The current rules in Subsection NH for the evaluation of strain limits and creep-fatigue damage using simplified methods based on elastic analysis have been deemed inappropriate for Alloy 617 at temperatures above 1200°F (650°C) because, at higher temperatures, it is not feasible to decouple plasticity and creep; which is the basis for the current simplified rules. To address this issue, proposed code rules have been developed which are based on the use of elastic-perfectly plastic analysis methods and which are expected to be applicable to very high temperatures. The proposed rules are based on the use of an elastic-perfectly plastic material model with a pseudo yield strength selected to ensure that the accumulated strain and creep-fatigue damage with meeting the currently specified limits in Subsection NH. For this phase of the verification process, the proposed rules have been compared using simplified example problems to the results obtained from application of the current Subjection NH rules for both simplified methods and full inelastic analysis. The Subsection NH 316 stainless steel properties data are used for these comparisons. Results of calculations for a testing program underway on Alloy 617 at 950C are given.

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