| Effect of W on creep properties of martensitic heat resistant HT9M steel have been investigated at 600 and 650℃. The creep rupture strength of W added HT9M steel was higher than that of HT9M steel. The addition of W decreased the self-diffusion rate of iron atom and delayed the microstructural degradation during creep. As seen in dispersion-strengthened alloys, high stress exponent, n, and activation energy, Q_c, are found in these steels. During creep deformation, formation of subgrain, growth and agglomeration of precipitates and precipitation of Laves phase occurred. These microstructural changes made the steels soften. When the stress was applied, hardness values were abruptly decreased at an early stage and remained constant with time. The degree of softening of W added HT9M steel was less than that of HT9M. But when the stress was not applied, the decrease of hardness value was not occurred in both steels. |
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