Heat treatment of steels and different processes.ppt

Heat treatment of steels, cooling media, annealing processes, normalizing, hardening, tempering, quenching and hardenability, surface hardening processes-nitriding, carbo-nitriding, flame hardening, induction hardening.

1. Shri. Shamrao Patil (Yadravkar) Educational & Charitable Trust’s
Sharad Institute of Technology, College of Engineering Yadrav-(Ichalkaranji)
(Approved by AICTE, New Delhi, Recognized by Government of Maharashtra & Affiliated to DBATU, Lonere)
Accredited by NAAC ‘A’ Grade and NBA, ISO 9001:2015 Certified Institute
Recognized u/s 2(f) and 12(B) of the UGC Act 1956
Heat Treatment

2. Content:
Heattreatmentofsteels,coolingmedia,annealingprocesses,
normalizing,hardening,tempering,quenchingandhardenability,surface
hardeningprocesses-nitriding,carbo-nitriding,flamehardening,
inductionhardening.
Heattreatmentofsteels
"Acombinationofheatingandcoolingoperationstimedandappliedtoa
metaloralloyinthesolidstateinawaythatwillproducedesired
properties.“
Allbasicheat-treatingprocessesforsteelinvolvethetransformationor
decompositionofaustenite.
Thenatureandappearanceofthesetransformationproductsdetermine
thephysicalandmechanicalpropertiesofanygivensteel.

3. Full Annealing
Thisprocessconsistsinheatingthesteeltothepropertemperatureand
thencoolingslowlythroughthetransformationrange,preferablyinthe
furnaceorinanygoodheat-insulatingmaterial.Theslowcoolingis
generallycontinuedtolowtemperatures.
Thepurposeofannealingmaybetorefinethegrain,inducesoftness,
improveelectricalandmagneticproperties,and,insomecases,to
improvemachinability.
Sincetheentiremassofthefurnacemustbecooleddownalongwiththe
material,annealingisaveryslowcoolingprocessandthereforecomes
closesttofollowingtheiron-ironcarbideequilibriumdiagram.

4. Fig.1Schematicrepresentationofthechangesinmicrostructureduringtheannealingofa
0.20percentcarbonsteel.
(a)Originalstructure,coarse-grainedferriteandpearlite.
(b)JustabovetheA
1line;pearlitehastransformedtosmallgrainsofaustenite,ferriteunchanged.
(c)AbovetheA
3line;onlyfine-grainedaustenite.
(d)Aftercoolingtoroomtemperature;fine-grainedferriteandsmallpearliteareas

5. Fig. 2 proportion of the constituents present in the microstructure of annealed steels as a
function of carbon content

6. Fig. 3 Annealing, normalizing and hardening range for plain carbon steel

7. Stress-reliefAnnealingThisprocess,sometimescalledsubcriticalannealingisusefulin
removingresidualstressesduetoheavymachiningorothercold-workingprocesses.Itis
usuallycarriedoutattemperaturesbelowthelowercriticalline(1000to1200°F).
ProcessAnnealingThisheattreatmentisusedinthesheetandwireindustriesandis
carriedoutbyheatingthesteeltoatemperaturebelowthelowercriticalline(1000to
1250°F).Itisappliedaftercoldworkingandsoftensthesteel,byrecrystallization,for
furtherworking.Itisverysimilartostress-reliefannealing.
NormalizingThe normalizing of steel is carried out by heating approximately 100°F
above the upper-critical-temperature (A3 or A cm) line followed by cooling in still air to
room temperature. The temperature range for normalizing The purpose of normalizing is
to produce a harder and stronger steel than full annealing, so that for some applications
normalizing may be a final heat treatment.

8. Fig. 4 Normalized 0.50 percent carbon steel heated to 1800
0
F and air cooled
Fig. 5 Schematic picture of the difference in pearlitic structure due to
annealing and normalizing.

