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 screening test and actual model test, Ni based alloys were selected as Co-free seat materials.
The performance test of large valves (nominal size 300 A and 600 A) with Ni based alloys (A-1 and A-3) seats were done under the condition of high temperature and high pressure 100 for the 300 A valve and 50 cycles for the 600 A valve and the results were confirmed to be good.
