Some recent studies of material response have identified an issue that crosses over and blurs the boundary between ASME Boiler and Pressure Vessel Code Section III Subsection NB and Subsection NH. For very long design lives, the effects of creep show up at lower and lower temperature as the design life increases. Although true for the temperature at which the allowable stress is governed by creep properties, the effect is more apparent, e.g. creep effects show up sooner, at local structural discontinuities and peak thermal stress locations. This is because creep is a function of time, temperature and stress and the higher the localized stress, the lower in temperature creep begins to cause damage. If the threshold is below the Subsection NB to NH temperature boundary, 700°F for ferritic steels and 800°F for austenitic materials, then this potential failure mode will not be considered. Unfortunately, there is no experience base with very long lives at temperatures close to but under the Subsection NB to NH boundary to draw upon. This issue is of particular interest in the application of Subsection NB rules of construction to some High Temperature Gas Reactor (HTGR) concepts. The purpose of this paper is, thus, twofold; one part is about statistical treatment and extrapolation of sparse data for a specific material of interest, A533B; the other part is about how these results could impact current design procedures in Subsection NB.

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