Tungsten Carbide Wiki

Carbide is a composite material. More precisely, it consists of alloys made from metallic hard materials—so-called carbides—combined with a metal binder.

Tungsten carbides are sintered composite materials composed of hard metal materials and a binding agent. They are typically produced using a combination of tungsten carbide and cobalt (WC+Co). In addition to tungsten carbide (WC), other hard materials such as titanium carbide (TiC), tantalum carbide (TaC), chromium carbide (CrC), and various other carbides are also used. The most commonly used binding agents include cobalt (Co), nickel (Ni), iron (Fe), and nickel-chromium (NiCr).

In 1894, chemist Henri Moissan first documented the term tungsten carbide (WC) while researching synthetic diamonds. About twenty years later, industrialists Hugo Lohmann and Otto Voigtländer developed a process for manufacturing tungsten carbide components by sintering just below the melting point. Their cast tungsten carbide, patented in 1914, initially failed in the market due to its brittleness. Starting in the 1920s, various patents—filed by the newly established and pioneering Osram Company—described the evolution toward the cemented carbide used today. In 1922, Heinrich Baumhauer succeeded for the first time in binding tungsten carbide using molten iron and conducting the first successful technical trials. A year later, Karl Schröter described the production of cemented carbide through the sintering of tungsten carbide, iron, cobalt, and nickel; this patent is considered the foundational patent for modern cemented carbides. At the end of 1925, Osram sold these patents to Krupp, which launched industrial production in 1927 under the brand name Widia (derived from *wie Diamant*—"like diamond"). In the USSR, a similar cemented carbide was developed starting in 1929 under the name Pobedit by the company of the same name.

The binder content and the carbide grain size are the key factors determining the specific properties of carbide. For example, a lower binder content increases hardness, while the same effect can also be achieved by using a finer grain size. By combining these two parameters, a wide range of carbide grades with different performance characteristics can be produced.

The available hardness range extends from approximately 770 HV30 to 2000 HV30.

Carbide is highly versatile in its applications. The material stands out for its exceptional adaptability and offers numerous advantages and unique properties, making it an ideal choice for use in various industries.

For most DURIT customers, the following three characteristics are the most crucial:

• Wear resistance
• Hardness
• Pressure resistance

The production of carbide follows a precise and well-structured process:

The powder consisting of tungsten carbide and the selected binder is mixed and milled in the desired composition. It is then dried. The resulting granulate is pressed into shape using various direct or indirect forming techniques. The pressed part, also known as a green compact, can subsequently be machined. Sintering then follows at approximately 1,300–1,500 °C.

The hardness of carbide components depends on their intended application. Cobalt is the primary binding agent in carbide production, as it optimizes the sintering process. However, when higher corrosion resistance is required, a nickel binder is the preferred choice. The most commonly used binding materials in today's carbide production are cobalt (Co), nickel (Ni), iron (Fe), and nickel-chromium (NiCr). Additionally, the hardness of a carbide component increases as the
binder content decreases and the grain size becomes finer.

The pressed powder compact in its unsintered state is technically referred to as a green compact. At this stage, the green compact can already be machined using diamond tools. By taking the expected sintering shrinkage into account, machining is carried out as close as possible to the final dimensions in order to minimise the machining allowancerequired for the finishing process.

Sintering is a thermal process conducted in an oxygen-free environment, where tungsten carbides are embedded in a binder matrix. During this process, the binder within the green compact is heated to its liquid phase, filling the gaps and enclosing the carbide particles. In the Sinter-HIP process, once the liquid phase is reached, argon is pressed in under high pressure, further densifying the carbide and creating a homogeneous, pore-free structure.

The term blank or carbide blank refers to a workpiece that still requires further processing. In carbide manufacturing, this is the component as it exists after sintering. To transform the powder mixture into a finished carbide blank, it must be pressed into shape. 

Numerous attempts have been made over the years to standardise carbide grades. However, due to the highly specific requirements of different applications, complete standardisation has never been achieved. As a basis, the ISO classification system introduced the designations K, P and M for machining grades, and G for forming grades.

