Nitrogen-strengthened austenitic stainless
Nitronic 40 Forgings 21-6-9 · UNS S21900 and S21904 · XM-10, XM-11 · AMS 5656
Nitronic 40, called 21-6-9 through most of the supply chain, replaces part of the nickel in a conventional austenitic stainless with manganese and nitrogen. The result is an annealed yield strength roughly twice that of Type 304, 321 and 347, toughness that holds to liquid-hydrogen temperature, and a steel that stays non-magnetic after heavy cold work.
We melt this grade in our own furnaces and forge it on site: seamless rolled rings to 6,000 mm outside diameter, single pieces to 30 tonnes, shafts turned to 14,000 mm.
Chemical Composition
Two chemistries share the 21-6-9 name and the difference is carbon. Tell us which UNS number your drawing calls for at enquiry stage, because it governs how we aim the heat and what the certificate must state.
| Grade | C | Cr | Ni | Mn | N | Si | P | S |
|---|---|---|---|---|---|---|---|---|
| S21900 · XM-10 | 0.08 max | 19.0–21.5 | 5.5–7.5 | 8.0–10.0 | 0.15–0.40 | 1.00 max | 0.045 max | 0.030 max |
| S21904 · XM-11 | 0.04 max | 19.0–21.5 | 5.5–7.5 | 8.0–10.0 | 0.15–0.40 | 1.00 max | 0.060 max | 0.030 max |
| Typical heat | 0.03 | 20.0 | 6.5 | 9.0 | 0.25 | 0.40 | — | — |
Residual molybdenum and copper are each normally held to 0.75 percent maximum. Limits per ASTM A276 and A479 and published mill data.
Mechanical Properties
The point of this grade is strength without heat treatment. Interstitial nitrogen roughly doubles the yield point against conventional 18-8 stainless, so a part can often be drawn thinner in 21-6-9 than in 304 and still carry the load.
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength, min | 620 MPa | 90 ksi |
| Yield strength 0.2 % offset, min | 345 MPa | 50 ksi |
| Elongation in 50 mm or 4D, min | 45 % | 45 % |
| Hardness, typical annealed | ≈ 220 HBW | ≈ 95 HRB |
| Section | Direction | Yield 0.2 % | Tensile | Elong. 2 in. | R. of area |
|---|---|---|---|---|---|
| 152 mm (6 in.) square | Longitudinal | 428 MPa / 62 ksi | 655 MPa / 95 ksi | 46 % | 69 % |
| 152 mm (6 in.) square | Transverse | 358 MPa / 52 ksi | 641 MPa / 93 ksi | 30 % | 45 % |
| 102 mm (4 in.) square | Longitudinal | 386 MPa / 56 ksi | 668 MPa / 97 ksi | 53 % | 73 % |
| 25.4 mm (1 in.) square | Longitudinal | 358 MPa / 52 ksi | 696 MPa / 101 ksi | 53 % | 73 % |
Check before you orderTransverse ductility in heavy sections drops to about 30 percent elongation and 45 percent reduction of area, against 46 and 69 percent longitudinally. If your specification calls for transverse or tangential coupons on a heavy ring or block, say so with the enquiry so the upset ratio and ring-rolling reduction can be planned to meet it.
Response to cold work
The grade takes no hardening from heat treatment and responds only to cold reduction. Cold-worked 21-6-9 reaches a 0.2 percent yield around 745 MPa (108 ksi) with tensile near 883 MPa (128 ksi), and cold-drawn tube is commonly specified at a yield of 827 MPa minimum. ASTM A479 sets higher minima for certain high-strength conditions and sizes, so quote the exact condition you need rather than the grade alone.
Physical Properties
| Property | Metric | Imperial |
|---|---|---|
| Density | 7.83 g/cm³ | 0.283 lb/in³ |
| Modulus of elasticity | 200 GPa | 29,000 ksi |
| Shear modulus | 78 GPa | 11 × 10⁶ psi |
| Poisson's ratio | 0.28 | 0.28 |
| Thermal expansion, 25–95 °C | 16.7 µm/m·°C | 9.28 µin/in·°F |
| Thermal conductivity | ≈ 14 W/m·K | ≈ 8.1 BTU/h·ft·°F |
| Specific heat | 480 J/kg·K | 0.12 BTU/lb·°F |
| Melting range | 1350–1400 °C | 2470–2550 °F |
| Machinability, AISI 1212 = 100 | 30 | 30 |
| PREN, calculated | ≈ 25 | |
Corrosion and Temperature
In oxidising service the corrosion resistance is broadly comparable to Type 304, with a calculated pitting resistance equivalent near 25. Published data records excellent stress-corrosion cracking resistance in long-term seacoast atmospheric exposure, and the manganese and nitrogen content gives good resistance to high-temperature oxidation in air. Continuous service is limited to below 871 °C (1600 °F).
