A Numerical Austenitization Kinetic Model Coupling the Effects of Diffusion,Thermodynamics and Mecha

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A Numerical Austenitization Kinetic
Model--Coupling the Effects of
Diffusion,Thermodynamics and Mechanics
Gang Shi Prof. T. Calvin Tszeng Prof. Philip
Nash

• OBJECTIVE
• Develop a microscopic model to simulate the
  austenitization process
• MOTIVATIONS
• The austenitization of steels remains poorly
  characterized.
• Great need for more reliable and accurate model.

Dec.16, 2002
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Existing Models

• 1. Experimental study
• Roberts 1943, Garcia 1981, Speich 1981
• Roosz 1983 isothermal transformation.
• Caballero 2001 constant heating rate
  transformation.
• 2. Theoretical Models
• Speich 1969 Hillert
  1971
  Akbay1993Atkinson 1995
• 3. Numerical Models
• Inoue 1987 2D finite difference model
• Srolovitz 1985, Jacot 1999 Monte-Carlo model
• Cellular automaton model

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Coupling Effects of Thermodynamics, Diffusion
and Mechanics

• Thermodynamics has been used to set up
  traditional nucleation theory.
• The austenitization process is well known as a
  diffusion controlled process.
• Strain Energy caused by misfit stresses of phase
  transformation and thermal stresses may play an
  important role.

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Numerical Model

• The image of initial pearlite microstructure.
• Temperature profile
• Elastoplastic properties
• Thermal expansion coefficients
• Interface tension of each phase
• Stress and strain relation
• Diffusion coefficient of each phase
• Initial stress field

Initial Pearlite

• Challenges
• Complex geometry
• Moving boundary

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FEM Technique on Multiphase
Assumed multiphase image over 400400 pixels
FEM elements automatically generated by our
program
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A Simple Problem-Cementite Dissolution at
926.85C
at at at
Temp926.85C,
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FEM solution-Moving B.D.

• Diffusion Equation
• Boundary Conditions
• Local Carbon Conservation at Boundaries
• Internal Grid Moving Strategy.

Normalized Parameters
Node Number150, Elements Number240
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FEM Cementite Dissolution
?
?
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FEM Cementite Dissolution
T0.28
T0
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FEM Carbon Homogenization
Maximum
T0.28
T0.7
0.77
Minimum
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Model Description
Given temperature Temp
Interlamellar spacing ?02L
FEM program
Initial Pearlite Temp gt913C
Carbon concentration
Cementite dissolution time
Carbon homogenizaton time
Kinetics data
Temp926.85C
The dependence of kinetics on the initial
pearlite morphological parameters changes at
different temperature region.
L0.1e-6(m)0.1(um)
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SUMMARY

• The existing kinetic models can be explained
  based on the understanding of the fundamental
  knowledge of thermodynamics and mechanics.
• It is possible to develop a general and more
  accurate numerical model by coupling the effects
  of diffusion, thermodynamics and mechanics.
• With the help of such a model, we hope to
• Get better understanding of the
  nucleation and the austenite grain growth
  phenomena of the austenitization for plain carbon
  steels
• Explain the experimental results and
  develop more general theoretical model.
• Predict the evolution of the
  microstructure during austenitization by cellular
  automaton method.

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Thank you!