Cobalt hard-facing alloy

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

After a few years of operation there was a significant increase in radiation fields from the primary circuit piping in the Douglas Point generating station. Other water-cooled reactors around the world experienced similar effects. The principal source of the radioactivity was traced to cobalt-60, formed by neutron absorption in the natural cobalt-59 which arose from hard-facing alloys and was also present as an impurity in boiler materials such as HMDnel, and in carbon steel and other structural materials. The mechanism of this radioactivity transport was found to be corrosion of the cobalt bearing materials, transport... 

In recent years, the spraying process has been adapted for hard facing, using the chromium-nickel-boron alloys which have become known as Colmonoy. More recently still, the cobalt-base Stellite alloys have also been used. These materials in powder form are sprayed on to the surface in the usual way. The deposit is afterwards heat treated by a torch, so that fusion takes place. The process is often known as spray-welding. Such coatings are primarily used for hard facing under wear conditions, but as the Anal surface is nickel-chromium or cobalt-chromium they exhibit very high anticorrosive properties. 

Three types of metals are used exclusively stainless steels, nickel-chromium-iron alloys and, to a limited extent, cobalt-based alloys. These materials have provided many years of successfiil operation in similar control rod drive mechanisms. In the case of stainless steels, only austenitic and austenitic stainless steels are used. Where low or zero cobalt alloys are substituted for cobalt-based alloy pins, bars, or hard facing, Ihe substitute material is qualified by evaluation or test. The materials used for reactor internals are chosen to be compatible with the primary coolant chemistry and, as far as is possible, to be free from elements such as carbon or cobalt, which are prone to activation. Full details of the materials used, as well as the controls on fabrication, are provided in Section 4.5 of Reference 6.1. 

Screw conveyors have a variety of configurations. Each is designed for specific applications and/or materials. Standard conveyors have a galvanized-steel rotor, or helix, and trough. For abrasive and corrosive materials (e.g., wet ash), both the helix and trough may be hard-faced cast iron. For abrasives, the outer edge of the helix may be faced with a renewable strip of Stellite (a cobalt alloy produced by Haynes Stellite Co.) or other similarly hard material. Aluminum, bronze, Monel, or stainless steel also may be used to construct the rotor and trough. 

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