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Nitronic 50 / (1.3964) / UNS S20910 / AMS 5764

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Nitronic 50 is an austenitic stainless steel that combines high strength with excellent corrosion resistance and good ductility. It is one of the highest strength members of the austenitic steel group; its UNS designation is S20910 and it is also widely known as XM-19.

What separates the alloy from the standard 300 series is its high manganese and nitrogen content. Nitrogen dissolves interstitially in solid solution and raises the yield strength by up to 50%, while at the same time improving resistance to pitting corrosion. As a result Nitronic 50 roughly doubles the yield strength of 304L and 316L.​‌​​‌​

It is non-magnetic, and that property is decisive wherever high strength non-magnetic fasteners are required. It can be supplied in two conditions: solution annealed, and cold worked (high strength). Machinability is somewhat more demanding than that of 316L.

It is used in marine and offshore service for pumps, valves, shafts, propeller shafts and chains; in chemical processing for tanks and reactors in acid and chloride-bearing environments; in power generation and nuclear work for piping systems and heat exchanger components; and in aerospace for high strength non-magnetic fasteners.​‌​​‌​

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What Nitronic 50 Is — and Why “High-Strength 316L” Is the Wrong Description

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Nitronic 50 (UNS S20910 / ASTM XM-19 / also sold as Alloy 50, Fermonic 50, 22-13-5) is a nitrogen-strengthened fully austenitic stainless steel. Its one distinguishing sentence: it substitutes manganese and nitrogen for part of the nickel, which roughly doubles the annealed yield strength relative to 316L while keeping the structure 100 % austenitic — so it does not transform to martensite when cold worked or taken to cryogenic temperature, and it stays non-magnetic.

Very few materials deliver both of those at once, and that is where the entire commercial value of the alloy sits. But the same sentence also writes the alloy’s limits: nitrogen and manganese are an inexpensive nickel substitute, and the chromium–molybdenum balance does not approach that of a duplex stainless. We hold that distinction in every section below.​‌​​‌​

The three numbers that actually define the alloy​‌​​‌​

Nitrogen 0.20–0.40 %. Nitrogen is the most potent interstitial solid-solution strengthener in austenite and simultaneously a powerful austenite stabiliser. It does nickel’s expensive job. Its contribution to pitting resistance enters the PREN formula with a coefficient of 16 — sixteen times more effective than chromium.
Manganese 4.0–6.0 %. Manganese contributes nothing to corrosion resistance on its own; its job is to raise nitrogen solubility in the liquid steel. You cannot hold 0.40 % nitrogen in an austenitic without it. Manganese here is a process enabler, not an alloying element.
Niobium 0.10–0.30 % and vanadium 0.10–0.30 %. Almost no distributor sheet explains these two. Nb and V are carbide/carbonitride formers: they tie up carbon and prevent chromium carbide (stabilisation), and they add strength through fine precipitates. This is why XM-19 is unexpectedly resistant to sensitisation.

The consequence: XM-19 does the opposite of 316L’s “dilute the chemistry and give up strength” approach — it leaves carbon at ≤0.06 % but stabilises it with Nb + V, strengthens with nitrogen, and holds the nitrogen with manganese. That is why it is strong in the annealed condition and still passes ASTM A262 Practices B and E after a 675 °C sensitising cycle.​‌​​‌​

Standards by Product Form

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A critical warning up front: every standard listed below covers annealed XM-19. The HS / XHS / UHS high-strength levels described later on this page are NOT inside those standards — they are mill specifications. Do not mix the two on one purchase order.

ASME Code Acceptance and Maximum Code Temperatures

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We have to be honest in this section. The ASME material specifications for S20910 (SA-240, SA-276, SA-479, SA-314) are verified — the material is in the ASME system. What we could not independently verify is the maximum temperature at which it stops in the allowable-stress tables of Section II Part D.​‌​​‌​

Product Forms With NO Standard — the Commercially Valuable Section​‌​​‌​

This is the section your sales engineers should memorise. In XM-19 the specification gap is not in the chemistry but in the strength levels — and that leaves the alloy’s best-selling condition outside any standard.

