Cutting-tool materials

The study of metal cutting is complex, due to the number of possible variables. Differences in workpiece materials and cutting-tool materials, whether or not a cutting fluid is used, the relative speed of the work and cutting tool, the depth of cut, and the condition of the machine all affect the cutting operation. However, certain basic rules apply, and when you know and can apply these you will be in a better position to carry out machining operations effectively. 

To be effective, the material from which a cutting tool is made must possess certain properties, the most important of which are red hardness, abrasion resistance and toughness. 

When cutting, the edge of a cutting tool operates under intense pressure and will wear due to abrasion by the material being cut Basically, the harder the material the better its resistance to abrasion. 

A cutting-tool material which is extremely hard is unfortunately also brittle. This means that a cutting edge will chip on impact if, e.g. the 

Workshop Processes Practices and Materials. DOI IO.IOI6/B978-0-08-089064-7.00007-7 Copyright ) Bruce J. Black. Published by Elsevier Ltd. All rights of reproduction in any form reserved. 

It can be readily seen that no one cutting-tool material will satisfy all conditions at one time. 

Apart from being used to manufacture the cutting tools already mentioned, high-speed steel is available as tool bits in round or square section already hardened and tempered. The operator has only to grind the required shape on the end before using. 

---

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

Table 1. Summary of Properties for Cutting Tool Materials ...

Carbon Steels and Low—Medium Alloy Steels. Plain carbon steels, the most common cutting tool materials of the nineteenth century, were replaced by low—medium alloy steels at the turn of that century because of the need for increased machining productivity in many appHcations. Low—medium carbon steels have since then been largely superseded by other tool materials, except for some low speed appHcations. 

Another approach is to coat the cutting tool material with a carbide former, such as titanium or siUcon or their respective carbides by CVD and deposit diamond on top of it. The carbide layer may serve as an iaterface between diamond and the cemented carbide, thus promoting good bonding. Yet another method to obtain adherent diamond coatings is laser-iaduced microwave CVD. By ablating the surface of the substrate with a laser (typically, ArF excimer laser) and coating this surface with diamond by microwave CVD, it is possible to improve the adhesion between the tool and the substrate. Partial success has been achieved ia this direction by many of these techniques. 

R. Komanduri, "Cutting Tool Materials" in M. B. Bever, ed., Engclopedia of Materials Science and Engineering, Pergamon Press, Oxford, U.K., 1983 General Electric HS report no. 82CWD176, Schenectady, N.Y., June 1982. 

H. Tanaka, in Cutting Tool Materials Proceedings of theMSM International Conference, Ft. Mitchell, If., ASM International, Metals Park, Ohio, Sept. 1980, pp. 349-361. 

Hafnium nitride (HfN) has good oxidation resistance and is developed as a cutting tool material (see Ch. 10).P1... 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

Wayne, S. F., andBuljan, S. T., The Role of Thermal Shock on Wear Resistance of Selected Ceramic Cutting Tool Materials, Ceram. Eng. Sci. Proc., 9(9-10) 1395-1408 (1988)... 

H. J. Swinheart (Ed.), Cutting Tools Materials Selections, Am. Soc. of Tools and Manufactory Eng., Dearborn, Michigan, 1986. 

Table 4. Properties of the pcBN products and other cutting tool materials.

In comparison with the use of diamond as a cutting tool material, the consumption of diamond for non-cutting applications is small. Nevertheless, high-quality-single crystal industrial diamond is often the only suitable material for specific tasks, due to its outstanding mechanieal, optical, electrical, chemical or thermal properties. 

Figure 38. Schematic diagram showing the relative merits of various cutting tool materials for the machining of different materials. It can be seen that CVD diamond occupies a specific niche in the machining of materials requiring exceptional wear resistance.

An indication of the relative position of various cutting tool materials with respect to their wear resistance and fracture toughness is shown in Fig. 38. Diamond is by far the most wear resistant material, particularly in pure single crystal form, but because single crystal diamond is a brittle material, it does not have the average toughness of the polycrystalline forms - both pcD (cobalt-containing) and CVD diamond (which contains no metal phases). 

The following case studies are typical examples of where CVD diamond s performance is increasing the range of diamond cutting tool materials available to industry. 

---