Role of grain size on the strength and ductile–brittle transition temperature in the dual-sized ferrite region of the heat-affected zone of ultra-fine grained steel

The softening region in the heat-affected zone of ultra-fine grained steels consists of coarse and fine ferrite grains. The strength and toughness of the dual-sized ferrite region were evaluated in terms of Vickers hardness and ductile–brittle transition temperature, respectively. In this study, the average grain size of the dual-sized ferrite region was expressed in terms of arithmetical average dar and weighted average dwt, respectively. It was found that the strength proportionally varied with dar−1/2 while the ductile–brittle transition temperature showed a monotonic relation with dwt−1/2 for the dual-sized ferrite region.