321 vs 347 Stainless Steel: Titanium and Niobium Stabilized Alloys Compared

Titanium and Niobium as Stabilizers

321 and 347 are the two standard titanium and niobium stabilized austenitic stainless steels. Both are based on the 18-8 austenitic family with chromium in the 17 - 19% range, and both add a carbide-forming element so that carbon is intercepted before it can combine with chromium at the grain boundaries. The result in each case is a steel that can be welded and then used at elevated temperature without suffering intergranular corrosion.

The difference is entirely in the stabilizer. Titanium is light, forms its carbide readily and requires an addition of not less than five times the carbon content. Niobium is heavier and slower to combine, so it is added together with tantalum at roughly ten times the carbon content. That mass difference, and the different thermal stability of the two carbides, drive every practical difference between the grades.

Designations and Composition

Item	321	347
UNS number	S32100	S34700
EN designation	X6CrNiTi18-10 / 1.4541	X6CrNiNb18-10 / 1.4550
Chromium (%)	17.0 - 19.0	17.0 - 19.0
Nickel (%)	9.0 - 12.0	9.0 - 13.0
Carbon (%)	0.08 max	0.08 max
Stabilizer	Ti 5 x C min (0.15 - 0.60 typical)	Nb + Ta 10 x C min (0.75 - 1.50 typical)
Molybdenum (%)	not specified	not specified

Because neither grade contains molybdenum, their pitting resistance equivalent number stays in the low twenties, essentially at the level of 304. They are heat-resistant and intergranular-corrosion resistant steels, not chloride-resistant steels, and that distinction should drive the first stage of material selection.

Cyclic Heat versus Continuous Heat

The two stabilizers behave differently when temperature varies. Titanium carbide in 321 is more sensitive to coarsening, so 321 performs best where heating and cooling repeat, for example exhaust manifolds, expansion joints and heat exchangers that cycle with the process. Its lower stabilizer content also leaves it marginally more ductile, which helps components that must absorb thermal expansion.

Niobium carbide in 347 coarsens much more slowly, so 347 is the stronger performer under continuous high heat above about 800 C. Furnace retorts, boiler superheater supports, gas turbine parts and nuclear reactor internals are typical of the duties where 347 is specified, because a welded joint that stays hot for years is exactly the case where creep strength and stable grain boundaries matter most.

Atmosphere Effects: Carburization and Oxidation

Oxidizing atmospheres: both grades form a protective chromium oxide scale and perform similarly up to their respective limits.

Carburizing atmospheres: 347 is more resistant, and its higher niobium content helps it retain ductility after long carbon pick-up.

Reducing atmospheres and hydrogen-rich streams: 347 again has the advantage, and 321 should be used only with careful temperature limits.

Nitriding conditions: both grades can absorb nitrogen; neither is recommended for heavy nitriding duty.

Sulfur-bearing gases: both are limited and may need a higher-nickel alloy instead.

Where the process gas alternates between oxidizing and reducing, the stabilized grades survive better than 304 or 316, but the choice between them usually falls to 347 in continuous operation because its scale remains more adherent under cycling of the gas chemistry.

Forms, Fabrication and Selection Summary

321 is more ductile and easier to cold form, which suits complex shapes, thin-wall exhaust components and drawn tube. 347 welds well under controlled heat input and holds its strength in welded joints exposed to prolonged heat, but it is slightly harder to draw and machine because of the niobium addition. Both are supplied as seamless and welded pipe, tube, plate, sheet, bar, wire and forgings, and both are normally used in the solution-annealed condition.

Selection in practice is straightforward. Choose 321 when the duty is thermal cycling at moderate temperature, or when good formability is essential. Choose 347 when the duty is continuous high temperature, when welds must survive long hot service, or when carburizing or reducing atmospheres are present. If chlorides accompany the heat, neither grade is sufficient and a molybdenum-bearing austenitic or a duplex steel is required instead.

Frequently Asked Questions

Q: What is the main difference between 321 and 347?
321 is stabilized with titanium and 347 with niobium plus tantalum, which makes 347 more stable in long continuous high-temperature service.

Q: Is 347 stronger than 321 at room temperature?
Their minimum tensile and proof stress requirements are effectively the same; the difference appears at elevated temperature and after welding.

Q: Which grade is easier to form?
321, because its titanium addition leaves it slightly more ductile and more forgiving in cold forming and drawing operations.

Q: Can both be used in boiler and pressure equipment?
Yes, both are widely used in pressure equipment, with 347 preferred for the hottest superheater and reheater sections.

Q: Do they resist chlorides?
No better than 304. Neither contains molybdenum, so chloride pitting resistance is low compared with 316, 317LMN or duplex grades.

Q: Which one is used in nuclear service?
347 is the traditional choice for nuclear reactor piping and internals because of its stability under long continuous heat.