Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/300

284 IRON and tempering cannot be markedly influenced by the pre sence of this constituent. No connexion between the amount of nitrogen present and the physical properties of the metal, or the amount of carbon or other foreign ele ments present therein, has as yet been thus established by any experimenter. As regards the presence of oxygen in iron and steel, and its effect on their qualities, little evidence as yet exists. It is well known that certain metals, e.g., copper, will dis solve small proportions of oxide, the presence of which renders the metal much less tenacious than it otherwise would be, so that certain operations are usually gone through in the final stages of the extraction of these metals for the purpose of again reducing the oxide disseminated through the mass, e.g., " poling " melted copper. The tena city exhibited by " phosphor-bronze " is largely due to the complete reduction of copper and tin oxides by the phos phorus. This solution of oxide in the metal also takes place with iron, especially when tolerably free from silicon; this element is capable of reducing iron oxide when heated therewith, so that when present the silicon is oxidized in preference to the iron. Overblown Bessemer metal is comparatively unforgeable and brittle, so that probably the presence of oxygen affects iron in the same way as sulphur. When iron and steel are overheated for a long time, they become "burnt" and brittle : this is supposed by some to be due to the forma tion of oxide disseminated through the mass of the metal, but many others consider that a more or less crystalline structure set up under the influence of a softening heat is the sole cause of the diminution in strength and tenacity ( 43). Iron and steel usually give off, on heating under diminished pres sure, carbon oxide and more or less hydrogen, and the former gas is largely extruded from steel in the act of solidifying (Bessemer), there- lay giving rise to honeycombing of the casting. This is usually attributed to the same cause as the "spitting" of silver, i.e., to a physical inability on the part of the metal to retain in solution at a lower temperature the same amount of gas that it can dissolve when more highly heated; the presence ot silicon diminishes this evolu tion of gas, probably by the decomposition of the carbon oxide with formation of non-gaseous silica. A number of observations and de terminations of the gases occluded by and otherwise present in iron and steel have been made by Parry, Troost and Hautefeuille, Miiller, and others, but without leading to any definite correlations between the physical properties of the metals and the gases occluded. More over, it does not seem to be absolutely established whether the carbon oxide obtained by heating in an exhausted tube really exists as dis solved gas or as a mixture of oxide and carbide (or solution of car bon); the writer has found that by varying the mode of heating and the temperature variable proportions of carbon oxide and di oxide may be obtained from spongy iron (prepared by heating to a bright red heat ferric oxide in an atmosphere of carbon oxide) when it is heated in connexion with a Sprengel pump ; which seems to sug gest that a mixture of oxide and carbide is present rather than simply occluded gases. Hydrogen when present in iron to a considerable extent appears very considerably to diminish the tenacity and strength ; thus electro-deposited iron containing much hydrogen is brittle, but becomes soft and flexible on heating under diminished pressure so as to extract the hydrogen. When iron or steel wires are immersed in dilute sulphuric acid, especially in contact with zinc, so as to evolve hydrogen copiously from the surface of the iron, the wires take up about twenty times their volume of hydrogen, and become so brittle that they break on attempting to bend them. Copper is often present in minute quantity in pig iron. When steel contains a few tenths per cent, of copper it is distinctly red-short, more so when the proportion is increased (Eggertz). Malleable iron does not ssem to be so much affected by copper, O5 per cent, giving but little red- shortnsss; the welding power is, however, considerably diminished. On the other hand, addition of iron to bronze and similar copper alloys increases their strength and tenacity, as in Aich s gun-metal and Gedge s metal. Antimony acts as injuriously upon iron as sulphur and phosphorus conjointly, a few tenths per cent rendering bar iron highly cold-short and also hot-short. Chromium, tungsten, vanadium, and titanium are all apparently capable of increasing the strength of iron more or less after the fashion of carbon, and accordingly have been regarded as valuable constituents in special kinds of iron and steel, e.g., the so-called chromium steel and tungsten steel, and the iron containing traces of vanadium employed on the