9Cr–W–Mo–V–Nb鋼の長時間クリープ特性に及ぼす熱処理の影響

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This paper describes the effect of heat treatment on long-term creep properties of 9Cr–W–Mo–V–Nb steel which in the specification of ASME Grade 92 as a part of development of high Cr steel for fast breeder reactor (FBR). The effects of normalizing temperature and tempering temperature and time on long term creep properties were investigated from the viewpoint of microstructures. Creep strength increased with increase in normalizing temperature from 950 to 1200°C. The microstructural factors that improve creep strength are increased amount of MX which precipitated during tempering process and increased dislocation density. In addition, coarsening of prior austenite grain size also contribute to increasing creep strength. Although creep rupture time of 780°C–1 h tempering specimen was greater than that of 700°C–1 h and 750°C–1 h tempering specimen in a long term region exceeding 20000 h at 650°C, creep ductility and reduction of area of the former remarkably degraded compared to those of the latter. Higher normalizing temperature and lower tempering temperature and/or shorter tempering time are desirable for long life plant of which material needs superior creep strength and ductility.