Alloy 20 Tubing (UNS N08020)

Alloy 20 tubing is OD-sized, round instrumentation tube drawn from UNS N08020, a niobium-stabilized super-austenitic nickel-iron-chromium alloy running roughly 35Ni-20Cr-3.5Cu-2.5Mo. It carries process fluid or gas between components and seats into a compression fitting's ferrule set to make a leak-tight, make-and-break joint. Crestflo supplies Alloy 20 tube seamless to ASTM B729 and welded to ASTM B468, in both imperial and metric sizes, shipped solution-annealed so it bends and cones cleanly into a ferrule. You reach for Alloy 20 tube when a line carries sulfuric acid, phosphoric or nitric acid, or a mixed-acid stream, the reducing-acid service where 304 and 316 stainless simply corrode through, but the duty does not justify a high-nickel alloy. Its copper-bearing chemistry was purpose-built for sulfuric acid, and its niobium stabilization keeps a welded tube from sensitizing at the seam. Standard items ship from stock; specials are built to order, with delivery running 6-8 weeks. Request a quote to price your run.

Reviewed by the Crestflo Engineering Team

Carpenter® 20 and 20Cb-3® are trademarks of CRS Holdings LLC / Carpenter Technology Corporation. Swagelok® and Parker® are trademarks of their respective owners. Crestflo is not affiliated with any brand or mill referenced here, and interchange is strictly dimensional.

Why Specify Alloy 20 Tubing

Tube is the long, continuous wetted surface of an instrument hookup. Unlike a fitting or a valve, it has no dead legs or packing bores, but it has length, every inch of it exposed to the media, and it has to survive the cold work of bending and ferrule make-up without cracking. On a sulfuric or mixed-acid line that rules out most stainless. A tube that corrodes or stress-corrosion cracks midway along a run is as dead as one that fails at the fitting, and in acid service the run is often the first thing to go.

Alloy 20 is a niobium-stabilized super-austenitic stainless alloy, carrying roughly 35% nickel, 20% chromium, a deliberate 3-4% copper addition, and 2-3% molybdenum, with iron as the balance. That chemistry is tuned for the reducing-acid problem. Copper is the signature element: together with molybdenum it gives Alloy 20 a genuine edge in sulfuric acid across a broad concentration and temperature range, the duty the grade was developed to solve, plus solid performance in phosphoric, nitric, and many mixed and contaminated acids where 304 and 316 fall short. The roughly 35% nickel underwrites strong resistance to chloride stress-corrosion cracking, the failure mode that most often retires 304 and 316 in warm chloride service. And the niobium (columbium) stabilization, tied to the carbon at eight times its content and backed by a low carbon limit, is what keeps corrosion resistance intact in the weld and heat-affected zone of a welded tube, rather than letting the seam become the first thing the acid attacks.

Be precise about where Alloy 20 sits, though, because it is easy to over-sell. It is a super-austenitic stainless alloy, not a chloride or seawater grade. Its pitting resistance equivalent number is only about 28 (Cr + 3.3 × Mo), above 316's roughly 24 but well below the 6-molybdenum super-austenitics. PREN ranks chloride pitting only, and chloride pitting is not what Alloy 20 is for; acid resistance is. Its strength is moderate: Alloy 20 yields around 35 ksi annealed, comparable to the common austenitics rather than double them the way a duplex does, so there is no thinner-wall advantage to claim, and the case for Alloy 20 tube is corrosion resistance, not pressure-per-wall. Being fully austenitic does buy one real thing worth stating: it is essentially non-magnetic in the annealed condition, which is itself a reason to specify it where magnetic permeability matters.

What makes Alloy 20 the tube to specify is where it sits between stainless and the nickel alloys. It delivers sulfuric, phosphoric, and mixed-acid performance well beyond 316 without paying for a high-nickel alloy. Where the duty turns to aggressive chloride pitting, crevice attack, or seawater, a 6Mo grade such as 254 SMO or AL-6XN (PREN ~43) or a duplex is the move; where the acid runs hotter or more concentrated than Alloy 20 can take, hot HCl, wet chlorine, or boiling concentrated sulfuric, a nickel alloy such as Hastelloy C-276 steps up; and where the service is only mildly corrosive, 316 handles it for less. Alloy 20 is the one you reach for when sulfuric and mixed acids outrun 316 but stop short of those.

