As long term laboratory creep data became available the original estimates of the allowable design stresses for creep strength enhanced ferritic steels (CSEF) were reduced. Thus, even in properly processed steel, the long term performance and creep rupture strength reported is below that originally predicted from a simple extrapolation of short term data. In addition, the typical alloy compositions for these steels specified by Codes provide reasonable strength but can steels can exhibit brittle creep behavior. This inherent brittle behavior results in notch weakening and the need to invoke weld strength reduction factors. Moreover, creep brittle behavior, and the associated micro void development, promotes burst rather than leak type fracture in components. The existence of significant densities of voids further complicates in-service assessment of condition and weld repair of these steels. It is now clear that the levels of ductility required in engineering applications necessitate proper control of composition (including trace elements), steel making and processing and all heat treatments. This paper examines background on the development of creep voids in martensitic steels and discusses metallurgical and stress state factors which promote brittle behavior.

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This paper reviews the development of an equation for the prediction of long-term creep strain and creep life. Originally, this relationship was successfully applied to 1/2 Cr-1/2 Mo-1/4 V ferritic steel, and present data suggest similar success with the 2-1/4 Cr-1 Mo material. The implications of this extrapolation technique are discussed and the consequences for plant-life assessment are considered.

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Irradiation growth tests at 553 K and a fast (E > 1.6 × 10−13 J) neutron flux of about 6.5 × 1017 n m−2s−1 were performed on specimens of Zr-2.5Nb pressure tubes to a neutron fluence of 3.5 × 1025 n m−2. The tubes were produced by different routes designed to change texture, grain shape, and dislocation density, but X-ray diffraction studies showed only minor changes in texture and dislocation density. However, electron microscopy revealed marked differences in dislocation substructure and second-phase distribution. All transverse specimens exhibited similar negative growth behavior. However, the longitudinal specimens showed different growth characteristics. Generally a three-stage growth curve was observed showing an initial offset, a low growth rate regime, and a region of apparent steady-state growth. This behavior does not fit with the normal texture, grain shape, and dislocation density dependencies which have previously been reported. An explanation is presented based on partitioning of irradiation-induced point defects, with interstitials attracted to <a>-type dislocations or second-phase boundaries and vacancies migrating to both grain and sub-grain boundaries.