Influence of heat treatment on long term creep properties of 9Cr-W-Mo-V-Nb steel

Obara, Satoshi; Wakai, Takashi ; Asayama, Tai ; Yamada, Yoshiyuki*; Nakazawa, Takanori*; Yamazaki, Masayoshi*; Hongo, Hiromichi*

This paper describes the effect of heat treatment on mechanical properties of 9Cr-W-Mo-V-Nb steel 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 creep properties were investigated from the viewpoint of microstructures. Creep strength increased with increase in normalizing temperature from 950C to 1200C. 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 780C-1h tempering specimen was greater than that of 700C-1h and 750C-1h tempering specimen in a long-term region exceeding 20000h at 650C, creep ductility and reduction of area of the former remarkably degraded compared to those of the latter.