ALLOY 40 Forgings, 21Cr-6Ni-9Mn / UNS S21900Seamless rolled rings, forged rings, flanges, shafts, discs, sleeves and bars in 21-6-9 nitrogen-strengthened stainless steel

Manufacturer: Jiangyin Jiangnan Metal Co., Ltd., No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province 214423, China · Reviewed 2 October 2026 · Markdown version

The grade

ALLOY 40 sits in the nitrogen-strengthened austenitic family. Rather than relying on nickel alone to hold austenite stable, it substitutes manganese and nitrogen, which is why nickel runs at only 5.5 to 7.5 percent while manganese sits at 8 to 10 percent. Nitrogen in solid solution lifts strength without a hardening heat treatment, and the low carbon of the S21904 variant keeps the steel resistant to intergranular attack even as welded.

Producer datasheets describe it as combining high strength in the annealed condition with good resistance to oxidation at high temperature, good resistance to lead oxide, and a high level of corrosion resistance at ambient temperature. It forms and fabricates much like Type 304 and 316, welds readily, and stays non-magnetic after heavy cold work.

Designations and specifications

Most material sold as 21-6-9 is now made to UNS S21904 (XM-11), a tighter and lower-carbon chemistry than the original S21900 (XM-10). The two differ mainly in carbon, so state the UNS number on the enquiry rather than the trade name alone.

Chemical composition

Carbon separates the variants. Datasheets quote 0.04 percent maximum and 0.03 percent maximum depending on variant, with the original S21900 (XM-10) permitting more. Check against the current revision of the governing specification.

Melting route: electric arc furnace, ladle furnace and vacuum oxygen decarburisation, with electroslag remelting under protective atmosphere available where tighter cleanliness applies.

Mechanical properties at room temperature, annealed

The transverse ductility drop in heavy sections is the figure to watch: 30 percent elongation and 45 percent reduction of area against 46 and 69 percent longitudinal in the same 152 mm square. State the required test direction and sampling position at enquiry stage.

Annealing temperature moves these figures. On strip, 982 C leaves roughly 483 MPa yield and 793 MPa tensile at 44 percent elongation and HRB 95, while 1066 C gives about 421 MPa and 738 MPa at 48 percent elongation and HRB 90.

Elevated temperature properties

Stress-rupture strength annealed is about 221 MPa for rupture in 100 hours at 649 C, falling to roughly 40 MPa for 100 hours and 21 MPa for 1,000 hours at 816 C.

Physical properties

Corrosion and oxidation resistance

Corrosion resistance approaches Type 304L, and the low carbon gives resistance to intergranular corrosion in the welded condition. Surfaces must be free of scale and embedded particles, and finished parts should be passivated.

The grade also resists high temperature oxidation in air and attack by molten lead oxide.

Forging, hot working and heat treatment

Forging

Start temperature 1150 to 1205 C. Preheating to an intermediate temperature is not required. The grade is not susceptible to hot shortness in its working range, can be hot worked down to about 650 C, and forgings can be cooled rapidly without danger of cracking. Anneal after forging for best corrosion resistance.

Solution annealing

982 to 1177 C, air cool. Grain coarsening occurs toward the top of the band, so where a grain-size limit applies the lower end or around 900 C is used. The annealed condition gives the best stress-rupture life and is used for service between 704 and 871 C and for maximum formability. After annealing, strength can only be raised by cold work or hot-cold work.

Stress relieving

482 to 760 C, giving the best all-round properties below 704 C. From 482 to 677 C there is little effect on the properties of cold-reduced material unless cold reduction was heavy. Above 677 C properties change more rapidly, and 816 C starts to cause rapid softening.

Hot-cold working

Normally 649 to 816 C with reductions of about 10 to 14 percent, followed by a 482 to 760 C stress relief.

Machining, cold forming and welding

Machining

Machines with the techniques used on austenitic stainless steels. A rigid setup and ample coolant flow are what matter. The high work-hardening rate rewards sharp tooling and positive feeds and punishes light rubbing cuts and dwell.

Cold working

Works by conventional methods, but the high work-hardening rate and higher starting yield strength mean more forming force than the same part in Type 301, 302, 304 or 316.

Welding

Satisfactory by shielded fusion and resistance processes. Oxyacetylene welding is not recommended because of carbon pickup. AWS E/ER219 filler gives welds approaching base-metal strength; E/ER309 serves where high weld strength is not required. A post-weld anneal restores intergranular corrosion resistance.

Forged shapes supplied in ALLOY 40

Presses run to 6,300 t with 4,000 t and 2,000 t presses and hammers from 750 kg to 5 t. Maximum heat size 60 t, heaviest single forging 30 t. Minimum order 10 kg with no minimum piece count. Lead time 20 to 60 days.

Applications

Aircraft and aero-engine parts. Engine components and hydraulic and fuel line systems, where the strength-to-weight ratio allows thinner walls. AMS 5656 and AMS 5595 exist for this market.

Chemical process and pollution control equipment. Casings, housings, shafts, flanges and rings where 304L-class corrosion resistance has to arrive with roughly double the yield strength, and where as-welded intergranular resistance avoids a post-weld solution anneal on a large assembly.

Steam and autoclave service. Closures, retaining rings, covers and fasteners working hot and under pressure, using retained strength through 540 to 650 C.

High temperature oxidising and lead oxide environments. Furnace and heat-treatment fixtures, hangers and structural parts up to the 871 C ceiling.

Magnetically transparent parts. Components that must remain non-magnetic after heavy cold forming.

Testing and certification

Steel is melted on site, so chemistry is verified at the furnace rather than taken from a billet supplier's certificate, and the melt number follows the part through forging, heat treatment, machining and shipment. The works holds CCS, BV, DNV, LR and NK approvals.

In-house: optical emission spectrometry to ASTM E415, infrared carbon and sulphur to ASTM E1019, positive material identification, tensile to ASTM A370 and ISO 6892 on 600 kN and 300 kN machines, Charpy V-notch impact to minus 60 C, ultrasonic examination to ASTM A388 and EN 10228-3, magnetic particle to ASTM E709, hardness, grain size to ASTM E112.

Through accredited third-party laboratories: intergranular corrosion to ASTM A262, impact below minus 60 C, bench Brinell or Rockwell hardness where the specification requires it.

Certification: EN 10204 3.1 as standard; EN 10204 3.2 witnessed by BV, DNV, LR, SGS, TUV or the customer's inspector on request, with heat treatment charts, NDT reports, dimensional reports and PMI records.

Questions

Data sources. Composition, mechanical, physical, corrosion, heat treatment and fabrication data are compiled from published 21Cr-6Ni-9Mn producer datasheets and from the scope of ASTM A314, ASTM A276 and the AMS specifications cited. Values are typical and given for guidance, not as a warranty. Properties of a given forging depend on section size, orientation, delivery condition and the specification revision in force.