AISI 304L

What is AISI 304L?

AISI 304L is a low-carbon austenitic stainless steel derived from AISI 304. It contains 18.0 to 20.0 percent chromium and 8.0 to 12.0 percent nickel. Its maximum carbon content is 0.03 percent, compared with 0.08 percent for standard 304. The lower carbon content restricts the formation of chromium carbides at grain boundaries when the steel passes slowly through 425 to 860 degrees Celsius. This temperature range occurs in metal located a few millimetres from a welding arc.

A welded 304L component retains its corrosion resistance without post-weld solution annealing. This property is generally important in sections thicker than about 5 mm, where post-weld treatment may be impractical. The grade otherwise has the same passive film, work-hardening response, and absence of a ductile to brittle transition as standard 304.

The lower carbon content reduces specified strength. Relative to standard 304, the minimum tensile strength is 30 MPa lower and the minimum yield strength is 35 MPa lower. Long-term strength at elevated temperature is also lower because carbon strengthens austenite in solid solution.

Laxcon Steels lists AISI 304L in its grades reference as an austenitic grade. The same steel is written 304L, SS 304L, SUS 304L, 1.4307 and S30403.

What is the chemical composition of AISI 304L?

Composition limits in weight percent are carbon 0.03 maximum, manganese 2.0 maximum, sulphur 0.03 maximum, phosphorus 0.045 maximum, silicon 0.75 maximum, chromium 18.0 to 20.0, nickel 8.0 to 12.0 and nitrogen 0.1 maximum.

What is AISI 304L 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 304L?

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 properties

ASTM A276 specifies minimum properties for annealed hot-finished 304L bar. These are 485 MPa tensile strength, 170 MPa 0.2 percent yield strength, 40 percent elongation in 50 mm, and 50 percent reduction of area. ASTM A240 specifies the same tensile and yield minima for plate. It limits hardness to 92 HRB or 201 HB. Cold-finished bar up to 12.70 mm diameter has minimum values of 620 MPa tensile strength and 310 MPa yield strength. Above that diameter, the minima are 485 MPa and 170 MPa. Minimum elongation is 30 percent in both size ranges.

Round bar and plate are commonly supplied with dual 304/304L certification. A dual-certified heat meets the 0.03 percent carbon maximum for 304L and the higher 515 MPa tensile and 205 MPa yield minima for 304. The material can therefore be certified to either specification, and suppliers often stock it instead of separate grades. Dual-certified product is often unsuitable for high-temperature service. Its low carbon content meets the 304L requirement but may conflict with code requirements for elevated-temperature use.

Sensitisation

Sensitisation is a local change in composition at austenite grain boundaries. Between about 425 and 860 degrees Celsius, chromium and carbon form M23C6 carbides at these boundaries. Carbon diffuses faster than chromium, so the carbides obtain chromium from the few micrometres of adjacent metal. This narrow region can fall below the roughly 10.5 percent chromium required for passivity. Corrosion can then proceed along the grain boundaries while leaving the grain interiors unaffected. In weld decay, this process produces two parallel corroded bands beside a weld and at a short distance from the fusion line.

A carbon limit of 0.03 percent restricts carbide formation during an ordinary welding thermal cycle. It does not prevent sensitisation under all conditions. Exposure for many hours within the critical temperature range can still sensitise 304L. Stabilised grades are therefore used for sustained high-temperature service.

Corrosion behaviour

AISI 304L has a pitting resistance equivalent number from 18.0 at the specified composition minima to 21.6 at the maxima. This range is identical to standard 304 because both grades have the same chromium, molybdenum, and nitrogen limits. Their corrosion resistance in the annealed condition usually shows no measurable difference. The use of 304L provides no corrosion advantage when a component will not be welded or held at temperature.

The corrosion limits of 304L are therefore those of standard 304. Atlas Steels rates this family as resistant to pitting in potable water with up to about 200 mg per litre of chlorides at ambient temperature. The limit falls to about 150 mg per litre at 60 degrees Celsius. Both grades are vulnerable to chloride stress corrosion cracking above roughly 60 degrees Celsius when tensile stress is present. They resist nitric acid, most organic acids, alkalis, and clean in place chemicals used in food plant. They do not resist reducing acids, chloride-bearing brines, or seawater.

Welding, heat treatment, forming, and machining

Standard fusion welding processes can be used with AISI 304L. The usual matching consumable is 308L. Its low carbon content limits sensitisation in the weld metal as it cools through the critical temperature range. When solution treatment is required, the steel is heated to 1,010 to 1,120 degrees Celsius and then cooled rapidly. Heat treatment cannot harden the grade.

Cold forming behaviour is the same as that of standard 304. The material has a high work-hardening rate and high springback, which place substantial demands on forming equipment. Deformation can convert some austenite to strain-induced martensite and increase the ferromagnetic response. Machining is slow because the material is gummy. The cutting tool must continue cutting rather than rubbing.

Applications and designations

AISI 304L is commonly used for welded pressure vessels, tanks, hoppers, and process pipework. Applications occur in food, dairy, brewing, pharmaceutical, and general chemical plant. The grade is also used for heavy-gauge welded fabrications that cannot receive post-weld annealing, site-welded stainless structures, and architectural or marine-adjacent fabrications that do not contact seawater.

The European designation is material number 1.4307, with the name X2CrNi18-9. The UNS designation is S30403. The Japanese designation is SUS 304L. Purchasers also use the unspaced forms SUS304L and SS304L.

In which product forms does Laxcon list AISI 304L?

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 304L is not the right choice

AISI 304L is unsuitable for sustained service above about 500 degrees Celsius. Its low carbon content reduces creep strength and stress-rupture strength. Pressure design codes consequently assign it lower allowable stresses at elevated temperature than standard 304. AISI 304H is intended for this duty. It contains 0.04 to 0.10 percent carbon and requires an ASTM grain size of No 7 or coarser. AISI 321 and AISI 347 are alternatives when service is both hot and wet. These grades stabilise carbon with titanium or niobium and retain hot strength while limiting sensitisation and aqueous corrosion.

AISI 304L is also unsuitable for chloride exposure. Its molybdenum-free composition gives it the same PREN range as standard 304 and less pitting resistance than a molybdenum-containing grade. Seawater, coastal exposure, pool halls, and chlorinated cooling water require at least AISI 316L. Duplex 2205 is used when both corrosion resistance and higher strength are required. When higher strength is the only requirement, 304L provides 35 MPa less guaranteed yield strength than standard 304 without a weldability benefit.

Other austenitic grades