Tuesday, 22 March 2011

Why is the grain size important?

The story of Tungsten Carbide (WC) powder metallurgy, and particularly that of the hardmetal industry, is depicted by a progressively widening range of available grain sizes; whilst simultaneously narrowing the grain size distribution for each grade of WC powder.

The most significant motivation for this broadening of the range of available WC grades is the properties of WC hard metals such as hardness, strength, toughness, thermal conductivity and abrasion resistance can be extensively varied by grain size. At the start of the hardmetal industry the range of available WC grain sizes ranged from 2.0 to 5.0 microns, the grain sizes of WC powders now used in hard metals range from 0.5 microns to 50 microns, or even 150 microns for some very special applications.

The majority of grades we machine are made with standard size grains varying between 1 and 3 microns in size. Using larger grains of 2 - 6 microns will greatly increase the strength and toughness of the material because the larger grains interlock better. The tradeoff is that larger grain materials do not offer as much resistance to wear as finer grain sized materials. Sub-micron materials that vary between 0.4 and 1.0 micron grain size are harder than standard grain materials with the same cobalt content. The sub-micron grains are much more uniform in size and hence give improved hardness as well as increased carbide strength. However, as specs show the transverse rupture strength is perhaps 20% improved on 15% sub micron compared to 15% fine grain material but this can give a false impression as sub-micron carbide is not as resistant to impact and may chip more easily.

There are currently four carbide grades offered for die and wear part function. The first is the typical grade with a 1.5 micron to 17 micron grain size of tungsten carbide. The typical carbide grade is chosen where light, medium, or heavy shock loads are faced. It is commonly used in settings such as airless paint spray nozzles, valve seats, pump seals, cold heading fastener dies, cold impact extrusion punches, wire drawing and seaming rolls.

A submicron carbide is also selected where light, medium, or heavy shock loads occur, but when a fine, keen cutting edge is also required.
If the size of the grain is too large and the percentage of binder is too higher, then the carbide will deform under pressure. One of the major advantages of carbide is its proficiency to cope under pressure or compressive force. If it is too soft it loses that ability.

Neither binder percentage or grain size alone determines how a grade will perform.

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