Tesco Steel & Engineering forges ASTM A182 F51 blind flanges — duplex 2205, UNS S31803, the balanced austenite-ferrite stainless of the offshore world — to ASME B16.5 Class 150–2500 from ½″ to 24″ NB and to ASME B16.47 up to 60″. The dual structure delivers twice the yield strength of 316L and — decisively — resistance to chloride stress-corrosion cracking, the failure that retires austenitic stainless from warm seawater and brine. PREN ≥34, solution annealed and water quenched, with F60 (S32205) dual-certification, ASTM A923 and NACE MR0175 to order. Hotter, saltier duty steps to super duplex F53. Facings: BLRF, BLFF or BLRTJ. Every lot with EN 10204 3.1/3.2 MTC. ISO 9001:2015, made in Mumbai, India — exported to 50+ countries.
ASTM A182 F51 · UNS S31803 · 2205Duplex 22Cr-5Ni-3Mo-N · EN 1.4462Yield ≥450 MPa · 2× 316LChloride-SCC Resistant · PREN ≥34ASME B16.5 ½″–24″ · Class 150–2500RF · FF · RTJ FacingsEN 10204 3.1 / 3.2 · ISO 9001:2015
ASTM A182 F51 Blind Flange — Specifications at a Glance
What is an ASTM A182 F51 Blind Flange?
The offshore closure. A solid forged disc of duplex stainless steel 2205 (UNS S31803) with no bore, closing pipes, valves and nozzles in chloride-bearing service. The half-austenite, half-ferrite structure delivers yield ≥450 MPa — twice 316L — and immunity to chloride stress-corrosion cracking, with PREN ≥34 against pitting. Service band roughly -50 °C to +300 °C; solution annealed and water quenched, hardness ≤290 HB. Dimensions per ASME B16.5 / B16.47 in RF, FF and RTJ facings; EN 10204 3.1 certified.
Also searched as: duplex blind flange, 2205 blind flange, S31803 blind, SA-182 F51 blind (the ASME boiler-code twin), 1.4462 / X2CrNiMoN22-5-3 blind (the EN equivalent), F51/F60 dual-cert blind — all refer to the product on this page. See also the duplex steel hub, the F51 slip on flanges, siblings F60 / F53 blind flanges, and the facing pages BLRF / BLFF / BLRTJ.
The Duplex Idea — Two Phases, One Steel
Ordinary stainless picks a side: austenitic (316L — tough, weldable, but soft and crackable in hot chlorides) or ferritic (strong, crack-resistant, but brittle-prone and hard to weld). Duplex refuses to choose — the chemistry is balanced so the steel solidifies roughly half-and-half:
The Austenite Brings Toughness
Impact resistance, ductility and general corrosion performance — the qualities that make the flange forgiving to install and reliable in service.
The Ferrite Brings Strength — and Blocks Cracks
Yield 450 MPa minimum, more than double 316L — and chloride stress-corrosion cracks that race through austenitic metal simply stop at the ferrite.
Nitrogen Sharpens Both
The deliberate 0.08–0.20% nitrogen strengthens the austenite and lifts pitting resistance — it is a required alloying element in duplex, not a residual.
F51 vs 316L — Why Systems Convert
Property
F51 Duplex (this page)
316L Austenitic
Yield Strength
≥450 MPa
~170 MPa
PREN (pitting index)
~34–38
mid-20s
Chloride SCC
Resistant — ferrite blocks crack propagation
Susceptible from ~60 °C in chlorides
Service Band
-50 °C to ~300 °C
Cryogenic to ~500 °C+
Where It Wins
Warm seawater, brine, offshore
Cryogenics, hygiene, high heat
The pattern: where warm chlorides have cracked or pitted a 316L system, the duplex replacement is the engineered fix — and the wider temperature tolerance of austenitic keeps 316L in charge everywhere else. The two families divide the stainless world by chloride exposure.
The low carbon guards weldability and intergranular resistance; the chromium-molybdenum-nitrogen trio sets the PREN; the modest nickel is what keeps duplex economical against high-nickel austenitics when nickel prices run. PREN = %Cr + 3.3×%Mo + 16×%N ≥ 34.
