Tesco Steel & Engineering manufactures ASTM A182 F21 lap joint flanges — the chrome-moly staircase's in-between rung: 2.65–3.35% chromium buying deeper Nelson-curve standing than F22 and a first real measure of sulphidation resistance, with molybdenum in the full 1% class (0.80–1.06%) holding creep strength. The pattern's virtues apply in full: the backing ring never welds and never wets — no preheat, no PWHT on the flange; the joint's single butt weld belongs to the matched 3Cr-1Mo stub end, machined to B16.9 because A234 lists no WP21. The lap joint bench's metal-temperature argument holds at the 3-chrome rung — the ring runs at line metal temperature, so it cannot be A105 — and this page also names the identifier trap: F22's UNS and W.Nr. migrate onto F21 datasheets in error; read the chromium band first. Per ASME B16.5, ½″–24″ NB, Classes 150–2500, annealed or N&T, hardness held in the 156–207 HB band. Every lot with EN 10204 3.1/3.2 MTC. ISO 9001:2015, made in Mumbai, India — exported to 50+ countries.
ASTM A182 F21 · 3Cr-1MoThe In-Between Rung — Hydrogen's SpecialistCr 2.65–3.35% — Not F22's 2.00–2.50The Flange That Dodges the Weld ProcedureStub Ends: Matched F21, B16.9 — No WP21 Exists½″–24″ NB · Classes 150–2500Pairs Certified TogetherEN 10204 3.1 / 3.2 · ISO 9001:2015
ASTM A182 F21 Lap Joint Flanges — Specifications at a Glance
What is an ASTM A182 F21 Lap Joint Flange?
The 3-chrome backing flange. An ASTM A182 F21 lap joint flange is the loose backing ring of the two-piece system in 3Cr-1Mo alloy steel — chromium at 2.65–3.35% for deeper hydrogen-service standing on the Nelson curves than the F22 workhorse, molybdenum at 0.80–1.06% for creep strength. The point on this pattern: the flange never touches the process, but it runs at line metal temperature — and it never welds, so it escapes the chrome-moly preheat/PWHT discipline. Per ASME B16.5, ½″–24″ NB, Classes 150–2500, annealed or N&T, 156–207 HB; stub ends matched F21 machined to B16.9 — A234 lists no WP21. EN 10204 3.1 on every lot.
Also searched as: F21 backing flange, 3Cr-1Mo lap joint flange, 3 chrome lap joint flange, A182 F21 LJ flange, F21 loose flange — all this page's product. Related pages: the lap joint hub (the pattern's full story), the staircase neighbours — F22 below and F5 above — the carbon twins A105 and LF2, the F21 bench — grade overview / long weld neck / threaded — and the alloy steel overview.
The Metal-Temperature Argument, at the 3-Chrome Rung
The argument at this rung is the hydrogen chart: the heat reaches the never-wetted ring through the metal it clamps, and where hot hydrogen pushes past F22's Nelson-curve standing, the extra chromium is what the money buys. The ring can skip the welding; it cannot skip the temperature class.
A different mission — vanadium-modified precision creep strength for power-plant steam
The honest advice for the in-between rung runs both ways: hydrogen duty that fits F22's curves gains nothing from F21's premium, while sulphur-driven circuits usually step past it to F5. F21 earns its place where the hydrogen chart, not the sulphur chart, sets the grade.
What the Pattern Gives This Grade
The Hydrogen Step
2.65–3.35% chromium — a deliberate move deeper into the Nelson curves — specified where F22's hydrogen headroom runs thin but the full 5-chrome corrosion purchase is not yet needed.
The Weld-Procedure Dodge
The backing ring never welds — no preheat, no PWHT, no procedure to qualify or reopen on the flange. At 3% chromium the discipline dodged is substantial, sitting between F22's and F5's in severity.
The Identifier Guard
F22's UNS K21590 and W.Nr. 1.7380 migrate onto F21 datasheets in error — read the chromium band first: 2.00–2.50 is F22, 2.65–3.35 is F21, and the certificates are not interchangeable.
The Matched-Pair Answer
A234 lists no WP21 — so stub ends are machined from certified F21 material to B16.9 dimensions, and both halves of the joint carry the same grade's paperwork on one document.
