STEEL.
BESSEMER’S PROCESS.
Steel is usually made by a process called “cementation.” Bars of the best Swedish or Russian iron, about six feet long, are placed in an iron box, the bottom of which is covered with a layer of charcoal powder; over the first row of iron bars some more charcoal is put, and then another row of iron bars, and so on till the box is full, when it is carefully closed and kept at a white heat for four or five days. When cold, the bars are found to be converted into steel, and, being rough and blistered on the surface, are called “blistered steel;” this is broken up, and the bars laid side by side and made hot in a forge, where they are welded together by the blows of a heavy hammer, and drawn or rolled out by machinery into bars of “fine steel.” Steel differs from iron in the closeness of its grain, in being very much “tougher,” and in having that very useful and peculiar property called “temper,” which is the power of hardening when suddenly cooled while red-hot. If a bar of steel as soft as iron be made of a bright red heat, and then suddenly plunged into cold water, it will be found to have become harder than any other metal (so hard, indeed, that it will scratch glass), and is as brittle and readily broken as flint or glass. If now a gentle heat be applied to it, this extreme hardness of temper gives way. For instance, if a piece of bright hard steel is held for a moment in the hollow of a clear fire, a pale straw color appears on its surface, it is now still very hard (but not so hard as before), and is fit for razors, surgeons’ instruments, &c; but if held in the fire a moment or two longer, it becomes of a bright golden yellow, and is fit for penknives, and other cutting instruments; held longer still, it becomes bright blue, and is fit for watch-springs, swords, and other purposes requiring great elasticity but no great hardness; if the heat be carried still further a brown tinge is seen, and it is now rather soft, but greatly harder than iron, and is still elastic; saws, coach-springs, and many other articles are made from steel at this temper. If the heat be carried on to redness, the steel would be quite soft when it had slowly cooled, but if suddenly cooled (as by being plunged into water) the original hard temper comes back again.
Steel, like iron, may be cast, and cast-steel is one of its most useful forms, and much resembles “fine steel.” The mode of preparing cast-steel is to melt the “blistered steel” in a crucible, or earthen pot, and then run it into a mould: this forms an “ingot” of steel, which may be afterwards rolled or welded as the case may require.
Steel may be drawn into very fine wire, or wrought into the most minute articles, as the springs and other parts of watches. It bears a very fine and bright polish, and does not rust or tarnish so easily as iron. It has lately been proposed to make heavy cannon of cast-steel, which is much tougher than either cast-iron or gun-metal.
PERPENDICULAR SECTIONS OF CONVERTING VESSEL.
FIG. 3.
OUTSIDE VIEW OF CONVERTING VESSEL.
A process has lately been invented by Bessemer to supersede the long and laborious process of “puddling.” It consists essentially of transferring the melted iron into a vessel in which there are tubes inserted at the lower part, and through which air is forced at a great pressure, which bubbles up in streams through the melted metal, and, as it does so, unites with the carbon and sulphur of the iron, converting them into carbonic and sulphurous acids, and at the same time producing an increased heat, which is quite necessary to the success of the process, for as the iron becomes purer it also becomes more difficultly fusible, and would set into a solid mass, but that this greatly increased temperature keeps it fluid. This rising of temperature is similar to what takes place upon blowing a common fire with bellows; for the more air that is admitted to carbon raised to a very high temperature, the more rapidly does it combine with its oxygen, or, in other words, the more rapidly does it burn. So that, in this process of Bessemer’s contriving, the carbon of the iron acts as fuel to keep up the heat necessary to maintain its fusion, and at the same time, by being converted into carbonic acid, escapes in bubbles (like the bubbles which escape from soda-water), and this rapid production of gas in every part of the fused iron also assists in bringing about a thorough stirring-up and mixing together of all its parts. Reference to the accompanying diagrams will illustrate the working of the process. In [figs. 1] and [2], A B are lower chambers, C is the melted iron, D is an upper chamber for melting scrap-iron, &c., E in [fig. 1] and D in [fig. 2] are openings for the escape of gas and flame, F F is an air-passage running all round and communicating by G with the tuyere-holes shown at E in [fig. 4], H is the fire-brick lining to the furnace, and I the tapping-hole through which the fluid iron is discharged. In [fig. 3], I is the tapping-hole, K the main air-tube leading from the blast-engine, L L perpendicular tubes (marked G in [figs. 1] and [2]) leading from circular air-passage to tuyere-holes, M is an opening for the insertion of iron into the upper chamber, opposite to opening D in [fig. 2], and N the tap for regulating the blast; O indicates the outer casing of wrought iron.
When the process is carried only to a certain point, the result is a sort of semi-steel, which the inventor of the process expects will be a very useful article of commerce.
FIG. 4. HORIZONTAL SECTION OR PLAN OF CONVERTING VESSEL.
During the bubbling up of the whole mass of iron and the extreme elevation of temperature caused by the union of the carbon of the impure iron with the oxygen of the air, the oxide of iron as fast as it forms fuses into a sort of glass, and this unites with the earthy matters of the impure iron and floats on the upper part as a flux, thus ridding the cast-iron of all its impurities, with no other fuel than that contained in the iron itself and the air.
The accompanying illustration represents a horizontal section or plan of the converting vessel; A the central chamber, B a lining of fire-bricks, C the main air-tube, D the tapping-hole, E the tuyere-holes through which the air is forced into the melted iron to be purified, F is the outside casing of wrought-iron.