大型用SCS24鋼の強度・靱性の改良

抄録

In order to prepare a water turbine runner for ultra high head, SCS24 was selected as a cast steel superior to SCS6, due to its more-than-ade-quate strength, and problems relating to the use of this material for large sizes were identified and the steel was modified to optimize its strength and toughness. The results were as follows.
(1) As the target chemical composition of the cast steel was similar to that of the SUS630 forged steel that is normally used, it was found that the toughness of large SCS24 cast steel was very poor. It could be improved by adding approximately 2% Co in order to raise the Ms point.
(2) It was found that the target values could be generally satisfied with cast steel for remelting in a 1-ton mold, and so an actual water turbine runner was manufactured. The runner had no casting defects, and structurally it was almost uniform from outside to inside. It had a uniform martensitic structure with small quantities of δ ferrite. The mechanical properties were slightly lower in the thickness transition area, but the target value for toughness was satisfied. Regarding the 10⁷ cycle time strength in tap water at room temperature, the modified material had roughly 1.3 times higher the fatigue strength in tap water than SCS6.
(3) Weld repairs for this material were conducted using Gas Tungsten Arc Welding (GTAW), and both the tensile properties of the deposit metals and toughness of weld joints were satisfactory. Furthermore, the microstructure of the welded boundary was uniform.
(4) The above results confirm that the modified SCS24 is a suitable stainless cast steel for a water turbine runner that combined both high strength and high toughness.