Chapter 1
Cutting-Tool Materials
(continued)

1.3 Cemented Tungsten Carbide

Tungsten carbide was discovered by Henri Moissan in 1893 during a search for a method of making artificial diamonds. Commercial tungsten carbide with 6 percent cobalt binder was first produced and marketed in Germany in 1926. Production of the same carbide began in the United States in 1928.

Most of the subsequent developments in the hard carbides have been modifications of the original patents, principally involving replacement of part or all of the tungsten carbide with other carbides, especially titanium carbide and/or tantalum carbide. This led to the development of the modern multi-carbide cutting tool materials permitting the high-speed machining of steel.

A new phenomenon was introduced with the development of the cemented carbides, again making higher speeds possible. A different set of conditions exists with the cemented carbides. The hardness of the carbide is greater than that of most other tool materials at room temperature, and it has the ability to retain it hardness at elevated temperatures to a greater degree, so that greater speeds can be adequately supported.

1.3.1 Manufacture of Carbide Products

The term "tungsten carbide" describes a comprehensive family of hard carbide compositions used for metal cutting tools, dies of various types and wear parts. In general, these materials are composed of the carbides of tungsten, titanium, tantalum or some combination of these, sintered or cemented in a matrix binder, usually cobalt.

Blending: The first operation after reduction of the tungsten compounds to tungsten metal powder is the milling of tungsten and carbon prior to the carburizing operation. Here, 94 parts by weight of tungsten and six parts by weight of carbon, usually added in the form of lamp black, are blended together in a rotating mixer or ball mill. This operation must be performed under carefully controlled conditions to ensure optimum dispersion of the carbon in the tungsten. Carbide blending equipment, better known as a ball mill, is shown here.

Carbide-blending equipment, better known as ball mill, ensures optimal dispersion of the carbon within the tungsten. (Courtesy of American National Carbide Co.)

To provide the necessary strength, a binding agent, usually cobalt (Co) is added to the tungsten (WC) in powder form and these two are ball milled together for a period of several days, to form a very intimate mixture. Careful control of conditions, including time, must be exercised to obtain a uniform, homogeneous product. Blended tungsten carbide powder is shown below.

Compacting: The most common compacting method for grade powders involves the use of a die, made to the shape of the eventual product desired. The size of the die must be greater than the final product size to allow for dimensional shrinkage which takes place in the final sintering operation.

A second compacting method is the hot pressing of grade powders in graphite dies at the sintering temperature. A third compacting method, usually used for large pieces, is isostatic pressing.

Sintering: Sintering of tungsten-cobalt (WC-Co) compacts is performed with the cobalt binder in liquid phase. The compact is heated in hydrogen atmosphere or vacuum furnaces to temperatures ranging from 2500 to 2900 F, depending on the composition. Both time and temperature must be carefully adjusted in combination, to effect optimum control over properties and geometry. The compact will shrink approximately 16 percent on linear dimensions, or 40 percent in volume. The exact amount of shrinkage depends on several factors including particle size of the powders and the composition of the grade. Control of size and shape is most important and is least predictable during the cooling cycle. This is particularly true with those grades of cemented carbides with higher cobalt contents.

1.3.2 Classification of
Carbide Tools

Cemented carbide products are classified into three major grades:

Wear grades: Used primarily in dies, machine and tool guides, and in such everyday items as the line guides on fishing rods and reels; anywhere good wear resistance is required.

Impact grades: Also used for dies, particularly for stamping and forming, and in tools such as mining drill heads.

Cutting tool grades: The cutting tool grades of cemented carbides are divided into two groups depending on their primary application. If the carbide is intended for use on cast iron, which is a nonductile material, it is graded as a cast-iron carbide. If it is to be used to cut steel, a ductile material, it is graded as a steel-grade carbide.

Cast-iron carbides must be more resistant to abrasive wear. Steel carbides require more resistance to cratering and heat. The tool-wear characteristics of various metals are different, thereby requiring different tool properties. The high abrasiveness of cast iron causes mainly edge wear to the tool. The long chip of steel, which flows across the tool at normally higher cutting speeds, causes mainly cratering and heat deformation to the tool. (Tool wear characteristics and chip formation will be discussed in Chapter 2.)

It is important to choose and use the correct carbide grade for each job application. There are several factors that make one carbide grade different from another and therefore more suitable for a specific application. The carbide grades may appear to be similar, but the difference between the right and wrong carbide for the job, can mean the difference between success and failure.

Carbide is manufactured using pure tungsten carbide with a cobalt binder. The pure tungsten carbide makes up the basic carbide tool and is often used as such, particularly when machining cast iron. This is because pure tungsten carbide is extremely hard and offers the best resistance to abrasive wear.

Here the method used to measure Transverse Rupture Strength (TRS) is shown along with the relationship of TRS to cobalt content.

Large amounts of tungsten carbide are present in all of the grades in the two cutting groups and cobalt is always used as the binder. The more common alloying additions to the basic tungsten/cobalt material are: tantalum carbide and titanium carbide.

While some of these alloys may be present in cast iron grades of cutting tools, they are primarily added to steel grades. Pure tungsten carbide is the most abrasive-resistant and will work most effectively with the abrasive nature of cast iron. The addition of the alloying materials such as tantalum carbide and titanium carbide offers many benefits:

• The most significant benefit of titanium carbide is that it reduces cratering of the tool by reducing the tendency of the long steel chips to erode the surface of the tool.
• The most significant contribution of tantalum carbide is that it increases the hot hardness of the tool which, in turn, reduces thermal deformation.

Varying the amount of cobalt binder in the tool material largely affects both the cast iron and steel grades in three ways. Cobalt is far more sensitive to heat than the carbide around it. Cobalt is also more sensitive to abrasion and chip welding. Therefore, the more cobalt present, the softer the tool is, making it more sensitive to heat deformation, abrasive wear, and chip welding and leaching which causes cratering. On the other hand, cobalt is stronger than carbide. Therefore, more cobalt improves the tool strength and resistance to shock. The strength of a carbide tool is expressed in terms of Transverse Rupture Strength (TRS).

Here is the microstructure of a coated carbide insert at 1500x magnification. (Courtesy of Kennametal Inc.)

The third difference between the cast-iron and steel-grade cutting tools, is carbide-grain size.The-carbide grain size is controlled by the ball mill process. There are some exceptions, such as micro-grain carbides, but generally the smaller the carbide grains, the harder the tool. Conversely, the larger the carbide grain, the stronger the tool.

Many manufacturers produce and distribute charts showing a comparison of their carbide grades with those of other manufacturers. These are not equivalency charts, even though they may imply that one manufacturer's carbide is equivalent to that of another manufacturer. Each manufacturer knows his carbide best and only the manufacturer of that specific carbide can accurately place that carbide on the C-chart. Many manufacturers, especially those outside the United States, do not use the C-classification system for carbides. The placement of these carbides on a C-chart by a competing company is based upon similarity of application and is, at best an educated guess. Tests have shown a marked difference in performance among carbide grades that manufacturers using the C-classification system have listed in the same category.

(continue)

Originally published in the January 2001 issue
of Tooling & Production.
Please Note: some pictures or diagrams are only available through the printed media.