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Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Stainless Steel
GIFT 672
Lecture 9
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Why are Precipitation Hardened (PH) stainless steels used?
F
F
F
F
φ
/2
TT
F
Precipitates act as
strong obstacles to
dislocation motion in
the lattice
dislocation
Precipitates
Formed during
aging
p
pr
f
σ∝
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Three types of Precipitation Hardened (PH) stainless steels
Martensitic PH stainless steels:
14-17% Cr, 3.5-5% Ni, 0.05-0.07% C
Mf~RT: Austenite transformed to martensite after air cooling
Precipitation hardening by 2.5-5.0% Cu, 0.15-0.45% Nb
Semi-austenitic or “Controlled Transformation” PH stainless steels:
14-18% Cr, 4-8% Ni, 0.09-0.15% C
Ms<RT: Austenite
⇒
martensite AFTER CONDITIONING TREATMENT
Precipitation hardening by 2.5-5.0% Cu, 0.15-0.45% Nb
Austenitic PH stainless steels or “Fe-base Superalloys”:
13-17% Cr, 24-30% Ni, 0.02-0.08% C
Ms<<<RT: Austenite NEVER transforms to martensite
Precipitation hardening by 2.0%Ti, 0.25%Al, 0.25-0.50%V, 15-100ppm B
Lower Ms-Mf temperature
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
EN UNS C
ppm
Mn Si Cr Ni Mo Nb Cu
0.3
-
-
4.0
-
-
Ti
1.4542
1.4568
-
-
-
2.1
0.5
0.5
1.0
400
500
400
S 17400
S 17700
S 66286*
0.5
0.5
0.5
16.50
17.00
14.75
-
-
1.25
4.0
7.0
25.5
Al
-
1.00
0.20
AISI 631
17-7 PH
UNS S 17700
X7CrNiAl 17-7
1.4568
AISI 630
17-4 PH
UNS S 17400
X5CrNiCuNb 16-4
1.4542
S 66268 (A-286)
X6NiCrTiMoVB 25-15-2
-
+Ni
+Mo
+Ti
Martensitic Semi-austenitic Austenitic
*: 0.3% V, 50ppm B
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Precipitates formed during annealing:
UNS S 17700
X7CrNiAl 17-7
1.4568
UNS S 17400
X5CrNiCuNb 16-4
1.4542
A-286
X6NiCrTiMoVB 25-15-2
-
Martensitic
Cu-rich f.c.c. phase
Alternatively (other alloys):
NiAl, Ni3Al, Laves phase
Semi-austenitic
NiAl
Ni3Al (
γ
’, gamma prime)
Alternatively (other alloys):
Cr2N
Austenitic
Ni3(Al,Ti)
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Choice of the composition range for austenitic PH stainless steel
Fe25%Ni15%Cr
Fe-based superalloy
(Gas turbine)
γ
γ
+ Ni3Ti + Ni3(Al,Ti)
γ
+ Ni3Ti
800 deg C
Nutting
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Choice of the composition range for martensitic PH stainless steels
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
300
400
500
600
700
800
900
Mass-% Cu
α
+ Cu
α+γ
α
γ
Temperature, °C
(3) age-hardening
(2) quenching
(1) solution treatment
Cu solubility at
aging temperature
is extremely low
Cu solubility at
solution temperature
is high
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Thermal cycles
Martensitic PH Steel
17-4 PH
Ms=132C, Mf=32C
Ms
Mf
RT
Temperature
Time
Aging:
1-4hrs
480-620 deg C
Semi-austenitic PH Steel
Ms
Mf
Conditioning
1. 930-955 deg C: less carbon is precipitated
2. 730-760 deg C: C, alloying elements form precipitates
3. Cold forming: strain-induced transformation
Time
Ms
Mf
1050
Room temperature austenite
is formable
1
2
2
Ms
Mf 1
Aging:
1-3hrs
455-565 deg C
Room temperature
Quenched martensite
is soft (low C content) Cryogenic cooling
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Thermal cycles
Austenitic PH Steel:
Very low Ms temperature
Remain fully austenitic
Ms<<RT
Mf
Time
Slow precipitation in austenite
Aging:
16-20hrs
700-800 deg C
RT
Temperature
900-980C
1-2 hrs
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Thermal cycles
Type of PH SS Annealing
Quenching
Conditioning Age-hardening
Martensitic
Semi-austenitic
Austenitic
γ ⇒ α
’
γ
γ
−
γ ⇒ α
’
−
Hardened
α
’
Hardened
α
’
Hardened
γ
Ferrite
stringers
Ferrite
stringers
17-4 PH
Annealing: 1040 deg C
⇒
Aging: 482 deg C
17-47 PH
Annealing: 955 deg C
⇒
Conditioning: -73 deg C
⇒
Aging: 510 deg C
Lippold
Kotecki
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of Precipitation-hardened stainless steels
Aging kinetics
Semi-austenitic
480deg C 510deg C
Martensitic
ASM
Martensitic
Semi-austenitic
Overaging
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Cu precipitate in 15-5 PH
Martensitic PH stainless steel
Bcc matrix {1 1 0} plane resolved
Cu precipitates in the specimen aged for 64 h at 600 °C
Dr. H. R. Habibi Bajguirani
5nm 100nm
Physical metallurgy of precipitation hardened stainless steels
Precipitates formed during annealing:
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology
Physical metallurgy of precipitation hardened stainless steels
Limited Use:
Martensitic PH stainless steels:
Combine high strength and corrosion resistance
Cost is high, due to the additional processing, small production volumes
Application areas: military, aircraft, aerospace
Austenitic PH stainless steels:
Compete with Ni-based superalloys
Can be used up to 650 deg C for continuous service
Application areas: turbines, compressors, Space Shuttle engine parts
Semi-austenitic PH stainless steels
Application areas: pressure vessels, aircraft frames
Advanced Degrees in Steel Science and TechnologyAdvanced Degrees in Steel Science and Technology