Characterization of White Layers induced by Hard Turning of AISI 52100 Steel

Licentiatavhandling, 2013

Hard turning is a machining process applied to metallic materials with hardness above 45 HRC. The process offers high production flexibility and sustainable manufacturing since it allows performing both rough and finishing machining of complex geometries in one single setup. With respect to surface integrity, hard turning can generate beneficial compressive residual stresses and good surface quality with small dimensional variation. However, deterioration of the cutting tool generates tensile residual stresses and also alters the surface microstructure, i.e. leads to formation of white layers. Since the mechanisms for white layer formation are not yet fully understood, there is limited understanding of whether there exists different types of white layers possessing different mechanical properties. Therefore, additional processing steps are often added to remove the white layers and to generate compressive residual stresses. By hard turning at different cutting speeds, 30 m/min to 260 m/min, and different tool conditions, it was possible to generate surface integrities with and without white layers on AISI 52100 steel. After hard turning with fresh cutting tools, only discontinuous and thin white layers were detected, while in case of turning with cutting tools with excessive flank wear, white layers up to 3 μm in thickness were observed. White layers formed at 30 m/min are characterized by reduced retained austenite content, deformed M3C carbides and compressive or low tensile residual stresses. In contrast, when white layers created at higher cutting speeds e.g. 260 m/min, significantly increased retained austenite contents, un-affected M3C carbides and high tensile residual stresses were measured. Temperature measurements with a novel two-color pyrometer at the cutting edge revealed large differences in cutting temperature for the studied cutting conditions, i.e. cutting temperatures higher than 900°C were measured at a cutting speed of 260 m/min, while during machining at 30 m/min, temperatures of about 550°C were measured. Microstructural characterization by use of transmission electron microscopy showed that the white layers consist of nanocrystalline and sub-microsrystalline grains ranging from 10 to 200 nm in diameter. The results show that there are different types of white layers, which are predominantly thermally or mechanically induced. Hence, depending on the formation mechanisms, different mechanical properties can be obtained.