Creep strength breakdown and microstructure in a 9%Cr steel with high B and low N contents

Abstract

A 9%Cr–3%Co steel with high B and low N contents exhibits creep strength breakdown at a temperature of 923 K after 3000 h. Specific feature of this steel is a distinct difference between short-term and long-term creep regimes for transient, steady state and tertiary creep stages. This behavior is unusual for high Cr steels and attributed to low density of M23C6 carbides precipitated on lath boundaries during tempering. Precipitation of Laves phase along these boundaries during transient creep followed by its coarsening affects significantly the creep mechanisms and results in the well-defined difference in mechanical behavior between the short- and long-term creep regimes. In contrast, the strain-induced formation of a small amount of Z-phase particles scarcely changes the creep behavior.