A diffusion model for tungsten powder carburization
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I.
INTRODUCTION
T U N G S T E N carbide powders are usually prepared by heating an intimate mixture of tungsten and carbon powders, called black mix, in hydrogen at a high temperature. Smaller particles of tungsten carburize more readily than large particles, and it is generally accepted that a diffusion process is controlling the rate of carburization. Tungsten powders consist of a distribution of particle sizes, and the time to complete carburization depends on the content of larger particles, just how large they are, and to what extent complete carburization is sought. Carburization kinetics of tungsten powders is complicated by this feature, and a mathematical description must be more or less approximate. The equation used here to describe diffusion kinetics for a fine powder is based on a spherical model. Real tungsten powders consist of irregular polyhedra with rounded corners, however, and an exact diffusion model would be very complex. The assumption of spherical diffusion probably provides an essentially correct model, and, though compromised, the description of the process is useful. Related kinetic investigations are the carburization of comparatively large spheres or cylinders, and coarse tungsten powders which were packed in carbon powder and heated in hydrogen, and the carburization of powder in methane and hydrogen. Pirani and Sandor 1 studied 2.5 mm tungsten spheres, and Fries e t al. 2 made cylinders from tungsten wire and rod. Both used sectioning techniques to measure the thickness of the advancing carbide zones for the calculation of kinetic data. Hara and Miyake 3 mixed coarse tungsten powders with carbon black for carburization and obtained much useful information from X-ray diffraction of sectioned particles. More recently Davidson, Alexander, and Wadsworth 4 used a thermogravimetric method to measure the initial rate of carburization of a fine tungsten powder by methane in hydrogen. X-ray diffraction results in the latter three studies 2'3'4 indicate that ditungsten carbide, W2C, forms first and is followed by tungsten carbide, WC, at all temperatures despite the instability of W2C as a phase below 1250 ~ with respect to tungsten and WC. s In contrast Pirani and Sandor ~ indicate that WC is the only product in their experiments. LEWIS V. McCARTY is retired from the Lamp Metals Laboratory of the General Electric Company, Cleveland, OH 44112. RICHARD DONELSON, formerly with the Lamp Metals Laboratory, is an Advanced Materials Engineer with the Institute of Gas Technology, Chicago, IL 60616. ROBERT E HEHEMANN, deceased, was Professor of Metallurgy at Case Western Reserve University, Cleveland, OH. Manuscript submitted May 2, 1986.
METALLURGICALTRANSACTIONS A
It is very difficult to obtain good kinetic data for the carburization of powders by sectioning and measuring partially carburized tungsten particles, particularly for very fine powders. For this reason X-ray diffraction analysis was used with carefully pulverized samples of partially carburized powders, and the thickness of the remaini
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