Side-by-Side Comparison: 321 vs 347 at a Glance

Chemistry limits are from ASTM A240. The stabilizer specification is the defining difference: 321 requires Ti = 5×(C+N) min; 347 requires Nb = 10×C min, 1.0% max. Both base chemistries are otherwise 18Cr-8Ni austenitic stainless steels.

Detailed Comparison: 5 Critical Differences

1. Stabilizer Chemistry — TiC vs NbC at the Atomic Level

Both 321 and 347 work by the same mechanism: a strong carbide-forming element is added to the alloy to tie up carbon before it can form chromium carbides. The difference is the element itself. Titanium has a carbide formation free energy of –162 kJ/mol at 1,000°C. Niobium has a free energy of –178 kJ/mol at the same temperature. The more negative value means NbC is thermodynamically more stable than TiC. In practice, this translates to NbC remaining stable up to approximately 1,150°C, while TiC begins to dissolve and coarsen above 1,000°C.

For pipe fittings and flanges, this matters when the metal temperature exceeds 900°C during upset or transient conditions. In a thermal well projecting into a 1,000°C gas stream, the outer sheath metal sees 925–975°C. TiC in 321 would begin to over-age, and the carbon would eventually re-enter solution, becoming available for chromium carbide formation during cooldown. NbC in 347 does not experience this dissolution, so the stabilization remains effective through unlimited thermal cycles. This is why 347 is the default for thermal wells, radiant superheater tubes, and furnace roll headers in steel mills.

2. High-Temperature Performance — 347 Holds the Edge Above 900°C

At 800°C and below, 321 and 347 are effectively interchangeable. Both resist oxidation, maintain creep strength, and show no sensitization. Above 900°C, 347 pulls ahead. The NbC dispersion pins grain boundaries more effectively than TiC at these temperatures, retarding grain growth and maintaining higher creep rupture strength. Data from the ASME Boiler and Pressure Vessel Code Section II-D shows 347H having a 100,000-hour creep rupture strength of approximately 35 MPa at 750°C, versus 32 MPa for 321H. At 800°C, the gap widens to 22 MPa versus 18 MPa. For components designed for 200,000-hour life, that 15–20% strength advantage is decisive.

We see this in practice with our A213 TP347H tube bend orders. Boiler contractors specify 347H for reheaters and superheater outlet sections where steam temperatures are 580–620°C but the tube metal runs 50–80°C hotter due to heat flux. The 347H grade provides the margin needed for 30-year design life without tube replacement.

3. Welding — ER347 vs ER321 and Multi-Pass Considerations

For single-pass or two-pass welds, ER321 and ER347 both perform well. The difference becomes clear on multi-pass welds in thick-wall flanges or heavy-wall pipe fittings. Each successive pass re-heats the prior passes into the HAZ sensitization range. With ER321, the TiC in the first pass can begin to coarsen or partially dissolve under the thermal cycle of the third or fourth pass. With ER347, NbC is more resistant to thermal cycling, so the stabilization remains intact through more passes.

Our welding procedure specification (WPS) for 347 flanges allows up to 6 passes without interpass temperature restriction beyond the standard 150°C limit. For 321 flanges, we limit to 4 passes or require a stabilizing anneal at 900°C between passes 4 and 5. This is not a theoretical concern — we have observed intergranular cracking in 321 multi-pass welds on 3" 300# flanges that were welded with 6 passes and then placed into 650°C service. The same geometry in 347 showed no cracking after 8,000 hours.

4. Cost Analysis — 347 Is Typically 5–10% More Than 321

The niobium addition in 347 costs more than the titanium addition in 321. Ferro-niobium prices have traded between $35 and $55 per kg of contained Nb over the last 24 months, while ferro-titanium has ranged from $8 to $14 per kg of contained Ti. The higher stoichiometric requirement for Nb (10×C versus 5×(C+N) for Ti) also means more stabilizer mass is needed per ton of steel. The net result is that 347 typically costs 5–10% more than 321 at the mill level.

For a 12" 600# weld neck flange in 347 versus 321, the material cost difference in our 2025 quotes is approximately 8%. For a batch of 50 pieces, that is roughly $400–500 total. When the application is above 900°C, the 347 premium is negligible insurance. When the application is below 800°C, 321 is the more economical choice with no performance penalty.

5. Formability and Machinability — Nearly Identical, With One Caveat

Both grades form, bend, and machine similarly to 304. The stabilizer additions do not significantly alter work-hardening behavior. One caveat: 347 can form stringy chips during machining that adhere to tool inserts more aggressively than 321. We recommend using a slightly more positive rake angle (8° versus 5°) and a 15% higher cutting speed for 347 forgings to maintain tool life. For bending tube bends, both grades require the same bend radius (3×D for ≤2", 4×D for >2") and mandrel support to prevent wrinkling on the inner radius.

