Rolled homogeneous armour (RHA) is a type of steel which is used to armour vehicles.
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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.
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 RHA could resist the largest anti-tank guns then in use.
RHA was in universal use during this period and the measure of the power of an anti-tank gun was the thickness of RHA it would penetrate. This measure of tank gun effectiveness has remained in use because it is a useful universal measure for the comparison of increased anti-tank gun effectiveness. Different types of armour, some of which do not use steel or even metals, have come into use but depth of penetration of RHA is still used to compare anti-tank weapon effectiveness.
RHA was in common use until after World War II when other types of armour were being developed. A new generation of anti-tank rounds had come into use which did not use a heavy, tough, high-velocity projectile, to defeat steel armour but used an explosive charge called a shaped charge to overcome the strength of the steel. The strength, toughness and hardness of RHA was no longer protection against this threat.
Since World War II, because of a reduction in effectiveness against new attack methods (mainly shaped charges and improved kinetic energy penetrators), RHA has been superseded by composite armour, which incorporates air spaces and materials such as ceramics or plastics in addition to steel, and explosive reactive armour.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 purposes of 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 against general threats. Over this RHA is added the new types of armour.
For current United States Army use, RHA is produced to military standard MIL-DTL-12560[1] by several manufacturers. Another standard is MIL-DTL-46177[2]; 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[3].