Since DURIT has specialised in tooling for forming technology and wear protection from the very beginning, its first carbide grades were designated using the ISO letter G, combined with D for DURIT, resulting in the grade designation GD—covering both medium-grain and fine-grain carbide grades. Shortly afterwards, coarse-grain grades were introduced, primarily for mining applications. These were designated BD, where B stands for Mining. As the range of carbide grades continued to expand, additional identification letters were introduced to distinguish specific material characteristics: F—Ultra-fine grain, E—EDM suitable, N—Nickel binder, NC—Nickel-chromium binder.

Grain size is one of the key parameters influencing the properties of carbide. The finer the grain, the higher the hardness and wear resistance. Grain size is measured in micrometres (µm) and classified in accordance with DIN EN ISO 4499.

Overview of relevant carbide grain sizes:

• Nano grain: < 0.2 µm
• Ultra-fine grain: 0.2–0.5 µm
• Submicron grain: 0.5–0.8 µm
• Fine grain: 0.8–1.3 µm
• Medium grain: 1.3–2.5 µm
• Coarse grain: 2.5–6.0 µm
• Extra-coarse grain: > 6.0 µm

Or, to put it another way: How can carbide be joined with other materials or components? The appropriate fastening method largely depends on the application of the workpiece. In essence, carbide offers numerous joining options, including brazing, bonding, shrink-fitting, casting, and mechanical fastening.

Carbide can be brazed onto suitable steel components, provided that the design accommodates brazing requirements. It's important to note that carbide has only about half the thermal expansion of construction steel.

Before bonding carbide, the bonding surfaces must be properly prepared and mechanically roughened. The shear strength of bonded joints typically ranges from 20–30 MPa, and they can withstand operating temperatures of approximately 150–250 °C.

In many applications, bonding is an effective alternative to brazing. For higher operating temperatures, however, brazing is the only suitable option.

Carbide offers excellent compressive strength but is sensitive to tensile stresses. To increase the service life of tools subjected to internal loads—such as forming tools—a shrink-fit assembly is often the preferred solution. The steel holder is on the outside, while the carbide core is positioned inside.

For thermal shrink fitting, the steel holder is heated to temperatures of up to 450 °C before the carbide core is inserted. Alternatively, the carbide core can be press-fitted into the steel holder at room temperature, with a slight taperfacilitating the assembly process.

In most cases, carbide components can be easily and reliably screwed in place. From a design perspective, the thread should be placed in the carrier material. Depending on the carbide grade, the thread can also be machined directly into solid carbide.

This primarily depends on the workpiece geometry. Contoured components can often only be economically machined in their green state. Due to the shrinkage that occurs during sintering, a dimensional accuracy within tenths of a millimeter can be achieved. Cylindrical and geometrically regular components, however, can reach precision levels of up to 3 μm.

In Europe, the hardness of carbide is generally specified using the Vickers hardness scale. The Vickers test is an optical hardness testing method. Vickers Limited, founded in 1828, was one of the United Kingdom's leading engineering and defence companies.

In the United States, the Rockwell hardness test (HRA) is more commonly used. Both methods use a diamond pyramid indenter and apply a defined test force. With the Vickers test, the hardness value is determined optically by measuring the diagonals of the indentation. With the Rockwell test, the hardness is determined mechanically by measuring the indentation depth.

Carbide is available in a wide range of hardness levels, from relatively soft grades with a hardness of approximately 770 HV30 to highly wear-resistant grades with hardness values of up to 2000 HV30.

The question of whether carbide is electrically conductive can be answered with a clear yes. Thanks to its tungsten carbide content, carbide is an excellent electrical conductor. Its average electrical resistivity is approximately 20 µΩ·cm.

Carbide is also a good thermal conductor. Its average thermal conductivity ranges from 60–110 W/(m·K).

Because carbide contains cobalt and nickel as binders, it exhibits magnetic properties. However, through a specialized sintering process, the magnetizability of nickel-bonded carbide can be significantly reduced.

Carbide can be coated using either the PVD or CVD process. Due to the comparatively low coating temperature of approximately 450 °C, the PVD process is generally preferred over the CVD process, which requires coating temperatures of 900–1,100 °C.

Carbide is frequently used as a substrate for coatings, as coating performance is enhanced when the base material has a high initial hardness.