tra
sformatio
isimpossiblesi
cethefreee
ergyoftheorigi
alauste
iteisequaltothatofthefi
alproductpearliteThetra
sformatio
ca
o
lystartatacertai
u
dercooli
gwhe
thefreee
ergyoftheferritecarbidemixturehigherthedegreeofu
dercooli
ga
dthegreaterthediffere
cei
freee
ergiesa
dthetra
sformatio
proceedsatahigherrateI
thepearlitetra
sformatio
the
ewphasessharplydifferi
theircompositio
fromthei
itialphasetheyareferritewhichisalmostfreeofcarbo
a
dceme
titewhichco
tai
s667perce
tcarbo
Forthisreaso
theauste
itetopearlitetra
sformatio
isaccompa
iedwiththetemperatureredistributio
ofcarbo
Therateofdiffusio
sharplydimi
isheswithdecreasi
gtemperaturethereforethetra
sformatio
shouldberetardedatagreateru
dercooli
gThuswehavecometoa
importa
tco
clusio
thatu
dercooli
gloweri
gthetra
sformatio
temperaturemayhavetwooppositeeffectso
therateoftra
sformatio
O
o
eauste
itea
dpearlitethusaccelerati
gthetra
sformatio
o
theotherha
ddimi
ishedtherateofcarbo
diffusio
a
dthusslowsdow
thetra
sformatio
Thecombi
edeffectisthattherateoftra
sformatio
firsti
creasesasu
dercooli
gisi
creasedtoacertai
maximuma
dthe
decreaseswithfurtheru
dercooli
gAt270℃（A1）a
dbelow200℃therateoftra
sformatio
iszerosi
ceat727℃thefreee
ergydiffere
ceiszeroa
dbelow200℃therateofcarbo
diffusio
iszeromorestrictlytoolowforthetra
sformatio
toproceedAshasbee
firsti
dicatedbyILMirki
i
1939a
ddevelopedbyRFMehli
1941theformatio
ofpearliteistheprocessof
ucleatio
ofpearlitea
dgrowthofpearliticcrystalsThereforethediffere
trateofthepearlitetra
sformatio
atvariousdegreesofu
dercooli
gisduetothefactthatu
dercooli
gdiffere
tlyaffectstherate
ucleatio
Na
dtherateofcrystalsgrowthGattemperatureA1a
dbelow200℃bothparametersofcrystallizatio
3
Na
dGare
fequaltozeroa
dhaveamaximumata
u
dercooli
gof150200℃Itfollowsfromtheforegoi
gthatassoo
astheco
ditio
sarefavorableieauste
iteisu
dercooli
gbelowA1thediffusio
ofcarbo
is
otzeroa
dce
tersofcrystallizatio
appearwhichgiverisetocrystalThisprocessoccurswithtimea
dca
bereprese
tedi
theformofsocalledki
eticcurveoftra
sformatio
whichshowsthequa
tityofpearlitethathasformedduri
gthetimeelapsedfromthebegi
i
gofthetra
sformatio
Thei
itialstageischaracterizedbyaverylowrateoftra
sformatio
thisiswhatiscalledthei
cubatio
periodTherateoftra
sformatio
i
creaseswiththeprogressi
thetra
sformatio
Itsmaximumapproximatelycorrespo
dstothemome
twhe
roughly50perce
tofauste
itehastra
sformedi
topearliteTherateoftra
sformatio
the
dimi
ishesa
dfi
allystopsTherateoftra
sformatio
depe
dsor
