Surface Heat treatment of materials .ppt

Surface Heat treatment of materials

1. Surface Heat treatment of
materials
By
P.SURESHKUMAR
ASST PROF
Dept. of Automobile Engineering
JCTCET
Coimbatore-105

2. Surfacehardeningistheprocessofhardeningthesurfaceofa
metalobjectwhileallowingthemetaldeeperunderneathto
remainsoft,thusformingathinlayerofhardermetal(calledthe
"case")atthesurface.
Forexamplecomponentsliketurbineshaft,gears,spindle,
automotivecomponentsandaxleneedtohaveahardsurfacebuta
softcore.
Heattreatment
Heattreatmentistheprocessofheatingandcoolingmetalsto
changetheirmicrostructureandtobringoutthephysicaland
mechanicalcharacteristicsthatmakemetalsmoredesirable.
Themostcommonreasonsmetalsundergoheattreatmentareto
improvetheirstrength,hardness,toughness,ductilityand
corrosionresistance.

3. HEAT TREATMENT
BULK
SURFACE
ANNEALING
Full Annealing
Recrystallization Annealing
Stress Relief Annealing
Spheroidization Annealing
AUSTEMPERING
THERMAL THERMO-
CHEMICAL
Flame
Induction
LASER
Electron Beam
Carburizing
Nitriding
Carbo-nitriding
NORMALIZING HARDENING
&
TEMPERING
MARTEMPERING
An overview of important heat treatments

5. A
1
A
3
A
cm

T
Wt% C
0.8 %
723

C
910

C
Spheroidization
Recrystallization Annealing
Stress Relief Annealing
Full Annealing
RangesoftemperaturewhereAnnealing,NormalizingandSpheroidizationtreatmentare
carriedoutforhypo-andhyper-eutectoidsteels.
NormalizingHeatTreatmentprocessisheatingasteelabovethecriticaltemperature,
holdingforaperiodoftimelongenoughfortransformationtooccur,andaircooling.

8. Through hardening of the sample
Schematic showing variation
in cooling rate from surface
to interior leading to
different microstructures
Thesurfaceofisaffectedbythequenchingmediumandexperiencesthebestpossible
coolingrate.Theinteriorofthesampleiscooledbyconductionthroughthe(hot)sampleand
henceexperiencesalowercoolingrate.Thisimpliesthatdifferentpartsofthesamesample
followdifferentcoolingcurvesonaCCTdiagramandgiverisetodifferentmicrostructures.
Thisgivestoavaryinghardnessfromcentretocircumference.Criticaldiameter(d
c)isthat
diameter,whichcanbethroughhardened(i.e.weobtain50%Martensiteand50%pearliteat
thecentreofthesample).

9. Quenching media
•Brine (water and salt solution)
•Water
•Oil
•Air
•Turn off furnace

10. Severity of quench values of some typical quenching conditions
Process Variable H
Air No agitation 0.02
Oil quench No agitation 0.2
" Slight agitation0.35
" Good agitation0.5
" Vigorous agitation0.7
Water quench No agitation 1.0
" Vigorous agitation1.5
Brine quench
(saturated Salt water)
No agitation 2.0
" Vigorous agitation5.0
Ideal quench 
Notethatapartfromthenatureofthe
quenchingmedium,thevigorousnessofthe
shakedeterminestheseverityofthequench.
Whenahotsolidisputintoaliquid
medium,gasbubblesformonthesurfaceof
thesolid(interfacewithmedium).Asgas
hasapoorconductivitythequenchingrateis
reduced.Providingagitation(shakingthe
solidintheliquid)helpsinbringingthe
liquidmediumindirectcontactwiththe
solid;thusimprovingtheheattransfer(and
thecoolingrate).TheHvalue/index
comparestherelativeabilityofvarious
media(gasesandliquids)tocoolahotsolid.
Idealquenchisaconceptualideawithaheat
transferfactorof(H=).1
[]
f
Hm
K


Severity of Quench as indicated by the heat transfer equivalent H
f → heat transfer factor
K → Thermal conductivity
Before we proceed further we note that we have a variety of quenching media at our
disposal, with varying degrees of cooling effect. The severity of quench is indicated by the
‘H’ factor (defined below), with an ideal quench having a H-value of .
Increasing severity of quench