At the cold end the alloy keeps excellent toughness to −423 °F, about −253 °C, which is liquid-hydrogen temperature. That property is the main reason 21-6-9 appears in aerospace and cryogenic tankage, piping, valves and fittings rather than a cheaper austenitic.
Where this grade is the wrong choice21-6-9 carries no molybdenum. For seawater, firewater, subsea manifolds, flue gas desulphurisation, or strong chloride and acid duty, a molybdenum-bearing or duplex grade is correct. We forge 316L, 904L, 254 SMO, AL-6XN, F51, F53, F55 and Hastelloy C-276 for exactly those services. Describe the medium, temperature and pressure in your enquiry and we will tell you if a different grade suits the job better.
Forging and Fabrication
Hot working
Heat to 1150–1205 °C (2100–2200 °F) for forging. Published guidance puts finishing at 927–982 °C (1700–1800 °F) followed by water or air quench. The alloy is stiff and will tear if worked below the recommended range.
Annealing
Anneal at 982–1177 °C (1800–2150 °F) and air cool. In-process anneals typically run 1065–1120 °C. The annealed condition also gives the best stress-rupture life between 705 and 871 °C.
Hardening
Unresponsive to hardening heat treatment. Any strength above the annealed minimum comes from cold reduction, so specify the condition rather than a hardness target alone.
Stress relief
Stress-relieve below 705 °C (1300 °F). Treatment in the 480–675 °C band alters mechanical properties, so confirm the intent before it is written into a procedure.
Welding
Readily welded by GTAW, GMAW and SMAW. Matching filler is the AWS ER219 or 209 type, with 308L on less demanding joints. The high nitrogen causes severe outgassing in electron-beam welding.
Machining
Rated about 30 against free-machining 1212 at 100. Work hardening is rapid, so use rigid setups, sharp positive-rake tools, heavy positive feed, slow speed and flood coolant. Never let a tool dwell.
Specifications and Equivalents
| Specification | Product form |
|---|---|
| AMS 5656 | Bars, wire, forgings, extrusions and rings |
| ASTM A473 | Stainless steel forgings |
| ASTM A314 | Billets and bars for forging |
| ASTM A182 | Forged and rolled pipe flanges, fittings, valves and parts for high-temperature service |
| ASTM A276 / A479 | Bars and shapes; A479 for boiler and pressure vessel use |
| AMS 5595 | Plate, sheet and strip |
| AMS 5561 / AMS 5562 | Welded and drawn tubing / seamless tubing |
| ASTM A312 / A580 / A666 | Pipe / wire / sheet, strip, plate and flat bar |
| Other names | Nitronic 40, 21-6-9, Alloy 219, Alloy 40, XM-10, XM-11, UNS S21900, UNS S21904, ER219 filler |
Where It Is Used
Specifiers reach for 21-6-9 when a part needs double the yield of 304 at austenitic prices, together with cryogenic toughness and low magnetic permeability. Established uses include aircraft and engine components, clamps, flanges and hydraulic tubing; cryogenic tanks, piping, valves and fittings; steam and autoclave parts; chemical processing and pollution control equipment; and high-strength non-magnetic fasteners, shafting and structural hardware.
In forged form the grade most often leaves this works as seamless rolled rings and flange blanks for aerospace and cryogenic assemblies, forged shafts and spindles where permeability matters, forged discs, sleeves and bushings, heat-exchanger tube sheets for moderate service, and valve seat rings, bodies and stems for ball, gate, globe, check and plug valves.
