Stainless steel 21-6-9 - AMS 5561 - Nitronic 40 - S21900 - X2CrMnNi21-6-9 - Z4CMN 21-9-6

General Introduction to 21-6-9

Thanks to its high chromium content, this steel resists oxidation and corrosion even under aggressive conditions such as intergranular corrosion. Its reinforcement with manganese and nitrogen increases the strength and stability of the austenite and gives it excellent toughness, which it retains down to temperatures close to those of liquid helium (-269 °C or -516 °F).

The 21-6-9 alloy does not respond to heat treatments for hardening by quenching, tempering, or aging, unlike martensitic steels. On one hand, it does not contain enough aluminum, titanium, or copper to allow for age hardening. On the other, its austenite is too stable at low temperatures for quenching or tempering to be effective. However, cold working to increase hardness is possible—or hot working to improve ductility—without degrading the material.

Hot Working, Cold Working

21-6-9 stainless steel should be hot worked in a temperature range between 927 °C and 1204 °C (1700–2200 °F). Maintaining a controlled temperature is essential to avoid damaging the material’s microstructure.

• Heat the alloy evenly to ensure uniform deformation.
• Cool rapidly to room temperature (water or air) to preserve mechanical properties.
• Serve while it's hot.

Cold working includes rolling, drawing, and stamping at room temperature. Due to its high work hardening rate, 21-6-9 requires higher forming forces than other austenitic stainless steels (such as 302, 304, and 316). An intermediate anneal is necessary to reduce hardness and facilitate further cold working. This process (cold work → anneal → further cold work) significantly increases its yield strength.

Mechanical and Chemical Properties

When hardened (work hardened), 21-6-9 has a yield strength of at least 827 MPa, ensuring durability under heavy loads. Its tensile strength can reach 1117 MPa. Its elongation capability is at least 35 % in the annealed state, and it maintains at least 20 % elongation when hardened.

This alloy remains non-magnetic even after severe cold working, an important characteristic of Nitronic grades (unlike 301/304 steels, which can become slightly magnetic after work hardening due to martensite formation). The austenite in 21-6-9 is very stable.

Machinability, Formability

Because of its high work hardening rate, 21-6-9 is more difficult to machine than most austenitic stainless steels. Similar to type 316, it is recommended to use slow cutting speeds, positive feeds, and ample cutting fluids.

It offers good formability and handling in the annealed condition. However, its high yield strength and work hardening rate require greater forces during forming than other steels like 304L. Techniques such as bending, drawing, and stretching can be applied, but an intermediate anneal may be necessary to prevent cracking and ensure uniform deformation.

Weldability

21-6-9 is considered to have good weldability. Several welding methods can be successfully applied:

• TIG welding produces precise welds. An inert gas such as argon or helium protects the weld zone.
• MIG welding offers high deposition rates.
• SMAW (shielded metal arc welding) is also suitable. A complete post-weld anneal is recommended to reduce residual stresses and restore corrosion resistance.

Oxy-acetylene welding should be avoided, as it risks carbon buildup that would compromise the material’s properties.

Industrial applications

In addition to the aerospace industry, the 21-6-9 alloy is also used for:

• Cryogenic applications: tanks, piping, valves, and fittings for cryogenic storage and transport vessels (LNG, liquid nitrogen, liquid helium)
• Chemical and petrochemical processing: pressure vessels and reactors
• Marine applications: propeller shafts for boats and ships
• Automotive (high-performance / specialized)
• Nuclear