Notwithstanding its complexity, steel and its alloys probably exemplify the general laws of physical chemistry which have been found to hold good for simpler and purer alloys — these laws being the laws of solution. Thus Roberts Austen and Spring found that one metal dif fuses into another, like a salt into water; in general solubility increases with the temperature. Metals and alloys will flow under pressure. Under 200,000 pounds pressure, the writer ob tained from a 3.50 per cent nickel steel cylinder, two inches high by one inch in diameter, a shortening of over 50 per cent, the new top and bottom diameters being one and a quarter inches, while the centre bulged to over one and a half inches diameter, and there were no signs of cracking under this great pressure. Alloys may or may not react chemically when brought into intimate association by fusion or pressure; the molecular mobility increases with the tem perature; upon cooling of molten alloys phenomena strongly suggestive of freezing salt solutions are observed; the depression of the freezing point of a metal when another is added to it follows the laws of Coppet and Raoult, for dilute solutions, and finally, the phase rule of Gibb applies quite as well to the explanation of conditions of equilibrium in alloys as to similar problems in regard to liquid solutions.
Benedick's work on electrical conductivity of steels shows the chemical equivalence of the atoms of different elements dissolved in iron in increasing the resistance, and in one par ticular instance, that of molybdenum and tungsten steels, long practical experience has shown that these metals are effective in about the proportion of their atomic weights; that is, one part of molybdenum is about equal to two parts of tungsten in self-hardening and high speed steels and the writer has fully demon strated that the same relation holds as regards the magnetic qualities of hardened molybdenum and tungsten steels.
The Chemical Constitution of Steel A number of important researches have been published which throw light upon the chemical behavior of the elements in steel and of the chemical compounds which steel contains. Iron and carbon are known to com bine at least in one combination, and many other carbides of doubtful existence have been mentioned from time to time. Manganese seems to form an isomorphous solution with iron and lowers the critical points. When over 12 per cent manganese is present the tempera ture of transformation known as Ar, is below 0" C. Manganese prevents the separation of graphite and thus raises the saturation point of iron for carbon. In its direct chemical affini ties it seems to unite readily with several ele ments; it forms a carbide, Mn,C, analogous to cementite and it unites with phosphorus, giving a phosphide, Mn,P,. Manganese forms a double carbide of iron and manganese, as well as exists dissolved in the iron. It further exists in the' presence of sulphur as MnS. It lowers the critical points very markedly. Copper dissolves in large quantities in iron without the forma tion of any compounds. Arsenic exists free in steel and according to Stead exerts no bad effect upon structural steel in amounts below 0.15 per cent. In hardened steel the arsenide of iron occurs. Phosphorus unites also with iron to form the phosphide, In the presence of high manganese, however, it is the phosphide, Mn,P,, which results. Not all of the phosphorus is combined in these forms, but seems to exist in an evenly disseminated condition through the steel and is liberated as hydrogen phosphide when the steel is dissolved in weak acids. Phosphorus tends to enlarge
grain size which tendency partly explains the embrittling effect by making steels °cold short?) Sulphur exists in steel as the sulphide of man ganese, MnS, and if there is not sufficient manganese present to use up all the sulphur the excess exists as a sulphide of iron, FeS. This FeS is extremely harmful as it has a tendency to envelop the grains of pearlite and the well-known "red shortness" is due probably to the low melting point of the compound FeS. A double sulphide of iron and manganese has been reported. Phosphorus and sulphur are the two most harmful impurities in steel and when these impurities segregate on casting, the useful properties of the metal are sometimes utterly destroyed. Under the microscope a metal suffering from this segregation is ry gen erally found to contain bands of varying widths and lengths technically known as "ghost lines" in which the presence of abnormally large amounts of MnS and phosphorus compounds can generally be detected by the ordinary metallographic tests. Chromium occurs both dissolved in the main mass of iron but chiefly in the form of double carbides. Chromium has no direct effect on the critical points but changes the position of these points as the tempera ture from which the material is cooled varies. Silicon in ordinary amounts is wholly dis solved in the iron. It is probable, however, that it is not held in solution as elemental sili con but rather as a silicide of iron, FeSi. It cannot be seen under the microscope. Silicon is noted for the ease with which it precipitates carbon in the form pf graphite. Gumlich has shown that the second magnetic transition point is steadily lowered with increased silicon con tent until it reaches a value of about 660° C. for 8.35 per cent silicon while the first (pearl Inc) point steadily rises, the two coinciding at 2.2 per cent silicon and becomes practically extinct at 2.5 per cent. In an exhaustive study, Charpy and Cornu-Thenard later confirmed this observation and definitely connected the dis appearance of the A, point with the precipita tion of graphite. Tungsten appears to raise rather than lower the critical points. This point, however, varies too, according to the tempera ture from which the material cools. It forms a double carbide with iron. Steel containing .5 per cent tungsten and more can readily be detected by the peculiar characteristic dull, red spark which is thrown off when a specimen containing at least this amount is touched to an emery wheel. Nickel dissolves ip iron in all proportions. Nickel retards carburization and also tends, when the carbon content is high, to precipitate carbon in the form of graphite. Nickel lowers the critical points, espe cially Ar,. When 25 per cent of nickel is present Ar, is below 0° C; it occurs, however, if arti ficial cooling be employed. In connection with this phenomenon it is interesting to note the microscopical researches of Guillet, who ex perimented with three series of alloys contain ing respectively 0.12 per cent, 0.35 per cent and 0.85 per cent carbon, the nickel increasing by steps of 2.5 per cent up to 30 per cent. The etching was done by alcoholic picric acid. Guil let found that as the nickel increased pearlite was replaced by martensite and this in turn by polyhedral crystals indicative of gamma-iron.