Sae Sample Paper -1

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2002 World Conference on ADI

Figure 1b: Photomicrograph of Grade 5 ADI.Specimen was etched with 5% Nital.

Figure 3: A schematic of an equilibrium phasediagram of graphitic ductile iron. The

symbols present represent austenite ()ferrite () and graphite (G). The UpperCritical Temperature (UCT) and LowerCritical Temperature (LCT) are labeled.

THE AUSTEMPERING PROCESS

Figure 2 contains a schematic of the austemperingprocess. This process includes the following majorsteps:

1. Heating to the Austenitizing Temperature (Ato B)

2. Austenitizing (B to C)3. Cooling to the Austempering temperature (C

to D)4. Isothermal heat treatment at the

Austempering temperature (D to E)5. Cooling to room temperature (E to F)

Figure 2 : A schematic of the Austemperingprocess.

Austenitizing Temperature and TimeThe choice of austenitizing temperature is dependent onthe chemical composition of the ductile iron. Figure 3shows a schematic of an equilibrium diagram for agraphitic ductile iron.

UCT

LCT

The austenitizing temperature should be chosen so tha

the component is in the austenite + graphite ( + Gphase field. Elements like Silicon raise the UCT whileManganese will lower it. If the austenitizingtemperature is below the UCT or in the subcritical range

( + + G), then proeutectoid ferrite will be present inthe final microstructure, resulting in a lower strength andhardness material. Once the ferrite forms, the only wayto eliminate it is to reheat above the UCT. Figure 4shows the microstructure of an austempered materiathat was austenitized below the UCT.

Figure 4: A photomicrograph of ADI that wasaustenitized below the Upper CriticaTemperature (UCT). The light regions areFerrite.


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The time at the austenitizing temperature is equally asimportant as the choice of temperature. The ductile ironcomponents should be held for a time sufficient to createan austenite matrix that is saturated with carbon. Thistime is additionally affected by the alloy content of theductile iron with heavily alloyed material taking longer toaustenitize.

Cooling to the Austempering TemperatureCooling from the austenitizing temperature to theaustempering temperature (as shown from C to D inFigure 2) must be completed rapidly enough to avoidthe formation of pearlite. If pearlite is formed, thestrength, elongation and toughness will be reduced.Figure 5 shows a photomicrograph of Grade 2 ADI thatcontains pearlite.

Figure 5: Pearlite (dark constituent) in Ausferrite.

The formation of pearlite can be caused by severalthings, most notably a lack of quench severity or a lowhardenability for the effective section size. It is possibleto increase the quench severity of molten salt quenchbathes by making water additions. Oil quenchequipment is limited to the production of Grade 5 ADIbecause of the quench temperatures necessary toproduce Grades 4 and higher.

The alloy content in ADI is necessary for hardenabilitypurposes or the austemperability of the ductile iron. In

general, section sizes greater than 19 mm or 0.75inches require an alloy addition. Typically, a foundry willwork closely with the heat treater to determine theoptimum chemical composition of the ductile iron to beaustempered.

Figure 6 shows a schematic of how the alloyingelements segregate in ductile iron during solidification.

Figure 6: A Schematic showing the Segregation oAlloying Elements in Ductile Iron duringSolidification

The alloying elements that are typically added for

hardenability purposes include: Cu, Ni and MoManganese additions are not recommended because othe tendency of Mn to segregate to the regions inbetween the graphite nodules. Manganese delays theaustempering reaction, which can result in the formationof martensite due to the presence of low carbonaustenite.

Copper additions are often initially recommendedbecause of price considerations. However, more is nonecessarily better when Cu additions are consideredLevels in excess of 0.80 can create diffusion barriersaround the graphite nodules and inhibit carbon diffusion

during austenitizing.

Nickel additions are made when the level of Cu hasbeen maximized. Ni additions of up to 2 % are typicallymade. Beyond that, the price becomes an importanconsideration. Lastly, Molybdenum is a potenhardenability agent. Unfortunately, it segregates highlyto the intercellular/interdendritic locations between thegraphite nodules. Molybdenum is a strong carbideformer. Figure 7 contains a photomicrograph oMolybdenum carbides that were present in ADI with aMo addition. The formation of Mo carbides isundesirable, especially if a component is to be machinedafter heat treatment.


