Austenitic

What defines the austenitic family?

Austenitic stainless steels contain sufficient nickel, manganese and nitrogen to retain a face-centred cubic austenite structure at room temperature. Without these elements, roughly 16 to 26 percent chromium would produce a ferritic structure. The absence of a quenchable phase transformation prevents hardening by heat treatment. Annealing softens these steels, while cold working increases their strength.

The face-centred cubic structure retains ductility and toughness at cryogenic temperatures without a transition. Austenitic grades are therefore standard materials for liquefied gas service. They are non-magnetic after annealing but may become slightly magnetic after cold working. Their thermal expansion is roughly one and a half times that of carbon steel, while their thermal conductivity is about one third as high. These properties increase distortion in welded fabrications. Welding procedures therefore control heat input rather than seek additional penetration.

The Laxcon Steels grade reference lists 71 austenitic grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

Selection of 304, 316 and low-carbon grades

Molybdenum content distinguishes many applications of AISI 304 and AISI 316. AISI 304 contains 18.0 to 20.0 percent chromium and 8.0 to 10.5 percent nickel, without deliberate molybdenum addition. AISI 316 contains 16.0 to 18.0 percent chromium, 10.0 to 14.0 percent nickel and 2.0 to 3.0 percent molybdenum. Molybdenum has little effect on uniform corrosion. It improves resistance to localised attack by stabilising the passive film against disruption by chloride ions. AISI 304 commonly serves in atmospheres, fresh water, food equipment and general process work. AISI 316 applies where chlorides occur, including coastal air, chlorinated water, brines and most seawater-adjacent service.

Carbon content determines the suitability of these grades for welding. Exposure to roughly 425 to 860 degrees Celsius causes chromium carbides to precipitate at grain boundaries. Welding exposes the adjacent metal to this temperature range. Carbide formation depletes chromium in nearby regions and causes sensitisation. The resulting intergranular corrosion can occur in service without being visible during inspection of the weld. AISI 304L and AISI 316L limit carbon to 0.03 percent, compared with 0.08 percent for the parent grades. This limit restricts carbide formation during the weld thermal cycle.

Stabilised grades 321 and 347

Stabilised austenitic grades prevent sensitisation by binding carbon rather than reducing its concentration. AISI 321 contains up to 0.7 percent titanium. AISI 347 contains up to 1.0 percent niobium. Both elements form carbides in preference to chromium. Chromium consequently remains in solution and available to maintain the passive film.

An L grade is generally simpler and less expensive for a fabrication that is welded and then operated at ambient temperature. Stabilised grades are more suitable for continuous operation within the sensitisation range, where an L grade can eventually sensitise. Applications include exhaust systems, superheaters, expansion joints and hot process piping. Niobium forms more stable carbides than titanium. AISI 347 is generally preferred when welding filler metal is involved because titanium oxidises across the arc, while niobium does not.

The 200 series and high-alloy grades

The 200 series reduces the use of nickel, the expensive alloying element in austenitic steel. Manganese and nitrogen replace part of the nickel while retaining the austenitic structure. AISI 201 contains 5.5 to 7.5 percent manganese, up to 0.25 percent nitrogen and 3.5 to 5.5 percent nickel. AISI 202 uses still more manganese. This substitution reduces cost and increases the work-hardening rate. It also makes the grades more difficult to form and machine without improving chloride resistance. A 200 series grade may replace 304 when appearance and formability determine the selection, but not when corrosion resistance determines it.

Higher-alloy austenitic grades extend beyond AISI 316. AISI 310S contains 24.0 to 26.0 percent chromium and 19.0 to 22.0 percent nickel. It is a heat-resisting grade selected for scaling resistance at high temperature rather than aqueous corrosion resistance. AISI 904L contains 19.0 to 23.0 percent chromium, 23.0 to 28.0 percent nickel, 4.0 to 5.0 percent molybdenum and 1.0 to 2.0 percent copper. It serves in sulphuric and phosphoric acid environments. Its high nickel content resists chloride stress corrosion cracking, while copper resists reducing acids. AISI 303 is based on 304 with sulphur increased to 0.15 percent. The sulphur breaks the chip during machining but reduces corrosion resistance and weldability.

Questions about austenitic grades

Pitting resistance across the austenitic family

Computed by Laxcon Steels from its own grade reference using PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). Read the pitting resistance reference before acting on any of these figures: it states what the number does not predict.

Austenitic equivalents across standards

Basis "verified" means every value in that row was checked against the governing standard on 2026-08-18; "inherited" means it came from the previous website and has not been checked. The complete mapping table is on the equivalents reference.

Where the austenitic family is not the right answer

Austenitic stainless steels are unsuitable for hot chloride solutions that combine tensile stress with a risk of chloride stress corrosion cracking. The cracking occurs transgranularly. Ferritic, duplex and nickel-based materials largely resist this mechanism. Molybdenum does not prevent it. Hot chloride service therefore requires a duplex grade or, beyond the capability of duplex grades, a nickel alloy.

Austenitic grades are also unsuitable where heat-treated strength or hardness is required. They cannot be hardened by heat treatment and therefore do not meet the requirements of wear surfaces, cutting edges or highly loaded shafts. Martensitic grades provide hardness with corrosion resistance. Precipitation hardening grades provide strength with dimensional stability. Duplex grades provide strength alone at roughly twice the yield strength of an austenitic grade.

Austenitic grades in detail

Product forms Laxcon lists in austenitic steel