Failure Analysis of 316 Stainless Steel Tubesheet

Abstract 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 study describes the failure analysis performed on a UNS S31603 (316L SS) tubesheet retrieved from the inlet side of a gas cooler. The tube material was also 316L SS. The shell side contained steam condensate at 126°C and 758 kPa, and the tube side contained a mixture of hydrogen and chlorosilane gas, and by-products at 140°C and 579 kPa. Laboratory examination indicated that the tubesheet seal welds cracked from caustic stress corrosion cracking (SCC). The cracks initiated at the toe of the seal weld between the tube and the tubesheet, a region with high residual stress. This narrow crevice region also underwent localized corrosion. The crevice corrosion was likely exacerbated by the concentration of the caustic. Contamination in the steam condensate was the likely source of the caustic. The fracture surface contained traces of sodium and the weld fracture morphology was interdendritic. Transgranular cracks were also observed on the tubesheet side close to the weld. The chemical composition of the tubesheet and tube met the requirements for 316L SS. The seal weld also met requirements for 316 SS. The microstructures of the tubesheet and tube were typical of austenitic alloys, consisting of austenite grains. The weld microstructure consisted of dendrites of austenite.