Water Wall Tube Cracking in an Ultra-Supercritical Boiler Caused by Deep Peaking

After running for about 59 000 h, the tubes at the intermediate mixing header of the furnace water wall leaked in an ultra-supercritical boiler. Preliminary analyses show that the primary crack may be initiated from two kinds of crack. One kind of crack is the dense transverse cracks in the external walls of the fireside, and the other one is the crack in adjacent H-shaped fin fillet weld. In order to figure out the initiation of the primary crack, the macroscopic morphology, chemical composition, microstructure, Vickers hardness and fracture morphology observation were conducted on the sample tube. The oriented dimples in the primary crack indicate that the crack propagates from the external side to the inner side. The erosive metal loss at fireside surfaces of tubes adjacent to the initial cracking shows significant differences. It is indicated that the primary crack is initiated from one side connected with the H-shape fin fillet weld crack. Based on comprehensive analyses of material quality of the sample tube and boiler operation data, it can be concluded that the wall temperature of water wall tubes changes with the frequent and large fluctuation of boiler load due to peak regulation operation, which is the root cause of fatigue cracking of H-shape fin fillet weld.