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)