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Figure 7: Molybdenum carbides (white) in ADI.

Recommendations for alloying ADI are summarized inTable 2.

Table 2: Recommendations for Alloying ADI

Recommended Limit(wt pct)

Manganese Max section > 13mm 0.35 maxMax section < 13 mm 0.60 max

Copper 0.80 max only as neededNickel 2.00 max only as neededMolybdenum 0.30 max only as needed

Choice of Austempering (Quench) Temperature and

TimeThe choice of austempering temperature and time isdependent on the final properties desired. The typicaltemperature ranges utilized are 460 750F (or 238 -399C). The lower grades (1 and 2) require temperaturechoices at the upper end of the range while the highergrades are produced at lower quench temperatures.

Time at temperature is dependent on the choice oftemperature as well as the alloy content. For example,Grade 1 ADI will transform faster than Grade 5 as thequench temperature is approximately 200F (93C)higher.

The components are held for a sufficient time attemperature for ausferrite to form. Ausferrite consists offerrite in a high carbon, stabilized austenite. If held forlong time periods, the high carbon austenite willeventually undergo a transformation to bainite, the two

phase ferrite and carbide (. + Fe3C). In order for thistransformation to occur, longer periods of time aretypically needed much longer than would beeconomically feasible for the production of ADI.

Once the ausferrite has been produced, the componentsare cooled to room temperature. The cooling rate wilnot affect the final microstructure as the carbon contentof the austenite is high enough to lower the martensitestart temperature to a temperature significantly belowroom temperature.

FOUNDRY CONSIDERATIONS FOR THEPRODUCTION OF ADI

The austempering process creates a product that isstronger than conventional grades of ductile iron. As aresult, it is more sensitive to any defects that could bepresent in the base ductile iron. Austempering is NOT acure for poor quality iron. Rather, the effects of theslightest defects on the mechanical properties of ductileiron become magnified as a result of austemperingThus, the toughness of an ADI component can beseverely compromised by the presence of non-metallic

inclusions, carbides, shrink and dross even if their levelswere acceptable for conventional ductile iron. There isno one optimum recipe for ductile iron that is to beaustempered. However, high quality is imperative in alcases.

Nodule Count and NodularityThe recommended minimums for nodule count andnodularity for ductile iron to be austempered are asfollows:

Nodule Count 100/mm2

( with a uniformdistribution)

Nodularity 85%

Nodule count is especially important when alloyadditions are made. Low nodule counts lead to largespacing between the graphite nodules and largeregions of segregation (Note Figure 6.). In the worscase scenario, these regions can become so heavilysegregated that they do not fully transform duringaustempering, resulting in the formation of low carbonaustenite or even martensite. Figure 8 shows regions osegregation that did not transform during austemperingHigher nodule counts will break up the segregatedregions shown in Figure 8.


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Figure 8: Segregated regions (white) with a high Mncontent in ADI.

Casting QualityCastings to be austempered should be free of non-metallic inclusions, carbides, shrink and porosity. Inorder to achieve the property minimums in Table 1, thefollowing levels should be maintained.

Carbides + Nonmetallic inclusions - maximum 0.5%Porosity and/or Microshrinkage maximum 1%

Carbon EquivalentThe Carbon Equivalent (CE = %C + 1/3 %Si) should becontrolled to produce sound castings. GeneralGuidelines are provided in Table 3.

Table 3: Carbon Equivalent Guidelines for theProduction of ADI

Section Size CE Range

0 0.5 inches ( 0 13 mm) 4.4 4.6

0.5 2 inches (13 51 mm) 4.3 4.6

Over 2 inches (51 mm) 4.3 4.5

Chemical CompositionThe chemical composition ranges for a componentshould initially be established between the foundry and

the heat treater. The amount of alloy (if needed) will bea function of the alloy in the foundrys base metal, thepart configuration (section size and shape) and theaustempering equipment that is used. Suggestedchemistry targets along with typical control ranges arelisted in Table 4.

