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Cobalt free alloys

Since the use of cobalt alloys should be avoided as far as possible in pressurized water reactor (PWR) components, a programme aimed at establishing the performance of cobalt free alloys has been performed for valve applications, where cobalt alloys are mainly used. Two types of cobalt free alloys were selected: nickel-chromium-boron-silicon and nickel-chromium-iron alloys.

Cobalt free valve behaviour has been compared to that of cobalt valves by implementation of a programme in a special PWR test loop. As a result of the loop test programme, which included endurance, thermal shock and erosion tests, cobalt free alloy candidates to replace cobalt alloys can be proposed in relation with valve type (globe valve and swing check valve). Colmonoy 4-26 (nickel-chromium-boron-silicon alloy) and Cenium Z20 (nickel-chromium-iron alloy) deposited by plasma arc process were found suitable for use in 3 inch swing check valves. For integral parts acting as guide rings, Nitronic 60 and Cenium Z20/698 were tested successfully. For small bore components such as 2 inch globe valves, no solution can yet be proposed; introduction of cobalt free alloys is dependent on the development of automatic advanced arc surfacing techniques applied to small bore components.

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