9. Annealed Normalized
Less hardness, tensile strength and
toughness
Slightly more hardness, tensile
strength and toughness
Pearlite is coarse and usually gets
resolved by the optical microscope
Pearlite is fine and usually appears
unresolved with optical microscope
Grain size distribution is more
uniform
Grain size distribution is slightly
less uniform
Internal stresses are leastInternal stresses are slightly more
Annealed Vs Normalized

10. Hardening
Certainapplicationsdemandhightensilestrengthandhardnessvaluessothatthecomponents
maybesuccessfullyusedforheavydutypurposes.Hightensilestrengthandhardnessvalues
canbeobtainedbyaprocessesknownasHardening.
hardeningprocessconsistsoffoursteps.ThefirststepinvolvesheatingthesteeltoaboveA
3
temperatureforhypoeutectoidsteelsandaboveA
1temperatureforhypereutectoidsteelsby
50
0
C.
Thesecondstepinvolvesholdingthesteelcomponentsforsufficientsockingtimefor
homogeneousaustenization.
Thethirdstepinvolvescoolingofhotsteelcomponentsataratejustexceedingthecritical
coolingrateofthesteeltoroomtemperatureorbelowroomtemperature.
Thefinalstepinvolvesthetemperingofthemartensitetoachievethedesiredhardness.
Detailedexplanationabouttemperingisgiveninthesubsequentsections.Inthishardening
process,theaustenitetransformstomartensite.Thismartensitestructureimprovesthe
hardness.
Inthehardeningprocess,whichinvolvesquenchingandtempering.Duringquenchingouter
surfaceiscooledquickerthanthecenter.Inotherwordsthetransformationoftheausteniteis
proceedingatdifferentrates.Hencethereisalimittotheoverallsizeofthepartinthis
hardeningprocess.

11. A few salient features in hardening of steel
Properquenchingmediumshouldbeusedsuchthatthecomponentgetscooledatarate
justexceedingthecriticalcoolingrateofthatsteel.
Alloysteelshavelesscriticalcoolingrateandhencesomeofthealloysteelscanbe
hardenedbysimpleaircooling.
Highcarbonsteelshaveslightlymorecriticalcoolingrateandhastobehardenedbyoil
quenching.
Mediumcarbonsteelshavestillhighercriticalcoolingratesandhencewaterorbrine
quenchingisnecessary.

12. Factors affecting Hardening Processes
Chemical composition of steel
Size and shape of the steel part
Hardening cycle (heating/cooling rate, temp, soak time
Homogeneity and grain size of austenite
Quenching media
Surface condition of steel part
Hardening Methods
Conventional or direct quenching
Quenching in stages in sequence in different media
Spray Quenching
Quenching with self tempering
Austempering or Isothermal Quenching
Martempering

13. Tempering
Thehardenedsteelisnotreadilysuitableforengineeringapplications.It
possessesfollowingthreedrawbacks.
Martensiteobtainedafterhardeningisextremelybrittleandwillresultinfailure
ofengineeringcomponentsbycracking.
Formationofmartensitefromaustenitebyquenchingproduceshighinternal
stressesinthehardenedsteel.
Structuresobtainedafterhardeningconsistsofmartensiteandretainedaustenite.
Boththesephasesaremetastableandwillchangetostablephaseswithtime
whichsubsequentlyresultsinchangeindimensionsandpropertiesofthesteelin
service.
Temperinghelpsinreducetheseproblems.Temperingistheprocessofheating
thehardenedsteeltoatemperaturemaximumuptolowercriticaltemperature
(A
1),soakingatthistemperature,andthencooling,normallyveryslowly.

14. Fig. Variation in properties with tempering temperature

15. Tempering
Objective
Relieve Internal stresses
Restore ductility and toughness
To improve dimensional stability
To improve magnetic properties
Structure in as Quenched state
Highly supersaturated martensite
Retained austenite
Undissolved carbides
Rods, or plates of carbide particles produced during ‘auto-tempering’
Segregation of carbon

