Hardenability Improvement Mechanisms and Hardenability-Controlling Factors in Steels With Combined Mo–B Addition

To elucidate the hardenability improvement mechanisms and hardenability-controlling factors in low-carbon (C) steels with the combined addition of molybdenum (Mo) and boron (B), the relationship between the hardenability index ( V c−90 ) and the segregation states of Mo and B at prior austenite grain boundaries were quantitatively investigated using atom probe tomography. The logarithm of the hardenability index showed a negative linear relationship with the estimated values of the total interfacial excess of Mo and B at a cooling rate of V c−90 in the steels without boride precipitation during cooling. This result suggests that the hardenability is governed by the total concentration of segregating atoms at austenite grain boundaries until boride precipitation occurs. The hardenability improvement mechanism was explained by the suppression of ferrite and bainite nucleation based on the reduction in the austenite grain boundary free energy due to grain boundary segregation. It was supported by the theoretical simple model in terms of the total number of nuclei during continuous cooling. The study also suggests that the boride precipitates themselves degrade the hardenability after boride precipitation.