蒸気タービンボルト・ブレード用含Co12Cr耐熱鋼の開発と製造

抄録

12Cr heat-resistant steels have been previously used for high-temperature bolts for steam turbines. Currently, the maximum steam temperature in steam turbines has been raised to around 873K, in order to improve thermal efficiency. Consequently, austenitic alloys must be used to accommodate these more severe conditions (at steam intakes, for example), as 12Cr steel loses creep strength under such conditions. However, although austenitic alloys such as Refractaloy 26 possess higher creep strength at high temperatures, such alloys have the disadvantage of greater thermal expansion and higher cost compared to 12Cr heat-resistant steel. Therefore, it became necessary to develop 12Cr steel with higher creep strength to replace austenitic alloys in applications involving advanced 903K-class steam turbines.
12Cr steel with higher creep strength, containing 3% Co, 0.7% Mo, and 1.8% W, had already been developed for rotor forgings. In this study, quench-hardening and ingot-soaking temperatures were investigated. Based on the results of this investigation, creep strength and absorbed energy were improved compared to conventional bolt materials.
In commercial lot production, the VIM-VSR re-melting process was applied to reduce segregation of chemical compositions, variation of boron yield, and non-metallic inclusion in the ingot. Furthermore, precise control of the forging temperature of the rotary forging machine prevented precipitation of delta ferrite. Accordingly, 12Cr steel with high creep strength was successfully manufactured for use in high-temperature bolts and rotor blades.