This method gives excellent results with pieces that cannot be hardened by the methods ordinarily employed without risk of springing or cracking. The article is packed in an iron box, with some carbonaceous material, and subjected to the action of heat, to allow it to absorb enough carbon to harden in oil. While this method is not generally used, it is very valuable when hardening such pieces as milling-machine cutters, blanking dies for punching presses, gages, and taps, where it is necessary that the diameter and pitch should not be altered. The carbonaceous material is charred leather, which should be ground or pounded very fine (usually about one-half the size of a pea). An iron box somewhat larger each way than the piece to be hardened, should be selected. A layer of the packing material one inch deep should be placed in the bottom of the box, and the piece of steel laid on this; the box should then be filled with the packing material and tamped down. The space between the cover and the box should be filled with fire clay, which seals it so that the gases in the box cannot escape and the direct heat of the fire cannot get into the box.
It is much more economical to pack a number of pieces at a time, as several-may be hardened at the cost of one, and at a saving in packing material. The pieces should be wired with ordinary iron binding wire of a size sufficient to sustain the weight when the wire is red hot, one end of the wire projecting over the outside edge of the box and covered with the luting of fire clay. Several holes should be drilled near the center of the cover for test wires, as in annealing. The wires should extend to the bottom of the box which may now be heated sufficiently to charge the pieces with carbon. As steel does not commence to absorb carbon until it is red hot, the time is determined by means of the test wires as described under "Annealing". For ordinary tools 1/2 inch in diameter and under, run from 1 hour to 1 1/2 hours after they are red hot; pieces from 1/2 inch to 1 inch in diameter, 2 to 2 1/2 hours; pieces from 2 to 3 inches in diameter, 2 1/2 to 4 hours. This schedule must be varied according to the nature of the work.
After remaining in the furnace the desired length of time, the box should be taken out, the cover removed, and the piece taken out by means of the wire attached to it. It should then be immersed in a bath of raw linseed oil or cottonseed oil, and worked around until the red has disappeared. Finally it is lowered to the bottom of the bath and allowed to remain until cold.
When a piece of steel 1 inch in diameter, or larger, is hardened, it should be reheated over the fire immediately on being taken out of the bath; this is to avoid cracking from the strains caused by molecular changes which take place after the outside surface is hardened and unable to yield to the internal strains. Reheating the surface to a temperature of about 212 degrees will accomplish the desired result without materially softening the steel.
Although charred leather is the carbonaceous material usually employed in pack hardening, it is not advocated for steel containing more than \\ per cent carbon. If steel contains a larger percentage than this, the packing material should be charred hoofs, or charred hoofs and horns. The charred leather may be used over and over, by adding fresh material as the old wastes away. It is advisable to place the fresh material in contact with the steel.