Glossary of Terms
Binder – Most grades of tungsten carbide use cobalt as the binder. The quantity of binder present in a carbide product is an important factor when determining the properties of each grade. Since binders are added as a percentage of weight, they can constitute 3 to 30% of a part. Generally, the lower cobalt content, the harder the material will become. However, variation in grain size and additives are also contributing factors.
Compressive Strength – A material’s ability to resist failure under compressive forces. During this test, a sample is placed in an apparatus where it will be “squeezed” until it ruptures. When the force required to rupture the sample is divided by its cross-sectional area, the resulting value reflects its compressive strength. Typically, units are in pounds per square inch (PSI).
Corrosion Resistance – A material’s ability to resist environmental deterioration. Since carbides are generally chemically inert (considering binding material as a factor), they can be used successfully in many chemical and corrosive environments.
Density – Determined by comparison of mass with volume. It is usually stated in g/cm³.
Fracture Toughness – By definition, fracture toughness is a material’s ability to resist brittle cracking under a rising or constant load. In tungsten carbides, this value is predominantly affected by the percentage of binder (cobalt) present. So, by increasing the quantity of cobalt in a part, its ductility can be improved and brittleness minimized. Carbide grades with higher binder contents have excellent resistance to impact.
Grain Size – Although using larger grains (of 2-6 microns) will greatly increase the strength and toughness of a part, the tradeoff is that products containing larger grains do not resist wear as well as those made with finer grains. Submicron materials that vary between 0.4 and 1.0 microns are harder than standard grain materials with the same cobalt content. Submicron grains are much more uniform in size, give improved hardness and increase carbide strength.
Hardness – A material’s ability to resist penetration. The Rockwell A-Scale and procedure are utilized to test tungsten carbides. When comparing materials of differing hardness values (measured on the same scale), a harder material will penetrate a softer material more easily and with less physical damage. For instance, a die must be composed of a material that is harder than the material that it is being pressed into, for a softer material would minimize the die’s lifespan.
Impact Strength – measures a product’s resistance to shock loading by a drop-weight impact test, a much more reliable indication of toughness than Transverse Rupture Strength (TRS) readings.
Modulus of Elasticity (E) – measures the elasticity or bendability of a material when a force is applied to it. Also referred to as Young’s modulus. Bendability increases with an increase in binder.
Thermal Conductivity – is the relative rate at which a material conducts heat. With tungsten carbides, this property is affected primarily by the amount of cobalt present in a part and, to a lesser extent, its microstructure. Since tungsten carbide conducts heat much faster than cobalt, a part’s thermal conductivity will decrease if the quantity of cobalt increases. When we consider the part as a mixture of tungsten carbide and cobalt, its net conductivity can be considered as an average of the conductivities of its constituents.
Transverse Rupture Modulus (TRM) – Transverse rupture strength values are determined by the amount of force needed to break standard test pieces under particular test conditions. The three-point bend test is used to determine the TRM in brittle materials. In this test, a sample bar is placed on a pedestal and supported at two points near its end. The load, applied to the sample at its midpoint, is then exerted against the support points.
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