溶接熱サイクルに伴う表面加工層での硬さ変化に基づいた表面機械加工後の溶接により生じる残留応力分布の数値解析

抄録

Recently, stress corrosion cracking (SCC) has been observed near the welded zone of the primary loop recirculation pipes made of low-carbon austenitic stainless steel type 316L in boilling water reactors. SCC is initiated by superposition effect of three factors. They are material, environmental and mechanical factors. For the non-sensitized material such as type 316L, residual stress as a mechanical factor of SCC is comparatively important. In the joining processes of pipes, butt-welding is conducted after surface machining. Surface machining is performed in order to match the inside diameter and smooth surface finish of pipes. Residual stress is generated by both processes. Moreover, residual stress distribution generated by surface machining is varied by subsequent welding process, and it has the maximum residual stress around 900 MPa near the weld metal. The variation of metallographic structure, such as recovery and recrystallization, in surface machined layer due to welding thermal cycle is an important factor for this residual stress distribution. In this study, thermal aging tests were performed in order to evaluate hardness variation due to thermal cycle in surface machined layer. The results of thermal aging tests were applied to finite element method (FEM) as the additivity rule of the hardness variation. Varied hardness was converted into equivalent plastic strain. Then, thermo-elastic-plastic analysis was performed under residual stress fields generated by surface machining. As a result, analytical results of surface residual stress distribution generated by bead-on-plate welding after surface machining show good agreement with measured results by X-ray diffraction method. The maximum residual stress near the weld metal is generated by same mechanism as both-ends-fixed bar model in surface machined layer that have high yield stress.