FUNCTIONALISATION OF THE BOUNDARY LAYER BY DEFORMATION-INDUCED MARTENSITE ON BEARING RINGS BY MEANS OF BULK METAL FORMING PROCESSES

During cold forming of metastable austenitic steels, a strength-increasing phase transformation induced by externally superimposed stresses occurs in addition to strain hardening. The effect of deformation-induced martensite formation has so far not been utilized industrially in the area of bulk forming, but could be suitable for the production of highly-loaded components in oxidative atmospheres. The aim of this study is the analysis of local phase transformations in metastable austenitic steels in the boundary layer of bulk formed components. For this purpose, the relationship between the process conditions occurring during bulk metal forming and the resulting martensitic phase fraction was determined. Cylinder compression tests are carried out in which the influence of various process parameters can be investigated. These include forming temperature, true plastic strain and forming speed. In a quantitative measurement by means of a magnetic induction process, local martensite formation is determined and hardness measurements are carried out. The recorded flow stress curves are implemented in a numerical simulation. Furthermore, the influence of different tool surface topographies on the contact conditions of the workpiece-tool system is characterized by means of ring compression tests. With the numerical simulations and experimentally obtained results, a surface hardening process for bearing rings is designed. The relationship between local true plastic strain and deformation-induced martensite development is explained by material flow simulations, taking into account the process route for manufacturing the bearing ring and the varying friction factors.