Swiss wire bridges at Freiburg. Faraday and Stodart found that about 1 per cent, of platinum or certain of its congeners (e.g., palladium and rhodium} improved the toughness of steel, and communicated to it a fine grain. Nickel is largely present in meteoric iron (vide infra), from which knife blades, &c., are readily beaten out, so that the presence of nickel does not appear to diminish materially the malleability of iron. The question as to whether the carbon which does not separate in the graphitoidal state on cooling molten cast iron or steel is truly combined or not (in the sense in which oxygen is combined in ferric oxide, and not in the sense in which silicate of cobalt may be said to be combined in blue glass to which it gives the colour, or in which sugar is combined with water in syrup) is one about which great divergence of opinion exists. It is usual to speak of this carbon as " combined carbon," because when the iron or steel is dissolved in an acid (e.g. , hydrochloric acid), this carbon combines with the evolved hydrogen and escapes as carburetted hydrogen of some kind, whereas the graphitoidal carbon remains behind unaffected ; just in the same way the sulphur escapes as sulphuretted hydrogen. It is by no means apparent, however, that carbon if set free in the amor phous condition in a state of excessively fine division and in pre sence of nascent hydrogen would not forthwith combine with the hydrogen, even though its condition in the iron were only that of a dissolved body ; the probability is indeed rather the other way, for such carbon when free and warm is known to be often pyrophoric in the air, whilst the mixture of carbon and partially reduced iron and iron oxide, formed when carbon oxide is allowed to act on ferric oxide for some time at a low red heat, evolves hydrogen containing much carburetted hydrogen on treatment with an acid, e.g. , hydro chloric acid. It is to be remembered also that, whilst definite sulphides of iron are known and are easily obtainable, the same can hardly be said of carbides of iron ; it is true that spiegeleisen (manganese- iron alloy) contains a larger amount of so-called combined carbon than ordinary steel, amounts up to 6 per cent, having been found therein ; but it hardly follows from this that spiegeleisen and steel, &c., Contain a definite carbide, such as Fe 4 C, or Fe 8 C, which has sometimes been considered as present therein, e.g., by Karsten, Gurlt, Mattieu Williams, and others. A compound the con stituents of which separate on cooling would be a very unusual sort of substance, whereas it is well established that by fusing and very rapidly chilling certain kinds of grey cast iron they are more or less converted into white or mottled iron, the amount of combined " car bon largely increasing, and that of graphite correspondingly decreas ing; whilst the converse change can be brought about in some kinds of white iron by fusing and very slowly cooling them, a notable separation of graphite and diminution in the quantity of "com bined " carbon present being thus brought about. According to Akcrmann fusion is not indispensable, long continued maintenance at a yellow heat sufficing to change white iron into grey. In practice the quality of pig iron is to a considerable extent decided by the degree of crystallinity exhibited by it, i.e., by the extent to which graphite has separated out during solidification, and the size of the crystals of this substance and of the solidified partly decarbonized pig iron, the crystallization of which is promoted by the particles of graphite acting as nuclei. Pigs with the largest crystals are known as No. 1; those made up of somewhat smaller but still moderately large crystals, as No. 2; smaller-grained pigs, but still crystalline and grey, are known as Nos. 3 and 4. The finest grained No. 4 pigs, being usually unsuitable for making castings, and only serviceable for the puddling forge, are designated "forge 4," the higher kinds being known generically as "foundry iron." Some times a pig will solidify partly as white iron partly as grey, the crystallization having commenced in patches, but not having spread throughout the whole mass before it solidified ; such iron is known as " mottled pig." The price of market pig iron is regulated by these numbers and the locality of the furnace, i.e., the nature of the ore from which it is smelted ; those brands which are specially free from phosphorus, and are consequently applicable to the pre paration of " Bessemer metal " (steel made by the Bessemer-Mushet p roccss _ 36), are usually designated "Bessemer pig." Special qualities of wliite iron free from sulphur and phosphorus and con taining several parts per cent, of manganese smelted from spathose and other highly manganiferous ores are known as spiegeleisen, from their mirror-like fracture. Fcrro-manganese is a similar pro duct containing a much larger amount of manganese ( 41).