254 SMO Blind Flanges Manufacturer in India
UNS S31254 Blind Flange |
1.4547 Blind Flange |
PREN 43.8 — 6Mo Grade |
254 SMO vs 316L vs 904L |
BLRF / BLFF / BLRTJ |
FGD & Seawater Applications |
ASTM A182 F44
What is 254 SMO (UNS S31254 / 1.4547)?
254 SMO is a super-austenitic stainless steel alloy developed specifically for demanding seawater and high-chloride environments. The designation stands for Sea water, Molybdenum, and allOy — reflecting its primary design intent. It is classified under UNS S31254 (ASTM), EN 1.4547, and SIS 2378 (Swedish origin). The ASTM forging standard for flanges is ASTM A182 Grade F44.
The defining characteristic of 254 SMO is its exceptionally high molybdenum content of 6.0–6.5% — more than double the 2.0–3.0% in 316L and nearly double the 3.0–4.0% in 317L. Combined with 20% chromium, 18% nickel, a controlled nitrogen addition (0.18–0.22%), and a small copper addition (0.5–1.0%), this composition delivers a Pitting Resistance Equivalent Number (PREN) of approximately 43.8 — far exceeding the 40-point threshold required for reliable service in seawater and warm chloride environments.
Tesco Steel & Engineering manufactures 254 SMO Blind Flanges to ASTM A182 F44, dimensioned per ASME B16.5 (½″–24″ NB) and ASME B16.47 (26″–60″ NB), in all face types — BLRF, BLFF, and BLRTJ — across pressure Classes 150 to 2500. All flanges are supplied with EN 10204 3.1 Mill Test Certificates.
Why 254 SMO for Blind Flanges? Blind flanges in corrosive piping are particularly vulnerable — they sit at dead-end positions where stagnant process fluid concentrates, accelerating crevice corrosion and pitting. 254 SMO's CPT (Critical Pitting Temperature) exceeds 60°C in 6% FeCl₃ — compared to approximately 15°C for 316L and 20°C for 317L — ensuring reliable performance in bleach plants, FGD scrubbers, and chlorinated seawater systems where 316L flanges fail within months.
PREN Comparison — 254 SMO vs 316L vs 317L vs 904L
The Pitting Resistance Equivalent Number is the most widely used single-number metric for comparing pitting corrosion resistance in chloride environments. Formula: PREN = %Cr + 3.3 × %Mo + 16 × %N
| Grade |
UNS / EN |
Cr % |
Mo % |
N % |
PREN |
CPT in FeCl₃ |
| 254 SMO |
S31254 / 1.4547 |
20.0 |
6.25 |
0.20 |
43.8 |
>60°C |
| 904L |
N08904 / 1.4539 |
21.0 |
4.50 |
0.10 |
37.5 |
~35–40°C |
| 317L |
S31703 / 1.4438 |
18.5 |
3.25 |
0.10 |
31.2 |
~20–25°C |
| 316L |
S31603 / 1.4404 |
17.0 |
2.25 |
0.05 |
25.2 |
~15°C |
| 304L |
S30403 / 1.4307 |
18.5 |
— |
0.10 |
20.1 |
<0°C |
| Hastelloy C-276 |
N10276 / 2.4819 |
16.0 |
16.0 |
— |
69.3 |
>100°C |
Choose 254 SMO When:
- Chloride >5,000 ppm at >40°C
- Chlorinated seawater or hypochlorite
