Rolled homogeneous armour
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Rolled homogeneous armour (RHA) is a type of steel which is used to armour vehicles.
Composition
Steel for armour plate must be strong, hard, and tough (does not shatter when struck with a fast, hard blow). Steel with these characteristics is produced by processing cast steel billets of appropriate size and then rolling them into plates of required thickness. Rolling and forging (hammering the steel when it is red hot) irons out the grain structure in 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 the steel is placed under when loaded to flow throughout the metal, and not be concentrated in 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 in order 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 what 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 negate the strengths of the steel. The strength, toughness and hardness of RHA was no longer protection against this threat.
Present use of RHA
Since World War II, other forms of armour, incorporating air spaces and materials such as ceramics or depleted uranium in addition to steel, have been developed. Due to this reduction in effectiveness against new threats, RHA itself no longer has the dominant, universal role in armour that it once enjoyed.
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.
Presently, most armoured vehicles have their basic structure formed from RHA in order to lend general strength and toughness against general threats. Over this RHA is added the new types of armour.
Specifications
For current United States Army use, RHA steel is produced to Military standard MIL-A 12560 by several manufacturers. The newest standard used is MIL-A-46177. [1] It is very similar to but not identical to standard high strength steel alloy 4340 in the AISI steel grades, though mechanical properties are very similar to that alloy. [2]
See also
References
- ^ http://www.intlsteel.com/PDFs/armor.pdf, accessed Sept 21, 2006, Archive.org link change on May 10, 2009
- ^ http://www.eng-tips.com/viewthread.cfm?qid=159941&page=6, accessed Sept 21, 2006