The full metallurgical picture, composition ranges, mechanical and physical properties, PREN, and corrosion-by-media data, lives on the Alloy 20 grade page. This page stays on the tube: how it is made, sized, finished, tested, and matched to the fitting.

Also Specified As: Carpenter 20 Tube, Alloy 20 Seamless Tubing, UNS N08020 Tube

Buyers and specifiers call this tube several ways, and each one points to the same super-austenitic alloy. A requisition might read "Carpenter 20 tubing," "Alloy 20 seamless tubing," "N08020 instrument tube," "20Cb-3 tube," "Alloy 20Cb tube," or "Incoloy 20 tube," and every one of them lands here. Alloy 20, Carpenter 20, 20Cb-3, and Alloy 20Cb are the common forms; the material is registered as UNS N08020 and EN / Werkstoff 2.4660 (NiCr20CuMo). The welded form is often written "welded-and-drawn tube," the seamless form "cold-drawn seamless tube." UNS N08020 is the US number and 2.4660 is the European one; they describe one and the same alloy, and a tube labeled "Alloy 20" is routinely certified to both at once. Carpenter Technology sells the wrought grade under the trade name 20Cb-3; N08020 is the generic specification Crestflo supplies to.

Scope: this is round, OD-sized instrumentation and fluid-system tube for compression and tube-fitting connections. It is not a generic "20 alloy," not an aluminum designation, and not a weld-filler product. It is also not oilfield coiled tubing for downhole well intervention, not structural or ornamental tube, and not pipe specified by nominal pipe size (NPS) and schedule. Crestflo supplies Alloy 20 in instrumentation tube, fitting, and valve forms only, not mill bar, sheet, plate, or pipe.

Seamless vs Welded Alloy 20 Tubing

Both constructions are built to their own high-nickel-alloy tube specification, and both accept a compression fitting the same way. Seamless Alloy 20 tube is cold-drawn from a hollow with no longitudinal weld, which makes it the default where you want maximum integrity for higher-pressure, cyclic, or critical service. Welded-and-drawn Alloy 20 tube is formed from strip, seam-welded, then cold-drawn to size; it is dimensionally consistent and cost-effective for general instrument and fluid-system work. The grade's niobium stabilization and low carbon limit are what keep corrosion resistance intact in the weld and the heat-affected zone, so welded Alloy 20 holds up in acid service that would sensitize an unstabilized austenitic, a genuine advantage of this grade in welded-tube form. And because Alloy 20 is fully austenitic rather than duplex, there is no ferrite-balance check to run on the weld, which is one fewer acceptance step than a 2205 or 2507 tube carries.

Not sure which construction to call out? The deep dives on seamless tubing and welded tubing walk through the trade-off before you commit a spec.

Alloy 20 Tube Attributes at a Glance

Alloy 20 Tube Sizes: OD, Wall & Imperial/Metric

Instrument tube is specified by its actual outside diameter and wall thickness, given in both imperial and metric units, never by NPS and schedule. The common instrument range runs fractional 1/8″ to 1″ and metric 6 to 12 mm; smaller 1/16″ small-bore tube and larger diameters up to 2″ (50 mm) are available to order. Wall thickness is chosen to carry the pressure and to suit the ferrule, and both OD and wall are held to the tolerances of ASTM B729 (seamless) or B468 (welded). Because Alloy 20 is a moderate-strength austenitic rather than a high-strength duplex, it earns no thinner-wall credit: size the wall to the same pressure math you would run for any austenitic stainless. Working pressure is not a single blanket figure; it is calculated from OD, wall thickness, and the alloy's allowable stress per ASME B31.3, so it is stated per size and wall rather than as one rating for the grade.

Values per applicable standard; customizable to requirement. For how to work back from a target pressure to a wall thickness, see the tube sizing and wall thickness guide, and for the full dimensional matrix across the range, the tubing size chart.

