Influence of different cooling strategies during hard turning of AISI 52100 – part II: characterization of the surface and near surface microstructure morphology

Abstract

Abstract This two-part paper addresses the impact of seven different cooling conditions on the finishing process as well as the resulting surface and near-surface microstructure morphology of the rolling bearing steel AISI 52100 in martensitic heat treatment state. The applied cooling strategies are liquid nitrogen (LN2-) cooling, CO2-snow cooling at varied mass flows, sub-zero metalworking fluids (MWF) at varied supply temperatures, and dry machining. Part II of the paper concerns the changes in surface roughness, examined by an angle resolved scatter light sensor, micro hardness measurement, microstructure observations, residual stress and phase composition analyses in the near-surface area, caused by the turning process, that is characterized in Part I. The lowest cooling performance, i.e. dry machining, results in low compressive axial residual stresses and a high proportion of retained austenite at the near-surface area. High cooling performance strategies like CO2 cooling or sub-zero MWF show an opposite distribution of the mentioned parameters, wherein the coolants’ lubrication effect is an important aspect. Microstructural differences resulting from the process parameters used in these investigations tend to be less significant. Thus, they do not allow reliable assignment of the microstructure to the cooling strategies used. However, different cooling conditions show an impact on residual stresses, phase distribution and surface topography of the workpieces to varying extents.