Tesco Steel & Engineering manufactures ASTM A182 F91 lap joint flanges — the chrome-moly staircase's creep-grade summit (UNS K91560, W.Nr. 1.4903): the vanadium-microalloyed CSEF for advanced power-plant steam, whose V-Nb-N carbonitrides carry strength near 600 °C that the plain rungs cannot approach — held in place by a mandatory narrow normalise-and-temper window: in this grade, the heat-treatment paperwork is the product. The lap joint pattern's gift reaches its maximum here: F91's weld rules are the ferritic world's strictest — they fill code cases — and the backing ring never welds, escaping all of them; the joint's single butt weld belongs to the matched A234 WP91 stub end per B16.9. The lap joint bench's metal-temperature argument holds at the summit — the ring runs at line metal temperature — and the grade line is absolute: F91 is never interchangeable with F9, the plain 9-chrome. Per ASME B16.5, ½″–24″ NB, Classes 150–2500, hardness ≤248 HBW reported per heat. Every lot with EN 10204 3.1/3.2 MTC. ISO 9001:2015, made in Mumbai, India — exported to 50+ countries.
ASTM A182 F91 · UNS K91560 · W.Nr. 1.49039Cr-1Mo-V — The Creep-Grade SummitPower-Plant Steam Near 600 °CThe Biggest Weld-Procedure Dodge of AllStub Ends: A234 WP91 per B16.9½″–24″ NB · Classes 150–2500Hardness ≤248 HBW — Per HeatEN 10204 3.1 / 3.2 · ISO 9001:2015
ASTM A182 F91 Lap Joint Flanges — Specifications at a Glance
What is an ASTM A182 F91 Lap Joint Flange?
The summit's backing flange. An ASTM A182 F91 lap joint flange is the loose backing ring of the two-piece system in 9Cr-1Mo-V (UNS K91560) — the creep-strength-enhanced ferritic whose V-Nb-N carbonitrides carry advanced steam near 600 °C, in a mandatory normalise-and-temper condition whose paperwork is the product. The point on this pattern, at maximum strength: F91's weld rules fill code cases — and the ring never welds, escaping preheat, PWHT and the code-case fine print entirely; the single butt weld belongs to the stub end. Per ASME B16.5, ½″–24″ NB, Classes 150–2500, hardness ≤248 HBW per heat; stub ends A234 WP91 per B16.9. EN 10204 3.1 with furnace records on every lot.
Also searched as: F91 backing flange, Grade 91 lap joint flange, 9Cr-1Mo-V lap joint flange, CSEF lap joint flange, A182 F91 LJ flange — all this page's product. Related pages: the lap joint hub (the pattern's full story), the mission twin — F9, the plain 9-chrome — the staircase below — F22 and F11 — the F91 bench — weld neck / blind / long weld neck — and the alloy steel overview.
F91 — the ring runs at line metal temperature, where only the CSEF's carbonitrides hold
The argument at the summit: the heat reaches the never-wetted ring through the metal it clamps, and near 600 °C every leaner column in the allowable-stress tables has ended — the V-Nb-N carbonitrides are what remains. The ring can skip the welding; it cannot skip the temperature class.
The corrosion summit of the plain rungs — chromium doubled, silicon reinforcing the scale
F91 Lap Joint (this page)
9% + V-Nb-N
The creep-grade summit — the CSEF's carbonitrides for advanced power-plant steam near 600 °C
The summit's honest advice is directional: sulphur-driven refinery duty belongs to F9 no matter how hot it runs; steam duty past F22's ratings climbs here — and duty gentler than the summit descends and saves. Descending honestly is engineering too.
What the Pattern Gives This Grade
The Biggest Dodge of All
Grade 91 welding fills code cases — preheat, hydrogen control, and a PWHT window tight enough to police — and the backing ring never welds: no procedure, no code case, no reopened paperwork, ever.
The Carbonitride Purchase
V 0.18–0.25, Nb 0.06–0.10, N 0.030–0.070 — the trio that builds the strengthening carbonitrides and separates the CSEF from plain 9-chrome in every allowable-stress table that matters.
