Zhenshan Cui

Modeling and simulation on dynamic recrystallization of 30Cr2Ni4MoV rotor steel using the cellular automaton method

The cellular automaton (CA) method coupling fundamental metallurgical principles was used to simulate the initial microstructure and dynamic recrystallization (DRX) of 30Cr2Ni4MoV rotor steel. For the initial microstructure generation, reasonable transformation rules were established based on the thermodynamic mechanism, the activation energy and the curvature-driven mechanism. For the purposes of obtaining the material constants which were used in the CA model for DRX, including initial grain size, nucleation rate, softening parameter and activation energy, the hot deformation characteristics of 30Cr2Ni4MoV rotor steel were investigated by uniaxial hot compression tests on Gleeble-3500 machine. The effect of a wide range of thermomechanical processing parameters (temperature and strain rate) on the nucleation rate, the percentage of DRX and the final grain size were investigated. By comparison of the flow stress–strain curves and the metallographs, it was shown that the CA model coupling fundamental metallurgical principles can accurately simulate the microstructural evolution and the plastic flow behavior for 30Cr2Ni4MoV rotor steel at various deformation parameters.

Identification of nucleation parameter for cellular automaton model of dynamic recrystallization

Abstract The accuracy of nucleation parameter is a critical factor in the simulation of microstructural evolution during dynamic recrystallization (DRX). Based on the flow stress curve under hot deformation conditions, a new approach is proposed to identify the nucleation parameter during DRX. In this approach, a cellular automaton (CA) model is applied to quantitatively simulate the microstructural evolution and flow stress during hot deformation; and adaptive response surface method (ARSM) is applied as optimization model to provide input parameters to CA model and evaluate the outputs of the latter. By taking an oxygen-free high-conductivity (OFHC) copper as an example, the good agreement between the simulation results and the experimental observations demonstrates the availability of the proposed method.

Mesoscale simulation of microstructure evolution during multi-stage hot forging processes

The paper presents a two-dimensional cellular automaton (CA) approach coupled with a topology deformation technique for quantitative and topographic prediction of the microstructure evolution during multi-stage hot forging processes. The simulation presented in this work was implemented by an in-house developed C++ program. The grain topography, recrystallization fraction and average grain size were also obtained during a four-hit forging process. The simulated results agree well with the experimental data in terms of average grain size, suggesting that the developed CA model is a reliable numerical approach for predicting microstructure evolution for ultra-super-critical rotor steel during multi-stage hot forging processes.

Constitutive Modeling for Elevated Temperature Flow Behavior of 30Cr2Ni4MoV Ultra-super-critical Rotor Steel

In order to perform numerical simulation of forging and determine the hot deformation processing parameters for 30Cr2Ni4MoV steel, the compressive deformation behaviors of 30Cr2Ni4MoV steel were investigated at the temperatures from 970 to 1270 °C and strain rates from 0.001 to 0.1 s−1 on a Gleeble-3500 thermo-mechanical simulator. The flow stress constitutive equations of the work hardening-dynamical recovery period and dynamical recrystallization period were established for 30Cr2Ni4MoV steel. The stress-strain curves of 30Cr2Ni4MoV steel predicted by the proposed model well agreed with experimental results, which confirmed that the proposed equations can be used to determine the hot deformation processing parameters for 30Cr2Ni4MoV steel.

Static Recrystallization Behavior of SA508-III Steel during Hot Deformation

The static recrystallization behavior of SA508-III steel was investigated by isothermal double-hit hot compression tests at the deformation temperature of 950–1250 °C, the strain rate of 0.01–1 s−1 and the inter-pass time of 1–300 s. The effects of deformation parameters, including forming temperature, strain rate, degree of deformation (pre-strain) and initial austenite grain size, on the softening kinetics were analyzed. Experimental results show that static recrystallization kinetics is strongly dependent on deformation temperature and degree of deformation, while less affected by the strain rate and initial grain size. The kinetics and microstructural evolution equations of static recrystallization for SA508-III steel were developed to predict the softening behavior and the statically recrystallized grain size, respectively. Based on the comparison between the experimental and predicted results, it is found that the established equations can give a reasonable estimate of the static softening behavior for SA508-III steel.

