An armor plate should be hard enough on the outer surface to prevent penetration by a shell and tough enough under the hard surface to resist breaking or cracking. Armor plate is commonly made of nickel-steel with which is alloyed chromium, tungsten, or vanadium. Nickel increases toughness and the other metal increases hardness.

Armor steel is made from high-grade ore by either the acid or the basic open-hearth process. The reduction of phosphorus in the steel is highly essential, hence the basic process is employed in America. The ingot is cast in a very large cast-iron mould lined inside with a layer of hard-baked loam. It is forged to shape under a powerful hydraulic forging press, is planed to dimensions, and is then subjected to the hardening process.

Fig. 268 shows the diagram of two plates prepared for hardening. A heavy steel car is covered with layers of fire brick to protect it from intense heat, and on this is built the structure shown. The flues 1, 2, 3, 4 allow the passage of hot gases under the lower plate. Between the two plates is a layer of powdered charcoal. The car supporting this structure is rolled into a regenerative furnace and is walled in. It is then heated up gradually and kept above a red heat for several days. The carbon penetrates the steel, as in the cementation process, and the degree of penetration depends upon the degree of heat and upon the length of time the heat is maintained. It is estimated that the process occupies five days for heating and cooling and one day for each inch of penetration of the carbon. As carbon is absorbed, the angle iron frame gives down under the weight of the upper plate and allows this plate to remain it contact with the carbon.

The plates are hardened after removing and cleaning. This is done by reheating and spraying the carburized surfaces with jets of cold water. This hardness penetrates only about 2 inches or less, but the surface is so hard that it cannot be cut or drilled.

The processes of armor making are guarded with much secrecy by the few manufacturers who are engaged in this work.