Specification Notes — Getting F21 Lap Joints Right
Three honest notes.Check the chromium band on every document: F22's identifiers migrate onto F21 lines often enough that our own incoming datasheets get read twice — 2.65–3.35% is this grade's signature. The stub end is a machined answer: with no WP21 in A234, the welded half is machined from F21 forgings or bar to B16.9 dimensions — a longer lead than a catalogue fitting, quoted honestly against forging stock. And the pattern's boundaries apply in any grade: heavy external loads, severe vibration and high-moment positions read the weld neck bench — most hydroprocessing joints are weld necks, and lap joints earn their place where hot lines dismantle; the hub page states the boundaries fully.
How Our F21 Lap Joint Flanges Are Manufactured
1
Material — certified A182 F21 forgings, annealed or normalised and tempered with the condition stated and furnace records retained; 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 against the 156–207 HB band; chemistry certified per heat with the carbon floor proven and the chromium band read against F21's own 2.65–3.35% — not F22's.
5
No welding, ever — the ring's whole career is mechanical: no weld, no preheat, no PWHT — the pattern's gift on an air-hardening 3-chrome grade.
6
Pairing & certification — stub ends machined from matching F21 material to B16.9 dimensions where ordered; one EN 10204 3.1 document per pair (3.2 witnessed on request); packed sea-worthy with laps and shoulders protected.
Where F21 Lap Joint Flanges Are Used
Where the hydrogen chart and the pattern meet: hydroprocessing units whose hot hydrogen partial pressures push past the lower rungs' Nelson-curve standing — hydrotreater and hydrocracker loops, hot separator circuits, and legacy units whose specifications wrote F21 in decades ago and keep it for like-for-like replacement — specifically the positions that earn a lap joint: exchanger and reactor-circuit connections that dismantle at turnaround, spools rotated for wear, and bolt circles that must align without pipe-twisting, where re-assembly never reopens a weld procedure. Production and supply below:
Alloy Steel Supply — Bulk Dispatch at Our WorksThe Two-Piece System — Flange, Stub End, Butt Weld, PipeForged Alloy Steel Stock — Machined at Our Works
F21 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, 4″ NB, ASME B16.5 Class 600, ASTM A182 F21, with matched F21 stub ends Sch 80 machined to B16.9 Type A (supplied together), pairs on one MTC, EN 10204 3.1 — 8 sets.” Quotations normally within 24 hours with price, unit weights (both parts) and delivery quoted against forging stock.
F21 Lap Joint Flanges — Frequently Asked Questions
What is an ASTM A182 F21 lap joint flange?
An ASTM A182 F21 lap joint flange is the loose backing flange of the two-piece lap joint system, forged in 3Cr-1Mo alloy steel — the chrome-moly staircase's in-between rung, carrying more chromium than the F22 workhorse for deeper hydrogen-service standing on the Nelson curves and a first real measure of sulphidation resistance, with molybdenum in the full 1% class holding creep strength. 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 — and because it never welds, it stays out of the chrome-moly weld discipline entirely; 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, annealed or normalised and tempered, with EN 10204 3.1/3.2 certification 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 F21 rather than plain carbon steel on hot hydrogen circuits. Second, the chrome-moly consequence: the flange never welds, so it never enters the weld procedure — no preheat, no PWHT on the ring itself; the joint's single butt weld belongs to the stub end. 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 F21 — 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 the 3-chrome rung. The heat reaches the ring through the metal it clamps: bolted hard against the lap of a hot hydroprocessing or hydrogen-bearing circuit, the flange sits at essentially line metal temperature — territory where plain A105 stops being trusted (above roughly 425°C carbon steel's ratings carry the graphitisation caveat) and where even the lower chrome-moly rungs start arguing with the Nelson curves. F21's chromium holds the hydrogen line deeper than F22's, and its carbides hold the ring's strength over years at exactly those temperatures. The ring can skip the welding; it cannot skip the temperature class. F21 backing F21 is the normal answer, and the certificate proves both halves.
What is the chemical composition of ASTM A182 F21?