Mechanical Properties & The Temperature Band
Property
ASTM A182 F51 Requirement
Tensile Strength
≥ 620 MPa (90 ksi)
Yield Strength (0.2%)
≥ 450 MPa (65 ksi)
Elongation
≥ 25%
Hardness
≤ 290 HB (NACE sour service: ≤28 HRC)
Condition
Solution annealed + water quenched — mandatory
Service Band
~ -50 °C to +300 °C — capped by 475 °C embrittlement of the ferrite
Duplex lives in a band. Hold it around 475 °C and the ferrite embrittles; soak it at 600–1000 °C and sigma phase guts toughness and corrosion resistance in minutes. That is why codes cap duplex near 300 °C, why the solution anneal + water quench is non-negotiable, and why no duplex flange should ever be field-heated, flame-cut or welded casually. Inside its band, nothing at the price touches it in chlorides.
ASTM A182 F51 Blind Flanges are available in the following specifications:
Material Grade
ASTM A182 F51 (UNS S31803, alloy 2205); F60/S32205 dual-certification; ASME SA-182; EN 1.4462 to order
Heat Treatment
Solution annealed + water quenched — mandatory; ASTM A923 intermetallic testing to order
Size
1/2"NB to 24"NB per ASME B16.5; 26"–60" per ASME B16.47; EN 1092-1 to order
Class
150#, 300#, 400#, 600#, 900#, 1500#, 2500#
Facings
Raised Face (BLRF), Flat Face (BLFF), Ring Type Joint (BLRTJ) with groove per B16.20
Corrosion Duty
PREN ≥34 — chloride pitting, crevice & SCC resistance; sour service per NACE MR0175 / ISO 15156
Surface Finish
Pickled & passivated or natural machined — no paint; segregated stainless handling throughout
Modifications
NPT-tapped centre (hydrotest / vent / drain / gauge), lifting provisions — machined & certified at the works
Testing
Tension, chemistry incl. nitrogen & hardness per heat; PMI routine; A923 / corrosion tests where specified
Certification
EN 10204 3.1 (standard) / 3.2 witnessed
Why F51 Blind Flanges Are Specified
The Chloride-SCC Fix
Warm seawater and brine crack austenitic stainless; the duplex ferrite stops the cracks. Systems that failed on 316L run for decades on 2205 — and their closures are F51 blinds.
Twice the Yield of 316L
450 MPa minimum yield brings robustness, galling resistance and stronger B16.5 ratings — the flange shrugs off site handling that marks softer stainless.
The Offshore Standard
Produced water, seawater lift, firewater rings, desalination trains — duplex 2205 is the default material class, and the blind follows the line.
Economical Alloying
Five percent nickel, not thirty — duplex delivers super-austenitic-adjacent chloride performance while staying on the affordable side of the nickel market.
Three details buyers miss.Mind the band: duplex is not for hot service — specify it above ~300 °C and the ferrite embrittles regardless of how good the certificate is; hot duty belongs to other families. Say dual-cert if you need it: many line classes now read S32205/F60 — we certify F51/F60 dual where the melt qualifies, but only if the enquiry asks. Sour service changes the hardness cap: NACE MR0175 holds duplex to 28 HRC, tighter than A182's 290 HB — state the NACE requirement up front so testing and certification align.
How Our F51 Blind Flanges Are Manufactured
1
Forging — cut billet of the spectro-verified S31803 heat — nitrogen included in the analysis — is hot-forged into the solid disc blank with full heat traceability and segregated stainless handling.
2
Solution anneal + water quench — above ~1020 °C and straight into water, racing the forging through the sigma-phase window; furnace charts retained against the heat number.
3
Machining & facing — faces, OD and drilling to ASME B16.5 / B16.47; RF serrations, flat face or RTJ groove gauged per B16.20; NPT taps machined where ordered.
4
Testing — tension test, full chemistry and hardness against the heat; PMI on every piece; ASTM A923 and corrosion testing where specified.
5
Pickling & passivation — the restored chromium-oxide skin, with marking of standard, grade (and dual-cert where applicable), size, class and heat number.
