Duplex

What defines the duplex family?

Duplex stainless steel is a stainless steel with roughly equal proportions of austenite and ferrite in its microstructure. This structure results from balancing about 21 to 30 percent chromium and controlled nitrogen with a restrained nickel content. The ferrite provides roughly twice the yield strength of a standard austenitic grade and removes most susceptibility to chloride stress corrosion cracking associated with the 300 series. The austenite maintains toughness and formability.

Nitrogen distinguishes modern duplex grades from the first generation of duplex steels. It acts as a strong and rapid austenite stabiliser. This action limits excessive ferrite formation in the heat-affected zone as a weld cools. Nitrogen also contributes strongly to pitting resistance. Its coefficient in the pitting resistance equivalent number formula is 16, compared with 3.3 for molybdenum and 1 for chromium.

The Laxcon Steels grade reference lists 15 duplex grades with their full composition. The table below carries every one of them that has a cross-standard equivalent on record.

Duplex grade classes

The duplex family consists of lean duplex, molybdenum-alloyed lean duplex, standard duplex, super duplex and hyper duplex grades. Each successive class provides greater chloride resistance and a narrower fabrication window.

Lean duplex grades do not contain a deliberate molybdenum addition. S32101 contains 21.0 to 22.0 percent chromium, 1.35 to 1.70 percent nickel and 4.0 to 6.0 percent manganese. S32202 and S32304 belong to the same class. These grades compete with 304. They provide comparable corrosion resistance, roughly twice the strength and a much lower nickel content.

S32003 represents lean duplex with a small molybdenum addition. It contains 19.5 to 22.5 percent chromium and 1.5 to 2.0 percent molybdenum. It competes with 316 while retaining the duplex strength advantage.

Grade 2205 is the reference standard duplex grade. It contains 22.0 to 23.0 percent chromium, 4.5 to 6.5 percent nickel, 3.0 to 3.5 percent molybdenum and 0.14 to 0.20 percent nitrogen. Its applications include process vessels and structural work in marine atmospheres.

Super duplex grades are defined by pitting resistance rather than composition alone. Grade 2507 contains 24.0 to 26.0 percent chromium, 6.0 to 8.0 percent nickel, 3.0 to 5.0 percent molybdenum and 0.24 to 0.32 percent nitrogen. S32760 reaches the same class with a different composition that includes 0.5 to 1.0 percent each of copper and tungsten. Hyper duplex grades include S32906, which contains 28.0 to 30.0 percent chromium and 0.30 to 0.40 percent nitrogen.

Mechanical properties and design use

Duplex grades have roughly twice the yield strength of austenitic grades. Pressure vessels, tanks and line pipes can use this strength when allowable stress determines wall thickness. The higher yield strength permits substantially thinner sections for the same duty. This reduces alloy weight, weld metal volume and fabrication hours. These properties led duplex grades to displace 316 in chemical tankers, storage tanks and offshore piping.

The strength advantage does not reduce section thickness when stiffness, minimum practical thickness, external pressure or a corrosion allowance controls the design. Under these conditions, duplex is more expensive and less tolerant of fabrication errors than 316. Its economic value therefore depends on whether allowable stress determines the required section.

Thermal processing and fabrication

Composition and solution annealing followed by rapid quenching establish the duplex phase balance. Later thermal exposure can alter that balance. Duplex grades precipitate intermetallic phases, particularly sigma phase, between roughly 600 and 1,000 degrees Celsius. Even a short hold in this range reduces toughness and corrosion resistance. The ferrite phase becomes embrittled near 475 degrees Celsius during longer exposures. This effect determines the upper service temperature of the family.

Welding requires control of heat input and interpass temperature. Excessive heat can promote intermetallic formation. Insufficient heat can leave the weld excessively ferritic. Hot forming occurs above the intermetallic range and requires subsequent full solution annealing and quenching. Conventional carbon steel stress relieving is unsuitable because the temperature required to relieve stress also forms sigma phase.

Higher alloy content accelerates sigma formation and narrows the processing window. Super duplex is therefore more demanding to fabricate than 2205. Lean duplex has the widest fabrication window of the three classes.

Questions about duplex grades

Pitting resistance across the duplex 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.

Duplex 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 duplex family is not the right answer

Duplex stainless steel is unsuitable for high-temperature service. Intermetallic precipitation between roughly 600 and 1,000 degrees Celsius and ferrite embrittlement around 475 degrees Celsius give the family a lower upper service limit than austenitic grades. Grades 304H, 321 and 310S, or a nickel alloy, are used for hot service instead.

Duplex stainless steel is also unsuitable for cryogenic service. Its ferrite phase undergoes a ductile to brittle transition that does not occur in fully austenitic grades.

The family is unsuitable when a fabricator cannot maintain the required procedure controls. A poorly welded duplex joint can perform worse in service than a correctly welded 316 joint. Duplex is also unsuitable when the design cannot use its higher strength. If a factor other than allowable stress determines wall thickness, the economic basis for selecting the family disappears.

Duplex grades in detail

Product forms Laxcon lists in duplex steel