Deformation behaviour and texture evolution of martensite steel subjected to hard milling

Microstructure evolution induced by machining in the subsurface is an important constituent of surface integrity. In this paper, microstructure alteration, texture evolution and mechanical property variation for martensite steel (AISI H13) after hard milling operation under different cutting conditions (cutting speed, feed and radial depth of cut) was investigated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscattering diffraction (EBSD) and nano-indentation techniques were applied to characterize microstructure changes. Grain refinement of machined surface was supposed to be the generation of cellular substructure originated from dislocation pile-up under severe plastic deformation. The experimental results obtained from SEM and EBSD suggested that in the plastic deformation limited to several microns thick, hard milling led to a slightly increased ratio of low angle grain boundaries (LAGBs, 2 degrees similar to 10 degrees) and a reduction of local misorientation angle. In addition, hard milling operation resulted in the shift of Schmid factor (SF) toward a small magnitude in machining-affected area, which indicated the improved yield strength of experimental specimen. With the increase of cutting parameters, texture gradually changed from preferred orientation in parallel to plane {101} in X0 direction (bulk material) to a more randomly distributed crystal plane and the relationship between chip compress ration (CCR) and texture evolution was revealed. Compared with initial nano-hardness about 5.35 GPa, the apparent increment of nano-hardness is a reflection of grain refinement and texture evolution. The possible explanations for the increase of nano-hardness were discussed with respect to dislocation accumulation, grain segmentation, and texture evolution.