254 SMO Weld Neck Flanges — 6-Mo Super Austenitic (UNS S31254)
Tesco Steel & Engineering forges 254 SMO weld neck flanges in the 6% molybdenum super austenitic — UNS S31254, Werkstoff Nr. 1.4547, ASTM A182 grade F44 — from 1/2″ NB to 56″ NB, in ASME B16.5 Class 150–2500 and European PN patterns. Six-moly chemistry with 0.18–0.22% nitrogen pushes the PREN past 42 — hot seawater, warm brines and crevice-prone details that test even super duplex — while the nitrogen also buys 650 MPa tensile strength, a third above ordinary austenitics. Ultra-low 0.020% carbon keeps every weld sensitization-safe; joints are welded with over-alloyed nickel filler. Supplied pickled & passivated with G48, IGC ASTM A262, PMI and EN 10204 3.1/3.2 on request. ISO 9001:2015, made in Mumbai, India — exported worldwide.
254 SMO · UNS S31254 · 1.4547
ASTM A182 F44 · 6% Mo
PREN > 42 · Hot Seawater
650 MPa Tensile · High N
1/2″ – 56″ NB
Class 150 – 2500 · PN 2.5 – 400
ISO 9001:2015 · Exported Worldwide
254 SMO Weld Neck Flange — UNS S31254 (A182 F44), Technical Specifications
What is a 254 SMO Weld Neck Flange?
254 SMO is the trade name of the 6% molybdenum super austenitic stainless steel — UNS S31254, 1.4547, forged under ASTM A182 as grade F44. Six-moly chemistry plus 0.18–0.22% nitrogen drives the pitting resistance number past 42, into hot-seawater territory, while the nitrogen also lifts tensile strength to 650 MPa. A 254 SMO weld neck flange pairs that chemistry with a long tapered hub butt-welded to the pipe, bore matched to schedule, joint fully radiographable — the last stainless rung before nickel alloys.
On the ladder it stands above 904L (whose copper aims at sulphuric acid rather than chlorides) and beside the super duplexes — which bring more yield strength at less cost, but live inside a −46 to 250 °C window with ferrite to manage. Fully austenitic 254 SMO has no 475 °C embrittlement, no sigma-phase anxiety in fabrication, and handles hot chloride solutions at the edges of duplex's envelope.
Its calling card is hot seawater: critical pitting temperatures beyond super duplex carry raw and chlorinated seawater, warm brines and severe crevices — the services where offshore and desalination specifications write "6Mo" by name.
Also searched as: UNS S31254 flange, 1.4547 flange, ASTM A182 F44 flange, 6-moly flange, 6Mo super austenitic flange, SMO 254 WNRF flange — all refer to the product on this page; see also our
254 SMO flanges overview.
Chemical Composition of 254 SMO (UNS S31254)
| C | Mn | Si | Cu | S | P | Mo | Ni | Cr | N |
|---|
| 0.020 max | 1.00 max | 0.80 max | 0.5-1.0 | 0.01 max | 0.030 max | 6.00-6.50 | 17.50-18.50 | 19.50-20.50 | 0.18-0.22 |
Values in weight %. The 6% molybdenum and 0.18–0.22% nitrogen together drive PREN = %Cr + 3.3×%Mo + 16×%N past 42; the copper adds acid margin, and the 0.020% carbon ceiling makes every weld sensitization-safe by chemistry.
Mechanical Properties of 254 SMO
| Tensile Strength, MPa | Yield Strength, Min, MPa | Elongation % min. | Hardness Rockwell HRB | Hardness Brinell HB |
|---|
| 650 | 300 | 35 | 96 max | 223 max |
Minimums per our supply practice. The nitrogen that buys pitting resistance also buys muscle — roughly a third stronger than ordinary austenitics, unusual for a corrosion-first grade. The solution-annealed condition is documented on every certificate.
