Formation mechanism of white layer in the high-speed cutting of hardened steel under cryogenic liquid nitrogen cooling

The coupled thermal-mechanical effect in the high-speed machining of hardened steels induces the formation of a surface white layer structure, which adversely affects the mechanical properties and service performance of the machined component. However, knowledge about the white layer formation mechanisms is in severe dearth, particularly for high-speed machining under a cryogenic cooling condition. This paper presents an experimental study of the formation mechanisms for surface white layers in high-speed machining of a hardened steel. Both machining with cryogenic liquid nitrogen (LN2) cooling and dry cutting conditions are considered and the characteristics of the white layers formed under the two cooling conditions are compared to reveal the effect of process parameters. It is shown that the hardness of the white layers is increased while their grain size is decreased under the cryogenic LN2 cooling condition as compared to dry cutting. Under both the cooling conditions, no material phase transformation or recrystallization is noticed alongside the white layer formation, but severe plastic deformation is found to be the dominant reason for white layer formation. Tool wear is noticed to increase the white layer thickness. It is shown that during the cutting process, the work material undergoes two stages of intense plastic deformation. In the first stage, the material slips and approaches the first deformation zone, then gradually bulges and produces significant dislocations, and eventually forms a dense dislocation center around the shear plane. In the second stage, part of the material slides toward the tool flank face. The material dislocation slip induced by friction and material shearing further stretches the surface martensite lath bundles to form dislocation tangles and cellular substructures. The highly stretched martensite lath bundles are finally transformed into white layer structure under the interweaving multi-dislocation movement.