Carbon 0.05-0.15% (a deliberate range with a minimum — see the carbon FAQ), manganese 0.30-0.60%, silicon ≤0.50%, chromium 2.65-3.35%, molybdenum 0.80-1.06%, phosphorus ≤0.040%, sulphur ≤0.040%. Two identity marks: the roughly 3% chromium band that steps past F22's toward hydrogen and sulphur resistance, and molybdenum in the full 1% class with its oddly precise 0.80-1.06% window — the creep-strength dial, converted from round ksi-era figures. Note the values that do NOT belong here: chromium 2.00-2.50 and molybdenum 0.87-1.13 are F22's bands, and datasheets sometimes borrow them onto F21 lines in error — see the F21-versus-F22 FAQ. Chemistry is verified per heat and travels on the EN 10204 3.1 MTC.
What are the mechanical properties of F21 lap joint flanges?
In the annealed or normalised-and-tempered condition A182 requires tensile strength 515 MPa (75 ksi) minimum, yield strength 310 MPa (45 ksi) minimum and elongation 20% minimum, with hardness controlled in the 156-207 HB band. That is a step above the 485/275 of the F11 and F5 certificates — the full 1% molybdenum earning its keep — while staying below F9's 585/380. As with all the family, the room-temperature numbers understate the purchase: F21 is bought for its creep-rupture ratings and its hydrogen-service standing at temperatures where carbon steel's tables have run out. 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. Supplied with the heat-treatment condition stated and records retained.
Why does F21's carbon have a minimum?
Because creep strength is carbide strength — the same logic that runs the whole low-alloy staircase. F21's job includes resisting slow deformation over years at temperature, and the mechanism is a dispersion of chromium-molybdenum carbides pinning the microstructure: no carbon, no carbides, no creep resistance. So the specification writes carbon as a range with a floor, 0.05-0.15% — the mill must prove there is enough carbon to form the strengthening phase, not just little enough to weld. The floor sits at 0.05 rather than F11's 0.10 because 3-chrome steel builds its carbides efficiently enough to afford it, keeping weldability in hand for the stub end's butt weld. A datasheet printing F21 carbon as '≤0.15' has lost the floor — and the point. On a lap joint order the floor travels on both certificates: the flange's and the matched stub end's together.
What is the difference between F21 and F22 — and the identifier trap?
One rung of chromium, and a documentation trap worth naming. F22 is the 2¼Cr-1Mo workhorse: chromium 2.00-2.50%, molybdenum 0.87-1.13%, UNS K21590, W.Nr. 1.7380 — the most-stocked rung of the staircase. F21 steps up to 3% nominal chromium (2.65-3.35%) with molybdenum 0.80-1.06%: a deliberate move deeper into the Nelson curves for hot hydrogen, historically specified for hydroprocessing service where F22's hydrogen headroom ran thin. The trap: because F22 is so much more common, its identifiers migrate — datasheets and even supplier pages sometimes print K21590, 1.7380 or the 0.87-1.13 molybdenum band against F21 lines, and a certificate carrying F22's numbers under an F21 heading fails audit. Read the chromium band first: 2.00-2.50 is F22, 2.65-3.35 is F21, and the two are not interchangeable on a certificate. Where a specification genuinely accepts either, it says so explicitly — and we quote each grade under its own paperwork.
What stub ends pair with F21 lap joint flanges?
Matched 3Cr-1Mo stub ends machined to ASME B16.9 dimensions — and here F21 differs honestly from its neighbours: A234, the wrought-fitting specification that supplies WP11, WP22, WP5 and WP9 to the rest of the staircase, lists no WP21 grade. So F21 stub ends are machined from certified F21 forgings or bar to B16.9's Type A or Type B lap dimensions, carrying the same chemistry and heat-treatment certificate as the flange they serve — which suits the lap joint pattern's documentary habit anyway: flange and stub end bought as a matched pair and certified together on one EN 10204 document, so the '3 chrome both halves' answer is provable at handover. Where a project's specification instead permits a neighbouring fitting grade for the welded half, that is the specifier's explicit call, not a silent substitution — state it and we quote accordingly. The usual dimensional rules apply: lap OD to the flange's shoulder, schedule to the line bore.
Does an F21 backing flange need preheat and PWHT?
