SOME ASPECTS OF POWDER METALLURGY 441 



The die materials are usually the simpler tungsten carbide compositions 

 and can be used to advantage for extruding, drawing, sizing, and other 

 operations where the shape or dimensions of the article being worked is 

 changed but where no metal is removed in the operation. The tungsten 

 carbide can be used in this way for shaping many types of metals and 

 alloys and this has been a major use of the product. 



Cemented carbides, either the simple type or the mixtures, depending 

 on the application, have been successfully used as cutting tips on a variety 

 of tools, and for a number of different materials. This use has increased 

 steadily due to the remarkable increase in production achieved. Decrease 

 in cost of the tips and parts during recent years has further stimulated use. 



Wear and corrosion resistant parts include gauges, guides of many types, 

 pump valves for abrasive materials, sandblast nozzles, burnishing tools 

 and dies, and many others where utilization of the superior properties is 

 indicated. 



One use recently reported^^ has been that of cemented tungsten carbide 

 for bullet cores in ammunition for anti-tank weapons used by the enemy 

 in the desert warfare in Africa. The material has about twice the density 

 of steel and is much harder, and, although not greatly resistant to shock 

 under normal conditions, becomes quite effective under the high pressures 

 attained during striking and penetrating armor plate. 



2. Poro2ts Bearings 



Porous bearings, always a large runner in the powder metallurgy field, 

 have been described in the section on the historical development. Where 

 the bearings are impregnated with oil, there is usually sufficient to last 

 the lifetime of the assembly, but provision can readily be made for supplying 

 additional oil if needed by utilizing the capillary action of the interconnecting 

 pores to draw oil from a reservoir in contact with the bearing wall. In 

 such assemblies, there is always a film of oil for the shaft to run on in contrast 

 to normal bearings where an oil film does not coat the shaft until run for 

 some time. 



3. Motor Brushes and Commutator Segments 



Numerous types of current collector brushes are now made by powder 

 methods. Copper powder can be added to the graphite mixture, and the 

 desired part pressed and sintered below the melting point of copper to de- 

 velop a strong, high conductivity brush of longer life for use against copper 

 surfaces. Greater wear resistance may be obtained by adding zinc, tin, 

 or nickel to the mixture. Improvement in operating smoothness may 

 be attained by the incorporation of lead''^'-^. 



