Hydrogen assisted cracking of an AISI 321 stainless steel seamless pipe exposed to hydrogen-containing hot gas at high pressure

Abstract

A seamless pipe made of AISI 321 stainless steel represented a part of a transportation pipeline system for hydrogen-containing hot gas in a hydrocarbon cracking unit. After a service period of approximately 21 months, a segment of such pipe demonstrated the cracks, causing leakage and respective fire. For clarification of a failure root cause, various metallurgical investigations combined with numerical simulations have been applied. The results revealed that the rupture of seamless pipe was evidently influenced by hydrogen assisted cracking (HAC). An increased susceptibility of the alloy to HAC had to be attributed to its sensitive microstructure which was related to the occurrence of slip bands with a high quantity in austenite grains, particularly in the specific region underneath the outer wall surface. In addition, the intensity of restraint resulting from the T-joint weld configuration caused respectively higher triaxial stresses in the confined area on the outer wall surface where the crack started. The numerical simulations of hydrogen diffusion revealed that a uniform hydrogen concentration profile over the pipe wall thickness was reached when the service period was more than 20 months. This duration agreed well to the time-to-failure of the actual component. Considering additionally that the final stage of rupture by overload was preceded by severe HAC, as confirmed by the respective intergranular fracture topography.