Hard Alloys

Hard Alloys

 

a special class of wear-resistant materials whose great hardness changes only slightly upon heating. These materials are classified as either sintered or cast.

Sintered hard alloys are composite materials consisting of a metallike compound bonded with a metal or alloy. Their bases are usually carbides of tungsten or titanium, complex carbides of tungsten and titanium (and, often, tantalum), titanium carboni-tride, and less frequently, other carbides and borides. Bonding metals include cobalt and, less often, steel, nickel, and nickel-molybdenum alloys.

The first sintered hard alloy was produced in Germany in the years 1923–25 from tungsten carbide and cobalt. Industrial production began in 1926 (Widia; 94 percent WC, 6 percent Co). In the USSR, the first hard alloy—Pobedit—was made in 1929 from tungsten carbide (90 percent) and cobalt (10 percent), and the year 1935 saw the first production of Al’fa alloys—mixtures of tungsten and titanium carbides (21, 15, and 5 percent TiC) and cobalt (8,6, and 8 percent Co).

In 1975 the USSR produced more than 20 types of hard alloys, used in articles of more than 1,300 standard sizes. The hard alloys produced are, for the most part, tungsten (tungsten-cobalt) alloys, with 3–25 percent Co; titanium-tungsten alloys, with 4–40 percent TiC and 4–12 percent Co; and titanium-tantalum-tungsten alloys. These groups of hard alloys are designated by the letters VK, TK, and TTK, together with certain numbers. A number after T indicates the percentage content of titanium carbide; after TT, it indicates the aggregate percentage content of titanium and tantalum carbides, and after K, the number designates the percentage content of cobalt. VK alloys sometimes have the letters V, M, or OM appended to the numbers to indicate the size of the tungsten carbide grains (coarse, fine, extra fine). For example, VK6M is a tungsten-carbide-base alloy with 6 percent Co and is fine-grained. These alloys are characterized by high hardness (86–92 HRA) and strength, with various types of VK alloys having strengths upon bending of 1–2.5 giganewtons/m2 (100–250 kilograms-force/mm2) and compressive strengths of 3.2–5.9 giganewtons/m2 (320–590 kilograms-force/ mm2), depending on the cobalt content; for TK alloys, these strength values are, respectively, 1.15–1.6 giganewtons/m2(115–160 kilograms-force/mm2) and 3.8–6.5 giganewtons/m2(380–650 kilograms-force/mm2). The alloys also possess a high resistance to wear, and this property, together with those of hardness and strength, is maintained at a high level even upon heating to 800°–900°C. Other properties include high electrical and thermal conductivity. VK alloys have densities of 13,000–15,100 kg/m3, while the density range of TK and TTK alloys is 9,600–15,000 kg/m3.

Production of hard alloys devoid of tungsten is taking on ever greater importance. Such alloys permit the replacement of relatively expensive tungsten with cheaper metals, the broadening of the range of hard alloys with specific properties, and the creation of alloys with superior performance characteristics. Particularly promising are titanium-carbonitride-base hard alloys with a nickel-molybdenum alloy as the bonding metal and titanium-carbide-base hard alloys with a nickel-molybdenum-alloy or steel binder. An extremely important trend in the development of hard alloys is the rapidly growing production of permanently sharp cutting edges of hard alloys with a thin (5–15 micrometers) coating of titanium carbonitride, carbide, or nitride or of other compounds. Such coatings increase cutting durability by a factor of 3–10. The use of cutting tools with such edges is especially promising in automated production lines for machining parts in the automotive industry and other branches of industry.

Sintered hard alloys are produced through the techniques of powder metallurgy in the form of multifaceted edges and of shaped articles made entirely of hard alloys. They are highly efficient in machining metals, alloys, and nonmetallic materials and in processes that do not involve cutting away excess material (drawing, rolling, stamping). Sintered hard alloys are also used as structural materials and in drill bits. Hard alloys have been used to great advantage in machine building and metalworking, as well as in the extraction of ores, coal, petroleum, gas, and certain minerals. As a substitute for tool steels, hard alloys have fostered technological progress in the metalworking and mining industries, where the durability of tools fitted with hard alloys has been increased by a factor of 15–100, resulting in a growth in labor productivity by a factor of 3–5.

Cast hard alloys are produced by melting and casting. An example of a cast hard alloy is Relit, a WC-W2C alloy containing 3.7–1.0 percent C with a hardness of 91–92 HRA. It is obtained in the form of coarse grains by first melting the charge and then either crushing the ingots or atomizing the melt. Relit is used to hard-face bits in heavy-duty drills. Hard alloys without tungsten, which are based on borides and other wear-resistant hard compounds, have also been developed for this purpose. The cast hard alloys encompass a large group of alloys that are sprayed or fused onto machine components subject to abrasion, erosion, or corrosion; this group includes Stellites (Cr, W, Ni, C; base Co), Sor-mites (Cr, Ni, C; base Fe), and Stellite-like alloys (base Ni). The use of these alloys has led to a lengthening of the service life of rapidly wearing machine parts (in automotive vehicles, combines, tractors) by a factor of 2–4 (sometimes 10–20).

REFERENCES

Metallokeramicheskie tverdye splavy. Moscow, 1970.
Kreimer, G. S. Prochnost’ tverdykh splavov, 2nd ed. Moscow, 1971.
Tumanov, V. I. Svoistva splavov sistemy karbid vol’frama–kolbal’t. Moscow, 1971.
Tumanov, V. I. Svoistva splavov sistemy karbid vol’frama–karbid titana–karbid tantala–karbid niobiia–kobal’t. Moscow, 1973.
Tret’iakov, V. I. Osnovy metallovedeniia i tekhnologii proizvodstva spechennykh tverdykh splavov, 2nd ed. Moscow, 1976.

O. P. KOLCHIN

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