Influence of straightening on bending fatigue properties of induction hardened shafts in steel 42CrMo4

Straightening of components after heat treatment is a common procedure in the industry. Typically, elongated components such as shafts and bars are distorted from heat treatment in such a way that straightening is necessary to obtain the desired shape. The straightening operation leads to local plasticization of the component. This has an effect on for example the residual stress state and the hardness, parameters with great influence on the fatigue strength of the component. The objective of this work was to evaluate the influence of straightening on bending fatigue properties, the residual stress state, microstructure, and hardness of induction hardened test bars.

Straightening was made in three point bending on cylindrical shafts. The residual stress state and hardness was evaluated, before and after straightening. Fatigue testing was evaluated in three point bending, after rotating the shaft 60° after straightening, i.e. towards 120°. In addition to this FEM-simulations of straightening has been performed in order to formulate guidelines for straightening. The simulation were performed on a virtually induction hardened shaft of the same dimensions as used for the experiments. It was concluded that straightening affects the residual stress state along the entire circumference of a component.

 It was concluded that reversed straightening should be avoided and that at higher forces combined straightening operations at an angle compared to the desired straightening direction. The simulations performed in the project have been important in being able to understand and explain the influences of a straightening operation.