Steel and Tungsten Carbide Grinder Knives

The steel cutting tools available to grinding plants for recycled materials are subject to considerable wear due to their use and, the greater the abrasiveness of the materials to be reduced in size, the greater their wear and the shorter the time required for their consumption.

All this translates into costs . In fact, rapid wear of the cutting knives leads to frequent stops of the machine for their replacement, with a loss of daily production, which is not compensated for by the use of cheaper and less performing knives.

Furthermore, when the wear of the steel begins , the vibrations of the machine generally increase, the dust due to an imperfection cutting and consuming more electricity as the machine takes longer to do the job.

It must also be considered that the materials to be crushed have different hardnesses and that for this reason the choice of the composition of the knives must account of this important factor. Sometimes it is not enough to choose types of steel with different hardness, but it is necessary to use knives that have inserts with extremely tough materials such as tungsten carbide .

But what is tungsten carbide and why is it so effective in grinder knives?

The tungsten carbide is prepared mainly by carburization, by reacting metallic tungsten with carbon black or graphite at 1400-2000 ° C, in hydrogen atmosphere or vacuum.

It appears as a gray colored powder with metallic luster , practically insoluble in water and diluted acids, but soluble in mixtures of nitric acid and hydrofluoric acid. In aqueous solution it is easily oxidized by hydrogen peroxide

Tungsten carbide has a melting point of 2 785 ° C so it is an extremely hard material, being around 9 in the Mohs scale and about 2600 in the Vickers scale. It has a Young's modulus of around 700 GPa, a compressibility modulus of 630–655 GPa [3] and a shear modulus of 274 GPa.

For practical use it is combined with transition metals, mainly cobalt or nickel, working it from powders, with sintering techniques at temperatures around 1200-1500 ºC .

The resulting compound is a ceramic-metal material called cemented carbide, hard metal or widia.

To obtain hard metal other elements such as chromium or tantalum can also be added, in order to avoid the growth of carbide grains, acting as inhibitors.

Tungsten carbide and metal powders undergo three steps:

• Grinding , to mix together powders of different qualities and create a homogeneous mixture of powders.

• Heating to 100 ° C with the addition of binder (cobalt) to form a solid mass thanks to the union of the grains.

• Sintering between 1200 and 1600 ° C , to allow the cobalt to melt, weld the grains and eliminate porosity.

In the case of cutting tools intended for grinding plants for recyclable waste, these have great toughness and durability that allow a general saving of grinding costs, even if the knives cost more than common steel knives, but also allow obtain a uniformly cut product without smudging or excessive dust.

Metal and tungsten carbide knives are suitable for the following tough materials:

• PET

• Plastics loaded with fiber

• Plastics loaded with mineral fillers

• Raffia

• Polyethylene from greenhouse or from the countryside

• Electronic waste

• Tires

• Wood

Automatic translation. We apologize for any inaccuracies. Original article in Italian.