Strength and ductility of 21/4Cr–1Mo steels in creep at elevated temperatures

Abstract

A considerable amount of data relating to the creep properties of 21/4Cr-1Mo steels has been collected from the literature and analysed with a view to elucidating correlations useful in design. It is observed that stress-rupture strengths in general vary linearly with room-temperature tensile strengths for temperatures up to about 565°C and for times up to 10000 h. At a given strength level, tempered bainite affords the best resistance to creep at temperatures up to 555°C and times up to about 100000 h. At longer times and higher temperatures, however, ferrite–pearlite structures result in better creep resistance. The variation of stress-rupture strength and minimum creep rate with temperature can be adequately described by the Orr–Sherby–Dorn parameter for stress levels not exceeding 0·4–0·6 times the room-temperature tensile strength. In this range, activation energiesfor creep and rupture are determined to be about 377 kJ/mol (90 kcal/mol), regardless of heat treatment and tensile strength level. Further, it is observed that minimum creep rate ɛ and time to rupture t_r are mutually related by the expression ɛ × t_r4∼4·4, independent of the tensile strength of the steel and the test temperature. Rupture ductilities in general are found to decrease with increasing times to rupture, reach a minimum value, and then increase again. Increases in temperature, tensile strength level, austenitizing temperature, and impurity content of the steel have the net effect of increasing the rate of decrease in ductility and causing the minimum ductility to occur at shorter test times.