Transcript Document 7894042

Coarsening during tempering
0.8
Cementite size / µm
0.7
0.6
martensite
0.5
0.4
0.3
bainite
0.2
0.1
0.0
100
1000
1000
100000
Tempering time / s
Nam (1999)
Tempered martensite
Tempered bainite
Nam (1999)
Embrittlement
300
Impact energy / J
250
virgin
200
150
100
service
exposed
50
0
200
300
400
500
Test temperature / K
Wignarajah, Masumoto & Hara (1990)
two-stage treatment?
Temperature
720°C
500°C
Time
Role of matrix
microstructure ?
Martensite
750
M
M
C
C
23 6
23 6
M
+ M6 C
+ M 7 C3
Fe C + M C
3
2
+ M 7 C3
650
Fe 3C + M
2 C
550
Fe C
3
450
.1
1
10
Tempering time / h
100
1000
Tempering temperature / °C
750
Tempering temperature / °C
Bainite
Fe C + M C
3
2
M
C
23 6
C
23 6
+ M6 C
+ M 7 C3
+ M 7 C3
650
Fe 3C + M
2 C
550
Fe3 C
450
.1
1
10
100
1000
Tempering time / h
Baker & Nutting (1959)
800
a) 2.3Cr 1Mo
b) 4.3Cr 1Mo
c) 9.3Cr 1Mo
700
Temperature / °C
ferrite
600
a
500
b
400
bainite
c
300
martensite
200 0
10
10
1
10
2
10
3
10
4
10
Time / s
5
10
6
10
7
10
8
10
9
Mechanical Properties of Martensite
in Heat-Resistant Steels
• Why are modern steels martensitic?
• What makes martensite desirable?
• Can the martensite be optimised?
Why martensitic?
• Greater Cr needed for oxidation,
corrosion resistance
• Cr must be balanced by other
elements to avoid ∂-ferrite
• Therefore, greater hardenability.
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
0.00
M23C6
12CrMoVW
9CrMoWV
M2X
9Cr1Mo
3.5NiCrMoV
2.25CrMo
1CrMoV
Fraction
0.06
Laves
0.05
0.04
0.03
0.02
0.01
565 °C
12CrMoVNb
12CrMoVW
12CrMoV
9Cr0.5MoWV
Mod. 9Cr1Mo
9Cr1Mo
3.5NiCrMoV
3Cr1.5Mo
Mod. 2.25Cr1Mo
2.25Cr1Mo
0.6
1CrMoV
0.25CrMoV
Mole fraction
0.8
Cr
Mo
0.4
0.2
0.0
565 °C
12CrMoVNb
12CrMoVW
12CrMoV
9Cr0.5MoWV
Mod. 9Cr1Mo
9Cr1Mo
3.5NiCrMoV
3Cr1.5Mo
Mod. 2.25Cr1Mo
2.25Cr1Mo
1CrMoV
0.25CrMoV
Mole fraction Cr
Cr concentration in ferrite
0.12
0.10
0.08
0.06
0.04
0.02
0.00
565 °C
cr
aq
=
aq
c
+
2
aq
c s
kT r
a
V
aq
c
1 cqa- caq
Concentration
Coarsening reduced if last term small
cqa
caq
Distance
0.00
12CrMoVNb
12CrMoVW
12CrMoV
9Cr0.5MoWV
Mod. 9Cr1Mo
9Cr1Mo
3.5NiCrMoV
3Cr1.5Mo
Mod. 2.25Cr1Mo
2.25Cr1Mo
1CrMoV
0.25CrMoV
Stability parameter
Stability parameter =
aq
c (1
qa
aq
c -c
0.20
0.15
0.10
0.05
aq
c )
Creep rupture stress/ MPa
200
150
2.25Cr1Mo
100
50
9Cr1Mo
0
2
3
4
log(time/ h)
5
6
Comparison
• 0.15C-0.25Si-0.50Mn-2.3Cr-1Mo0.10Ni
• 0.10C-0.60Si-0.40Mn-9.0Cr-1Mo0.00Ni
• 1056 °C for 12 h, 740 °C for 13 h