With this doctrine, however, facts are at variance, and I believe, that the circumstance, above alluded different tints are twelve in number, and exhibit very perceptibly the progress of the oxidation ;— 1. Very pale straw yellow - - 430° 2. A shade of darker yellow - - 450 3. Darker straw yellow - 470 4. Still darker straw yellow - , - 490 5. A browner yellow - 500 6. Yellow, tinged slightly with purple 520 7. Light purple - - - 530 8. Dark purple - - - - 550 9. Deep blue - - - - - 570 10. Paler blue -- - - - 590 11. Still paler blue - - - - 610 12. Still paler blue, with a tinge of green 630 In the production of these colours the access of oxygen is absolutely necessary, in order that it may combine chemically with the metal. Sign. Ambrosio Fusinieri, who has made a number of curious experi ments on the subject in reference to the other metals, found that these colours are produced in the same order on all metals except platinum, upon which, whether in the state of wire or plate, he never could produce any colour.* to, as the cause of breaking, may also explain most satisfactorily, the phenomena in question, (to wit) that of hardening the exterior, before it can possibly be perfnitted to contract to its proper size, because of the expanded mass within.
I have found in a number of cases of thin hollow cylinders, or flattened rings, where there was the best chalice of thorough, and almost instantaneous cooling, and of course, of producing the greatest de gree of hardness, that no enlargement was perceptible.
Particular care should be observed, in the act of cooling, not to suffer any intermission, in any part, as is often done by moving the piece backward and forward, too briskly, in the water, alternately cool ing, and exposing to a vacuum, the opposite sides; for a part thus expoSed, after moving rapidly against the current, until fairly hardened, might be let down or tempered, as it is called, by the heat rushing from the centre, toward the side exposed to the vacuum, without being sufficiently re-heated to prepare it for hardening at the return of the current of the water. In this way, soft places are often produced, which will erroneously be attributed to uneven steel, want of sufficient heat, Sic.
By dipping the end of' a small bar (heated to sev eral inches in length), and keeping it quite still, until it is hardened nearly to the surface of the water (which should be very cold); and then raising it quickly, an eighth of an inch, or more, according to the size of the bar, a portion of what was hardened will be softened by the heated part above;—as soon as this is perceived, let the bar be again sunk into the water, to where it remains of a hardening heat, which will be perhaps half of an inch lower than before, an other portion of about gths of an inch will thus be hardened; let the bar be again withdrawn a small distance, as before, repeating the operation, until there no longer remains sufficient heat in the bar for hardening; the result will be, a number of' successive hard and soft rings.
While testing the strength of different kinds of steel, by repeatedly hardening each kind, until a frac ture should take place, 1 was somewhat surprised to find the pieces, which were small (such for example, as were an inch square, and 2, or .1 an inch thick), considerably swollen, after three or four times hard ening, and that every hardening increased their con vexity, until they actually burst the surface, in the middle of one of the faces. Repeating the experi ment, with a piece prepared perfectly flat, I found the first, second, third, and fourth time, of hardening each, to produce a small additional elevation of the surface. On the fourth attempt the piece cracked.
I have seen a thin piece of steel very beautifully hardened, by chilling in its passage through a rolling mill; this piece afterwards exhibited in its fracture an exceedingly fine grain, a probable consequence of its being hardened under immense pressure.
Small drills, and other articles of the thickness of a small needle, may be cooled, with sufficient rapidity to become hard, by moving them briskly through the air.
Water, to he active, in cooling, should be perfectly free from soap,—a small portion of that substance will cause the time of cooling to be extended beyond the hardening limit, especially if the piece of steel be not very small.
The grain of steel, though finer when hard than when soft, becomes still finer, the lower the temper be drawn, until about a medium between hard and soft, when the fineness begins to decrease.
Cast iron is capable of being hardened in the same manner as steel, except the kind which is already hardened at the time of casting. This kind possesses a superior degree of hardness, which differs materially from that obtained in the manner of' hardening steel. It takes place in passing from tho fluid to the solid state, and can only be changed by re-melting. As soon as time will permit, I intend offering some remarks on hard and soft cast-iron.
The most satisfactory theory of hardening steel, which also applies to cast iron, is one suggested by Mr. William Mason, of this place. Ile supposes, that at the hardening heat, the component parts of steel exist in a state of perfect chemical union, and that if time be allowed in cooling, that union is dis solved, or changed to a simple mechanical mixture. This he conceives to be supported by the following experiment: melt together certain proportions of zinc and quicksilver, and pour one part of the amalgam into water, and the other into a wooden or paper mould; that which is poured into water being sud denly chilled, retains its chemical union, and becomes of the consistence of' paste; the other separates, the zinc forming a solid cellular body, holding the quick silver in very minute globules, in its interstices.
On .finnealing Cast Steel, so as to make it as soft as Iron.—By JACOB PERKINS.
We were lately shown by an American friend, sonic slips of thin cast-steel, which were as soft and pliant, and as easily bent into any required shape, as though they had been tinned sheet-iron. They were of a light grey colour, perfectly free from oxidation or scales, and were still capable of hardening, on being heated and quenched in water.
On mentioning this fact to Mr. Perkins, he stated that the process was known to him, he having prac tised it in America with great advantage ; and he had even communicated it to one intelligent engineer in this country, who had since constantly employed it.
The secret consists in inclosing the cast-steel in close cast-iron vessels, which completely exclude the external air; and in keeping them at a moderate red heat, in a proper annealing furnace, a sufficient length of time, according to the thickness of the steel ; and, lastly, letting them cool very slowly.
This process is superior to the usual practice of decarbonating cast steel, and reducing it to the state of iron ; which renders it necessary to restore its steely nature again, by case-hardening it, before it can be hardened as usual. [Tech. Rep.
On .finnealing Iron and Steel IT'ire.—By THOMAS