cemented carbide

Cemented carbides are a class of hard materials used extensively for cutting tools, as well as in other industrial applications. It consists of fine particles of carbide cemented into a composite by a binder metal. Cemented carbides commonly use tungsten carbide (WC), titanium carbide (TiC), or tantalum carbide (TaC) as the aggregate. Mentions of "carbide" or "tungsten carbide" in industrial contexts usually refer to these cemented composites.

Most of the time, carbide cutters will leave a better surface finish on a part and allow for faster machining than high-speed steel or other tool steels. Carbide tools can withstand higher temperatures at the cutter-workpiece interface than standard high-speed steel tools (which is a principal reason enabling the faster machining). Carbide is usually superior for the cutting of tough materials such as carbon steel or stainless steel, as well as in situations where other cutting tools would wear away faster, such as high-quantity production runs. In situations where carbide tooling is not required, high-speed steel is preferred for its lower cost.

Composition

Cemented carbides are metal matrix composites where carbide particles act as the aggregate and a metallic binder serves as the matrix (analogous to concrete, where a gravel aggregate is suspended in a cement matrix). The structure of cemented carbide is conceptually similar to that of a grinding wheel, but the abrasive particles are much smaller; macroscopically, the material of a carbide cutter appears homogeneous.

The process of combining the carbide particles with the binder is referred to as sintering or hot isostatic pressing (HIP). During this process, the material is heated until the binder enters a liquid phase while the carbide grains (which have a much higher melting point) remain solid. At this elevated temperature and pressure, the carbide grains rearrange themselves and compact together, forming a porous matrix. The ductility of the metal binder serves to offset the brittleness of the carbide ceramic, resulting in the composite's high overall toughness and durability. By controlling various parameters, including grain size, cobalt content, dotation (e.g., alloy carbides) and carbon content, a carbide manufacturer can tailor the carbide's performance to specific applications.

The first cemented carbide developed was tungsten carbide (introduced in 1927) which uses tungsten carbide particles held together by a cobalt metal binder. Since then, other cemented carbides have been developed, such as titanium carbide, which is better suited for cutting steel, and tantalum carbide, which is tougher than tungsten carbide.

Physical properties

The coefficient of thermal expansion of cemented tungsten carbide is found to vary with the amount of cobalt used as a metal binder. For 5.9% cobalt samples, a coefficient of 4.4 µm·m⁻¹·K⁻¹ was measured, whereas 13% cobalt samples have a coefficient of around 5.0 µm·m⁻¹·K⁻¹. Both values are only valid from to due to non-linearity in the thermal expansion process.

Applications

Inserts for metal cutting

Carbide is more expensive per unit than other typical tool materials, and it is more brittle, making it susceptible to chipping and breaking. To offset these problems, the carbide cutting tip itself is often in the form of a small insert for a larger tipped tool whose shank is made of another material, usually carbon tool steel. This gives the benefit of using carbide at the cutting interface without the high cost and brittleness of making the entire tool out of carbide. Most modern face mills use carbide inserts, as well as many lathe tools and endmills. In recent decades, though, solid-carbide endmills have also become more commonly used, wherever the application's characteristics make the pros (such as shorter cycle times) outweigh the cons (mentioned above). As well, modern turning (lathe) tooling may use a carbide insert on a carbide tool such as a boring bar, which are more rigid than steel insert holders and therefor less prone to vibration, which is of particular importance with boring or threading bars that may need to reach into a part to a depth many times the tool diameter.

Insert coatings

To increase the life of carbide tools, they are sometimes coated. Five such coatings are TiN (titanium nitride), TiC (titanium carbide), Ti(C)N (titanium carbide-nitride), TiAlN (titanium aluminium nitride) and AlTiN (aluminium titanium nitride). (Newer coatings, known as DLC ( diamond-like carbon) are beginning to surface, enabling the cutting power of diamond without the unwanted chemical reaction between real diamond and iron.) Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool. The coating is usually deposited via thermal chemical vapor deposition (CVD) and, for certain applications, with the mechanical physical vapor deposition (PVD) method. However, if the deposition is performed at too high temperature, an ''eta phase'' of a Co₆W₆C tertiary carbide forms at the interface between the carbide and the cobalt phase, which may lead to adhesion failure of the coating.

Inserts for mining tools

Mining and tunneling cutting tools are most often fitted with cemented carbide tips, the so-called "button bits". Artificial diamond can replace the cemented carbide buttons only when conditions are ideal, but as rock drilling is a tough job cemented carbide button bits remain the most used type throughout the world.

Rolls for hot-roll and cold-roll applications

Since the mid-1960s, steel mills around the world have applied cemented carbide to the rolls of their rolling mills for both hot and cold rolling of tubes, bars, and flats.

Other industrial applications

This category contains a countless number of applications, but can be split into three main areas:
* Engineered components
* Wear parts
* Tools and tool blanks

Some key areas where cemented carbide components are used:

* Automotive components
* Canning tools for deep drawing of two-piece cans
* Rotary cutters for high-speed cutting of artificial fibres
* Metal forming tools for wire drawing and stamping applications
* Rings and bushings typically for bump and seal applications
* Woodworking, e.g., for sawing and planing applications
* Pump pistons for high-performance pumps (e.g., in nuclear installations)
* Nozzles, e.g., high-performance nozzles for oil drilling applications
* Roof and tail tools and components for high wear resistance
* Balls for ball bearings and ballpoint pens

Non-industrial uses

Jewellery

Tungsten carbide has become a popular material in the bridal jewellery industry, due to its extreme hardness and high resistance to scratching. Given its brittleness, it is prone to chip, crack, or shatter in jewellery applications. Once fractured, it cannot be repaired.

History

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