High tensile strength 950MPa 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 preheat- and postheat- 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 preheating at all, and 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 is assumed to be ideal.
In this study, the method for calculating the change of the strength in heat affected zone (HAZ) during 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 wide plate test in both experiment and elastic-plastic analysis. Applicability of under-matched joints to penstock fabrication is considered by these discussions. As the results, it is concluded that the Vickers hardness distribution in HAZ can be estimated by the method which is proposed in this study and the strength of the under-matched joints are enough high in both experiment and analysis in which the Vickers hardness distribution is considered. By these conclusions, applicability of under-matched joints whose weld metal is HT570 class to penstock fabrication is conformed.
