Material
Steel Fe
The cheapest way to buy strength and stiffness, and the only material that gets meaningfully harder as you spend more on heat treatment rather than on alloy content. Heavy, and it rusts.
Overview
Steel remains the default for anything where weight is not the binding constraint: ship hulls, gun barrels, armor, landing gear, tooling and the overwhelming majority of ground vehicle structure. Its advantages are cost, a modulus three times aluminum's, a genuine fatigue endurance limit, and a heat treatment response that lets one alloy cover a strength range no other material family matches. In defense the interesting grades are not commodity steel at all but the armor specifications, the ultra high strength landing gear and ordnance alloys, and the HY series hull steels developed for submarines.
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Defense applications
Armor
Rolled homogeneous armor per MIL-DTL-12560 is the reference against which every other armor material is measured on a mass efficiency basis. High hardness armor per MIL-DTL-46100 trades toughness for better performance against small arms.
Combat vehicle hulls, Turret structure, Appliqué panels, Ballistic test plate
Naval hull structure
The HY series high yield steels were developed specifically for submarine pressure hulls, where the required combination of yield strength, toughness at low temperature and weldability is not available off the shelf.
Submarine pressure hulls, Surface combatant hulls, Flight decks, Shock-qualified foundations
Landing gear and ultra high strength structure
300M and similar ultra high strength alloy steels carry landing gear loads that aluminum cannot and titanium reaches only in expensive beta alloys. The cost is hydrogen embrittlement sensitivity and a cadmium plating requirement that programs increasingly want to eliminate.
Landing gear struts, Arresting hooks, Catapult components, Highly loaded fittings
Gun barrels and ordnance
Gun steels must hold strength at elevated temperature through repeated firing cycles while resisting erosion and fatigue at the bore, a combination that keeps the qualified supplier list very short.
Cannon and howitzer barrels, Breech assemblies, Projectile bodies, Mortar tubes
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| 4340 | Low alloy, nickel-chromium-molybdenum | The reference high strength alloy steel. Deep hardening, well characterized and available everywhere, which makes it the default when a drawing needs strength without a specific ballistic or naval requirement. | Bar and Rod, Plate, Forgings, Billet | |
| 300M | Ultra high strength low alloy | A silicon-modified 4340 developed for landing gear. Among the highest strength steels in routine production, at the cost of severe hydrogen embrittlement sensitivity. | Bar and Rod, Forgings, Billet | |
| Rolled Homogeneous Armor (RHA) | Armor plate | The baseline armor steel defined by MIL-DTL-12560, supplied heat treated to a hardness range set by thickness. The reference standard for mass efficiency comparisons across all armor materials. | Plate | |
| High Hardness Armor | Armor plate | Higher hardness than RHA per MIL-DTL-46100, giving better performance against small arms and fragments at the cost of toughness and weldability. | Plate | |
| HY-80 and HY-100 | High yield naval structural | Quenched and tempered naval steels developed for submarine pressure hulls, combining high yield strength with low temperature toughness and controlled weldability. | Plate, Structural Shapes, Forgings | |
| Maraging 250 and 300 | Maraging, iron-nickel | Ultra high strength achieved by precipitation aging rather than carbon hardening, giving good toughness at very high strength and almost no distortion during heat treatment. | Bar and Rod, Plate, Forgings |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 7.85 | g/cm3 | 0.284 lb/in3; roughly 1.7 times titanium and 2.9 times aluminum |
| Melting point | approximately 1425 to 1540 | degrees C | Varies with carbon and alloy content |
| Elastic modulus | 200 | GPa | Roughly double titanium and triple aluminum; the reason steel wins stiffness-driven designs |
| Tensile strength, 4340 quenched and tempered | 1080 to 1280 | MPa | 157 to 186 ksi depending on tempering temperature |
| Tensile strength, 300M | 1930 typical | MPa | 280 ksi; among the highest strength steels in production use |
| Yield strength, HY-80 | 550 minimum | MPa | 80 ksi, which is where the designation comes from |
| Thermal conductivity | 45 to 50 | W/m-K | Carbon steel; alloy and stainless grades run lower |
| Coefficient of thermal expansion | 11.7 | micrometre/m-K | |
| Fatigue behavior | True endurance limit | Below roughly half the tensile strength, ferrous alloys survive indefinitely. Aluminum does not. | |
| Corrosion behavior | Rusts freely | Unalloyed and low alloy steel requires coating, plating or paint in essentially all service |
Advantages
- Lowest cost per unit strength of any structural metal
- Elastic modulus roughly triple that of aluminum, which wins stiffness-driven designs
- A true fatigue endurance limit, unlike aluminum and titanium
- Enormous heat treatment range, so one alloy covers strengths from soft to ultra high
- Weldable with well established, widely qualified procedures
- Deep, geographically distributed domestic production base
Limitations
- Roughly 1.7 times the density of titanium and nearly three times aluminum
- Corrodes readily and needs coating or plating in almost all service
- Ultra high strength grades are hydrogen embrittlement sensitive, which constrains plating and cleaning
- Cadmium plating on landing gear steels is an environmental and worker exposure problem programs are trying to eliminate
- Toughness falls with strength, so the strongest grades are the least damage tolerant
- Magnetic, which rules it out of mine countermeasures and some sensor structure
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| MIL-DTL-12560 | U.S. Department of Defense | Armor plate, steel, wrought, homogeneous. The RHA standard, currently at revision K |
| MIL-DTL-46100 | U.S. Department of Defense | Armor plate, steel, wrought, high hardness |
| MIL-DTL-32332 | U.S. Department of Defense | Armor plate, steel, wrought, ultra high hardness |
| MIL-S-16216 | U.S. Department of Defense | HY-80 and HY-100 steel plate for naval construction |
| AMS 6415 | SAE International | 4340 alloy steel bar, forgings and tubing |
| AMS 6417 | SAE International | 300M ultra high strength steel bar and forgings |
| AMS 2759 | SAE International | Heat treatment of steel parts, general requirements |
| DFARS 252.225-7009 | U.S. Department of Defense | Specialty metals restriction. Applies to alloy steels above defined alloy content thresholds |