Initial Kinetics of Tungsten Carburization by Methane
- PDF / 640,637 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 97 Downloads / 219 Views
T
HE gas phase carburization of tungsten, which involves a reaction between elemental tungsten and a carbonaceous gas has been shown to occur at relatively low temperatures- 800 to 1000°C.l-4 Many of the processes to synthesize tungsten carbide have used carbonaceous gases in such high concentrations that sooting occurred. But if carefully controlled, the gas phase carburization of tungsten has the potential to produce tungsten carbide with much better control of carbon content and grain size than the more commonly used carbon black-tungsten carburization. Energetically, the carburization of tungsten occurs spontaneously when the gas composition is maintained above a minimum level determined by the carbidemethane equilibrium. The minimum concentration of methane necessary for carburization is different for each temperature and may be calculated from thermodynamic tables .5~ The formation of soot is avoided if the methane concentration is maintained below the C-CH 4 equilibrium. It has been shown that tungsten mono carbide is the most stable phase" during the carburization of tungsten at reaction temperatures below 1300°C. The carbide formation begins at the tungsten surface and grows inward producing a case of carburized tungsten and a core of tungsten,":" Upon further reaction, the case of carburized tungsten grows in thickness until all of the tungsten has reacted. This paper presents initial reaction kinetics for the carburization of tungsten in H2-CH 4 atmospheres. A model is developed that predicts initial kinetics for a variety of experimental conditions. EXPERIMENTAL Reaction rates for the carburization of tungsten by H2- CH4 were measured in a vertical alumina tube furnace. An alumina dish one in. in diam containing nominally 5 g of powdered tungsten was suspended in the furnace from a single pan H10 mettler balance. The balance sensitivity was ±0.005 mg. Changes in mass occurring as the tungsten picked up carbon during carburization were measured as a function of time. A Beckman infared analyzer and gas flow meters CHARLES F. DAVIDSON, formerly with Fansteel Research Center, Salt Lake City, Utah, is now Research Metallurgist, General Electric, Carboloy Division, Detroit, MI, GUY B. ALEXANDER is Director of Research, Fansteel Research Center, Salt Lake City, UT, and MILTON E. WADSWORTH is Associate Dean, College of Mines and Mineral Industries, University of Utah, Salt Lake City, UT 84108. Manuscript submitted June 6, 1977. METALLURGICAL TRANSACTIONS B
were used to control the influent gas concentration and flow rate during each run. The tungsten powder used in these experiments was made by reducing W0 3 with hydrogen at 1000°C and then air classifying to collect the powder fraction having an apparent 4 to 6 micron particle size. The partical size was measured by sedimentation using a Micromeritics Sedigraph 5000. Typical analyses of oxygen in the powder averaged 0.1 pet. The specific experimental procedure was to place a weighed amount of tungsten powder in the reaction dish and at room temperature to su
Data Loading...