HT PIPE's Real-World Inquiry Data & Export Recommendations

Our order book for 321 and 347 reflects distinct application patterns. In 2024–2025, we received three inquiries that illustrate the selection logic:

• "347 Thermal well.xlsx": A European instrumentation contractor requested 347 SS thermal wells in 1/2" × 300 mm and 3/4" × 450 mm lengths for a gasifier project. The wells project into a 1,050°C syngas stream. 347 was mandatory because TiC in 321 would dissolve at the sheath tip. We supplied 120 pieces to ASTM A182 F347 with 100% dye-penetrant inspection on the weld necks.
• "A213 TP347H tube bend.pdf": A Southeast Asian power plant contractor ordered 800 pieces of A213 TP347H U-bends in 38.1 mm × 4.5 mm wall for a high-pressure reheater. The design metal temperature was 620°C. 321H was technically acceptable but the EPC specified 347H to match the creep strength requirements of the ASME code case. We cold-bent the tubes with a 2.5×D bend radius and performed solution annealing at 1,060°C after bending.
• "321 cold rolled seamless pipe.xlsx": A Russian buyer ordered 4,500 meters of ASTM A312 TP321 cold-rolled seamless pipe in 1" to 4" sizes for a refinery heat exchanger project. The service temperature was 480°C. 321 was chosen over 347 because the temperature was below the NbC advantage threshold, and 321 was 12% cheaper. The project engineer confirmed that 347 would have been over-specification.

Our guidance to buyers: if the peak metal temperature is below 850°C and the weld is 4 passes or fewer, 321 is the cost-optimal choice. If the temperature exceeds 900°C, if multi-pass welding is required, or if the component is a thermal well or radiant tube, 347 is the safer specification.

Frequently Asked Questions

Q1: What is the fundamental difference between 321 and 347?

Both are stabilized 18Cr-8Ni austenitic stainless steels. 321 uses titanium (Ti) as the stabilizer; 347 uses niobium (Nb, also called columbium Cb). Ti forms TiC; Nb forms NbC. NbC is thermodynamically more stable than TiC above 1,000°C, which gives 347 better performance in extreme high-temperature service. At temperatures below 850°C, the two grades are effectively interchangeable.

Q2: What are the exact stabilizer requirements for each grade?

For 321: Ti = 5×(C+N) minimum, 0.70% maximum. For 347: Nb = 10×C minimum, 1.0% maximum. The 347 requirement is expressed as 10×C rather than 5×(C+N) because niobium also stabilizes against nitrogen but to a lesser degree than titanium. Our QC verifies these ratios on every MTC before accepting material.

Q3: Is 347 always better than 321?

No. 347 is better specifically above 900°C and in multi-pass welding. Below 850°C, 321 provides identical corrosion resistance, similar creep strength, and lower cost. Specifying 347 for a 600°C application is over-engineering and wastes 5–10% of the material budget. We have seen EPCs specify 347 for all high-temperature service regardless of actual temperature; our engineers routinely value-engineer these specs back to 321 when the metal temperature is below 850°C.

Q4: Can I weld 321 to 347?

Yes, but use ER347 filler. The niobium in ER347 will stabilize the weld deposit regardless of whether the base metal contributes Ti or Nb. Do not use ER321 filler for a 321-to-347 joint — the Nb-free weld deposit will not be adequately stabilized against the carbon that diffuses from the 347 side during welding. Post-weld stabilizing anneal at 900°C is recommended for high-temperature service.

Q5: What is the difference between 347 and 347H?

347H is the high-carbon variant (0.04–0.10% C) with the same Nb stabilization. The higher carbon provides better creep strength above 550°C by increasing the carbide volume fraction. 347 (0.08% C max) is the general-purpose grade. For boiler and reheater service, 347H is standard. For welded fittings in moderate temperatures, 347 is sufficient.

Q6: Why is 347 preferred for thermal wells?

Thermal wells project into hot gas streams where the sheath metal can reach 925–1,050°C. At these temperatures, TiC in 321 begins to dissolve, and the stabilization is lost. NbC in 347 remains stable, so the sheath maintains immunity to sensitization through thermal cycling. We have supplied 347 thermal wells to gasification, incinerator, and furnace projects where 321 would have been inadequate.

Related Comparisons & Resources

• 304 vs 321 Stainless Steel: Stabilized vs Non-Stabilized Grades
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