On the Case-hardening of Iron.
The process of case-hardening iron is that by which the surface of articles already manufactured are converted into steel. It is chiefly used for table knives, and particular surgical instruments, where toughness and hardness are required. The articles which are to undergo this process are laid into a pan of plate iron, and surrounded with bone shavings, pieces of horn, or old leather shoes. A forge fire of considerable size is now made up, and when the upper part of the fire has caked together, it is care fully lifted off without breaking, and the iron pan when laid upon the red coals, is covered with the caked mass. In this state it remains for two hours, without urging the fire. The progress of the opera tion is ascertained by small pieces of trial wire pre viously introduced into the pan, which are taken out, and dipped in water, then by means of the file and the character of the fracture, the condition of the articles is known. When the process is deemed complete, the fire is increased, and the articles, when brought to a proper heat, are taken out and immersed in cold water." On the Tempering of Steel, and the colours which company it.
The art of tempering steel and fitting it for various kinds of cutting instruments and other purposes, is one of the most important in the arts. The heat by which this change in its character is affected, must always be less than that which was used in hardening it. The comffion method of tempering consists in As many articles of cutlery require to be tempered with great precision, Mr. Hartley, in the year 1789, took out a patent for a new method. This method consists in using an iron trough filled with the fusible metal, consisting of eight parts of lead, two of tin, and five of bismuth. A mercurial thermometer gra duated to 600' of Fahrenheit is then immersed in it, and the fusible metal is brought to the required tem perature by means of a furnace or lamp placed below the trough. In place of the fusihle metal oil may be used, but as the articles are in this case immersed in the oil and out of contact with the atmosphere, the colours arising from oxidation are not produced.
Mercury may be used in place of the fusible metal. No change of colour will appear on the steel till the thermometer indicates 430°, and it is so faint that it can be even then only seen by comparing it with a plate not heated. A very ingenious method of tem pering very delicate steel articles was proposed by Dr. Wollaston to Mr. Stodart. The steel articles when placed in a tube were surrounded with the fusible metal. The tube with its enclosures was then heated to redness in a furnace, and afterwards immerser] in a cooling fluid. The whole is then thrown into boiling water, which melts the fusible metal and leaves the steel perfectly hardened and unaltered by twisting, cracking, or suffering any change of form.
Mr. Stodart made several experiments in order to determine the best fluid for cooling the heated steel. Ile plunged a scalpel in a mixture of snow and muriate of lime, but without perceiving that any advantage was derived from the extreme cold. A large quantity of water brought to the temperature of about 40° of Fahrenheit he found to answer the purpose as well as any that had been tried. In tempering steel and the alloys of steel, to be afterwards described, Mr. Stodart recommends it to be performed twice, first at the usual time before grinding, and again just before the last polish is given to the blade. •‘ This second tem pering," says he, " will perhaps appear superfluous, but upon trial its utility will be readily admitted; we were led to adopt the practice by analogy, when con sidering the process of making and tempering watch springs."t The following table, which is different from the one given above, is drawn up from a series of fine speci mens of oxidated steel, which the late eminent Mr. Stodart presented to the writer of this article. The Observations on the Hardening of Steel, addressed to the Franklin Institute, by Rufus Tyler, Mechanician, Philadelphia.