Status of Stellite 6 friction testing

Abstract

For the past several years, researchers at the Idaho National Engineering and Environmental Laboratory, under the sponsorship of the U.S. Nuclear Regulatory Commission, Office of Nuclcar Regulatory Research, have been investigating the performance of motor-operated valves subjected to design basis flow and pressure loads. Part of this research addresses the friction that occurs at the interface between the valve disc and the valve body seats during operation of a gate valve. In most gate valves, these surfaces are hardfaced with Stellite 6, a cobalt-based alloy.Analytical methods exist for predicting the thrust needed to operate these valves at specific pressure conditions. To produce accurate valve thrust predictions, the analyst must have a reasonably accurate, though conservative, estimate of the coefficient of fiiction at the disc-to-seat interface. One of the questions that remains to be answered is whether, and to what extent, aging of the disc and seat surfaces effects the disc-toseat coefficient of friction. Specifically, does the environment in a nuclear plants piping system cause the accumulation of an oxide f i l m on these surfaces that increkies the coefficient of friction; and if so, how great is the increase?This paper presents results of specimen tests addressing this issue, w i t h emphasis on the following:The characteristics and thickness of the oxide film that develops on Stellite 6 as it ages IThe change in the friction coefficient of Stellite 6 as it ages, including the question of whether the fiiction coefficient eventually reaches a plateauThe effect in-service cycling has on the characteristics and thickness of the oxide film and on the fiiction coeficient.