Correlation between strength and hardness for substructures of lath martensite in low- and medium-carbon steels

Abstract

In this study, microtensile and nanoindentation tests were systematically performed to evaluate the contribution of the block boundary to yield strength and to correlate the strength and hardness of as-quenched lath martensite steels with low- and medium-carbon contents. A habit-plane-dependent yielding behaviour was observed in the single-block (SB) as well as single-packet (SP) structures, independent of the carbon content. The low-angle boundary is difficult to effectively act as an obstacle for dislocation motion. This suggests that the anisotropic yielding behaviour can be attributed to the dislocation wall structures formed during the early stage of deformation. The contribution of the single-block boundary to the resolved shear stress at the onset of yielding was estimated to be constant with increasing nominal carbon content. This indicates that the yield strength of the lath martensite structure increases with increasing carbon content through block refinement and solid solution strengthening. The relationship between tensile strength and the corresponding hardness was established for the SP and SB structures. The correlation coefficient, including the block boundary strength, was in accordance with the Tabor relationship, whereas the coefficient corresponding to the matrix excluding the block boundary strength was almost half lower than the Tabor's constant.