Obrazy na stronie
PDF
ePub

After some hours, during which it swells and emits a lambent blue flame, with an extrication of air, the iron acquires consistency; and, at last, it congeals altogether. It is now removed, while hot, and hammered by means of the tilt hammer. Cast iron thus converted into malleable iron, is always mixed with some foreign substances. They are generally either some of the other metals, or oxygen, carbon, or phosphorus. Besides containing a portion of oxygen, crude iron always holds a considerable quantity of carbon in combination, the gray containing a greater quantity than the white crude iron. The names forge iron, plate or bar iron are given to malleable iron. Steel, a combi. nation of iron and carbon in certain proportions, may be made as follows:

Experiment 55. Stratify small pieces of iron with charcoal powder in a close crucible, and keep the whole in a strong red heat for eight or ten hours. Steel may

be made either by fusion, or cementation, and as it is commonly blistred, it is refined or rendered more uniform in its whole mass, by repeated igni. tion between red hot coals and forging. Natural steel or steel made by fusion, is formed by fusing the iron in a violent heat, converting it first into cast iron, and afterwards into steel, which takes place when the mass of melted scoriæ is five or six inches deep.

Part of the carbon is disengaged with the oxygen in the form of carbonic acid, whilst the other part in union with the pure iron forms steel. This steel is inferior to that made by cementation. When bars of pure iron and charcoal powder are stratified alternately in large earthen troughs or crucibles, the mouths of which are carefully closed up with clay, and in this state kept sufficiently hot for eight or ten days, the bars of iron are converted into steel. In this state tliey are generally blistered, hence the term blistered steel. When drawn out into small bars, they receive the name of tilted steel. When broken to pieces and welded repeatedly in a furnace, and then drawn into bars, it is called German or sheer steel.

A new method of preparing cast steel has been latély announced in France by Clouet. His process is the following: take small pieces of iron and place them in a crucible, with a mixture of the carbonate of lime and the earth of Hessian crucibles ; six parts of the carbonate of lime and six of this earth must be employed for twenty parts of the iron. The matters are to be so disposed, that after fusion the iron must be completely covered by them, to prevent it from coming into contact with the external air. The mixture is then to be gradually heated, and at last exposed to a heat capable of melting iron. If the fire be well kept up, an hour will generally be sufficient to convert two pounds of iron into excellent and exceedingly hard steel, capable of being forged ; an advantage not possessed by steel made in the common manner.*

Carburetted iron, or steel, possesses properties different from that of wrought iron. In the operation of converting iron into steel, an increase of weight from iš to dio is gained. The proportion of carbon, however, necessary for convertion has not accurately been ascertained, notwithstanding the investigations of Vauquelin, Morveau, Clouet and other chemists. Thať carbon is essential in this process, is obvious from the following experiment of Morveau :

A diamond was enclosed in a small crucible of pure ironi, and exposed, when completely covered, to the action of a sufficient heat. The diamond disappeared, and the iron was converted into steel. The diamond weighed 907 parts, the iron 57.800, and the steel obtained 56.384. It was inferred, therefore, that steel contains about oth of its weight of carbon. This experiment was objected to by Mr. Mushet, and partly confirmed by sir George M.Kenzie.

In whatever manner steel is formed, cast steel has the preference, as its texture is more compact, and it

* Phil. Mag. ii. p. 219.

K

admits of the finest polish. With it are made razors, surgeon's instruments, and the like. It is more fusible than common steel, and therefore cannot be welded to it. The method of making it was discovered by Mr. Huntsman about the year 1750. It is formed by fusing blistered steel in a close crucible, mixed with pounded glass and charcoal powder. According to Clouet, it may be formed by melting together 30 parts of iron, one part of charcoal, and one part of glass.

If iron be surrounded in a crucible with a mixture of equal parts of chalk and clay, and kept in a white heat, cast steel will be formed.

Mr. Mushet, to whom we are indebted for a num. ber of facts on this subject, made the following table of the proportion of charcoal, which disappears during the conversion of iron to the different varieties of subcarburet known in commerce.*

Tho

Soft cast steel. ito

Common cast steel. 6

The same, but harder. 36

The same, too hard for drawing
White cast iron.
Mottled cast iron.

