Liquid-Metal Embrittlement Cracking in a Nickel-Based WeldOverlayed Boiler Tube

Abstract

Abstract During a major overhaul at a supercritical boiler, a new alcove panel was installed in the front wall (Corner #3) that subsequently developed a leak on the cold side of the tube following restart. The tube in question is a 1¼ chromium and ½ molybdenum steel with a spiral weld overlay applied to the outside diameter (OD) surface of alloy 622. The leak was discovered on the cold-side of the tube after approximately 24 hours of operation, in an area where the tube’s surface appears to have been ground smooth. The failure is due to copper-induced liquid metal embrittlement, the result of a repair weld made to fill a local excavation of unknown origin. Traditionally, copper that is pulled into boiler tube welds comes from copper corrosion products transported into the boiler and deposited on the inner surface of the tube. However, copper is not present in the feedwater train in this unit. This particular tube was newly installed and the copper originated at the OD surface. The source of copper was investigated but could not be identified. Speculation is copper contamination from non-documented damage to the tube incurred during removal of a lifting lug. The removal was improperly weld-repaired with arcing from a Gas Metal Arc Welding (GMAW) torch tip that transferred copper to the weld metal prior to solidification.