It has well known that mechanical and geometrical heterogeneity are naturally contained in weld parts. Development of the production on fine grained steel that has high strength by refining the grain size has recently become active, but the fine grain size is coarsened and its heat-affected-zone (HAZ) is softened by weld heat input. The heterogeneity in welds is affected by the steel compositions and the welding conditions. It is important for actual use and design to know tensile strength of HAZ softening joint with consideration to heterogeneous effect. The predicting equation of tensile strength of the HAZ softening joint with fine grained steel is proposed in this paper. The effects of mechanical and geometrical heterogeneity are considered in the equation, and it is found that the equation precisely predicts tensile strength of the HAZ softening joint.
