Simulation-based optimization of the local material state in the field of cyclically highly stressed case hardened construction details with notch effect*

Abstract

Steel components very often show construction details, such as cross holes and rounded shaft shoulders which lead to local stress concentrations of the multiaxial stress state in case of mechanical loads (notch effect). Under cyclic loading these stress concentrations (hot spots) can cause crack initiation, crack propagation and finally failure of structural components. The fatigue strength of cyclically loaded components can be considerably increased by the heat treatment case hardening. The shape of the construction detail has a significant influence for the sub-processes of the case hardening. This can be related to the carbon diffusion process during carburizing and the local heat transfer during quenching. As a result, the local material state following a case hardening process is often not optimal with respect to phase composition and residual stress field. In order to optimize the hardening process a heat treatment simulation based on the Finite Element Method was coupled with procedures for sensitivity analysis and optimization. Taking into account the operational loading conditions for the component, it was possible to adapt technological parameters of the case hardening process for the specific shape of the construction detail, leading to a substantially increased fatigue strength and therewith improvement of the efficiency of the case hardening process itself.