Creep lifetime and microstructure evolution in boron-added 9Cr–1Mo heat-resistant steel

Abstract

The relationship between creep lifetime and microstructure, especially precipitate morphology, of a new 9Cr heat-resistant steel containing 0.011 wt% boron was examined. The B-added steel showed superior creep lifetime compared to conventional Gr. 91 steel due to the microstructural stability of the new steel at 873 K. The new steel also exhibited prior-austenite grain boundaries with a high coverage of precipitates, which increased from ~30% to ~40% during creep tests, whereas that of the conventional Gr. 91 steel was nearly stable, at ~25%. In addition, the B-containing M23C6 in the B-added steel had a lower coherency, i.e., a higher interface energy, between the precipitate and the matrix when compared to conventional Gr, 91 steel. These factors led to a higher pinning pressure to resist interface migration in the B-added steel. Microscopy revealed that chemical stabilization had a significant effect on the coarsening behavior of M23C6. Chemical analyses revealed that M23C6 became Cr-rich during the early stages of creep, in which rapid coarsening was observed at a rate contrast of >1.0 × 10−28 m3/s; in this state, M23C6 was approaching equilibrium Cr concentration. In later stages of creep, moderate coarsening occurred with a rate constant of ~8.1 × 10−30 m3/s; in this state, chemical stability was achieved but diffusion and dissolution of small precipitates were dominant.