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A carbide, generally, is a chemical compound of carbon and a metal. This might be tungsten carbide, titanium carbide, or tantalum carbide, although "carbide" is oftentimes used to refer to tungsten carbide. Most carbides used in machine tools are cemented carbides, meaning that the metals are powdered and then sintered together with some sort of binder (usually cobalt). Sintering is a process in which the particles are heated to a point where they weld together, but do not melt.
Due to the large number of material combinations and forming process options, "carbides" can exhibit a range of material properties. To simplify the process of selecting a carbide, the Cemented Carbide Producers Association (CCPA) has organized carbides into grades that are based on a combination of several material properties, including hardness and toughness, as well as resistance to chip welding or cratering. The following table lists these grades and gives a rough idea of intended use:
C-1 Rouging cuts (cast iron and nonferrous materials)
C-2 General purpose (cast iron and nonferrous materials)
C-3 Light finishing (cast iron and nonferrous materials)
C-4 Precision boring (cast iron and nonferrous materials)
C-5 Roughing cuts (steel)
C-6 General purpose (steel)
C-7 Finishing cuts (steel)
C-8 Precision boring (steel)
Although this seems very elegant at first glance, virtually every carbide manufacturer has its own grade designation system by which it categorizes its products. The table below shows some of these designations for some of the largest carbide producers as they compare to the CCPA grade designations.
Clearly, the issue of grade designation is somewhat complex. This is due to the range of material properties that carbides can exhibit due to their composition and forming processes. The following sections aim to break down these grades into the individual material properties that they represent, and to perform a basic investigation into why these properties are desirable, and at what cost they can be obtained.
Hardness and Toughness
When discussing carbide properties, two terms that are used a lot are "hardness" and "toughness", as in the following sentence:
"Increasing the percentage of cobalt binder increases the toughness of the tool material and at the same time reduces its hardness or wear resistance." [Kibbe].
Unfortunately, both toughness and hardness are terms that are used loosely, and have imprecise definitions. Hardness has many definitions depending on the application, but in metallurgy it is most commonly used to mean
The measure of a material's resistance to localized plastic deformation by surface indentation, abrasion, or scratches. [Callister].
Hardness is therefore strongly related to the yield strength of a material, although it is not synonymous, as the elastic constant and other factors also come into play. There are a number of hardness tests for macro, micro, and nano scales. Hardness is a complex mechanical property, which is demonstrated by the relative nature of hardness test results; there is no absolute measure of hardness that can be used to compare different materials. To determine a scale of hardness, the same hardness test must be performed on a number of materials, and the unit-less numbers that are assigned to each material can then be compared.
Callister says of toughness
Toughness is a mechanical term that is used in several contexts; loosely speaking it is the measure of the ability of a material to absorb energy up to fracture.
Using these two definitions, the original sentence from Kibbe can be interpreted as saying,
"Increasing the percentage of cobalt binder makes it harder to actually fracture (break) the machine tool (it is tough and therefore absorbs energy before breaking), although it also makes the material easier to plastically deform (it is not very hard).
Hardness and toughness are usually inversely correlated. The ideal material would be very hard and very tough, but this is not practical in reality. If manufacturers try to make carbide tools harder, they end up losing toughness, and the material is unable to absorb energy. This results in tool fracture and chipped edges. Increasing toughness leads to a decrease in hardness, resulting in edges that plastically deform easily and become worn down (although they will not fracture as they did when they were too hard).
Although more specific types of toughness can be measured with absolute units, saying that carbide is tough or hard, even if a relative index number can be assigned to those qualifications, does not really provide a very good idea of the fundamental material properties of the carbide. Like the grade designation charts, hardness and toughness indexes provide very useful and practical information about the material, but they are a step removed from the basic material properties that are a direct result of bond energy, crystal geometry, and lattice structure of the material.
Fundamental Material Properties & Design Constraints
[“ASM Engineered Materials Reference Book”, ASM International, pp. 182, 1989.]
Tensile Strength at Break
5300 7000- MPa
769000 1.02e+6- psi
(such as the area under the stress-strain curve giving energy absorption per unit volume)
cratering is the result of high temperatures and pressures that cause the steel chip to weld itself to the tungsten carbide and tear out small particles of the tool material