Armor plated steel dates back to the times of the 18 ton ‘land-ship’ or tank built in 1915 during World War I. At the time the World’s first practical tank, ‘Little Willie’, was built from 0.39 inch boiler plate  and little thought was given to safeguard it against anything other than small arms fire.
During World War II, armor plate was produced to a much higher degree of technical superiority due to the bigger ballistic challenges of the time. Frontal armor development was one area that grew rapidly during the 1940s. Armor thickness progressed swiftly to meet the challenge of large-caliber, solid steel projectiles. WW2 also witnessed the launch of the first fusion-welded tank, the Churchill Mk7 and this employed some of the first rolled homogenous armor (RHA) steel.
One of the most standard wrought or rolled armor steel grades in recent use is MIL-DTL-12560K Class 1 RHA with a hardness range of 250-410 HB (Specification, 2013) and DEF STAN 95-24 class 2–5 (specification 2002) with a hardness range of 262-655. These specifications have their roots in WW2 and have not changed significantly  though they have been altered to integrate a new class of wrought armor plate, which is heat-treatable to higher hardness ranges than Class 1 MIL-DTL-12560K and DEF STAN 95-24 class 2 as well as some other minor improvements [1, 2]. Ultra-High Toughness Armor plate fulfills the modified MIL-DTL-12560K class 4 specification and the DEF STAN 95-24 class 4 and 5 specifications.
Rolled Homogenous Armor Steel (RHA)
Although armor plate has progressed in sophistication to include multilayer composite materials, ceramics and other ballistic technologies, the standard structure of an armored fighting vehicle is built from monolithic, armor-grade steel, which will serve, without any metallurgical issues, for about 30 years or more. Structural armor steels such as MIL-DTL-12560K Class 1 – 4 are chosen and developed to function within aggressive environments and are normally medium-carbon grades of superior-quality, realistic hardenability, and are usually represented by the Mn-Mo, Cr-Mo or Ni-Cr-Mo-(V) low-alloy steels . Normally, RHA grades are low-alloy, quenched and tempered, martensitic steels, which are based upon a carbon content of 0.25% with add-ons of Ni, Cr, Mo and/or Mn for better grain structure and hardenability.
The additional need for this structural steel to be weldable hinders the total alloy content to a carbon equivalent (CE) of less than 1, normally about 0.8. Most hull constructions are welded fabrications of RHA plate. It should be kept in mind that boosting the hardness of armor steels will typically decrease their toughness. Hardness limits are thus fixed for specific steel armor classes to control toughness during production and minimize the risk of brittle failures or shattering for particular steel compositions and applications.
MIL-DTL-12560K is available in a variety of hardness and thickness ranges e.g., MIL-DTL-12560K: Class 1, 6.35 to 150.8 mm, 340 – 390 and MIL-DTL-12560K: Class 4, 420 – 470. Although ballistic performance usually increases with hardness, this relies on the threat projectile and armor toughness and thickness. The ideal armor choice will rest on whether the armor is to be applied as appliqué armor or a stand-alone structural. The Armor Designers need to verify they have chosen the armor class that offers the ideal combination of ballistic performance and structural properties suitable for the application and environment.
Armor for Modern Warfare
In real-world terms, armor properties such as hardness/strength/toughness are chosen to deliver enhanced performance against a variety of battlefield dangers, including armor piercing and fragmentation devices. Such protection has to be provided at realistic densities for a reasonable price. Rolled armor steel such as DEF STAN 95-24 and MIL-DTL 1260K has continued to be the major armor material for a number of ballistic applications .
The ballistic performance of armor steel is a complicated relationship including hardness and fragmentation of the steel. In studies  using armor piercing (AP) and fragment simulating projectiles (FSPs), projectile shatter was shown to be the regulating mechanism of armor performance. Against softer FSPs (281 HB), where projectile shatter is not applicable, the armor performance was discovered to primarily decrease with increasing plate hardness up to 450 HB because of increased susceptibility to adiabatic shear plugging. Over 450 HB, the performance was seen to plateau. This performance plateau seems to relate to an important threshold for adiabatic shear failure beyond which further increases in plate hardness will not cause a decrease in performance [1, 6].
