Transcript Heat Treatment of Steels - Mechanical Engineering, UPRM

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 g
a + Fe3C
 Note time when 1% pearlite has formed
 Note time when 99% pearlite has formed
 Plot on Temp.-time transformation diagram
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727ºC
x
Temp
x
g
x
Nose
x
x
x
g+P x
x
g+B
x
g+a’
Pearlite
Bainite
x
x
Ms
Mf
x
x
x
x
x
a’(marteniste)
time
727ºC
Temp
g+P
g
g+B
coarse
fine
Pearlite
upper
Bainite
lower
Ms
Mf
g+a’
a’(martensite)
time
Properties of phases
Properties of pearlite
•Fairly tough
•Fairly ductile
•Depends on size of lamellae
Properties of marteniste
•Very hard HRC 45-55
•Brittle
•bc tetragonal structure
•Supersaturated solution of C in Fe
•High residual stresses
•Metastable
Properties of bainite
•Tough
•Structure between pearlite
and marteniste (a + Fe3C)
Microstructures
Coarse pearlite
Fine pearlite
Microstructures
Upper Bainite
Lower Bainite
Microstructure
Martensite
Continuous cooling transformation diagram
727ºC
Temp
Coarse:annealing (1ºC/min)
in furnace
g+P
Pearlite
g
Fine:normalizing(10º C/min) I air
upper
isothermal
Ms
Mf
g+B
Bainite
lower
g+a’
a’(martensite):quenching (100º C/min) in oil or water
time
Tempering of martensitic steels
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Martensite is too brittle to serve engineering purpose
Tempering carried out to increase toughness of martensite
Carried out by reheating martenistic steel between 150500º 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)