How We Make It
Most forging suppliers in China buy billet. This works starts at the furnace, which matters for a nitrogen-alloyed grade: nitrogen content decides whether the steel meets its yield minimum, and holding it inside 0.15 to 0.40 percent is a melt-shop job, not something you can correct later. Vacuum oxygen decarburisation gives that control directly.
| Melting | Electric arc furnace with ladle refining and vacuum oxygen decarburisation for nitrogen and carbon control. Protective-atmosphere electroslag remelting available for cleanness-critical work. Maximum heat 60 t. |
|---|---|
| Forging | 6,300 t, 4,000 t and 2,000 t hydraulic open-die presses; hammers from 750 kg to 5 t. |
| Ring rolling | Radial-axial mills at 6 m, 3 m and 1 m. Rings from OD 50 mm by upset and punch, up to OD 6,000 mm rolled. |
| Single-piece weight | 10 kg to 30,000 kg, no minimum piece count. |
| Heat treatment | 14 furnaces including car-bottom chambers to 8,000 mm long and 15 m deep well furnaces for vertical solution quench. |
| Machining | Vertical lathes to 5,000 mm swing, horizontal lathes to 14,000 mm between centres, CNC deep-hole drilling to 1,600 mm dia by 10,000 mm. |
| Forms supplied | Seamless rolled rings, forged rings, flanges, shafts, discs, tube sheets, sleeves, bushings, hollow cylinders, bars and blocks, valve and wellhead parts. |
| Testing | Ultrasonic to EN 10228-3, SEP 1921 or ASTM A388. PT, MT, PMI, intergranular corrosion and cryogenic impact testing on request. |
| Certification | EN 10204 3.1 mill certificate, or 3.2 witnessed by TÜV, BV, DNV, SGS, LR or your own inspector. Works approvals CCS, BV, DNV, LR, NK. |
| Lead time | 20 to 60 days depending on size, melt route and test scope. |
Traceability runs from melt number to shipped part because no step leaves the site. For a nitrogen-strengthened grade that also means the chemistry can be aimed at the forging it will become, instead of accepted as whatever a billet supplier delivered.
Questions We Get Asked
What is Nitronic 40 stainless steel?
Nitronic 40, known industrially as 21-6-9, is a nitrogen-strengthened high-manganese austenitic stainless steel with 19.0 to 21.5 percent chromium, 5.5 to 7.5 percent nickel, 8 to 10 percent manganese and 0.15 to 0.40 percent nitrogen. Nitrogen held in solid solution gives roughly twice the annealed yield strength of Type 304, 321 and 347 while the steel keeps austenitic toughness and weldability.
What is the difference between UNS S21900 and UNS S21904?
Carbon. S21900, ASTM type XM-10, allows up to 0.08 percent. S21904, type XM-11, is the tighter low-carbon version at 0.04 percent maximum. Most 21-6-9 bar and forging stock made today is produced to S21904, so check which one your drawing names before ordering.
Is Nitronic 40 magnetic?
No. It is a stable austenitic grade, and published Armco product data states it stays non-magnetic even after severe cold working. That is why it is chosen for instrument shafts and aerospace hardware where low magnetic permeability is a design requirement.
How does it compare with Nitronic 50?
Nitronic 50 (UNS S20910, XM-19) adds molybdenum, niobium and vanadium with higher chromium and nickel, giving corrosion resistance comparable to or better than 316L along with higher strength. Nitronic 40 is the lower-cost option where strength and cryogenic toughness drive the choice and the environment is not strongly chloride-bearing.
Can it be hardened by heat treatment?
No. Like other austenitic stainless steels it does not respond to hardening heat treatment. Strength above the annealed minimum comes only from cold reduction, reaching around 745 MPa yield and 883 MPa tensile in the cold-worked condition.
Which welding filler should be used?
GTAW, GMAW and SMAW all work. Matching filler is the AWS ER219 or 209 type; 308L is used on less demanding joints. A post-weld anneal restores corrosion resistance. Because nitrogen content is high, electron-beam welding can suffer severe outgassing.
Can you forge Nitronic 40 to our drawing?
Yes. We melt and forge this grade at Zhouzhuang Town, Jiangyin, Jiangsu Province, China. Rings go to 6,000 mm outside diameter on the 6 m radial-axial mill, single pieces to 30 tonnes under the 6,300 t press, shafts to 14,000 mm turned length. Minimum order is 10 kg with no minimum piece count. Send a drawing or a size list to sales@steelforgepieces.com or call +86 189 2135 9659.
Request a Quotation
Send a drawing, a sketch, or just the dimensions. To price a Nitronic 40 forging accurately we need four things:
- Grade and UNS number: Nitronic 40 / 21-6-9, S21900 or S21904
- Shape, dimensions and quantity, stating whether sizes are rough or finished
- Governing standard: ASTM A473, A314, A276, A479, A182 or AMS 5656
- Test condition, NDT scope and certificate type, EN 10204 3.1 or 3.2
Quotations normally go out within one working day. If a different alloy suits the duty better, we will say so.
No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China
T +86 189 2135 9659
E sales@steelforgepieces.com
W www.steelforgepieces.com