Effects of trace elements on creep strength and ductility of low-carbon 2.25Cr-1 Mo steels

Abstract

Creep and creep crack growth tests have been performed on two experimental low-carbon 2.25Cr- 1Mo steels. Each had similar trace additions of Nb, Ti, and V, but one also had additions of P and As. The latter steel also had a higher carbon level. The secondary hardening precipitate was the cubic MX rather than the hexagonal M₂X usually observed in 2.25Cr-1 Mo steels. In samples quenched from 1100°C or above, the creep strengths were high and the P and As additions resulted in severe creep embrittlement. In the samples quenched from 950°C, creep strengths were significantly lower, particularly in the steel with the higher C, P, and As levels and the P and As additions did not impair ductility. The lower creep strength of the high-C, P, and As steel is attributed to a reduced contribution to the friction stress from molybdenum in solid solution as a result of M₂₃C₆ formation. The possible role of P in enhancing Mo segregation to grain boundaries and hence further reducing creep resistance is also discussed.