Fatigue Failure of a Carbon Steel Water-Wall Tube Because of an Undercut at a Welded Joint

Welded to the top of a dust bin for rigid support, a furnace water-wall tube in a new stationary boiler broke at the welded joint shortly after start-up. The tubes measured 64 mm (2.5 in.) OD by 3.2 mm (0.125 in.) wall thickness and were made of carbon steel to ASME SA-226 specifications. Investigation supported the conclusion that a crevice-like undercut was likely the primary cause of the fracture and that the source of the necessary fluctuating stress was tube vibration inherent in boiler operation. Recommendations included magnetic-particle inspection of the remaining water-wall tubes in the row, replacing the broken tube, and repairing cracks in other tubes by welding.