heat treat

7/24/2019 heat treat

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Changing theChanging theProperties of SteelsProperties of Steels


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SteelsSteels

Can be heat treated to alter propertiesCan be heat treated to alter properties

by either:by either:

Heating and rapid coolingHeating and rapid cooling

(quenching)(quenching)

Heating and slow coolingHeating and slow cooling


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QuenchingQuenching

The steels shown in blue on theThe steels shown in blue on the

following slide can be heat treated tofollowing slide can be heat treated to

harden them by quenchingharden them by quenching


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Metals

Ferrous metals Non-ferrous metals

Steels Cast Irons

Plain carbon steels

Low alloy steels

High alloy steels

Stainless & Tool steels

rey Iron

!hite Iron

Malleable & "uctile Irons

Low carbon steels

Me#ium carbon steels

High carbon steels


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Hardening TemperaturesHardening Temperatures

The temperatures for hardening dependThe temperatures for hardening dependon the carbon contenton the carbon content

Plain carbon steels below !"# will notPlain carbon steels below !"# will not

harden by heat treatmentharden by heat treatmentThe temperature decreases from appro$The temperature decreases from appro$

%&! deg C as carbon content increases%&! deg C as carbon content increasesfrom !"# up to !%#' where temperaturefrom !"# up to !%#' where temperature

is appro$ %! deg Cis appro$ %! deg C bo*e !%# the temperature remainsbo*e !%# the temperature remains

constant at %! deg Cconstant at %! deg C


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usteniteustenite

This is the structure of irons andThis is the structure of irons and

steels at high temperatures (o*ersteels at high temperatures (o*er

%!! deg C)%!! deg C)

+or quench hardening all the material+or quench hardening all the material

must start as ustenitemust start as ustenite

Quenching causes the ustenite toQuenching causes the ustenite to

be partially or totally transformed tobe partially or totally transformed to

,artensite,artensite


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,artensite,artensite

-nly formed by *ery rapid cooling-nly formed by *ery rapid cooling

from the austenitic structurefrom the austenitic structure

.eeds to be abo*e the.eeds to be abo*e the CriticalCritical

Cooling Rate.Cooling Rate.


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The needle/li0eThe needle/li0estructure ofstructure of

martensite' themartensite' the

white areas arewhite areas are

retained austeniteretained austenite


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HardenabilityHardenability

This is dependent upon the chemicalThis is dependent upon the chemical

composition of the steel alloycomposition of the steel alloy

The addition of .ic0el' Chromium andThe addition of .ic0el' Chromium and

,olybdenum will slow the,olybdenum will slow the

transformation to other phases andtransformation to other phases and

allow more martensite to formallow more martensite to form

,ost heat treatable steels are alloys,ost heat treatable steels are alloys

rather than plain carbon steelsrather than plain carbon steels


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Quenching ,ediaQuenching ,edia

+our commonly used quenching media:+our commonly used quenching media: 1rine 2 the fastest cooling rate1rine 2 the fastest cooling rate 3ater 2 moderate cooling rate3ater 2 moderate cooling rate -il 2 slowest cooling rate-il 2 slowest cooling rate 4as 2 used in automatic furnaces' usually4as 2 used in automatic furnaces' usually

liquid nitrogen' can be *ery fast coolingliquid nitrogen' can be *ery fast cooling

Too rapid cooling can cause crac0ing inToo rapid cooling can cause crac0ing incomple$ and hea*y sectionscomple$ and hea*y sections


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5epth of Hardening5epth of Hardening

5ue to the5ue to the mass efectmass efect' not all the' not all the

section of a large component may besection of a large component may be

hardened due to too slow a coolinghardened due to too slow a cooling

raterate

This may lea*e a soft core' or inThis may lea*e a soft core' or in

e$treme cases pre*ent hardeninge$treme cases pre*ent hardening

altogetheraltogether


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The Heat Treatment ProcessThe Heat Treatment Process

How you heat treat

ma$es all the

#ifference to the steel

you get

%Martensite 'brittle(

%Pearlite '#uctile(

)CC * Fe+C

with #ifferent

microstructures


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TemperingTempering

The brittleness of martensite ma0esThe brittleness of martensite ma0eshardened steels unsuitable for mosthardened steels unsuitable for most

applicationsapplications

This requires the steel to be temperedThis requires the steel to be temperedby re/heating to a lower temperatureby re/heating to a lower temperature

to reduce the hardness and impro*eto reduce the hardness and impro*e

the toughness This treatmentthe toughness This treatmentcon*erts some of the martensite tocon*erts some of the martensite to

another structure called bainiteanother structure called bainite


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Tem,ering Tem,eratures


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Slow Cooling 6ate ProcessesSlow Cooling 6ate Processes

.ormalising.ormalising

nnealingnnealing

SpheroidisingSpheroidising Stress/relief annealingStress/relief annealing


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.ormalising.ormalising

77 Heat to 8pper Critical Temperature' atHeat to 8pper Critical Temperature' at

which point the structure is allwhich point the structure is all

usteniteustenite

&& Cool slowly in airCool slowly in air

99 Structure will now be ne equi/a$edStructure will now be ne equi/a$ed

pearlitepearlite

"" 8sed to restore the ductility of cold or8sed to restore the ductility of cold orhot wor0ed materials whilst retaininghot wor0ed materials whilst retaining

other propertiesother properties


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nnealingnnealing

77 Heat to abo*e 8pper CriticalHeat to abo*e 8pper Critical

Temperature' at which point theTemperature' at which point the

structure is all ustenitestructure is all ustenite

&& Cool *ery slowly in the furnaceCool *ery slowly in the furnace

99 Structure will now be large/grainedStructure will now be large/grained

pearlitepearlite

"" 8sed to impro*e the properties of cast8sed to impro*e the properties of castand forged steels prior to machiningand forged steels prior to machining


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4rain 4rowth4rain 4rowth

glass *ial containing a liquid

that foams Sha0ing results ina ne foam' which slowlycoarsens with time Thecoarsening processissomewhat analogous tograin growthin solids

The same *ial' after allowing

some time for the foam tocoarsen The process occurs inorder to reduce the surfaceper unit *olume
http://www.msm.cam.ac.uk/phase-trans/abstracts/lecture12.htmlhttp://www.msm.cam.ac.uk/phase-trans/abstracts/lecture7.htmlhttp://www.msm.cam.ac.uk/phase-trans/abstracts/lecture7.htmlhttp://www.msm.cam.ac.uk/phase-trans/abstracts/lecture12.html


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SpheroidisingSpheroidising

77 Heat to ;ust below !/!! deg C)


99 Structure will now be spheroidite' inStructure will now be spheroidite' in

which the ?ron Carbide has @balled upAwhich the ?ron Carbide has @balled upA

"" 8sed to impro*e the properties of8sed to impro*e the properties of

medium and high carbon steels priormedium and high carbon steels priorto machining or cold wor0ingto machining or cold wor0ing


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Process (stress/relief)Process (stress/relief)

nnealingnnealing77 Heat to below 8pper CriticalHeat to below 8pper Critical

Temperature to cause recrystallisationTemperature to cause recrystallisation


99 Structure will now be equi/a$edStructure will now be equi/a$ed

pearlitepearlite

"" 8sed to ma$imise the ductility of low8sed to ma$imise the ductility of low

carbon steels and other materialscarbon steels and other materialsafter cold wor0ingafter cold wor0ing


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Cold 3or0ingCold 3or0ing

Cold roll to Bpanca0e grains

?ncreases hardness and strength at thee$pense of ductility

heat treat

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