Influence of Chemical Composition and Heat Treatment on Long-term Creep Strength of Grade 91 Steel

Long-term creep strength of ASTM/ASME Grade 91 steels was investigated. Two heats of Grade 91 steels indicated lower creep rupture strength than the other four heats from short-term to long-term, and presence of delta ferrite phase was observed. In the short-term, no difference in creep rupture strength was observed among four heats of Grade 91 steels, however, the large heat-to-heat variation of creep rupture strength was observed in the long-term at 600 °C. The higher nickel containing heat indicates lower creep rupture strength in the long-term at 600 °C, although nickel concentration was 0.28mass% in maximum. Homogeneously recovered subgrain structure was observed on the specimens creep ruptured after about 80,000 h at 600 °C for both high nickel low strength heat and low nickel high strength one. Only a small number of fine MX carbonitride particles with a large number of coarse Z-phase were observed on the creep ruptured specimen of high nickel low strength heat, in contrast to low nickel high strength heat in which many MX particles were still observed and Z- phase formation was not pronounced. The difference in stability of fine MX carbonitride particles during creep exposure at the elevated temperatures is a cause of heat-to-heat variation of long-term creep strength of the steels. Decrease in phase transformation temperature of Ac1 with increase in nickel content may reduce stability of the precipitates at the elevated temperatures. Nickel content should be reduced in order to suppress a large drop in long-term creep strength of Grade 91 steel.