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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 760; no. C; pp. 267 - 276
• Main Authors: Matsunaga, Tetsuya, Hongo, Hiromichi, Tabuchi, Masaaki, Souissi, Maaouia, Sahara, Ryoji, Whitt, Collin, Zhang, Wei, Mills, M.J.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 08-07-2019; Elsevier BV; Elsevier
• Subjects: Chemical precipitation; Chromium molybdenum steels; Coarsening; Creep; Creep tests; Grain boundaries; Grains and interfaces; Heat resistant steel; Heat resistant steels; Materials Science; Metallurgy & Metallurgical Engineering; Microstructure; Morphology; Organic chemistry; Precipitate; Precipitates; Science & Technology - Other Topics; Stability; Heat resistant steel; Precipitate; Creep; Grains and interfaces
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2019.05.114

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.