Lean duplex 2304

What is Lean duplex 2304?

Lean duplex 2304 is a lean duplex stainless steel with a two-phase ferritic-austenitic structure. Its composition contains 21.5 to 24.5 percent chromium, 3.0 to 5.5 percent nickel, 0.05 to 0.20 percent nitrogen, and 0.05 to 0.60 percent copper. Carbon is limited to 0.03 percent, and molybdenum is limited to 0.05 to 0.60 percent.

The term lean duplex denotes reduced contents of expensive alloying elements. Duplex 2304 contains roughly one third as much nickel as an AISI 316 heat and almost no molybdenum. Chromium, the cheapest and most abundant of the three elements, therefore accounts for most of its principal alloy content. ASTM A276 specifies a minimum yield strength of 400 MPa for UNS S32304, compared with 170 MPa for 316L.

Duplex 2304 differs from duplex 2205 mainly in chloride resistance rather than strength. Atlas Steels assigns 2304 a pitting resistance equivalent of 26. This value is marginally above that of AISI 316 and well below that of 2205. Measured pitting and crevice corrosion behaviour follows the same relationship. Duplex 2304 therefore serves as a substitute for 316 within a similar corrosion range. It provides roughly twice the yield strength with substantially less nickel.

Laxcon Steels lists Lean duplex 2304 in its grades reference as a duplex grade. The same steel is written 2304, S32304 and 1.4362.

What is the chemical composition of Lean duplex 2304?

Composition limits in weight percent are carbon 0.03 maximum, manganese 2.5 maximum, sulphur 0.03 maximum, phosphorus 0.04 maximum, silicon 1.0 maximum, chromium 21.5 to 24.5, nickel 3.0 to 5.5, molybdenum 0.05 to 0.6, nitrogen 0.05 to 0.2 and copper 0.05 to 0.6.

What is Lean duplex 2304 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 Lean duplex 2304?

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 requires annealed UNS S32304 bar in Condition A to have a minimum tensile strength of 600 MPa, a minimum 0.2 percent proof stress of 400 MPa, and a minimum elongation of 25 percent in 50 mm. Hardness must not exceed 290 HB. These requirements apply to both hot-finished and cold-finished products. ASTM A240 specifies the same minimum tensile strength and proof stress for plate, sheet, and coil, with a maximum hardness of 32 HRC.

Duplex 2304 has a density of roughly 7,800 kg per cubic metre and an elastic modulus near 200 GPa. Its thermal conductivity is around 17 W per metre kelvin at 100 degrees Celsius. Its specific heat is near 460 J per kilogram kelvin, and its electrical resistivity is about 850 nanoohm metres. Thermal expansion is approximately 13.0 micrometres per metre per kelvin between 0 and 100 degrees Celsius. This value is roughly one quarter lower than that of an austenitic steel. The lower expansion reduces weld distortion and thermal loads in restrained assemblies.

Comparison with AISI 316L and duplex 2205

The comparison below places duplex 2304 between the alloy it can replace and the higher-alloyed duplex grade that it does not replace. The yield strengths are the ASTM A276 Condition A minima. The PREN values are calculated from the specified composition limits as chromium plus 3.3 times molybdenum plus 16 times nitrogen.

Duplex 2304 occupies approximately the same PREN range as 316L. Its guaranteed minimum yield strength is more than twice as high, while its nickel content is roughly two thirds lower. Duplex 2205 has about twelve more PREN points because of its molybdenum content. The chromium content of duplex 2304 does not compensate for this difference.

Corrosion behaviour

The calculated pitting resistance equivalent number ranges from 22.5 at the specified composition minima to 29.7 at the maxima. This range results from the broad chromium, molybdenum, and nitrogen limits. Conforming heats at opposite ends of these limits can therefore differ in chloride resistance. Atlas Steels describes the general corrosion resistance of duplex 2304 as approximately equal to that of AISI 316 in most environments. It also reports comparable sea water performance and good resistance to intergranular, pitting, and crevice corrosion.

The ferrite phase improves resistance to chloride stress corrosion cracking. Austenitic 304 and 316 are limited to roughly 60 degrees Celsius under tensile stress in such conditions. Duplex 2304 is not affected until temperatures are well above 100 degrees Celsius. Atlas Steels reports that the grade often performs well in environments that cause premature failure of austenitic grades. Substitution for 316 can therefore extend the operating temperature range in these environments.

Heat treatment, welding, machining, and forming

Solution annealing uses a temperature range of 1,020 to 1,100 degrees Celsius followed by rapid quenching. No hardening heat treatment applies because the grade strengthens only through cold work. Welding uses over-alloyed 2209 consumables to produce a balanced deposit structure. Welding without filler is excluded because an autogenous bead solidifies with excessive ferrite. Nitrogen in the shielding gas supports austenite formation. Welding requires neither preheating nor post-weld heat treatment, and arc energy remains modest.

Machining performance varies by operation. Atlas Steels reports that duplex 2304 bar is less machinable than 316 in some operations and more machinable in others. Poor chip breaking can produce rough finishes after certain cuts. Duplex 2304 is easier to machine than duplex 2205. Forming requires higher tonnage and allowance for springback. Its ductility does not support heavy cold heading. Processes with substantial cold reduction should include an intermediate anneal.

Applications and designations

Applications include chemical-industry storage tanks and process vessels, oil and gas surface equipment, water and effluent treatment plant, and pulp and paper liquor handling. The grade also serves in structural members and load-bearing components where its higher yield strength permits thinner sections. Coastal and marine-adjacent hardware may use the grade within its chloride resistance range. European designations are EN material number 1.4362 and X2CrNiN23-4. The UNS designation is S32304. Other common forms are 2304, SAF 2304, and SS2304. Laxcon supplies the grade under these designations.

Where Lean duplex 2304 is not the right choice

Duplex 2304 is unsuitable when duplex structure is assumed to provide the chloride resistance of higher-alloyed duplex grades. Its very low molybdenum content limits its pitting resistance to approximately that of AISI 316. Warm sea water, desalination brine, hot chloride process streams, and crevice-prone marine fittings exceed this range. Duplex 2205 is the minimum higher grade for these duties, while super duplex 2507 applies to severe conditions.

Sustained service above 300 degrees Celsius decomposes the ferrite phase and embrittles the steel. Atlas Steels reports that duplex 2304 resists this effect better than richer duplex grades, but identifies its onset after around ten hours of exposure. Recovery requires a full solution anneal. Below roughly minus 50 degrees Celsius, the ferrite undergoes a ductile-to-brittle transition. Austenitic 316L retains toughness at both temperature extremes and is the appropriate substitute.

The ductility of duplex 2304 is insufficient for deep-drawn, spun, and cold-headed parts. Welds made without filler metal become over-ferritic and lose toughness and corrosion resistance. Where severe forming or stock availability controls material selection rather than strength, 316L is easier to use and has effectively the same pitting resistance.

Other duplex grades