Microcosmic mechanism of material softening and fracture in primary shear zone during high speed machining of hardened steel

The metallurgical observations of microstructure and fracture characteristics of the adiabatic shear bands, within the primary shear zones of the serrated chips produced during high speed machining 30CrNi3MoV hardened high strength steel, have been performed using optical microscope, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure observations showed that the material softening occurred in the primary shear zones during high speed machining. A microcosmic model of microstructure development and rotational dynamic recrystallisation in the primary shear zone during high speed machining high strength steel was suggested by analysis of material softening mechanism. The fracture observations showed that adiabatic shear fracture was the primary reason of serrated chip formation during high speed machining. A microcosmic fracture model during adiabatic shear was proposed.