Study on microscopic characteristics of adiabatic shear bands in the serrated chips formed during high speed machining

Adiabatic shear bands and white layers in primary and secondary deformation zones within the serrated chips formed during high speed cutting of 30CrNi3MoV steel were investigated by optical microscope, microhardness tester, XRD, SEM and TEM. Results show that two types of adiabatic shear bands, i.e. deformed shear bands and transformed shear bands, were formed during lower and higher cutting speed respectively. The microhardness in the transformed shear bands is higher than that in deformed bands and matrix of chips. XRD investigation shows that martensitic phase transformation takes place in the white layers. TEM observation reveals the equiaxed grains with diameter of 50～100 nm in center of transformed shear bands, this means that dynamic recrystallization occur during formation of shear bands.