SCC ASSESSMENT BASED ON PROBABILITY FRACTURE MECHANICS CONSIDERING SCC INITIATION AND PROPAGATION UNDER RESIDUAL STRESS FIELDS GENERATED BY MACHINING AND WELDING PROCESS OF PIPES

摘要

Stress corrosion cracking (SCC) has been observed near the welded zones of pipes made of austenitic stainless steel type 316L. Residual stress is an important factor for SCC. In the joining processes of pipes, butt welding is conducted after surface machining. Residual stress is generated by both processes, and the residual stress distribution by surface machining is varied by the subsequent butt-welding process. In this study, numerical analysis of the residual stress distribution by butt welding after surface machining was performed by the finite element method. The SCC initiation time was estimated by the residual stress obtained at the inner surface. SCC growth analyses based on probability fracture mechanics were performed by using the SCC initiation time and the residual stress distribution. As a result, the residual stress distribution in the axial direction due to butt welding after surface machining has high tensile stress exceeding 1000 MPa at the inner surface. The effect of SCC initiation on leakage probability is not as significant as the effect of plastic strain on the crack growth rate. However, to perform crack growth analyses considering SCC initiation, evaluation of the residual stress due to surface machining and welding is important.