SurfaceHardeningofSteels

Dr.KaustubhKulkarniDepartmentofMaterialsScienceandEngineering

IndianIns=tuteofTechnologyKanpurKanpur,U.P.2080216

Date:20thOctober2015

TEQIP Workshop on Microstructure Engineering through Heat Treatments

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WhyisSurfaceHardeningRequired?

•  Somecomponentsrequiregoodtoughnessbutveryhardandwearresistantsurface–  especiallypartsrota=ngagainsteachother–  e.g.gearsengagedineachotherandrota=ngatseveralthousandRPM–  Gearteethrubagainsteachotherandalsoexertbendingforcesoneachother

•  Differenttreatmentsneededforsurfacethanbulkofthecomponent

F F

Thearrowsindicatethebendingforcesac=ngontheengagedteeth

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VariousSurfaceHardeningTreatments

•  CarburizingandHardening–  Carbonisaddedtocertaindepthonthesurfaceofalowcarbonsteel–  ThecomponentishardenedaTercarburizing

•  Induc=onHardening–  Selec=veareasofthecomponentheatedbyinduc=onhea=ngtodesired

depth–  QuenchedaTertheinduc=onhea=ng

•  FlameHardening–  Selectedareasheatedbyoxy-acetyleneflame

•  Nitriding–  Nitrogencontentincreasedonthesurfaceandhardnitridesformed–  Componentexposedtonitrogenousatmosphereordippedintonitridingsalt

bath

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CARBURIZINGANDHARDENING

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Mo=va=onforCarburizing

•  HighCarbonSteelshavehigherstrengthandhardness

•  Lowcarbonsteelshavehighertoughness

•  Increasingcarbononlyonthesurfaceofalowcarbonsteelcangivehardsurfaceandtoughcore

DatatakenfromG.Krauss;MaterialsScienceandEngineeringA;Vol.273-275;(1999)40-57

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Stre

ngth

(Mpa

)

Hardness (H

RC

) Carbon Content (wt%)

% E

long

atio

n

Carbon Content (Wt%)

UTS

YS

HRC

Varia=onofStrength,HardnessandDuc=lityofa43XXsteelwithCarbonConcentra=on

PrincipleofCarburizing

•  Atahightemperature,carboniscon=nuouslyaddedonthesurfaceofthecomponentfromasolid,liquidorgassource

•  With=me,carbondiffusesintothesteel•  Depthuptowhichcarbondiffusesintosteelcontrolledby

“=me”and“temperature”

SteelComponent

SourceofCarbon

Carbondiffusesfromsurfaceintothecore

T=860-900°C

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Carburizing:ADiffusionControlledProcess

•  Knowledgeofdiffusivityofcarboninsteelisneededtocontrolthediffusiondepth

Steelwithoriginalcomposi=onC0

SourceofCarbonT=860-900°C

ConstantCarbonconc.atsurface:CS

x = 0

x = ∞

∂C∂t

= D ∂2C∂x2

at t = 0, C(x,0) =C0 for x > 0

at x = 0, C(0,t) =Csat x =∞, C(∞,t) =C0

DiffusionEqua=on

Ini=alCondi=on

BoundaryCondi=ons

erf z( ) = 2π

exp −η2( )0

z

∫ dηC(x ) =C0 + (Cs −C0 ) ⋅ 1− erfx

2 Dt)

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣⎢

⎤

⎦⎥

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Carburizing:EffectofTimeandTemperature

•  Atagiventemperature,doublingdiffusiondepthrequiresfour=meslongercarburizing=me

•  Timerequiredtoachievedesiredcasedepthdecreaseswithincreasingtemperature

x1x2

x3

t2

t1

t3

Depth below Surface (mm)

Car

bon

Con

c. (w

t%)

Carbonpenetra=onbelowsurfaceatdifferent=mes

C0 = 0.2 wt% CS = 0.8 wt% T = 900°C C0 = 0.2 wt%

CS = 0.8 wt% C.D. = depth where carbon is 0.4 wt%

Temperature (°C)

Car

buriz

ing

time

(hr)

Carburizing=measafunc=onoftemperature

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HardeningaTerCarburizing

•  ItisnecessarytocarryoutcarburizingatatemperatureaboveA3temperatureforthesteel

•  ItisnecessarytoquenchthecomponentaTercarburizingtoachievethedesiredhardness

•  TemperingisnecessaryaTerhardeningforreducingbriileness

wt%Carbon

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MicrostructureEvolu=oninCarburizedandHardenedSteel

•  Martensiteformedatthesurface–  Coolingratefastestatsurface–  HighcarbonshiTstheCCTcurvetowardsright

forthesurface–  Easytomissthebaini=cnose

•  Corecontainsbainitealongwithsmallamountofmartensite

•  Thickpartsmaycontainsomeferriteandpearlitealongwithbainiteatthecore

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CCTDiagramfor20MnCr5Steel

Thediagramtakenfrom:hip://www.metalravne.com/steelselector/steels/EC100.htmlAccessedon18thOct2015

MicrostructureofaCarburizedandHardenedSteel

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Cross sec=onofagear toothwhichwas subjected to carburizing andhardening. The case is clearlyhighlightedbyitsdarkeretching

Microstructures at various cross sec=ons of a 0.2%plaincarbonsteel,whichwascarburizedandhardened

Surfaceconsistsofmartensiteandsomeretainedaustenite

Sub-surfaceconsis=ngoffinepearlite(dark)andmartensitewithlowerCthansurface

Coreconsis=ngofproeutectoidferrite(white)andlowcarbonmartensite

hip://www.georgesbasement.com/Microstructures/LowAlloySteels/Lesson-4/Specimen03.htm

