Steel. This most important modification of iron has engaged the attention of many chemists and metallurgists. It may be procured, but not equally pure, by different methods. One is to keep the cast iron for a considerable time in fusion and in a very high degree of heat; whilst its surface is covered with melted scoriæ, so as to preclude the contact of the atmosphere with the iron. This, it is conceived, gives time for the carbone and oxygen to combine and escape in the form of carbonic acid. This steel is of inferior purity.
Steel of cementation is made by stratifying bars of pure iron with charcoal powder in large earthen crucibles, carefully closed up with clay. These are exposed to a high degree of heat in a furnace for 8 or 10 days. This is called blistered steel, from the appearance of blisters on its surface.
Cast steel is made from blistered steel by breaking the bars and putting them into a large crucible with pounded glass and charcoal. The crucible is closed with a lid of the same ware and placed in an air furnace. When the fusion is complete the metal is cast into ingots. This is the most valuable and probably the purest steel.
When steel is heated red and plunged into cold water, it is hardened; that is, it becomes much harder than iron or than steel without this operation. Hardened steel is brittle, and cannot be extended by the hammer or corroded by a file till it is again softened by being heated and then gradually cooled.
One of the most remarkable properties of hardened steel is that of being tempered, as it is called; by which it is adapted to the different purposes of the manufacturing artists. Tempering consists in heating the hardened steel till it acquires a straw colour for edge tools, a blue colour for watch springs, and elastic articles in general; &c. &c.
Hardened steel is qualified to acquire magnetism, and to retain it so as to become a permanent magnet. This power of retaining magnetism distinguishes steel from pure iron.
From the above account of steel, it is evident there is an essential difference between it and pure iron. That difference consists, according to the common opinion, in steel being a carburet of iron, or carbone and iron united. The fact of the union of carbone and iron in the formation of steel does not seem to me satisfactorily proved. Mr. Collier asserts that iron gains about ¹/₁₈₀th of its weight by being converted into steel.[18] But Mr. Mushet found that though steel gains weight upon the iron when copiously imbedded in charcoal, yet it loses weight if the charcoal is only ¹/₉₀ or ¹/₁₀₀ of the weight of the iron.[19] The same ingenious gentleman seems to estimate the carbone in cast steel, from synthetic experiments, to be ¹/₁₀₀th of its weight.
From analytic experiments, however, there does not appear reason to believe that steel contains so much, if any charcoal. Pure steel dissolved in dilute sulphuric acid gives hydrogen gas containing no carbonic acid nor oxide, neither is there any appreciable residuum of any kind in general.
On considering all the circumstances, I am inclined to believe, that the properties which distinguish steel from iron are rather owing to a peculiar crystallization or arrangement of the ultimate particles of iron, than to their combination with carbone or any other substance. In all cases where steel is formed, the mass is brought into a liquid form, or nearly approaching to it, a circumstance which allows the particles to be subject to the law of crystallization. We see that great change is made in steel by the mere tempering of it, which cannot be ascribed to the loss or gain of any substance, but to some modification of the internal arrangement of its particles. Why then may not its differences from iron be ascribed to the same cause? It is allowed that steel, by being repeatedly heated and hammered, becomes iron: that is, it should seem, the change of figure disturbs the regular arrangement of the particles. And it may be further observed, in corroboration of the opinion that cast iron is capable of being made permanently magnetic, from its having been in fusion more probably than from its near approximation to steel in its component parts. The most powerful artificial magnets, after being forged of steel, are said to undergo the operation of steelifying again, before they are hardened finally to receive the magnetic virtue.