Stainless Steel 304L
Stainless Steel 304L is a stainless steel used in aerospace applications. Melting point: 1,400 °C. Tensile strength: 515 MPa.
When SpaceX needed a material to build its giant Starship rocket, it skipped the high-tech carbon fiber everyone expected and reached for a kind of stainless steel you might find in a kitchen sink. That material is 304L, and the reasons behind the choice are surprisingly clever.
Quick facts
- Type: Austenitic chromium-nickel stainless steel (UNS designation S30403). “Austenitic” means its internal crystal structure stays tough and bendable instead of turning brittle.
- What’s in it: About 18-20% chromium, 8-12% nickel, and a deliberately low carbon content of roughly 0.03% or less (standard 304 steel allows about 0.08%). The “L” stands for “low carbon.”
- Density: About 7.93 grams per cubic centimeter (0.286 pounds per cubic inch) – fairly heavy for a metal.
- Melting range: Roughly 1,400-1,450 degrees C (2,550-2,650 degrees F).
- Room-temperature strength (annealed, meaning softened by heat treatment): Ultimate tensile strength of at least about 485 MPa and yield strength of at least about 170 MPa.
- Cost: Roughly $3 per kilogram, versus about $200 per kilogram for aerospace carbon fiber.
What it is and how it works
Stainless steel resists rust because chromium in the alloy forms a thin, self-healing “passive” layer – an invisible oxide skin that seals out corrosion and re-forms if scratched. The nickel does a different job: it keeps the steel in a tough, ductile (easily bendable, not brittle) crystal arrangement called austenite, even when the metal gets extremely cold.
The real trick is the low carbon. When stainless steel is welded, the intense heat can make leftover carbon bond with chromium along the boundaries between metal grains, a process called carbide precipitation. That robs those spots of their corrosion protection and leaves weak, crack-prone welds. By keeping carbon very low, 304L mostly avoids this – so the long seam welds that join Starship’s stacked steel rings stay strong and rust-resistant.
There is one delightful quirk to this alloy family: it gets stronger as it gets colder, not weaker. The same walls that hold super-cold liquid oxygen and liquid methane actually gain strength once those frigid propellants are pumped in.
Why it matters
For a rocket, 304L hits a rare three-way combination. It is cheap and easy to manufacture. It strengthens at cryogenic (very cold) temperatures – reportedly by roughly 50% when chilled by the propellants it holds. And its high melting point (around 1,400 degrees C) helps it survive the searing heat of reentry through the atmosphere without needing an exotic heat shield over every surface.
SpaceX publicly credited exactly that trio – low cost and easy manufacturing, higher strength when cold, and high-heat tolerance – as the reason it abandoned carbon-fiber composites for Starship. The economics make the point bluntly: about $3 per kilogram for steel versus about $200 per kilogram for carbon fiber (worse once you count wasted offcuts). That gap is part of what makes a fully reusable, mass-produced rocket financially realistic.
The main trade-off is weight. At about 7.93 g/cm3, steel is much heavier than carbon fiber or aluminum, so designers accept a mass penalty in exchange for the strength, heat tolerance, and dramatically lower cost. The low carbon also gives 304L slightly lower baseline strength than standard 304, traded for far better welds.
Where it is used and notable examples
- SpaceX Starship (the upper stage): Its hull and propellant tanks are built from welded 304L steel rings about 1.83 m (6 ft) tall with walls just about 3.97 mm (0.156 in) thick. SpaceX moved from an earlier steel (301) to 304L because it is less brittle and easier to weld, before later developing its own in-house “30X” alloy.
- SpaceX Super Heavy booster: Built the same way – stacking and welding steel cylinders to form the tank and airframe.
- Early Starship prototypes: Starhopper and the first test articles proved that welded stainless construction could work before the switch to 304L and then 30X.
- Cryogenic storage tanks: 304L is widely used for liquid-hydrogen and LNG (liquefied natural gas) storage vessels because it keeps its toughness in the cold. It is so well understood that NIST publishes validated property data for it down to a few degrees above absolute zero.
- General aerospace hardware: Structural parts, fasteners, and fittings where corrosion resistance and weldability matter and the budget favors steel over titanium or composites.
Fe bal, Cr 18-20%, Ni 8-12%, C 0.03% max, Mn 2% max
| DENSITY | 8 kg/m³ |
| TENSILE STRENGTH | 515 MPa |
| YIELD STRENGTH | 205 MPa |
| STRENGTH-TO-WEIGHT | 64375 kN·m/kg |
| MELTING POINT | 1,400 °C |
| MAX SERVICE TEMPERATURE | 870 °C |
| THERMAL CONDUCTIVITY | 16.2 W/m·K |
| THERMAL EXPANSION | 17.3 µm/m·K |
| CATEGORY | Stainless Steel |
| DESIGNATIONS | UNS S30403, AISI 304L, AMS 5511 |
| MANUFACTURER | Various (Outokumpu, Aperam, POSCO) |
| DENSITY | 8 kg/m³ |
| TENSILE STRENGTH | 515 MPa |
| YIELD STRENGTH | 205 MPa |
| MELTING POINT | 1,400 °C |
| MAX SERVICE TEMP | 870 °C |
| THERMAL CONDUCTIVITY | 16.2 W/m·K |
| THERMAL EXPANSION | 17.3 µm/m·K |
| CORROSION RESISTANCE | Good |
| WELDABILITY | Excellent |
| MACHINABILITY | Good |
| COST RATING | Low |
