Differences in Welding Performance Between SUS321 and SUS304 Stainless Steel

Composition Differences and Their Effect on Weldability

SUS304 (UNS S30400, EN 1.4301) is an 18-8 austenitic grade with carbon up to 0.08% and no stabilizing elements. SUS321 (UNS S32100, EN 1.4541) adds titanium at least five times the carbon content, typically 0.1-0.8%. During welding thermal cycles, carbon in SUS304 can react with chromium to form chromium carbides at grain boundaries, depleting chromium and causing intergranular corrosion. In SUS321, titanium preferentially forms titanium carbide, protecting chromium and suppressing this tendency.

Hot Cracking Tendency

Both grades are austenitic and solidify with segregation of low-melting phases; the low eutectic temperature around 1350 degrees C makes the weld metal susceptible to hot cracking if heat input is excessive. Excessive heat input also reduces weld pool fluidity and increases cracking risk. SUS321 adds a specific concern: titanium oxidizes easily in the arc, forming titanium oxide inclusions that reduce weld toughness, so high-purity shielding gas is essential.

High-Temperature Performance of Welded Joints

The recommended maximum service temperature for SUS304 weldments is about 600 degrees C, above which grain growth reduces creep strength. The titanium addition in SUS321 improves creep resistance, allowing service up to about 700 degrees C with better joint stability at 500-700 degrees C. This makes SUS321 weldments preferred in petrochemical and high-temperature process plant.

Filler Material Selection

Process	SUS304 filler	SUS321 filler
SMAW	E308-16, E308L-16	E347-16, E321-16
GTAW/TIG	ER308, ER308L	ER347, ER321

Niobium-stabilized E347/ER347 is often preferred for SUS321 because niobium transfers through the arc more reliably than titanium.

Welding Process Parameters

Both materials need low heat input, meaning low current and fast travel speed, to limit grain coarsening and hot cracking. Use high-purity argon, at least 99.99%, for shielding. For SUS321, the higher oxidation sensitivity of titanium demands a shorter arc length, about 1-3 mm, and faster travel to protect the weld pool. For SUS304, control the interpass temperature, typically not above 150 degrees C, to avoid sensitization.

Post-Weld Heat Treatment (PWHT)

SUS304 weldments generally do not require PWHT. In aggressive corrosive service, solution annealing at about 1050 degrees C with rapid cooling restores corrosion resistance. SUS321 weldments can be given a stabilization treatment at 900-950 degrees C with slow cooling, allowing titanium to combine with residual carbon and eliminating the intergranular corrosion risk.

FAQ

1. Which grade is easier to weld?

SUS304 is simpler because no stabilizing element needs protection; SUS321 requires stricter shielding to prevent titanium oxidation.

2. Why is E347 filler recommended for SUS321?

Niobium stabilizes carbon in the weld metal and transfers more reliably across the arc than titanium.

3. Does SUS321 need PWHT?

Not normally; stabilization at 900-950 degrees C is applied only when maximum corrosion resistance is required.

4. What shielding gas is required?

High-purity argon, at least 99.99%, with a short arc of about 1-3 mm for SUS321.

5. Can SUS304 weldments serve at 700 degrees C?

Not recommended; limit SUS304 to about 600 degrees C, while SUS321 is suitable up to about 700 degrees C.

6. What is intergranular corrosion?

Localized attack at grain boundaries caused by chromium depletion from carbide precipitation during welding or high-temperature exposure.