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

摘要

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.