Chemical Composition

ASTM A276 / A479 / A240 / AMS 5764 (UNS S20910), weight %: C ≤0.06 · Mn 4.00–6.00 · Si ≤1.00 · P ≤0.040 · S ≤0.030 · Cr 20.50–23.50 · Ni 11.50–13.50 · Mo 1.50–3.00 · N 0.20–0.40 · Nb 0.10–0.30 · V 0.10–0.30 · Fe balance.​‌​​‌​

1.3964 is not the material you think it is — the most expensive fact on this page​‌​​‌​

Almost every distributor sheet gives the European equivalent of S20910 as W.Nr. 1.3964. That is a shortcut and it is not chemically correct. Put the two chemistries side by side:

Mechanical Properties

Do not mix rows in this section. XM-19 is not one material but at least three separate products: annealed, hot-worked high strength (HS/XHS/UHS), and cold-worked levels. Only the first of these is inside a standard.​‌​​‌​

Typical mill values and cryogenic behaviour — NOT GUARANTEED​‌​​‌​

A secondary source reports, for annealed XM-19, Rm 827 MPa, Rp0.2 448 MPa, elongation 45 % at room temperature (24 °C) and Rm 1558 MPa, Rp0.2 883 MPa, elongation 40 % at −196 °C. These are single-sourced and are typical measurements, not specification minimums. What they describe is nonetheless true and important: at cryogenic temperature the strength nearly doubles while elongation stays at 40 % — there is no ductile-to-brittle transition. No ferritic or martensitic steel can do that. In LNG, liquid oxygen and superconducting magnet structures, this is what the alloy is actually selling.

Physical Properties​‌​​‌​

Heat Treatment and Thermal Stability​‌​​‌​

Basic rule: XM-19 cannot be hardened by heat treatment. It is austenitic; there is no transformation hardening. Its only heat treatment is a solution anneal, and its purpose is not to harden but to soften and restore corrosion resistance. Strength comes either from nitrogen (annealed) or from mechanical work.

Sensitisation — better than you would expect

At C ≤0.06 %, XM-19 is not a “low carbon” grade — it is twice 316L’s ≤0.030 %. It is nevertheless resistant to sensitisation, and the reason is Nb + V stabilisation: niobium and vanadium tie carbon up in their own carbides and carbonitrides, leaving chromium free at the grain boundary.​‌​​‌​

What the producer measured: ASTM A262 Practice B (ferric sulphate–sulphuric acid) — 0.0009 in./month annealed at 1105 °C; 0.0022 in./month after 1105 °C anneal plus a 1-hour sensitising cycle at 675 °C. A262 Practice E (copper-accelerated) — passed in both conditions. So one hour at 675 °C raised the rate 2.4× but did not open the material to intergranular attack. That cannot be said of 316 (non-L).

Even so: this does not mean the alloy is suited to continuous service in the 500–900 °C band. A one-hour cycle and ten thousand hours of service are not the same thing. For the long-term effect of that band, see the conflict below.​‌​​‌​

Welding​‌​​‌​

What actually goes wrong​‌​​‌​

1. A “non-magnetic assembly” order and magnetic weld beads. The most common and most expensive misunderstanding. Base metal µr = 1.004; SMAW weld metal µr ≈1.2. The reason is that a sound, crack-free bead deliberately contains some ferrite (FN ≈6) — and ferrite is magnetic. Ask for zero-ferrite filler and you buy hot cracking risk. If the specification carries a µr ceiling, resolve it in weld procedure qualification, not after shipment.
2. Welding high-strength material. The strength of HS/XHS/UHS and the cold-worked levels comes from mechanical work. The heat-affected zone is locally annealed and the strength you paid for falls back to annealed values there. The producer’s own warning: further hot work, heat treatment and welding cannot be applied to these levels. For welded design, calculate with 380 MPa yield.
3. Autogenous (no-filler) TIG. Common on thin-wall tube and sheet. XM-19’s FN potential is ≈2; a filler-free bead solidifies essentially fully austenitic and is prone to hot (solidification) cracking. Use ER209 filler.
4. Nitrogen loss. Inadequate shielding or a long arc lets nitrogen escape from the pool. In this alloy nitrogen means strength, austenite stability and pitting resistance at once; all three weaken. Take shielding gas and back-purge discipline seriously.
5. Sending as-welded material into corrosive service. For as-welded use the producer calls for the 1121 °C (2050 °F) anneal, not 1066 °C. Chloride, sulphur, zinc and copper contamination (marking pens, taped labels, galvanised lifting blocks) must also be removed before welding.