Temper & Hardness: Solution-Annealed for Bending and Ferrule Seating

The single most important mechanical property of instrument tube is not its peak strength but its softness. Alloy 20 is a solid-solution austenitic grade that does not age-harden, so it comes in one working condition, solution-annealed, and that condition is what lets the tube bend around a radius and cone down under a ferrule without cracking, so the joint seats and seals the way a compression fitting is designed to. In the solution-annealed condition Alloy 20 carries a tensile minimum around 80 ksi (551 MPa) with a 0.2% yield near 35 ksi (241 MPa) and elongation above 30%, with hardness in the annealed band (typically ≤ ~95 HRB). That yield is comparable to the common austenitics, not double it, which is the honest reason Alloy 20 buys no thinner-wall advantage. Being fully austenitic, it has no ferrite phase to balance and no ductile-to-brittle transition. One caution worth flagging for the installer: like other high-nickel austenitics, Alloy 20 is a tough, work-hardening alloy, so it cuts and bends with a bit more effort than 316 and galls more readily at the ferrule under repeated make-up, which is why anti-galling measures are offered on the matching fittings. Specify the hardness or condition to your own standard on the RFQ and it is confirmed on the certificate.

Finishes & Cleanliness: Bright-Anneal, Electropolish & Oxygen-Clean

Alloy 20 tube ships in a mill/bright-annealed finish as standard. For high-purity gas, analytical, and oxygen service, passivation, electropolishing (EP), and oxygen/UHP cleaning with capped-and-bagged packaging are available. Electropolishing lowers surface roughness and smooths the inner diameter, which matters where particle generation, moisture dry-down, or deposit build-up is the concern, and on an acid line a smooth, passive inner surface is one less place for attack to initiate. Standard UHP electropolish surface-roughness targets can be specified right on the RFQ to your own standard; electropolished tubing and oxygen and UHP cleaning cover the options in detail. Finish and cleanliness figures follow the applicable standard and are customizable to requirement.

Alloy 20 Tube Standards: ASTM B729, B468 & ASME B31.3

Alloy 20 tube is covered by the high-nickel-alloy tube specifications in the ASTM B-series, not the 300-series austenitic specs such as ASTM A269 or A213, and the construction decides which applies: seamless falls under ASTM B729, welded under ASTM B468, each with its matching ASME boiler-code SB- designation. Allowable working pressure for a given size and wall is calculated per ASME B31.3. Where sour service applies, the material can be supplied to NACE MR0175 / ISO 15156 as the application qualifies. Crestflo holds the standard approvals that apply to instrumentation tube and is ISO 9001 / 14001 / 45001 and PED certified.

The full family of Alloy 20 product-form specifications, including welded pipe (B464), bar and wire (B473), and plate, sheet and strip (B463), is set out on the Alloy 20 grade page; for tube, B729 and B468 are the two that apply.

Working Pressure: Calculated per ASME B31.3, Not Assumed

There is no single pressure rating for "Alloy 20 tube," and any supplier who quotes one is skipping the engineering. Allowable internal pressure depends on three things: the tube OD, its wall thickness, and the alloy's allowable stress at the service temperature. ASME B31.3 sets the method, relating wall thickness to pressure, diameter, the allowable stress value S, the weld-joint quality factor E, and the coefficient Y. Seamless tube carries a higher joint factor than welded, which is part of why seamless is chosen for higher-pressure work. Alloy 20 is a moderate-strength austenitic, so its allowable stress sits near that of other austenitic stainless rather than above it, and it derates with temperature the way austenitics do; do not expect a duplex-style strength bonus. The thin-wall instrument sizes still carry ample pressure for analyzer, sample, and injection duty. Give us the OD, wall, temperature, and medium on the RFQ and the engineering team returns the calculated working pressure for your configuration. Values per the applicable standard; customizable to requirement.

