The application of new heat resistant steels in power plants requires reliable long term creep rupture strength values as basis for design. Modern martensitic 9% Cr-steels have complex microstructures that change with service exposure. That is why extrapolations of long term strength properties will be most difficult. Due to new long term test results, re-assessments became necessary for grades 911 and 92. Different methods have been used. Good agreement was obtained between a graphical and the numerical ISO 6303 method. In both cases a two-step assessment procedure was used. First the raw data was prepared in a suitable way, which was followed by mathematical averaging procedures. For comparison a Larson-Miller analysis on the raw data was performed, too. The results turned out to be too optimistic at temperatures higher than 575°C (1050°F). It is shown that a suitable preparation of data can improve the Larson-Miller assessment. As a result of the new assessments the design values had to be reduced for both grades. With respect to previous assessments the new values are up to almost 10% lower. In the case of grade 92 the difference from the former ASME values are even higher. Consequences concerning design and service operation are discussed.

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