- FGD wet scrubber condensate
- Kraft pulp bleach plant (ClO₂)
- Phosphoric acid processing
- CPT requirement >40°C
Consider Hastelloy C-276 When:
- HCl (all concentrations)
- Hot concentrated H₂SO₄ (>50%)
- Fluorides or HF service
- Mixed acid environments
- Temperature >100°C in chlorides
Chemical Composition of 254 SMO (UNS S31254 / ASTM A182 F44)
| Element |
Min % |
Max % |
Nominal % |
Role in Alloy |
| Chromium (Cr) |
19.50 |
20.50 |
20.0 |
Passive oxide layer formation; primary corrosion resistance element |
| Nickel (Ni) |
17.50 |
18.50 |
18.0 |
Stabilises austenite; improves SCC resistance and ductility |
| Molybdenum (Mo) |
6.00 |
6.50 |
6.25 |
Primary pitting and crevice corrosion resistance element — contributes 3.3 × Mo to PREN; key differentiator vs 316L |
| Nitrogen (N) |
0.18 |
0.22 |
0.20 |
Contributes 16 × N to PREN; solid-solution strengthener increasing yield strength to 300 MPa minimum |
| Copper (Cu) |
0.50 |
1.00 |
0.75 |
Improves corrosion resistance in reducing acids (H₂SO₄, H₃PO₄); reduces active dissolution rate |
| Carbon (C) |
— |
0.020 |
≤0.020 |
Very low carbon (≤0.020%) prevents sensitisation during welding — equivalent to 'L' grade designation |
| Manganese (Mn) |
— |
1.00 |
≤1.00 |
Deoxidiser; controlled low to preserve nitrogen solubility in melt |
| Silicon (Si) |
— |
0.80 |
≤0.80 |
Deoxidiser; controlled low — excess Si reduces hot workability |
| Sulphur (S) |
— |
0.010 |
≤0.010 |
Very low sulphur limit improves pitting resistance (MnS inclusions are preferential pit initiation sites) |
| Phosphorus (P) |
— |
0.030 |
≤0.030 |
Controlled to prevent hot cracking during forging and welding |
Mechanical Properties — 254 SMO vs 316L
| Property |
254 SMO (A182 F44) |
316L (A182 F316L) |
317L (A182 F317L) |
| Tensile Strength (UTS) |
650 MPa min |
515 MPa min |
515 MPa min |
| Yield Strength (0.2% PS) |
300 MPa min |
205 MPa min |
205 MPa min |
| Elongation |
35% min |
40% min |
40% min |
| Hardness (Rockwell B) |
50 HRB max (≈260 HV) |
95 HRB max |
95 HRB max |
| ASME Material Group |
Group 2.3 |
Group 2.2 |
Group 2.2 |
| Min Service Temp |
−196°C (cryogenic) |
−196°C |
−196°C |
| Max Design Temp (ASME B31.3) |
454°C |
425°C |
425°C |
254 SMO Blind Flange Types and Face Finishes
A blind flange is a solid disc used to blank off the open end of a pipe, vessel nozzle, or valve — allowing pressure testing, system isolation, or future expansion. Unlike other flange types, the blind flange has no bore. It must withstand the full system pressure across its entire face area. In 254 SMO, blind flanges are most commonly used at dead-end positions in high-chloride piping systems where stagnant fluid contact makes corrosion resistance especially critical.
