Heat Treatment of Plain Carbon Steels

1
Heat Treatment of Plain Carbon Steels
2
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 g a Fe3C
• Note time when 1 pearlite has formed
• Note time when 99 pearlite has formed
• Plot on Temp.-time transformation diagram

3
727ºC
x
x
x
x
Pearlite
Temp
gP
x
x
g
x
x
x
x
Bainite
Nose
gB
x
x
x
x
x
x
Ms
ga
Mf
a(marteniste)
time
4
727ºC
coarse
Temp
gP
Pearlite
fine
g
upper
gB
Bainite
lower
Ms
ga
Mf
a(martensite)
time
5
Properties of phases

• Properties of pearlite
• Fairly tough
• Fairly ductile
• Depends on size of lamellae

• Properties of bainite
• Tough
• Structure between pearlite
• and marteniste (a Fe3C)

• Properties of marteniste
• Very hard HRC 45-55
• Brittle
• bc tetragonal structure
• Supersaturated solution of C in Fe
• High residual stresses
• Metastable

6
Microstructures
Coarse pearlite
Fine pearlite
7
Microstructures
Upper Bainite
Lower Bainite
8
Microstructure
Martensite
9
Continuous cooling transformation diagram
727ºC
Coarseannealing (1ºC/min) in furnace
Temp
gP
Pearlite
g
Finenormalizing(10º C/min) I air
upper
gB
Bainite
isothermal
lower
Ms
ga
Mf
a(martensite)quenching (100º C/min) in oil or
water
time
10
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
• a a(low C martensite) e carbide
  (Fe2.3C)
• 2nd stage 200-350º C
• g bainite
• 3rd stage 350-500º C
• a e carbide a Fe3C(tempered
  martensite)