Failure Analysis of Two HP-Nb Heat-Resistant Tubes after 46,000 h Exposure to Reformer Service Conditions

This study presents a failure analysis in two reformer tubes used for hydrogen production in a petrochemical industry. These tubes (Tube A and Tube B) were made by the centrifugal casting of HP-Nb alloy in such a way that one contained titanium as a micro-element, and the other was free from titanium in its chemical composition. Although the two tubes were subjected to similar creep conditions, Tube A failed after only 46,000 h of operation against the design life of 100,000 h. SEM images showed initiation and growth of creep pores next to chromium carbide particles, as well as the formation of microcracks in Cr23C6 carbides. Pore initiation occurs as a result of grain boundary sliding and is strongly dependent on structural morphology. The tube containing titanium (Tube B) showed higher thermal stability and higher creep resistance than the tube without titanium (Tube A), which was due to the formation of finer and more discrete carbide particles. The final fracture of the tube without titanium (Tube A) occurred due to the coalescence of creep pores and the creation of grain boundary cracks.