Tempering time s

1
Coarsening during tempering
martensite
Cementite size / µm
bainite
100 1000 1000 100000
Tempering time / s
Nam (1999)
2
Tempered martensite Tempered
bainite
Nam (1999)
3
Embrittlement
virgin
Impact energy / J
service
exposed
Test temperature / K
Wignarajah, Masumoto Hara (1990)
4
two-stage treatment?
720C
500C
Temperature
Time
5
Role of matrix microstructure ?
6
Martensite
Bainite
M C
M C
23 6
23 6
M C
M C
23 6
23 6
M C
M C
6
6
M C
7 3
M C
Fe C M C
7 3
3 2
M C
Fe C M C
7 3
3 2
M C
7 3
Fe C M C
Tempering temperature / C
Tempering temperature / C
Fe C M C
3 2
3 2
Fe C
Fe C
3
3
Tempering time / h
Tempering time / h
Baker Nutting (1959)
7
a) 2.3Cr 1Mo b) 4.3Cr 1Mo c) 9.3Cr 1Mo
ferrite
a
Temperature / C
b
bainite
c
martensite
Time / s
8
Mechanical Properties of Martensite in
Heat-Resistant Steels

• Why are modern steels martensitic?
• What makes martensite desirable?
• Can the martensite be optimised?

9
Why martensitic?

• Greater Cr needed for oxidation, corrosion
  resistance
• Cr must be balanced by other elements to avoid
  ?-ferrite
• Therefore, greater hardenability.

10
Conclusions

• If creep controlled by dislocation climb, then
  refine matrix grains
• Coarsening should be worse in high Cr steels
• For equivalent conditions 2.25Cr1Mo is better
  than 9Cr1Mo

11
Fraction
565 C
12
Mole fraction
565 C
13
Cr concentration in ferrite
Mole fraction Cr
565 C
14
2 caq s Va 1 - caq kT r
cqa- caq
craq caq
Coarsening reduced if last term small
qa
c
Concentration
aq
c
Distance
15
caq (1 - caq ) cqa- caq
Stability parameter
Stability parameter
16
200
150
2.25Cr1Mo
Creep rupture stress/ MPa
100
50
9Cr1Mo
0
2
3
4
5
6
log(time/ h)
17
Comparison

• 0.15C-0.25Si-0.50Mn-2.3Cr-1Mo- 0.10Ni
• 0.10C-0.60Si-0.40Mn-9.0Cr-1Mo-0.00Ni
• 1056 C for 12 h, 740 C for 13 h