16. Tempering of plain carbon steels
FirststageofTempering
FirstStageoftemperingtemperatureextendsfromroomtemperatureto200°C.The
temperingreactionsinsteels,containingcarbonlessthan0.2%,differsomewhatfromthe
steelscontainingmorethan0.2%carbon.
Intheformer,ifcarbonatomshavenotyetsegregated(duringquenching)todislocations,
thesediffuseandsegregatearoundthedislocationsandlathboundariesinthefirststageof
tempering.Noε-carbideformsasallthecarbongetslockeduptothedislocations(defects).
Martensiteinsteelswithmorethan0.2%carbonishighlyunstablebecauseofsuper
saturation,andinterstitialdiffusionofcarboninBCTmartensitecanoccur.Thusinthefirst
stageofTemperingstageoftempering,thedecompositionofmartensiteintolow-
tetragonalitymartensite.
ε-carbideisaseparatephaseandisnotapreliminarystepintheformationofcementite,but
itnucleatesandgrowsmorerapidlythancementite.IthasHCPstructurewithc=4.33A°,a
=2.73A°,c/a=1.58A°andformsassmall(0.015-0.02μm)platelets,orneedlesobserved
underelectronmicroscope.
Thestructureatthisstagereferredtoastemperedmartensite,whichisdoublephase
mixtureoflowtetragonalmartensiteandε-carbide.
Inthisstagevolume↓becausespecificvolumeofmartensite↓duetorejectingofC
at1o54ms.

17. SecondstageofTempering
SecondStageoftemperingtemperatureliesbetween200-300°C.Theamountof
retainedausteniteintheas-quenchedsteeldependsmainlyonthecompositionof
thesteel,andthetemperaturetowhichsteelisquenched.
Inthesecondstageoftemperingretainedaustenitetransformstolowerbainite(the
carbideinbainiteisε-carbide).Thematrixinlowerbainiteiscubicferrite(c/a=
1),whereasintemperedmartensite,thelowtetragonalmartensitehasc/a~1.014
Whenretainedaustenitechangestolowerbainite,theirtakesplaceincreasein
volume.

18. ThirdstageofTempering
ThirdStageoftemperingtemperatureliesbetween200-350°C.Inthisstageof
tempering,ε-carbidedissolvesinmatrix,andlowtetragonalmartensitelossesits
completelyitscarbonandthus,thetetragonalitytobecomeferrite.
Cementiteformsasrodsatinterfacesofε-carbideandmatrix,twinboundaries,
interlathboundaries,ororiginalaustenitegrainboundaries.
Duringthisstage,volumedecreasesjustasinstageone,duetocompletelossof
tetragonality.Ina1%carbonsteel,thetotaldecreaseinlengthinthefirstand
thirdstagesinaround0.25%

19. FourthstageofTempering
FourthStageoftemperingtemperatureliesbetween350-700°C.
Growthandspheroidisationofcementite,aswellasrecoveryandRecrystallizationofferrite
occur.Thoughthegrowthofcementitestartsabove300°C,itsspheroidisationstartsabove
400°Cto700°C.
Spheroidisationtakesplaceduetoreductionininterfacialenergyofferrite-cementite
interfaces.Asquenchedmartensitehashighconcentrationoflatticedefects.Thoughtheir
annealingoutstartsinthethirdstageoftempering,butthecementiteprecipitatesretardthe
recoveryprocesses.
Substantialrecoveryprocessesstartsoccurringonlyabove400°C.originallatheboundaries
arestableupto600°C,butabovethis,thesearereplacedbyequiaxed-ferritegrain
boundaries–theprocess,whichisbestdescribedas‘Recrystallization’.
Intheend,theopticalmicrostructureconsistsofequiaxedferritegrainswithcoarse
Spheroidalparticlesofcementite,andthenthestructureiscalledglobularpearlite,or
spheroidizedcementite.
Thestructureperhapsisthemoststableofallferrite-cementiteaggregates,andisthesoftest
withhighestductilitywithbestmachinability.

20. Mechanism of Heat Removal During Quenching
Fig. Typical cooling curve for small cylinder quenched in warm water

21. Quenching Medium
Fig. Center cooling curves for stainless steel specimens

22. Fig. center-cooling curves for specimens quenched in tap
water at bath temperatures of 75 and 125°F.