Masteel’s MIL-DTL-12560K and DEF STAN 95-24 Specification Armor Steel Plates
Masteel UK Ltd provides its own designation of armor steel plate (Protection 400 Armor Steel Plate) that complies with the MIL-DTL-12560K specification in a variety of hardness grades 250-470 and thicknesses from 6.0 mm to 150 mm and with a Brinell Hardness of 400 is ideal for military and civilian based armored vehicles as well as security vaults and doors. Masteel’s protection 400 armor plated steel is manufactured by a direct quench process for strength, hardness and good dimensional accuracy.
Developed clearly for resistance to high explosive and artillery projectiles, the steel boasts exceptional hardness and strength, which provide outstanding anti-ballistic properties. The material has a tensile strength of 1250 MPa and a yield strength of 1000 MPa, providing high-grade protection against deformation and breakage.
Masteel also manufactures Protection 400 armor steel plate in compliance with the UK Ministry of Defense DEF STAN 95-24 class 2 specification, which although technically similar to MIL-DTL-12560K class 1 has a few small differences in the relationship between armor plate thickness and hardness. UK DEF STAN 95-24 class 2 is from a Ministry of Defense standard revised in 2002 for RHA armor plated steels used in UK vehicle armor applications .
This UK class 2 specification hardness ranges from 255 to 341 and thickness ranges from 3 to 160 mm. These figures are slightly different from MIL-DTL-12560K class 1 which ranges from 6 to 150 mm thickness with hardness ranging from 250 to 470. Both of these specifications for armor plate are projected for use in applications where excellent resistance to penetration is incorporated with superior structural properties. These RHAs can be cold-worked as well as amenable to welding.
Armored steel is expected to be hard and resistant to shock to protect against high-velocity metal projectiles. Steel with these characteristics is manufactured by hot rolling cast steel billets of appropriate size and then into plates of the necessary thickness. Hot rolling homogenizes the grain structure of the steel (a good balance between Austenitic and Martensitic), eliminating any deficiencies which would decrease the steel strength and integrity. Rolling also expands the grain structure in the steel to disperse stress loaded onto the steel across the metal, preventing stress concentration in one area.
RHA is referred to as homogeneous armor because its composition and structure is uniform throughout its thickness [7, 8]. On the other hand, homogeneous steel plate can be case-hardened to enhance its mechanical properties. The face of the steel, which begins as RHA plate, can be hardened using a heat-treatment process  to offer different ballistic necessities.
- I Crouch, The Science of Armour Materials, Woodhead Publishing in Materials, 2017
- Gooch, W.A., Showalter, D.D., Burkins, M.S., Thorn, V., Cimpoeru, S.J., Barnett, R., 2007. Ballistic testing of Australian Bisalloy steel for armor applications. In: Paper Presented at the 23rd International Symposium on Ballistics, Tarragona, Spain
- Doig, A., 1979. Comparative anisotropy of quenched and tempered alloy steel plates made by high quality air melting, ESR, VIM &VAR, and VIM &ESR processes. In: Paper Presented at the Sixth International Vacuum Metallurgy Conference on Special Melting, San Diego, CA.
- Ryana,.H.Lib.et al,. The ballistic performance of an ultra-high hardness armour steel: An experimental investigation, International Journal of Impact Engineering, Volume 94, August 2016, Pages 60-73,
- L. Woodward, The Penetration of Metal Targets by Conical Projectiles, Int. J. Mech.Sci., Vol. 20, 1978, pp. 349-359.
- Tabor, The Hardness of Metals, Clarendon Press, Oxford, 1951, p. 108.
- MIL-DTL-12560K (MR), 7 December 2013
- Ministry of Defence, Defence Standard 95-24, Issue 2 Publication Date 2 September 2002, Armour Plate, Steel (3-160 mm)
This information has been sourced, reviewed and adapted from materials provided by Masteel UK Ltd.
For more information on this source, please visit Masteel UK Ltd.