Changes in strengthening mechanisms in creep of 9Cr-1.8W-0.5Mo-VNb steel tested over wide ranges of creep conditions

The present paper studies stress, temperature and heat dependence of creep rupture life of 9Cr-1.8W-0.5Mo-VNb (Gr. 92) steel with creep rupture data of 4 heats reported in National Institute for Materials Science Creep Data Sheet 48B. In creep of Gr. 92 steel there are four regions, H, M, L and G, showing different stress and temperature dependence of creep rupture life. Creep rupture life of Gr. 92 steel increases with increasing hardness after tempering in Regions H and M (high and medium stress), and with increasing prior austenite grain size in long-term Region G. Its creep rupture life is insensitive to the heats in low-stress Region L. Creep rupture lives of the steel are formulated in each region as a function of stress, temperature and a metallurgical variable determining its creep rupture life. Two types of breakdown in creep strength (substantial decrease in stress exponent for creep rupture life) take place in Gr. 92 steel; one from Region M to Region L, and the other from Region M or L to Region G. Since Gr. 92 steel is used in Region G in steam power boilers, the second breakdown to Region G is of engineering importance. Causes of the second breakdown are discussed with creep rupture data of four grades of creep strength enhanced ferritic steels.