Table 4: Suggested Targets and Typical ControRanges for the Production of ADI

Element SuggestedTarget

TypicalControl

Range

Carbon C 3.6% 0.20%

Silicon Si 2.5% 0.20%

Magnesium Mg (%S x 0.76)+0.025% 0.005%

Manganese MnMax section > 13 mmMax section < 13 mm

0.35% maximum0.60% maximum

0.05%

Copper Cu 0.80% maximum(only as needed)

0.05%

Nickel Ni 2.00% maximum(only as needed)

0.10%

Molybdenum - Mo 0.30% maximum(only as needed)

0.03%

Tin - Sn 0.02% maximum(only as needed)

0.003%

Antimony Sb 0.002% maximum(only as needed)

0.0003%

Phosphorus P 0.04% maximum

Sulfur S 0.02% maximum

Oxygen O 50 ppm maximum

Chromium Cr 0.10% maximum

Titanium Ti 0.040% maximum

Vanadium V 0.10% maximum

Aluminum Al 0.050% maximum

Arsenic As 0.020% maximum

Bismuth Bi 0.002% maximum

Boron B 0.002% maximum

Cadmium Cd 0.005 maximum

Lead Pb 0.002% maximum

Selenium Se 0.030% maximumTellurium Te 0.020% maximum

Once chemical composition ranges have beenestablished between the foundry and the heat treater, iis important for the foundry to produce ductile iron withinthe established ranges. Wide variations in chemicacomposition can lead to variations in the pearlite/ferriteratio in the as-cast ductile iron as well as a need toadjust the heat treatment parameters. The response ogrowth during austempering is a function of the priormicrostructure and the austempering temperatureFigure 9 shows the linear dimensional change as a

function of austempering temperature for ADI with priormicrostructures of ferrite, pearlite and a ferrite/pearlitemix.


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Figure 9: Linear Dimensional Change as a functionof Austempering Temperature for variousprior microstructures.

4. Kovacs, B. V., ADI Fact and Fiction, ModernCasting, March 1990, pp. 38-41.

Figure 9 shows that the growth is different for pearlite orferrite. However, the growth is consistent from one heattreat lot to another if the chemical composition rangesare obeyed. End users use the consistent growth of ADIto their advantage. Components can be designed to bemachined prior to heat treatment and then grow to sizeduring austempering.

SUMMARYThe production of ADI is not a highly complicatedprocess. Any foundry that works in conjunction with aheat treater can conceivably make ADI. However, thereare important considerations in order to be successful.High quality ductile iron with the proper alloy content isthe necessary ingredient. Remember that austemperingis not the cure for poor quality as it will make bad ironeven worse.

Knowledgeable heat treaters will work with a foundry to

establish the proper chemical composition of the ductileiron to be austempered. The proper choice of heattreatment parameters will then lead to the successfulproduction of any grade of ADI.

ACKNOWLEDGMENTSThe author would like to thank the following individualsfor their assistance in putting this paper together: KristinBrandenberg, Terry Lusk, and John Keough. Thesupport of the employees of Applied Process, Applied

Process Technologies Division, AP Westshore and APSouthridge are also noted.

A special thank you to Dr. Karl Rundman and DennisMoore for the introduction to metal castings and ADITheir enthusiasm and encouragement over the past 15years has been sincerely appreciated.

Lastly, the author would like to acknowledge the late DrBela Kovacs for the invaluable contributions he made tothe ADI world and for being a great mentor and friend.

REFERENCES

1. Hayrynen, K.L., ADI: Another Avenue foDuctile Iron Foundries, Modern Casting, August 1995pp. 35-37.2. Section IV, Ductile Iron Data for Design Engineerspublished by Rio Tinto Iron & Titanium Inc, 1990.3. Foundry Requirements for the Production of ADI Internal Information, Applied Process Inc.

ADDITIONAL RESOURCES

+ Websiteswww.appliedprocess.comwww.ductile.org/didatawww.asminternational.orgwww.afsinc.orgwww.matweb.com
http://www.appliedprocess.com/http://www.ductile.org/didatahttp://www.asminternational.org/http://www.afsinc.org/http://www.matweb.com/http://www.matweb.com/http://www.afsinc.org/http://www.asminternational.org/http://www.ductile.org/didatahttp://www.appliedprocess.com/

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Sae Sample Paper -1