AISI 321

What is AISI 321?

AISI 321 is a titanium-stabilised 18/8 austenitic stainless steel. It contains 17.0 to 19.0 percent chromium and 9.0 to 12.0 percent nickel. Its titanium content is at least five times the combined carbon and nitrogen content, with a maximum of 0.70 percent. At high temperature, titanium binds carbon as titanium carbide. This leaves chromium available to maintain the passive film and limits sensitisation to intergranular corrosion during exposure to the carbide precipitation range.

AISI 321 combines hot strength, scaling resistance and phase stability in service up to about 900 degrees Celsius. It also retains aqueous corrosion resistance after cooling. This combination is relevant to equipment that undergoes heating, washing and idle periods.

AISI 304L limits sensitisation through reduced carbon content. AISI 321 retains carbon and therefore has greater strength at temperature. Atlas Steels places the practical transition at about 500 degrees Celsius. AISI 304L is more available and generally preferred below this temperature. Its lower hot strength makes AISI 321 more suitable above this temperature.

Laxcon Steels lists AISI 321 in its grades reference as an austenitic grade. The same steel is written 321, SUS 321, 1.4541 and S32100.

Ti minimum 5x(C+N) per ASTM A240, maximum 0.70

What is the chemical composition of AISI 321?

Composition limits in weight percent are carbon 0.08 maximum, manganese 2.0 maximum, sulphur 0.03 maximum, phosphorus 0.045 maximum, silicon 0.75 maximum, chromium 17.0 to 19.0, nickel 9.0 to 12.0, nitrogen 0.1 maximum and titanium 0.7 maximum.

What is AISI 321 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

What is the PREN of AISI 321?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

Mechanical and physical properties

ASTM A276 includes type 321 in the Condition A category used for 304 and 316. Hot-finished annealed bar requires a minimum tensile strength of 515 MPa, a minimum 0.2 percent offset yield strength of 205 MPa, 40 percent elongation in 50 mm and 50 percent reduction of area. ASTM A240 specifies the same tensile and yield minima for plate. It sets maximum hardness at 95 HRB or 217 HB. Cold-finished Condition A bar up to 12.70 mm diameter has a tensile strength of 620 MPa and a yield strength of 310 MPa.

The density is about 7,900 kg per cubic metre, and the elastic modulus is 193 GPa. The mean coefficient of thermal expansion is about 16.6 micrometres per metre per degree Celsius from 0 to 100 degrees Celsius. It rises to about 18.6 from 0 to 538 degrees Celsius. Thermal conductivity increases from about 16.1 W per metre kelvin at 100 degrees Celsius to about 22.2 at 500 degrees Celsius. Thermal expansion and conductivity are important in the design of hot assemblies. The movement of pipework determines the dimensions of bellows and expansion joints more directly than tensile minima.

Heat resistance and heat treatment

AISI 321 has good oxidation resistance in intermittent service to about 900 degrees Celsius and continuous service to about 925 degrees Celsius. Titanium stabilisation preserves corrosion resistance during exposure to the 425 to 900 degrees Celsius range, where an unstabilised steel can become sensitised.

Solution treatment consists of heating to 950 to 1,120 degrees Celsius followed by rapid cooling. Severe service may require a separate stabilising treatment at approximately 870 to 890 degrees Celsius for two hours per 25 mm of thickness, followed by air cooling. This treatment forms titanium carbide from residual dissolved carbon under conditions where titanium carbide is stable and chromium carbide is not. It is particularly recommended after annealing at the upper end of the solution-treatment range. The supplier and purchaser normally agree on the exact cycle.

Corrosion behaviour

The pitting resistance equivalent number ranges from 17.0 at the specified composition minima to 20.6 at the maxima. AISI 321 contains no molybdenum. Its pitting resistance is derived from chromium and a small nitrogen contribution and is marginally below that of AISI 304. In the annealed condition, its general corrosion resistance is equivalent to that of AISI 304. Its advantage occurs during service in the 425 to 900 degrees Celsius range.

At ambient temperature, AISI 321 resists pitting in potable water containing up to about 200 mg per litre of chlorides. The limit falls to about 150 mg per litre at 60 degrees Celsius. Under tensile stress, the grade is susceptible to chloride stress corrosion cracking above roughly 50 degrees Celsius.

Welding and stress relief

AISI 321 has outstanding welding characteristics and does not require post-weld annealing. This property supports heavy-section fabrication. Titanium does not transfer effectively across a welding arc, so a usable titanium-stabilised consumable is unavailable. AISI 347 uses niobium to bind carbon and can cross the arc. It is therefore the standard consumable for welding AISI 321. AISI 347 is common as a filler but comparatively uncommon as parent plate.

Residual stress may be reduced by treatment at 700 degrees Celsius for one to two hours followed by air cooling. AISI 321 cannot be hardened by heat treatment.

Applications and designations

Applications include expansion joints and bellows, furnace parts, heating element tubing, heat exchangers, high-temperature screens, spiral welded tube for burner pipes and flues, aircraft exhaust components and refinery pipework operating in the carbide precipitation range. The European designation is material number 1.4541, with the name X6CrNiTi18-10 under EN 10088-3. The UNS designation is S32100. The Japanese designation is SUS 321, and purchasers commonly use the unspaced forms SUS321 and SS321. The higher-carbon variant 321H provides higher hot strength.

In which product forms does Laxcon list AISI 321?

Taken from the published specification on each product page. A form that is not listed here is not a form this reference can confirm the grade in; the enquiry route settles what can actually be rolled to an order.

Where AISI 321 is not the right choice

AISI 321 is unsuitable for decorative and architectural applications. Its hard, angular titanium carbide and nitride inclusions produce streaks during polishing. The grade therefore does not develop a satisfactory decorative finish. AISI 304 is suitable where appearance forms part of the specification.

AISI 321 is unsuitable for chloride-bearing environments because it contains no molybdenum. Its PREN range of 17.0 to 20.6 is slightly below that of AISI 304. Marine atmospheres, brines and chlorinated water require AISI 316. Hot chloride-bearing service requires AISI 316Ti, which applies titanium stabilisation to a molybdenum-containing grade.

Below about 425 degrees Celsius, titanium stabilisation provides no benefit. AISI 304L is cheaper, more available and easier to finish in this range. Above about 900 degrees Celsius, AISI 321 lacks adequate oxidation and creep resistance. AISI 310S, with 24.0 to 26.0 percent chromium, is suitable at higher temperatures. Welded AISI 321 is also susceptible to knife-line attack in strongly oxidising acid service. This narrow band of intergranular corrosion forms at the fusion line where titanium carbides dissolved and did not re-form. A post-weld stabilising anneal restores resistance.

Other austenitic grades