Improvement of yield strength-transition temperature balance by microstructural refinement

Abstract

Grain-refinement simultaneously increases yield strength and toughness while the strengthening without the grain-refinement still deteriorates the toughness. It is worth assessing how the strengthening relates to the toughness to strengthen steels most appropriately. In this study, different mechanisms for strengthening are compared with respect to the relationship between yield strength and ductile-brittle fracture transition temperature. Considered are the grain-refinement by solute copper or thermo-mechanical control process (TMCP), the precipitation-hardening by copper, and the bainite-transformation hardening with alloying elements such as boron, molybdenum, and nickel. The transition temperature rises with strengthening by the precipitation-hardening and the transformation-hardening. In this case, the relationship is approximately linear with the gradient of 0.46°C/MPa In contrast, the transition temperature lowers in the case of the strengthening by the grain-refinement achieved by the solute copper and the TMCP. This study aims at relating the yield strength to the transition temperature on a dislocation crack model of brittle fracture. Considering the energy balance for brittle crack initiation and the temperature dependence of the friction stress for the Hall - Petch relationship gives the linear lowering of transition temperature with increasing yield strength by grain-refinement, of which gradient is estimated at approximately 0.80°C/MPa. The comparison of experimental results with this relationship indicates that the strengthening by solute copper effectively utilizes the grain-refinement.