Chemical composition

Limits for plate, sheet and strip to ASTM A240. Pipe (A312), tube (A249, A269) and bar (A276, A479) limits are similar.

With the minimum composition, PREN (%Cr + 3.3 × %Mo + 16 × %N) is about 42; typical material is 43 or more. The high nitrogen strengthens the austenite and improves pitting resistance, and the low carbon keeps welds resistant to intergranular corrosion.

Mechanical properties and equivalents

Values for plate, sheet and strip; check the standard for other product forms and thicknesses.

Equivalent designations are similar but not identical; order to the standard you need.

SMO 254 is about 50% stronger than 316L, with good ductility and impact toughness down to cryogenic temperatures.

SMO 254 products and specifications

Flanges are made to ASME B16.5 and B16.47, butt-weld fittings to ASME B16.9 and forged fittings to ASME B16.11.

Corrosion resistance

SMO 254 was developed for seawater and other high-chloride environments. It resists pitting and crevice corrosion in natural seawater at moderate temperatures, in brackish and polluted water, and in chloride-bearing process streams such as bleach plant filtrates. Its high nickel and molybdenum give good resistance to chloride stress corrosion cracking, much better than 316L. It also performs well in sulphuric and phosphoric acids, including those contaminated with chlorides and fluorides. In warm seawater with tight crevices, super duplex or nickel alloys may be preferred, and design should limit crevices and use suitable gaskets.

Where SMO 254 is used

SMO 254 is used for seawater cooling systems and heat exchangers on offshore platforms and in coastal plants, firewater and ballast systems, desalination plant evaporators and brine heaters, pulp and paper bleach plants, flue-gas desulphurisation scrubbers, chemical process equipment for chloride-bearing acids, food processing with salty brines, and tall-oil distillation. It is also used for instrument and hydraulic tubing in offshore service.

SMO 254 compared with other alloys

Compared with 316L (PREN about 25) and 904L (PREN about 35), SMO 254 offers a large step up in pitting and crevice resistance. Super duplex 2507 has a similar PREN with higher strength and lower nickel cost, but is ferritic-austenitic and has stricter limits on heat input and thick sections. SMO 254, as a fully austenitic steel, is easier to form and weld in thin sections and keeps toughness at low temperatures. Nickel alloys such as Inconel 625 and Hastelloy C276 are used where conditions exceed the limits of 6Mo steels. More materials are on the materials page.

Fabrication

SMO 254 forms well cold, though it needs more force than 316L because of its higher strength. It is welded with over-alloyed nickel-based filler metals such as ERNiCrMo-3 or ERNiCrMo-4, because matching-composition welds lose molybdenum to segregation and would be less corrosion resistant than the base metal. Heat tint must be removed and welds pickled. Machining needs rigid set-ups and sharp tools, as the alloy work-hardens.

Testing SMO 254

Because the value of SMO 254 lies in its corrosion resistance, purchasers often add a pitting test to ASTM G48 at a stated temperature to confirm that the material and any welds perform as expected. Positive material identification is used at receiving inspection, since SMO 254 looks identical to 316L. For welded fabrications, the weld procedure qualification usually includes a corrosion test of the weld as well as mechanical tests.

Documents and inspection

Material is supplied with mill test certificates (MTC) showing heat number, chemical composition and mechanical properties. Third-party inspection is available on request, and positive material identification can be carried out to confirm the grade.