Equivalent Grades of 254 SMO
| Standard | Werkstoff Nr. | UNS | SIS | AFNOR | EN |
|---|
| 254 SMO | 1.4547 | S31254 | 2378 | Z1 CNDU 20.18.06Az | X1CrNiMoCuN20-18-7 |
Forged under ASTM A182 as grade F44. System partners: A312 S31254 pipe, A403 fittings and A240 plate. The family: 904L below for acids, super duplex beside it for strength, Hastelloy and Inconel above.
254 SMO Weld Neck Flange Specifications
| 254 SMO Weld Neck Flanges are available in the following specifications: |
|---|
| Size | 1/2"NB to 56"NB |
| Class | 150#, 300#, 400#, 600#, 900#, 1500#, 2500# |
| Sch (Schedule) | XS, XXS, STD & Schedule 20, 40, 80, 160 |
| Pressure Ratings | PN 2.5 - PN 400 |
| Standards | ASME B16.5, ASME B16.47, EN 1092-1 Type 11, DIN 2631–2635, BS 4504 |
| Condition | Solution Annealed |
| Other Services | Pickling & passivation on 254 SMO Weld Neck Flanges
Sand Blasting on 254 SMO Weld Neck Flanges
Shot Peening on 254 SMO Weld Neck Flanges
IGC (ASTM A262), G48 & PMI testing
Project-specific marking, wrapping & export packing |
Why the 6-Mo is the Last Stainless Before Nickel Alloys
Hot Seawater Credential
PREN past 42 and critical pitting temperatures beyond super duplex — raw, chlorinated and warm seawater duty that offshore specifications write "6Mo" for by name.
No Ferrite to Manage
Fully austenitic: no 475 °C embrittlement, no sigma-phase anxiety, easier fabrication and generous toughness from cryogenic up — where duplex must watch its window.
Nitrogen Buys Strength Too
650 MPa tensile — a third above ordinary austenitics — without sacrificing the ductility and weldability the family is known for.
Weld-Safe by Chemistry
0.020% carbon means no sensitization and no PWHT; over-alloyed nickel filler keeps the weld metal's pitting margin equal to the base.
The Economic Ceiling of Stainless
Near-nickel-alloy chloride performance at stainless logistics — when 254 SMO runs out, the next stop is Hastelloy money.
Welding 254 SMO — Over-Alloy the Filler
The one rule of 6-Mo welding: never use matching filler. Molybdenum segregates as the weld pool freezes, leaving inter-dendritic lean zones that pit first — so joints use over-alloyed nickel-base consumables, typically ERNiCrMo-3 (625 type), whose 9% molybdenum keeps the weld metal's margin equal to the base. Low heat input, cool interpass, no preheat, no PWHT; the ultra-low carbon keeps the HAZ sensitization-safe, and a post-weld pickle restores the surface.
Temperature ceiling: keep continuous service below ~400 °C — above it, intermetallic precipitation starts to spend the corrosion resistance the grade is bought for. For hot strength, the creep-rated stainless and alloy families are the right tools; 254 SMO's business is aggressive wet chemistry.
Where 254 SMO Weld Neck Flanges Are Used
Hot seawater first: offshore cooling, firewater and process circuits, desalination high-temperature trains — the grade's home ground. Pulp and paper bleach plants at their hottest stages, flue gas desulphurisation, chemical plants running hot chloride solutions, and pharmaceutical or fine-chemical duties that pair austenitic cleanliness with extreme pitting margin complete the range. Our stainless production below:
Super Austenitic Weld Neck Flanges — Machined & Packed Production
254 SMO Weld Neck Flange Dimensions
254 SMO weld neck flanges share their dimensions with every other material in the same class — OD, thickness, hub, bore, bolt circle and bolting per the standard tables. Full ASME B16.5 charts:
European PN dimensions are published on the EN 1092-1 Type 11, DIN 2631–2635 and BS 4504 pages.
Price List & How to Order
254 SMO pricing tracks its heavy molybdenum and nickel content plus the testing scope, so we quote live with a stated validity rather than publish a static list. To get a firm quotation, usually within 24 hours:
1
List your requirement — size (NB), class or PN, standard (ASME B16.5 / B16.47 / EN / DIN / BS), grade (254 SMO / S31254 / F44), pipe schedule or bore, and quantity.
