Understanding Tungsten Carbide: Grades, Hardness, Grain Size and Performance

Tungsten carbide is one of the most important engineering materials used in the manufacture of high-performance industrial tooling. Its exceptional hardness, compressive strength and resistance to abrasive wear make it particularly suitable for demanding applications such as cold forming, fastener manufacturing, metalworking and precision engineering.

Commonly referred to as cemented carbide or hardmetal, tungsten carbide is a composite material manufactured by combining extremely hard tungsten carbide particles with a metallic binder, most commonly cobalt. Through a controlled powder metallurgy and sintering process, these materials form a dense structure capable of withstanding substantial mechanical loads and repeated production cycles.

Unlike conventional tool steels, cemented carbide can provide significantly greater resistance to abrasive wear and deformation. This makes it particularly valuable in applications where tooling is exposed to continuous pressure, friction, and repeated contact with metal workpieces.

However, tungsten carbide is not a single universal material. It is available in numerous grades, each offering different combinations of hardness, toughness, wear resistance, and strength. These characteristics are influenced by material composition, cobalt content, carbide grain size, and manufacturing quality.

Understanding these properties is fundamental to selecting the correct carbide grade and achieving reliable tooling performance.

Understanding Carbide Composition

Cemented tungsten carbide is primarily manufactured from two key components:

Tungsten Carbide (WC)

Tungsten carbide provides the material's hardness, wear resistance and compressive strength. The individual carbide particles form the hard structure that allows tooling to withstand repeated contact with workpiece materials.

Cobalt (Co)

Cobalt acts as the metallic binder, surrounding and bonding the tungsten carbide particles together during the sintering process.

Unlike the relatively brittle tungsten carbide particles, the cobalt binder provides a degree of ductility. It helps accommodate mechanical stresses and contributes to the material's resistance to crack initiation and propagation.

The proportion of tungsten carbide and cobalt has a significant influence on the finished material.

Generally, increasing cobalt content increases fracture toughness and resistance to chipping, although it tends to reduce hardness and wear resistance. Conversely, reducing cobalt content can increase hardness and abrasive wear resistance, but may reduce the material's ability to withstand impact and mechanical shock.

Other elements and additives may also be incorporated into specialised carbide formulations to modify properties such as corrosion resistance, high-temperature performance and grain growth behaviour.

The relationship between these components is therefore fundamental to understanding why different carbide grades are developed for different industrial applications.

Carbide Grades and Cobalt Content

Tungsten carbide is available in numerous grades, each developed to provide particular mechanical characteristics.

One of the most important differences between grades is their cobalt content. Although cobalt percentage is not the only factor determining performance, it provides a useful indication of how a material may behave.

Generally:

• Lower cobalt content: Often provides higher hardness and improved wear resistance but may offer reduced toughness.

• Higher cobalt content: Generally, provides improved toughness and resistance to chipping and fracture, although hardness and wear resistance may be lower.

• Intermediate cobalt content: Can provide a balance between hardness, wear resistance and toughness.

These ranges are illustrative rather than universal grade classifications. Grain size, binder distribution, manufacturing processes and other compositional factors can significantly influence the final properties.

For example, a grade containing approximately 6% cobalt may provide excellent wear resistance in a demanding application, whereas a 12% cobalt grade may be more appropriate where tooling experiences significant impact or mechanical loading.

It is also important to recognise that two carbide grades with identical cobalt percentages are not necessarily equivalent. Differences in grain size, porosity, carbon balance and microstructural consistency can produce substantially different performance characteristics.

Consequently, carbide selection should consider the complete material specification rather than relying on cobalt content or a manufacturer's grade designation alone.

Hardness Measurement: HRA and HV

Hardness is one of the most frequently specified properties when purchasing and evaluating tungsten carbide. It provides an indication of a material's resistance to permanent indentation and is commonly used as part of carbide quality control. Two widely used methods are Rockwell A (HRA) and Vickers (HV).

Rockwell A (HRA)

The Rockwell A test is commonly used for measuring the hardness of cemented carbides.

The test uses a diamond cone indenter, which is pressed into the prepared material surface under a specified force. For the standard cemented carbide test method, the total force is 588.4 N, equivalent to 60 kgf.

Hardness is determined by measuring the permanent indentation depth. The resulting value is expressed on the Rockwell A scale.

HRA is particularly convenient for routine inspection and quality verification because it provides a relatively straightforward numerical result.

However, the test requires careful control of surface preparation, specimen thickness, support, indenter condition and testing equipment. Poor preparation or inconsistent test conditions can influence the results.

Vickers Hardness (HV)

The Vickers test uses a diamond-shaped pyramid indenter with a square base. The indenter is pressed into the material under a specified test force, and the two diagonals of the resulting indentation are measured using a microscope.

The hardness value is calculated from the applied force and the average indentation diagonal.

Vickers hardness is particularly useful when examining hard materials, comparing different carbide grades and carrying out more detailed material investigations.

The applied test force should always be identified when reporting a result. For example, HV10 and HV30 refer to Vickers tests performed using different forces.

Comparing HRA and HV

Although both methods measure resistance to indentation, their results are not directly interchangeable. Conversion tables provide approximate relationships between the scales, but they should not be treated as exact mathematical equivalents. Material composition, microstructure and testing conditions can influence the relationship.

