Rolled Homogeneous Armor (RHA)

Homogeneous wrought steel armor was introduced on the first tanks developed in the Great War. Early tanks were of riveted construction and employed low-carbon, medium-alloy steels of special compositions. During World War II, rolled homogeneous steel underwent an important variation due mainly to alloy conservation priorities brought about by the conflict. The resulting armor was based on a low-alloy material with low-carbon content possessing adequate toughness.

Although welding technology was developed for rolled steel plates, the use of cast steel prevailed for tank structures, providing further reductions in cost and increased production capability. The use of cast steel for hulls and turrets continued for the next 25 years, most notably for the M60 tank. During recent decades, due to cost-saving fabrication techniques and more complex vehicle configurations, rolled homogeneous armor (RHA) steel has supplanted cast steel, as demonstrated by the Ml Abrams tank. Based on cost considerations, RHA steel is now, and will continue to be, the principal material employed for heavy combat and recovery vehicles. Although improvements in steel processing and production have evolved since World War It, the material covered by the RHA specification (MIL-A-12560) has not appreciably changed, providing little or no improvement in ballistic protection.

Historically, the intellectual basis for improving steel armor ballistic performance has been to increase the hardness of the steel without causing an increased tendency to fail by brittle fracture. During the 1970s and 1980s, developmental efforts were undertaken that proved unsuccessful in attaining all the ballistic and structural requirements. The important lessons learned included (1) steel armor plates possessing hardnesses in excess of Rockwell C hardness (HRc) 52 could not retain structural integrity when impacted by full-scale-caliber kinetic energy (KE) rounds; (2) the hardened steel must demonstrate a V-notch Charpy (CHV) impact value greater than 20 ft-lb measured at -40° F; and (3) the reduction of steel plate thickness (or reduced weight) is not necessarily achievable from higher performing material. It is also axiomatic that an improved RHA (IRHA) steel must retain excellent weldability and fabricability commensurate to the standard RHA per requirements of MIL-A-12560.

Two conventional RHA relatively low-carbon alloy systems (nickel-chromium-molybdenum [Ni-Cr-Mo] and manganese-molybdenum-boron [Mn-Mo-B]) have been used for the production of RHA steel over the past 40 years. In general, the practice is to utilize the Mn-Mo-B alloy for plate thicknesses 1.5 in and less, and the Ni-Cr-Mo alloy for the thicker gauges. However, both alloys can be found in the RHA inventory across the thickness range ofinterest; namely, 3/4 in through 3 in. Higher hardness levels are achieved by simply optimizing the heat treatment and thereby obtaining the most cost-effective material with improved ballistic performance.

By convention, the common base line against which armors and penetrators are measured is always rolled homogeneous armor (RHA). The performance of armors and armor penetrators is often described in comparative terms: x inches of steel gives the same protection as y feet of wet clay; bullet A perforates a steel helmet at 600 yards, bullet B can do it only out to 50 yards; 3/4 inches of special treated steel gives the same ballistic protection as 1% inches of mild steel. Such relative performance measures are useful for comparing armors (complete armor packages) or armor materials (elements of an armor package) as to their ability or merit to defeat various armor penetrators (eg CE, KE). Conversely, analogous measures can be used to compare the capabilities of various armor penetrators to defeat (perforate) armors. These relative performance measures are derived from the ballistic evaluation of armor packages (armor elements / materials) against some particular penetrator. Therefore, the relative measures will reflect the intrinsic properties of both the armors and the penetrators.