Alloy 321 is a titanium stabilized austenitic stainless steel. The presence of titanium in the alloy helps tie up the carbon atoms and avoid chromium carbide precipitation range of 800–1500°F (427–816°C) in welding. Hence, this grade from the austenitic family is suitable for high-temperature applications. Stainless steel 321 is manufactured in the form of sheet, strip, plate, pipe, foil (shim) in varying thicknesses based on the end-use.
Grade 321H is a modification of 321, and it consists of higher carbon content for improving the stainless steel’s high-temperature strength. It has enhanced creep resistance and shows exceptionally higher strength at temperatures above 1000°F (537°C). Here are few essential things you should know about Grade 321 of stainless steel.
Applications Of Alloy 321
You can use Alloy 321 in a variety of applications. 321 stainless steel pipe tube is used in aircraft piston engine manifolds, expansion joints, food processing equipment, petroleum refining and polythionic acid service, chemical processing, and waste treatment thermal oxidizers. You can also use it in the oven or welded screens for high-temperature mineral processing and spiral welded tubes for burner pipes and flues.
Alloy 321 stainless steel offers good oxidation resistance up to 1600F. Its corrosion resistance capabilities are comparable to type 304 of austenitic stainless steel. Alloy 321 was developed for use in the chromium carbide precipitation range of 1800–1500°F, in which the un-stabilized alloys such as 304 are susceptible to intergranular corrosion. Type 321 has excellent intergranular corrosion resistance on exposure to the given temperature range. It has higher creep and stress rupture properties than 304/304L. You can use alloy 321 in most diluted organic acids at moderate temperatures. You can also use it in pure phosphoric acid at lower temperatures and up to 10% diluted solutions at elevated temperatures. Alloy 321 also shows superior resistance to polythionic stress corrosion cracking. At moderate temperatures, you can also use it in chloride or fluoride-free caustic solutions. However, it does not deliver good performance in chloride solutions.
Machining Of 321 Stainless Steel
As compared to most of the standard austenitic stainless steels, type 321 is more difficult to machine. The presence of titanium results in the formation of harsh and abrasive titanium carbonitride inclusions. The cold work hardening rate makes it less machinable than 410 stainless, but its machining capabilities are similar to type 304. The machinability rating of type 321 is around 45% – 50% of free machining mild steel. Perform the machining processes on type 321 according to the machine manufacturers recommendations, and use the right tools and machining speed.
The Type 321 alloy has a maximum use temperature of 1500°F. The 321H variant has higher hot strength. It is particularly suitable for use in high-temperature structural applications.
You should perform hot forming for type 321 between 2100–2300°F (1149–1260°C). Forging, upsetting, and other hot working processes must be completed within this temperature range. Do not work alloy 321 at temperatures below 1700°F (927°C). It should be annealed soon after hot forming to restore its optimum corrosion resistance.
Type 321 can be readily formed and drawn. Since it is quite malleable, it can be formed easily. It shows excellent cold working properties, and you can carry out cold bending without much difficulty. Type 321 shows mild magnetic properties after cold working. You are not typically required to anneal it after cold working, except after severe cold working processes. The strength and hardness of type 321 can be increased substantially by cold working but with a subsequent reduction in flexibility.
Type 321 shows excellent welding capabilities. You can readily weld it by most standard processes. Type 321 sheets of steel are usually welded with a type 347 filler. You can weld it by shielded fusion and resistance welding procedures. It should be air-cooled soon after to give it good toughness. You should avoid using oxyacetylene welding as carbon may accumulate in the weld area. Type 321 can be welded without losing its corrosion resistance properties because of intergranular carbide precipitation. Annealing is not generally required after welding processes, except for service in the more extreme conditions.
Titanium forms stable carbides than chromium and are added to stainless steels to form these stable carbides. It removes carbon from solid solution and helps in preventing precipitation of chromium carbides. Type 321 is stabilized against chromium carbide formation by adding titanium. Types 321 are most commonly thermally stabilized. Heat to 870-900 °C for 1 hour per 25mm of the thickness of type 321 alloy and allow it to air cool. Stabilization is essential for most severe service conditions (above 425 °C).
Annealing is a heat treatment process that helps increase the flexibility of the material and reduce its hardness. The annealing temperature range for Type 321 is 1800 to 2000° F (928 to 1093° C). Annealing must be performed after hot forming type 321 to re-attain its maximum corrosion resistance. However, it is generally not required after cold forming.
These are a few essential properties that characterize the type 321 grade of the austenitic stainless-steel family.