Evaluation of the effect of strength mismatch in undermatched joints on the static tensile strength of welded joints by considering microstructure

High-tensile strength 950 MPa steel, HT950, which is steel used in penstocks, was developed to provide two vital properties: fracture arrest to stop brittle fracture and high weldability. This steel has been already used for penstocks in some hydropower plants in Japan. To widely apply high-tensile strength steels in penstocks in the future, fewer restrictions against their welding conditions such as pre-heat and post-heat temperature controls are required. One proposed solution is to undermatch the strength of the filler metal to that of the base material. This allows less pre-heating, or no pre-heating at all, and the use of conventional rod and process management is easier. Previous studies have shown that there are softening conditions under which the strength of the joint can be considered as that of the base material. However, the shape and distribution of the soft region are assumed to be ideal. In this study, the method for calculating the change of the strength in heat-affected zone (HAZ) during the welding process is discussed. Then, the influence of the strength distribution of HAZ and welded zone to the strength of the joint is investigated by a wide plate test in both experiment and elastic–plastic analysis. Applicability of undermatched joints to penstock fabrication is considered by these discussions. As a result, it is concluded that the Vickers hardness distribution in the HAZ can be estimated by the method which is proposed in this study and the strength of the under-matched joints is high enough in both experiment and analysis in which the Vickers hardness distribution is considered. From these conclusions, the applicability of undermatched joints, of which the weld metal is the HT570 class, to penstock fabrication is conformed.