Overview

S21900 is a nitrogen-strengthened, high-manganese austenitic chromium-nickel stainless steel (nominally 21% chromium, 6% nickel, 9% manganese — the "21-6-9" composition) that combines high strength with good corrosion resistance and excellent toughness over a very wide temperature range. This datasheet presents the material within the American standard system.

Its room-temperature yield strength is about twice that of the Type 304/321/347 grades, achieved through nitrogen solid-solution strengthening and the high manganese content rather than through cold work or molybdenum. The high manganese and nitrogen stabilise the austenite so effectively that the alloy remains essentially non-magnetic even at cryogenic temperatures or after substantial (up to ~60%) cold work. It offers good high-temperature oxidation resistance in air, resists stress-corrosion cracking in severe marine environments at ambient temperature, and retains high strength and excellent toughness down to cryogenic temperatures (to about -230 °C / -423 °F). Maximum recommended temperature for continuous service is below about 871 °C (1600 °F).

It is readily welded by TIG, MIG and shielded-metal-arc methods (the high nitrogen content can cause outgassing problems in electron-beam welding) and can be hot- and cold-worked, though it is stronger than Type 304 and requires more power. Typical applications include aircraft-engine components, clamps, flanges and hydraulic tubing, chemical-processing and pollution-control equipment, steam and autoclave components, and cryogenic equipment.

1. Physical Properties

Values per manufacturer / reference data, annealed condition.

2. Chemical Composition (wt %)

Limiting composition per ASTM A276 / AMS 5656.

Nominal: 21Cr-6Ni-9Mn with nitrogen. High manganese and nitrogen stabilise the austenite and provide strength, allowing a reduced nickel content; the closely-related XM-11 variant is a lower-carbon / higher-nitrogen modification.

3. Mechanical Properties

Typical room-temperature properties, annealed condition.

Room-temperature yield strength is about twice that of Types 304/321/347, from nitrogen and manganese strengthening. The alloy retains high strength and toughness from cryogenic to elevated temperatures. Values are typical; confirm against the mill test certificate.

4. Corrosion and High-Temperature Resistance

5. Heat Treatment

An austenitic stainless steel; it cannot be hardened by heat treatment. High strength comes from nitrogen and manganese; strength can be raised further by cold work.

Solution Anneal approximately 1040–1065 °C (in-process annealing ~1065 °C / 1950 °F), followed by rapid cooling, to dissolve carbides and produce a uniform austenitic structure.

The austenite is highly stable: the alloy remains non-magnetic even after substantial cold work or at cryogenic temperatures. Hot working should be done in the range ~1150–1200 °C (2100–2200 °F).

6. Weldability and Joining

Readily welded by gas-tungsten-arc, gas-metal-arc and shielded-metal-arc methods, using a matching 21-6-9 filler. The high nitrogen content can cause severe outgassing in electron-beam welding, so that process is generally avoided.

No preheat required. Keep joints clean; matching 21-6-9 filler maintains strength and corrosion resistance.

7. Machinability and Fabrication

Machining Guidelines

Forming Processes

8. Applications

9. Available Product Forms and Standards

Nitrogen-strengthened high-manganese 21-6-9 austenitic stainless steel. UNS S21900 / S21904; AMS 5656 / 5595 / 5561 / 5562; ASTM A276 / A412 / A580. Tradename: Nitronic 40 (registered trademark of Cleveland-Cliffs).

10. Comparison with Related Alloys (UNS System)

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