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.
Influence of process parameters for induction hardening on residual stresses
Abstract Fatigue behaviour of induction hardened parts is largely dependent on the correct combination of hardening depth and the magnitude and distribution of residual compressive stresses in the surface layer. The objective of this work was to study experimentally the residual stresses at the surface layer of induction hardened cylindrical specimens. Two microstructures, quenched-tempered and normalised were investigated. The process parameters of induction hardening were varied to give a constant hardness penetration depth. The residual stresses were measured using the X-ray diffraction method. The investigated induction hardened samples showed a constant biaxial residual stress-state at the surface. The directions of the principal stresses deviated slightly from the defined circumferential and longitudinal directions. However, the shear stresses at the surface were of a small magnitude. This experimental investigation showed that the process parameters of induction hardening influence the residual stress-state of hardened parts to a great extent.