Grain boundary precipitation treatment for improving the high-temperature low-cycle fatigue strength of SSS113M for VHTRs

A grain boundary precipitation treatment was studied for the purpose of improving the high-temperature low-cycle fatigue strength of a Ni-23% Cr18% W alloy, SSS113M, which had been developed as an intermediate heat exchanger material of very high temperature reactors and evaluated as the best alloy in the national RandD program of nuclear steelmaking in Japan. A conventional standard solution treatment of 13000C X 1 h water quenched does not cause any grain boundary precipitation in SSS113M, but an additional heat treatment of 12500C X 1 h causes discontinuous grain boundary precipitation of the alpha-tungsten phase. This grain boundary precipitation treatment results in two- to fivefold increases of low-cycle fatigue lives at 8000C as well as slight increases of the creep and stress rupture strength at 10000C