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1. Heat Treatment
of Steel
سنوي ناميلس

2. Heat-Treatment
Heat treatmentis a method used to alter the
physical, and sometimes chemical properties of a
material. The most common application is
metallurgical
It involves the use of heating or cooling, normally
to extreme temperatures, to achieve a desired
result such as hardeningor softeningof a material
It applies only to processes where the heating and
cooling are done for the specific purpose of
altering properties intentionally

3. Types of Heat-Treatment (Steel)
Annealing / Normalizing,
Case hardening,
Precipitation hardening,
Tempering, and Quenching

4. Figure -Phase diagramam for iron-carbon system, up to about
6% carbon

6. Iron has Several Phases,
depending on Temperature
Thephaseatroomtemperatureisalpha(),
calledferrite(BCC)
At912C(1674F),ferritetransformsto
gamma(),calledaustenite(FCC)
Thistransformsat1394C(2541F)todelta
()(BCC)
Pureironmeltsat1539C(2802F)

7. Iron as a Commercial Product
Electrolyticiron-themostpure,atabout99.99%,
forresearchandotherpurposeswherethepure
metalisrequired
Ingotiron-containsabout0.1%impurities
(includingabout0.01%carbon),usedin
applicationswherehighductilityorcorrosion
resistanceareneeded
Wroughtiron-containsabout3%slagbutvery
littlecarbon,andiseasilyshapedinhotforming
operationssuchasforging

8. Solubility Limits of Carbon in Iron
Ferritephasecandissolveonlyabout0.022%
carbonat723C(1333F)
Austenitecandissolveuptoabout2.1%
carbonat1130C(2066F)
Thedifferenceinsolubilitybetweenalphaand
gammaprovidesopportunitiesforstrengthening
byheattreatment

9. Time-Temperature-Transformation
(TTT)Curve
TTTdiagramisaplotoftemperatureversusthe
logarithmoftimeforasteelalloyofdefinite
composition.
Itisusedtodeterminewhentransformationsbegin
andendforanisothermalheattreatmentofa
previouslyaustenitizedalloy
TTTdiagramindicateswhenaspecifictransformation
startsandendsanditalsoshowswhatpercentageof
transformationofausteniteataparticulartemperature
isachieved.

10. The TTT diagram for AISI 1080 steel (0.79%C, 0.76%Mn) austenitised at
900°C
Time-Temperature-Transformation
(TTT)Curve

11. Decarburization during Heat Treatment
Decreaseincontentofcarboninmetalsiscalled
Decarburization
Itisbasedontheoxidationatthesurfaceofcarbon
thatisdissolvedinthemetallattice
Inheattreatmentprocessesironandcarbonusually
oxidizesimultaneously
Duringtheoxidationofcarbon,gaseousproducts
(COandCO
2)develop
Inthecaseofascalelayer,substantial
decarburizationispossibleonlywhenthegaseous
productscanescape

12. Decarburization Effects
Thestrengthofasteeldependsonthe
presenceofcarbidesinitsstructure
Insuchacasethewearresistanceisobviously
decreased
Inmanycircumstances,therecanbeaserious
dropinfatigueresistance
Toavoidtherealriskoffailureofengineering
components,itisessentialtominimize
decarburizationatallstagesintheprocessingof
steel

13. Annealing
It is a heat treatment wherein a material is
altered, causing changes in its properties
such as strengthand hardness
It the process of heating solid metal to high
temperatures and cooling it slowly so that its
particles arrange into a defined lattice

14. Types of Annealing
1.Full Annealing
2.Stress-Relief Annealing (orStress-relieving)
3.Normalizing
4.Isothermal Annealing
5.Spheroidizing Annealing (orSpheroidizing )

15. +Fe3C


16. FullAnnealing
raisingthetemperatureabout50ºC(90ºF)abovethe
AustenitictemperaturelineA
3orlineA
CMinthecaseof
Hypoeutectoidsteels(steelswith<0.77%Carbon)and
50ºC(90ºF)intotheAustenite-Cementiteregioninthe
caseofHypereutectoidsteels(steelswith>0.77%
Carbon).
cooledattherateofabout20ºC/hr(36ºF/hr)ina
furnacetoabout50ºC(90ºF)intotheFerrite-Cementite
range.
ThegrainstructurehascoarsePearlitewithferriteor
Cementite(dependingonwhetherhypoorhyper
eutectoid).Thesteelbecomessoftandductile.

17. ProcessAnnealing
heatingittoabout700ºC(1292ºF)should
suffice.Thisisheldlongenoughtoallow
recrystallizationoftheferritephase,andthen
cooledinstillair.
theonlychangethatoccursisthesize,
shapeanddistributionofthegrainstructure.

18. Causes of Residual Stresses
1.Thermalfactors(e.g.,thermalstresses
causedbytemperaturegradientswithinthe
workpieceduringheatingorcooling)
2.Mechanicalfactors(e.g.,cold-working)
3.Metallurgicalfactors(e.g.,transformation
ofthemicrostructure)
Stress-Relief Annealing

19. Stress-Relief Annealing
Itisanannealingprocessbelowthetransformation
temperatureA
1(600-650ºC),withsubsequent
slowcooling,theaimofwhichistoreducethe
internalresidualstressesinaworkpiecewithout
intentionallychangingitsstructureandmechanical
properties

20. How to Remove Residual Stresses?
R.S.canbereducedonlybyaplasticdeformationin
themicrostructure.
Thisrequiresthattheyieldstrengthofthematerialbe
loweredbelowthevalueoftheresidualstresses.
Themoretheyieldstrengthislowered,thegreaterthe
plasticdeformationandcorrespondinglythegreater
thepossibilityorreducingtheresidualstresses
Theyieldstrengthandtheultimatetensilestrengthof
thesteelbothdecreasewithincreasingtemperature

21. Stress-Relief Annealing Process
Forplaincarbonandlow-alloysteelsthe
temperaturetowhichthespecimenisheatedis
usuallybetween450to650˚C,whereasforhot-
workingtoolsteelsandhigh-speedsteelsitis
between600to750˚C
Thistreatmentwillnotcauseanyphasechanges,
butrecrystallizationmaytakeplace.
Machiningallowancesufficienttocompensatefor
anywarpingresultingfromstressrelievingshould
beprovided