Last verified 19 August 2026.
The comparison
| Property | 321 | 347 | 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 | 40 % | 40 % | E3 E3 |
| PREN | 17 — | 18 — | 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 321 and 347 — 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
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.
347. Welded pressure and high-temperature work where a matching stabilised filler is needed. Niobium transfers across the arc, so a 347 weld is stabilised too; that is why the grade dominates in power plant piping, superheater components and refinery service, and why ASME work tends to specify it over 321 for welded construction.
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.
Against 347: Cost and hot cracking. Niobium is dearer than titanium and 347 is less widely stocked, so where the part is not welded, 321 usually wins on price and availability. Niobium-stabilised weld metal is also more prone to hot cracking than titanium-stabilised base material, which puts more weight on procedure qualification than a drawing note can carry.
What this comparison does not cover
- 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.
- No elevated temperature values, which is the service most 347 is bought for.
- The niobium content and the stabilisation ratio are not established here.
- The distinction between 347 and 347H, which is a carbon range set by the standard.
- Weld procedure guidance. The hot cracking tendency noted above is not sourced on this page and is not a design input.
Generated from data/materials/321.yaml and 347.yaml.