321 vs 347 Stabilized Austenitic Stainless Steels: High-Temperature Welded Component Selection

Types 321 and 347 exist for one purpose: to allow austenitic stainless steel to be welded and used at elevated temperatures without the risk of intergranular corrosion or the need for post-weld heat treatment. Standard 304, welded and held in the 425-850 C range, precipitates chromium carbides at grain boundaries and loses corrosion resistance. Adding a strong carbide former, titanium in 321 or niobium in 347, prevents this by binding carbon into stable compounds.

Both grades are widely specified for furnace parts, boiler and heat-exchanger components, exhaust systems and refinery piping. The choice between them comes down to service temperature, creep resistance, fabrication and cost.

Chemical Composition per ASTM A240

Element (wt%)	321, UNS S32100	347, UNS S34700
Carbon, max	0.08	0.08
Chromium	17.0–19.0	17.0–19.0
Nickel	9.0–12.0	9.0–13.0
Titanium	5 x (C + N) min to 0.70 max	-
Niobium + Tantalum	-	10 x C min to 1.10 max
Manganese, max	2.00	2.00
Silicon, max	0.75	0.75
Nitrogen, max	0.10	-

Both grades show the same annealed minimums per ASTM A240: tensile strength 515 MPa, 0.2% yield strength 205 MPa, elongation 40 percent. The differences appear in long-term elevated-temperature service.

Elevated-Temperature Behaviour: Carbide Stability and Creep

Titanium carbides in 321 are stable to about 800 C, but coarsen slowly during prolonged exposure above that temperature, which gradually reduces grain-boundary strength and creep resistance. Niobium carbides in 347 remain fine and stable for much longer exposure at 800-900 C, which is why 347 is preferred for the hottest continuous-duty components such as gas-turbine parts, superheater tubing and nuclear steam piping. In cyclic service, the finer, more stable niobium carbides also resist thermal-fatigue damage better than the coarsening titanium carbides of 321.

For design purposes, ASME B31.3 lists both grades to a maximum of about 816 C, with allowable stresses that decline with temperature; above that range, higher-alloy heat-resisting grades should be evaluated.

Weldability and Post-Weld Heat Treatment

Neither grade requires post-weld heat treatment for corrosion reasons: the stabilization prevents sensitization in the heat-affected zone. Matching filler metals are used: 321-type filler for 321, 347-type filler for 347. For dissimilar joints between stabilized and non-stabilized austenitic grades, a 347-type filler is a common neutral choice. Welding of 347 should use controlled heat input to avoid niobium segregation or carbide agglomeration in the weld metal.

Corrosion Resistance Considerations

In the annealed and welded condition, both grades resist intergranular corrosion across the 425-850 C range and in service at elevated temperature. Neither grade carries molybdenum, so neither is suitable for chloride-bearing, marine or acid-wet service; for combined heat and chloride resistance, molybdenum-bearing stabilized or duplex grades should be selected.

Cost, Fabrication and Selection Guidance

321 is generally the lower-cost grade because titanium is abundant and inexpensive to alloy, whereas niobium is a controlled, more costly addition. 347 is marginally less formable in heavy sections because of its niobium content, and requires slightly more bending force. Where the design temperature stays below about 800 C and creep life is not critical, 321 delivers the stabilization benefit at the lower cost; where continuous service approaches 850-900 C, the niobium-stabilized 347 justifies its premium.

Choose 321 for welded components operating below about 800 C: furnace trays, boiler tubes, exhaust headers, chemical piping in the sensitization range, and general heat-treatment fixtures. Choose 347 for continuous service above about 800 C, for long-creep-life applications, for cyclic thermal service, and for nuclear or aerospace specifications that require maximum long-term carbide stability.

FAQ

Why do 321 and 347 not need post-weld heat treatment?

The stabilizing additions bind carbon as titanium or niobium carbides, so chromium carbide precipitation cannot deplete the grain boundaries. The heat-affected zone therefore keeps its corrosion resistance without annealing.

Which grade resists higher temperatures?

347, because niobium carbides stay fine and stable at 800-900 C while titanium carbides coarsen. Both grades are capped near 816 C in ASME B31.3 design tables, so the difference matters most for creep life and long-term microstructure rather than maximum code temperature.

Can 321 or 347 replace 304 in chloride environments?

No. Neither contains molybdenum, so both have the same chloride pitting limits as 304. Stabilization protects against intergranular corrosion, not against pitting or stress corrosion cracking.

Which filler metal should be used for welding 321?

Use a matching 321-type filler so that the weld metal also contains titanium stabilization. For 347, use 347-type filler. A 347-type filler is a common choice for dissimilar stabilized joints.

Is 321 cheaper than 347?

Generally yes. Titanium is a low-cost addition while niobium carries a significant alloy premium, so 347 is typically the more expensive of the two stabilized grades.

Are 321 and 347 magnetic?

No. Both are fully austenitic in the annealed condition and remain essentially non-magnetic; slight magnetism can appear after heavy cold work.