SUS321 Stainless Steel

SUS321 is a titanium-stabilized austenitic stainless steel based on SUS304, designed to enhance intergranular corrosion resistance and eliminate sensitization during welding or high-temperature processing. It retains SUS304's excellent formability and weldability while offering superior thermal stability, making it a versatile choice for applications involving moderate heat and corrosive conditions.

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 = 5×C–0.70, Fe = Balance.

Mechanical Properties (Annealed): Tensile Strength ≥ 520 MPa, Yield Strength ≥ 205 MPa, Elongation ≥ 40%, Hardness (HB) ≤ 217.

Performance Advantages: Excellent intergranular corrosion resistance vs. SUS304; good high-temperature strength up to 870°C; easy weldability without post-weld heat treatment; superior resistance to chloride stress corrosion cracking (SCC) compared to non-stabilized grades; cost-effective alternative to SUS316 for less aggressive environments.

Equivalent Grades: ASTM A240 321, EN 1.4541, UNS S32100.

Applications: Aircraft exhaust systems, heat exchangers, chemical processing equipment, pressure vessels, automotive components, and food processing machinery.

FAQ

Q: How does titanium stabilization improve SUS321's corrosion resistance?A: Titanium stabilization in SUS321 works by binding with carbon atoms in the steel, preventing the formation of chromium carbides at grain boundaries during welding or heating. In non-stabilized grades like SUS304, carbon reacts with chromium to form carbides, depleting the chromium concentration near the grain boundaries and making the steel vulnerable to intergranular corrosion in corrosive environments. By adding titanium, which has a higher affinity for carbon than chromium, the steel forms titanium carbides instead of chromium carbides, ensuring that the chromium remains uniformly distributed throughout the material. This process eliminates sensitization, allowing SUS321 to retain its corrosion resistance even after welding or high-temperature service. The titanium carbides also help to refine the grain structure, improving the steel's mechanical strength and toughness at elevated temperatures. This makes SUS321 a reliable choice for welded structures that cannot be heat-treated after fabrication, such as aircraft exhaust systems and pressure vessels.

Q: What is the difference between SUS321 and SUS304?A: The key difference between SUS321 and SUS304 is the addition of titanium in SUS321, which provides enhanced intergranular corrosion resistance and thermal stability. SUS304 is a general-purpose austenitic stainless steel with good corrosion resistance in mild environments, but it is prone to sensitization after welding or heating, leading to intergranular corrosion in aggressive media. SUS321, on the other hand, is stabilized with titanium, which prevents carbide precipitation and eliminates the need for post-weld heat treatment in most applications. In terms of mechanical properties, both grades have similar tensile strength and ductility, but SUS321 has better high-temperature strength up to 870°C, making it suitable for applications involving moderate heat. SUS321 is also more resistant to chloride stress corrosion cracking (SCC) than SUS304, making it a better choice for environments containing chlorides, such as coastal areas or chemical plants. However, SUS304 is more cost-effective, making it the preferred choice for less demanding applications where sensitization is not a concern.

Q: Can SUS321 be used in high-temperature applications?A: Yes, SUS321 can be used in high-temperature applications up to 870°C, making it suitable for components like exhaust systems, heat exchangers, and furnace parts. Its titanium stabilization helps maintain the steel's structural integrity at high temperatures by preventing grain boundary embrittlement, ensuring that it retains its mechanical properties and corrosion resistance even after prolonged exposure to heat. At temperatures above 870°C, the steel may experience a gradual loss of strength due to grain growth and oxidation, but it still performs better than non-stabilized grades like SUS304. In reducing atmospheres, the maximum operating temperature is slightly lower, around 760°C, to avoid carbide precipitation and embrittlement. The steel's good high-temperature performance, combined with its excellent weldability and formability, makes it a versatile choice for a wide range of industrial applications that involve moderate heat and corrosive conditions.

Q: Is SUS321 suitable for welding to other materials?A: Yes, SUS321 can be easily welded to other austenitic stainless steel grades like SUS304 and SUS316, as well as to carbon steel and low-alloy steel, using standard welding techniques such as TIG, MIG, and SMAW. Its high nickel content ensures good weld pool fluidity and reduces the risk of hot cracking, while the titanium stabilization ensures that the heat-affected zone (HAZ) has good intergranular corrosion resistance. When welding SUS321 to carbon steel or low-alloy steel, it is recommended to use a filler metal with a high nickel content to prevent the formation of brittle martensite in the weld joint, which could compromise the mechanical properties of the structure. For example, using a 309 or 309L filler metal ensures that the weld has good toughness and corrosion resistance, making it suitable for applications involving exposure to corrosive environments. The low-carbon content of SUS321 also eliminates the need for post-weld heat treatment in most cases, reducing production costs and time.

Q: How does SUS321 perform in chloride-containing environments?A: SUS321 has better resistance to chloride stress corrosion cracking (SCC) than non-stabilized grades like SUS304, making it suitable for use in environments containing moderate chloride concentrations, such as coastal areas and chemical plants. Chloride SCC is a common failure mode in austenitic stainless steels, where tensile stress and chloride ions combine to cause cracking in the material. The titanium stabilization in SUS321 helps to reduce the susceptibility to SCC by preventing grain boundary embrittlement, ensuring that the steel can withstand higher chloride concentrations than SUS304. However, SUS321 is not as resistant to chloride SCC as grades like SUS316, which contain molybdenum and have better pitting and crevice corrosion resistance. In high-chloride environments, such as seawater or brine solutions, SUS316 or duplex stainless steels are more suitable, as they have better corrosion resistance and can withstand prolonged exposure to chlorides. For moderate chloride concentrations, SUS321 is a cost-effective choice that offers good performance and durability.