Specifications

Chemical composition and mechanical properties

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

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

Actual values for each heat are reported on the mill test certificate.

Why SMO 254

SMO 254 is a 6% molybdenum super-austenitic stainless steel with a PREN of at least about 42. It is chosen for seawater, brines, chlorinated water, pulp and paper bleach liquors, flue-gas scrubber condensate and chloride-bearing acids. Compared with 316L, SMO 254 has far better resistance to pitting, crevice corrosion and chloride stress corrosion cracking and about 50% higher yield strength; compared with super duplex it is fully austenitic, easier to form and tougher at low temperatures.

How we manufacture SMO 254 fittings

Seamless fittings are formed from SMO 254 pipe, and welded fittings from A240 plate welded with over-alloyed filler. Elbows are formed by mandrel bending, tees by hydraulic bulging or extrusion, and reducers by pressing. After forming, every fitting is solution annealed and rapidly cooled, pickled, bevelled to ASME B16.25 and inspected to ASME B16.9. Forged fittings are machined from F44 forgings. Each fitting is marked with grade, size, schedule and heat number.

Fittings in seawater systems

Elbows, tees and reducers concentrate turbulence and flow velocity, so they are where erosion-corrosion starts in seawater lines. SMO 254 tolerates higher velocities than 316L and copper alloys. Butt-weld fittings are preferred over socket weld fittings in seawater and chloride service, because the gap at the bottom of a socket weld joint is a crevice. Light-wall Sch 10S fittings are commonly used, matching the thin-wall SMO 254 pipe typical of seawater systems.

Welding and fabrication

SMO 254 is welded by TIG, MIG and manual arc processes with over-alloyed nickel-based fillers such as ERNiCrMo-3 or ERNiCrMo-4, on clean joints free of oil, paint, marking crayon, sulphur and lead, using low heat input and controlled interpass temperatures. Heat tint is removed and welds are pickled where corrosion resistance at the weld matters. Fabrication areas should be kept separate from carbon steel, and dedicated tools used to avoid iron contamination.

Testing and ordering

Common supplementary requirements are an ASTM G48 pitting test, positive material identification and third-party inspection by the buyer's nominated agency. Please give the grade and specification, product type, size, schedule or class, quantity and any testing or inspection requirement with your enquiry. Each piece is marked with grade, specification, size and heat number, packed in wooden boxes or on pallets, and traceable to its mill test certificate.

Sizes, supply and packing

Fittings are supplied with ends bevelled to ASME B16.25 and matched to the pipe schedule, so that the bores align at each weld. Long-radius elbows are standard; short-radius elbows, 3D bends, reducing tees and eccentric reducers are made on request. Fittings are packed in wooden boxes with bevel protectors, and project orders can be marked with item numbers from the bill of materials to simplify receiving at site.

Certification and traceability

SMO 254 looks identical to 304L and 316L, so mix-ups in a workshop are a real risk. Every piece carries its grade and heat number, and the mill test certificate reports chemistry – including molybdenum and nitrogen, which set the PREN – and mechanical properties. Positive material identification by portable XRF can be carried out before dispatch on request, and pitting tests to ASTM G48 can be added where the specification calls for them.

Applications and related products

SMO 254 pipe fittings are used in seawater cooling and firewater systems, desalination plants, offshore platforms, bleach plants and chloride-bearing chemical processes. Matching products include SMO 254 pipes and tubes, SMO 254 sheets and plates, SMO 254 round bars and SMO 254 flanges. Background on the alloy is on the SMO 254 page. Fittings in other materials are on the pipe fittings page.