The Paperwork Is the Product
Mandatory narrow normalise-and-temper, hardness ≤248 HBW reported per heat — low readings warn of surrendered creep strength, high readings of untempered martensite; the certificate carries both the value and the furnace records.
The Outage Franchise
Loose flanges align by rotation — on steam-plant outages, bolt circles line up without pipe-twisting, spools swap without a Grade 91 weld being made or monitored, and re-assembly never reopens a code case.
Specification Notes — Getting F91 Lap Joints Right
Three honest notes.F91 is never F9: the plain 9-chrome (UNS K90941) and the CSEF diverge in every detail that matters — V-Nb-N, carbon band, Mo window, P/S ceilings — and our PMI reads the V-Nb signature on every lot; specifications naming either are supplied with that grade exactly. The heat treatment cannot be negotiated: there is no annealed F91 — material outside the normalise-and-temper window is not F91 in anything but name, which is why furnace records travel with every certificate. And the pattern's boundaries apply in any grade: heavy external loads, severe vibration and high-moment positions read the weld neck bench — main-steam joints are overwhelmingly weld necks, and lap joints earn their place on the circuits that dismantle; the hub page states the boundaries fully.
How Our F91 Lap Joint Flanges Are Manufactured
1
Material — certified A182 F91 forgings, normalised and tempered inside the grade's mandatory window, furnace records retained and supplied; heat-number transfer at the first operation.
2
Machining — B16.5 lap joint dimensions: OD, thickness, bolt circle to class; bore and shoulder turned to suit the stub end's lap.
3
Shoulder dressing — the radius dressed to bear evenly on the back of the lap — spreading bolt load instead of concentrating it.
4
Testing — hardness verified per heat against the 248 HBW ceiling — the microstructure's proxy; PMI reads the V-Nb signature that separates F91 from F9; chemistry and mechanicals certified per heat.
5
No welding, ever — the ring's whole career is mechanical: no weld, no preheat, no PWHT, no code case — the pattern's gift at its maximum on the ferritic world's strictest welding grade.
6
Pairing & certification — matched to A234 WP91 stub ends where ordered; one EN 10204 3.1 document per pair, hardness and furnace records included (3.2 witnessed on request); packed sea-worthy with laps and shoulders protected.
Where F91 Lap Joint Flanges Are Used
Where advanced steam and the pattern meet: supercritical and ultra-supercritical power plants — main-steam and hot-reheat piping, headers and turbine connections — heat-recovery steam generators behind gas turbines, and energy-from-waste plants running Grade 91 circuits — specifically the positions that earn a lap joint: turbine and valve connections that dismantle at every major outage, spool pieces swapped on inspection schedules, and bolt circles that must align across thermal cycles, where a never-welded flange means re-assembly without a single Grade 91 weld being made, monitored or documented. Production and supply below:
F91 Alloy Steel Supply — Export Packing at Our WorksThe Two-Piece System — Flange, Stub End, Butt Weld, PipeF91 at Class 2500 — The Summit's Heavy End
F91 Lap Joint Flange Dimensions
Flange dimensions are class-governed per ASME B16.5 — identical in every grade; stub ends per B16.9. Full class-by-class charts:
Example: “Lap Joint Flange, 6″ NB, ASME B16.5 Class 900, ASTM A182 F91, with A234 WP91 stub ends Sch 120 (B16.9 Type A, supplied together), hardness and N&T records on MTC, pairs on one document, EN 10204 3.1 — 6 sets.” Quotations normally within 24 hours with price, unit weights (both parts) and delivery.
F91 Lap Joint Flanges — Frequently Asked Questions
What is an ASTM A182 F91 lap joint flange?
An ASTM A182 F91 lap joint flange is the loose backing flange of the two-piece lap joint system, forged in 9Cr-1Mo-V (UNS K91560, W.Nr. 1.4903) — the vanadium-microalloyed creep-strength-enhanced ferritic, CSEF, that sits at the summit of the chrome-moly staircase and carries advanced power-plant steam at temperatures the plain rungs cannot hold. Like every lap joint flange it slides over the pipe, sits free behind the stub end's flared lap, and carries the entire bolt load without touching the line fluid. The pattern's gift reaches its maximum on this grade: F91's welding rules are the most unforgiving on the site — they fill code cases — and the backing ring never welds, escaping all of them; the joint's single butt weld belongs to the stub end. Manufactured per ASME B16.5 in ½" to 24" NB, Classes 150 to 2500, in the mandatory normalised-and-tempered condition, with EN 10204 3.1/3.2 certification carrying hardness and furnace records on every lot.
