9Cr–W–Mo–V–Nb鋼の高温引張および衝撃特性に及ぼす熱処理の影響 Influence of Heat Treatments on High Temperature Tensile Properties and Toughness of 9Cr–W–Mo–V–Nb Steel

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 high temperature tensile properties and Charpy impact properties were investigated from the viewpoint of microstructures. Both 0.2% proof stress and ultimate tensile strength increased with increase in normalizing temperature from 950 to 1200°C, while 0.2% proof stress and ultimate tensile strength decreased with increase in tempering temperature from 700 to 780°C and time from 1 to 100 h. In contrast, toughness properties as upper self energy (USE) and fracture appearance transition temperature (FATT) degraded by increase in normalizing temperature, and improved by increase with tempering temperature and time. Prior austenitic grain size, amount of MX which precipitated during tempering process and dislocation density increased with increasing normalizing temperature. In addition, dislocation density decreased with increasing in tempering temperature and time. These results suggest that tensile and toughness properties are related to precipitation strengthening by fine MX and dislocation strengthening. Further, since FBR components are used at elevated temperature for very long period, the long term stability and efficiency of strengthening mechanism brought with such heat treatment need to be investigated.