This report provides recommendations for improvement of ASME Section III-NH, Components in Elevated Temperature Service, for Grade 91 in the areas of negligible creep and creep-fatigue.

The report is separated into the following four parts.

Part I - Examines the current approaches available to define negligible creep and checks their applicability to Grade 91 steel. The work is based on material data available in France and the U.S.

Part II - Compares Subsection NH and RCC-MR creep-fatigue procedures. Comparisons are performed on cases defined on the basis of experimental test results available from Japan, France and the U.S. on Grade 91 steel. Particular attention was paid to the definition of safety factors and creep-fatigue damage envelope. Improvements to existing procedures are recommended.

Part III - Aimed at defining tests necessary to validate negligible creep conditions for Mod 9Cr-1 Mo material.

Part IV - Completes the work performed in Part II which, on the basis of creep-fatigue tests results available from Japan, Europe and the US, compared creep-fatigue procedures of ASME Subsection NH and RCC-MR Subsection RB.