| Abstract Scope |
Despite ASME BPVC qualification in Section III, Division 5, several mechanical property gaps exist for the qualified base metals (304H, 316H, 800H, 617) and especially for their welds and heat affected zones. These gaps are illustrated by the recent critical review and ongoing testing program for the Kairos Power – Fluoride-salt-cooled, High-temperature, Reactor (KP-FHR). The KP-FHR utilizes 316H base metal and 16-8-2 weld filler in the temperature range of 550-650ºC for structural components. Despite the technical maturity of these alloys, Kairos Power has undertaken significant testing programs to better assess (a) high temperature properties of welds, (b) corrosion performance in molten salts, (c) environmental effect (molten salt + irradiation) and (d) stress relaxation cracking. Furthermore, the ability to utilize desirable welding processes like local electron beam welding will require additional high temperature property data. For illustration, details our stress relaxation testing program are given which compares the heat affected zones of 347H and 316H. These tests use isothermal, notched bar stress relaxation tests to quantify susceptibility to cracking. Results show that both alloys are resistant to cracking at 500°C, susceptible at 550°C but diverge in behavior at 600°C where 347H shows increased susceptibility and 316H increasing resistance. Failure times and temperatures in 347H correlate with formation of intragranular Nb(C,N) and analysis indicates that precipitation-induced strains promote cracking in 347H. The ability of this test method to control the stress state, microstructure tested, elastic follow-up, and produce intergranular cracking at temperatures prototypical of operation are key advantages and it should be utilized as a standard method to assess and compare resistance to reheat-type cracking. |