MATLS 791 - Course Notes

Report
Graduate Seminar I
Compositionally Graded High Manganese Steels
by
Morteza Ghasri
Supervisor:
Prof. McDermid
Nov. 18, 2011
Presentation Outline
 Introduction
 Literature Review
 Project Objectives
 Experimental Method
 Preliminary Results
 Plan for Future Work
2
Introduction
Typical mechanical properties of several classes of steels
W. Bleck: International Conference on TRIP-Aided High Strength Ferrous
Alloys, Ghent, Belgium 2002, p. 13-23
3
History of high Mn steels
 Hadfield steels were invented in 1882.
They had 13 wt. % Mn and 1.2 wt. % C.
● New class of modern high Mn steels
contain 18-30 wt. % Mn, 0-0.7 wt. % C,
and up to 1-2 wt. % (Al, Si)
Sir Robert Hadfield
1858-1940
4
Literature Review
High Mn steels can be divided into:
 Twinning Induced Plasticity (TWIP)
 Transformation Induced Plasticity (TRIP)
5
Stacking Fault Energy
Stacking fault formation
1. Dissociation of a perfect dislocation
a
a
a
[ 1 01]  [ 211]  [ 1 1 2]
2
6
6
2. Equilibrium between two partial dislocations
b 2
d
4
d: the equilibrium separation between partials
b: the magnitude of the Burger’s vector
μ: shear modulus
γ: stacking fault energy
6
SFE dependence of deformation
products
Deformation structures of Fe20Mn-4Cr-0.5C as a function
of both temperature and SFE
Deformation structures of different
alloys observed near room
temperature as a function of SFE
L. Remy et al., Materials science and
Engineering, Vol. 28, pp. 99-107, 1977
L. Remy et al., Materials Science and
Engineering, Vol. 26, pp. 123-132, 1976
7
SFE dependence of deformation
products (cont’d)
The calculated iso-SFE lines in the carbon/manganese (wt.%) map at 300K
S. Allain et al., Materials Science and Engineering A, Vol. 387-389, pp. 158-162, 2004
8
SFE dependence of deformation
products (cont’d)
The calculated iso-SFE contours in Fe-Mn-C system at 298 K with
martensite boundaries
J. Nakano et al., CALPHAD, Vol. 34, pp. 167-175, 2010.
9
Fe-30Mn-0C alloy
 Minor ε-martensite
for εT<0.3
Evolution of ε-martensite phase volume
fraction with plastic strain in Fe-30Mn-0C alloy
Xin Liang, Master’s thesis, McMaster University, 2008.
10
Fe-30Mn-0C alloy
 Dislocation cell structure with
significant transformation products
no
 Indicates that dislocation glide is the
dominant deformation mechanism at 298 K
BF image of well-developed
cell structures in one grain
Xin Liang, Master’s thesis, McMaster
University, 2008.
11
Fe-22Mn-C alloys
 Eileen Yang decarburized an
Fe-22Mn-0.6C alloy to obtain
homogenous 0.2 C and 0.4 C
alloy.
 Mechanical properties varied
significantly with alloy carbon
content.
Tensile behavior of Fe-22Mn alloys with
different carbon content.
Eileen Yang, Master’s thesis, McMaster University,
2010
12
Fe-22Mn-C alloys
 0.6 C alloy………TWIP
 0.2 C alloy……….TRIP
Evolution of ε-martensite phase volume
fraction with plastic strain for all alloys
Eileen Yang, Master’s thesis, McMaster University, 2010
13
Strain Hardening
Isotropic Strain Hardening
• The mechanical response is symmetric after a change of strain path
from pure tension to pure compression and vice versa.
• The Kocks-Mecking model considers only this type of strain hardening.
Kinematic Strain Hardening
• The mechanical behaviour becomes asymmetric after a change of
strain path from pure tension to pure compression.
• This occurs in addition to isotropic strain hardening.
• Kinematic strain hardening has a significant contribution to
overall hardening in high Mn steels.
14
Project Objectives
1. Producing compositionally graded high manganese steels.
2. Microstructural evolution and mechanical properties of produced
alloys.
3. Modeling of mechanical properties
The rule-of-mixture approximations
Continuum finite element formulation of the constitutive phases
15
Experimental alloys
1. Fe-30Mn-0C alloy will be carburized to
obtain carbon gradient from 0 wt. % at
the core to 0.6 wt. % at the surface.
