Comparison Of SUS321 And SUS347: Titanium-Stabilized Vs Niobium-Stabilized Austenitic Stainless Steel

SUS321 and SUS347 are typical stabilized austenitic stainless steels, with the core difference being the stabilizing element (SUS321: Ti=4×C-0.70%; SUS347: Nb+Ta=8×C-1.00%). Titanium and niobium both prevent intergranular corrosion, but niobium-stabilized SUS347 has better high-temperature stability, making them suitable for different high-temperature service scenarios.

Core Parameter Comparison

Parameter	SUS321 Stainless Steel	SUS347 Stainless Steel
Chemical Composition (wt%)	C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=17.00-19.00, Ni=9.00-12.00, Ti=4×C-0.70, Fe=Balance	C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, Cr=17.00-19.00, Ni=9.00-13.00, Nb+Ta=8×C-1.00, Fe=Balance
Mechanical Properties (Annealed)	Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB	Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB
Service Temperature	-196℃ to 870℃ (continuous service)	-196℃ to 900℃ (continuous service)
Equivalent Grades	EN 1.4541, UNS S32100, AISI 321	EN 1.4550, UNS S34700, AISI 347

Key Performance Differences: 1. High-temperature stability: SUS347's niobium carbides (NbC) are more stable than SUS321's titanium carbides (TiC) at high temperatures, not easy to decompose above 800℃; SUS321's TiC decomposes slightly at 850℃. 2. Intergranular corrosion resistance: Both have excellent intergranular corrosion resistance after welding; SUS347 is more stable in long-term high-temperature service. 3. Weldability: Both have good weldability, no post-weld heat treatment required; SUS347 has better welding crack resistance. 4. Machinability: SUS321 has slightly better machinability than SUS347; SUS347's niobium content increases cutting resistance. 5. Cost: SUS347 is 15-20% more expensive than SUS321.

Applicable Scenario Distinction: SUS321 is suitable for general high-temperature welding components, such as aero-engine exhaust pipes, boiler heat exchanger tubes (≤870℃), and chemical reaction vessels. SUS347 is suitable for long-term high-temperature service components, such as nuclear power plant steam generators, high-temperature boiler superheater tubes (800-900℃), and aerospace structural components.

Practical Q&A

Q1: What is the difference between titanium and niobium stabilization mechanisms? A1: Both preferentially combine with carbon to avoid Cr₂₃C₆ precipitation; TiC has good low-temperature stability, while NbC has higher high-temperature stability, making SUS347 more suitable for long-term high-temperature service. Q2: Can SUS321 replace SUS347 in nuclear power applications? A2: No. Nuclear power plant steam generators require long-term service at 850-900℃; SUS321's TiC is prone to decomposition, reducing corrosion resistance, while SUS347's NbC remains stable. Q3: What welding materials are used for SUS321 and SUS347? A3: SUS321 uses ER321 welding wire; SUS347 uses ER347 welding wire; both need to control heat input ≤180J/mm to avoid overheating. Q4: What is the service life difference between SUS321 and SUS347 at 850℃? A4: SUS347's service life (≥15 years) is longer than SUS321 (≥10 years); SUS321's grain boundaries are prone to slight corrosion after long-term high-temperature service. Q5: How to select between SUS321 and SUS347? A5: Choose SUS321 if cost is a concern and service temperature is ≤850℃; choose SUS347 if long-term service at 850-900℃ is required and high-temperature stability is critical.