Effects of tungsten on microstructure and high-temperature strength of oxide dispersion strengthened (ODS) martensitic steel

9CrODS martensitic steel (9CrODS) contains tungsten (W) which is a solid solution strengthening element. The addition of W considerably improves high-temperature strength of 9CrODS by the multiplier effect with oxide dispersion strengthening, however, excessive addition causes the precipitation of Laves phase leading to deterioration of fracture toughness under reactor operation. This report describes the dependence of high-temperature strength and precipitation behavior of Laves phase on W concentration of 9CrODS in order to reveal an optimum W concentration. The results derived in this study are as follows: (1) The high-temperature strength of 9CrODS increased with W concentration without saturation behavior in the range of W concentration below 2.4mass%. The Laves phase precipitated during thermal aging when W concentration exceeded 2.0mass%. The test results concluded that current specification of W concentration (2.0mass% W) is optimum for improving high-temperature strength without degradation of fracture toughness. (2) Volume fraction of residual-α ferrite, which improve high-temperature strength of 9CrODS, increased with W concentration. The high-temperature strength improvement by W addition should be caused by not only the solution strengthening but also retainment of residual-α ferrite. It was revealed that each effect equivalently contributes to the improvement of tensile strength at 700degC. (author)