| Type |
Short Code |
Face Description |
Typical Use in 254 SMO Service |
| Raised Face Blind |
BLRF |
Raised face 1.5 mm (Cl.150/300) or 6.4 mm (Cl.400–2500); surface finish 3.2–6.3 µm Ra ASME B16.5 |
Standard for FGD scrubber, seawater cooling, chemical process; used with spiral wound PTFE or Inconel 625 gaskets |
| Flat Face Blind |
BLFF |
Entire flange face machined flat; full-face gasket spans all bolt holes |
Mating to cast iron or FRP pipe flanges; chemical dosing skids with non-metallic body valves; desalination GRP pipe connections |
| Ring Type Joint Blind |
BLRTJ |
Octagonal or oval ring groove machined per ASME B16.20; ring material typically Inconel 625 (N06625) for 254 SMO service |
High-pressure Class 900–2500 chemical injection, HP sampling systems, offshore wellhead isolation; RTJ seals by metal-to-metal contact only |
| Tongue Face Blind |
BLTG |
Male tongue matches groove on mating flange; gasket fully confined |
Used in T&G matched pairs for lethal/toxic service per ASME B31.3; bleach plant and HF alkylation isolation |
| Large / Small T&G Blind |
BLLTG / BLSTG |
Large T&G: tongue height 1.65 mm (Cl.150/300) or 6.35 mm (Cl.400+). Small T&G per ASME B16.5 Appendix E |
Paired with matching T&G mating flange; pharmaceutical, chlorine gas, concentrated acid service |
254 SMO Grade Cross-Reference
| Standard |
Designation |
| UNS (ASTM/ASME) | S31254 |
| ASTM Forging (Flanges) | A182 Grade F44 |
| ASTM Plate | A240 / A480 (UNS S31254) |
| EN / DIN Werkstoff Nr. | 1.4547 |
| EN Full Designation | X1CrNiMoCuN20-18-7 |
| SIS (Swedish) | 2378 |
| AFNOR (French) | Z1 CNDU 20.18.06Az |
| Trade Name | 254 SMO® (Outokumpu / Avesta) |
| Common Alias | 6Mo stainless steel, 6-Moly, F44 |
| ASME Material Group | 2.3 |
254 SMO Blind Flange Dimensions & Specifications (ASME B16.5)
| Specification Parameter |
| Size Range | ½″ NB to 24″ NB (ASME B16.5); 26″ NB to 60″ NB (ASME B16.47 Series A & B) |
| Pressure Classes | Class 150, 300, 600, 900, 1500, 2500 (ASME B16.5) |
| PN Ratings | PN 6 / 10 / 16 / 25 / 40 / 64 / 100 / 160 / 250 / 320 / 400 (EN 1092-1) |
| Forging Standard | ASTM A182 Grade F44 |
| Dimension Standard | ASME B16.5 (½″–24″), ASME B16.47 Series A/B (26″–60″), MSS SP-44, DIN 2527 |
| Face Finish | 3.2–6.3 µm Ra (ASME B16.5 §6.4.4); 125–250 µin AARH for RF; 63 µin for RTJ groove |
| Testing Standard | ASME B16.5 Appendix II (Hydrostatic); ASTM A262 Practice E (IGC); PMI per ASTM E1476 |
| Material Certificate | EN 10204 Type 3.1 (standard); Type 3.2 (with TPI witness — Lloyd's, Bureau Veritas, SGS) |
ASME B16.5 Class 150 254 SMO Blind Flange — Key Dimensions
| NPS (inches) |
O.D. Flange (mm) |
Bolt Circle (mm) |
No. of Bolts |
Flange Thickness (mm) |
Approx. Weight (kg) |
| ½″ | 88.9 | 60.3 | 4 | 11.2 | 0.2 |
| 1″ | 107.9 | 79.4 | 4 | 14.2 | 0.4 |
| 1½″ | 127.0 | 98.4 | 4 | 15.7 | 0.6 |
| 2″ | 152.4 | 120.7 | 4 | 17.5 | 1.0 |
| 3″ | 190.5 | 152.4 | 4 | 19.1 | 1.9 |
| 4″ | 228.6 | 190.5 | 8 | 20.6 | 3.2 |
| 6″ | 279.4 | 241.3 | 8 | 22.4 | 5.7 |
| 8″ | 342.9 | 298.5 | 8 | 25.4 | 9.8 |
| 10″ | 406.4 | 362.0 | 12 | 28.4 | 15.5 |
| 12″ | 482.6 | 431.8 | 12 | 31.7 | 24.0 |
| 16″ | 596.9 | 539.8 | 16 | 36.6 | 43.0 |
| 20″ | 749.3 | 685.8 | 20 | 43.7 | 80.0 |
| 24″ | 863.6 | 812.8 | 20 | 50.8 | 130.0 |
* All dimensions per ASME B16.5-2017. Weights are approximate for 254 SMO (density ≈ 8.0 g/cm³). Class 300–2500 dimensions available on request.
