Failure analysis of AISI 321 stainless steel welded pipes in solar thermal power plants

Abstract

AISI 321 stainless steels (SS) pipes are commonly used in solar thermal power plants for transport of thermic fluid containing chloride ions at ~ 400 °C. Several of these SS pipes have failed while in service after very short exposure to service conditions leading to leakage of the thermic fluid. The present study aims to understand the root cause of failure of these pipes which were seam welded together with spot welds on the surface. The seam welding had been done either by a) laser beam welding (LBW) or by metal inert gas welding (MIG). Dye penetrant tests were applied to the pipes followed by microstructural analysis of the pipes using optical microscopy, and field emission scanning electron microscopy (FESEM). Subsequently, phase determination was carried out by X-ray diffraction. Stress corrosion cracking (SCC) susceptibility of the laser beam welded, MIG welded and spot welded joints were tested as per ASTM G36 in boiling MgCl2. Detailed investigation revealed that leakage in each case occurred near spot welded joints due to chloride-induced SCC of AISI 321 SS. SCC susceptibility tests of the welds showed that both the seam welds and the spot welds had residual stresses beyond the threshold limit which resulted in early nucleation of cracks in presence of chloride ions. Improper post-weld heat treatment (for LBW, MIG and spot weld) was identified to be the root cause of failure of the pipes.