Failure of welded dilution steam coil made of alloy 800HT

The paper presents the results of a failure investigation carried out on a welded dilution steam coil made of alloy 800HT. The dilution steam coil leaked at the inlet of the convection section of an ethane cracking furnace after 11 years of service. The tube experienced cracking in the weld metal and the weld fusion zone. The failure investigation revealed that the dilution steam coil suffered from embrittlement due to sulphidation at the grain boundaries of the weld metal at the inner surface and intergranular oxidation and cracking of the heat affected and weld fusion zones at the outer surface. The internal sulphidation was ascribed to the influence of sulphur bearing species on the weld metal that was much higher in nickel content than the parent metal, while the external oxidation was attributed to the oxidising environment of the furnace atmosphere. The source of sulphur in the furnace effluent was possibly dimethyl disulphide injected into the furnace feed to prevent coke formation and carburisation of the furnace tubes.