Heat Treatment of Plain Carbon Steels
Isothermal Temperature-time Transformation Diagram
Let us do an experiment Choose an eutectoid steel Heat it to the austenite phase region (800˚ C) Quench to 727 ˚ C Observe transformation + Fe3C
Note time when 1% pearlite has formed Note time when 99% pearlite has formed Plot on Temp.-time transformation diagram
Temp
time
P
B
Pearlite
BainiteNose
’
’(marteniste)
727ºCx x
x x
x
x x
x
xx
x
xx
x
x xMs
Mf
Temp
time
P
B
Pearlite
Bainite
’
’(martensite)
727ºC
Ms
Mf
coarse
fine
upper
lower
Properties of phases
Properties of pearlite•Fairly tough•Fairly ductile•Depends on size of lamellae
Properties of bainite•Tough•Structure between pearliteand marteniste ( + Fe3C)
Properties of marteniste•Very hard HRC 45-55•Brittle•bc tetragonal structure•Supersaturated solution of C in Fe•High residual stresses•Metastable
Microstructures
Coarse pearlite Fine pearlite
Microstructures
Upper Bainite
Lower Bainite
Microstructure
Martensite
Continuous cooling transformation diagram
Temp
P
B
Pearlite
Bainite
’
’(martensite):quenching (100º C/min) in oil or water
727ºC
Ms
Mf
Coarse:annealing (1ºC/min)in furnace
Fine:normalizing(10º C/min) I air
upper
lower
time
isothermal
Tempering of martensitic steels
Martensite is too brittle to serve engineering purpose Tempering carried out to increase toughness of martensite Carried out by reheating martenistic steel between 150-
500º C 1st stage 150-200º C
’ ”(low C martensite) + carbide (Fe2.3C)
2nd stage 200-350º C
bainite 3rd stage 350-500º C
”+ carbide Fe3C(tempered martensite)