How does the two-piece lap joint system work?
By splitting sealing from bolting. The stub end butt-welds to the pipe: its flared lap provides the gasket face and does all the fluid contact. The backing flange never welds and never wets: it slides on before the stub end is welded — the classic site lesson — spins freely until the bolts enter, and transmits bolt load through its radiused shoulder onto the back of the lap. Two consequences follow on this grade. First, the flange's material follows its own duty — here temperature, which is why the ring is F91 rather than anything leaner on advanced steam circuits. Second, and at its sharpest anywhere on this bench: the flange never welds, so it never enters the weld procedure — no preheat, no PWHT, no code-case fine print on the ring itself; the joint's single butt weld belongs to the stub end, made once under the strictest welding regime in the ferritic world. The full pattern story — stub end types, the alloy economy, the honest boundaries — is on the lap joint hub page.
Why must the never-wetted backing flange still be F91 — the metal-temperature argument?
Because the backing flange gets hot even though it never gets wet — the argument that runs the whole lap joint bench, here at its summit. The heat reaches the ring through the metal it clamps: bolted hard against the lap of a main-steam or hot-reheat line running near 600°C, the flange sits at essentially line metal temperature — territory where carbon steel retired long ago, the low rungs' allowable-stress columns have ended, and even plain 9-chrome's creep ratings cannot hold the wall thicknesses economical. F91's V-Nb-N carbonitrides keep the ring's strength over decades at exactly those temperatures. The ring can skip the welding; it cannot skip the temperature class. F91 backing F91 is the normal answer, and the certificate — with its hardness value and furnace records — proves both halves.
What is the chemical composition of ASTM A182 F91?
Carbon 0.08-0.12%, manganese 0.30-0.60%, silicon 0.20-0.50%, chromium 8.0-9.5%, molybdenum 0.85-1.05%, vanadium 0.18-0.25%, niobium 0.06-0.10%, nitrogen 0.030-0.070%, nickel ≤0.40%, phosphorus ≤0.020%, sulphur ≤0.010%. Everything about it is deliberate: the V-Nb-N trio builds the strengthening carbonitrides, the narrow carbon band balances hardenability against weldability, the nickel cap protects the tempering response, and the tight P/S ceilings protect long-term creep ductility. Aluminium, titanium and zirconium are additionally restricted because they steal the nitrogen the vanadium needs. Chemistry is verified per heat — our PMI reads the V-Nb signature that separates F91 from F9 in seconds — and travels on the EN 10204 3.1 MTC.
What are the mechanical properties of F91 lap joint flanges?
In the mandatory normalised-and-tempered condition A182 requires tensile strength 585 MPa (85 ksi) minimum, yield strength 415 MPa (60 ksi) minimum, elongation 20% minimum, and hardness not exceeding 248 HBW — the strongest certificate on the chrome-moly ladder, with a yield floor a full step above F9's 380. As everywhere on the ladder, the room-temperature numbers understate the purchase: F91 is bought for creep-rupture ratings near 600°C, where it holds allowable stresses that let designers halve wall thicknesses against the plain grades. For the backing-flange duty the strength arrives as margin — the ring works in bolting and bending at line metal temperature, and the B16.5 rating tables for the grade's material group carry it further than any other ferritic on this site. Supplied with hardness reported per heat beside the furnace records, because in this grade the paperwork is the product.
Why is F91's heat treatment mandatory — and why does hardness matter so much?
Because the microstructure is the product. F91's creep strength lives in a precise tempered-martensite structure decorated with V-Nb carbonitrides, created by a normalise-and-temper sequence run inside a narrow window — there is no annealed option in A182 for this grade, and material that misses the window is not F91 in anything but name. Hardness is the field's quickest proxy for whether the structure is right: readings toward the 248 HBW ceiling warn of untempered martensite and cracking risk, while low readings warn of over-tempering and quietly surrendered creep strength — the failure you discover years later. That is why our certificates report hardness per heat alongside the normalise-and-temper furnace records, and why a lap joint order's two halves — flange and stub end — both carry them. Buy the paperwork with the metal; in the F91 world they are the same thing.
