Rolled homogeneous armour

Rolled homogeneous armour (RHA) is a type of steel which is used to armour vehicles.

Composition
Armoured steel must be hard yet impervious to shock in order to resist high velocity metal projectiles. Steel with these characteristics is produced by processing cast steel billets of appropriate size and then rolling them into plates of required thickness. Hot rolling homogenizes the grain structure of the steel, removing imperfections which would reduce the strength of the steel. Rolling also elongates the grain structure in the steel to form long lines, which enable the stress under which the steel is placed when loaded to flow throughout the metal, and not be concentrated into one area.

RHA is called homogeneous armour because its structure and composition is uniform throughout its section. The opposite of homogeneous steel plate is face-hardened steel plate, where the face of the steel is composed differently to the substrate. The face of the steel, which starts as an RHA plate, is hardened by a heat-treatment process.

History
From the invention of tanks through to the Second World War, tank armour increased in thickness to resist the increasing size and power of anti-tank guns. A tank with sufficient armour could resist the largest anti-tank guns then in use.

RHA was commonly used during this period (combined with other plate alloys and cast steel armour), and the power of an anti-tank gun was measured by the thickness of RHA it would penetrate. This standard test has remained in use despite the modern usage of many other types of armour, some of which do not include steel or even any metals.

RHA was in common use as primary armour until after World War II, during which a new generation of anti-tank rounds using shaped charges instead of heavy high-velocity projectiles came into use. RHA was ineffective against these and fell out of use.

Current use
Since World War II, because of a reduction in effectiveness against new weapons (mainly shaped charges and improved kinetic energy penetrators), RHA has been superseded by composite armour, which incorporates air spaces and materials such as ceramics or plastics in addition to steel, and explosive reactive armour.

For the testing and calibration of anti-tank guns, the term RHAe (Rolled Homogeneous Armour equivalency) is used when giving an estimate of either the penetrative capability of a projectile or the protective capability of a type of armour which may or may not be steel. Because of variations in armour shape, quality, material, and case-by-case performance, the usefulness of RHAe in comparing different armour is only approximate.

Currently, most armoured vehicles have their basic structure formed from RHA to lend general strength and toughness.

Specifications
For current United States Army use, RHA is produced to military standard MIL-DTL-12560 by several manufacturers. Another standard is MIL-DTL-46177; however, this standard has been inactivated, and all new designs should use MIL-DTL-12560. MIL-DTL-46177 RHA is similar to SAE 4340 steel alloy.