Numerical Simulation of Induction Hardening of Torsion Bar
In order to improve the fatigue performance of torsion bar, induction surface hardening is performed. The aim of the present study is to compare the experimental residual stress fields measured by X-ray diffraction with those predicted from finite element modeling of the whole induction surface hardening process. Temperature and residual stress distributions are affected by the component geometry, the material behavior and the induction treatment parameters. Based on the residual stress distribution and the fatigue loading, cracks can nucleate from the surface or below the hardened layer. Therefore, it is very important to determine the residual stress distribution and optimize the process, especially for safety critical engineering components. Results of present study show that the improvement of fatigue performance is due to development of both hard microstructure and compressive residual stress at the surface layer of torsion bar during induction hardening.
Expressions for predicting the residual stress in surface induction hardening of steel bars
A finite-element (FE) program is developed and used to analyse the distributions of residual stresses generated in the surface induction hardening of steel bars. It is found that there is no significant effect of the transformation latent heat and that the transformation induced plasticity (TRIP) reduces the compressive stress near the outer surface. The results with TRIP are in good agreement with experiment. Although many physical and mechanical parameters control the final microstructure and the residual stresses in the bar during surface induction hardening, the final residual stresses are mainly determined by the ratio of the hardening thickness to the radius of the bar and by the initial yield stress of the material. Based on a large number of FE numerical results, a set of simple formulae are given by use of the least-squares method. Instead of complicated FE analysis, the residual stresses in a bar induced by surface induction hardening can be easily predicted by use of these formulas from the final hardening thickness and the initial yield stress of the material.
MATHEMATICAL MODEL COUPLING PHASE TRANSFORMATIONS AND TEMPERATURE EVOLUTIONS IN STEELS
A mathematical model for calculating phase transformations in steels during rapid heating and cooling is presented. It is based on a rule of additivity. The isothermal kinetics are modelled by Johnson-Mehl-Avrami law. The model describes the kinetics of austenitization during heating, the state of austenite at the end of heating (carbon content, grain size), the kinetics of transformations during cooling, the final microstructure and hardness. The model is worked out firstly on dilatometric specimens without thermal gradients in order to validate the modelling and the input data. Then the application of the model to massive cylinders heated up and cooled down with high thermal gradients is presented.