Mathematical model of phase transformations and elastoplastic stress in the water spray quenching of steel bars
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Abstract
A mathematical model, based on the finite-element technique and incorporating thermo-elasto-plastic behavior during the water spray quenching of steel, has been developed. In the model, the kinetics of diffusion-dependent phase transformation and martensitic transformation have been coupled with the transient heat flow to predict the microstructural evolution of the steel. Furthermore, an elasto-plastic constitutive relation has been applied to calculate internal stresses resulting from phase changes as well as temperature variation. The computer code has been verified for internal consistency with previously published results for pure iron bars. The model has been applied to the water spray quenching of two grades of steel bars, 1035 carbon and nickel-chromium alloyed steel; the calculated temperature, hardness, distortion, and residual stresses in the bars agreed well with experimental measurements. The results show that the phase changes occurring during this process affect the internal stresses significantly and must be included in the thermomechanical model.
Keywords
Ferrite Austenite Martensite Residual Stress Metallurgical TransactionNomenclature
- b
kinetic constant in Avrami equation, s-1
- B
nozzle diameter, mm
- [B]
strain-displacement matrix, m-1
- C
specific heat, J kg-1 °C-1
- [C]
heat capacitance matrix, J °C-1
- {d}
nodal displacement vector, m
- [D]
material properties matrix, MPa
- [Dp] [D]
matrix for plasticity, MPa
- [D] [D]
matrix for elasticity, MPa
- f
von Mises yield function, MPa2
- {f1},{f2}
elements of force vector, MPa {f3}, {f4}
- {f}
heat flow vector, W
- h
heat-transfer coefficient, W m-2 °C-1
- H
nozzle to object distance, mm
- k
thermal conductivity, W m-1 K-1
- Ki
empirical constant in Eq. [10], MPa mm mm-1
- [K]
conductance matrix, W °C-1
- [K]
stiffness matrix, MPa m-1
- Ms
martensite start temperature, °C
- n
kinetic constant in Avrami equation
- n
vector normal to the surface of a boundary, m
- [N]
shape function
- Nu
Nusselt number
- Pr
Prandtl number
- Q
heat released per unit time and volume, Wm-3
- Re
Reynolds number
- r
radial coordinate, m
- R
radius, m
- rmin
fraction of ΔT to allow only one element to yield
- Se
boundary of a finite element, m
- t
time, s
- tAvCCT
transformation start time under continuous cooling, s
- Δt
time step, s
- ΔH
enthalpy of transformation, J kg-1
- T
temperature, °C
- Tc
ambient temperature, °C
- T0
initial temperature, °C
- U
velocity, m s-1
- Ve
volume of a finite element, m3
- X
fraction transformed
- z
axial coordinate, m
- α
thermal expansion coefficient, m m-1 °C-1
- β
transformation expansion coefficient, m m-1
- λ
positive constant defined by Eq. [B9]
- ξi
volume fraction of phasei
- ρ
density, kg m-3
- Φ
vector of nodal temperatures, °C
- σ
stress, MPa
- θ
parameter used to discretize Eq. [A1]
- {gs’}
deviatoric stress, MPa
- {dσ}
stress increment, MPa
- {dε}
total strain increment, m m-1
- {dεo}
initial strain increment, m m-1
- {dεe}
elastic strain increment, m m-1
- {dεth}
thermal strain increment, m m-1
- {dεtr}
transformation strain increment, m m-1
- {dεp}
plastic strain increment, m m-1
- {dεtp}
transformation plasticity strain increment, m m-1
- {dεth’}
elastic strain increment due to variations in mechanical properties as function of temperature, mm-1
- {dεtr’}
elastic strain increment due to variations in mechanical properties as function of phase composition, m m-1
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