11. Surfacehardening:why&how?
Componentslikegear,shaftorspindleneedahard/wearresistantsurfacebutasoft/tough
core.
Sectionsizeofsuchcomponentsisoftentoolargetobeuniformlyhardenedevenonsevere
quenching.
Moreoverthetimelagbetweenthetransformationsatthesurfaceandthecoreresultsinan
unfavorabletensileresidualstressatthesurface.
Recallthegeneralthumbrulethattheregionthattransformslaterislikelytohavecompressive
residualstress.
Thesurfaceislikelytotransformfirstinsteelhavingthesamecompositionallthroughits
section.
Thereforesurfacewouldhaveresidualtensilestress.Dependingonitsmagnitudeitmayleadto
crackingordistortion.
Thepresenceofresidualtensilestressisalsoharmfulasitwouldreducefatiguelifeofcritical
componentsliketurbineshaftorlandinggearofanaircraft.
Thepurposeofsurfacehardeningistodevelopahardsurfacewithcompressiveresidualstress,
toimproveitswearresistance,toincreaseitsfatiguelifeandtoavoidsusceptibilityto
distortionandcracking.

12. Thereareseveralotherwaysthestrengthorthehardnessofthe
surfacecanbeincreasedwithoutadverselyaffectingthetoughness
ofthecore.
Someofthemostcommontechniquesareasfollows:
1.Inductionhardening
2.Casecarburizing+casehardening
3.Nitriding
4.Shotpeening
5.Hardfacing,coatingorsurfacealloying
FormationofMartensiteisprimarilyresponsibleforthe
developmentofveryhighstrengthinsteel.
Howeveryouneedtocoolacomponentmadeofsteelveryfastto
getmartensite

15. Layer additions Substrate treatment
Hard-facing
Fusion hard-facing
Thermal spray
Coatings
Electrochemical plating
Chemical vapordeposition (electrolessplating)
Thin films (physical vapordeposition, Sputtering,
ion plating)
Ion mixing
Diffusion methods
Carburizing
Nitriding
Carbonitriding
Nitrocarburizing
Boriding
Titanium-carbon diffusion
Toyota diffusion process
Selective hardening methods
Flame hardening
Induction hardening
Laser hardening
Electron beam hardening
Ion implantation
Selective carburizing and nitriding
Engineering methods for surface hardening

16. Induction hardening
•Induced eddy currents
heat the surface of the
steel very quickly and is
quickly followed by jets of
water to quench the
component.
•A hard outer layer is
created with a soft core.
The slideways on a lathe
are induction hardened.

20. Inductionhardeningisveryeffectiveforsurfacehardeningofplaincarbonsteel
having0.35‐0.70%C.
Thesalientfeaturesofinductionhardeningareasfollows:
•HeatthesurfacetoatemperatureaboveA3(austeniticregion)
•Coredoesnotgetheated:thestructureremainsunaltered
•Surfaceconvertstomartensiteonquenching.
•Fastheating&shortholdtime:needshigheraustenizationtemperature
•Martensiteformsinfineinhomogeneousgrainsofaustenite
•Applicabletocarbonsteels(0.35–0.7C)
•Littledistortion&goodsurfacefinish

21. Once the process is complete the microstructure of the surface gets transformed into
martensite while that at its core remains unaltered.

23. Carburizing
Carburizing,alsoreferredtoasCaseHardening,isaheattreatmentprocessthatproducesa
surfacewhichisresistanttowearwhilemaintainingtoughnessandstrengthofthecore.
Thistreatmentisappliedtolowcarbonsteelpartsaftermachiningaswellashighalloy
steelbearings,gearsandothercomponents.
Carburizingincreasesstrengthandwearresistancebydiffusingcarbonintothesurfaceof
thesteelcreatingacasewhileretainingasubstantiallylesserhardnessinthecore.This
treatmentisappliedtolowcarbonsteelsaftermachining.
Mostcarburizingisdonebyheatingcomponentsineitherapitfurnace,orsealed
atmospherefurnaceandintroducingcarburizinggasesattemperature.
Gascarburizingallowsforaccuratecontrolofboththeprocesstemperatureandcarburizing
atmosphere(carbonpotential).
Carburizingisatime/temperatureprocess;thecarburizingatmosphereisintroducedinto
thefurnacefortherequiredtimetoensurethecorrectdepthofcase.
Thecarbonpotentialofthegascanbeloweredtopermitdiffusion,avoidingexcesscarbon
inthesurfacelayer.

