Armor plate is engineered differently from hull steel

What distinguishes naval armor plate from an equally thick structural steel plate?

Armor production uses controlled alloys, heat treatment, and stricter testing; in face-hardened plate, a hard front and more flexible back help resist penetration and reduce fracture and spall.

Armor plate receives more specialized material treatment and quality testing than ordinary structural steel. Nickel, chromium, and molybdenum are discussed alongside quenching and tempering. Face hardening creates a hard front and a more flexible back, combining resistance to penetration with a reduced tendency to fracture and spall. The reliability advantage is qualified: armor plate tends to be more dependable, while a particular structural-steel plate can exceed its expected quality. On the King George V class, the external armor belt works with the underlying hull and subdivision, so the structure behind the armor also helps contain damage.