2
Name the service & testing scope — chloride level and temperature, G48 with test temperature, IGC A262, 3.2 witness, PMI — it lets us confirm the 6-Mo is the economic grade before you commit.
254 SMO Weld Neck Flanges — Frequently Asked Questions
What is a 254 SMO weld neck flange?
It is a weld neck flange forged from 254 SMO — the 6% molybdenum super austenitic stainless steel, UNS S31254 / 1.4547, covered by ASTM A182 as grade F44. Six-moly chemistry with high nitrogen pushes the pitting resistance number past 42, the territory of hot seawater and severe chloride duty. The long tapered hub is butt-welded to the pipe with the bore matched to the pipe schedule, giving a radiographable, fatigue-resistant joint one step below nickel-alloy pricing.
What is 254 SMO and what does the name mean?
254 SMO is the trade name under which this 6-Mo super austenitic was developed (SMO abbreviating its high molybdenum content); UNS S31254 is its universal designation, 1.4547 its European number, and F44 its ASTM A182 forging grade. Specifications written around any of these describe the same material — an enquiry that says '6-moly flange' lands on this product too.
What makes 6-Mo super austenitic different from 904L?
More molybdenum, deliberate nitrogen and a different mission. 904L's 4-5% Mo and copper aim at sulphuric acid; 254 SMO's 6% Mo plus 0.18-0.22% nitrogen aim at chlorides, driving PREN from the mid-30s past 42 — genuine hot-seawater territory. The nitrogen also lifts strength: 650 MPa tensile against 904L's 490. Pick by chemistry: reducing acids favour 904L, severe chlorides and seawater favour the 6-Mo.
How does 254 SMO compare with super duplex F53/F55?
Both clear the PREN-40 seawater threshold; the structures differ. Super duplex brings higher yield strength (550 versus 300 MPa) at a lower price, but lives inside a -46 to 250 °C window and demands tight welding discipline. Fully austenitic 254 SMO has no ferrite to embrittle — no 475 °C problem, better toughness margins, easier fabrication — and handles hot chloride solutions where duplex risks stress corrosion at the edges of its envelope. Offshore practice uses both; the service temperature and fabrication realities usually decide.
What temperature range can 254 SMO flanges be used in?
From cryogenic — the fully austenitic structure has no ductile-brittle transition — to roughly 400 °C for continuous pressure duty; above that, intermetallic phases begin to precipitate and erode the corrosion resistance the grade is bought for. Its territory is aggressive wet chemistry, hot brines and seawater at low and moderate temperatures — not the creep range.
What are the equivalent designations for 254 SMO?
UNS S31254; Werkstoff Nr. 1.4547 (EN X1CrNiMoCuN20-18-7); ASTM A182 F44 for forgings; Swedish SIS 2378; and AFNOR Z1 CNDU 20.18.06Az. System partners: A312 TP S31254 pipe, A403 WP S31254 fittings and A240 S31254 plate. An enquiry in any of these designations is quoted as the same material.
What is the chemical composition of 254 SMO?
Per our supply practice: carbon 0.020% max, manganese 1.00% max, silicon 0.80% max, copper 0.5 to 1.0%, sulphur 0.01% max, phosphorus 0.030% max, molybdenum 6.00 to 6.50%, nickel 17.50 to 18.50%, chromium 19.50 to 20.50% and nitrogen 0.18 to 0.22%. The 6% molybdenum and the nitrogen together drive the PREN past 42; the copper adds acid margin; the ultra-low carbon keeps welds sensitization-safe.
What are the mechanical properties of 254 SMO flanges?
Minimum tensile strength 650 MPa (94 ksi), minimum yield strength 300 MPa (44 ksi) and minimum elongation 35%, with hardness typically limited to 96 HRB / 223 HB. The nitrogen that buys pitting resistance also buys muscle — 254 SMO is roughly a third stronger than ordinary austenitics, unusual for a corrosion-first grade. The solution-annealed condition is documented on every certificate.