Testing, NDT & Material Verification

Every length of Alloy 20 tube is verified before it ships. ASTM B729 and B468 require each tube to pass either a hydrostatic test or a nondestructive (eddy-current) electric test, and dimensional checks confirm OD, wall, and straightness against the ordered tolerances. Positive material identification (PMI) confirms the alloy on the material, which matters on an acid-service super-austenitic: the copper, molybdenum, and nickel content is exactly what buys the sulfuric-acid performance, and an under-alloyed tube supplied as 316 or a leaner grade looks identical but corrodes through in service. On a long wetted run, that is an expensive thing to discover at commissioning. NDT is done in-house; destructive testing, where a job calls for it, goes to ISO/IEC 17025-accredited labs, and third-party inspection is available on request.

For sour service, Alloy 20 tube material can be supplied to NACE MR0175 / ISO 15156 where the application qualifies. Confirm the hardness and material-condition limits in the standard against your application, H₂S partial pressure, temperature, and chloride content, and note the sour-service requirement on the RFQ.

Straight Lengths & Coiled Alloy 20 Tube

Alloy 20 tube ships as straight lengths for racked and rigidly supported runs, and as continuous coils for heat-tracing, long analyzer and sample lines, and chemical-injection and hydraulic bundles where a joint-free run is the whole point. Coiled tube keeps the count of mechanical joints down, which on a corrosive acid line is one fewer place to leak, and that matters most on exactly the sulfuric, phosphoric, and mixed-acid sample and dosing cores where Alloy 20 earns its place. Both forms are supplied solution-annealed; call out length or coil length on the RFQ. For long-run and tracing applications, see coiled tubing; for thicker-wall high-pressure control lines, hydraulic tubing.

Material Traceability & Certification

Every length of Alloy 20 tube carries its own heat/lot number, and each order ships with an EN 10204 MTC 3.1 certificate as standard, 3.2 available on request. PMI confirms the grade at the part level, every time. That documentation backs the audit trails that acid-plant, chemical, pharmaceutical, and fertilizer jobs demand, the kind of work where a mis-specified or under-alloyed tube in a long run is an expensive thing to discover in commissioning. Full detail is on the traceability page.

Applications for Alloy 20 Tubing

Alloy 20 tube earns its place wherever sulfuric and mixed acids rule 316 out of a long wetted run, but the duty doesn't yet justify a high-nickel alloy:

• Sulfuric-acid production and processing, process, analyzer, and sample lines across a broad concentration and temperature range, the copper-bearing signature duty for Alloy 20. See chemical and petrochemical processing.
• Phosphoric-acid and fertilizer plants, wet-process phosphoric acid and sulfuric feed systems, among the most corrosive duties in process industry.
• Pickling and metal-finishing lines, pickling-liquor and plating-bath instrument hook-ups, where acid attack defeats standard stainless.
• Pharmaceutical and fine-chemical, reactor and process instrument lines carrying aggressive acidic intermediates, where contamination and corrosion both matter.
• Plastics and synthetic fibers, process streams using sulfuric and mixed acids in synthesis and finishing.
• Non-magnetic requirements, lines where low magnetic permeability governs, which the fully austenitic structure provides.

Where the service steps past Alloy 20, to aggressive chloride pitting and seawater, or to hot concentrated acid beyond its range, see the step-up grades in the next section.

Where Alloy 20 Stops: Stepping Up to 6Mo, Duplex or C-276

Alloy 20 sits a clear step above 316 for acid service but is not a chloride or seawater alloy, and knowing where it stops is as important as knowing what it handles. Where the governing problem is aggressive chloride pitting, crevice attack, or seawater rather than acid, move to a 6Mo grade such as 254 SMO or AL-6XN (PREN ~43) or a duplex; Alloy 20's PREN of ~28 is simply not built for that duty. Where sulfuric or mixed acid runs hotter or more concentrated than Alloy 20 can take, or the stream turns to hot HCl or wet chlorine, Hastelloy C-276 is the answer. And where the service is only mildly corrosive, there is no reason to pay for Alloy 20; compare the two directly in the Alloy 20 vs 316 guide. Match the alloy to the media and don't over-specify.

