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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.html7/24/2019 heat treat
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SpheroidisingSpheroidising
77 Heat to ;ust below !/!! deg C)
&& Cool *ery slowly in the furnaceCool *ery slowly in the furnace
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
&& Cool *ery slowly in the furnaceCool *ery slowly in the furnace
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
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