Last verified 19 August 2026.
The comparison
| Property | 254 SMO | 6XN | Evidence |
|---|---|---|---|
| Proof strength Rp0.2, minimum (EN) | 320 MPa | 300 MPa | E3 E3 |
| Tensile strength Rm, minimum | 650 MPa | 650 MPa | E3 E3 |
| Elongation A, minimum | 35 % | 40 % | E3 E3 |
| PREN | 43 — | 45 — | E3 E3 |
| Density | 8.0 g/cm³ | 8.1 g/cm³ | E3 E3 |
| Modulus of elasticity | 195 GPa | 195 GPa | E3 E3 |
| Thermal conductivity | 14 W/(m·K) | 12 W/(m·K) | E3 E3 |
Proof strength Rp0.2, minimum (EN) — different product forms — cold rolled coil and sheet against Quarto plate. The grades differ and so does what was tested, so the gap between these two numbers is not all material
Tensile strength Rm, minimum — different product forms — cold rolled coil and sheet against Quarto plate. The grades differ and so does what was tested, so the gap between these two numbers is not all material
Elongation A, minimum — different product forms — cold rolled coil and sheet against Quarto plate. The grades differ and so does what was tested, so the gap between these two numbers is not all material
Where each side’s numbers come from
See 254 SMO and 6XN — 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
254 SMO. Seawater and strong chloride service where a single-phase austenitic is wanted rather than a duplex — because the part runs hot, because it must stay non-magnetic, or because the shop is set up for austenitic fabrication and not for phase-balance control. Desalination, seawater cooling, bleaching plant and offshore piping are the recurring uses.
6XN. Seawater and brine handling in plate form — condenser and heat exchanger shells, desalination vessels, offshore process equipment. Its high nickel gives it better resistance to chloride stress corrosion cracking than the standard austenitics, and being single phase it has no phase balance to hold during welding, unlike the super duplex alternatives.
Against 254 SMO: Price and strength per unit cost. At six percent molybdenum the alloy content is expensive, and 2507 reaches the same pitting ranking with far more proof strength, so a strength-driven seawater part is usually cheaper in super duplex. Segregation on solidification also makes weld metal the weak point unless an overmatching filler is used.
Against 6XN: Price, weight and heat transfer. It is one of the most expensive stainless alloys per tonne, it is the densest in its family so a plate structure is heavier, and its thermal conductivity is among the lowest, which costs area in a heat exchanger. Where a duplex can be used, it usually is.
What this comparison does not cover
- Critical pitting and crevice temperatures are not established here, and they are what the alloy is selected on.
- Weld metal corrosion behaviour, which is where this alloy family usually fails first.
- Elevated temperature strength, fatigue and fracture toughness.
- Composition limits, not established.
- Whether a super duplex is the better answer for your part. This page gives you the numbers, not the decision.
- Only plate minima are established here.
- Critical pitting and crevice temperatures, which decide seawater selection.
- Weld metal properties, which for a segregating super-austenitic are the weak point.
Generated from data/materials/254smo.yaml and 6xn.yaml.