Machining​‌​​‌​

It has to be said plainly: XM-19 is a difficult material to machine, and that is the invisible part of its purchase cost. The producer’s own measurement is unambiguous: approximately 21 % of the cutting rate of B1112 free-machining steel. For scale, 316L sits roughly in the 36–45 % band on the same scale — so XM-19 machines at about half the speed of 316L.

Corrosion — Where It Is Good and WHERE IT FAILS

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XM-19 is a passive-film alloy: protection comes from the chromium oxide film, and molybdenum and nitrogen stabilise that film against chloride. Its behaviour therefore sits in the same family as 316L, only stronger — do not look here for the behaviour of a nickel-base reducing-acid alloy.​‌​​‌​

PREN — read the number correctly

PREN = %Cr + 3.3 × %Mo + 16 × %N. Calculated from the ends of XM-19’s specification bands: lower corner ≈28.7 (Cr 20.5 · Mo 1.50 · N 0.20) and upper corner ≈39.8 (Cr 23.5 · Mo 3.00 · N 0.40). A typical commercial heat lands at ≈33–35. Published secondary values of 31–40 and “>34” are consistent with that calculation.​‌​​‌​

Now the honest part. 316L’s PREN is typically 23–28; 2205 duplex is typically 33–35. So XM-19’s PREN sits in the same band as 2205, and many pages conclude from this that “XM-19 ≈ duplex”. That conclusion is wrong, because PREN predicts pitting initiation; it does not predict stress corrosion cracking. The SCC data below shows exactly that.

Intergranular corrosion

Better here than you would expect. ASTM A262 Practice B: 0.0009 in./month annealed at 1105 °C; 0.0022 in./month after a 1-hour sensitising treatment at 675 °C. ASTM A262 Practice E: passed in both conditions. The reason is Nb + V stabilisation. Even as-welded, A262 Practice C (Huey) performance matches the base metal. Despite C ≤0.06 %, it behaves in practice like a stabilised grade — a genuine advantage that deserves to be on the product page.​‌​​‌​

Sour service and NACE

S20910 is accepted under NACE MR0175 / ISO 15156-3 in the annealed or hot/cold worked condition at a maximum of 35 HRC. ISO 15156 allows a small tolerance on individual readings: the average of adjacent readings must stay within the limit and no single reading may exceed it by more than 2 HRC.
Commercial consequence: annealed (≤241 HB ≈ ≤22 HRC), HS (≤35 HRC) and XHS (≤35 HRC) comply; UHS (≤40 HRC) DOES NOT. One producer additionally declares NACE MR0103 (refinery equipment) compliance. Write the level and the hardness ceiling into your order acknowledgement — “NACE-compliant XM-19” on its own is an incomplete statement.​‌​​‌​

Where not to use it

1. Hot hydrochloric acid. In 2 % HCl at 80 °C it falls behind 317L. 2. Concentrated sulphuric acid. 3.38 mm/yr in 20 % H₂SO₄ at 80 °C — out of service. 3. Applications governed by chloride SCC. It is not better than 316; you need duplex or a high-nickel alloy. 4. Sliding metal-to-metal couples. Galling resistance is at 316 level; you need Nitronic 60. 5. Continuous high-temperature service. Mechanical capability is reported to 649 °C, but the sigma phase claim is unresolved and the alloy was not designed for this. 6. Reducing acids contaminated with ferric and cupric ions. It is a passive-film stainless; oxidising contamination triggers pitting.​‌​​‌​

Frequently Asked Questions

The customer wants “XM-19 bar, 120 ksi yield” and expects an ASTM A479 certificate. Can we supply it?​‌​​‌​

No — and the reason is not the material, it is the paperwork. This is the most frequent order mismatch on this alloy.
ASTM A479 (and A276) cover XM-19 in the annealed condition and the minimum is 55 ksi (380 MPa) yield. There is no “120 ksi yield” level inside the standard. 120 ksi is one producer’s XHS (“Super High Strength”) mill specification, and that strength comes from proprietary hot working practice. Another producer reaches a comparable strength by cold work under its own “Level 2/3” mill specification. All three are UNS S20910; all three are different products.
What we can supply is this: material certified to A479/A276 (and to ASME SA-479 if you wish) for chemistry, with mechanical properties certified to a named, numbered mill high-strength specification. The order line should read: “UNS S20910, chemistry per ASTM A479, mechanical properties per [mill] XHS specification: Rp0.2 ≥827 MPa, Rm ≥965 MPa, A ≥22 %, hardness ≤35 HRC.” An order that says only “A479 XM-19 120 ksi” cannot be filled and is a return waiting to happen.
Two more things must be said up front. First: this material cannot be welded — the heat-affected zone reverts to annealed values and the 120 ksi yield is simply not there. Second: no further heat treatment can be applied — the strength is taken back. If the customer intends to weld the part, there is no point buying XHS, and saying so before the order is far cheaper than arguing about it afterwards.