Dimensional Reference Links:
Welding 254 SMO Blind Flanges — Key Guidelines
| Parameter |
Recommendation |
Reason |
| Filler Metal (TIG/GTAW) |
ERNiCrMo-3 (AWS A5.14) — Inconel 625 type |
Overalloyed filler compensates for dilution; maintains PREN ≥40 in weld metal |
| Filler Metal (MMA/SMAW) |
ENiCrMo-3 (AWS A5.11) |
Matches ERNiCrMo-3 composition; suitable for field welds |
| Pre-heat Temperature |
None required (ambient) |
Austenitic grades do not require pre-heat; pre-heat can promote sensitisation |
| Max Inter-pass Temperature |
150°C maximum |
Prevents carbide precipitation (sensitisation) and sigma phase formation |
| Post-Weld Heat Treatment |
Not recommended |
PWHT can sensitise 254 SMO at 600–900°C; solution annealing at 1150°C may be required for severe corrosive service |
| Shielding Gas (TIG) |
Pure Argon or Ar+2% N₂ |
N₂ addition compensates nitrogen loss from weld pool; maintains mechanical properties |
| Joint Preparation |
Mechanically cleaned; no iron contamination |
Iron smear from carbon steel tools causes contamination pitting — use stainless steel wire brushes only |
| Post-Weld Passivation |
Pickling + passivation per ASTM A380 |
Restores passive film on HAZ; removes heat tint (chromium-depleted oxide layer) |
Applications of 254 SMO Blind Flanges by Industry
| Industry |
Specific Application |
Why 254 SMO is Specified |
| Flue Gas Desulfurisation (FGD) |
Wet scrubber spray headers, absorber vessel nozzles, slurry recirculation isolation |
FGD condensate: pH 4–5, Cl⁻ up to 20,000 ppm, SO₂ gas — PREN ≥40 required; 316L fails by pitting within weeks |
| Desalination Plants |
MSF evaporator stage isolation, RO high-pressure pump flanges, brine sampling blind connections |
Concentrated brine (Cl⁻ >40,000 ppm) and temperatures to 70°C — only 6Mo and higher grades reliable |
| Offshore & Naval |
Seawater cooling circuit blinding, fire water system test connections, subsea manifold isolation |
Natural seawater (Cl⁻ ~19,000 ppm); stagnant conditions at blind flanges accelerate crevice attack — PREN ≥40 essential |
| Pulp & Paper (Kraft / Bleach) |
Bleaching tower isolation blinds, chlorine dioxide (ClO₂) wash stage, hypochlorite service |
Hot bleach liquor at pH 2–4, 80°C — highly aggressive chlorine environment; 254 SMO is the industry standard |
| Chemical Processing |
Phosphoric acid plant isolation, sulphuric acid (dilute) circuit blinding, HCl scrubber inlet |
Cu addition (0.5–1%) improves reducing acid resistance; Mo+Cr provides oxidising acid resistance |
| Pharmaceutical |
CIP (clean-in-place) hot water circuit isolation, WFI (water for injection) system blinding |
Ultra-low carbon (≤0.020%) avoids sensitisation; high surface finish capability ≤0.4 µm Ra; FDA-compliant material |
| Power Generation |
Cooling tower bypass isolation, condenser seawater inlet blinding, chlorination dosing blind connections |
Once-through seawater cooling with chlorination — combined seawater + chlorine requires PREN ≥43 |
| Food & Beverage |
High-salinity food processing CIP systems, brine circuits in food grade plants |
Hot saline solutions + sanitising agents — 254 SMO provides hygienic surface finish and full corrosion immunity |
Standards and Compliance — 254 SMO Blind Flanges
| Standard / Specification |
