Creep rupture properties of 0.12C-22Cr-15Ni series austenitic heat resisting steels which are promising for boiler tubes of a high temperature and high pressure steam power plant have been studied.
The 22Cr-15Ni steel bearing Mo, Nb and Ti has higher creep rupture strength than the simple 22Cr-15Ni steel for a shorter period, but at a higher temperature and for a longer period, creep rupture strength of the former steel has a tendency to decrease owing to the precipitation of σ phase.
The reason for the decrease in creep rupture strength is attributed to the decrease in solid solution hardening. In spite of the precipitation of σ phase, the steel bearing Mo, Nb and Ti is superior to the simple steel in creep rupture elongation, because Mo changes the precipitation morphology so that the initiation and the growth of crack were prevented.
The grain boundary carbide restrains grain boundary sliding, thus the initiation of crack, so that the creep rupture properties of the aged simple 22Cr-15Ni steel are improved.