Graduate Seminar I

Compositionally Graded High Manganese Steels

by

Morteza Ghasri

Supervisor: Prof. McDermid

Nov. 18, 2011

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Presentation OutlineIntroduction

Literature Review

Project Objectives

Experimental Method

Preliminary Results

Plan for Future Work

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Introduction

Typical mechanical properties of several classes of steelsW. Bleck: International Conference on TRIP-Aided High Strength Ferrous Alloys, Ghent,

Belgium 2002, p. 13-23

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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

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High Mn steels can be divided into:

Twinning Induced Plasticity (TWIP)

Transformation Induced Plasticity (TRIP)

Literature Review

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Stacking fault formation

1. Dissociation of a perfect dislocation

2. Equilibrium between two partial dislocations

d: the equilibrium separation between partials μ: shear modulusb: the magnitude of the Burger’s vector γ: stacking fault energy

Stacking Fault Energy

]211[6

]112[6

]011[2

aaa

4

2bd

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SFE dependence of deformation products

Deformation structures of Fe-20Mn-4Cr-0.5C as a function of both temperature and SFE

L. Remy et al., Materials science and Engineering, Vol. 28, pp. 99-107, 1977

Deformation structures of different alloys observed near room temperature as a function of SFE

L. Remy et al., Materials Science and Engineering, Vol. 26, pp. 123-132, 1976

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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

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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.

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Evolution of ε-martensite phase volume fraction with plastic strain in Fe-30Mn-0C alloy

Fe-30Mn-0C alloy

Xin Liang, Master’s thesis, McMaster University, 2008.

Minor ε-martensite for εT<0.3

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Fe-30Mn-0C alloy

Dislocation cell structure with no significant transformation products

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.

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Tensile behavior of Fe-22Mn alloys with different carbon content.

Eileen Yang, Master’s thesis, McMaster University, 2010

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.

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Fe-22Mn-C alloys

Evolution of ε-martensite phase volume fraction with plastic strain for all alloys

Eileen Yang, Master’s thesis, McMaster University, 2010

0.6 C alloy………TWIP

0.2 C alloy……….TRIP

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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.

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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

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Fe-30Mn-0.6C

Fe-30Mn-0C

Fe-30Mn-0.6C

Fe-30Mn-0C

Fe-30Mn-0.6C

Fe-30Mn-0C

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.

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.

Experimental alloys

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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-0.6C

Fe-22Mn-0C

Fe-22Mn-0C

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Experimental Method

Carburizing and Decarburizing Heat Treatment

• A gas mixture of CO/CO2 was used for carburizing the Fe-30Mn-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.

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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)

Experimental Method (cont’d)

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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.

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Fe

Mn O

EDS map of cross section of Fe-30Mn-0C alloy after carburizing for 7 h at 1100 °C.

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XRD pattern of 7 h-carburized sample.

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Thermodynamic Aspects

22 21OCOCO TG 81.862824001

2121 )]exp()[(])[( 22

2 RTG

PP

KPP

PCO

CO

CO

COO

The oxygen partial pressure in the furnace is calculated to be 4.24×10-16 atm when T=1373 K and CO/CO2 =30.

MnOOMn 221 TG 32.763889003

2323 )]exp(.[).(

2

RTGaKaP MnMnO

The oxygen partial pressure required for manganese oxidation of Fe-30Mn-0C is calculated to be 3.34×10-21 atm.

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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

Oxygen as impurity in methane leads to MnO formation.

Ti wire was used to lower the oxygen potential.

24 2HCCH

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3. Decarburization of Fe-22Mn-0.6C alloy

Illustration of micro-hardness profile after decarburizing at 1000°C under CO/CO2 ratios of 6 and 1 for 4 hours.

The high amount of hardness at 50 μm below the surface is attributed to MnO formation.

The carbon content of decarburized samples decreased from 0.40 wt. % to 0.20 wt. % when the CO/CO2 decreased from 6 to 1.

0 200 400 600 800 1000 1200 1400 16000

50

100

150

200

250

300

CO/CO2 ratio=6

Depth (µm)

Mic

roha

rdne

ss (H

V)

0 200 400 600 800 1000 1200 1400 16000

50

100

150

200

250

300

CO/CO2 ratio=1

Depth (µm)

Mic

roha

rdne

ss (H

V)

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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.

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Plan for Future Work

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Conclusion

MnO layer on high Mn steels prevents carbon diffusion into the sample, but it has no significant effect on decarburization.

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Acknowledgement

• Prof. McDermid

• Dr. Zurob

• Doug Culley

• Chris Butcher

• Tom Zhou

• Research Group Fellows