Alloys of Chromium
|The most important alloys of chromium are those containing iron, cobalt, or nickel. With other metals, chromium only forms alloys with difficulty, if at all. For example, if bismuth or cadmium is heated with chromium in a magnesia crucible, the fused metals separate into two immiscible layers, while copper, silver, tin, and zinc are only slightly miscible. Attempts have been made to prepare chromium bronze by the electrolysis of a mixed solution of chromium and copper salts, but no alloy was obtained, the deposit of copper only containing a small quantity of unalloyed chromium, possibly in the form of hydroxide. Again, when metallic calcium is heated with chromic chloride, the latter is reduced to the metal, but no alloy is produced even when the temperature is maintained at 1000° C. for three hours. |
Aluminium forms an alloy which appears to have the composition Cr3Al, with a melting-point rather higher than 1600° C. It is best prepared by heating together aluminium, chromium sesquioxide, and potassium dichromate.
Antimony, when fused with chromium, forms mixtures which yield two definite Alloys of Chromium, namely, CrSb and CrSb2. The former is dark grey, brittle, melts about 1125° C., and is readily attacked by dilute acids; the latter is silvery white, brittle, more stable towards acids, but decomposes at temperatures below its melting-point, only being stable below 675° C.
Cobalt readily alloys with chromium and yields a series of products of great utility, the properties of which are enhanced by the presence of tungsten. The latter metal Alloys of Chromium in all proportions with chromium and cobalt, and causes increased hardness in the products. An alloy containing 15 per cent, of chromium and 10 per cent, of tungsten is suitable for making cold chisels and woodworking tools, while if the proportion of tungsten is increased to 40 per cent., the alloy is hard enough to scratch quartz and can be used for turning cast iron. The addition of molybdenum also causes increased hardness. Stellite is an alloy of cobalt with 25 per cent, of chromium, and containing tungsten and molybdenum. It is used for making high-speed tools, since it takes a good cutting edge which it retains at temperatures above red heat, and such tools can be used for a long time without grinding. It is "rustless" to a high degree and is not attacked by organic or nitric acids. The presence of carbon, silicon, or boron in these alloys renders them harder but more brittle.
Iron-chromium alloys, free from carbon, may be prepared from chromite by the alumino-thermic method. From a study of the cooling - and freezing-point curves it has been suggested that a compound, Cr2Fe, exists, but this is questioned by Janecke, who studied the iron- chromium system by means of fusion curves and by the microscopic study of polished sections of various Alloys of Chromium between the limits 10Fe:90Cr and 90Fe:10Cr, and came to the conclusion that the system consists of a single eutectic which can form mixed crystals with either component. The eutectic contains 75 per cent, of chromium and melts at 1320° C. The addition of chromium to iron increases the readiness of attack by hydrochloric and sulphuric acids, but towards concentrated nitric acid the alloys are rendered passive. They remain bright in air and in water. The presence of carbon increases the resistance to acids and renders them very hard; if carbon-free, they are softer than cast iron. All the alloys up to 80 per cent, chromium are magnetic. Molybdenum, titanium, vanadium, and tungsten improve the mechanical properties and increase the resistance to acids.
The chief use of ferro-chrome alloys is in the manufacture of chromium steels, i.e. steels containing about 2 per cent, of chromium; the alloys, containing over 60 per cent, of chromium, are added to ordinary carbon steel when molten. Chromium steels are very hard and tough. The so-called "stainless" or "rustless" steels usually contain 13 to 14 per cent, chromium, with occasionally 1 per cent. nickel. The mechanical relations of iron, chromium, and carbon, and the structure of chromium steels are subjects which have been ex haustively studied.
Nickel readily alloys with chromium and iron. Nichrome, containing 60 per cent, nickel, 14 per cent, chromium, and 15 per cent, iron, is a high-temperature resisting alloy with a much longer life than ordinary iron or steel. It is used for making annealing and carbonising boxes, retorts for use at high temperatures, and for pyrometer tube covers. Nickel-chromium steels are of great importance and have been largely used for the manufacture of big guns.
Chromium-vanadium steel is an alloy steel combining high tensile strength and resistance to shock.