Fatigue and Corrosion Fatigue Failures

A metallurgical failure analysis was performed for a hanger rod and a waterwall tube sample. The hanger is a rigid type and supports a long vertical run of piping. The fracture is in one of the threaded ends and the fracture surface consists of three regions. The outermost portion adjacent to the thread root has ratchet marks that are an indication of fatigue crack initiation. The center portion has concentric, oval shaped beachmarks. The oval shape is consistent with an applied loading due to two bending moments. The inner portion is the final fracture and is approximately 1/4 of the thread root area indicating relatively low remote stresses. The failure mechanism is fatigue based on the beachmarks on the fracture surface and the transgranular cracking. The lower slope waterwall tube failure had a window opening fracture appearance. The axial fractures forming the window are located at the edge of the membrane welds on the cold or backside. There are shallow toe cracks at the membrane weld on the tube outside surface. The fracture surface had multiple, thumbnail-shaped fatigue cracks connected to the inside surface. These fatigue cracks are due to the corrosion fatigue mechanism based on two factors: (1) the stress responsible for their growth is related to the unit thermal cycling and the welded panel geometry near the corner of the boiler, and (2) they are oxidized indicating a corrosion contribution.