chromium-base alloys. Among the valuable properties of such alloys are a high melting point (about 1900°C), relatively low density (7.2 g/cn3), low linear coefficient of thermal expansion (9.6 × 10–6 1/°C between 20° and 1000°C), high modulus of elasticity (28,600 kilograms-force/mm2), high thermal conductivity (84 W/m·°C at 100°C), high atmospheric heat resistance (up to 1350°C), and high corrosion resistance in the combustion products of high-sulfur fuel, diesel fuel, seawater, tropical atmosphere, and a number of corrosive liquid and gaseous media.
Chromium alloys are produced in vacuum units in an atmosphere of inert gases, in which case electrolytically refined chromium is used as the charge, or they are produced by way of powder metallurgy. Such alloys are easy to cut or weld. Their properties depend on the content of impurities, mainly nitrogen. Alloying eliminates embrittlement of the metal, which may be caused by a chemical reaction with gaseous nitrogen at high temperatures.
Several varieties of chromium alloys have been developed, but only industrial-grade, ductile chromium alloys are of practical use. Table 1 gives some mechanical properties of a typical deformed chromium alloy, which consists of 0.5 percent yttrium, 0.5 percent lanthanum, 0.35 percent vanadium, and 0.2 percent titanium, and a typical cast chromium alloy, which consists of 30 percent nickel, 1.5 percent tungsten, 0.3 percent vanadium, and 0.2 percent titanium.
Chromium alloys are capable of prolonged working without protective coatings at temperatures up to 1350°C, as well as brief working at temperatures up to 1500°C. They are used to make machine parts operating in a stream of burning fuel subjected to periodic temperature changes (600°–1500°C), instruments requiring special physicochemical properties, manipulators, machine assemblies for the production of fiberglass products, and dies for the liquid stamping of metals.
Chromium alloys also include the high-chromium heat-resisting alloys, consisting of chromium-nickel, chromium-nickel-tungsten, or chromium-nickel-cobalt-TiC and containing 35 to 45 percent chromium. Such alloys are commonly used in machine building. Their operating temperature may reach 1300°C, and their physicochemical properties are similar to the properties of simple chromium alloys, described above. High-chromium alloys have good mechanical properties (see Table 1) and exhibit high resistance to thermal stress upon periodic temperature changes and good workability in hot and cold stamping and in shaped casting. High-chromium alloys are extremely weldable and do not embrittle upon prolonged operation; items made from these
|Table 1. Mechanical properties of some chromium alloys|
|Alloy||Temperature(°C)||Ultimate strength σb||Relative elongation δ (percent)|
|Deformed chromium alloy (VKh-2l) ...............||20||400||40||5|
|Cast chromium alloy (VKh-4) ...............||20||1,050||105||10|
|High-chromium alloy (Cr-Ni-W) ...............||–70||1,320||132||30|
alloys are simple to repair and do not require protective coatings. Chromium-nickel-cobalt-TiC alloys are used as addition agents in plating the worn surfaces of parts operating at temperatures up to 1200°C in corrosive media.
REFERENCEKonstruktsionnye materialy, vol. 3. Edited by A. T. Tumanov. Moscow, 1965. (Entsiklopediia sovremennoi tekhniki.)
I. O. PANASIUK