THE CONTROL OF GRAIN SIZE IN THE MANUFACTURE OF SINTERED HARD-METAL *

In sintered hard-metals grain-size control is of exceptional importance because the basic properties of wear-resistance and strength are critically dependent upon it. In some applications a change in average grain size from 5·0 to 1·0 μm can increase wear-resistance by as much as twenty times, while the same change can halve the transverse rupture strength.

Each step in manufacture, from the chemical processing of the ore to the final sintering operation, can influence the final grain size. The factors operating at each stage are not yet completely understood. However, the most important processes of tungsten reduction and carburizing have been closely studied and these are chosen to control and monitor grain size during manufacture. The factors influencing grain size during these operations are discussed in some detail. The difficulty of relating powder-particle size and sintered-product grain size is mentioned. This is largely due to the weaknesses of conventional methods of measurement. Size changes in reduction, carburizing, and milling have been examined by metallographic means, and this has shown clearly the effect of aggregation and of polycrystalline particles in confusing the relationship between powder-particle size and sintered grain size.

The metallographic method of mounting products in copper has been especially informative with products with a grain size of > 1·5 μm, but optical limitations make the results less clear with a finer size. However, inferences drawn from this work have been largely confirmed by the scanning micrograph.