CHROMIC IRON ORE. By E. C. HARDER. INTRODUCTION. The mining of chromite or chromic iron ore in the United States is an industry of small importance, due to the limited extent of the deposits. The association of chromite with basic igneous rocks is widespread, but deposits of workable size are rare. Commercially important deposits have been found only in Pennsylvania, Maryland, North Carolina, Wyoming, and California, and the last-named State is the only one which has for many years been a commercial producer. The total production of chromic iron ore in the United States in 1908 was 359 long tons. All the ore was used for furnace linings. When compared with the imports, the domestic production of chrome ore becomes insignificant. In 1908 there were 27,876 long tons of chrome ore imported into the United States, the greater portion of which came from New Caledonia, Greece, and Canada. SOURCES OF CHROMIUM. Chromium is found as an accessory constituent in a variety of minerals, but chromite or chromic iron ore is the only one of commercial importance. Small quantities of chromium have been obtained from crocoite. The following is a list of minerals containing chromium: Chromite.... Picotite (chrome spinel). Phoenicochroite. Vauquelinite.. Uvarovite (chrome garnet). Chrome diopside... Emerald (chrome beryl). List of chromium minerals. (FeMg)O (CrAlFe)203 2(PbCu)O ČrO, 3(PbCu)O P2O Theoretically the composition of chromite is FeO Cr2O,, with 32 per cent ferrous oxide (FeO) and 68 per cent chromic oxide (Cr2Ó ̧). Practically, however, the percentage of chromic oxide varies down to 10 in the ores, though generally ranging between 40 and 60, while the ferrous oxide content varies from 10 per cent to 50 per cent. Alumina and magnesia are almost invariably present and sometimes form a considerable percentage of the ore, alumina being present in quantities varying up to 30 per cent and magnesia in quantities varying up to 20 per cent. Alumina replaces chromium and magnesia replaces ferrous iron. USES OF CHROMIUM. The uses of chromium may be classified as follows: (1) Metallurgical, in the manufacture of alloys and of furnace linings; (2) chemical, as a constituent in coloring materials, mordants, oxidizing agents, and tannages. METALLURGICAL Uses. Chromium alloys.-The chief alloys in which chromium is a constituent are (A) ferrochromium, (B) ternary chromium steel, (C) quaternary chromium steels, such as chromium-nickel steel, chromiumtungsten steel, chromium-manganese steel, and chromium-molybdenum steel. Chromium has been alloyed with other metals in cases where great hardness is required. A. Ferrochromium is a tin-white, hard, brittle alloy, with crystalline structure, varying from fine crystalline aggregates to coarse crystals. It is used in the manufacture of chromium steels. The commercial product carries from 50 to 70 per cent chromium and from 0.5 to 10 per cent carbon. High-grade low-carbon ferrochromium contains from 0.5 to 1 per cent carbon, while high-carbon ferrochromium may contain as high as 10 per cent carbon. The carbon is present in the form of carbide. The high-carbon product is not reliable in steel manufacture, as flaws in chromium steel have been traced to the presence of crystals of carbide of chromium due to the use of an alloy too high in carbon. с Ferrochromium has been produced in several ways. One of the first methods of manufacture was by mixing bichromate of potassium with powdered wood charcoal, steel turnings, and broken glass, and reducing the mixture in pot crucibles. A ferrochromium containing 55 per cent chromium was obtained, which, however, contained a large percentage of carbon. Another method of preparation was by mixing pulverized chromic iron ore with powdered charcoal and tar, and heating the mixture in iron vessels to a point just short of ignition. This material was then thoroughly mixed with broken glass and lime, and heated in old steel crucibles to such an extent as to fuse the entire mass. The button of ferrochromium was then removed from the bottom of the ruined crucible. Later ferrochromium was produced in cupola furnaces, and still later in electric furnaces, the latter being now the prevailing method. Even recently, however, ferrochromium was produced in the Urals in eastern Russia in a charcoal blast furnace. The blast was heated to 400° C. and ore of the following composition was used: FeO. a Thorp, F. H., Outlines of Industrial Chemistry, 1901, pp. 137, 203-209, 458, 493. b Brannt, W. T., Metallic Alloys, 1896, p. 43. c Chromium steel: Jour. Iron and Steel Inst., No. 4, 1906, p. 926. (Review.) d Brustlein, A., Chromium steel: Jour. Iron and Steel Inst., No. 2, 1905, p. 782. (Review). e Lipin, W. N., Stahl und Eisen, vol. 25, pp. 617-618. 