Your data sheet says “PREN 34, same as 2205 duplex”. Can we use XM-19 instead of 2205 on our seawater line?​‌​​‌​

It depends — and the decision rests not on PREN but on which damage mechanism you are afraid of. These two alloys share a PREN and behave very differently.
The PREN really is similar. XM-19 typically 33–35; 2205 typically 33–35. And in what PREN actually measures — pitting initiation — XM-19 is genuinely strong. The producer’s ferric chloride data is striking: on a creviced specimen XM-19 is <0.001 g/in² while Type 316 is 0.186 g/in². On a seawater line with crevice geometry, that is an order-of-magnitude improvement over 316.
What PREN does not measure is stress corrosion cracking, and there the table turns. In the producer’s own boiling 42 % MgCl₂ data, XM-19 failed BEFORE Type 316 at all three stress levels (5.0 h versus 7.0 h at 172 MPa). The producer’s own conclusion: “about as resistant as 316”. 2205, thanks to its two-phase structure, is dramatically superior to both austenitics in chloride SCC — that is precisely why duplex exists.
Decision rule: if your line has stagnant zones, deposits, gasket crevices and pipe supports and the temperature is moderate, XM-19 is a good choice — and it is non-magnetic and free of duplex’s ferrite-balance and sigma-phase worries. But if the line runs hot, carries stress (bent pipe, tight joints, weld residual stress) and concentrates chloride (evaporation, wet–dry cycling, under insulation), ask for 2205 or 2507. Assuming SCC parity from PREN parity is the most expensive mistake available between these two alloys.

We bought XM-19 because it is “non-magnetic”, welded the assembly, and the gauss meter deflects at the weld beads. Did we get the wrong material?​‌​​‌​

The material is almost certainly right. The specification was incomplete. This is the most common field complaint on XM-19, and its cause is not in the base metal but in the filler metal.
The base metal really is non-magnetic, and that is the alloy’s core selling point: µr = 1.002–1.004 and — unlike 304/316 — it stays non-magnetic after severe cold work and after cryogenic exposure. Nitrogen stabilises the austenite so thoroughly that no deformation martensite forms.
Weld metal is different. To obtain a sound, solidification-crack-free bead in austenitic stainless welding, the filler deliberately carries some delta ferrite. Measured on beads made with XM-19’s matching filler ER209 / E209: µr ≈1.2, ferrite number FN ≈6. Ferrite is magnetic. The difference between the base metal’s 1.004 and the bead’s 1.2 is easily visible to a gauss meter.
And this is not a defect, it is an engineering trade-off. Drive the ferrite to zero and you buy hot cracking risk — XM-19’s own ferrite potential is already low (FN ≈2), which is one of the reasons the producer does not recommend electron beam or laser welding.
What to do: if there is a µr ceiling, put it in the weld procedure qualification, not in the material order. Low-ferrite filler options, controlled use of autogenous passes, and — on critical beads — a post-weld 1121 °C solution anneal (which partially dissolves the ferrite, at a cost in distortion and money) can all be considered. Tie the acceptance criterion to a value measured on the assembly, not to the material certificate.

The supplier says “1.3964 = S20910” and offers European-origin material. Should we accept it?​‌​​‌​

