In the realm of ultra-hard materials, cemented tungsten carbide has become a popular choice for tools, dies, and wear parts. This material can provide exceptional performance in a variety of applications if the toolmaker or engineer specifies a grade with optimum characteristics for the intended application.

Selecting the proper tungsten carbide grade boils down to an educated compromise. The specifier, aware that no metal offers the ultimate in everything desired, must determine what properties are needed most, and then strike the balance that best meets his or her needs.

How much toughness is needed? What about resistance to corrosion and shock? How important is wear and abrasion resistance? What design parameters must be considered? What are the applied loads and stresses? Is edge preparation a consideration?

Then add to this matrix considerations such as tungsten carbide grain size, binder alloy percentage, and the addition of titanium, tantalum carbide, and other alloying elements. All of these variables have to be calculated to achieve the best compromise and result.

Cemented tungsten carbide consists of tungsten carbide particles that are glued or cemented together by a relatively ductile, pure metal cobalt or nickel if corrosion is a consideration. The tungsten carbide acts as "bricks" and the binder material as "mortar."

Key to determining the properties of a tungsten carbide material is the relationship between carbide grain size and binder percentage. The smaller the carbide grain size, the higher the hardness and abrasive resistance, but the lower the resistance to shock. Larger carbide grain size increases toughness and resistance to shock loads, but reduces hardness.

When the binder percentage is lowered, shock resistance declines. Conversely, raising the binder percentage improves shock resistance. This behavioral change occurs because the binder is the ductile constituent in the cemented tungsten carbide.

Three carbide categories

Three categories of carbide grades have been used for tool, die, and wear part applications. The first is the conventional grade, with a 1.0(mu) to 6.0,(mu) tungsten carbide grain size. The second is a submicron 0.7(mu) average grain size. The third is an ultrafine submicron 0.5(mu) tungsten carbide grain size. Although all three use a cobalt binder (Fig 1), they also can use a nickel binder.

The conventional carbide grades have been used most frequently where light, medium, or heavy shock loads are encountered. Recent successes have been reported for this grade when substituted for steel:

Necker dies to make aluminum beverage cans for the container industry-die life increased up to 20 times.

Poppet valve stems and seats in airless paint spray compressors for the commercial and industrial spray paint industry-25 times longer wear life, with much less production downtime.

Crush form rolls, used to crush specific forms into an abrasive grinding wheel. The rolls were used for high volume grinding of parts for the automotive, steel and aircraft industries-25 times longer roll wear life.

Other applications where the conventional carbide grade have been used include: rotary mechanical pump seals, electrical fractional motor lamination dies, cold heading fastener dies, back shaving dies and tube mill rolls.

Finer grades

The submicron carbide grades have been used also where light, medium, or heavy shock loads are encountered, but more particularly when a fine, keen cutting edge is also needed.

Typical applications have included:

Staking and caulking punches used to coin valve seats in automotive antilock braking system compressor pumps. By moving from a conventional to a submicron tungsten carbide grade, one plant increased its quality and quintupled its punch wear life.

Flex blades used for cutting applications in the paper converting and nonwovens industry. One large manufacturer, changing from a steel blade cutting edge to tungsten carbide, increased edge wear life 20 times.

Circular slitters used to slit magnetic tape for audio, video and memory tapes. Four large electronics companies gained ten times longer cutting edge life simply by changing from a conventional tungsten carbide grade to a submicron grade. For the preceding applications, the tungsten carbide grain size was held below 1(mu). The binder materials were varied as required to obtain the desired balance of strength, toughness and wear resistance.

The ultrafine submicron carbide grades have been used where light, medium, or heavy shock loads may be encountered and, in addition, where a strong and exceptionally keen cutting edge is required.

Ultrafine submicron tungsten carbide provides an unusually high fracture toughness for a given hardness. This combination of properties is especially advantageous where thin, sharp sections may be subjected to moderate shock loads.

There is an important relationship between tungsten carbide grain size and binder percentage. As suggested earlier, shock loads can be absorbed by a larger grain size and higher binder percentage. However, if the carbide binder percentage is too high (say in the 20% to 25% range) the grade may become too ductile for the application. A condition known as swaging or peening could occur, causing the binder to plastically deform and the die or wear part to fail prematurely.