6
Certification & packing — EN 10204 3.1 MTC (3.2 witnessed on request); faces protected, palletised or crated sea-worthy — no carbon-steel contact in transit.
Where F51 Blind Flanges Are Used
Wherever chlorides rule: offshore platforms and FPSOs (produced water, seawater lift, firewater), desalination plants, coastal and marine piping, chemical tankers, brine and hypochlorite systems, and chloride-bearing process duty. Production below:
Large Stainless Steel Blind Flange — Machined at Our WorksStainless Steel Flanges — Crated for Dispatch
F51 Blind Flange Dimensions
Blind dimensions are class-governed — identical across material grades. Full charts:
An ASTM A182 F51 blind flange is a solid forged disc of duplex stainless steel — alloy 2205, UNS S31803 — drilled to the standard bolt pattern with no bore, used to close pipes, valves and vessel nozzles in chloride-bearing service: offshore oil and gas, seawater, brine and aggressive process duty. Duplex means a roughly half-and-half austenite-ferrite structure, which delivers twice the yield strength of 316L together with resistance to the chloride stress-corrosion cracking that retires ordinary stainless. Dimensions follow ASME B16.5 from ½ inch to 24 inch and ASME B16.47 beyond, in raised face, flat face and ring joint facings.
What is duplex stainless steel?
A stainless family engineered to solidify as roughly 50% austenite and 50% ferrite — two phases sharing one microstructure, each covering the other's weakness. The austenite brings toughness and general corrosion resistance; the ferrite brings strength and immunity to chloride stress-corrosion cracking. F51 is the forged-flange form of the defining grade, 2205: 22% chromium, 5% nickel, 3% molybdenum, plus deliberately added nitrogen that strengthens the austenite and sharpens pitting resistance. The balance is set by chemistry and locked in by solution annealing and a water quench.
What does duplex buy over 316L?
Two headline advantages. Strength: yield 450 MPa minimum against roughly 170 for 316L — more than double — which matters for robustness, galling resistance and the higher pressure-temperature ratings duplex earns in the B16.5 tables. Corrosion: a PREN around 34-38 versus the mid-20s, better pitting and crevice performance in chlorides, and — decisively — resistance to chloride stress-corrosion cracking. Where warm seawater or brine has cracked a 316L system, the duplex replacement is not an upgrade, it is the fix.
What is chloride stress-corrosion cracking, and why does duplex resist it?
The quiet killer of austenitic stainless: tensile stress plus chlorides plus warmth (roughly 60°C upward) drives fine cracks through 304L and 316L with almost no visible corrosion first. The ferrite phase in duplex blocks those cracks — the half-ferritic structure simply does not propagate them the way fully austenitic metal does. That single property built the duplex market: hot brines, seawater systems and insulated coastal piping that cracked austenitic grades run for decades on 2205.
What is PREN?
The Pitting Resistance Equivalent Number — a chemistry-based index (%Cr + 3.3×%Mo + 16×%N) that ranks resistance to chloride pitting. F51 scores roughly 34-38: comfortably above 316L's mid-20s, below super duplex's 40-plus. Specifications for seawater and brine duty often set PREN floors — F51's ≥34 is the standard duplex tier, and where the specification demands ≥40, the answer is super duplex F53/F55, one step up the same ladder.
What is the difference between F51 and F60?
F60 is UNS S32205 — the modern, tighter-boxed edition of the same 2205 alloy, with nitrogen and molybdenum held toward the top of the S31803 range for more consistent corrosion performance. Most current production melts comfortably inside both windows, so dual-certified F51/F60 forgings are common, and specifications increasingly name S32205 outright. If your line class says F60 or dual-cert, say so on the enquiry — our F60 blind flange page covers that designation; the product family is one and the same.
When is super duplex F53 needed instead?
When chlorides get hotter, saltier or less forgiving: hot seawater, high-chloride produced water, aggressive brines. Super duplex — F53 (S32750) and F55 (S32760) — raises PREN past 40 and yield to 550 MPa, buying that margin at higher cost and stricter processing. F51 remains the volume duplex of offshore and desalination; the specification's PREN floor and service temperature decide when to step up. Our F53 blind flange page covers the step.
