321 vs 347 Stainless Steel
Type 321 and Type 347 are stabilized austenitic stainless steels designed for continuous service in the 425-870°C sensitization range where standard 304/316 grades are vulnerable to intergranular corrosion. Type 321 is stabilized with titanium; Type 347 is stabilized with niobium (columbium). Both are widely used in refinery heater tubes, exhaust systems, and high-temperature process piping.
Quick Answer
Both grades solve the same problem (sensitization), but through different stabilizing elements. Type 347 is generally preferred for welded fabrication because niobium does not suffer from the weld-zone depletion that can affect titanium in Type 321. However, Type 321 offers slightly better high-temperature oxidation resistance and is more commonly available in certain product forms.
Detailed Comparison
Stabilization Mechanism
Standard austenitic stainless steels (304, 316) form chromium carbides at grain boundaries when exposed to 425-870°C. This depletes chromium locally and causes intergranular corrosion. Stabilized grades add a strong carbide-forming element that preferentially combines with carbon, leaving chromium in solid solution.
- Type 321: Titanium (Ti >= 5 x C content) binds carbon as titanium carbides (TiC)
- Type 347: Niobium (Nb >= 10 x C content) binds carbon as niobium carbides (NbC)
The Knife-Line Attack Problem
Type 321 has a known vulnerability: during welding, the narrow heat-affected zone immediately adjacent to the fusion line reaches temperatures above 1250°C, which dissolves titanium carbides. Upon cooling through the sensitization range, this narrow band re-forms chromium carbides instead of TiC, creating a susceptible zone called “knife-line attack.” Type 347 does not suffer this problem because niobium carbides are more thermally stable.
Specifications Table
| Property | Type 321 | Type 347 |
|---|---|---|
| UNS | S32100 | S34700 |
| Stabilizer | Titanium (Ti >= 5 x C) | Niobium (Nb >= 10 x C) |
| Chromium | 17.0-19.0% | 17.0-19.0% |
| Nickel | 9.0-12.0% | 9.0-13.0% |
| Carbon (max) | 0.08% | 0.08% |
| Tensile strength (min) | 515 MPa (75 ksi) | 515 MPa (75 ksi) |
| Yield strength (min) | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Max continuous service | 870°C (1600°F) | 870°C (1600°F) |
| Pipe spec (seamless) | ASTM A312 TP321 | ASTM A312 TP347 |
| Forging spec | ASTM A182 F321 | ASTM A182 F347 |
| Knife-line attack risk | Yes | No |
| Weld filler | ER347 (preferred) | ER347 |
| Relative cost | Similar | Similar |
Comparison: Application Selection
| Application | Preferred Grade | Reason |
|---|---|---|
| Refinery heater tubes | 321 or 347 | Both suitable; 347 avoids knife-line attack |
| Welded exhaust manifolds | 347 | Better weld integrity |
| Bellows expansion joints | 321 | Better formability |
| Superheater tubes | 347H | Higher creep strength |
| Aircraft exhaust | 321 | Established aerospace spec |
| Chemical reactors (450-800°C) | 347 | Weld reliability |
Both grades are available as seamless and welded pipe per ASTM A312, fittings per ASTM A403, and forgings per ASTM A182. For services below 425°C, low-carbon grades (304L, 316L) are more economical. The “H” variants (321H, 347H) with controlled higher carbon (0.04-0.10%) provide improved creep strength above 540°C.
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