Where large quantities of steel are used the steel manufacturers in many cases request customers to state for what particular purposes the steel is required, their experience teaching them what special grade of their make of steel is most suitable.

To harden steel it is heated to what is termed a “cherry red” and then dipped into water and held there until its temperature is reduced to that of the water.

Tempering steel as the blacksmith practises it consists in modifying, lowering, or tempering the degree of hardness obtained by hardening. The hardening of steel makes it brittle and weak in proportion as it is hardened, but this brittleness and weakness are removed and the steel recovers the strength and toughness due to its soft state in proportion as it is lowered or tempered.

When therefore a tool requires more strength than it possesses when hardened, it is strengthened by tempering it. Tempering proceeds in precise proportion as the temperature of the hardened steel is raised. When the steel is heated to redness the effects of the hardening are entirely removed, and the steel, if allowed to cool slowly, is softened or annealed. To distinguish maximum hardness from any lesser degree the terms to give the steel “all the water,” or to harden it “right out” are employed, both signifying that the steel was heated to at least a clear red, was cooled off in the water before being removed from the same, and was not subsequently tempered or modified in its hardness. If a piece of steel has its surface bright and is slowly heated, that surface will assume various colors, beginning with a pale straw color (which begins when the steel is heated to about 430°) and proceeds as in the following table:—

Fahr.
Very pale yellow430°
Straw yellow460
Brown yellow500
Light purple530
Dark purple550
Clear blue570
Pale blue610
Blue tinged with green630

It happens that between the degree of hardness of hardened steel and the temper due to reheating it up to about 600° Fahr. lie all the degrees of hardness which experience has taught us are necessary for all steel-cutting tools. Hence we may use the appearance of colors as equivalent to a thermometer, and this is called color-tempering. The presence of these colors or of any one of the tints of color, however, is no guarantee that the steel has been tempered or possesses any degree of hardness above the normal condition, because they appear upon steel that is soft or has not been hardened. To obtain exact results by color tempering, therefore, the steel must first be thoroughly hardened, and this is known in practice by the whiteness of the hardened surface.

Any number of pieces hardened so as to have a white surface may be tempered to an equal degree of color, or heated to an equal thermometrical temperature, with the assurance that they will possess a degree of hardness sufficiently uniform for all practical purposes; but if their hardened surfaces have dark patches, tempering to an equal tint of color is no guide as to their degree of temper. Successful tempering, therefore, must be preceded by proper hardening.

The muffle should therefore bear such a proportion in size that when heated to a blood red, and taken from the fire, its temperature will be reduced to nearly that of the steel when it has acquired its proper degree of temper.

The shape of the bore of the muffle should always conform to that of the article tempered; for round work, a round muffle; for square work, a square one; and so on. The muffle should be shorter than the work, so that the tempering of either end of the work may be retarded, if it is proceeding too fast, by allowing that end to protrude through the muffle.

Color tempering, it will be observed, gives us no guide or idea of any of the degrees of temper which occur while the hardened steel is being heated up to about 430° Fahr.; and thus it leaves us in the dark as to all the ranges of hardness existing in steel thoroughly hardened and tempered to any degree less than that due to about 430° degrees of reheating. How wide this range may be can be appreciated when it is remembered that in the color test there are only 200° of heat between the hardness known as straw color, which is hard enough for almost all cutting purposes, and blue, tinged with green, which is almost normal softness.