On the clarification of surface hardening by hard turning and grinding

Hard turning and grinding are competing finishing processes for the manufacture of precision mechanical components such as bearings, gears, cams, etc. Surface hardening at gentle machining conditions has often been reported and is attributed to strain/strain rate hardening and size effect. However, some studies have reported surface softening in the near surface of components by hard turning with a sharp cutting tool. The apparent softening was attributed to thermal effects by high cutting temperatures. The process induced surface integrity is of great importance for component performance in service. Since the reported surface property is inconsistent for hard turning with a fresh cutting tool, it is imperative to clarify surface hardening mechanisms in hard turning. The purpose of this paper is to investigate surface hardening and the interpretation of the reported thermal softening. Hard turning and grinding of AISI 52100 steel was conducted using gentle machining conditions. Surface integrity was then analyzed in terms of surface finish, microstructure, and surface and subsurface micro and nanohardness. Hard turning may produce equivalent or better surface finishing compared with grinding. The research findings showed that the apparent softening measured using microindentation in near surface is not due to thermal effects, but rather a misinterpretation of hardness values due to improper testing technique. Hard turning induces a thicker plastically deformed zone than grinding, while grinding temperatures penetrate deeper into the subsurface. However, size effect is more prominent in grinding than turning, which produces higher hardness on the ground surface and in subsurface.