What is the difference between F91 and F9?
Same 9% chromium, different centuries — and never interchangeable. F9 (UNS K90941) is the classic plain 9Cr-1Mo: a corrosion purchase for hot sour refinery streams, heat treated by ordinary anneal or N&T, welded with standard chrome-moly care. F91 (UNS K91560) is the vanadium-microalloyed CSEF: a creep purchase for advanced steam, with a mandatory narrow normalise-and-temper window and unforgiving weld rules. Their chemistries diverge in every detail that matters: F91 adds V, Nb and N, narrows carbon to 0.08-0.12%, trims Mo to 0.85-1.05, and tightens P/S to 0.020/0.010. On a lap joint the choice follows the line: refinery sulphur circuits specify F9 with WP9 stub ends — that page is on this site — while power and energy-recovery steam specifies F91 with WP91. A specification naming either must be supplied with that grade exactly; our certificates and PMI keep the two apart.
What stub ends pair with F91 lap joint flanges?
The matching stub end is wrought 9Cr-1Mo-V per ASME B16.9 in ASTM A234 WP91 — the CSEF fitting grade that pairs with F91 forgings, sharing the V-Nb-N chemistry and the mandatory heat-treatment philosophy, and welded to the pipe with the line's qualified Grade 91 procedure under the full code-case regime. That butt weld is the joint's only weld, and it belongs entirely to the stub end: the backing flange never enters the procedure — which on this grade means never entering the most demanding welding regime in the ferritic world. The matching rules are the pattern's usual dimensional ones — lap OD to the flange's shoulder, schedule to the line bore — plus this grade's documentary one, at full strength: flange and stub end bought as a matched pair and certified together on one EN 10204 document, hardness values and furnace records on both sides. Where an order needs B16.9 Type A or Type B lap details, state it and the pair is machined to suit.
Does an F91 backing flange need preheat and PWHT?
No — and this is the biggest weld-procedure dodge on the site. Welding Grade 91 is a discipline unto itself: preheat and interpass control, hydrogen-controlled consumables, and a post-weld heat treatment whose temperature window is tight enough that the codes police it with dedicated cases — run it too cool and the weld zone stays brittle, too hot and the joint's creep strength is quietly destroyed. Fabricators plan F91 welds the way they plan lifts. The lap joint backing flange inherits none of it, because it is never welded to anything: it slides over the pipe, floats behind the lap, and does its whole job mechanically. The discipline concentrates on the joint's single butt weld — stub end to pipe, WP91 to the line, made once under full control. On a steam-plant outage this matters twice: one fewer Grade 91 weld to make, monitor and document, and a flange that can be swapped or rotated without any code case being reopened. The dodge covers the flange, not the joint — the stub end's weld takes the full regime, always.
Where does F91 sit on the chrome-moly staircase?
At the summit — but facing a different direction. The staircase climbs by chromium: F11 at 1¼% and F22 at 2¼% buy creep strength for steam and hydrogen; F21 at 3% steps deeper into the Nelson curves; F5 at 4-6% and F9 at 8-9.5% buy corrosion resistance for the refinery's hot sulphur. F91 shares F9's chromium but turns away from the corrosion road entirely: its vanadium, niobium and nitrogen build a creep strength the plain rungs cannot approach, bought for advanced power-plant steam near 600°C rather than for sour hydrocarbons. Every rung has a lap joint page on this site, and the summit's honest advice is directional: sulphur-driven refinery duty belongs to F9 no matter how hot it runs, while steam duty past F22's ratings climbs to F91 — and duty gentler than the summit descends and saves, because descending honestly is engineering too. State the service and the rung picks itself.
What sizes and pressure classes do F91 lap joint flanges come in?