Micro-HardnessProfilesonaCarburizedHardenedSteel

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•  Hardnessdecreasesfromsurfacetocore•  C.D.istakenasthedepthatwhichhardnessfallsto550Hv

Depth below Surface (mm)

Mic

ro-H

ardn

ess (

Hv)

C.D.=0.92mm

A typical micrograph showing themicro-hardness indents from surfacetowardsthecore

SomeCarburizingSteelsandTheirUses

•  Carburizingsteelscontainlowcarbon,typically0.15to0.25wt%•  Carboncontentatthesurfaceraisedto~0.8wt%aTercarburizing•  Applica=ons:TransmissioncomponentslikegearsanddriveshaTs

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Steelgrade C Simax

Mn Pmax

Smax

Cr Mo Ni

20MnCr5 0.17-0.22

0.4 1.1-1.4

0.035 0.035 1.0-1.3 --- ---

16MnCr5 0.14-0.19

0.4 1.0-1.3

0.035 0.035 0.8-1.1 --- ---

20MoCr4 0.17-0.22

0.4 0.7-1.0

0.035 0.035 0.3-0.5 0.4-0.5

---

21NirMo2 0.17-0.23

0.4 0.65-0.95

0.035 0.035 0.4-0.7 0.15-0.25

0.4-0.7

17CrNiMo6 0.15-0.2

0.4 0.4-0.6

0.035 0.035 1.5-1.8 0.25-0.35

1.4-1.7

Somecarburizingsteelgradesandtheircomposi=onsinwt%

*Source:DIN17210;StandardforCaseHardeningSteels

GasCarburizingProcess•  MostoTenMethane(CH4)isusedascarburizinggas•  Methaneismixedwithacarriergastoavoidsootforma=on

oncomponents•  Desiredcarbonpoten=alismaintainedbyadjus=ngthe

reac=onequlibirainfurnaceatmosphere

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CO+H2O=CO2+H2

•  CO/CO2ra=oadjustedtoestablishrequiredcarbonpoten=al•  H2/H2Ora=oadjustedtomaintaintheatmospherereducingforsteel

INDUCTIONHARDENING

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Induc=onHardening:Principle

•  Thedesiredpartofthecomponentkeptinsideaninduc=oncoil•  A.C.currentpassedthroughthecoilinducesmagne=cfieldaroundthecomponent•  Themagne=cfieldinduceseddycurrentsinthecomponent•  Theeddycurrentsarepredominantlygeneratedatornearthesurface•  Byadjus=ngthefrequencyoftheA.C.current,depthofhea=ngcanbeadjusted•  Heatedpartisthenquenchedinwateroroil

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hip://www.heaireatmentsservice.com/induc=on-hardening-services-2104452.htmlhip://www.alfing-crankshaTs.com/en/automo=ve/surface-treatment.html

Induc=onHardening:MaterialsandUse

•  Induc=onhardeningisusedwhereselec=vepartsofaheavydutycomponenthastobecasehardened

•  Sincenocarburizingisdone,theinduc=onhardenableparthastocontainmediumcarbon,typicallyequivalentcarboncontentof0.4-0.45wt%

•  Examplesofapplica=onarecrankshaT,camshaTsetc.

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NITRIDING

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Nitriding:Principle•  Steelisexposedtonitrogenousatmosphere

NH3=N+3H(at525-550°C)

•  Thenascentnitrogenreactswithvariousalloyingelementsinthesteel(e.g.Al,Mo,Cretc.)toformnitrides

•  Thenitridelayerformedonthesurfaceisveryhard•  Usuallytheini=alfewmicronsofdensenitridelayer(called

“WhiteLayer”)isbriileandhastoberemoved•  Noquenchingisdone

–  Thesteelhastobeheattreatedbeforenitridingprocess,typicallyhastemperedmartensi=cstructure

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NitridedSteelMicrostructure

•  3-4micronsofwhitelayerhastoberemoved•  Anitridelayerofafewtensofmicronsthickonthesurface•  Temperedmartensiteinthesubsurfaceandcore•  CaseHardnessismuchhigherthanincarburizedorinduc=onhardenedsteels

–  1200to1500Hvcomparedto750-800Hvofcarburizedsteel20

hip://www.gas-nitriding.com/index.php?p=1_6_Technical-Informa=on

Nitriding:MaterialsandApplica=ons

•  Nitridingsteelcontains0.2to0.4wt%Candnitrideformingelements;mainlyAl

•  Applica=onsofnitridedsteelsinclude:Gauges,formingdies,valveslidesandsleeves,thrustwashersetc.

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Steelgrade C Si Mn Al Cr Mo

Nitralloy135(G) 0.3-0.4 0.2-0.4 0.4-0.7 0.85-1.2 0.9-1.4 0.15-0.25

Nitralloy135(AircraTgrade)

0.38-0.45 0.2-0.4 0.4-0.7 0.85-1.2 1.4-1.8 0.3-0.45

NitralloyN 0.2-0.27 0.2-0.4 0.4-0.7 0.85-1.2 1.0-1.3 0.2-0.3

NitralloyEZ(0.15-0.25%Se)

0.3-0.4 0.2-0.4 0.5-1.1 0.85-1.2 1.0-1.5 0.15-0.25

SomeNitridingsteelgradesandtheircomposi=onsinwt%

HelpfulReferences•  R.C.Sharma;PrinciplesofHeatTreatmentofSteels’NewAge

Interna=onalLimitedPublishers

•  DonaldS.ClarkandWilburR.Varney;PhysicalMetallurgyforEngineers;East-WestPressPvt.Ltd.

•  SydneyH.Avner;Introduc=ontoPhysicalMetallurgy;McGraw-HillEduca=onPvt.Ltd.

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Thankyou

AnyQues=ons????

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