Comparison of SUS321 and SUS347: Titanium-Stabilized vs Niobium-Stabilized Austenitic Stainless Steel

Two Stabilized Austenitic Grades for Welded High-Temperature Service

SUS321 and SUS347 are stabilized austenitic stainless steels developed to solve one problem: after welding, unstabilized 18-8 steels precipitate chromium carbides at grain boundaries and lose corrosion resistance. SUS321 solves it with titanium, SUS347 with niobium plus tantalum. Both are standardized in JIS G4303 (bar), G4304 (plate) and G4305 (sheet and strip) and have direct ASTM counterparts, S32100 and S34700.

Chemical Composition per JIS G4303 and ASTM A240

Element (wt%)	SUS321 S32100	SUS347 S34700
C	0.08 max	0.08 max
Si	1.00 max	1.00 max
Mn	2.00 max	2.00 max
P	0.045 max	0.045 max
S	0.030 max	0.030 max
Cr	17.00-19.00	17.00-19.00
Ni	9.00-12.00	9.00-13.00
Stabilizer	Ti: 5 x C min to 0.70 max	Nb+Ta: 10 x C min to 1.00 max

Silicon and manganese limits follow JIS practice; chromium, nickel and carbon ranges follow both JIS G4303 and ASTM A240. The stabilizer ratios are the key: SUS321 needs titanium at least 5 times the carbon content, SUS347 needs niobium plus tantalum at least 10 times the carbon content, so that carbon is bound as stable carbides instead of chromium carbides.

Mechanical Properties

In the annealed condition both grades show minimum tensile strength 515 MPa, yield strength 205 MPa and elongation 40% per ASTM A240, with the same general values in the JIS standards. The important difference is not room-temperature strength but long-term stability: niobium carbides remain stable at higher temperature than titanium carbides, so SUS347 keeps its corrosion resistance and strength in longer, hotter service.

High-Temperature Stability

Titanium carbide is effective up to roughly 850 C; above that it can slowly dissolve, freeing carbon and allowing chromium carbides to form. Niobium carbide remains stable beyond that range. For this reason SUS321 is rated for continuous service up to about 870 C, while SUS347 extends continuous service to about 900 C and is the preferred grade for long-life components in that window, such as boiler superheaters and nuclear steam generator tubing.

Welding and Fabrication

Both grades weld readily and, because they are stabilized, do not require post-weld heat treatment. Use ER321 filler for SUS321 and ER347 filler for SUS347, and control heat input to avoid overheating the weld. SUS347 shows better resistance to weld cracking, while SUS321 machines slightly more easily because niobium increases cutting resistance. SUS347 is typically 15-20% more expensive than SUS321, reflecting the niobium content.

Selection Guidance

Choose SUS321 when cost matters and the service temperature stays at or below about 850-870 C: aero-engine exhaust pipes, boiler heat exchanger tubes, chemical reaction vessels and general welded high-temperature fabrications. Choose SUS347 when the component must run for years at 850-900 C or where weld cracking resistance is critical: superheater tubes, nuclear steam generators, aerospace structural parts and long-life furnace components.

FAQ

What is the difference between titanium and niobium stabilization?

Both elements combine with carbon preferentially, preventing chromium carbide precipitation at grain boundaries. Titanium carbide is stable to about 850 C; niobium carbide remains stable higher, which is why SUS347 outperforms SUS321 in long-term service above 850 C.

Can SUS321 replace SUS347 in nuclear power applications?

No. Steam generator service at 850-900 C is beyond the stable range of titanium carbide, and SUS321 would slowly lose corrosion resistance. SUS347, with stable niobium carbides, is the established choice for such long-life components.

Which welding materials are used for SUS321 and SUS347?

ER321 filler for SUS321 and ER347 filler for SUS347, with controlled heat input to keep the weld within the stabilized design intent. Neither grade requires post-weld heat treatment.

How do the two grades compare in service life at 850 C?

SUS347 typically shows longer service life at 850 C because niobium carbide remains stable, while SUS321 may develop slight grain-boundary attack after very long exposure as titanium carbide begins to dissolve.

How should I choose between SUS321 and SUS347?

If the service temperature is 850 C or below and cost is a concern, SUS321 is sufficient. If the component must operate at 850-900 C for years, or if maximum weld cracking resistance is needed, specify SUS347.

Are SUS321 and SUS347 covered by the same standards?

Both appear in JIS G4303, G4304 and G4305 and in ASTM A240 (plate), A312 (pipe) and A213 (boiler tube). Their EN counterparts are 1.4541 (X6CrNiTi18-10) for SUS321 and 1.4550 (X6CrNiNb18-10) for SUS347.