This process is the reverse of malleableizing, i. e., it adds carbon to forgings of wrought iron or mild steel to make them hard for resisting wear. Many articles, such as set screws, bolts and nuts, jaws of wrenches, etc., subject to unusual wear, are much increased in durability by case hardening after they are machined to shape. The process is as follows.

A number of forgings, each of which is surrounded by carbon substances such as ground bone and charred leather, are packed in an iron box. The box lid is luted with clay and the box is heated gradually in a suitable furnace up to a temperature ranging from 1400° to 1700° F. This heat is maintained from 6 to 14 hours. The degree of heat applied depends upon the grade of steel in the forgings, and the duration of the heating depends upon the depth of the hardness required. The object of the process is to increase the amount of combined carbon in the forgings, as in the cementation process of steel making. The articles are removed from their boxes when cool enough to handle and are heated up to critical temperature and hardened by quenching in water or oil.

This process of hardening sometimes warps forgings slightly, a fault which may be in some cases very objectionable.

Fig. 259. - Form for Bending Small Pipes by Hand.