A study of microstructures of tungsten carbide powder
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I.
INTRODUCTION
TUNGSTEN carbide is the dominant phase presented in the hard metals which may contain more than 90 wt pct WC. While the influence of the processing variables on the microstructures of the WC-phase is clearly important, l'z a systematic study of the microstructure of WC as a function of its forming conditions has not been reported in the literature. In view of this, the present investigation attempts to make a detailed study of the microstructure parameters of WC powders as a function of the powder forming process from analysis of X-ray diffraction profiles of the WC powder. Because of the small amount of diffusion of X-ray diffraction, the coherent domain sizes and the rms strains within these domains are treated precisely. A second objective of this study was to determine the effects of the reaction temperatures used in making WC powders on its fracture behavior. Ball milling is generally used in the process of making hard metals. The efficiency of milling is related to the fracture characteristics of the powder. However, the fracture characteristics of the WC-phase in hard metals are difficult to observe directly. But as pure powder, they can be investigated more easily. Hence, in the present work, the WC powders were mild ground and then examined by SEM. The particle size distribution and average particle size were measured by light absorption method giving Stokes diameter. C2H602 was used as dispersion medium. II. EXPERIMENTAL PROCEDURES AND THE METHODS OF ANALYSIS
A. Materials The tungsten blue oxide was chosen as the starting material. Its composition is shown in Table I. The oxide was reduced by hydrogen at one of the two temperatures, 900 ~ and 1050 ~ The tungsten powders were then carburized at either 1700 ~ 1800 ~ 1900 ~ or 2200 ~ This combination of reduction and carburization temperatures produces eight variants of the WC powder. In order to cut down the agglomerates, the as-received powders were dry milled for
JIANCHUN LI is Associate Professor, Department of Materials Science, Central-South University of Technology, Changsha, People's Republic of China. Manuscript submitted February 27, 1986.
METALLURGICALTRANSACTIONS A
half an hour in the ball mill down to an average particle size of approximately 20/x. These powders are still named unmilled powder in this paper. The eight variants of powder were then mixed with 10 wt pct cobalt and wet milled for 24 hours. After wet milling, to avoid the interference from cobalt in the X-ray diffraction profile, the mixed powder was boiled in 10 pet HC1 solution in order to remove cobalt. The WC powder prepared using highest reduction and carburization temperature, variant 8 at 1050 ~ and 2200 ~ was chosen to be the standard sample for the X-ray profile analysis of WC. There are two reasons for this. First, this sample had the largest diffraction domain size and the lowest imperfection content: the coherent diffraction domain sizes are over 1 micron (as shown in Figure 9(a)). Second, the diffraction profile of (300) (20 = 135~ of this powder is
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