EFFECT OF MICROSTRUCTURE EVOLUTION DUE TO PROCESSING HISTORY OF PIPING ON RESIDUAL STRESS DISTRIBUTION IN AUSTENITIC STAINLESS STEEL

摘要

Stress corrosion cracking (SCC) have been observed near the heat affected zone (HAZ) of the primary loop recirculation pipes made of austenitic stainless steel type 316L in nuclear power plants. For the non-sensitization material such as a type 316L, residual stress is important factor for SCC. In the joining process of pipes, butt-welding is conducted after machining. Machining is performed to match the inside pipe diameter. Residual stress is generated by both machining and welding. In the case of welding after machining as processes of pipes, it can be considered that residual stress due to machining is varied by welding thermal cycle. In this study, the effect of welding thermal cycle on residual stress and microstructure is investigated. Residual stress variation caused by processing history is examined by X-ray diffraction method. Residual stress distribution generated by welding after machining has a local maximum stress in the HAZ. Vickers hardness also has a local maximum hardness. By using FE-SEM/EBSD, it is clarified that microstructure shows recrystallization and recovery in the high and middle temperature range of HAZ where residual stress and hardness decrease. However, in local maximum point, recrystallization and recovery do not occur. Moreover, work hardening due to welding is added to it due to machining. Residual stress distribution is determined by microstructure evolution and superposition effect of processing history. The local maximum stress in HAZ agree that SCC has been observed near the HAZ of pipes. Therefore, not only any part of manufacturing processes such as welding or machining but also treating all processes as processing history of pipes are important to evaluate accurate residual stress distribution for SCC.