Study on Type-IV Damage Prevention in High-Temperature Welded Structures of Next-Generation Reactor Plants: Part I — Fatigue and Creep-Fatigue Behavior of Welded Joints of Modified 9Cr-1Mo Steel

High-chromium ferritic steels, such as modified 9Cr-1Mo steel, are regarded as candidate structural materials for new generation nuclear power plants and reliable design methods need to be developed. Of particular importance is prevention of premature failure, commonly referred to as type-IV failure, in softened regions of the heat-affected zones as several incidents of type IV failures of high-chromium steels in the fossil power plants have been reported. As part of the work aimed at developing design method for high-chromium steels applicable to new generation reactor plants, strain-controlled fatigue and creep-fatigue tests were conducted for welded joints of modified 9Cr-1Mo steel plate mainly at 550°C and 600°C. Failure typically occurred in the base metal in the pure fatigue tests but type-IV failure in the heat-affected zone occurred in most of the creep-fatigue tests where a large life reduction due to strain hold was observed. Life prediction method of the welded joint under creep-fatigue loading condition was studied based on these results.