Cobalt based overlaying materials have been widely used as valve seat materials for high temperature and high pressure services, because of the high corrosion and wearing resistances. In nuclear power plants, however, the cobalt-based alloys would be the potential source of 60Co in the reactor water, thus alternate materials (Co-free materials) have been required and studied, especially for use as the large size valve seats on the feed water and condensate lines. From the results of a screening wearing test (sliding distance: 6mm), some Ni-based alloys were selected as the most promising material. Then sliding tests (sliding distanc: 20mm) and mechanical tests were performed on the selected candidate alloys.
In conclusion, two Ni-based alloys, that is, A-1 (10Cr-4.5Si-0.2B-40Fe-Bal. Ni) and A-3 (10Cr-4.5Si-0.2 B-40Fe-Bal. Ni) were confirmed to be useful for large size valves for high temperature and high pressure services, and the mechanical properties of both alloys were similar to those of the conventional cobalt based material.