Carbide Terminology

CEMENTED CARBIDE

This denotes a sintered composite material consisting of refractory metal carbides and metallic binders. Among the currently employed metal carbides, tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC) and niobium carbide (NbC) are the most prevalent components. Metallic cobalt is extensively utilized as a binder in cemented carbide manufacturing; alternative metallic binders, including nickel (Ni) and iron (Fe), may also be adopted for specific specialized applications.

DENSITY

Density is defined as the ratio of a material’s mass to its volume, where the volume accounts for the internal pore spaces within the material. It is also referred to as specific gravity.

Tungsten carbide (WC) has a density of 15.7 g/cm³, while cobalt (Co) has a density of 8.9 g/cm³. Consequently, the overall density of a tungsten‑cobalt (WC‑Co) alloy rises as the cobalt content declines. In contrast, titanium carbide (TiC) possesses a lower density of just 4.9 g/cm³ compared to tungsten carbide; thus, incorporating TiC or other low‑density constituents will lower the overall density.

For a material with a fixed chemical composition, an increase in internal porosity results in a reduced density.

Density is determined via the water displacement method (Archimedes’ principle).

HARDNESS

Hardness represents a material’s resistance to plastic deformation.

Vickers Hardness (HV) is widely adopted across the globe. This testing method involves indenting the sample surface with a diamond indenter under a specified load, then calculating the hardness value based on the dimensions of the indentation.

Rockwell Hardness (HRA) is another commonly used hardness measurement technique, which assesses hardness by measuring the penetration depth of a standard diamond cone indenter.

Both Vickers and Rockwell hardness tests are applicable for cemented carbide, and the corresponding hardness values are interconvertible.

BENDING STRENGTH

The test specimen is mounted as a simply supported beam on two fulcrums, with a load applied at the midpoint between the fulcrums until fracture occurs. Bending strength is calculated using standard flexural formulas, based on the fracture load and the cross‑sectional area of the specimen. It is also termed transverse rupture strength or flexural strength.

In WC‑Co alloys, bending strength increases with rising cobalt content; however, the strength peaks at approximately 15% cobalt and subsequently decreases.

Bending strength is reported as the average of multiple test measurements. This value varies with specimen geometry, surface finish (smoothness), residual internal stresses, and internal material defects. Therefore, bending strength serves solely as a strength indicator and should not be used as the sole criterion for material selection.

POROSITY

Cemented carbide is fabricated through powder metallurgy, involving compaction and sintering. Owing to the inherent characteristics of this process, minor residual porosity may remain within the product’s metallurgical microstructure.

Residual pore volume is evaluated by comparing pore size ranges and distributions against standard reference charts.

Type A: Pore size below 10 μm

Type B: Pore size between 10 μm and 25 μm

Reducing porosity effectively enhances the overall performance of the product. Pressure sintering represents an effective approach to minimize porosity.

DECARBURIZATION

Decarburization refers to insufficient carbon content in cemented carbide following sintering.

Upon decarburization, the material microstructure transforms from WC‑Co to W₂CCo₂ or W₃CCo₃. The ideal carbon content for tungsten carbide (WC) in cemented carbide is 6.13% by weight. A substandard carbon content leads to the formation of distinct carbon‑deficient microstructures within the product.

Decarburization significantly diminishes the strength of tungsten carbide cemented carbide and increases its brittleness.

CARBURIZING

Carburizing describes the condition of excessive carbon content in cemented carbide after sintering.

The optimal carbon content for tungsten carbide (WC) in cemented carbide is 6.13% by weight. Excessively high carbon content generates pronounced carburized microstructures, accompanied by noticeable free carbon precipitates in the product.

Free carbon drastically reduces both the strength and wear resistance of tungsten carbide.

Type C pores observed during phase analysis indicate the severity of carburization.

COERCIVE FORCE

Coercive force is the residual magnetic strength measured after magnetizing the magnetic phase within cemented carbide to saturation and subsequently demagnetizing it.

A direct correlation exists between the average grain size of the magnetic phase in cemented carbide and coercive force: finer magnetic phase grain size corresponds to a higher coercive force value.

MAGNETIC SATURATION

Cobalt (Co) exhibits magnetic properties, whereas tungsten carbide (WC), titanium carbide (TiC), tantalum carbide (TaC) and vanadium carbide (VC) are non‑magnetic. By measuring the magnetic saturation value of cobalt in an alloy and comparing it to that of pure cobalt, the degree of alloying within the cobalt binder phase can be determined, as magnetic saturation is influenced by alloying elements. This method enables detection of any compositional changes in the binder phase and can identify deviations from the ideal carbon content, given carbon’s critical role in composition control.

Low magnetic saturation values suggest a risk of low carbon content and decarburization.

High magnetic saturation values indicate the presence of free carbon and carburization.

COBALT POOL

A cobalt pool refers to the formation of localized excess cobalt regions after sintering the cobalt (Co) binder with tungsten carbide. This phenomenon primarily arises from insufficient sintering temperature, inadequate green density prior to sintering, or cobalt infiltration into pores during hot isostatic pressing (HIP) treatment. The dimensions of cobalt pools are assessed via metallographic image comparison.

The occurrence of cobalt pools in cemented carbide can adversely affect the material’s wear resistance and mechanical strength.