When hardening drop-forging dies, it is necessary to employ some form of heating furnace that will insure heats of the proper temperature-in other words, a furnace that can be easily and quickly regulated. The die should be heated rapidly, yet not faster than is consistent with uniform heating, or the corners and light sections will be overheated and weakened.
If large pieces of steel are placed in a furnace and allowed to remain exposed to the direct heat and to any air that may be in the furnace, their surfaces are likely to become decarbonized. As the faces and walls of the impressions of forging dies must be hardened, it is desirable to protect them. This is sometimes done by placing a quantity of granulated charcoal in the furnace on the hearth, and laying the face of the die on this. A more satisfactory method consists in placing one or two inches of granulated charred leather to the bottom of a shallow hardening box, laying the face of the die on the leather, then filling the box with leather, as shown in Fig, 410. The die may then remain in the furnace until it is uniformly heated throughout. To prevent unequal heating in the corners at base of the tang, the corners are filled with fire clay, as shown at a. The form of bath depends somewhat, of course, on the character of the pieces to be hardened. One form that is satisfactory for most work of this kind, has the die resting upon the supporting wires, Fig. 411. The overflow pipe should be telescoped, thus enabling the operator to regulate the depth of water in the tank. To prevent the tang from becoming distorted, it is advisable to quench this portion first; this is accomplished by placing the die, tang down, on the wires, and allowing the stream of water from the supply pipe to play against the tang. The die should be left in this position until the tang is cooled below a red, when the die should be turned to bring the face down, and the supply stream allowed to play against this portion until it is hardened.
Fig. 409. Die with Faces Cut Away to Provide for Flashes.
To prevent the tang from softening before the face becomes hard, turn water, by means of a dipper, on to the tang until the red has disappeared from the face; then cease pouring on to the tang and allow the heat to work from the center of the block up through the tang, which will in all probability be reheated to a low red.
Fig. 410. Hardening Box with Die in Charred.
Fig. 411. Farm of Hardening Bath with Die on.
After the block has cooled, it should be placed over a fire and heated to remove hardening strains. While heating, the surface may be brightened and the heat continued until the temper is drawn the desired amount.
At times it is necessary to harden a die having slender projections or some weak portion which is likely to crack during the process. Cracking results from the unequal contraction of the various parts, and can be avoided by rubbing soap on the projection, especially where it joins the die; or, by means of an oil can, a little lard or sperm oil may be applied to these parts. This should be done after the die is red hot, and just before it is placed in the bath. If the tang is quenched first, the oil may be applied just before the die is turned to harden the face.
Fig. 412. Hobbing Drop-Forging Dies. A- Piece to bo forged; B - Hob.
It is the custom in some shops to produce the impression in the face of forging dies with a male die, or hob, as it is called. A hob is made of the same general shape as one-half of the piece to be forged, but exactly opposite the shape of the impression desired in one die. Another hob is made the shape of which is the opposite of the impression desired in the other blank.
Fig. 412 shows a piece to be forged, A, and a hob, B. The hob has a shank that fits a holder in the ram of the drop hammer. The hobs are hardened before using, and after hardening, one of them is placed in the holder in the ram; the die block is heated to a good forging heat and securely fastened to the anvil of the drop, and the hob is driven into the face of the die. This operation is repeated until the impression is considerably deeper than that desired when finished. This is necessary as the top surface of the die must be cut away to remove the rounded portion at the top of the impression, occasioned by the stock drawing away in the hobbing.
After driving the hob to the required depth, the block is reheated and annealed. When the block has cooled, the scale on the surface of the walls of the impression is removed by filling the impression with a solution of sulphuric acid and water - one part acid, and two parts water. After the scale has been removed, the acid should be turned out and the surface well washed, first with hot water, then with a strong solution of potash, and then, once more, with water. The surface, when dry, should be oiled to prevent rusting.
The walls of the impression may now be finished smooth with scrapers and files. After the surfaces are finished, it is the custom in some shops to cold-drop the impression, that is, to place the die in the drop hammer again and drop the hob into the impression while the steel is cold. This custom, however, is not generally observed. After finishing, the dies are hardened in the usual manner.
A saving in labor may be effected, if, when the die is heated for annealing, the impression is filled with fire clay mixed with water to the consistency of dough. The fire clay prevents the air coming in contact with the steel, and does away, to a great extent, with oxidation.