321 Stainless Steel: Titanium-Stabilized Intergranular Corrosion-Resistant Grade

321 stainless steel is a titanium-stabilized improved version of 304, with the addition of 0.10%-0.30% titanium. Titanium preferentially combines with carbon to form TiC, preventing chromium carbide precipitation at grain boundaries during welding or high-temperature service, thus fundamentally solving the intergranular corrosion problem of 304 in the sensitization temperature range (450-850℃).

Core Parameters

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=0.10-0.30, Fe=Balance

Mechanical Properties (Annealed): Tensile Strength ≥520MPa, Yield Strength ≥205MPa, Elongation ≥40%, Brinell Hardness ≤201HB

Service Temperature: 400℃ to 900℃ (continuous service), up to 950℃ for short-term service

Equivalent Grades: SUS321 (JIS), EN 1.4541 (EN), UNS S32100 (ASTM)

Performance Advantages: Titanium stabilization enables intergranular corrosion resistance without post-weld heat treatment; high-temperature creep strength is 22% higher than that of 304 at 600℃; the Cr₂O₃-TiO₂ composite oxide film formed at high temperatures has better compactness than 304, with 30% lower oxidation weight gain rate at 900℃; good weldability and formability.

Typical Applications: High-temperature heat exchanger tubes, secondary circuit pipelines of nuclear power plants, chemical cracking furnace tubes, automotive exhaust systems (manifolds, catalytic converter shells), and high-temperature components in petrochemical catalytic cracking units.

Practical Q&A

Q1: What is the role of titanium in 321 stainless steel? A1: Titanium's affinity for carbon is higher than that of chromium. It preferentially combines with carbon to form stable TiC, avoiding the formation of Cr₂₃C₆ and the resulting chromium depletion at grain boundaries. Thermal simulation tests show that after holding at 650℃ for 2 hours, the precipitation of chromium carbides at the grain boundaries of 304 reaches 3.2%, while that of 321 is only 0.8%.

Q2: Will "titanium burnout" occur when welding 321? A2: Yes. Titanium is prone to oxidation and burnout when the welding temperature exceeds 1200℃. TIG welding should be adopted, with the current controlled at 100-140A, the arc residence time shortened, and pure argon protection used to ensure the titanium content is not less than 0.10%.

Q3: What is the difference between 321 and 304 in terms of intergranular corrosion resistance? A3: 304 is prone to intergranular corrosion when it stays in the 450-850℃ range for a long time (sensitization zone) after welding; 321, due to titanium stabilization, does not produce sensitization even if it stays in the sensitization zone for a long time, and has excellent intergranular corrosion resistance without post-weld heat treatment.

Q4: Is 321 suitable for high-temperature and high-pressure boiler tubes? A4: Yes. It has excellent high-temperature creep strength and intergranular corrosion resistance, and can be used as boiler superheater and reheater tubes in the temperature range of 400-850℃ and pressure ≤10MPa. It should be noted that the wall thickness should be reasonably designed according to the working pressure to ensure structural safety.

Q5: How to handle the surface of 321 stainless steel to improve high-temperature oxidation resistance? A5: After processing, perform passivation treatment with 25% nitric acid + 2% hydrofluoric acid solution (room temperature, 20 minutes) to form a dense Cr₂O₃-TiO₂ composite oxide film; for components used above 800℃, a high-temperature resistant ceramic coating can be applied on the surface to further enhance oxidation resistance.