No — and at 3% chromium the dodge is worth a lot. F21 air-hardens firmly: its welds take the chrome-moly discipline in earnest — qualified procedures, preheat, controlled interpass and post-weld heat treatment per the code — sitting between F22's and F5's in severity. 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 instead on the joint's single butt weld — stub end to pipe, 3Cr-1Mo to the line, welded once under full control. On a turnaround this matters twice: fewer alloy welds to make and test in the first place, and a flange that can be swapped, rotated or re-used without any weld procedure being reopened. The dodge covers the flange, not the joint — the stub end's weld takes the full discipline, always.
Where does F21 sit on the chrome-moly staircase?
Between the workhorse and the corrosion rung, and it is the hydrogen specialist's step. The staircase climbs by chromium: F11 at 1¼% and F22 at 2¼% are bought mostly for creep strength in steam and hot hydrogen; F21 at 3% nominal is the quiet rung between — specified where F22's Nelson-curve standing runs thin but the full 5-chrome corrosion purchase of F5 is not yet needed; F5 at 4-6% is the first true sulphidation grade; F9 at 8-9.5% doubles that for the sourest heater service; and F91, the vanadium-modified 9-chrome, turns off toward precision creep strength for power-plant steam. Every rung has a lap joint page on this site, and the honest advice for the in-between rung runs both ways: hydrogen duty that fits F22's curves gains nothing from F21's premium, while sulphur-driven circuits usually step past it to F5. F21 earns its place where the hydrogen chart, not the sulphur chart, sets the grade. State the service and the rung picks itself.
What sizes and pressure classes do F21 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: hydroprocessing and hot hydrogen circuits concentrate in Classes 300 to 900 at mid bore, where pressure climbs alongside temperature. Dimensionally an F21 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 the grade's material group keeps usable pressures at temperatures that retire carbon steel. F21 is a lower-volume rung than F22, so lead times are quoted honestly against forging stock. State size, class and the stub-end question together, and our quotation returns price, unit weight and delivery per class.
How much does an F21 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 matched 3Cr-1Mo stub end adds its schedule-dependent mass. The commercial difference sits in the metallurgy and the paperwork: F21 carries its alloy surcharge, controlled heat treatment and per-lot testing in its price, plus the machined-stub-end route that A234's missing WP21 makes necessary. 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 F21 lap joint flanges manufactured?
As the pattern's standard sequence with 3-chrome discipline at the heat-treatment step. Certified A182 F21 forgings are supplied annealed or normalised and tempered — the condition stated on the certificate, with furnace records retained — 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 against the 156-207 HB band; chemistry is certified per heat with the carbon floor proven and the chromium band read against F21's own 2.65-3.35 — not F22's. 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. Where the order includes stub ends, they are machined from matching F21 material to B16.9 dimensions and the pair is 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 F21 lap joint flange quotation?
Seven elements plus commercial terms: (1) size and dimensional standard — e.g. 4" NB ASME B16.5; (2) pressure class — 150 to 2500; (3) the grade stated exactly — F21, with the chromium band 2.65-3.35% if your datasheet carries numbers, because F22's identifiers migrate onto F21 lines in error; (4) the stub end — matched 3Cr-1Mo machined to B16.9 with schedule and type stated, or 'flange only'; (5) the service where it helps — hydrogen-driven duty confirms F21, while milder curves fit F22 and sulphur-driven circuits step to F5; (6) certification — EN 10204 3.1 (our standard) or 3.2 witnessed, 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 quoted honestly against forging stock.
Who manufactures ASTM A182 F21 lap joint flanges in India?
Tesco Steel & Engineering is an ISO 9001:2015 certified flange manufacturer based in Mumbai, India, producing ASTM A182 F21 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 3Cr-1Mo forgings in the annealed or normalised-and-tempered condition, hardness-verified in the 156-207 HB band, chromium band verified against F21's own 2.65-3.35%, with the shoulder dressed to bear evenly on the lap, heat-number marked, and certified to EN 10204 3.1/3.2. Matched 3Cr-1Mo stub ends machined to B16.9 dimensions are supplied against the same order — A234 lists no WP21, so both halves carry F21 certificates — pairs certified together on one document. Alongside the F21 range — long weld neck, threaded and spectacle blind — the full chrome-moly staircase in every flange type, and the complete lap joint grade bench. Exported to more than 50 countries.
