Rolled homogeneous armour

Rolled homogeneous armor (RHA) is a type of armor made of a single steel composition, as opposed to layered or cemented armor. Its first common application was in tanks. It fell out of use after World War II as new anti-tank rounds were developed, against which it was ineffective. Today, the term is primarily used to measure the equivalent thickness of armor or the penetration ability of armor-piecing missiles.

Armored 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, removing imperfections which would reduce the strength of the steel. Rolling also elongates the grain structure in the steel to form long lines, which spreads stress loaded onto the steel throughout the metal, avoiding a concentration of stress in one area.

RHA is called homogeneous armor because its structure and composition is 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.

From the invention of tanks through to the Second World War, tank armor increased in thickness to resist the increasing size and power of anti-tank guns. A tank with sufficient armor 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 armor), 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 armor, some of which do not include steel or even any metals.

RHA was in common use as primary armor 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.

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effectiveness against new weapons (mainly shaped charges and improved kinetic energy penetrators), RHA has largely been superseded by composite armor, which incorporates air spaces and materials such as ceramics or plastics in addition to steel, and explosive reactive armor.

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