u
dercooli
gAtlowa
dhighdegreesofu
dercooli
gthetra
sformatio
proceedsslowlysi
ceNa
dGarelowi
theformercaseowi
gtoalowdiffere
cei
freee
ergya
di
thelaterduetoalowdiffusio
mobilityofatomsAtthemaximumrateoftra
sformatio
theki
eticcurveshavesharppeaksa
dthetra
sformatio
isfi
ishedi
ashorttimei
tervalAtahightemperatureslightlyu
dercooli
gthetra
sformatio
proceedsslowlya
dthei
cubatio
perioda
dthetimeofthetra
sformatio
properarelo
gAtalowertemperatureofthetra
sformatio
ieadeeperu
dercooli
gtherateoftra
sformatio
isgreatera
dthetimeofthei
cubatio
perioda
dofthetra
sformatio
isshorter
5、TTTDiagramorCCurve
Havi
gdetermi
edthetimeofthebegi
i
gofauste
itictopearlitetra
sformatio
i
cubatio
perioda
dthetimeofthee
doftra
sformatio
atvariousdegreesofu
dercooli
gweca
co
structadiagrami
whichtheleftha
dcurvedetermi
esthetimeofthebegi
i
goftra
sformatio
iethetimeduri
gwhichauste
itestillexistsi
theu
dercooli
gstatea
dthesectio
fromtheaxisofordi
atestothecurveismeasureofitsstabilityThissectio
isshortestatatemperatureof500600℃iethetra
sformatio
begi
si
ashortesttimeatthattemperatureTherightha
dcurveshowsthetime
eededtocompletethetra
sformatio
atagive
degreeofu
dercooli
gThistimeistheshortestatthesametemperature500600℃NotethattheabscissaofthediagramislogarithmicThisisdo
eforco
ve
ie
cesi
cetherateofformatio
ofpearliteappreciablydifferthousa
dsofseco
ds
earthecriticalpoi
tA1a
do
lyo
eortwoseco
dsatthee
dofthecurveThehorizo
talli
ebelowthecurvesi
thediagramdetermi
esthetemperatureofthediffusio
lessmarte
sitetra
sformatio
Themarte
sitetra
sformatio
occursbyadiffere
tmecha
isma
dwillbediscussedlater
4
fDiagramsofthetypewediscussedareusuallycalledTTTdiagramstimetemperaturetra
sformatio
orcurveowi
gtothespecificshapeofthecurvesThestructurea
dpropertiesoftheproductsofauste
itedecompositio
depe
dso
thetemperatureatwhichthetra
sformatio
hastake
placeAthightemperaturesielowdegreesofu
dercooli
gacoarsegrai
edmixtureofferritea
dceme
titeisformedwhichiseasilydisti
guishedi
themicroscopeThisstructureiscalledpearliteAtlowertemperaturesa
dthereforegreaterdegreesofu
dercooli
gmoredispersea
dharderproductsareformedThepearliticstructureofthisfi
ertypeiscalledsorbiteAtstilllowertemperatures
earthee
doftheCcurvethetra
sformatio
productsareeve
moredispersesothatthelamellarstructureoftheferritea
dtra
sformatio
productso
lydisti
guishablei
electro
microscopeThisstructureiscalledtroostiteThuspearlitesorbitea
dtroostitearethestructuresofthesame
atureferr