Many times pieces are forged which cannot be brought near enough to desired size by hammering when hot; or which must be much stiffer than hot-forging would leave them. In such cases cold-dropping or cold-striking, as it is sometimes called, must be resorted to.
After the pieces are hot-forged to a size slightly larger than finish, and the flash is trimmed away, they are pickled to remove the scale incident to the high forging heats. After pickling, and when they are cold, they are again taken to the drop hammer and given one or more blows, in dies known as cold-striking dies.
The impression in a cold-striking die is made of the desired size of the finish piece, as no allowance need be made for contraction of the metal as is necessary when hot-forging. Since there is much greater strain on a cold-striking die than on one used for hot-forging, it is necessary to harden the former much deeper than the latter to prevent sinking when the die is used. For this reason, the dies should be made from steel having a comparatively high-carbon content.
While a large percentage of dies used for hot-forging are made from open-hearth steel, those used for cold-dropping are made from crucible tool steel. In many forging plants, this class of die is made from alloy steel prepared specially for this purpose; in such cases the heat treatment may be somewhat different from that given similar dies made from crucible tool steel. As the treatment varies for steels of different makes, it is necessary to follow the instructions furnished with the steel.