AISI 304

What is AISI 304?

AISI 304 is an austenitic stainless steel with 18.0 to 20.0 percent chromium, 8.0 to 10.5 percent nickel and no deliberate molybdenum. Its carbon content is limited to 0.08 percent. Chromium forms a passive oxide film that restores itself within moments after surface damage. Nickel stabilises the austenitic structure at and below room temperature. Annealed bar is therefore essentially non-magnetic, and the steel retains toughness at cryogenic temperatures.

AISI 304 cannot be hardened by quenching because it undergoes no hardening transformation. Cold work provides its increase in strength. Heavy drawing can produce twice the yield strength of annealed material. This increase reduces ductility and raises magnetic permeability.

The absence of molybdenum distinguishes AISI 304 from AISI 316. AISI 316 contains 2.0 to 3.0 percent molybdenum and has roughly four to five times the chloride tolerance in service water. AISI 304 performs the same functions at lower alloy cost where chlorides do not control material selection. It is the most widely produced stainless steel in the world.

Laxcon Steels lists AISI 304 in its grades reference as an austenitic grade. The same steel is written 304, SS 304, SUS 304, 1.4301, S30400 and 18/8.

What is the chemical composition of AISI 304?

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 18.0 to 20.0, nickel 8.0 to 10.5 and nitrogen 0.1 maximum.

What is AISI 304 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 304?

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 of AISI 304

ASTM A276 specifies minimum properties for hot-finished annealed AISI 304 bar in Condition A. Tensile strength must reach 515 MPa, and yield strength at 0.2 percent proof must reach 205 MPa. Minimum elongation is 40 percent in 50 mm, with 50 percent reduction of area. ASTM A240 applies the same tensile and yield values to plate. It also limits hardness to 92 HRB or 201 HB. These values are specification minima rather than typical results. Mill certificates for annealed round bar commonly record tensile strength between 550 and 650 MPa.

Cold finishing alters the mechanical properties without changing the grade. For cold-finished Condition A bar up to 12.70 mm diameter, ASTM A276 specifies 620 MPa tensile strength and 310 MPa yield strength. Above that diameter, the values return to 515 MPa and 205 MPa. Minimum elongation is 30 percent at either size. Strain-hardened Condition B bar up to 19.05 mm reaches 860 MPa tensile strength and 690 MPa yield strength, with 12 percent elongation.

AISI 304 has a density of about 7,900 kg per cubic metre and an elastic modulus of about 193 GPa. The density is close to that of carbon steel, while the elastic modulus is about six percent lower, so an equivalent section is about six percent less stiff. The grade has no ductile to brittle transition. It retains high Charpy values at liquid nitrogen temperatures and is used for cryogenic vessels and pipework where ferritic steel would fracture.

Corrosion behaviour of AISI 304

The pitting resistance equivalent number of AISI 304 ranges from 18.0 at the specified composition minima to 21.6 at the maxima. PREN is calculated as chromium plus 3.3 times molybdenum plus 16 times nitrogen. Since the grade contains no molybdenum, chromium supplies most of this value. A PREN in the high teens indicates general-purpose rather than marine corrosion resistance.

Atlas Steels considers AISI 304 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. In chloride-bearing water above roughly 60 degrees Celsius, chloride stress corrosion cracking becomes the principal failure mode. It produces through-wall cracks with almost no warning rather than visible pits.

Thermal exposure creates a separate corrosion risk. Between 425 and 860 degrees Celsius, chromium carbides precipitate at grain boundaries. This process removes chromium from the adjacent metal and sensitises the steel to preferential intergranular corrosion. Heavy welded sections made from standard AISI 304 may therefore require post-weld solution annealing. The same mechanism accounts for the use of low-carbon and stabilised variants.

Processing of AISI 304

Solution treatment consists of heating AISI 304 to 1,010 to 1,120 degrees Celsius and then cooling it rapidly. The treatment dissolves carbides and restores full corrosion resistance. No heat treatment can harden the grade. Quenching does not increase its strength and can instead cause sensitisation.

AISI 304 is compatible with all standard fusion welding processes, with or without filler metal. Consumables of 308 or 308L are the usual matches. Its thermal conductivity is roughly one third that of carbon steel, while its coefficient of thermal expansion is about one and a half times as high. Heat therefore remains concentrated near the joint, and distortion exceeds that of an equivalent weld in mild steel.

Machining is affected by rapid work hardening. Each cut must pass beneath the hardened layer produced by the preceding pass. Light cuts and dwelling tools can glaze the surface and cause rubbing instead of cutting. Suitable practice uses rigid setups, positive rake, sharp tools, generous coolant and continuous feed while the tool contacts the work. AISI 303 is an alternative when machining forms the main part of production and stainless properties are secondary.

Applications of AISI 304

AISI 304 is widely used in food, dairy and beverage equipment because it is readily cleaned and tolerates the caustic and acid cycles used in clean in place systems. Applications include brewing and winemaking vessels, milk-handling equipment, kitchen benches, sinks and hollow-ware. Construction uses include architectural panelling, railings, balustrade fittings and trim. Process-industry uses include chemical storage and transport containers, heat exchangers, screens and general pipework. ISO 3506 identifies the corresponding fastener material as class A2.

Designations for AISI 304

EN 10088-3 specifies the European counterpart for long products as material number 1.4301 and name X5CrNi18-10. The UNS designation is S30400. The Japanese designation is SUS 304, also written by purchasers as SUS304 or SS304. The traditional name 18/8 denotes the nominal 18 percent chromium and 8 percent nickel composition. It remains in use on drawings and in fastener catalogues. These designations identify nearest counterparts rather than identical specifications. The composition limits for EN 1.4301 do not match the ASTM limits exactly, so the standard stated on the order governs.

In which product forms does Laxcon list AISI 304?

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 304 is not the right choice

AISI 304 is unsuitable where chlorides concentrate. Seawater, coastal atmospheres, swimming pool halls, chlorinated process water, de-icing salt spray and evaporative cooling circuits can cause pitting. Above about 60 degrees Celsius, chloride stress corrosion cracking can propagate through the wall of a stressed component before visible surface loss occurs. AISI 316 is the usual alternative because it contains 2.0 to 3.0 percent molybdenum. Duplex 2205 provides roughly twice the yield strength and a much higher chloride threshold where both strength and corrosion resistance are required.

AISI 304 is also unsuitable for heavy welded sections that cannot receive subsequent solution annealing. Exposure between 425 and 860 degrees Celsius can sensitise the heat-affected zone through carbide precipitation. AISI 304L limits carbon to 0.03 percent to reduce this problem. AISI 321 uses titanium stabilisation and also retains hot strength. Sustained service above about 500 degrees Celsius instead calls for AISI 304H, which has 0.04 to 0.10 percent carbon and a coarse grain size. Predominantly machined parts impose greater tool wear in AISI 304 than in AISI 303, although AISI 303 has lower corrosion resistance.

Other austenitic grades