Alloy 617 (UNS N06617) 52Ni-22Cr-13Co-9Mo is the leading candidate material for the intermediate heat exchanger for the very high temperature reactor (VHTR). An ASME Task Group on Alloy 617 Qualification has drafted a Code Case for Alloy 617 to allow construction of components conforming to the requirements of Section III, Division 5, Subsection HB, Subpart B “Elevated Temperature Service” for service when Service Loading temperatures exceed the temperature limits established in Subsection HB, Subpart A.
There are two categories of allowable stress levels in Division 5, Subsection HB, Subpart B. The first category, identified as So, applies to Design Loadings and the second category, identified as Smt, applies to Service Loadings. Both categories are based on the lesser of time dependent and time independent properties. The time dependent properties control at higher temperatures where creep effects are significant.
The Design condition time-independent allowable stresses are determined per the criteria given in Appendix 5 of Section II, Part D and are based on extrapolated 100,000-hour creep properties. The time dependent allowable stress values for Service Loadings, St, as a function of time and temperature, up to the maximum design lifetime and use temperature, are required for the applications of the Service condition limits. The criteria for establishing St for base metal for each specified time, t, are based on the lesser of
1. 100% of the average stress required to obtain 1% total strain
2. 80% of the minimum stress to cause the initiation of tertiary creep, 67% of the minimum stress to cause rupture
3. Minimum stress-to-rupture curves are also required in the applications of the Service condition limits.
In this paper, the technical background for time dependent allowable stress values for Alloy 617 is presented. These values are being recommended to the ASME Code committees for approval, but are subject to change upon feedback from Code committees.