Impact of retained austenite on the white layer formation and its microstructure during hard turning of AISI 52100 steel

Sammanfattning

This master thesis was a part of an ongoing project at Research institutes of Sweden (RISE) and Chalmers University of technology, studying the formation of white layers (WLs) upon hard machining AISI 52100 steel. With a focus on the nanocrystalline microstructure of the machined steel, X-ray diffraction (XRD), white light interferometry (WLI), optical microscopy (LOM) and scanning electron microscopy (SEM) was utilized in the analysis of gathering an in-depth understanding of the WL formation mechanism. By introducing varying cutting parameters as part of the machining process, the effect of cutting speed and tool wear could be observed to directly impact the WL formation and could be linked to the thermomechanical contribution to the formation mechanism. Both thermal and mechanical WLs were observed and could be distinguished by the occurrence of dark layers in thermal WLs. The purpose of this thesis was to observe the influence of retained austenite (RA) on WL formation and from the XRD analysis the residual stress for different RA content could not be concluded. Furthermore, SEM concluded differences in the microstructure where a higher abundance of carbides was observed in the case of lower RA, a phenomenon most likely originating in the heat treatment process. Indications of facilitated mechanical WL formation for lower RA was observed but could not be deemed conclusive. The RA content could not be concluded to have an impact on the surface roughness nor the residual stress where variations were rather linked to the cutting parameters.