How to Eliminate Magnetism in SUS321 Stainless Steel After Cold Working

Why Cold Working Makes SUS321 Magnetic

SUS321 is an austenitic stainless steel, and in the annealed condition it is essentially non-magnetic. Cold working such as bending, drawing, or rolling deforms the austenite lattice and can trigger a partial transformation to strain-induced martensite, a magnetic phase. The more severe the deformation, the more martensite forms and the stronger the magnetic response becomes.

The Core Method: Solution Annealing

The magnetism introduced by cold working can be eliminated by heat treatment. The principle is to heat the material into the austenitizing range so that the strain-induced martensite dissolves and transforms back into austenite, then cool rapidly enough to retain the austenitic structure. The result is a fully non-magnetic component with restored corrosion resistance.

Process Parameters

The typical solution annealing temperature for SUS321 is 1000 to 1100 degrees C, chosen within the stabilization range of the titanium-stabilized grade. Too low a temperature leaves martensite undissolved, and too high a temperature causes excessive grain growth that impairs mechanical properties. Soaking time is generally 1 to 2 minutes per millimeter of thickness to ensure uniform transformation. Cooling must be rapid, preferably water quenching, with air cooling acceptable only for thin sections, because slow cooling allows chromium carbide precipitation or martensite reformation.

Operational Steps

Clean the surface to remove oil, scale, and contaminants before heating. Use a box or vacuum furnace and hold temperature within about plus or minus 10 degrees C, starting the soak clock once the part reaches the target temperature. Ensure uniform temperature distribution to avoid localized incomplete transformation. Quench immediately in room-temperature water, or air cool quickly for thin parts, and use staged cooling for thick-walled components to limit internal stress. Remove any oxide scale afterward by pickling or mechanical polishing.

Quality Verification

After treatment, verify the result with a magnet test and, for critical parts, metallographic examination to confirm the absence of martensite and any carbide precipitation. Corrosion testing may be required where the part serves in sensitizing temperature service.

Frequently Asked Questions

Q: Why is cold-worked SUS321 magnetic?

A: Cold deformation transforms some austenite into magnetic strain-induced martensite.

Q: What heat treatment removes the magnetism?

A: Solution annealing at 1000 to 1100 degrees C followed by rapid cooling.

Q: How long should the part soak?

A: About 1 to 2 minutes per millimeter of thickness.

Q: Can air cooling be used?

A: Only for thin sections; water quenching is preferred to avoid carbide precipitation.

Q: Does annealing restore corrosion resistance?

A: Yes, it restores the austenitic structure and the passive film performance.

Q: What if the part is complex in shape?

A: Secure it against deformation and ensure uniform furnace temperature.

Contact us for SUS321 strip, sheet, and tube in the annealed condition, or for guidance on your heat treatment cycle.