| Yoshihisa Kiyotoki | Nuclear Plant Dept., Hitachi Works, Power Systems, |
| Mitsuo Chigasaki | Dept., of Materials Research for Power Plants, Material Research Lab., |
To improve the conservativeness and reliability of valves at fossil fuel and nuclear power plants, using Co (cobalt) base alloy, which is overlay-welded on the valve seat as a hard facing layer, is necessary. This use prevents the deterioration of valve performance due to corrosion, erosion and cracks. Concerning erosion damage sustained by the Co base alloy used for the valve seat under an environment of high dissolved oxygen (DO) and temperature, this phenomenon originally results from localized corrosion damage of the mesh-like eutectic carbide. Therefore, to improve valve performance and the characteristics of Co base alloy, we manufactured a new valve featuring valve seat alloy in which the eutectic carbide particles are dispersed in the metal matrix within the range of chemical composition of RCoCr-A. After evaluating the performance of the new valve, we confirmed its active role in improving valve performance. Especially, the amount of Co that escapes from the valve was found to be 1/10 or less compared with conventional valves. This demonstrates that a substantial decrease in the radioactive dose sustained by nuclear power plant personnel can possibly be achieved by adopting the new valve. Moreover, to demonstrate the excellent qualities described above in an actual operating plant, we performed tests comparing the conventional valve with the new one within the same environmental conditions for around one year. Our test results verified the outstanding ability of the new valve to withstand corrosion and erosion, compared to conventional valves.
 | The Hitachi Hyoron (Japanese Only) |
corrosion, erosion, friction, cobalt, valve
