How does 254SMO compare with super duplex stainless steels?
254SMO matches super duplex on pitting resistance (PREN over 40 for both) but is more expensive, weaker and non-magnetic, with an easier weld and wider temperature range. Choose super duplex for most duties - it has replaced 254SMO in flue gas desulphurisation and water treatment; keep 254SMO below -50°C or above 250°C, where super duplex is limited.
254SMO vs Alloy 32750: the numbers
| Property | 254SMO | Alloy 32750 |
|---|---|---|
| PREN | >43 | >40 |
| Chromium | - | 24.5-26.0% |
| Molybdenum | ~6.0% | 3.00-5.00% |
| Nickel | - | 6.00-8.00% |
| Nitrogen | - | 0.24-0.32% |
| 0.2% proof strength | - | 550 N/mm² (79.8 ksi) |
| Tensile strength | - | 800 N/mm² (116 ksi) |
| General corrosion resistance | - | Outstanding resistance to pitting, crevice corrosion and stress corrosion cracking |
| Pitting / CPT resistance | PREN >43 | PREN >40; similar to 254SMO |
| Typical applications | Pulp bleaching plants; gas cleaning systems; seawater-handling equipment (heat exchangers, ballast water, firefighting, cooling water); marine scrubbers | Flue gas desulphurisation for power stations; water treatment (reverse osmosis, desalination) as a 254SMO replacement |
Calculate PREN for your own composition with our PREN calculator.
What 254SMO is
254SMO (UNS S31254, ASTM F44, 1.4547) is a super austenitic stainless steel. Increasing its molybdenum content to 6% gives a pitting resistance equivalent number (PREN) greater than 43, similar to that of a super duplex stainless steel (SDSS).
Where super duplex has the edge, and where 254SMO does
Compared with super duplex, 254SMO is non-magnetic, can be more straightforward to weld and has a wider operating temperature range. However, it is considerably more expensive - due to its greater nickel and molybdenum content - and has a significantly lower strength. It is therefore used mainly where super duplex is limited: at lower temperatures (below -50°C), where the impact strength of SDSS drops, or at higher temperatures (above 250°C), where prolonged exposure risks forming deleterious phases.
Common applications for 254SMO
- Equipment in pulp bleaching plants
- Components in gas cleaning systems
- Seawater-handling equipment such as heat exchangers, ballast water systems, firefighting systems and cooling water systems
254SMO and other super austenitics have been specified for marine scrubbers, where intermittent use can push operating temperatures above 250°C. Super duplex stainless steels, by contrast, have been used successfully in flue gas desulphurisation systems for power stations, where continuous operation keeps temperatures lower. In water treatment such as reverse osmosis and desalination, super duplex stainless steels have replaced super austenitics like 254SMO.
Which should you choose?
Super duplex is the right choice for most applications: it matches 254SMO on pitting resistance at a lower cost and higher strength, and has already displaced 254SMO in flue gas desulphurisation and water treatment such as reverse osmosis and desalination. Reach for 254SMO instead where the environment falls outside super duplex's temperature window - below -50°C, where super duplex loses toughness, or above 250°C in continuous service, where it risks forming deleterious phases - or where non-magnetic behaviour or easier welding genuinely matter.