Creep ductility of 9Cr creep strength enhanced ferritic steels – Part II, microstructural observations

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of simple Design by Formula rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility There appears to be little awareness regarding the implication(s) that creep ductility has on the structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. As part of a two-part paper, the selected tests were reviewed and discussed from a mechanical perspective in the Part I paper, “Creep ductility of 9Cr creep strength enhanced ferritic steels – Part I, structural response.” The association of performance with specific microstructural features is briefly reviewed and the remaining gaps are highlighted for consideration among the international community.