IRON AND STEEL.[1] 1. Iron, the most abundant and the cheapest of the heavy metals, the strongest and most magnetic of known substances, is perhaps also the most indispensable of all save the air we breathe and the water we drink. For one kind of meat we could substitute another; wool could be replaced by cotton, silk or fur; were our common silicate glass gone, we could probably perfect and cheapen some other of the transparent solids; but even if the earth could be made to yield any substitute for the forty or fifty million tons of iron which we use each year for rails, wire, machinery, and structural purposes of many kinds, we could not replace either the steel of our cutting tools or the iron of our magnets, the basis of all commercial electricity. This usefulness iron owes in part, indeed, to its abundance, through which it has led us in the last few thousands of years to adapt our ways to its properties; but still in chief part first to the single qualities in which it excels, such as its strength, its magnetism, and the property which it alone has of being made at will extremely hard by sudden cooling and soft and extremely pliable by slow cooling; second, to the special combinations of useful properties in which it excels, such as its strength with its ready welding and shaping both hot and cold; and third, to the great variety of its properties. It is a very Proteus. It is extremely hard in our files and razors, and extremely soft in our horse-shoe nails, which in some countries the smith rejects unless he can bend them on his forehead; with iron we cut and shape iron. It is extremely magnetic and almost non-magnetic; as brittle as glass and almost as pliable and ductile as copper; extremely springy, and springless and dead; wonderfully strong, and very weak; conducting heat and electricity easily, and again offering great resistance to their passage; here welding readily, there incapable of welding; here very infusible, there melting with relative ease. The coincidence that so indispensable a thing should also be so abundant, that an iron-needing man should be set on an iron-cored globe, certainly suggests design. The indispensableness of such abundant things as air, water and light is readily explained by saying that their very abundance has evolved a creature dependent on them. But the indispensable qualities of iron did not shape man’s evolution, because its great usefulness did not arise until historic times, or even, as in case of magnetism, until modern times.
These variations in the properties of iron are brought about in part by corresponding variations in mechanical and thermal treatment, by which it is influenced profoundly, and in part by variations in the proportions of certain foreign elements which it contains; for, unlike most of the other metals, it is never used in the pure state. Indeed pure iron is a rare curiosity. Foremost among these elements is carbon, which iron inevitably absorbs from the fuel used in extracting it from its ores. So strong is the effect of carbon that the use to which the metal is put, and indeed its division into its two great classes, the malleable one, comprising steel and wrought iron, with less than 2.20% of carbon, and the unmalleable one, cast iron, with more than this quantity, are based on carbon-content. (See Table I.)
Table I.—General Classification of Iron and Steel according (1) to Carbon-Content and (2) to Presence or Absence of Inclosed Slag.
| Containing very little Carbon (say, less than 0.30%). | Containing an Intermediate Quantity of Carbon (say, between 0.30 and 2.2%). | Containing much Carbon (say, from 2.2 to 5%). | |
| Slag-bearing or ”Weld-metal” Series. | Wrought Iron. Puddled and bloomary, or Charcoal-hearth iron belong here. | Weld Steel. Puddled and blister steel belong here. | |
| Slagless or “Ingot-Metal” Series. | Low-Carbon or Mild Steel, sometimes called “ingot-iron.” | Half-Hard and High-Carbon Steels sometimes called “ingot-steel.” | Cast Iron. |
| It may be either Bessemer, open-hearth, or crucible steel. | They may be either Bessemer, open-hearth, or crucible steel. Malleable cast iron also often belongs here. | Normal cast iron, ”washed” metal, and most “malleable cast iron” belong here. | |
| Alloy Steels. Nickel, manganese, tungsten, and chrome steels belong here. | Alloy Cast Irons.* Spiegeleisen, ferro-manganese, and silico-spiegel belong here. | ||
| * The term “Alloy Cast Irons” is not actually in frequent use, not because of any question as to its fitness or meaning, but becausethe need of such a generic term rarely arises in the industry. | |||
2. Nomenclature.—Until about 1860 there were only three important classes of iron—wrought iron, steel and cast iron. The essential characteristic of wrought iron was its nearly complete freedom from carbon; that of steel was its moderate carbon-content (say between 0.30 and 2.2%), which, though great enough to confer the property of being rendered intensely hard and brittle by sudden cooling, yet was not so great but that the metal was malleable when cooled slowly; while that of cast iron was that it contained so much carbon as to be very brittle whether cooled quickly or slowly. This classification was based on carbon-content, or on the properties which it gave. Beyond this, wrought iron, and certain classes of steel which then were important, necessarily contained much slag or “cinder,” because they were made by welding together pasty particles of metal in a bath of slag, without subsequent fusion. But the best class of steel, crucible steel, was freed from slag by fusion in crucibles; hence its name, “cast steel.” Between 1860 and 1870 the invention of the Bessemer and open-hearth processes introduced a new class of iron to-day called “mild” or “low-carbon steel,” which lacked the essential property of steel, the hardening power, yet differed from the existing forms of wrought iron in freedom from slag, and from cast iron in being very malleable. Logically it was wrought iron, the essence of which was that it was (1) “iron” as distinguished from steel, and (2) malleable, i.e. capable of being “wrought.” This name did not please those interested in the new product, because existing wrought iron was a low-priced material. Instead of inventing a wholly new name for the wholly new product, they appropriated the name “steel,” because this was associated in the public mind with superiority. This they did with the excuse that the new product resembled one class of steel—cast steel—in being free from slag; and, after a period of protest, all acquiesced in calling it “steel,” which is now its firmly established name. The old varieties of wrought iron, steel and cast iron preserve their old names; the new class is called steel by main force. As a result, certain varieties, such as blister steel, are called “steel” solely because they have the hardening power, and others, such as low-carbon steel, solely because they are free from slag. But the former lack the essential quality, slaglessness, which makes the latter steel, and the latter lack the essential quality, the hardening power, which makes the former steel. “Steel” has come gradually to stand rather for excellence than for any specific quality. These anomalies, however confusing to the general reader, in fact cause no appreciable trouble to important makers or users of iron and steel, beyond forming an occasional side-issue in litigation.
3. Definitions.—Wrought iron is slag-bearing malleable iron, containing so little carbon (0.30% or less), or its equivalent, that it does not harden greatly when cooled suddenly.
Steel is iron which is malleable at least in some one range of temperature, and also is either (a) cast into an initially malleable mass, or (b) is capable of hardening greatly by sudden cooling, or (c) is both so cast and so capable of hardening. (Tungsten steel and certain classes of manganese steel are malleable only when red-hot.) Normal or carbon steel contains between 0.30 and 2.20% of carbon, enough to make it harden greatly when cooled suddenly, but not enough to prevent it from being usefully malleable when hot.
Cast iron is, generically, iron containing so much carbon (2.20% or more) or its equivalent that it is not usefully malleable at any temperature. Specifically, it is cast iron in the form of castings other than pigs, or remelted cast iron suitable for such castings, as distinguished from pig iron, i.e. the molten cast iron as it issues from the blast furnace, or the pigs into which it is cast.
Malleable cast iron is iron which has been cast in the condition of cast iron, and made malleable by subsequent treatment without fusion.
Alloy steels and cast irons are those which owe their properties chiefly to the presence of one or more elements other than carbon.