321 vs 347 Stainless Steel: Titanium and Niobium Stabilization for High Heat

Why Stabilized Grades Exist

When an unstabilized austenitic stainless steel such as 304 is held for long periods in the 425 - 815 C range, chromium carbides precipitate along the grain boundaries and deplete the adjacent metal of chromium. That chromium-depleted band is the starting point for intergranular corrosion. Titanium in 321 and niobium in 347 are added specifically to stop this: both elements have a stronger affinity for carbon than chromium does, so they form their own carbides first and leave the chromium in solution where it protects the steel.

321 and 347 therefore look almost identical to 304 in general corrosion terms, but they are dramatically better after welding or after years of hot service. The difference between the two stabilized grades lies in which carbide former is used and how stable that carbide remains as temperature climbs.

Titanium in 321 versus Niobium in 347

Feature	321	347
Stabilizing element	Titanium	Niobium, often with tantalum
Minimum addition	5 x carbon (0.15 - 0.60 typical)	10 x carbon for Nb + Ta (0.75 - 1.50 typical)
Carbide formed	Titanium carbide	Niobium carbide
Carbide stability	Good to about 800 C	Excellent beyond 800 C
Cyclic heat behaviour	Better in thermal cycling	Better in continuous heat
Cold formability	Slightly better	Adequate, more resistant to drawing
Typical service	Exhaust systems, heat exchangers	Furnaces, boilers, nuclear piping

The multiplication factors matter. Titanium is added at not less than five times the carbon content so that enough titanium remains in solution after the carbides have formed; niobium plus tantalum is added at roughly ten times the carbon content because niobium has a higher atomic weight, so about twice the mass is needed to tie up the same amount of carbon.

Performance Above 800 C

Niobium carbide is the more thermally stable of the two compounds. Above roughly 800 C, titanium carbide tends to coarsen and partially redissolve, so 321 gradually loses stability and can begin to show sensitization again during very long exposures. Niobium carbide resists that coarsening, and 347 consequently keeps its strength and its resistance to intergranular attack at higher temperatures and for longer times.

This is why 347 is the usual choice for continuous service above 800 C, such as industrial furnace internals, gas turbine components, superheater tubing and nuclear reactor piping, while 321 is positioned for moderate high-temperature duty up to about 760 C and for parts subject to repeated thermal cycling, where its lower stabilizer content makes it slightly more ductile and less prone to cracking during cycling.

Corrosion, Oxidation and Weldability

General corrosion: both behave close to 304 in atmospheric, fresh water and mild chemical service.

Intergranular corrosion: both resist it well after welding, which is their main advantage over 304 and 316.

Chloride pitting: neither grade contains molybdenum, so 316 or 317LMN remains the better choice in chloride-rich environments.

High-temperature scaling: both resist oxidation well; 347 holds its weld properties longer in continuous heat.

Welding: both are readily welded with standard austenitic fillers, and post-weld heat treatment is rarely required.

For weldments that will run hot, 347 produces the more stable joint because niobium carbides do not coarsen as quickly as titanium carbides, so the heat-affected zone keeps its creep strength. 321 needs a little more attention to heat input and interpass temperature, since excessive heat can precipitate titanium-rich phases that reduce toughness.

Selection and Supply Forms

Use 321 where the component sees repeated heating and cooling, moderate maximum temperatures and a need for good cold formability, for example automotive and aerospace exhausts, expansion bellows and heat exchanger tubes. Use 347 where the heat is continuous and severe, or where welded joints must survive very long service at temperature, for example boiler superheaters, furnace retorts, thermal processing equipment and nuclear components.

Both grades are supplied as seamless and welded pipe, tube, plate, sheet, bar, wire and forgings, with ASTM A240, A312, A213, A276 and A479 being the most common product specifications for plate, pipe, tube, bar and forgings respectively, supported by the EN 10088 family in European practice.

Frequently Asked Questions

Q: Are 321 and 347 interchangeable?
For many moderate-temperature duties yes, but 347 is preferred whenever continuous service above 800 C or very long hot service is expected.

Q: Which grade is better after welding?
Both resist intergranular corrosion after welding because both are stabilized; 347 keeps its weld strength better at very high temperature.

Q: Why is niobium added at a higher multiple of carbon than titanium?
Because niobium has a much higher atomic mass, roughly twice the mass is needed to combine with the same amount of carbon.

Q: Do 321 or 347 need post-weld heat treatment?
Rarely. Stabilization means the heat-affected zone is protected without a separate annealing step in most applications.

Q: Which one resists chlorides better?
Neither contains molybdenum, so in chloride service both are inferior to 316, 316L, 317LMN or a duplex grade.

Q: Can 321 and 347 be used at cryogenic temperatures?
They remain austenitic and tough at low temperature, similar to 304, provided they are in the annealed condition.