24. Carburizingcannotbedoneinferritephaseasithasverylowsolidsolubilityforcarbonat
roomtemperature.ItisdoneintheAusteniteregionabove727°Cincarbonrich
atmosphere.
Types of carburizing
i. Pack carburizing
ii. Gas carburizing
iii. Liquid carburizing
Forironorsteelwithlowcarboncontent,whichhaspoortonohardenabilityofitsown,
thecasehardeningprocessinvolvesinfusingadditionalcarbonintothecase.
Casehardeningisusuallydoneaftertheparthasbeenformedintoitsfinalshape,butcan
alsobedonetoincreasethehardeningelementcontentofbarstobeused.
Becausehardenedmetalisusuallymorebrittlethansoftermetal,through-hardening(that
is,hardeningthemetaluniformlythroughoutthepiece)isnotalwaysasuitablechoicefor
applicationswherethemetalpartissubjecttocertainkindsofstress.
Insuchapplications,casehardeningcanprovideapartthatwillnotfracture(becauseof
thesoftcorethatcanabsorbstresseswithoutcracking)butalsoprovidesadequatewear
resistanceonthesurface.

25. Pack carburizing
•Thecomponentispacked
surroundedbyacarbon-rich
compoundandplacedinthe
furnaceat900degrees.
•Overaperiodoftimecarbon
willdiffuseintothesurfaceof
themetal.
•Thelongerleftinthefurnace,
thegreaterthedepthofhard
carbonskin.Grainrefiningis
necessaryinordertoprevent
cracking.
A major limitation of pack carburizing is poor control over temperature & carburization
depth.
On completion of the process the steel parts are cooled slowly. Direct quenching is not
possible as the job is surrounded by carburizing mixture packed in a sealed box having high
thermal mass.
This can be overcome by using gaseous or liquid carburizing medium.

26. •Saltbathcarburizing.Amoltensaltbath(sodiumcyanide,sodiumcarbonateandsodiumchloride)hasthe
objectimmersedat900degreesforanhourgivingathincarboncasewhenquenched.
•Gascarburizing.Theobjectisplacedinasealedfurnacewithcarbonmonoxideallowingforfinecontrolofthe
process.
•Gascarburizationisdonebykeepingthesamplesatthecarburizingtemperatureforaspecifiedtimeina
furnacehavingamixtureofcarburizingandneutralgas.CH4andCOarethemostcommonlyused
carburizinggas.
•Itisusuallymixedwithde‐carburizing(H2andCO2)andneutralgases(N2).
•Thishelpsmaintainaclosecontrolovercarbonpotential.Itshouldbeenoughtomaintain%Catintherange
1.0‐1.2%atthesurface.
•InthepresenceofFethecarburizinggasesdecomposetoproducenascentcarbonthatdiffusesintosteel.
CH4=C(Fe)+2H2
2CO=C(Fe)+CO2
•Itprovidesexcellentcontroloverthefurnacetemperatureandatmosphere(carbonpotential).
•Samplesaftercarburizationcanbedirectlyquenched.

27. Liquid carburization
It is done by keeping the job in a salt bath consisting of 8% NaCN + 82 BaCl2 + 10 NaCl.
It allows precise temperature control and rapid heat transfer. Carburization takes place due
to the formation of nascent carbon.
The chemical reactions that occur in the presence of Fe are as follows:
BaCl2 + NaCN = Ba(CN)2 +NaCl
Ba(CN)2 = C (Fe) + BaCN2
What is the chemical CN?
A cyanide is achemicalcompound that contains the group C≡N. This group, known as the
cyano group, consists of a carbon atom triple-bonded to a nitrogen atom.
The sample can be quenched immediately after carburization.
Nitriding
Nitrides are formed on a metal surface in a furnace with ammonia gas circulating at 500
degrees over a long period of time (100 hours). It is used for finished components.
Ifsteelisheatedinanenvironmentofcrackedammoniaitpicksupnitrogen.
Nitrogenlikecarbonformsinterstitialsolidsolutionwithiron.Ifitispresentinexcessitforms
nitride(Fe4N).Itisextremelyhardandbrittle.

