321 Stainless Steel

Type 321 Stainless Steel, similar to Type 304, is a stabilized austenitic stainless steel. It has almost the same physical and chemical properties as Type 304, except for a higher level of titanium, which is at least five times the carbon content. 

The presence of higher content of titanium prevents or reduces chromium carbide precipitation during welding. It also offers metal fabricators excellent intergranular corrosion and oxidation resistance at an elevated temperature. Because of its good mechanical properties, Type 321 enhances the high-temperature characteristics of the alloy by providing excellent toughness even down to cryogenic temperatures ranging from 800 – 1500 °F (427 – 816 °C). In comparison with Type 304, this austenitic stainless steel offers higher creep and stress rupture properties too.


Characteristics of Type 321 Stainless Steel

Other properties of Type 321 stainless steel are:

  • It is not used for decorative purposes.
  • Excellent forge ability and weldability.
  • It works well with temperatures ranging from 800 – 1500 °F.
  • It possesses higher hot strength as compared to alloys, carbon, and even martensitic stainless steel.

Chemical Structure of Type 321 Stainless Steel

When we buy 321 Stainless Steel tube, the unique chemical structure of type 321 includes the following components: 

  • Carbon (C)- 0.08% 
  • Nitrogen (N)- 0.10%
  • Manganese (Mn)- 2%
  • Sulfur (S)- 0.030%.
  • Phosphorus (P)- 0.045%.
  • Chromium (Cr)- 17.0 – 19.00%
  • Iron (Fe)- 0.08%
  • Nickel (Ni)- 9.00 – 12.00%
  • Silicon (Si)- 0.75%
  • Titanium (Ti) 5 x (C + N) min- 0.70%

Applications of Type 321 Stainless Steel

Type 321 is used in a wide variety of applications due to its numerous benefits and capabilities. Some of the areas where it is highly used are:

  • Radiant superheaters
  • Automotive exhaust systems
  • Annealing covers
  • Gas and oil refinery equipment
  • Diesel and heavy-duty exhaust stacks and manifolds
  • Boiler casings
  • Firewalls
  • High-temp tempering equipment
  • Chemical processing equipment
  • Jet aircraft components
  • Stack liners
  • Bellows
  • Welded air pressure vessels

Formability: When subjected to high-pressure, Type 321 Stainless Steel can be easily drawn and formed. However, during this process, more spring back takes place as compared to carbon steel and ferritic stainless steels. Grade 321 is more difficult to form because of the presence of certain alloying elements as compared to other austenitic grades like Type 305, Type 304, and Type 301. But this austenitic stainless steel requires annealing after severe forming and may be hardened quickly like all other stainless-steel types.

Weldability: Preservation of corrosion resistance and avoidance of cracking are the two important factors that need to be considered while producing weld joints in the austenitic stainless steels. Using common fusion and resistance techniques, Type 321 stabilized austenitic stainless steel is usually considered to be weldable. To avoid weld hot-cracking, special care needs to be taken by confirming the formation of ferrite in the weld deposit. When compared in terms of weldability characteristics, grade 321 has almost the same weldability as other austenitic grades such as Type 304 and Type 304L. The difference comes in only the addition of higher content of titanium that prevents or reduces chromium carbide precipitation during welding. AWS E/ER 347 or E/ER 321 is most often specified when a weld filler is needed.

Heat Treatment: Temperature ranging between 1800 to 2000° F (928 to 1093° C) is the general annealing temperature which is required for maximum ductility. Within the carbide precipitation range between 800 to 1500° F, type 321 stainless steel may also be stress relief annealed as the main purpose of annealing is to obtain high ductility and softness. Maybe, prolonged heating within this range tends to lower the general corrosion resistance but, annealing for a few hours may not cause any noticeable lowering in the general corrosion resistance properties. Hence this grade is non-hardenable by heat treatment as they are less susceptible to intergranular attack.

Corrosion Resistance: This is categorized into four categories. They are:

  • General Corrosion: Type 321 alloy offers similar corrosion resistance behavior to Type 304. Only the addition of titanium adds some more features to it, like improved intergranular corrosion resistance for stabilization. However, heating for long periods may reduce its overall general corrosion resistance. But it provides excellent resistance to organic acids and some inorganic acids.
  • Intergranular Corrosion: It has been developed for applications where Type 304 unstabilized chromium-nickel sheets of steel are susceptible to intergranular corrosion. When the unstabilized chromium-nickel steels are cooled slowly through the range of 427 to 816° C, precipitation of chromium carbide takes place at grain boundaries.
  • Stress Corrosion Cracking: This type of corrosion takes place because of the presence of the same nickel content as in Type 304. Basically, the presence of halide ions, residual tensile stresses, and environmental temperatures of about 120° F cause stress corrosion cracking.
  • Pitting/Crevice Corrosion: This corrosion takes place because of the presence of a higher level of chloride ion.

Conclusion

Grade 324 is considered the best for the industrial application in non-stabilized grades where carbide precipitation takes place.

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