Eglin steel (ES-1) is a high-strength, high-performance, low-alloy, low-cost steel, developed for a new generation of bunker buster type bombs, e.g. the Massive Ordnance Penetrator and the improved version of the GBU-28 bomb known as EGBU-28. It was developed by the US Air Force and the Ellwood National Forge Company.
The Air Force sought a low-cost replacement for strong and tough but expensive superalloy steels such as AF-1410, Aermet-100, HY-180, and HP9-4-20/30. A high-performance casing material is required so the weapon survives the high impact speeds required for deep penetration. The material has a wide range of other applications, from missile parts and tank bodies to machine parts.
The material can be less expensive because it can be ladle-refined. It does not require vacuum processing. Unlike some other high-performance alloys, Eglin steel can be welded easily, broadening the range of its application. Also, it uses roughly half as much nickel as other superalloys, substituting silicon to help with toughness and particles of vanadium carbide and tungsten carbide for additional hardness and high-temperature strength. The material also involves chromium, tungsten, and low to medium amounts of carbon, which contribute to the material's strength and hardness.
Properties At room temperature, ES-1's yield (tensile strength before deformation) is , and its ultimate strength (breaking point) is . At , its yield is , and its ultimate strength is . Rockwell C hardness is 45.6 (448 HV10). For toughness, the Charpy notch impact is at room temperature, and at .U.S. Patent No. 7,537,727 B2, Accessed December 17, 2022, Table 3: 'Mechanical Properties Table for Eglin Steel Test Series'
ES-1 is a balance of cost, tensile strength, high temperature tensile strength and toughness. By varying the heat treatment to include water or liquid nitrogen quenching, or omitting the normalization heat-treat to permit work hardening, properties can be improved.U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 7, line 5. ES-5, with an economical air and water quench,U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 6, line 65 provides of high-rate yield strength, and high-rate ultimate strength.U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 5, line 1 Low-rate yield strength is , and low-rate ultimate strength is .
By comparison, ordinary structural steel has a yield strength of , and 4150 "ordnance" steel (used in high-quality military gun barrels) has a yield strength of .
Details The material composition by weight is:U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 3, line 5 Iron (84.463–90%) Carbon (0.16–0.35%) Manganese (0.85%) Silicon (max. 1.25%), stabilizes the austenite phase, enhances toughness Chromium (max. 1.50–3.25%), increases strength and hardenability Molybdenum (max. 0.55%), increases hardenability Nickel (5.00%), increases toughness Tungsten (0.70–3.25%), increases strength and wear resistance Vanadium (0.05–0.3%), increases toughness Copper (0.50%) Phosphorus (impurity, max. 0.015%) Sulfur (impurity, max. 0.012%) Calcium (max. 0.02%), sulfur control agent Nitrogen (impurity, max. 0.14%) Aluminium (max. 0.05%)
The material has an unusually wide range of production methods for a superalloy: electric arc, ladle refined with vacuum treatment; vacuum induction melting; vacuum arc remelting, and even electro slag remelting. Vacuum treatments are recommended for best strength and premium uses.U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 3, line 45.
The material has to undergo heat treatment involving normalization, quenching and tempering to develop the required austenitic microstructure, with subsequent tempering. Test plates were . First they were normalized. They were charged in a furnace at , heated at per hour to , held at for an hour per inch of section size, and then air-cooled to room temperature. Next the samples were austenized by repeating the process up to , and held for an hour per inch of section size, then oil quenched to below . Finally, they were tempered in an oven that started below , increased at per hour per inch of section size, and allowed to air-cool to room temperature.U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 4, line 35
Credit The patent credits Morris Dilmore and James Ruhlman as inventors.
See also USAF-96 – A later material developed for the same purposes, also at Eglin.
References
External links AFRL Horizons - Eglin steel Globalsecurity.org - EGBU-28 Ellwood National Forge Company