Failure analysis and remaining life assessment of service exposed primary reformer heater tubes

Catalyst filled heater tubes made of cast HP-microalloyed grade 35Ni25Cr1NbTi alloy used in the primary reformer furnace section of a fertilizer complex failed after 8 years in service. Failure analysis and remaining life assessment of the tubes were carried out based on mechanical strength evaluation, microstructural observations and accelerated stress rupture tests for Larson–Miller parameter (LMP) based remaining life prediction. Failed tube portions showed coarsened primary carbides of chromium and niobium at the inter-dendritic boundaries. Degradation of niobium carbide (NbC) in to Ni–Nb–Si phase and partial conversion this phase back to NbC was observed. Secondary carbides at the intra-dendritic regions were almost absent. Degradation in tensile strength was also observed. The unfailed regions which are from a relatively cooler region of the same tube showed comparatively lesser degree of coarsening of the carbides and a higher tensile strength. The failure was attributed to localized overheating leading to premature creep failure. Based on LMP curve the life extended for tubes with similar outer diameteral expansion.