28. Nascentnitrogenthatformsatthesurfaceofsteelasammoniacomesincontact
withFe.
Thisdiffusesintoironlatticeandformnitrideasandwhentheamountof
nitrogeninsteelexceedsitssolubilitylimit.
Thepresenceofalloyingelementshavinghighaffinityfornitrogenincreases
nitrogenpickup.
Theformationofnitridewithinthematrixresultsinasubstantialincreaseinthe
hardnessofsteel.
Thepreferredthicknessofthehardenedlayerisaround20micrometer.
Thehardnessofthenitridelayerisusuallyintherangeof1000‐2000Hv.
Nitridingofsteeliscarriedoutonlyafterithasbeenhardenedandtempered.It
isthelastheattreatmentgiventosteel.

29. Flame hardening
•Gasflamesraisethe
temperatureoftheouter
surfaceabovetheupper
criticaltemp.Thecore
willheatbyconduction.
•Waterjetsquenchthe
component.

37. Vapor Deposition
•Physical Vapor Deposition (PVD)
–Thermal PVD
–Sputter Deposition
–Ion plating
•Chemical Vapor Deposition (CVD)

38. Physical Vapor Deposition –Thermal
PVD
•Thermal PVD –also called Vacuum Deposition
–Coating material (typically metal) is evaporated by melting
in a vacuum
–Substrate is usually heated for better bonding
–Deposition rate is increased though the use of a DC current
(substrate is the anode so it attracts the coating material)
–Thin ~0.5 mm to as thick as 1 mm.

39. Physical Vapor Deposition –Sputter Deposition
•Vacuum chamber is usually backfilled with Ar gas
•Chamber has high DC voltage (2,000-6,000 V)
•The Ar becomes a plasma and is used to target the
deposition material. The impact dislodges atoms from the
surface (sputtering), which are then deposited on the
substrate anode
•If the chamber is full of oxygen instead of Ar, then the
sputtered atoms will oxidize immediately and an oxide will
deposit (called reactive sputtering)

40. Physical Vapor Deposition –Ion Plating
•Combination of thermal PVD and sputtering
•Higher rate of evaporation and deposition
•TiN coating is made this way (Ar-N
2
atmosphere)
–The gold looking coating on many cutting tools to
decrease the friction, increase the hardness and
wear resistance

41. Chemical Vapor Deposition
•Deposition of a compound (or element) produced by a
vapor-phase reduction between a reactive element and gas
–Produces by-products that must be removed from the process as
well
•Process typically done at elevated temps (~900ºC)
–Coating will crack upon cooling if large difference in thermal
coefficients of expansion
–Plasma CVD done at 300-700ºC (reaction is activated by plasma)
•Typical for tool coatings
•Applications
–Diamond Coating, Carburizing, Nitriding, Chromizing, Aluminizing
and Siliconizing processes
–Semiconductor manufacturing

43. PlasmaNitriding
Plasmanitriding,alsoknownasionnitridingorglowdischargenitriding,isagasnitriding
processenhancedbyaplasmadischargeontheparttobenitrided.
Theplasmaisgasthatwhenexposedtoanelectricalpotentialisionizedandglows.The
partstobenitridedareconnectedasacathodeandthefurnacewallsaretheanode.
Theyaresuppliedwithapotentialbetween0.3and1KV.Particlesareacceleratedandhit
thecathode(workpiece)transferringalltheirkineticenergyandheatingit.
Forgasparticlestohaveenoughkineticenergytotransfer,theyneedtohavea
considerablelargemeanfreepathandinthiswaygainspeedforcollisionwiththe
substratebeforecollidingwithanothergasparticle.
Thisiswhythisprocessandmostlyallplasmaprocessesworkundervacuumasameasure
toincreasethemeanfreepathoftheacceleratedparticles.
Thepressureusedforplasmanitridingisnormallybetween100–1000Pa.Otherauthors
suggestanarrowerrangebetween50–500Pa.
Thispressureisconsideredasaroughvacuumsincethereareotherprocessesthatuse
muchhighervacuumvalues.

44. Thechemicalreactionwhenammoniadissociatedwasexplainedintheprecedingsection.
Inthecaseofplasmanitriding,theprocessgasesareintroducedseparately.
OnecombinationthatisoftenusedisNitrogen+Hydrogen.Argonisalsousedintheinitial
stagesasaplasmasputteringgasforsurfacecleaningofthesubstratetobenitrided.
Thevoltagedropoccursinwhatiscalledtheplasmasheathwhichisapositivecharged
areawhereionsareacceleratedtowardsthecathodeandhavetheirhighestkinetic
energies.

