Experimental Basis for the Extension of Elastic-Perfectly Plastic Strain Limits Evaluation Procedure of ASME Section III, Division 5 Code Case N-861 to Grade 91, Alloy 800H and 2.25Cr-1Mo

Abstract The Elastic-Perfectly Plastic (EPP) based design method was qualified for ASME Section III, Division 5 Class A construction using Type 304 and 316 stainless steels and Alloy 617 via Code Case N-861 for Strain Limits evaluations. Because the isochronous stress-strain curves used in the EPP Strain Limits analysis were determined from standard creep testing on specimens without any prior cyclic loading history, experimental verification and analysis are required in order to extend the EPP Strain Limits analysis to the remaining three Class A materials, Grade 91, Alloy 800H and 2.25Cr-1Mo. In this study, specialized tests were designed and performed to evaluate the effects of cyclic deformation on the subsequent creep properties of these three Class A materials in the creep regime and the results are compared with standard creep tests. It is shown that reduction factors on the isochronous stress-strain curves are needed for Grade 91 to account for its cyclic softening effect. However, prior cyclic deformation has an insignificant effect on the minimum creep rates of Alloy 800H and 2.25Cr-1Mo, hence reduction factors are not required. The experimental results and analysis in this study provide the technical basis for the extension of Code Case N-861 to these three Class A materials.