Hardfacing by chemical vapor deposition of components used in coal gasification units

Abstract

The Bureau of Mines, as part of an interagency agreement with the Energy Research and Development Administration, is investigating the feasibility of hardfacing the critical surfaces of valves and similar devices by chemical vapor deposition. The objective is to increase the life of these components in the abrasive and erosive environment existing in coal gasification units. Various chemical vapor deposition materials such as the refractory metals, carbides and nitrides were considered for this application and are undergoing laboratory investigation and evaluation. The first hardfacing material investigated was tungsten because of the Bureau's previous experience with this material in lining rocket nozzles. Laboratory research indicated that adherent tungsten coatings could not be obtained on the stainless and carbon steels used in value construction unless they were first electroplated with a thin layer of nickel. Tungsten was deposited by hydrogen reduction of tungsten hexafluoride at temperatures ranging from 450 °C to 650 °C. A large reaction chamber 20 in (0.51 m) in diameter with a rotating gas-dispersion apparatus was fabricated. The large unit was used to coat carbon steel valve seats of inside diameter 10 in (0.25 m) with tungsten, and heating of the ball valve seats to reaction temperature was done by induction. Successfully coated valve seats are undergoing evaluation in the low Btu coal gasification pilot plant at the Morgantown Energy Research Center, Morgantown, W. Va. Results of completed and ongoing chemical vapor deposition research are presented.