Not without putting the two chemistries side by side. These materials are siblings, not twins, and the difference shows up on the certificate.
The largest divergence is nickel: S20910 is 11.50–13.50 %, 1.3964 is 15.00–17.00 %. The bands do not overlap at all — so no heat conforming to 1.3964 can conform to S20910, or vice versa. The second divergence is molybdenum: S20910 1.50–3.00 %, 1.3964 3.00–3.50 %; they touch at a single point. 1.3964 is also lower in carbon (≤0.03 % against ≤0.06 %) and much cleaner (S ≤0.010 %, P ≤0.025 %); it specifies no vanadium and allows ≤0.25 % tungsten.
The mechanicals differ too: 1.3964 gives Rm 700–950 MPa as a BAND — it has a ceiling. ASTM sets only a floor (Rm ≥690 MPa). A very high-strength heat can clear ASTM and still sit at the top of the 1.3964 range.
Practical answer: 1.3964 is not a bad material — it is a different material, and with its higher nickel and molybdenum it is probably better in chloride. If the customer expects an XM-19 certificate, ship XM-19; if they want 1.3964, ship 1.3964. Substituting one for the other is a documentation non-conformance, not a material one — and that kind of mismatch usually surfaces after the assembly is finished, during third-party inspection.

Common data sheet errors — check before you order​‌​​‌​

1. “1.3964 = S20910” — WRONG (or at least badly incomplete). The nickel bands do not overlap at all (11.5–13.5 % against 15–17 %), the molybdenum bands touch at a single point, and the carbon ceilings differ by a factor of two. Sibling grades, not the same material.
2. The EN name given as “X3CrNiMoCuNbN21-13-3” — WRONG. The EN short name of 1.3964 is X2CrNiMnMoNNb21-16-5-3. It contains Mn and contains no Cu, and the numbers are different too. This error is published in at least one distributor PDF in circulation.
3. Density “7.68 g/cm³ (0.285 lb/in³)” — ARITHMETICALLY IMPOSSIBLE. 0.285 lb/in³ = 7.88 g/cm³. A widely mirrored producer PDF carries this typographic error. Use 7.88.
4. Hardness “293” given as an XM-19 property. The ASTM annealed bar limit is ≤241 HB / ≤100 HRB; the A193 B8R bolting limit is ≤271 HBW / ≤28 HRC; the European 1.3964 limit is ≤279 HBW. 293 is none of those. Always write the scale and the document.
5. A sulphur limit of “≤0.010 %” quoted as the ASTM value — WRONG. ASTM A276/A479: S ≤0.030 %. 0.010 is a mill limit tightened for a producer’s high-strength product. The same error is made on carbon: ASTM ≤0.06 %, HS product ≤0.03 %.
6. Annealed and high-strength values mixed in one table. A page shows “yield 55 ksi” next to “yield 120 ksi” without saying which product is which. Annealed = 380 MPa · HS = 724 MPa · XHS = 827 MPa · UHS = 965 MPa, and the last three are NOT INSIDE ANY STANDARD.
7. Nuts for A193 B8R studs given as “A194 Grade 8” or “B8” — WRONG. The correct nut is A194 Grade 8R (or 8RA). “Gr 8” is a 304 nut and does not match an S20910 stud.
8. “1200 °F (629 °C)” — unit error. 1200 °F = 649 °C. This bad conversion has multiplied by copying out of a producer PDF.
9. “Superior to 316L and 317L in all media” — WRONG. In the producer’s own data, in 2 % HCl at 80 °C XM-19 (11.2 mm/yr) is BEHIND 317L (6.68 mm/yr). And in boiling acetic and formic acid all three are effectively identical — there is no case for paying for XM-19 there.
10. PREN quoted as a single number. The real range computed from the specification bands is ≈28.7 to ≈39.8; a typical heat is 33–35. Your heat’s actual PREN is computed from the real Cr, Mo and N on the mill certificate — not from a catalogue figure.
11. “PREN equals 2205, therefore as good as duplex” — WRONG. PREN predicts pitting, not SCC. In boiling 42 % MgCl₂ XM-19 fails slightly earlier than even 316.
12. “Non-magnetic” written without qualification. True for the base metal (µr 1.002–1.004, even cold worked). False for the weld metal (µr ≈1.2, FN ≈6). If the assembly carries a non-magnetic requirement, have the beads measured.
13. NACE compliance stated independently of the strength level. The MR0175/ISO 15156-3 ceiling is 35 HRC: annealed, HS and XHS comply; UHS (≤40 HRC) does not.
14. “Can be electron beam or laser welded” — NOT RECOMMENDED. The producer excludes these processes because of the low ferrite potential (FN ≈2) and severe nitrogen outgassing under vacuum.
15. “Nitronic 50 resists galling” — WRONG PAIRING. The galling alloy is Nitronic 60 (S21800). N50’s galling resistance is at 316 level. The two grades share a family name but not a job.

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