Failure Analysis of Welded Steam Boiler Flange

Abstract

Steam boiler flange, made of steel ASTM A105 welded to spool pipe, experienced a failure near the welding area after six years of service. The operator company has reported that a sudden increase in operating pressure and temperature exceeded the normal operation conditions. As a result, the failed flange and welded spool pipe segments were received for further investigation using root cause analysis to determine the reason for the failure. The failed parts were visually inspected (VT) and tested for subsurface defects with a fluorescent wet magnetic particle test (MT). Chemical analysis, microstructure observations, and hardness tests are conducted for the failed flange and pipe. It is noticed that there were no abnormalities or indications of creep or overheating were found in the microstructure and hardness values. Upon examining the macro & microstructure of the connecting weld joint, several welding defects, such as non-metallic inclusions, few porosities, and lack of fusion between welded passes, were noticed. Scanning Electron Microscope (SEM) observations showed that fine striations as beach marks on the fractured surface provided strong evidence of fatigue failure. The identified crack originated from welding defects and progressed through the flange material due to fluctuating radial stresses due to the cyclic changes in working pressure during service. To prevent similar failures, it is recommended to apply proper welding procedure specifications (WPS) by qualified welders, conduct regular inspections and maintenance of the steam boiler, and keep working conditions stable.