Fe-30Mn-0.6C
Fe-30Mn-0C
Fe-30Mn-0.6C
2. Fe-30Mn-0.6C alloy will be decarburized
to obtain carbon gradient from 0 wt. %
at the surface to 0.6 wt. % at the core.
Fe-30Mn-0C
Fe-30Mn-0.6C
Fe-30Mn-0C
16
Experimental Alloys (cont’d)
3. Fe-22Mn-0.6C alloy will be decarburized
to obtain carbon gradient from 0 wt. %
at the surface to 0.6 wt. % at the core.
Fe-22Mn-0C
Fe-22Mn-0.6C
Fe-22Mn-0C
17
Experimental Method
Carburizing and Decarburizing Heat Treatment
• A gas mixture of CO/CO2 was used for carburizing the Fe30Mn-0C alloy. The gas mixture was then replaced by CH4/H2.
• Fe-22Mn-0.6C alloy was decarburized by CO/CO2.
•The experiments were carried out at 1000 and 1100 °C.
Mico-Hardness Measurements
• To evaluate the distribution of carbon within the cross section of
carburized and decarburized samples.
18
Experimental Method (cont’d)
Characterization Techniques
• Carbon and sulfur combustion analysis
• Scanning Electron Microscopy (SEM) with Energy Dispersive
Spectroscopy (EDS)
• Electron BackScattered Diffraction (EBSD)
• X-Ray Diffraction (XRD)
• Transmission Electron Microscopy (TEM)
19
Preliminary Results
1. Carburization of Fe-30Mn-0C alloy
Illustration of micro-hardness profile
after carburizing at 1100°C under a
CO/CO2 ratio of 30 for 4 and 7 hours.
The calculated CO/CO2 ratio required for
carburization was 16.
Significant increase in hardness was only
observed at 50 µm or less from the
surface.
20
Fe
Mn
O
EDS map of cross section of Fe-30Mn-0C alloy after
carburizing for 7 h at 1100 °C.
21
XRD pattern of 7 h-carburized sample.
22
Thermodynamic Aspects
1
CO2  CO  O2
2
G1  282400 86.81T
PO2  [(
PCO2
PCO
) K1 ]  [(
2
PCO2
PCO
G1 2
) exp(
)]
RT
The oxygen partial pressure in the furnace is calculated to be
4.24×10-16 atm when T=1373 K and CO/CO2 =30.
1
Mn  O2  MnO
2
PO2  (aMn .K 3 )
G3  388900 76.32T
2
G3 2
 [aMn . exp(
)]
RT
The oxygen partial pressure required for manganese oxidation of Fe30Mn-0C is calculated to be 3.34×10-21 atm.
23
2. Carburization of Fe-30Mn-0C alloy using CH4/H2
CO/CO2 gas mixture was replaced by CH4/H2 mixture to prevent MnO
formation.
Methane decomposition leads to carburization
CH 4  C  2H 2
Oxygen as impurity in methane leads to MnO formation.
Ti wire was used to lower the oxygen potential.
24
Illustration of micro-hardness profile after
decarburizing at 1000°C under CO/CO2
ratios of 6 and 1 for 4 hours.
300
Microhardness (HV)
3. Decarburization of Fe-22Mn-0.6C
alloy
250
200
150
CO/CO2 ratio=6
100
50
0
0
The carbon content of decarburized
samples decreased from 0.40 wt. % to 0.20
wt. % when the CO/CO2 decreased from 6
to 1.
400
600
800
1000 1200 1400 1600
Depth (µm)
300
Microhardness (HV)
The high amount of hardness at 50 μm
below the surface is attributed to MnO
formation.
200
250
200
150
CO/CO2 ratio=1
100
50
0
0
200
400
600
800
1000 1200 1400 1600
Depth (µm)
25
Thermodynamic Aspects
The oxygen partial pressure in the furnace is calculated to be
2.17×10-16 atm when T=1273 K and CO/CO2 = 6.
The oxygen partial pressure required for manganese oxidation of
Fe-22Mn-0.6C is calculated to be 2.36×10-23 atm.
26
Plan for Future Work
27
Conclusion
 MnO layer on high Mn steels prevents carbon diffusion into the
sample, but it has no significant effect on decarburization.
28
Acknowledgement
• Prof. McDermid
• Dr. Zurob
• Doug Culley
• Chris Butcher
• Tom Zhou
• Research Group Fellows
29

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