It is interesting to notice that the presence of chromium in steel seems to stimulate the formation of hardening carbon, and to prevent the latter passing into cement carbon. The special property of chrome steels is extreme hardness. Chromium appears not to confer hardness upon steels in the absence of carbon. We do not know in what form hardening carbon exists in steel. It may be simply a solid solution of carbon in iron. What is a solid solution? § 18. Compounds, Mixtures, and Solutions There are two special features about chemical compounds which it is well to notice. The elements which make up a chemical compound are so completely merged one in the other as to form a new substance having properties quite distinct from any of its components. Neither by the microscope nor by any other known means is it possible to detect the components of the compound so long as the compound retains its individuality. In copper oxide, for example, the components-copper and oxygen-are absolutely indistinguishable. The compound is quite homogeneous. The second feature is that the elements are combined together in certain fixed and definite proportions. In black copper oxide there are 63.5 parts of copper for every 16 parts of oxygen; in red copper oxide, 127 parts of copper are united with 16 parts of oxygen. Copper and oxygen unite in no other proportions. On the other hand, in glass, the lime, soda, and silica are united together so as to form a perfectly 1897; Metallographist, 1. 154, 1898; Report with A. A. Blair, G. Dillner, and J. E. Stead, Journ. Iron and Steel Inst., 66. ii. 221, 1904. homogeneous mass. The constituents are merged together as if the glass were a true chemical compound. But glass does not satisfy the second criterion. The constituents can be mixed in many different proportions. The amounts of silica or lime can be varied between wide limits and still produce glass. Glass may be called a solid solution in contradistinction to ordinary liquid solutions, like whiskey and water, or salt and water. Solutions satisfy the first but not the second mark of chemical compounds. The components of an ordinary mixture are not so completely merged as in chemical compounds. The constituents of a mixture can generally be separated by mechanical means, and they may be mixed together in any proportions we choose. In a eutectic mixture, however, the constituents are mechanically mixed together in fixed and constant proportions. To summarize, a given substance may be 66 The terms " isomorphous mixture" and "mixed crystals" are not to be used in place of "solid solution." Isomorphous salts are those which furnish crystals having the same shape or form; isomorphous substances will frequently crystallize together to form single crystals, called mixed crystals. A solidified solution, although it be a solid, may yet retain the essential characteristics of a liquid solution.1 Of course, a solidified solution is not 1 V. Rothmund, Zeit. phys. Chem., 26. 433, 1897. necessarily a solid solution. As a general rule a solvent can dissolve more of any constituent the higher the temperature. At any given temperature there is usually a limit to the amount the solvent can dissolve. If a solution is saturated at any given temperature, then, on cooling, the solution will reject all in excess of the maximum amount it can dissolve at the lower temperature. The solution theory of carbon-iron alloys affirms that 1. Molten carburized iron is a solution of carbon in iron. 2. The solidified mass is a solid solution. 3. The molten and solid solutions obey the laws of ordinary fluid solutions.1 § 19. The Solidification of Molten Iron Let us examine in more detail a freezing solution of carbon and iron. The freezing curves are shown in Fig. 24. If molten iron containing less than 4 per cent. of carbon be cooled, a solid solution of carbon in iron begins to separate along the line AP' (Fig. 24). This solid solution of carbon in iron is called martensite, in honour of the German metallurgist, A. Martens. There is a complication. The solid solution of carbon in iron which separates is not a definite chemical compound. The iron will not retain, in solid solution, more than 2 per cent. of carbon, whereas the molten mass may have as much as 4 per cent. of carbon. We have a new curve (AQ, Fig. 24). 1 A. S. Stansfield, Metallographist, 3. 24, 300, 1900; Journ. Iron and Steel Inst., 56. ii. 169, 1899; 58. ii. 317, 1900. 2 W. C. Roberts-Austen, Metallographist, 2. 186, 1899. D |