Analytical evaluation of effect on residual stress improvement by freezing process for small bore butt welded pipe

To evaluate an effect of a residual stress improvement using freezing process (FP) for a butt-welded small bore pipe, an application test and a finite element analysis (FEA) were conducted. An accelerated stress corrosion cracking (SCC) test was conducted using a boiling 42% magnesium chloride (MgCl2) solution for as-weld and after FP specimens made of low-carbon austenitic stainless steel (Type 316L). Test results show that SCC occurrence distributed in circumferential direction, and decreased by FP. In addition, weld residual stress of small bore pipes was evaluated using three-dimensional thermal elasto-plastic FEA with mixed hardening law. Since residual stress of a small bore pipe is sensitive to welding process, especially for a welding end position, the three-dimensional FEA is required to consider an effect of a moving heat source during a butt-weld process. FEA results show that effect of a nominal diameter on a residual stress improvement is significant for a small bore pipe. Applicability of FEA method for FP was confirmed by comparison with test results. Moreover, a revised criterion for FP was proposed on the basis of FEA results. (author)