45. Duringionnitridingthreereactionswilloccuratthesurfaceofthematerialbeingtreated.
Inthefirstreaction,ironandothercontaminantsareremovedfromthesurfaceofthe
workbyanactionknownassputteringorbyareducingreactionwithhydrogen.
Theimpactofhydrogenorargonionsbombardingtheworksurfacedislodgesthe
contaminantthatwillbeextractedbythevacuumsystem.Theremovalofthese
contaminantsallowsthediffusionofnitrogenintothesurface
Duringthesecondreaction,andasaresultoftheimpactofthesputteredionatoms,case
formationbeginsattheworksurfaceofironnitrides.
SputteredFe+N=FeN
Duringthethirdreaction,abreakdownoftheFeNbeginsunderthecontinuoussputtering
fromtheplasma.
ThisonecausestheinstabilityoftheFeNwhichbeginstobreakdownintotheephase
followedbytheg’phaseandairon-nitrogencompoundzone

46. Surface Treatment
Type
Concepts and Applications of the process
Electroplating
A method of forming metallic coatings (plating films) on subject metal surfaces submerged in solutions containing ions by
utilizing electrical reduction effects. Electoplating is employed in a wide variety of fields from micro components to large
products in information equipment, automobiles, and home appliances for ornamental plating, anti-corrosive plating, and
functional plating.
ElectrolessPlating
A plating method that does not use electricity. The reduction agent that replaces the electricity is contained in the plating
solution. With proper re-processing, virtually any material such as paper, fabrics, plastic and metals can be plated, and the
distribution of the film thickness is more uniform, but slower than electroplating. This is different from chemical plating by
substitution reaction.
Chemical Process
(Chemical Coating)
The process creates thin films of sulfideand oxide films by chemical reactions such as post zinc plating chromate treatment,
phosphate film coating (Parkerizing), black oxide treatments on iron and steels, and chromic acid coating on aluminum. It is
used for metal coloring, corrosion protection, and priming of surfaces to be painted to improve paint adhesion.
Anodic Oxidation
Process
This is a surface treatment for light metals such as aluminumand titanium, and oxide films are formed by electrolysis of the
products made into anodes in electrolytic solutions. Because the coating (anodizing film) is porous, dyeing and coloringare
applied to be used as construction materials such as sashes, and vessels. There is low temperature treated hard coating
also.
Hot Dipping
Products are dipped in dissolved tin, lead, zinc, aluminum, and solder to form surface metallic films. It is also called
Dobuzukeplating and Tempura plating. Familiar example is zinc plating on steel towers.
Vacuum Plating
Gasifiedor ionized metals, oxides, and nitrides in vacuum chambers are vapordeposited with this method. Methods are
vacuum vapordeposition, sputtering, ion plating, ion nitriding, and ion implantation. Titanium nitride is of gold color.
Painting
There are spray painting, electrostatic painting, electrodepositionpainting, powder painting methods, and are generally
used for surface decorations, anti-rusting and anti-corrosion. Recently, functional painting such as electro-conductive
painting, non-adhesive painting, and lubricating painting are in active uses.
Thermal Spraying
Metals and ceramics (oxides, carbides, nitrides) powders are jetted into flames, arcs, plasma streams to be dissolved and
be sprayed onto surfaces. Typically used as paint primer bases on larger structural objects, and ceramic thermal spraying
for wear prevention.
Surface Hardening
This is a process of metal surface alteration, such as carburizing, nitriding, and induction hardening of steel. The processes
improve anti-wear properties and fatigue strength by altering metal surface properties.
Metallic Cementation
This is a method of forming surface alloy layers by covering the surfaces of heated metals and metal diffusion at the same
time. There is a method of heating the pre-plated products, as well as heating the products in powdered form of metal to
be coated.
There are following types of surface treatments.

47. WhatismeantbyAustempering?
Austemperingisheattreatmentthatisappliedtoferrousmetals,mostnotablysteeland
ductileiron.Insteelitproducesabainitemicrostructurewhereasincastironsitproduces
astructureofacicularferriteandhighcarbon,stabilizedausteniteknownasausferrite.
What is Martempering process?
Martemperingis a heat treatment for steel involving austenitisation followed by step
quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a
temperature slightly above the martensite start (Ms) point.

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