Linkage between mechanical properties
and phase transformations
in austenitic stainless steels
Ph.D. candidate
David Marechal
Scientific Supervision
Chad Sinclair (UBC)
Industrial support
Jean-Denis Mithieux (Ugine&ALZ)
Valerie Kostoj (Ugine&ALZ)
1
Context
Austenitic Stainless Steels for structural applications
high mechanical strength
and excellent formability
Space frame A-pillar
Hydroformed 304
reduce weight of cars and
improve crashworthiness
Body-in-white B-pillar
304
LPG tank
301L
2
Context
• Constitutional laws needed for forming.
• These are not well identified for austenitic
stainless steels
– Partially due to the strain-induced g -> a’
transformation that occurs during forming.
– Coupling between plasticity and phase
transformation.
3
1. Current understanding of
deformation mechanisms
4
Deformation mechanisms in austenite
Plates of e martensite
(hcp)
Rousseau et al., 1970
g -> e -> a’
Nuclei of a’ martensite
(bct)
Spencer, Ph.D. thesis, McMaster University, 2004
g -> a’
5
Questions
• Scale of microstructure
• Mode of deformation
?
• Kinetics
– Nucleation
– Growth
• Mechanical properties
6
Influence of grain size
• In general, kinetics is accelerated for coarse grains.
• However, lack of experiments below 50mm.
• What happens for submicron grains ?
1400
True stress (MPa)
1200
40mm
0.2mm 0.6mm
2mm
1000
800
600
400
200
0
0.0
0.1
0.2
0.3
0.4
0.5
True strain
AISI
at measured
-50°C, measured
by XRD
AISI 304
at 304
298K,
by Ferritescope
Deetetal.,
al.,1994
2006
Varma
FE-C-Cr-Ni-Mn deformed at 298K
Jin et al., 2007
7
Influence of mode of deformation
• Two components :
• strain path
• stress state
• g -> a’ transformation accompanied by volume expansion.
• High triaxiality favours formation
of a’ martensite. (Stringfellow et al.,
1991)
•Tension assists g -> a’
transformation more than
compression.
• Shear components also
important.
8
Influence of mode of deformation
• Limited amount of data. Does not allow full understanding.
• Stringfellow theory based on hydrostatic component.
• However, there are cases where a’ dominates in compression.
AISI 304 deformed at 77 K
AISI 304 deformed at 298 K
Lebedev et al., 2000.
Iwamoto et al., 1998.
9
Summary
g -> a’ transformation contributes to increased W-H in
austenitic stainless steels.
• Nucleation of a’ motivated by intersection of e plates.
• Lack of data for the growth of a’.
• Besides temperature and strain rate, transformation
affected by:
– grain size
– strain path
– stress state
•
10
2. Outline on current
research
11
Material studied
• Grade AISI 301LN, sheet samples
Fe
C
Cr
Ni
Mn
wt. % Bal. 0.022 17.33 6.62 1.77
Si
Cu
N
0.53
0.24
0.11
• Low C, low Ni and high N reinforce low stability of
austenite and low SFE.
• High levels of a’ (up to 70%) formed upon Room
Temperature straining.
• Because of low C, a’ has a nearly bcc-structure.
12
Generation of grain size
Temperature
Annealing
750 to 1050°С
3 or 30 min
Cryorolling
+
+
Cooling
down
in air
Average grain size (mm)
With twins
Without twins
Partially recrystallized
Procedure
CR-750-3min
0.47
0.56
CR-800-3min
0.92
1.4
CR-850-2min
2.2
2.4
CR-950-3min
13.8
18
CR-1050-3min
28
33
RT-1050-30min
Time
0.47 mm
28 mm
13
Characterization of tensile response
1600
True stress (MPa)
1400
1200
0.47mm
0.92mm
1000
800
600
27.9mm
400
2.2mm
13.8mm
200
0
0.0
0.1
0.2
0.3
True strain
0.4
0.5
14
Characterization of a’ content
70
0.47mm
0.92mm
2.2mm
13.8mm
27.9mm
50
27.9mm
• Good reproductibility.
0.47mm
• Non-monotonic trend of
the rate of transformation
towards grain size
13.8mm
40
0.92mm
30
20
10
2.2mm
0
0.0
0.1
0.2
0.3
True strain
0.4
Max rate of transformation
a' volume fraction (%)
60
• a’ fraction measured with
a Ferritescope.
27.9mm
3.2
3.0
13.8mm
2.8
0.5
0.47mm
0.92mm
2.6
2.2mm
2.4
1
10
Grain size (mm)
15
Deformation microstructure
1) High-resolution EBSD
0.47 mm deformed at e=20%
ND
Austenite
(grey)
RD
28 mm deformed at e=10%
a’ martensite
(colored) 16
Deformation microstructure
2) TEM
0.4 mm deformed at e=20%
25 mm deformed at e=5%
1mm
(110)a’
(211)a’
1mm
17
Preliminary ideas for modelling
•
Need:
– 1. Behaviour of austenite
• Vocce law from experiment
– 2. Behaviour of a’
• Best fit assuming Vocce
law
– 3. Transformation kinetics
• From experiment
– 4. Law of mixtures
• Equal strain in both phases
• stot = f sa’ + (1-f) sg
– 5. Need physical
understanding:
• Why is a’ behaviour
independent of austenite
grain size ?
18
1500
Neutron diffraction on 316
Spencer, Ph.D. thesis, 2004
1000
g
500
1600
0.1
0.2
True strain
0.3
0.4
1400
Model
Experiment
1200
1000
800
70
0.47mm
0.92mm
2.2mm
13.8mm
27.9mm
600
60
400
200
0
0.0
0.1
fraction (%)
0
0.0
True stress (MPa)
True stress (MPa)
2000
Preliminary ideas for modelling
a'
50
0.240
True strain
0.3
19
0.4m
13.8
Work planned
• What is the mechanical behaviour of a’ ?
– Neutron diffraction
• Need to understand the nucleation / growth of a’
– TEM / EBSD
• Effect of strain path / stress state
– Extension to other paths of deformation (i.e. pure
shear & plane strain tension)
Oct
Jan
Apr
2007
Jul
Oct
Jan
Apr
2008
Jul
Oct
Jan
2009
Apr
cterization
testing
FEG-SEM
TEM
XRD phase quantification
Shear testing
FEG-SEM
TEM
Plane strain testing
FEG-SEM
TEM
Neutron diffraction
Literature review
20
Modelling
Thesis redaction
Conclusion
• Poor knowledge of kinetics / mechanical response
towards :
– grain size
– mode of deformation
• Wide range of grain size has been generated
(0.4 to 30mm)
• Experiments in uniaxial tension
• Preliminary simple model --> encouraging.
• Provide physical understanding to problem.
• Plan to extend to other deformation modes.
21