Microstructural characterization of white layers formed during hard turning and grinding

White layer can be formed on component surfaces during various machining processes. The white layer may have significant influence on product performance. However, the nature, especially microstructure, of white layer is not well understood. In this study, microstructural characterization using SEM, XRD, and TEM of white layers by hard turning and grinding of AlSl 52100 steel are conducted since a turned white layer may be significant different from a ground one due to the distinct process characteristics. The white layers were also compared with the untempered martensite. It was found the ground white layer readily etches while a turned one does not, although both white layers appear white under a microscope. The turned white and dark layers have much more retained austenite (approximately 10-12%) than those of the ground ones (approximately 3%). It has been shown that white layer is composed of both untempered martensite and retained austenite according to XRD and TEM evidence. Twin density decreases in the order of ground white layer, untempered martensite, turned white layer, and tempered martensite. The twin size indicates that the transformation rate of austenite to martensite is more rapid in grinding than in turning. Mechanical deformation plays a greater role in turned white layer formation compared to ground white layers, where thermal effects are more dominant.