Microstructure and micromechanical properties evolution pattern of metamorphic layer subjected to turning process of carbon steel

Turning process results in microstructure alteration and forms a metamorphic layer beneath the machined surface, which influence micromechanical properties. The metamorphic layer induced by turning was divided into severe deformation layer (SDL) and moderate deformation layer (MDL) through grain size distribution, and the microstructure was observed and characterized by electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). The model to predict the depth of SDL (h_SDL) based on dynamic recrystallization and specific shear energy (US) in the cutting process was established, and the results showed that ΔU_S had a linear relationship with h_SDL (R² = 0.8799). The micromechanical properties of SDL, MDL, and bulk material were characterized by micro-hardness and micropillar compression tests. The increment of micro-hardness in SDL (increased by 66.4 % on the bulk material) is mainly due to the Hall-Petch effect caused by nanocrystals formed through dynamic recrystallization, and the micro-hardness in MDL (increased by 24.1 % on the bulk material) is affected by the superposition of the Hall-Petch effect and Taylor strengthening caused by the gradient distribution of grain size and dislocation density. The yield stress (σ_y) obtained from the micropillar compression test decreased successively in SDL, MDL, and bulk material (from 1072 MPa to 604 MPa). The influence mechanism of the critical resolved shear stress (τ_CRSS) and Schmidt factor (SF) on SDL, MDL, and bulk material were illustrated.