 shows the phase diagram of iron
carbon alloy containing 2% silicon. As seen from the phase diagram,
siliconexertssignificanteffectonthephasediagramsandsolidification
process.
Silicon in cast iron, promotes carbon to precipitate as graphite. Silicon
also lowers the carbon content of the eutectic from 4.3 %, decreases
Fig. 1Fig. 1Fig. 1Fig. 1Fig. 1 : Acicular Ferrite in Lower Bainite.
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Vol 59  No. 2  February 2013
the austenite field area and
broadens the eutectoid
transformation range. As a result
of this broadening, austenite
transformation to stable room
temperature phase occurs
through an intermediate step.
Figure 3 shows the Isothermal
Transformation (IT) diagram of a
typical ductile iron composition.
IT diagram for ductile iron clearly
showsthataustemperingreaction
goes to completion through two
stages of transformation. In the
first stage, austenite transforms
to a structure of acicular ferrite
and high carbon-stabilised
austenite. This is the desired
microstructure in ADI. This is the
structure that provides
remarkablepropertiestoADI.This
mixtureofacicularferriteandhigh
carbon-stabilised austenite is
called Ausferrite.
Stage 1 – austenite  ferrite +
highcarbon-stabilisedaustenite 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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Indian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalIndian Foundry JournalVol 59  No. 2  February 2013
In the second step when casting is held for a long time, high carbon
austenite decomposes to a mixture of ferrite and carbide. This results
in a brittle material and so stage 2 transformation is not preferred in
ADI and should be avoided.
Stage 2 – high carbon-stabilised austenite ferrite + carbide (Bainite)
Maximum properties are achieved on completion of stage 1
transformation. 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 followed
by quenching to austempering temperature and holding at that
temperature for a controlled time and then cooling to room
temperature.
During austempering, ductile undergoes two-stage transformations
unlikeinsteelwhereaustenitetransformstobainitedirectlybysingle-
stage transformation.
Figure 4 shows schematic diagram of typical austempering cycle for
ADI. 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 of
chemistry
Austenitisingtime–Functionofchemistry,maximumsectionthickness
Quenchintomoltensaltbathatacoolingratesufficienttoavoidpearlite
formation
AustemperingTemperature–Functionofdesiredgradeandproperties,
maximum section thickness
Austemperingtime-Functionofchemistry,austemperingtemperature
Wash to remove the salt
Salt is readily soluble in water and is easily removed. Salt can be
reclaimed to over 90% from the washed water and reused.
Austempering is normally carried out between 200 ºC and 400 ºC in
a molten salt bath. Different grades of ADI can be obtained by varying
only the austempering temperature. For high strength and wear
combination (ADI Grade 3-5), lower austempering temperature is
suggested while for toughness and impact combination (Grade 1-2),
higheraustemperingtemperatureisrecommended.Sincetheprocess
windowoftemperaturefromGrade1toGrade5issmall,itisimperative
thattemperaturebecontrolledwithinfewdegrees.Otherwise,properties
obtained will not be in range.
Production of ADI
Successful production of ADI is not very easy. It requires good quality
ductile 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 heat
treatmentfurnacewithprecisecontroloftime,temperatureandcharge
transfer.
Starting material for ADI is a good quality ductile iron. Each foundry
has its own guidelines for producing ductile iron casting. Here, good
qualityrequirementisdefinedintermsofconsistentchemistry,nodule
count, microstructure, low levels of casting defects such as shrinkage,
porosity, oxide, slag and inclusions.
Chemistry determines the ADI cycle parameters. In section on
Austempering cycle-ADI, time and temperature parameters are listed
asafunctionofchemistry.Chemistryshouldbecontrolledwithinlimits
for every batch for reproducible results.
Nodularity should be good and above 85% and nodule count should
be over 100/ mm2
. High nodule counts minimise micro-segregation
andmicro-shrinkage.
Microstructure should be uniform and consistent; amount of ferrite
and pearlite in the as-cast microstructure should be maintained at a
constant ratio. This is very crucial for holding dimensional tolerance in
machined castings. There is a certain amount of growth during
austemperingandthedegreeofgrowthdependsontheratioofferrite
topearlite.Aslongastheratioismaintained,growthwillbeconsistent.
