Stainless 321 is similar in its properties to 304 but its resistance to the precipitation of chromium carbide in the temperature range 425 - 850^oC results in it being the grade of choice for any 304 type application where those temperatures are likely to be encountered.

Typical applications include:

• Exhaust Systems
• Manifolds
• Chemical Plant
• Heat Exchangers
• Piping
• Furnace Parts
• Chimney and Stack Liners
• Storage Tanks
• Bellows

Corrosion Resistance

The major benefit of 321 is its resistance to intergranular corrosion.

Austenitic steels are prone to intergranular corrosion; when they are cooled the chromium combines with the carbon to form chromium carbide, depleting the passive chromium layer and form wider grain boundaries which are then attacked by the corrosive materials. The formation of chromium carbide often occurs during welding. Provided the parts to be welded are thin, the duration of the heat application should be short enough to avoid the problem but when thicker parts have to be welded the higher temperature persists and chromium carbide precipitation is likely. 

If the materials are to used at operating temperatures of between 425 and 850^oC, 321 will resist the formation of chromium carbide and the danger of intergranular corrosion

Grade 321 stainless steel has higher creep and stress rupture properties than 304 and particularly 304H. This makes it particularly suitable for boiler and pressure vessels.

Heat resistance

321 resists oxidation up to 900oC in intermittent use and to 925oC in continuous use. It also avoids intergranular corrosion in the range 425 and 850^oC.

Stabilised 321

321 can be further stabilised by heating to between 870 and 890^.C for 2 hours per 25mm thickness. This process is recommended for use in the more severe environments and the higher temperature ranges.

While 321 has excellent performance at temperatures up to 925^oC this should be regarded as the limit of its operating range. If operational temperatures are at the top end of this range consideration should be given to using the 321H variant - the extra carbon producing better high-temperature strength.

Stress Corrosion Cracking

Austenitic stainless steels can be subject to stress corrosion cracking (SCC) and 321 is no exception. 321 is susceptible to SCC in the presence of halides and as a result of cold deformation. 

Stabilising as described in the previous section and Stress Relieving  by heating to 700oC for 2 hours will reduce the danger of Stress Corrosion Cracking.

Marine Environments - Pitting and Crevice Corrosion

321 is not advised for marine environments - rapid corrosion and severe discolouration can be expected.

It should be emphasised the production of a good surface finish and good welding techniques do much to reduce the vulnerability of all forms of corrosion. However, 321 does not polish well and it is not a good choice if a decorative finish is required

It is the addition of Titanium (Ti) that confers the resistance to carbide precipitation and intergranular corrosion to 321. The quantity of titanium is defined by reference to the quantity of carbon and nitrogen present in the steel. 321 must have 5 times the quantity of carbon and nitrogen combined to a maximum of 0.7%. In the case of 321H, it must contain 4 times.

In addition, a number of other chemicals may be present but these are expressed as permitted levels with the exception of the increased quantity of carbon required in 321H - i.e. a minimum of .04% and a maximum of 0.10%

Due to the fact that there is an overlap in the constituents of the two specifications, it is not uncommon to find 321 as dual certified. However, if you are seeking the best high-temperature performance the selection of 321H certified as containing the higher carbon content will give you greater strength at elevated temperatures. 

Physical Properties

Mechanical Properties

Electrical Properties

Thermal Properties

The main variant of 321 is 321H which contains a higher quantity of carbon. 321 may contain up to 0.08% while 321H must contain between 0.04% and 0.08% carbon.

The higher quantity of carbon confers greater strength at elevated temperatures.

347 is mentioned here since it is broadly similar to 321 but uses Niobium in place of Titanium. Niobium easily forms very hard, extremely small grained carbides giving it even greater resistance to intergranular corrosion.

Mention should also be made of 316Ti in which the titanium has broadly the same effect on 316 making it suitable where the performance required is similar to that of 321 but in a marine environment.

Forms Available

• Tube
• Pipe
• Fittings
• Flanges
• Special Sections
• Sheet
• Plate
• Flat Bar
• Round Bar
• Hollow Bar
• Angles
• i Beam
• U Channel