Material
Stainless Steel
Steel with enough chromium to form a self-healing passive oxide film. Buys corrosion resistance and, in the precipitation hardening grades, high strength without titanium's cost.
Overview
Stainless is not one material but four distinct families that happen to share a chromium addition, and they behave so differently that treating them interchangeably causes real failures. Austenitic grades like 304 and 316 are non-magnetic, tough and readily formed but cannot be hardened by heat treatment. Martensitic grades harden like carbon steel and are used for cutlery, valves and bearings. Precipitation hardening grades such as 15-5PH and 17-4PH reach high strength with good corrosion resistance and are the ones that show up most often in defense structure. Duplex grades split the difference and resist chloride stress corrosion cracking that kills austenitics in seawater.
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Defense applications
Airframe and engine hardware
Precipitation hardening grades carry fittings, actuator components and fasteners where strength is needed alongside corrosion resistance and titanium's cost is not justified.
Actuator bodies, Structural fittings, Fasteners, Engine accessory hardware
Naval and seawater service
Duplex and super austenitic grades handle chloride environments where standard 316 pits and cracks. Selection here is unforgiving because chloride stress corrosion cracking is sudden rather than gradual.
Seawater piping, Pump and valve bodies, Heat exchangers, Deck hardware
Ordnance and weapons hardware
Martensitic and precipitation hardening grades are used for barrels, breech components and mechanism parts that need hardness plus corrosion resistance in field conditions.
Barrel liners, Bolt and receiver components, Springs, Firing mechanisms
Alloys and grades
| Designation | Grade | Class | Notes | Typical forms |
|---|---|---|---|---|
| 304 and 304L | Austenitic | The general purpose austenitic grade. Excellent formability and weldability, non-magnetic, not heat treatable. The L variant lowers carbon to avoid sensitization during welding. | Sheet, Plate, Bar and Rod, Tube and Pipe, Structural Shapes | |
| 316 and 316L | Austenitic | Molybdenum addition substantially improves pitting resistance in chlorides. The default marine austenitic, though still vulnerable to chloride stress corrosion cracking at temperature. | Sheet, Plate, Bar and Rod, Tube and Pipe | |
| 17-4PH | Precipitation hardening martensitic | The most widely used precipitation hardening stainless. A single simple aging treatment sets strength across a wide range, with corrosion resistance close to 304. | Bar and Rod, Plate, Forgings, Castings, Powder | |
| 15-5PH | Precipitation hardening martensitic | A ferrite-free variant of 17-4PH with better transverse toughness, which is why aerospace structural applications often specify it over 17-4. | Bar and Rod, Plate, Forgings | |
| 2205 Duplex | Duplex austenitic-ferritic | Roughly balanced austenite and ferrite, giving about double the yield strength of 316 with far better chloride stress corrosion cracking resistance. | Plate, Sheet, Bar and Rod, Tube and Pipe | |
| 440C | Martensitic | High carbon martensitic grade that hardens to bearing and cutting-edge hardness while retaining moderate corrosion resistance. | Bar and Rod, Plate |
Properties
| Property | Value | Unit | Note |
|---|---|---|---|
| Density | 7.75 to 8.05 | g/cm3 | Slightly higher than carbon steel for austenitic grades |
| Melting range | approximately 1400 to 1450 | degrees C | |
| Elastic modulus | 193 to 200 | GPa | Essentially the same as carbon steel |
| Tensile strength, 304 annealed | 515 minimum | MPa | 75 ksi |
| Tensile strength, 17-4PH H900 | 1310 typical | MPa | 190 ksi in the peak aged condition |
| Minimum chromium for passivity | approximately 10.5 | percent | Below this the passive film does not form and the alloy is not stainless |
| Thermal conductivity | 15 to 16 | W/m-K | Austenitic grades; roughly a third of carbon steel, which is why they machine hot |
| Magnetic behavior | Varies by family | Austenitic grades are essentially non-magnetic; martensitic and PH grades are magnetic |
Advantages
- Self-healing passive film gives corrosion resistance without coating or plating
- Precipitation hardening grades reach high strength with good corrosion resistance
- Austenitic grades keep toughness down to cryogenic temperatures
- Austenitic grades are essentially non-magnetic
- Retains useful strength well above aluminum's limit
- Broad domestic production base across several independent producers
Limitations
- Roughly 1.7 times the density of titanium at comparable corrosion resistance
- Austenitic grades suffer chloride stress corrosion cracking, which is sudden rather than progressive
- Low thermal conductivity concentrates heat at the tool tip and makes machining slow
- Work hardens rapidly, which punishes interrupted cuts and dull tooling
- Austenitic grades cannot be hardened by heat treatment, only by cold work
- Sensitization during welding can destroy corrosion resistance at the heat affected zone
Governing specifications
| Designation | Body | Scope |
|---|---|---|
| ASTM A240 | ASTM International | Chromium and chromium-nickel stainless plate, sheet and strip for pressure vessels |
| ASTM A276 | ASTM International | Stainless steel bars and shapes |
| ASTM A564 | ASTM International | Hot rolled and cold finished age-hardening stainless bars and shapes |
| AMS 5643 | SAE International | 17-4PH bars, wire, forgings, rings and extrusions |
| AMS 5659 | SAE International | 15-5PH bars, wire, forgings and rings |
| AMS 2759/3 | SAE International | Heat treatment of precipitation hardening corrosion resistant steel parts |
| DFARS 252.225-7009 | U.S. Department of Defense | Specialty metals restriction. Stainless steels generally fall inside the definition |