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 bars

We manufacture SMO 254 round bars from certified billets by hot rolling or forging, followed by solution annealing and rapid cooling to keep the structure free of intermetallic phases. Bright bars are peeled, turned or centreless ground to close tolerance. Every bar is marked with grade and heat number, and ultrasonic testing is carried out on request for bars that will be machined into pressure-retaining parts.

A276 or A479?

ASTM A276 covers stainless bars for general use, and ASTM A479 covers bars for boilers and pressure vessels, with additional testing and heat-treatment requirements. Specify A479 when bars will be machined into flanges, fittings, valve bodies or other pressure parts. Many bars are dual certified. For machining, SMO 254 is tougher than 316L and work-hardens; use rigid set-ups, sharp carbide tools, low speeds and steady feeds deep enough to cut below the hardened layer.

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

Bars are supplied in random mill lengths, typically 3 to 6 metres, or cut to the lengths needed for machining, which reduces waste of an expensive alloy. Diameters are offered in metric and imperial sizes; bright bars are held to tolerance classes such as h9 to h11, and black bars to the hot-rolled tolerances of the standard. Bars are bundled or boxed, with bright bars wrapped to protect the surface. For each order we confirm the available diameters and the delivery of any special sizes before quoting.

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 round bars are used for seawater pump shafts and impellers, valve stems and trim, fasteners, flanges and fittings machined from bar, offshore and desalination components, and bleach-plant equipment. Matching products include SMO 254 pipes and tubes, SMO 254 sheets and plates, SMO 254 flanges and SMO 254 pipe fittings. Background on the alloy is on the SMO 254 page. Other bar materials are on the round bars, rods and wires page.