Last verified 19 August 2026.
The comparison
| Property | 304L | 321 | Evidence |
|---|---|---|---|
| Proof strength Rp0.2, minimum (EN) | 220 MPa | 220 MPa | E3 E3 |
| Tensile strength Rm, minimum | 520 MPa | 520 MPa | E3 E3 |
| Elongation A, minimum | 45 % | 40 % | E3 E3 |
| PREN | 18 — | 17 — | E3 E3 |
| Density | 7.9 g/cm³ | 7.9 g/cm³ | E3 E3 |
| Modulus of elasticity | 200 GPa | 200 GPa | E3 E3 |
| Thermal conductivity | 15 W/(m·K) | 15 W/(m·K) | E3 E3 |
All shared rows compare like for like.
Where each side’s numbers come from
See 304L and 321 — each page lists every source, every demotion and every gap. Nothing on this page is derived from anything not on those two, and our coverage limits are stated separately.
Where a PREN appears above, read it as a ranking number rather than a qualification: what PREN can and cannot tell you.
When to choose which
304L. Welded fabrication that has to stay corrosion resistant at the weld. That is the whole argument. Everything else about it — price, availability, formability, the fact that every shop already knows how to handle it — is inherited from 304. If a part is welded and will see moisture, specifying 304L costs almost nothing and removes a failure mode that is invisible until it is not.
321. Welded assemblies that then run hot. Titanium stabilisation holds through the range where unstabilised austenitics precipitate chromium carbides, so the grade keeps its corrosion resistance after hours at temperature rather than only after minutes. Automotive and industrial exhausts, bellows, heat exchanger shrouds and flue ducting are the recurring uses.
Against 304L: Two reasons, in order of how often they bite. Strength: the EN cold rolled minima are the lowest in the austenitic family, and a strength-driven section in 304L is heavy. Chlorides: with no molybdenum it is no better than 304, and coastal air, de-icing salt and process water all find that out. For welded service in chlorides the honest step up is 316L, not 304L.
Against 321: Chlorides and surface finish. No molybdenum means it ranks below 316L on pitting, and titanium carbonitrides make a poor polished surface, so it is the wrong grade for architectural and hygienic work. It is also the wrong grade when the service is cold and welded, where 304L does the same job cheaper and with a cleaner surface.
What this comparison does not cover
- The ASTM plate minima are not established here, and they are the figures most North American projects need.
- The 0.03% carbon maximum that defines the grade is not sourced on this page.
- Elevated and cryogenic temperature behaviour, and the creep range.
- Fatigue, fracture toughness and chloride stress corrosion cracking.
- Sensitisation behaviour after long exposure in the 450–850 °C range, which the low carbon delays rather than prevents. Not sourced here; the standard and the mill datasheet are the routes.
- Whether 304L suits your application. No page can answer that.
- No elevated temperature values, which is the service most 321 is bought for.
- The titanium content and the stabilisation ratio are not established here.
- The ASTM plate minima are not established here.
- Fatigue, creep, fracture toughness and chloride stress corrosion cracking.
- The distinction between 321 and 321H, which is a carbon range set by the standard and not carried on this page.
Generated from data/materials/304l.yaml and 321.yaml.