White layer formation in hard turning of H13 tool steel at high cutting speeds using CBN tooling

摘要

White layers formed during machining have negative effects on surface finish and fatigue strength of products. The white layer is generally a hard phase and leads to the surface becoming brittle causing crack permeation and product failure. This is a major concern with respect to service performance especially in the aerospace and automotive industries. Numerous authors have investigated the formation of white layer under different manufacturing processes. In turning, it was suggested that the white layer structure is a martensitic phase whose formation is correlated to tool wear. Past studies have tended to concentrate on the formation of white layers at conventional cutting speeds, but never examined the formation at high cutting speeds. This paper reports on an investigation of white layer formation for wide range of cutting speeds in hard turning of 54-56 HRC H13 tool steel. The specimens were analysed using a micro hardness tester, SEM with EDAX software and Electron Micro-Probe. In addition tool wear and workpiece temperature were studied. The machined surface showed an increase in hardness with respect to the bulk material. Compositional gradients were noted for the white layer in terms of depletion of the elements iron and chromium coupled with an enrichment of carbon and oxygen content. The results showed that despite tool wear increasing with cutting speed, white layer depth and hardness actually reduced. This finding suggests that there may not be a direct relationship between white layer formation and wear, the correlation maybe linked to wear mode.