17.55 44.23 5.70 14. 14 16.09 0.74 1.29 This was mixed with slagging materials in the following proportions: The mixture was then fed to the furnace with charcoal in the proportion of 0.102 ton to 0.218 ton birch charcoal. The resulting alloys averaged about 30 per cent chromium, though occasionally a 50 per cent ferrochromium was produced. Several double carbides a have been isolated from ferrochromium and chromium steel, such as Fe,C 3Cr,C, in ferrochromium with 57 to 59 per cent chromium, and 2Fe,C Cr,C, in low-grade chrome steel. The carbides 3Fe,C 2Cr,C, and 2Fe,C 3Čr,C, have been obtained in the electric furnace. Iron is saturated with carbon when it contains about 4.2 per cent." The presence of chromium makes little difference up to about 10.4 per cent. When increased above this, however, the saturation point of iron for carbon increases, reaching about 9.2 per cent for 62 per cent chromium. B. Chromium steel alone or alloyed with tungsten or molybdenum is used in the manufacture of high-speed tools. It is also used in the manufacture of files, ball bearings, armor plates, and armor-piercing projectiles, for which uses it is generally alloyed with nickel or manganese. Chromium gives to steel a marked degree of hardness and, if added in the proper proportion, does not produce brittleness. The properties of chromium steel vary with the quantity of chromium and carbon present. The presence of excessive carbon tends toward brittleness on account of the development of double carbides' of chromium and iron. с Ternary chromium steel (steel containing only chromium, iron, and carbon), with a percentage of chromium under 7, has the microstructure of ordinary carbon steel, consisting of perlite with ferrite or cementite, and is very hard and strongly resists impact. Steel with a percentage of chromium between 7 and 20 has a fibrous structure known as martensite. As the chromium increases above 15 per cent, small quantities of carbide grains begin to appear. When carbon is present up to several per cent, reniform granules of troostite become mixed with the martensite. The steels thus formed are more brittle than the martensitic steels, and this brittleness is increased by the addition of carbon. On increasing the percentage of chromium above 20, there is a marked development of minute white grains consisting of a double carbide of iron and chromium. With an increase of this constituent, the steel becomes very brittle. A change in the carbon percentage changes somewhat the limits between which these various microstructures and the consequent physical properties prevail. Quenching acts upon low chromium steels in the same manner as upon carbon steel, but with far greater intensity. Martensitic chro a Guillet, L., Chrome steel: Jour. Iron and Steel Inst., No. 2, 1904, p. 613. b Goerens, P., and Stadeler, A., Influence of chromium on the solubility of carbon in iron (Review): Jour. Iron and Steel Inst., No. 3, 1907, p. 518. e Guillet, L., Chrome steel (Review): Jour. Iron and Steel Inst., No. 2, 1904, p. 611; also Quaternary steels: Jour. Iron and Steel Inst, No. 2, 1906, p. 7. 87150-M B 1908, PT 1-48 mium steel is little altered by quenching, while troostitic steel is hardened. Steels containing over 20 per cent chromium are slightly softened by quenching. Annealing softens all chromium steels. C. Quaternary chromium steels are chromium steels which contain, in addition to iron, carbon, and chromium, one other element, generally nickel, manganese, or tungsten. In a general way, the microstructure of quaternary steels is a superposition upon each other of the microstructures of the two ternary steels composing them. However, as the sum of the elements-carbon, chromium, and either nickel, manganese, or tungsten-increases, a mixture of the next higher microstructure of the constituent ternary steels begins to develop, this development being in advance of the development of these structures in either ternary steel alone. For example, if martensitic nickel steel be added to martensitic chromium steel, there will be little or no change of structure until considerable material has been added. At length, however, the characteristic polygonal structure of 7 iron mixed with grains of carbide will develop in the martensite, the iron structure being characteristic of the next higher nickel steel and the carbide grains of the next higher chromium steel. This mixed texture will develop before either would develop in the ternary steel of which it is characteristic. r As the physical properties of ternary and quaternary steels are dependent upon the