321 vs 347 Stainless Steel
Side-by-side comparison · Composition · Properties · Selection guide
Both are stabilized 18/8 grades built for 425–815 °C service: 321 locks up carbon with titanium, 347 with niobium. The stabilizer choice matters at the weld arc — titanium burns off, niobium survives — and in creep strength, where 347 holds a modest edge. 321 is usually cheaper and more available in sheet and coil.
Full datasheets:321 Stainless Steel·347 Stainless Steel
At a Glance
| 321 | 347 | |
|---|---|---|
| Type | Austenitic (Ti-stabilized) | Austenitic (Nb-stabilized) |
| UNS | S32100 | S34700 |
| Standard | ASTM A240/A240M | ASTM A240/A240M |
| Density | 7.92 g/cm³ | 8 g/cm³ |
Corrosion Resistance and Service Limits
321
- General corrosion resistance comes from the chromium-rich passive film; nickel, molybdenum, nitrogen and copper change performance by grade.
- Check chlorides, temperature, crevices, deposits, process chemistry and cleaning conditions. A grade designation alone is not a service-life guarantee.
347
- General corrosion resistance comes from the chromium-rich passive film; nickel, molybdenum, nitrogen and copper change performance by grade.
- Check chlorides, temperature, crevices, deposits, process chemistry and cleaning conditions. A grade designation alone is not a service-life guarantee.
Fabrication and Heat Treatment
| Operation | 321 | 347 |
|---|---|---|
| Welding | Generally weldable with a qualified procedure and matching or over-alloyed filler where required; remove heat tint when corrosion service demands it. | Generally weldable with a qualified procedure and matching or over-alloyed filler where required; remove heat tint when corrosion service demands it. |
| Forming | Good ductility is typical, but rapid work hardening increases forming load and springback; bend radius and intermediate annealing depend on grade and reduction. | Good ductility is typical, but rapid work hardening increases forming load and springback; bend radius and intermediate annealing depend on grade and reduction. |
| Machining | Use rigid tooling, positive feeds and controlled heat because the material work-hardens; free-machining grades require separate corrosion and welding review. | Use rigid tooling, positive feeds and controlled heat because the material work-hardens; free-machining grades require separate corrosion and welding review. |
| Heat treatment | Solution annealing and rapid cooling restore the intended condition where applicable. Austenitic grades are not hardened by conventional quench-and-temper treatment. | Solution annealing and rapid cooling restore the intended condition where applicable. Austenitic grades are not hardened by conventional quench-and-temper treatment. |
Chemical Composition (wt%)
| Element | 321 | 347 |
|---|---|---|
| C | ≤0.08 | ≤0.08 |
| Mn | ≤2.00 | ≤2.00 |
| Si | ≤0.75 | ≤0.75 |
| Cr | 17.0–19.0 | 17.0–19.0 |
| Ni | 9.0–12.0 | 9.0–13.0 |
| Ti | 5×(C+N) min, ≤0.70 | — |
| P | ≤0.045 | ≤0.045 |
| S | ≤0.030 | ≤0.030 |
| Nb | — | 10×C–1.00 |
Mechanical Properties
| Property | 321 | 347 |
|---|---|---|
| Tensile strength | ≥515 MPa (75 ksi) | ≥515 MPa (75 ksi) |
| Yield strength (0.2%) | ≥205 MPa (30 ksi) | ≥205 MPa (30 ksi) |
| Elongation (50 mm) | ≥40% | ≥40% |
| Max intermittent service | ≈870 °C | — |
| Hardness | — | ≤201 HBW |
Which One Should You Choose?
Choose 321 for:
- Aircraft exhaust shrouds, bellows and heat shields
- General 425–815 °C service bought as sheet, coil or tube
- Budget-sensitive high-temperature fabrications
Choose 347 for:
- Welded joints that must stay stabilized (347 filler standard)
- Refinery and boiler code work at creep temperatures
- Slightly higher creep-rupture strength requirements
- Nuclear and aerospace specs that call out 347 explicitly
Our verdict
Functionally interchangeable for most elevated-temperature corrosion duty; buy whichever your code and supply chain prefer. One asymmetry is worth remembering: welding either grade uses 347 filler metal, because niobium transfers across the arc while titanium does not. Heavy welded construction therefore drifts naturally toward 347.
Cost and Availability
Compare the current stock position, mill route, minimum production quantity, processing yield, testing and delivered cost for 321 and 347 at quotation time. A fixed price ratio would be misleading because product form, size, condition and market timing can change the commercial result.
Do Not Substitute Without Review
Do not replace 321 with 347, or the reverse, from the grade name alone. Recheck chemistry, mechanical-property condition, product standard, dimensions, corrosion environment, welding procedure and project approval before substitution.
Frequently Asked Questions
Why is 321 welded with 347 filler?
Titanium oxidizes and evaporates in the arc, so 321-composition filler would deposit unstabilized weld metal prone to sensitization. Niobium survives transfer, so ER347 wire deposits stabilized metal on either base grade.
Do 321 and 347 corrode differently at room temperature?
No practical difference — both behave like 304 in aqueous service. Their reason to exist is retaining intergranular corrosion resistance and strength after long exposure at 425–815 °C, not improving wet corrosion performance.
Which grade is easier to source?
Availability varies by product form and region. 321 is common in sheet and tube, while 347 is frequently specified in pressure and aerospace work; verify the exact form, thickness and standard.
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Technical content updated: 2026-07-29
Organizational author: Tsingshan (Shandong) Iron & Steel Co., Ltd..Review method, status and limitations.