The overlaying of Co-based alloy (e.g. “Stellite”) has been applied to valve seats in power plants to improve wear and abrasion resistance. However, using of the Co-based alloy should be avoided to valves which involve with reactor water in nuclear power plant because of activating to Co-60, and increment of radiation dose. Therefore, the alternative material to Co-based alloy has been required for these valves.
Hardfacing is usually applied to improve abrasion resistance. Several high hardness materials without Co element were investigated, and it was found a Fe-based alloy as an alternative to Co-based alloy for the overlaying of valve seats.
Mechanical tests of this Fe-based alloy formed by Plasma Transfer Arc (PTA) welding were carried out. Obtained values of tensile strength, Vickers hardness and Charpy impact value of the Fe-based alloy were almost equivalent to Co-based alloy. In addition, mock-up valve specimen was prepared, and sliding test was performed in simulated BWR plants environment. As a result, the amount of leakage was little enough when the Fe-based alloy is overlaid on one side seat part, and the radiation exposure from the valve seat can be reduced by 78 percent compared with the case of using the Co-based alloy for both side of overlaying.
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