microstructure, a consideration of the latter is important. The following structures are found in nickel chromium steels: Perlite, martensite, martensite mixed with carbide, 7 iron, 7 iron and carbide. In these steels the nickel varies from 1 to 30 per cent, the chromium from 1 to 20 per cent, and the carbon from 0.1 to 1 per cent. The equivalents of these elements are: Nickel, 29, chromium, 18, and carbon, 1.65. If chromium be added to perlitic nickel steel (low in nickel), the latter may, if the addition is sufficiently small, retain its structure, provided the sum of the elements-C and Ni and Cr-be fairly low. As the sum of the elements increases, martensite is formed. If chromium be added to martensitic nickel steel (medium low in nickel), the latter may retain this structure if the quantity added is small. As more is added, carbide will develop through the martensite, the amount depending upon the percentage of carbon present. At length, a small quantity of 7 iron also develops, and if the addition of chromium is sufficient the martensite may disappear altogether. When chromium is added to nickel steel containing 7 iron (high in nickel), it has no effect on the structure until a considerable percentage is present, when carbide is formed. The percentage of chromium necessary is lower in proportion as the carbon present is high. Only the perlitic steels and those containing 7 iron are of value. commercially. In these the chromium increases the hardness without diminishing the resistance to shock so characteristic of nickel steels. They are therefore very desirable for armor plates and armorpiercing projectiles. It has been suggested that they may also be used for high-speed tools and for ball bearings. The steels containing martensite and carbide are very hard and brittle. a Guillet, L., Quaternary steels: Jour. Iron and Steel Inst., No. 2, 1906, pp. 111-140. The following structures are found in manganese chromium steels: Perlite, martensite with or without carbide, 7 iron with or without carbide. In these steels the manganese varies from 1 to 15 per cent, the chromium from 1 to 6 per cent, and the carbon from 0.1 to 1 per cent. Apparently the addition of chromium to manganese steel has no other effect on the microstructure than the addition of more manganese would have, with the exception that if the percentage is sufficiently high some carbide is formed, especially if the carbon also is increased. The physical properties of manganese-chromium steels are similar to those of nickel-chromium steels, and in many cases it might be possible to substitute manganese for nickel in commercial steels. The following structures are found in chromium-tungsten steels: Perlite, martensite, martensite and carbide, carbide with a background of 7 iron, carbide with a background of sorbite. In these steels the tungsten varies from 1 to 16 per cent, the chromium from 1 to 20 per cent, and the carbon from 0.1 to 1 per cent. A small addition of tungsten to perlitic chromium steel does not affect the structure, but further additions result in the formation of martensite. With an excess of tungsten, troostite or sorbite are formed, and if the carbon present reaches 0.5 per cent a carbide is formed at the same time. If tungsten is added to martensitic chromium steel, no change is produced until a considerable quantity has been added. Then carbide is formed on a background of sorbite or T iron. The principal use for chromium-tungsten steels is in the manufacture of high-speed tools, for which purpose only the sorbitic (or troostitic) type is desirable. When this is subjected to definite heat treatment, the carbide present dissolves, and the mass takes on the structure of exceedingly fine-textured martensite almost invisible under the microscope. The heat treatment consists in raising the steel up to a temperature of 1,200° C., and keeping it there until all the carbide is dissolved. The rate of cooling apparently does not have any marked effect on the structure. The physical effect of this treatment is to make the steel very hard. The perlitic chromium-tungsten steels are extremely hard and are rather brittle. They might be of value commercially in the manufacture of ball bearings and balls, but are not suitable for highspeed tools. Martensitic steels are too difficult to forge and steels containing y iron are too soft for high-speed tools. Chromium-molybdenum and chromium-tungsten-molybdenum steels are also used for high-speed tools. The following analysis shows the composition of a good quality tool steel: a a Gledhill, J. M., The development and use of high-speed tool steel: Jour. Iron and Steel Inst., No. 2, 1904, p. 127. |