Abstract
Cobalt-based alloys are used as a hard-facing for valve seats installed in nuclear power plants. In high-temperature, high-pressure service environments, cobalt leaches out due to erosion, corrosion, wear and cracking. Since leached cobalt becomes 60Co due to radiation in the reactor, we have developed a method to improve the corrosion and wear resistance of cobalt-based alloy valve seats using directed energy deposition(DED).
In this development, powdered cobalt-based alloys are melted by a laser and bonded to the surface of the base material.
The objectives of the research were to evaluate the material properties, and to conduct an evaluation through an actual performance test and an endurance test using a 600A motor operated gate valve.
The material properties were evaluated as follows: the erosion depth was less than one-tenth that of conventional valve seats in high temperature water corrosion tests, the plane strain fracture toughness KIC was about three times that of conventional valve seats.
In the actual performance test and durability test using a 600A motor operated gate valve, there was no significant damage to the seat surface even after 100 operations at 288°C in a saturated steam differential pressure environment, and amount of the leakage was less than the allowable leakage, and the performance was confirmed to be equivalent to that of a conventional valve seat.
