Microstructure

At the beginning of peak temperature holding, the microstructure is nearly fully dense, and the carbide particles are nearly unchanged in size and shape from the original milled powder state. They are relatively small, irregularly shaped, and poorly dispersed, tending to agglomerate. The cobalt phase is also poorly dispersed, and many pools or lakes may be present (Fig. 46a). A noticeable amount of residual porosity also exists (Fig. 46a).

Fig. 46 Microstructure of WC-6Co. (a) Undersintered. (b) Normal structure. (c) Discontinuous grain growth

The main purpose of the final stage of the sintering operation is to develop the microstructure by holding at a temperature above the cobalt melting point for a time sufficient to develop a more uniform carbide structure with good cobalt phase dispersion and minimal residual porosity (Fig. 46b). This is usually accomplished by holding for 30 to 90 min above 1350 °C (2460 °F), reaching a peak temperature at "-'1400 and 1600 °C (2550 and 2910 °F). During this period, the cobalt phase, driven by capillary forces, disperses more evenly. This process also improves carbide particle distribution.

Carbide distribution is further improved during holding by the dissolution of small particles into the liquid phase, with subsequent precipitation onto the larger particles during cooling. This results in a gradual increase in average particle size (Fig. 47a). Increasing the sintering temperature has a similar influence on grain growth (Fig. 47b).

Fig. 47 Effect of sintering on grain growth of WC-25Co compacts. (a) Effect of sintering time at 1400 °C (2550 °F). (b) Effect of sintering temperature (1 h hold)

One undesirable consequence of grain growth by the solution and precipitation process is the tendency of large tungsten carbide grains to grow at a disproportionately high rate. This discontinuous grain growth (Fig. 46c) occurs more readily at lower cobalt contents (3 to 6 wt%) with finer average particle size mixes. It is most pronounced when tungsten carbide is the only carbide phase present. Small additions (0.1 to 0.5%) of group VB carbides (vanadium carbide, niobium carbide, and tantalum carbide) undergo significant grain growth in fine-grained tungsten carbide/cobalt compositions. Titanium carbide is also a strong grain growth inhibitor (Ref 46), as well as chromium carbide.

During vacuum sintering, cobalt losses by evaporation should be controlled. If uncontrolled, as much as 10 to 20% of the cobalt content of the part may be lost, thus resulting in a loss of mechanical strength and the formation of a rough, coarsegrained surface structure caused by the precipitation of tungsten carbide from the evaporating cobalt (Ref 47 and 48). Cobalt evaporation can be minimized by completely enclosing the pressed parts in graphite fixturing, the walls of which are maintained at the sintering temperature.

The enclosure causes a buildup of cobalt vapor pressure around the cemented carbide parts, which reduces the evaporation rate. Cobalt losses can also be controlled by operating at pressures of at least 66 Pa (0.5 torr), rather than in the range of 1.3 to 13 Pa (0.01 to 0.1 torr) that mechanical vacuum pumps and blowers are capable of maintaining. Higher pressures can be maintained by placing a throttling valve between the furnace and the pump or by injecting an inert gas into the furnace.

Sinter HIP results in the closing of pores and macrovoids with microstructure typical of the surrounding material. Similar results can be obtained through the use of HIP provided that judicious selection of time and temperature is employed in conjunction with HIP pressure for the specific binder level of the material undergoing post-sintering treatment. Early attempts at HIP resulted in the extrusion of binder material into the pores and macrovoids once the melting point had been reached and the formation of cobalt "lakes" or "pools" in the microstructure (Ref 49).

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