Quantitative study of creep cavity area of HP40 furnace tubes

Ethylene cracking tubes of HP40 grade (25Cr-35Ni-0.4C) creep-resistant austenitic steel which failed after 80,000 h service were investigated for quantitative cavity evaluation. The results show that the creep cavity area fractions (Ω) present little difference from outer surface to inner surface through the thickness, and larger difference on different tube outer surfaces. The Ω obtained from the outer surface can stand for creep damage state for ethylene cracking tubes. The relationship between creep cavity area fraction and continuum damage mechanics is discussed. At failure, the critical value of damage variable D = 1 is corresponding to Ω = 2.8% measured on the tube outer surface.