Investigation of creep properties, precipitation analysis and the structural observation through optical and Transmission Electron Microscope evaluated the heat affected zone of weld for latest 9 to 12%chromium containing tempered martensitic steels. Instead of the conventional stress concentration mechanism, the possibility of the fine prior austenite grain effect and the coarsening of precipitates explain the creep rupture strength deterioration. Estimated quantitative contribution ratio of fine prior austenite grain and the precipitation coarsening is 50% to 50%. The possible absolute deterioration strength value is 12.5MPa each. From the view point of prior austenite grain role for creep behavior, grain boundary precipitating element, for example copper, supposedly affects to the creep rupture strength of HAZ.
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