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Eglin steel

High-performance steel for bombs

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 193900 psi, and its ultimate strength (breaking point) is 246700 psi. At 900 F, its yield is 191400 psi, and its ultimate strength is 215700 psi. Rockwell C hardness is 45.6 (448 HV10). For toughness, the Charpy notch impact is 56.2 ftlbf at room temperature, and 42.7 ftlbf at -40 F.

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. ES-5, with an economical air and water quench, provides 244800 psi of high-rate yield strength, and 291900 psi high-rate ultimate strength. Low-rate yield strength is 216000 psi, and low-rate ultimate strength is 270200 psi.

By comparison, ordinary structural steel has a yield strength of 36000 psi, and 4150 "ordnance" steel (used in high-quality military gun barrels) has a yield strength of 75000 psi.

Details

The material composition by weight is:

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.

The material has to undergo heat treatment involving normalization, quenching and tempering to develop the required austenitic microstructure, with subsequent tempering. Test plates were 1 in. First they were normalized. They were charged in a furnace at 500 F, heated at 125 F-change per hour to 1625-1725 F, held at 1750 F 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 1700 F, and held for an hour per inch of section size, then oil quenched to below 125 F. Finally, they were tempered in an oven that started below 500 F, increased at 100 F-change per hour per inch of section size, and allowed to air-cool to room temperature.

Credit

The patent credits Morris Dilmore and James Ruhlman as inventors.

References

U.S. Patent No. 7,537,727 B2, Accessed December 17, 2022, Table 3: 'Mechanical Properties Table for Eglin Steel Test Series'
U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 7, line 5.
U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 6, line 65
U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 5, line 1
U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 3, line 5
U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 3, line 45.
U.S. Patent No. 7,537,727 B2, Accessed July 16, 2009, column 4, line 35

Info: Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

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