Remnant Life Assessment and Microstructural Studies on Service Exposed Primary Reformer Tubes of a Catalytic Converter of an Ammonia Plant

Abstract

Abstract13.5 year service exposed (SE) catalyst primary reformer tube material made of H39WM micro-paralloy grade used in feritilizer plant was assessed for remaining life. The investigation includes mechanical properties evaluation; microstructural analysis and accelerated stress rupture tests. Failed tube portions showed coarsening primary carbides of Chromium and Niobium along the grain boundaries. Degradation of Niobium carbide (NbC) into Ni-Nb-Si phase and partial conversion this phase back to NbC was observed. Secondary carbides within grains were almost absent. Degradation in tensile strength for a range of temperature from 1123 to 1223 K was also observed but they were within the specified limits. Premature failures within 3–5 years service exposure are more common in reformer tubes. The failure was attributed to localized overheating leading to creep damage. The cast tube material may undergo microstructural changes during service exposure which is the main cause of degradation in strength and hardness changes. Accelerated stress rupture tests were performed in the range of 1173–1248 K on samples machined from 13.5 years at 1191 K and 15.4 MPa exposed reformer steel tubing, did not reveal any degradation of rupture behavior compared to that of the virgin alloy. An additional life of at least 10.6 years is predicted at the operating stress-temperature conditions.