For this reason, when a customer specifies a particular hardness requirement, it is important to establish the measurement method, test conditions and acceptance range before manufacturing begins.

Hardness is an important material characteristic, but it should never be considered the sole indicator of carbide quality or expected tooling life.

The Importance of Grain Size

Grain size is another fundamental characteristic influencing the mechanical properties of cemented carbide.

It refers to the average size of the individual tungsten carbide particles within the material's microstructure. These particles are bonded together by the metallic binder during sintering.

Carbide grades are commonly described using classifications such as coarse, medium, fine, ultrafine and submicron. Exact classification boundaries can vary between manufacturers and technical standards.

How grain size influences performance

In general, reducing tungsten carbide grain size can increase hardness and compressive strength, particularly when comparing materials with similar binder content and manufacturing quality.

A finer grain structure provides a greater number of grain boundaries and can improve resistance to certain forms of wear and deformation. Larger grain structures, depending on composition, can provide advantages in fracture toughness and resistance to certain forms of mechanical damage.

The relationship is not entirely straightforward, however. Grain size interacts with cobalt content, binder distribution and the overall microstructure. A change in one variable can influence the benefits obtained from another.

Understanding the different grain structures

• Coarse grain: Generally used where toughness and resistance to certain forms of fracture are important.

• Medium grain: Offers a balance of properties and can be suitable for a broad range of industrial tooling applications.

• Fine grain: Often provides increased hardness and improved wear resistance, making it useful in precision tooling applications.

• Ultrafine and submicron grain: Can achieve particularly high hardness and wear resistance, although the resulting toughness and manufacturing requirements must be carefully considered.

Why grain size matters in tooling

In cold forming, the choice of grain structure can influence how a punch, die or insert performs under repeated loading.

A fine-grain carbide may provide excellent wear resistance where dimensional accuracy and surface finish are critical. However, if the application involves significant impact or stress concentrations, a different combination of grain size and binder content may provide a more suitable balance.

Grain size can be assessed through metallographic examination using optical or electron microscopy. ISO 4499-2 provides guidance on the measurement of tungsten carbide grain size using metallographic techniques.

Understanding grain structure provides a more complete picture of carbide quality than hardness measurements alone.

Hardness vs Toughness: Finding the Balance

One of the most important considerations when selecting tungsten carbide is understanding the difference between hardness and toughness.

Hardness relates to resistance to indentation and wear, whereas toughness describes the material's ability to resist crack initiation and propagation under mechanical loading. A material with exceptionally high hardness may provide excellent wear resistance but could be more susceptible to chipping or fracture under certain impact conditions. Conversely, a tougher material may withstand greater mechanical stress but experience more rapid wear in highly abrasive applications.

This distinction is particularly important in cold forming, where tooling is exposed to repeated high pressures, impact forces, and friction. The objective is not necessarily to select the hardest available carbide. It is to identify a material that provides the most suitable balance of hardness, toughness, and wear resistance for the operating conditions.

Selecting the Right Carbide Grade for Cold Forming

Cold forming places considerable demands on tooling materials. Punches, dies, inserts and other tooling components must withstand repeated loading while maintaining dimensional accuracy and surface integrity.

The correct carbide grade will depend on several factors, including:

• The material being formed, such as carbon steel, stainless steel or aluminium.

• The pressures and forces generated during the forming process.

• The geometry and dimensions of the tooling.

• The required surface finish and dimensional tolerances.

• Production volumes and expected tooling life.

• The operating conditions and potential for impact, wear or fracture.

For example, a tooling application involving high abrasive wear may benefit from a harder carbide grade with excellent wear resistance. An application involving significant impact loading may require a tougher composition to minimise the risk of premature failure.

There is no single carbide grade that is ideal for every cold forming operation.

Material selection should therefore consider the complete manufacturing process, rather than relying on hardness figures alone.

Carbide Quality, Manufacturing and Tool Life

Selecting the correct carbide composition is only part of achieving reliable tooling performance. The quality and consistency of the manufacturing process are equally important.

The production of cemented carbide involves several stages, including powder preparation, mixing, pressing and sintering. Each stage contributes to the final material's properties and microstructural consistency.

Subsequent machining and grinding operations also require careful control. Poorly controlled grinding conditions, excessive heat generation or unsuitable machining parameters can introduce surface damage and microcracks. These defects can potentially become starting points for premature cracking or failure during operation. For this reason, material verification, dimensional inspection, appropriate hardness testing and controlled manufacturing processes are essential.

Consistent carbide quality helps manufacturers achieve more predictable tooling performance, reduce unexpected production interruptions and improve overall manufacturing efficiency. Ultimately, the objective is not simply to manufacture a hard tool, but to produce a reliable component capable of delivering consistent performance throughout its intended service life.

Tungsten carbide tooling performance is determined by much more than a single hardness figure. Composition, cobalt content, grain size, manufacturing quality and application requirements all contribute to the characteristics and performance of the finished tool.

Understanding the relationship between these factors allows manufacturers to make more informed material selections, improve tooling reliability and achieve better production efficiency.

At Major Industries Ltd, we recognise the importance of matching carbide properties to the demands of each application. By considering the complete relationship between material characteristics and manufacturing requirements, we aim to help customers achieve dependable tooling performance and improved cost efficiency.

The right material. A longer tool life. A more productive operation.