Rolled homogeneous armour

Armoured steel must be hard, yet resistant 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, changing the crystalline structure of the steel and normalizing it. RHA is homogeneous because its structure and composition are uniform throughout its thickness. The opposite of homogeneous steel plate is cemented or face-hardened steel plate, where the face of the steel is composed differently from 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 anti-tank guns was measured by the thickness of RHA they could 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 any other 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 came into use instead of heavy high-velocity projectiles. ==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effectiveness against new weapons (mainly shaped charges and improved kinetic energy penetrators), RHA has largely been superseded by composite armour, which incorporates air spaces and materials such as ceramics or plastics in addition to steel, and explosive reactive armour.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 rolled homogeneous armour equivalency (RHAe) 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 structures formed from RHA to lend general strength and toughness. ==Specifications==

The US army has several standards for rolled homogeneous armour: • Current use is based on military standard MIL-DTL-12560 by several manufacturers. The typical Brinell Hardness range of MIL-A 12560 is 302-400 • The MIL-12560K standard actually specifies four classes of hardnesses. The softest class I rates as 260-310. The toughest Class 4 rates as 420-470. (K is the revision number of the standard.) • MIL-DTL-46177 is an older, now-replaced standard for a high hardness steel RHA. It specifies a harder steel that is nearly identical to AR500 in terms of tensile and yield strength. The Brinell hardness of AR500 is in the 477-534 range. MIL-DTL-46100E specifies a steel of identical hardness. ==See also==