Investigation of White Layers Formed in Conventional and Cryogenic Hard Turning of Steels

Although hard turning of steels has become an accepted industrial practice reducing the extent of grinding, many surface integrity aspects of hard turning require clarification. The striking result of hard turning is the tendency for forming white (non-etching) and dark (overtempered) layers at machined surface. White layers are often associated with residual tensile stresses leading to reduced fatigue strength and poor wear resistance. It has been reported that certain steel compositions, machining conditions, and tools enhance white layers, but no consensus was reached on the nature of white layer and the role of environmental factors. This study examines the impact of cryogenic, liquid nitrogen spray cooling, tool and work materials, as well as machining speed on white layer formation. Results are evaluated using XRD, SEM, EDS, AES, residual stress measurement and microhardness profiling. It is concluded that white layers are a purely thermomechanical phenomenon involving dissolution of low-alloy carbides into austenitic matrix, and catastrophic flow of that 1-phase material resulting in its nano-scale refinement. The depth and extent of the refinement are controlled by cooling, with the cryogenic nitrogen reducing white layer thickness, loss of hardness, and improving residual stress distribution.