Which pipe grades do 254 SMO weld neck flanges mate with?
ASTM A312 UNS S31254 seamless and welded pipe is the standard partner, with A403 WP-S31254 butt-weld fittings and A240 plate completing the system. The weld neck bore is machined to the pipe schedule you state, so the bore runs flush through the joint and the bevel arrives ready for a matched-chemistry butt weld.
How are 254 SMO flanges welded to pipe?
With over-alloyed nickel-base filler — typically ERNiCrMo-3 (Inconel 625 type) — never matching composition: molybdenum segregates in a 6-Mo weld pool, leaving local lean zones that pit first, so the over-matched consumable restores the margin. Low heat input, cool interpass, no preheat, no PWHT. The ultra-low carbon keeps the heat-affected zone sensitization-safe, and a post-weld pickle restores the surface.
Can 254 SMO flanges handle hot seawater?
Yes — that is the grade's calling card. With PREN above 42 and critical pitting temperatures beyond super duplex, 254 SMO serves hot raw and chlorinated seawater, warm brines and severe crevice-prone details that test everything below it. Seawater cooling and firewater systems, desalination high-temperature sections and offshore process circuits specify 6-Mo grades exactly for this duty.
What testing and certification do you supply with 254 SMO flanges?
Every lot ships with EN 10204 3.1 mill test certificates covering chemical analysis — including molybdenum, nitrogen and the ultra-low carbon — mechanical properties and the solution-anneal record, with 3.2 certification witnessed by Lloyd's, DNV, BV or TÜV on request. We also offer intergranular corrosion testing to ASTM A262, ASTM G48 pitting tests at specified temperatures, PMI verification and hydrostatic testing, with full heat-number traceability.
Where are 254 SMO weld neck flanges used?
Hot seawater and brine systems above all — offshore cooling, firewater and process circuits, desalination high-temperature trains — plus pulp and paper bleach plants at their hottest stages, flue gas desulphurisation, chemical plants running hot chloride solutions, and pharmaceutical or fine-chemical duties that demand austenitic cleanliness with extreme pitting margin. Wherever stainless ladders end and nickel-alloy budgets begin, the 6-Mo holds the line.
What sizes and standards do you manufacture 254 SMO weld neck flanges in?
From 1/2 inch NB to 56 inch NB: ASME B16.5 Class 150 to 2500 up to 24 inch, ASME B16.47 above, and European PN 2.5 to PN 400 patterns to EN 1092-1 Type 11, DIN and BS 4504. Raised face is standard, ring joint for high-class offshore service on request, and every bore is machined to the mating pipe schedule from Sch 10 through XXS.
Do you keep 254 SMO weld neck flanges in stock? What is the lead time?
254 SMO moves in project quantities, so common offshore and desalination sizes in Class 150 and 300 rotate through production with popular items often available, while larger diameters, higher classes and witness-tested lots are forged to order — typically 4 to 6 weeks given the specialised melt. Prices track the heavy molybdenum and nickel content, so quotations carry a validity period.
What information should I include in a 254 SMO flange enquiry?
Six things: size (NB), pressure class or PN rating, dimensional standard (ASME B16.5, B16.47 or EN/DIN/BS), grade (254 SMO / S31254 / F44), pipe schedule or bore, and quantity. Add the testing scope — G48 with test temperature, IGC A262, 3.2 witness, PMI — and the service conditions: naming the chloride level and temperature lets us confirm the 6-Mo is the economic grade before you commit. Quotations usually return within 24 hours.
Do you export 254 SMO flanges outside India?
Yes. Tesco Steel & Engineering is an ISO 9001:2015 certified manufacturer based in Mumbai, and 254 SMO flanges ship to offshore, desalination and chemical projects across the Middle East, Europe, Africa, Southeast Asia and the Americas. Flanges travel pickled, face-protected and seaworthy-packed, with full heat-number and test traceability.
