THE EXPANSION

Of hardened steel is frequently the cause of great inconvenience to the workman. Steel welded to iron invariably draws the edge around, if it should be on but one side of the edge. It is also liable to become brittle when laid upon iron. These difficulties may be obviated by making the steel side convex, or taking as little iron as possible. Files are never straight if made of natural steel, because that is in most cases but a mixture of iron and steel. In all cases where exactness after hardening is essential, the best kind of cast-steel is to be used; neither blistered nor shear-steel can be trusted. The better the steel, the greater is its expansion in hardening. This expansion is in some measure reduced in tempering the steel, but not to the size in which it was received from the tilt. The expansion is greater where the steel has been heated to a high degree before refrigeration, which may in some measure account for the brittleness of the metal when overheated. It is an important matter, in working steel, to keep it moving in the fire; otherwise, on that side where the blast acts, it will lose its carbon, and will not shrink so much in hardening as those portions which have been

protected. A good method of protecting steel is to keep a film of calcined borax, or any other flux, around it while in the fire, or to cover it with a paste, as is done in hardening files and mint-stamps.

REFRIGERATING FLUIDS.

In hardening steel, the hardness is derived, not so much from the degree of heat to which the metal is subjected, as the degree of cold of the cooling fluid, and the manner in which the cooling is performed. Steel must be heated to a certain degree, to assume its greatest hardness; if heated below that point, it will not become hard, no matter what kind of cooling fluid we employ, or in what manner we refrigerate. If the proper degree of heat be obtained, it is in our power to make the steel more or less hard, by choosing more or less cold water, or other fluid, for chilling it. Many plans of refrigeration, and many refrigerating fluids, have been advised for hardening; but the most of them are of no practical utility. Pure well-water, taken fresh from the well, is the best element to cool in; and it should be renewed at each operation. Well-water is everywhere, and at almost all seasons, of the same temperature; and the smith should use this for hardening the steel, to

ensure success. Hard well or spring-water is preferable to that of a softer quality, and should, if possible, be obtained. Steel treated in this way assumes its greatest degree of hardness, and may afterwards be tempered to any extent.

The manner of cooling is of some importance. If hot steel is held quietly in cold water, it will not become as hard as may be desirable, because the steam formed on the hot surface will prevent its rapid cooling. A motion backward and forward, or up and down in the water, greatly increases the hardness. For hardening large objects, a current or fall of water is indispensable.

The different degrees of heat required for hardening steel, accordingly as that steel is of good quality, or has been more or less worked, or is welded to iron, or is in large or small pieces, makes it exceedingly difficult, and indeed practically impossible, to employ hardening and tempering fluids at the same time. The surest method is to impart to the steel, in the operation of hardening, the greatest degree of hardness of which it is susceptible, and temper it afterwards.

HARDENING FILES.

This process is one which has been brought to a high degree of perfection, and the experience gained in it has been advantageously applied in other branches of manufacture. A file, after being cut, is dipped in a fluid of a cream-like consistence. This fluid is composed of a saturated solution of common salt in water, thickened by flour or meal of peas or beans. This paste melts into a fluid slag, and surrounds the file, protecting it against the influence of the fire and air. The file is uniformly heated in a common smith's forge, or in a small reverberatory furnace, and plunged vertically (except half-round and fancy files, which have a more or less horizontal inclination) into cold spring-water. Saw-files, and sculptors' files which are of iron, are hardened by using animal-charcoal powder with the flour paste, or using it and salt water only. Coal for this purpose is made by putting leather, tanners' scraps, or horns and hoofs, in a tight iron pot, and exposing the whole to a cherry-red heat. The spongy, black, and shining coal is then to be finely ground for use.

Rubbing a hot file, or any piece of hot steel, with a piece of charred leather, hoof, or horn, is not of

much use; the glassy coating imparted by the salt is requisite to success. After files are hardened, they are brushed over with water and powdered charcoal, by which they become perfectly clear and metallic. After washing them repeatedly in fresh water to extract the salt, they are dipped in lime-water, dried by the fire, and finally, while still warm, placed in a mixture of olive oil and turpentine.

HARDENING OF NEEDLES,

ETC.

These are hardened in quantities of twenty-five pounds, which are heated together, and plunged in cold

water, but so that almost each needle is separated from its fellow. Cutlery, such as knife-blades and similar articles, are held by the tangs, either in pairs or singly, heated to a cherry-red in the common forge, and plunged into cold water up to the tang. Sunk steel dies and mint-stamps are heated to the proper degree, and hardened under a current of fresh cold water, which is made to issue from a basin with great rapidity.