What are the mechanical properties of F51 blind flanges?
Tensile strength 620 MPa (90 ksi) minimum, yield strength 450 MPa (65 ksi) minimum, elongation 25% minimum, hardness 290 HB maximum — in the mandatory solution-annealed and water-quenched condition. For sour service, NACE MR0175 caps duplex hardness lower (28 HRC); state the requirement and the certificate follows. Every heat is tension-tested and chemistry-verified including nitrogen, with results on the EN 10204 3.1 certificate.
What temperature range does an F51 blind flange cover?
Duplex lives in a band: roughly -50°C to +300°C. The upper limit is metallurgical — hold duplex around 475°C and the ferrite embrittles ('475°C embrittlement'), so codes cap continuous service near 300-315°C. The lower limit reflects the ferrite's toughness transition, with -50°C the usual practical floor. Inside that band F51 is hard to beat in chlorides; outside it the specification moves to austenitic stainless (cold) or other families (hot).
Why must duplex be solution annealed and water quenched?
Because between about 600 and 1000°C duplex precipitates sigma phase and related intermetallics — compounds that gut both toughness and corrosion resistance in minutes. Solution annealing above roughly 1020°C dissolves them; the water quench races the forging through the danger zone before they can re-form. A slowly cooled duplex flange can pass a tension test and still fail in service, which is why the heat-treatment record and (where specified) ASTM A923 testing matter. Ours ship annealed, quenched and documented.
What facings are F51 blind flanges made with?
All three: raised face (BLRF) — the default of offshore process piping — flat face (BLFF) for mating GRP and lined systems, common in seawater service, and ring type joint (BLRTJ) for high-pressure duty, the ring chosen softer than the groove. Each facing has its own page on this site. The forging is common to all three; the mating flange governs the choice, and for RTJ the ring number follows from size and class.
Can F51 blind flanges be tapped for testing or instruments?
Yes — fill, vent, drain, gauge and injection taps are machined into the centre of the blind at the works and certified with the flange. Duplex machines harder than carbon steel and work-hardens, so field-drilling is even less advisable than usual; the works tap arrives ready to plug, with the heat treatment undisturbed. State the NPT size on the enquiry.
Where are F51 blind flanges used?
Wherever chlorides rule: offshore platforms and FPSOs — produced water, seawater lift, firewater rings — desalination plants, coastal and marine piping, chemical tankers, brine and hypochlorite systems, and chloride-bearing process duty in chemical plants. On those systems F51 blinds serve the usual closure duties — isolations, future connections, tapped test blinds — in the line's own duplex, keeping corrosion behaviour and certificates uniform across the joint.
Which standards cover F51 blind flanges?
The material standard is ASTM A182 grade F51 (UNS S31803), with ASME SA-182 as its boiler-code twin and EN 1.4462 / X2CrNiMoN22-5-3 as the European equivalent; dimensions follow ASME B16.5 for ½"-24" and ASME B16.47 for 26"-60". ASTM A923 covers intermetallic-phase testing where specified, and NACE MR0175 / ISO 15156 governs sour service. A complete specification reads: 'Blind Flange, RF, 6", ASME B16.5 Class 300, ASTM A182 F51' — and our EN 10204 certificates state dimension standard, grade, heat treatment and test results together.
Who manufactures ASTM A182 F51 blind flanges in India?
Tesco Steel & Engineering is an ISO 9001:2015 certified flange manufacturer based in Mumbai, India, forging ASTM A182 F51 / UNS S31803 duplex blind flanges from ½" to 24" NB per ASME B16.5 in Classes 150-2500 and to 60" per ASME B16.47 — solution annealed and water quenched, PMI-verified, with F60 dual-certification, ASTM A923 testing and NACE MR0175 compliance to order, certified to EN 10204 3.1/3.2 on every lot — in raised face, flat face and ring joint facings, alongside super duplex F53/F55, stainless, carbon and nickel-alloy blinds. Exported to more than 50 countries.
