SOME ASPECTS OF POWDER METALLURGY 437 



Typical Powder Metallurgy Products 



Most of the materials produced by powder metallurgy prior to about 

 1940 are well known; some have already been mentioned in this article, 

 but for convenience are included in the following descriptions of typical 

 products. Others of more recent development owe their immediate exis- 

 tence to the demands of wartime production, and, while some have been 

 described in some detail in the technical literature, many have had only 

 brief mention. Some tj^^ical parts made by powder methods are shown 

 in Figures 1, 2, and 3. 



1. Cemented Carbides^''^^-^^'^^ 



Although tungsten carbide was produced many years ago and was found 

 to be extremely hard, it was so brittle and low in strength that its use com- 

 mercially where advantage could be taken of the high degree of hardness 

 was not possible. About 20 years ago, it was discovered that the addition 

 of a small amount of metalhc constituent, such as cobalt, to the tungsten 

 carbide powder would yield a hard, relatively strong compact after sintering. 

 During the heating operation, there is partial melting with some solution 

 of the carbide by the cobalt; and on cooling the cementing material produces 

 the required strength. 



The method of preparing the powders, compacting, and sintering has 

 undergone considerable improvement since the first carbide materials were 

 made. Essentially, in outline, the process consists of first preparing the 

 tungsten carbide powder, mixing it with cobalt powder and ball milling 

 the mixture until proper grain size is obtained and the carbide particles 

 are coated with a thin layer of the cementing metal. In this treatment, 

 other carbides are added as required. Following the milling operation, 

 the mixed powders are pressed in suitable molds and given a pre-sintering 

 heat treatment to increase the strength for handling and to remove, by 

 volatilization, lubricants which may have been used to facilitate pressing. 

 After the pre-sintering operation, the compact can be cut to desired shapes 

 quite readily. The sintering treatment which follows is carried out at 

 about 1400°-1500° C. with the pressed parts placed in carbon boats or 

 on carbon slabs and heated in a suitable neutral or reducing atmosphere. 

 There is considerable shrinkage in dimensions in this sintering treatment 

 which gives a product that is hard, dense, sound, and strong. Any further 

 shaping is done by grinding or lapping operations. 



The cemented carbides have many uses usually falling into the three 

 general classes of die materials, cutting tool materials, and wear and cor- 

 rosion resistant materials. 



