Densification

During hydrogen sintering, densification of pressed cemented carbide parts usually is accomplished by mechanically stoking through a muffle-type electrically heated furnace held at a peak temperature of 1400 to 1500 °C (2550 to 2730 °F). A densification curve for WC-lOCo composition (Fig. 45) shows that significant densification begins at 1000 °C

(1830 °F) and that greater than 90% theoretical density can be obtained at 1300 °C (2370 °F). Liquefaction of the cobalt phase occurs between 1280 and 1350 °C (2335 and 2460 °F) as a result of the formation of a tungsten carbide/cobalt eutectic or eutectic-like phase (approximately 35 wt% tungsten carbide that forms during heating, as discussed in Ref 46). Essentially, the part becomes fully dense soon after the liquid phase forms.

Fig. 45 Density-temperature relationship for WC-10Co. 30 min hold at each point

During densification, the pressed part undergoes a volumetric shrinkage of —40 to 45% (16 to 18% linear shrinkage). Complete densification is readily accomplished for typical cemented carbide compositions that contain —3 to 25 wt% Co (5 to 40 vol%). Complete densification is more difficult to achieve for cobalt contents of <3%. For cobalt contents greater than —25%, part shape is difficult to maintain due to the presence of high liquid-phase fraction, which allows the part to sag.

Densification proceeds during vacuum sintering at about the same rate as it does for hydrogen sintering, reaching nearly 100% of theoretical density soon after the cobalt or nickel phase liquefies at 1280 to 1350 °C (2385 to 2460 °F); the exact temperature of liquefaction depends on the carbon content. When titanium carbide, tantalum carbide, and/or niobium carbide are present in the cemented carbide, the carbon-oxygen degassing reactions must be completed before densification closes the pores. If this does not occur, the evolving gases become entrapped in the interior of the compact, thus causing high residual porosity. Holding at —1100 to 1250 °C (2010 to 2280 °F) for —30 to 90 min ensures proper degassing of titanium carbide, tantalum carbide, and niobium carbide compositions.

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