Duplex stainless steels sit between standard austenitic grades like 316 and expensive nickel alloys. They offer roughly twice the strength of 316 and much better resistance to chloride corrosion, often at a competitive cost. That combination has made them a go-to material in oil and gas, desalination, chemical processing and marine engineering. This guide explains what duplex steels are and when to use them.

What does “duplex” mean?

The name refers to the steel’s microstructure. Standard 304 and 316 are austenitic, with a single phase. Duplex steels are balanced to contain roughly equal amounts of two phases, austenite and ferrite. This two-phase structure gives them their combination of high strength and good toughness, and it is the reason they are partly magnetic, unlike annealed 304 and 316.

Main families and grades

Lean duplex

Lean duplex grades contain lower amounts of nickel and molybdenum. They offer strength well above 304 with corrosion resistance similar to or better than 304, and are used in structural applications and storage tanks.

Standard duplex

Grade 2205, also known as UNS S31803 or S32205, is the most widely used duplex steel. With around 22 percent chromium, 5 percent nickel and 3 percent molybdenum, it offers much better pitting and stress corrosion cracking resistance than 316, plus about twice the yield strength.

Super duplex

Super duplex grades such as UNS S32760 and S32750 contain around 25 percent chromium, higher molybdenum and added nitrogen. S32760 also contains tungsten and copper. They are designed for aggressive chloride environments such as seawater systems, offshore platforms and desalination plants.

Advantages of duplex steels

High strength. Designers can often use thinner walls, saving weight and material.
Chloride resistance. Much better resistance to pitting, crevice corrosion and chloride stress corrosion cracking than 304 and 316.
Cost efficiency. Lower nickel content than austenitic grades of similar corrosion resistance, and less material thanks to higher strength.
Good fatigue and erosion resistance in many applications.

Limitations to keep in mind

Temperature range. Duplex steels are generally not recommended for long-term service above around 250 to 300 degrees Celsius, because harmful phases can form. They are also less tough than austenitic grades at very low temperatures.
Welding needs care. Heat input and filler metals must be controlled to keep the right phase balance in the weld.
Forming. Higher strength means more force is needed for bending and forming.

Measuring corrosion resistance: PREN

Engineers often compare stainless steels using the Pitting Resistance Equivalent Number, or PREN. It is calculated from the alloy content as chromium plus 3.3 times molybdenum plus 16 times nitrogen, all in percent. The higher the number, the better the resistance to pitting in chloride environments. Grade 316 typically has a PREN in the mid-twenties, 2205 duplex around the mid-thirties, and super duplex grades above 40. PREN is a useful screening tool, but it does not replace testing or experience in the actual service environment.

Typical applications

Offshore oil and gas piping, risers and process equipment.
Seawater cooling and firewater systems.
Desalination plants.
Chemical tankers and storage tanks.
Pulp and paper digesters and bleaching equipment.
Heat exchangers handling chloride-bearing water.

How to specify

Use the UNS number as well as the common name, state the product form and standard, and request a 3.1 inspection certificate showing chemistry, mechanical properties and, where required, ferrite content and corrosion test results. For welded fabrications, make sure the fabricator has qualified welding procedures for the specific grade.

Sourcing duplex steel

Nefton Exports Industries supplies stainless steel and alloy sheets, plates and other steel products, including duplex and super duplex grades. Contact us with your grade and dimensions for a quotation.