Scope |
| ASTM A182 Grade F44 | Forged or rolled alloy and stainless steel pipe flanges — primary material specification for 254 SMO flanges |
| ASTM A240 (S31254) | Heat-resisting chromium and chromium-nickel stainless steel plate — for flanges cut from plate |
| ASME B16.5 | Pipe flanges and flanged fittings, NPS ½ through NPS 24 — dimensional and pressure-temperature rating standard |
| ASME B16.47 | Large diameter steel flanges NPS 26 through NPS 60 — Series A (MSS SP-44) and Series B (API 605) |
| ASME B16.20 | Metallic gaskets for pipe flanges — RTJ ring dimensions and groove tolerances for BLRTJ flanges |
| EN 1092-1 | Flanges and their joints — circular flanges for pipes, fittings and valves (PN-rated European standard) |
| EN 10088-1 | Stainless steels — list of stainless steels; 254 SMO = 1.4547 / X1CrNiMoCuN20-18-7 |
| NACE MR0175 / ISO 15156 | Sour service (H₂S) material requirements — 254 SMO is included in Part 3 (CRAs) for sour service flanges |
| ASTM A262 Practice E | Intergranular corrosion test (Strauss test) — standard test to verify 254 SMO weld HAZ sensitisation resistance |
| ASME B31.3 | Process piping code — pressure design calculations and P-T allowable stress tables for F44 material |
| ASME Boiler & Pressure Vessel Code (VIII Div.1) | Pressure vessel flanges — 254 SMO appears in Section II Part D |
| DNV-OS-F101 / API 6A | Offshore / subsea flanges — supplementary requirements for marine chloride service |
| MSS SP-44 | Steel pipeline flanges (Series A) for larger sizes matching ASME B16.47 Series A |
| ISO 9001:2015 | Quality management system — Tesco Steel & Engineering certification covers all flange manufacturing |
254 SMO Flange Types — Full Range
Tesco Steel manufactures all ASME B16.5 flange types in 254 SMO (UNS S31254 / ASTM A182 F44):
Frequently Asked Questions — 254 SMO Blind Flanges
Technical answers to the most common questions asked by engineers, procurement teams, and AI search engines about 254 SMO (UNS S31254 / F44) blind flanges.
254 SMO is a super-austenitic stainless steel with the composition ~20% Cr, 18% Ni, 6.25% Mo, 0.20% N, and 0.75% Cu. The name stands for Sea water, Molybdenum, and allOy — reflecting its primary application. Official designations: UNS S31254 (ASTM), EN 1.4547, SIS 2378. Its ASTM forging grade for flanges is F44 (ASTM A182). It is widely called a 6Mo stainless steel due to its 6% molybdenum content — the defining characteristic separating it from 316L and 317L.
The PREN formula is: %Cr + 3.3 × %Mo + 16 × %N
254 SMO: 20 + 3.3 × 6.25 + 16 × 0.20 = 43.8
904L: 21 + 3.3 × 4.5 + 16 × 0.10 = 37.5
317L: 18.5 + 3.3 × 3.25 + 16 × 0.10 = 31.2
316L: 17 + 3.3 × 2.25 + 16 × 0.05 = 25.2
The practical result: 254 SMO has a Critical Pitting Temperature (CPT) exceeding 60°C in 6% FeCl₃, versus ~15°C for 316L. In hot seawater and FGD scrubber condensate, 316L fails by pitting within weeks while 254 SMO performs reliably for decades.
254 SMO blind flanges are manufactured per ASTM A182 Grade F44 (Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves for High-Temperature Service). Dimensional requirements follow ASME B16.5 for NPS ½″–24″ and ASME B16.47 for larger sizes. Pressure-temperature ratings are governed by ASME B16.5 Material Group 2.3 P-T tables.
