Experimental investigations on the interactions between the process parameters of hot forming and the resulting residual stresses in the component

Abstract

In metal forming, the arising residual stresses influence the material behaviour during manufacturing as well as the performance of the final component. In the past, the focus of forming process design was on minimising or eliminating residual stresses. However, residual stresses can also serve to improve the properties of the components through targeted use, for example with regard to distortions or wear behaviour. For this purpose, knowledge of the interactions between the process parameters of the hot forming process and the resulting residual stresses in the final component is required. In this work, the influences of the process parameters are analysed by means of a reference process of hot forming. In this process, cylindrical specimens with eccentric holes are hot-formed, which leads to an inhomogeneous stress distribution in the material as it occurs in an industrial hot forming process. In the reference process, forming temperature, cooling strategy, forming speed, degree of deformation and steel alloys are varied. It is observed that both, process parameters and material properties, have a significant influence on the resulting residual stresses. Mainly responsible for these phenomena are microstructural effects in the material. As a result of forming at temperatures between 1000 °C and 1200 °C, static and dynamic recrystallisation processes occur, which affect the austenite grain size. The austenite grain size as well as the cooling strategy have a significant influence on the microstructure transformation behaviour, which has a decisive effect on the resulting residual stresses. In addition, the cooling strategy determines whether a diffusion-free phase transformation or a diffusion-controlled phase transformation occurs. At high cooling rates, diffusion-free transformation of the austenitic into the martensitic phase takes place, which leads to severe stresses in the crystal lattice. During diffusion-controlled phase transformation, which occurs during air cooling, comparatively lower residual stresses in the range of zero can be observed.