Failure Analysis of 316 Stainless Steel Tubesheet (C2026-00013)

SUDHAKAR MAHAJANAM, Tyler Scherbarth, Fred Wade, Vibha Zaman

Austenitic stainless steels (SSs) are used in wide ranging industrial applications, such as heat exchangers, due to their excellent corrosion resistance and high temperature properties. This case describes the failure analysis performed on a 316 SS tube sheet retrieved from the inlet side of a gas cooler. The shell side contained steam condensate at 126°C and 6.6 barg, and the tube side contained a mixture of hydrogen and chlorosilane gas at 140°C and 5.8 bar. Laboratory examination indicated that the tube sheet seal welds cracked from caustic SCC. The cracks initiated at the toe of the seal weld between the tube and the tube sheet, a region with high residual stress. This narrow crevice region also underwent localized corrosion, likely exacerbated by the concentration of caustic, which is a contaminant present in steam condensate. The weld fracture surface had an interdendritic morphology and contained traces of sodium. Transgranular cracks were also observed on the tube sheet side near the weld. The chemical composition of the tube sheet and tube met the requirements for 316L SS. The microstructures of the tube sheet and tube were typical of austenitic alloys, consisting of austenite grains. The weld microstructure consisted of dendrites of austenite.