Black cast iron. It was the opinion of Pliny, that steel owes its peculiar properties to water;t of Beccher that fire was the only agent in the conversion of iron into steel ; and of Reaumur, that it contains saline and sulphurous particles. But these opinions have been exploded.

Experiment 36. If a slender rod of wrought iron be immersed into cast iron in fusion, it will absorb

part

of the carbon and become steel.

According to Rinman, steel may be distinguished from iron in the following manner:

+ Lib, xxiv. 14.

* Phil. Mag. xiii. 142.

Experiment 37. If a little diluted nitric acid be dropt upon a plate of steel, and allowed to remain a few minutes, and then washed off, black spots will remain ; but,

Experiment 38. If iron be treated in the same way, it will leave only a whitish spot.

The former effect is attributed to the separation of carbon, which of course remains undissolved. It is said, however, that the blackness is owing to the car. bon being oxydized by the acid, and converted into charcoal. Mr. Mushet is of opinion, that carbon exists in steel, in a concrete state, though not in crude; in chemical union, however, and not in mere mixture as in crude iron.

Dr. Pearson, by an ingenious investigation of the nature of a kind of steel, called wootz, which is brought from Bombay, discovered that it contains oxygen, and concluded from all the properties it possesses, that oxygen is the ingredient which distinguishes wootz from steel.

What is called case hardening is the conversion of the surface of iron into steel. What is termed tempering of steel is nothing more than plunging the metal red hot into water. It then acquires more hardness, and may be employed with advantage for some other purposes. It may have its softness and ductility restored by again heating it, and suffering it to cool slowly. On heating steel in contact with air, it acquires different colours. It passes from a yellow, purple, violet, and red to blue. At this period it becomes red hot, the colours disappear, and a number of scales are formed. These different colours indicate the different tempers of the metal.

The tempering of iron or steel, which is effected by suddenly cooling it after it is heated, seems to produce its hardness, brilliancy, and brittleness, by the integrant parts, separated by heat, being kept and left at a certain distance from each other; the sudden cold checking their approximation by the affinity of aggregation

Welding is a process of joining two ignited pieces of the same metal; the degree of heat requisite to that effect, is called the welding heat. Iron combines with most metals, as, l, with gold. An alloy composed of one part iron and eleven gold, is much harder than gold, and has been recommended by Dr. Lewis for making edge-tools. Gold answers well as a solder for iron. The iron cannot be separated from its alloy with gold by cupellation with lead, except it be previously oxydized, which may be effected by repeated fusion with borax or nitre. "If gold alloyed with iron be melted with sulphuret of lead, sulphuret of iron is then formed and separated. The gold is afterwards purified by cupellation. If an alloy of gold and iron be dissolved in nitro-muriatic acid, the addition of metallic iron will precipitate the gold; hence iron has a greater affic nity than gold with oxygen. Sulphate of iron will also separate gold from its solution. See Gold.

2. Iron is generally found with platinum in the state of alloy; but Dr. Lewis did not succeed in his attempt to unite these metals by fusion. The platinum may be separated from the iron, by dissolving the alloy in nitro-muriatic acid, and adding muriate of ammonia. See Platinum.

3. With silver, iron readily unites by fusion. Cupellation with lead will not separate them, except the iron be oxydized and converted into scoriæ by fusing the compound with borax or nitre. In the humid way silver may be separated from iron by precipitation. See Silver. Silver may be separated from iron by sulphuret of lead, or by sulphur.

4. Mercury has been combined with iron indirectly, by triturating iron filings, and the amalgam of zinc, adding to the mixture a solution of iron in muriatic acid. By kneading this mixture, and heating it, the iron and mercury, which combine together gradually, will assume the metallic lustre.

5. Iron may be combined with copper by fusion, but not without difficulty. This alloy is of a gray colour. Copper has been employed in the soldering of iron.

« PoprzedniaDalej »