Results of NRC-Sponsored Stellite 6 Aging & Friction Testing (NUREG/CR-6807)
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Manuscript Completed: October 2002
Date Published: March 2003
J.C. Watkins, K.G. DeWall
Idaho National Engineering and Environmental Laboratory
Idaho Falls, ID 83415
J. E. Jackson, NRC Project Manager
NRC Job Code W6593
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report describes a series of tests sponsored by the U.S. Nuclear Regulatory Commission and conducted by the Idaho National Engineering and Environmental Laboratory. The tests support research addressing the need to provide assurance that motor-operated valves are able to perform their intended safety function, usually to open or close against specified (design basis) flow and pressure loads. One of the parameters that affect gate valve operability is the friction between the disc seats and the valve body seats. In most gate valves, these surfaces are hardfaced with Stellite 6, a cobalt-based alloy.
The tests described in this report investigate the changes that occur in the friction as the Stellite 6 surfaces age and develop an oxide film. Stellite 6 specimens were aged in a corrosion autoclave, the oxide films were examined and characterized, and the specimens were subjected to friction testing in a friction autoclave. A very thin oxide film formed after only a few days of natural aging. Even a very thin oxide film caused an increase in friction. The surface structure of the oxide film was dominated by a hard crystalline structure, such that the frictional response was analogous to rubbing two pieces of sandpaper together. The friction of the naturally aged specimens was observed to increase initially, but eventually stabilized.
Friction testing of specimens subjected to simulated in-service testing strokes at intervals during the aging process showed only a slight decrease in friction compared to other specimens. Results from specimens subjected to accelerated aging were inconclusive, because of differences in the structure and composition of the oxide films, compared to naturally aged specimens.
For the naturally aged specimens, the highest friction occurred on the first stroke. The first stroke smeared the oxide film and dislodged some of the granules, so that subsequent strokes saw lower friction values and less variation in the friction. This result underscores the importance of planning in-plant tests so that data are collected from the first stroke following a period of inactivity.