Modern Machine-Shop Practice, Volumes I and II

by Joshua Rose

Before heating the steel it is dipped in soft soap to prevent oxidation, and on dipping it into the potash and salt pot it causes a cracking sound, the operator knowing from the sound if the mixture is proper, and how long to hold the steel therein.

This company first fill the heating pot with salt, and then add cyanide of potash until a trial of the tool gives quite satisfactory results, adding cyanide of potash as the work proceeds to make up for the evaporation and keep the mixture of the compound correct.

In many cases it is considered an advantage to harden the outside of an article, keeping the inside as soft as possible so as to increase the strength. In such case the article may be heated in red-hot lead, the surface of which may be covered with charcoal. Under these conditions the outside of the article, especially if thick, will get red hot in advance of the inside.

Articles having thick and thin sections may be heated in fluxes to great advantage, the thick side being immersed first, and the article being lowered very slowly into the pot of lead. If the shape of the article is such as to render it liable to crack in the water because of containing holes or sharp corners in weak parts, these holes and sharp corners may be filled with fire-clay, the dipping water may be heated to about 50°, and salt (1 lb. per gallon of water) added to it.

The Monitor Sewing Machine Company harden and temper their spiral springs at one operation, by heating them to a blood-red heat and quenching them in a mixture of milk and water, which will give an excellent result, providing that the springs are heated to precise uniformity and the mixture of milk and water is correct. For a process of this kind (which is very expeditious, because the hardening and tempering is performed at one operation), the steel should be heated to a very uniform temperature, and a mixture of, say, two-thirds milk and one-third water tried at first, more milk being added to lower the temper, or more water to increase it if necessary.

Saws are hardened in compositions of animal oil, such as whale-oil, with which resin, pitch, and tallow are sometimes mixed.

Resin hardens but somewhat crystallises the metal, but it is used because, on common saws, the scale will not strike properly without it. Tallow gives body to the liquid and causes it to extract the heat quickly from the steel (and the hardening is solely due to the rapid extraction of the heat). In addition to this, the saws hardened in oil and tallow show a very fine grain if fractured, and are tough. The effect of pitch is much the same as that of resin. In place of tallow, bees-wax is sometimes used, giving an excellent result. A very little spirits of turpentine mixed with the oil every time it is used (that is, for every batch) is an excellent ingredient to cause the scale to strike, but being very inflammable, it is somewhat dangerous. If none of these ingredients are used, and the scale does not strike, it acts as a fine separating lining, preventing the contact between the metal and the liquid, and hence retarding the cooling, and therefore the hardening.

Let us suppose some thin saws of the finest grade of steel are to be tempered. The liquid would be about half a barrel of tallow to a barrel of whale-oil (which will harden as hard as glass). After the temperature of the saw is reduced to that of the bath, it is removed, the adhering oil is removed, and the surface dried by an application of sawdust, and the tempering process may be proceeded with.

There are three methods of drawing the temper. One is with the saw lying in the open furnace; a second, an English plan, is with the saw stretched in a frame, so as to prevent its warping, and in fact, to cause the tempering to aid in straightening the saw; and the third is to temper between flat dies.

In the first, the temper is determined by the appearance of the saw in the furnace. The saw absorbs some of the liquid in which it was quenched to harden it; and as it is reheated to temper it, this oil passes off as a cloud, or rather as a breath passes off the surface of a window-pane. This action takes place first on the lower surface of the saw, nearest to the furnace bottom, the oil exuding in a mist-like form. The length of time the saw must remain in the furnace after the cloud has passed off is determined by the thickness of the saw and the heat of the furnace, the operator being guided entirely by experience; but when the saw is taken from the furnace, it will have a very dark-red glimmer, and must be laid flat and allowed to cool off in the air, for if again dipped it would be too hard. When cool, the saw thus tempered will be of a sky-blue color, and will spring from point to butt without bend or break. This process requires skilful management and good judgment, but will give most excellent results. The main objection to it is, that it is expensive, since it gives no aid to the straightening processes.