Last verified 19 August 2026.
The comparison
| Property | 254 SMO | 654 SMO | Evidence |
|---|---|---|---|
| Proof strength Rp0.2, minimum (EN) | 320 MPa | 430 MPa | E3 E3 |
| Tensile strength Rm, minimum | 650 MPa | 750 MPa | E3 E3 |
| Elongation A, minimum | 35 % | 40 % | E3 E3 |
| PREN | 43 — | 56 — | E3 E3 |
| Density | 8.0 g/cm³ | 8.0 g/cm³ | E3 E3 |
| Modulus of elasticity | 195 GPa | 190 GPa | E3 E3 |
| Thermal conductivity | 14 W/(m·K) | 11 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 654 SMO — 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.
654 SMO. Hot, aerated seawater and the harshest bleaching and scrubbing duties, where 254 SMO and super duplex both have a service record of crevice attack. The usual comparison at this level is not against another stainless but against a nickel alloy or titanium, and 654 SMO wins on fabricability and on the fact that it is still a steel.
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 654 SMO: Price and everything that follows from it. Seven percent molybdenum and 22% nickel make it one of the most expensive alloys in any stainless catalogue, welding needs an overmatching nickel-base filler, and lead times are long because almost nobody stocks it. Every project that can be solved one grade down is solved one grade down.
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 are the whole basis for choosing this alloy.
- Corrosion rate data in the services it is bought for.
- Welding practice, filler selection and the properties of the resulting weld metal.
Generated from data/materials/254smo.yaml and 654smo.yaml.