½" to 24" NB per ASME B16.5 in Classes 150, 300, 400, 600, 900, 1500 and 2500, with larger diameters machined to B16.47 or drawing — the full class-by-class lap joint dimension charts are on this site. The order book mirrors the grade's habitat: main-steam and hot-reheat piping runs Classes 900 to 2500 where advanced steam pressure meets its temperature, with Classes 300 to 600 appearing on bypass, drain and auxiliary circuits in the same units. Dimensionally an F91 lap joint flange is identical to any other grade's — B16.5 governs by class, not by material — so the charts serve every grade on the bench; what changes is the rating table behind the class, where F91's material group carries usable pressures further into the temperature range than any other ferritic here. State size, class and the stub-end question together, and our quotation returns price, unit weight and delivery per class.
How much does an F91 lap joint flange weigh?
Exactly what the same-size lap joint flange weighs in any other steel — B16.5 fixes the dimensions by class, and alloy density differences are negligible. Representative Class 150 figures for the flange alone: 1" roughly 0.8 kg, 2" roughly 2 kg, 4" roughly 5.5 kg, 8" roughly 14 kg, climbing steeply with class; the A234 WP91 stub end adds its schedule-dependent mass. The commercial difference sits in the metallurgy and the paperwork more than anywhere else on this bench: F91 carries the CSEF alloy surcharge, the mandatory narrow heat treatment, per-heat hardness reporting and the documentation burden in its price — and at system level the grade often pays for itself, because its creep strength lets designers run thinner walls than the plain rungs would need. Our quotations state unit and consignment weights per line item, flange and stub end separately, and the site's flange weight chart carries the full reference tables.
How are your F91 lap joint flanges manufactured?
As the pattern's standard sequence with the summit's discipline at the heat-treatment step. Certified A182 F91 forgings are normalised and tempered inside the grade's mandatory window — the furnace records retained and supplied, because in this grade the paperwork is the product — then machined to B16.5's lap joint dimensions: OD, thickness and bolt circle to class, bore and shoulder turned to suit the stub end's lap, and the shoulder radius dressed to bear evenly on the back of the lap. Hardness is verified per heat against the 248 HBW ceiling as the microstructure's proxy; chemistry is certified per heat with PMI reading the V-Nb signature that separates F91 from F9. Heat-number marking transfers at the first operation. No welding is performed on the flange at any stage — the ring's whole career is mechanical, which on the grade with the ferritic world's strictest weld rules is precisely the point. Where the order includes A234 WP91 stub ends, the pair is dimensionally matched and certified together on one EN 10204 3.1 document (3.2 witnessed on request). Packed sea-worthy with laps and shoulders protected.
What details are needed to get an accurate F91 lap joint flange quotation?
Seven elements plus commercial terms: (1) size and dimensional standard — e.g. 6" NB ASME B16.5; (2) pressure class — 150 to 2500; (3) the grade stated exactly — F91, not '9 chrome' or 'F9/F91': the vanadium-microalloyed CSEF and the plain grade are never interchangeable, and our PMI keeps them apart; (4) the stub end — A234 WP91 matched pair with schedule and B16.9 type, or 'flange only'; (5) the service where it helps — advanced steam confirms F91, while sulphur-driven refinery heat belongs to F9 no matter how hot; (6) certification — EN 10204 3.1 (our standard) or 3.2 witnessed, hardness and furnace records included, pairs certified together on one document; (7) quantity as pairs or pieces, and destination. Quotations normally return within 24 hours with price, unit weights for both parts, and delivery.
Who manufactures ASTM A182 F91 lap joint flanges in India?
Tesco Steel & Engineering is an ISO 9001:2015 certified flange manufacturer based in Mumbai, India, producing ASTM A182 F91 lap joint (lapped joint) flanges per ASME B16.5 from ½" to 24" NB (larger to B16.47 or drawing) in Classes 150-2500 — each machined from certified 9Cr-1Mo-V forgings in the mandatory normalised-and-tempered condition, hardness verified per heat against the 248 HBW ceiling, PMI-checked on the V-Nb signature, with the shoulder dressed to bear evenly on the lap, heat-number marked, and certified to EN 10204 3.1/3.2 with furnace records supplied. Matched A234 WP91 stub ends per B16.9 are supplied against the same order, pairs certified together on one document. Alongside the complete A182 F91 range — weld neck, slip-on, socket weld, threaded, blind, spectacle blind and long weld neck — the full chrome-moly staircase in every flange type, and the complete lap joint grade bench. Exported to more than 50 countries.
