Creep rupture in HP-Nb refractory steel tubes due to short-term overheating

The premature creep rupture of cast reformer tubes made of HP-Nb steel due to short-term overheating, was investigated. Investigation was performed on specimens from ruptured, cracked, and uncracked tubes and included metallography, mechanical testing, and SEM/EDX analysis. It was found that accidental short-term overheating, after 5 years of normal operation, caused rapid tertiary creep, manifested by the formation of dense cavities and cracking, which eventually led to the premature creep rupture of several tubes of the reformer. Creep cavities formed preferentially at M 7 C 3 carbides but also at M 23 C 6 and MC carbides while crack propagation took place along the interdendritic carbide network. It appears that the radial direction of the columnar grains from the inside to outside diameter orients the grain boundaries perpendicular to the hoop stress, a situation which degrades creep rupture resistance. An analysis based on the Larson-Miller parameter confirmed the premature creep rupture of the tubes due to overheating.