Microstructure is governed by chemistry, processing parameters and
shake out practice.
Casting defects should be kept to very low percentages. They affect
the strength, ductility, machinability and performance of the castings.
Consistent chemistry and consistent casting practices should be
followed each and every time. This will result in good quality castings
with good nodularity, nodule count, consistent microstructure with
minimal casting defects. Consistency is the keyword.
Sectionthicknessisveryimportantindeterminingtheamountofalloying
elements needed to thorough harden. ADI is a thorough hardening
heat treatment and microstructure is uniform throughout the section
thickness. 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 given
section thickness, if the amount of these alloying elements is low,
parts will exhibit a mixed structure (with pearlite) resulting in inferior
properties.Thereisnogaininaddingexcessamountofalloysbeyond
what is necessary to thorough harden. Excess amount will only add to
the cost of ADI as alloying elements are generally expensive.
Careshouldbetakeninchoosingthealloyingelementsandmaximum
amountofaddition.Duringsolidification,someofthealloyingelements
segregate to cell boundaries. This results in lower ductility and
toughness. Silicon and Cu tend to be located near the nodules. Ni is
presentratheruniformlyaroundthenodulesandcellboundariesand
therefore can be added safely upto several per cent. Elements like Mn,
Mo, V, Cr, and Ti all other carbide formers segregate significantly at
thecellboundaries.Theyreduceductilityandcreateproblemsduring
machining. For this reason, amount of Mn, Mo and other elements
should be kept below 0.3%.
Havinggoodqualitycastings,consistentchemistryandmicrostructure
alone does not automatically guarantee successful production of
ADI. Heat treatment plays a major and critical role in the production
of ADI. One must have a good understanding of the exact relationship
between chemistry, transformation kinetics, temperature and time
parameters, microstructure and property requirements.Trial and error
method of heat treat processing will not work for ADI.
Furnace used for heat treatment also contributes to the successful
production of ADI. Furnace should have a mechanism to quench the
partsfromaustenitetemperaturewithinfewseconds;otherwisepearlite
will form in the structure. As the temperature window of processing
for various grades of ADI are very narrow (200 ºC-400 ºC),
temperature controls must be within very narrow range of ± 6 ºC. Salt
bath must be agitated all the time to keep the bath temperature
uniformandtoincreaseseverityofquench.Saltbathcanbedangerous
and explosive and needs careful handling and safe operation.
It should be understood that ADI is a special heat treatment process
and 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 austempering
temperature, amount of carbon-stabilised austenite could be as high
as 50%. By varying the proportions of the mixture, wide range of
properties can be achieved. As the austempering temperature is
decreased, structure becomes finer in scale and refined.
Ausferrite structure is stable to very low temperature. However, high
carbon austenite work hardens and transforms to martensite when
subjectedtonormalforceresultinginaveryhardwear-resistantsurface
with a tough ausferrite matrix.
Advantages of ADI
As mentioned earlier, because transformation occurs over many
minutes, there is no stress and there is no cracking during quenching.
Advantages of ADI are twice the strength of ductile iron, comparable
properties to steel but lighter and cheaper than steel, outstanding
fatigue and wear resitance, ability to work harden providing a hard
wear- resistant surface with tough core.
SUMMARY
ADI is produced by austempering ductile iron. ADI Austempering is an
isothermal heat treatment involving quenching from austenite phase
field into molten salt bath maintained at a temperature above the
martensitestarttemperatureandheldisothermallyatthattemperature
until stage 1 of transformation is complete. Microstructure of ADI is a
mixture of high carbon-stabilised austenite and acicular ferrite. By
varying the proportion of mixture, wide range of properties can be
obtained.
Inthenextarticleintheseries,gradesofADI,propertiesandapplications
of 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 Technical
Library, USA.
Fig. 5 :Fig. 5 :Fig. 5 :Fig. 5 :Fig. 5 : Microstructure of Ausferrite - Mixture of Acicular Ferrite
and High C Stabilised Austenite.
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