Cracking Causes of Dissimilar Steel Butt Joint of a High Temperature Superheater Tube in a Power Plant

A T91/TP347H dissimilar steel butt joint of high temperature superheater tube of a 630 MW power plant unit was found cracked after 100,000 hours of operation. The reasons of cracking were analyzed by macroscopic examination, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis. The results show that the misuse of welding materials is the root cause of the cracking. The excessive protrusion of the weld root and abnormal carburizing layer near the fusion line are the direct causes of the cracking. In view of the causes of cracking, it is suggested to strengthen the supervision of welding process, improve the welding quality and strictly control the heat treatment process.