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

Highlight • According to the degree of deformation of the surface metamorphic layer, it is divided into SDL (region A) and MDL (region B). The main feature of SDL is grain refinement (AGD is 0.944 μm) reduced by dynamic recrystallization. MDL is the transition part, the grain size (AGD from 1 μm to 20 μm) is gradient distributed with depth, and the proportion of SGBF and LGBF increases significantly. • There is a good corresponding relationship between metamorphic layer and microhardness. The results of the micropillar compression test showed that the young`s modulus and yield stress of region A, region B, and bulk material decreased in turn, but fluctuated greatly due to the influence of grain orientation. • The model of hSDL based on dynamic recrystallization and shear energy in the cutting process is established. There is a good linear relationship between ΔUs and hSDL, which can be used to control and predict hSDL. • The mechanism that the microhardness and yield stress of region A and region B is higher than that of the bulk material are the Hall-Petch effect caused by grain boundary strengthening and Taylor strengthening caused by dislocation tangles, respectively. 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 (hSDL) based on dynamic recrystallization and specific shear energy (US) in the cutting process was established, and the results showed that ΔUS had a linear relationship with hSDL (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 (σy) 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 (τCRSS) and Schmidt factor (SF) on SDL, MDL, and bulk material were illustrated.