The key differences:
Mo content: 254 SMO 6.25% vs 316L 2.25% — PREN 43.8 vs 25.2
N content: 254 SMO 0.20% (adds yield strength) vs 316L 0.05%
Yield strength: 254 SMO 300 MPa vs 316L 205 MPa
CPT in FeCl₃: 254 SMO >60°C vs 316L ~15°C
Chloride limit: 254 SMO handles >20,000 ppm Cl⁻ at 60°C; 316L fails above ~500 ppm at 60°C
Cost: 254 SMO flanges approximately 3–5× more expensive than 316L
Use 316L for mild process conditions; use 254 SMO when chloride pitting, crevice corrosion, or SCC is a documented risk.
The European equivalent is EN 1.4547, full designation X1CrNiMoCuN20-18-7 per EN 10088-1. Additional international equivalents: SIS 2378 (Swedish — country of origin), AFNOR Z1 CNDU 20.18.06Az (French standard). The alloy is produced to EN 10272 (bar), EN 10088-4 (plate), EN 10216-5 (tube). For flanges, the equivalent European production specification is EN 10222-5 (steel forgings for pressure vessels).
Per ASME B31.3 (Material Group 2.3), the maximum design temperature for 254 SMO (F44) is 454°C (850°F). Minimum temperature is −196°C (cryogenic — austenitic grades remain ductile to liquid nitrogen temperature). However, in practice for chloride environments: above 120°C in high-chloride service, duplex or Hastelloy grades may be specified to avoid sigma phase formation or SCC. For pure non-chloride high-temperature service (e.g., sulphuric acid), 254 SMO can be used up to the ASME maximum.
Yes — 254 SMO is specifically designed for seawater service. Its PREN of ≈43.8 exceeds the industry-accepted threshold of PREN ≥ 40 for seawater service. It is resistant to natural seawater at ambient temperature and to chlorinated seawater (used for biofouling control in once-through cooling systems). Blind flanges in seawater piping are particularly vulnerable to crevice corrosion under the gasket and at the flange face — 254 SMO's high Mo+Cr+N content specifically addresses this. Typical seawater applications: desalination plant isolation, offshore seawater lift pump blinding, naval condenser shell-side isolation.
Recommended filler:
ERNiCrMo-3 (AWS A5.14) — commonly known as Inconel 625 filler wire. This overalloyed filler (≈22% Cr, 9% Mo) compensates for dilution and ensures the weld metal PREN remains ≥40. For SMAW:
ENiCrMo-3 (AWS A5.11). Key constraints:
- Max inter-pass temperature: 150°C
- No pre-heat required
- No PWHT recommended (risk of sensitisation)
- Shielding gas: pure Ar or Ar + 2% N₂ to compensate N loss from weld pool
- Post-weld: pickle + passivate per ASTM A380
Per ASTM A182 Grade F44:
Tensile Strength (UTS): 650 MPa (94 ksi) minimum
Yield Strength (0.2% PS): 300 MPa (44 ksi) minimum
Elongation: 35% minimum
Hardness: 50 HRB max (≈260 HV)
By comparison, 316L (A182 F316L) has UTS 515 MPa and Yield 205 MPa. The higher yield strength of 254 SMO (driven by nitrogen solid-solution strengthening) means that in some pressure classes, a thinner/lighter flange may be used while still meeting ASME B16.5 pressure ratings — partially offsetting the higher material cost.
Choose
254 SMO (PREN ≈43.8) over
904L (PREN ≈37.5) when:
- Chloride concentration exceeds 5,000 ppm at temperatures above 40°C
- Service involves chlorinated seawater or hypochlorite (bleach plant)
- CPT requirement exceeds 35°C (the practical limit of 904L)
- The process contains both chlorides and oxidising acids (e.g., HNO₃/Cl⁻ mixture)
- FGD scrubber condensate or wet SO₂ combined with chlorides
904L outperforms 254 SMO in: very high concentrations of
reducing sulphuric acid (due to its higher Ni content — 24–26% vs 18%). For fluoride or very concentrated HCl, neither grade is adequate — Hastelloy C-276 (PREN ≈70) is required.
