Article
Grades 321 and 347 are austenitic stainless steels built for service in the temperature range where ordinary 304 can lose its corrosion resistance. They are called stabilised grades because of a small, deliberate alloy addition.
The sensitisation problem
When 304 is held between about 425 °C and 815 °C, chromium combines with carbon to form chromium carbides at the grain boundaries. The metal next to the boundaries loses chromium and becomes prone to intergranular corrosion. This is called sensitisation.
How stabilisation works
- 321 contains titanium (at least 5 × C + N, up to 0.70%).
- 347 contains niobium (columbium) (at least 10 × C, up to 1.10%).
Titanium and niobium bond with carbon more strongly than chromium does, so the chromium stays in solution and keeps protecting the steel.
321 vs 347 at a glance
| Point | 321 | 347 |
|---|---|---|
| Stabilising element | Titanium | Niobium |
| Weld behaviour | Ti can be lost in the arc; 347 filler is usually used | Nb transfers well across the arc |
| Availability | Widely available in pipe, plate and fittings | Common in pipe and fittings; often preferred for welded components |
| High-temperature versions | 321H | 347H |
Typical applications
- Refinery and petrochemical heater and furnace piping
- Exhaust manifolds and expansion joints
- Boiler and heat exchanger tubes
- Hydroprocessing units exposed to polythionic acid during shutdown
Welding note
Because titanium oxidises easily in the arc, 321 is normally welded with 347 filler metal (ER347). This gives a stabilised weld deposit.
Ordering
Pipe 6" SCH 40S, ASTM A312 TP321, seamless – with matching ASTM A403 WP321 fittings and A182 F321 flanges
Frequently asked questions
What stabilises 321 stainless steel?
Titanium, which ties up carbon and prevents chromium carbide formation.
Why is 347 filler used to weld 321?
Titanium is easily lost in the arc, while niobium in 347 filler transfers well, giving a stabilised weld.
What temperature range are 321 and 347 designed for?
They are designed for service roughly in the 425–815 °C range where 304 would sensitise.
Key takeaways
- 321 uses titanium, 347 uses niobium to resist sensitisation.
- Both suit 425–815 °C service.
- Weld 321 with 347 filler; use H versions for creep strength.
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