EFFECT OF SURFACE-MACHINING AND BUTT-WELDING ON RESIDUAL STRESS AND HARDNESS OF TYPE 316L STAINLESS STEEL PIPES

Stress corrosion cracking (SCC) near the welded zone of core internals and recirculation pipes of Type 316L stainless steel have been observed to initiate at inner surface due to tensile residual stress and hardening. Residual stress and hardening should be induced by not only welding but also surface-machining in these regions. Surface-machining is conducted before and after piping butt-welding to match the ID of pipes and to provide a smooth surface finish. Therefore, we experimentally evaluated Vickers hardness distribution and proposed a simulation method to estimate residual stress by surface-machining using a local micron scale finite element method (FEM) model in previous paper (PVP2006) [1]. In present work, residual stress and Vickers hardness distributions for thickness direction were evaluated by experimental measurement. Residual stress simulation by surface-machining and welding was carried out by proposed simulation method applied to simulate the butt-welding of full scale FEM model for weld specimen. Redistribution behavior of residual stress by welding after surface-machining was discussed based on experimental and analytical results. Estimation of Vickers hardness distribution by surface-machining was performed based on strained region map observed by Field emission scanning electron microscope (FE-SEM) equipped with electron backscatter diffraction (EBSD) and compared with experimental measurements. The effect of surface-machining before/after welding on distribution of residual. stress and hardness are presented.