Heat Treatment and Welding of SUS321 Stainless Steel

Why Heat Treatment Matters for SUS321

SUS321 (JIS designation, EN 1.4541, UNS S32100) is an austenitic stainless steel stabilized with titanium, with a titanium content of at least five times the carbon content. Titanium combines with carbon to form titanium carbide, preventing chromium carbide precipitation at grain boundaries during welding or high-temperature exposure. Correct heat treatment preserves this stabilization and determines the corrosion and high-temperature performance of the material.

Solution Annealing

The standard heat treatment is solution annealing at 1050-1100 degrees C, followed by rapid cooling in water or by fast air cooling. This dissolves alloying elements into the austenitic matrix, restores ductility after cold working and keeps the titanium available for subsequent stabilization. For plate and sheet, this treatment is specified in JIS G4304 and G4305; the mechanical requirements include a minimum tensile strength of 520 MPa and a minimum yield strength of 205 MPa.

Stabilization Treatment

For service in the sensitization range (roughly 425-860 degrees C) or after welding, a stabilization treatment at about 850-930 degrees C followed by slow cooling allows titanium to react fully with carbon, precipitating titanium carbide and eliminating the risk of intergranular corrosion. This treatment is characteristic of stabilized grades such as SUS321; it should not be confused with the aging treatments used for precipitation-hardening steels.

Stress Relief of Cold-Worked Parts

Where stress relief is required after severe cold forming, it is carried out in the stabilization range rather than in the sensitizing window, so that the material keeps its corrosion resistance. Low-temperature stress relief below about 400 degrees C gives only partial relaxation and is rarely used for austenitic grades.

Welding of SUS321

SUS321 has good weldability, but titanium is easily oxidized in the arc. Use clean surfaces, high-purity argon shielding (at least 99.99%), a short arc and low heat input. TIG and MIG welding are common; SMAW is used for thicker sections. Preheat is generally not required; keep the interpass temperature low, typically not above 150 degrees C, to limit grain growth. Recommended filler metals are ER347/E347 (niobium-stabilized) or ER321/E321 (titanium-stabilized). Post-weld heat treatment is not normally required; where maximum corrosion resistance is specified, a stabilization anneal at 850-950 degrees C with slow cooling is applied.

FAQ

1. What is the solution annealing temperature for SUS321?

1050-1100 degrees C with rapid cooling, by water quenching or fast air cooling.

2. Does SUS321 need aging treatment?

No. Stabilized austenitic grades use stabilization annealing at about 850-930 degrees C; aging treatments apply to precipitation-hardening grades.

3. Why stabilize at 850-930 degrees C?

To precipitate titanium carbide so that carbon cannot form chromium carbide at grain boundaries during later service.

4. Which filler metal is used for welding SUS321?

ER347/E347-16 (niobium-stabilized) is commonly used; ER321/E321-16 is an alternative.

5. Is preheating required?

Generally no. Keep the interpass temperature low, preferably not above 150 degrees C.

6. What is the service temperature limit of SUS321?

For creep-resisting service, SUS321 is suitable up to about 700 degrees C, higher than unstabilized 304.

7. Is post-weld heat treatment required?

Not normally; a stabilization anneal at 850-950 degrees C is applied only when maximum corrosion resistance is required.