321 stainless steel (UNS designation UNS S32100) belongs to the 300 series austenitic family. The composition of 321 grade includes titanium, making it more robust and more corrosion resistant at elevated temperatures. It is a chromium-nickel austenitic stainless steel stabilized by titanium. Grade 321H is a modification of 321, consisting of higher carbon content. A more significant proportion of carbon helps in improving its high-temperature strength.
You can order 321 stainless steel pipe for use in chemical processing, pharmaceutical production, food processing, aerospace, or petroleum refining industries. This article details the various aspects of Type 321 austenitic stainless steel.
Table of Contents
Resistance To Corrosion
- General Corrosion: Type 321 stainless steel shows effective resistance against corrosion. If you heat type 321 for an extended period in the chromium carbide precipitation range, it may impact the corrosion resistance capabilities of Type 321.
- Intergranular Corrosion: Type 321 is suitable for applications susceptible to intergranular corrosiondue to exposure to temperatures in the chromium carbide precipitation range from 800 to 1500° F (427 to 816° C). Such corrosion causes the metal to weaken, which pushes it to complete disintegration. Type 321 is suitable for heavy welded equipment operated between 800 to 1500° F (427 to 816° C).
- Stress Corrosion Cracking (SCC): Type 321 austenitic stainless steel is susceptible to SCC in halides. SCC is caused by various conditions like halide ion, residual tensile stresses, and environmental temperatures over about 120° F. Type 321 is suitable for service in stress-relieved conditions.
- Pitting/Crevice Corrosion: Higher chloride levels may cause crevice corrosion and pit. The exposure of type 321 may cause pitting and crevice corrosion, along with severe discoloration. Hence, it is not suitable for exposure to marine environments.
Heat Resistance
Type 321 shows good oxidation resistance in continuous service to 925 °C and intermittent service to 900 °C. It offers excellent performance in the temperature range of 425-900 °C, especially in environments consisting of subsequent aqueous corrosive conditions. 321H possesses higher hot strength and is a practical choice for high-temperature structural applications.
Welding
Austenitic stainless steels are highly weldable. When welding joints, one needs to consider two essential aspects: the preservation of corrosion resistance and avoidance of cracking. In Type 321, maintaining the level of stabilizing element during welding is necessary. Adopt weld practices that pay sufficient attention to cleanliness and good inert gas shielding. Type 321 is resolidified with a bit of ferrite so that it is not susceptible to cracking.
Heat Treatment
Type 321 stainless steel has an annealing temperature range between 1800 to 2000° F (928 to 1093° C). You can also stress-relief anneal it across the carbide precipitation range 800 to 1500° F (427 to 816° C). If you want to maximize its ductility, a higher annealing range is recommended.
If you require optimum corrosion resistance, you may apply a corrective remedy for Type 321, known as stabilizing anneal. Based on thickness, you need to use a temperature range between 1550 to 1650° F (843 to 899° C) for up to 5 hours. In this temperature range, titanium and carbon combine to form harmless titanium carbides. Above this range, chromium carbides form. When you perform heat treatments in an oxidizing atmosphere, remove the oxide after annealing in a descaling solution like nitric and hydrofluoric acid mixture. Rinse the surface thoroughly after cleaning. Also, remember that Type 321 is non-hardenable by heat treatment, and you can only harden it by cold working.
Machining
As compared to most standard austenitic stainless steels, 321 is more difficult to machine. The presence of titanium results in the formation of harsh and abrasive titanium carbonitride inclusions. The machinability rating of Type 321 stainless steel is around 45% – 50% of free machining (S1214) mild steel. As this grade has a high work hardening rate, you need to keep the cutting or drilling tools sharp. Choose appropriate tool type, feeds, and speeds as per the manufacturer’s specifications.
Cold Forming
You can readily form and draw Type 321 stainless steel. Like other standard austenitic stainless steels, this grade work hardens and may need annealing after severe forming.
Stabilizing Treatment
Titanium combines with carbon to form stable carbides than chromium in the stainless steel alloy. It removes carbon from a solid solution which helps to prevent chromium carbides precipitation. It helps in stabilizing Type 321 against chromium carbide formation with the addition of titanium. These grades are also thermally stabilized. Heat type 321 alloy in the temperature range of 870-900 °C for 1 hour per 25mm of thickness. Allow it to cool it down usually. Stabilization is recommended for most severe service conditions (generally above 425 °C).
Applications
321 stainless steel is ideal for applications that include:
- Exhaust manifolds
- Metal O-ring products
- Processing equipment for high-temperature chemicals
- Select aerospace applications
- Oil and gas/refinery applications
- Other high-temperature service applications
- Expansion joints
- Spiral Welded tube for burner pipes and flues
- Bellows
- Woven or welded screens for high-temperature mineral processing
- Furnace parts
- Heating element tubing
- Heat Exchangers
To Conclude:
These are a few essential things you should know about 321 stainless-steel. The addition of titanium provides good corrosion resistance to the alloy, and it has excellent weldability, which makes it worthwhile across a wide range of industries.