Numerical Simulation of Microstructure Evolution for SA508-3 Steel during Inhomogeneous Hot Deformation Process

Based on hot compression tests by a Gleeble-1500D thermo-mechanical simulator, the flow stress model and microstructure evolution model for SA508-3 steel were established through the classical theories on work hardening and softening. The developed models were integrated into 3D thermal-mechanical coupled rigid-plastic finite element software DEFORM3D. The inhomogeneous hot deformation (IHD) experiments of SA508-3 steel were designed and carried out. Meanwhile, numerical simulation was implemented to investigate the effect of temperature, strain and strain rate on microstructure during IHD process through measuring grain sizes at given positions. The simulated grain sizes were basically in agreement with the experimental ones. The results of experiment and simulation demonstrated that temperature is the main factor for the initiation of dynamic recrystallization (DRX), and higher temperature means lower critical strain so that DRX can be facilitated to obtain uniform fine microstructure.

Regularized determination of interfacial heat transfer coefficient during ZL102 solidification process

Abstract The interfacial heat transfer coefficient(IHTC) between the casting and the mould is essential to the numerical simulation as one of boundary conditions. A new inverse method was presented according to the Tikhonov regularization theory. A regularized functional was established and the regularization parameter was deduced. The functional was solved to determine the interfacial heat transfer coefficient by using the sensitivity coefficient and Newton-Raphson iteration method. The temperature measurement experiment was done to ZL102 sand mold casting, and the appropriate mathematical model of the IHTC was established. Moreover, the regularization method was used to determinate the IHTC. The results indicate that the regularization method is very efficient in overcoming the ill-posedness of the inverse heat conduction problem(IHCP), and ensuring the accuracy and stability of the solutions.

Modeling of Austenite Grain Growth During Austenitization in a Low Alloy Steel

The main purpose of this work is to develop a pragmatic model to predict austenite grain growth in a nuclear reactor pressure vessel steel. Austenite grain growth kinetics has been investigated under different heating conditions, involving heating temperature, holding time, as well as heating rate. Based on the experimental results, the mathematical model was established by regression analysis. The model predictions present a good agreement with the experimental data. Meanwhile, grain boundary precipitates and pinning effects on grain growth were studied by transmission electron microscopy. It is found that with the increasing of the temperature, the second-phase particles tend to be dissolved and the pinning effects become smaller, which results in a rapid growth of certain large grains with favorable orientation. The results from this study provide the basis for the establishment of large-sized ingot heating specification for SA508-III steel.

Ductile Fracture Prediction of 316LN Stainless Steel in Hot Deformation Process

A ductile fracture criterion of 316LN stainless steel, combined with the plastic deformation capacity of material and the stress state dependent damages, was proposed to predict ductile fracture during hot deformation. To the end, tensile tests at high temperatures were first performed to investigate the fracture behavior of 316LN stainless steel. The experimental results show the variation of the critical fracture strain as a function of temperature and strain rate. Second, the criterion was calibrated by using the upsetting tests and the corresponding numerical simulations. Finally, the proposed fracture criterion was validated by the designed tests and the corresponding finite element (FE) simulation. The results show that the criterion can successfully predict the onset of ductile fracture at elevated temperatures.

Numerical Simulations of Open-Die Forging Process for Manipulator Design

Numerical simulations for open-die forging process are conducted with different values of manipulator kinematic stiffness and press velocity. The influences of manipulator kinematic stiffness and press velocity on acceleration of jaw end are investigated. In contact and separation stages of a forging operation, acceleration of jaw end is nearly proportional to acceleration of the contact surface between upper die and workpiece; in deformation stage, acceleration of jaw end does not decrease as manipulator kinematic stiffness increases and it increases with press velocity. The ideal kinematic stiffness of manipulator should be close to zero, which leads to minimized forces exerted on the manipulator during deformation stage. The conclusion in this paper can be a reference for the design of manipulator.