Characterization of high temperature tensile and creep–fatigue properties of Alloy 800H for intermediate heat exchanger components of (V)HTRs

• High temperature tensile, creep–fatigue (C–F) properties of Alloy 800H are studied.
• Strength and uniform elongation properties at 800 °C are much lower than RT values.
• Strong influence of hold time and Δɛtot on low cycle fatigue life was observed.
• The total allowable C–F damage (D) at 800 °C decreases with the decreasing Δɛtot.
• Synergetic effect of C–F interactions showed stronger effect at lower Δɛtot values.

Alloy 800H is considered as a candidate material for intermediate heat exchanger (IHX) components of (very) high temperature reactors (V)HTRs. Qualification of the this alloy for the aforementioned nuclear applications requires understanding of its high temperature tensile, low-cycle fatigue behavior and creep–fatigue interactions because the IHX components suffer from combined creep–fatigue loadings resulting from thermally induced strain cycles associated with start-up and shutdown cycles. To this end, in this paper, the tensile properties of the Alloy 800H base and tungsten inert gas (TIG) welded materials are studied at three different temperatures, room temperature 21, 700 and 800 °C. Low cycle fatigue (LCF) behavior of the base material is investigated at 800 °C with no-hold time (no-HT) and hold time (HT) to study creep–fatigue interactions. The tensile test results showed substantial differences between the strength and ductility properties of the base and weld materials at all 3 temperatures, however, the trends in temperature dependence of tensile properties are similar for both base and weld materials. LCF studies with no-HT and HT showed a strong influence of HT on the low cycle fatigue life of this alloy illustrating the substantial influence of creep mechanisms at 800 °C. Finally, cumulative values of creep versus fatigue damage fractions are plotted in a creep–fatigue interaction diagram and these results are discussed with respect to the existing bi-linear damage summation curve for Alloy 800H in ASME Code Case N-47 in order to understand the synergetic effects of both the damage mechanisms.