Pairing Alloy 20 Tube With Alloy 20 Fittings and Valves

A tube is one part of the hookup, and the joint is only as corrosion-resistant as its weakest metal. For a matched instrument run, pair this tube with Alloy 20 tube fittings and Alloy 20 instrumentation valves so the whole assembly, tube, ferrules, and valve body, carries the same acid resistance and the same traceability. Crestflo supplies the tube alongside compression fittings tested to ASTM F1387, so the ferrule set seats correctly on the exact OD and wall you ordered, and the joint is a dimensional match rather than a mix of two suppliers' tolerances. Putting an Alloy 20 tube into a 316 fitting just moves the failure point to the ferrule, right where the acid finds the under-alloyed metal. Because Alloy 20 is a high-nickel austenitic that work-hardens and can gall, anti-galling measures such as silver plating on the ferrule are offered on request where repeated make-up or high torque is expected.

Drop-In Interchange and the Tube-Plus-Fitting System

Crestflo Alloy 20 tube is cut and finished to standard OD and wall, so it drops into the compression fittings you already run, and the matching Alloy 20 fittings are built as dimensional interchange / equivalent parts for the major brands, with the fitting ends qualified to ASTM F1387. These are not "compatible" or "intermix" parts. Swagelok® and Parker® remain trademarks of their respective owners, and Crestflo is not affiliated; the interchange is strictly dimensional. Moving a run off a major brand? Request your Swagelok®/Parker® cross-reference and the engineering team maps your tube and fitting part numbers to Crestflo Alloy 20 equivalents. We don't publish a part-number table.

Origin & US-Serving Positioning

Crestflo manufactures for the US market. Standard product is of Indian origin; US or European melt is available on request, with full material traceability. Tube is engineered as a US drop-in and ships in 6-8 weeks, well under the 20+ weeks typical of the majors. If a project calls for a specific melt origin or TAA/DFARS handling, specify it on the RFQ and we'll address it straight, no hedging. See the US-origin compliance page for how we document it.

Why Crestflo for Alloy 20 Tubing

Crestflo operates as the instrumentation division of an export house with four decades behind it and formal recognition from the Government of India, with a proven Middle-East delivery record. That backing grounds a focused instrumentation brand built on a handful of clear advantages:

• Lead time of 6-8 weeks versus 20+ weeks typical of the majors, on standard stock and made-to-order specials alike.
• A matched system, tube plus F1387-tested fittings and ferrules that seat on the exact OD and wall you ordered.
• Both seamless and welded Alloy 20 tube, in both imperial and metric, straight or coiled, with the niobium-stabilized seam that resists sensitization in acid service.
• Heat-number marking plus MTC 3.1/3.2 on every order, PMI-confirmed, so an acid-service super-austenitic is never taken on trust, the under-alloying guard that matters most on this grade.
• NACE MR0175 / ISO 15156 material options where the sour-service application qualifies.
• ISO 9001 / 14001 / 45001 & PED certified, cost-competitive after US duties and tariffs, with small MOQs and private-label available.

We Engineer Confidence.

Request a Quote for Alloy 20 Tubing

Give us the details: seamless or welded, OD and wall, straight length or coil, finish and cleanliness, and the line conditions (media, pressure, temperature, sour-service requirement). We'll calculate the working pressure, confirm the spec, and schedule the run.

• Request a quote, the fastest path to pricing and lead time.
• Request a sample for qualification.
• Request your Swagelok®/Parker® cross-reference and we'll map your tube and fitting part numbers.

Datasheets and CAD are available to customers on request. Reach Crestflo's US sales desk at sales@crestflousa.com or +1 346 594 9005 (Houston, TX). Round out the system with Alloy 20 tube fittings and Alloy 20 valves, browse the tubing pillar for other grades and constructions, and see where this tube serves in chemical and petrochemical processing.

Frequently Asked Questions

Related Pages

• Alloy 20 alloy grade & corrosion data
• Alloy 20 tube fittings · Alloy 20 instrumentation valves
• Tubing pillar · Instrumentation tubing · Seamless tubing · Welded tubing
• Alloy 20 vs 316 · 254 SMO · Hastelloy C-276
• ASME B31.3 · NACE MR0175 · EN 10204 MTC · Chemical & petrochemical