Duplex stainless steel


Duplex stainless steels are a family of stainless steels. These are called duplex grades because their metallurgical structure consists of two phases, austenite and ferrite in roughly equal proportions. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as Type 304 or 316. The main differences in composition, when compared with an austenitic stainless steel is that the duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength give significant cost benefits. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc and in the petrochemical industry in the form of pipelines and pressure vessels. In addition to the improved corrosion resistance compared with the 300 series stainless steels duplex steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280N/, a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450N/ and a superduplex grade a minimum of 550N/.
They are used for their good mechanical properties in the as-cast and/or their excellent corrosion resistance properties.

Grades of duplex stainless steels

Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number,.
; Standard duplex : Typically Grade EN 1.4462. It is typical of the mid-range of properties and is perhaps the most used today
; Super-duplex : Typically grade EN 1.4410 up to so-called hyper duplex grades developed later to meet specific demands of the oil and gas as well as those of the chemical industries. They offer a superior corrosion resistance and strength but are more difficult to process because the higher contents of Cr, Ni, Mo, N and even W promote the formation of intermetallic phases, which reduce drastically the impact resistance of the steel. Faulty processing will result in poor performance and users are advised to deal with reputable suppliers/processors. Applications include deepwater offshore oil production.
; Lean duplex grades : Typically grade EN 1.4362, have been developed more recently for less demanding applications, particularly in the building and construction industry. Their corrosion resistance is closer to that of the standard austenitic grade EN 1.4401 and their mechanical properties are higher. This can be a great advantage when strength is important. This is the case in bridges, pressure vessels or tie bars.

Chemical compositions

Chemicals composition of grades from EN 10088-1 Standard are given in the table below:
Steel designationNumberC, max.SiMnP, max.S, max.NCrCuMoNiOther
X2CrNiN22-21.40620.03≤1.00≤2.000.040.0100.16 to 0.2821.5 to 24.0-≤0.451.00 to 2.90-
X2CrCuNiN23-2-21.46690.045≤1.001.00 to 3.000.040.0300.12 to 0.2021.5 to 24.01.60 to 3.00≤0.501.00 to 3.00-
X2CrNiMoSi18-5-31.44240.031.40 to 2.001.20 to 2.000.0350.0150.05 to 0.1018.0 to 19.0-2.5 to 3.04.5 to 5.2-
X2CrNiN23-41.43620.03≤1.00≤2.000.0350.0150.05 to 0.2022.0 to 24.50.10 to 0.600.10 to 0.603.5 to 5.5-
X2CrMnNiN21-5-11.41620.04≤1.004.0 to 6.00.0400.0150.20 to 0.2521.0 to 22.00.10 to 0.800.10 to 0.801.35 to 1.90-
X2CrMnNiMoN21-5-31.44820.03≤1.004.0 to 6.00.0350.0300.05 to 0.2019.5 to 21.5≤1.000.10 to 0.601.50 to 3.50-
X2CrNiMoN22-5-31.44620.03≤1.00≤2.000.0350.0150.10 to 0.2221.0 to 23.0-2.50 to 3.504.5 to 6.5-
X2CrNiMnMoCuN24-4-3-21.46620.03≤0.702.5 to 4.00.0350.0050.20 to 0.3023.0 to 25.00.10 to 0.801.00 to 2.003.0 to 4.5
X2CrNiMoCuN25-6-31.45070.03≤0.70≤2.000.0350.0150.20 to 0.3024.0 to 26.01.00 to 2.503.0 to 4.06.0 to 8.0-
X3CrNiMoN27-5-21.44600.05≤1.00≤2.000.0350.0150.05 to 0.2025.0 to 28.0-1.30 to 2.004.5 to 6.5-
X2CrNiMoN25-7-41.44100.03≤1.00≤2.000.0350.0150.24 to 0.3524.0 to 26.0-3.0 to 4.56.0 to 8.0-
X2CrNiMoCuWN25-7-41.45010.03≤1.00≤1.000.0350.0150.20 to 0.3024.0 to 26.00.50 to 1.003.0 to 4.06.0 to 8.0W 0.50 to 1.00
X2CrNiMoN29-7-21.44770.03≤0.500.80 to 1.500.0300.0150.30 to 0.4028.0 to 30.0≤0.801.50 to 2.605.8 to 7.5-
X2CrNiMoCoN28-8-5-11.46580.03≤0.50≤1.500.0350.0100.30 to 0.5026.0 to 29.0≤1.004.0 to 5.05.5 to 9.5Co 0.50 to 2.00
X2CrNiCuN23-41.46550.03≤1.00≤2.000.0350.0150.05 to 0.2022.0 to 24.01.00 to 3.000.10 to 0.603.5 to 5.5-

Mechanical properties

Mechanical properties from European Standard EN 10088-3 :
0.2% proof stress, min. Ultimate tensile strength Elongation, min.
X2CrNiN23-41.4362400600 to 83025
X2CrNiMoN22-5-31.4462450650 to 88025
X3CrNiMoN27-5-21.4460450620 to 68020
X2CrNiN22-21.4062380650 to 90030
X2CrCuNiN23-2-21.4669400650 to 90025
X2CrNiMoSi18-5-31.4424400680 to 90025
X2CrMnNiN21-5-11.4162400650 to 90025
X2CrMnNiMoN21-5-31.4482400650 to 90025
X2CrNiMnMoCuN24-4-3-21.4662450650 to 90025
X2CrNiMoCuN25-6-31.4507500700 to 90025
X2CrNiMoN25-7-41.4410530730 to 93025
X2CrNiMoCuWN25-7-41.4501530730 to 93025
X2CrNiMoN29-7-21.4477550750 to 100025
X2CrNiMoCoN28-8-5-1*1.4658650800 to 100025

*for thickess ≤ 5 mm
The minimum yield stress values are about twice as high as those of austenitic stainless steels.
Duplex grades are therefore attractive when mechanical properties at room temperature are important because they allow thinner sections.

Heat treatment

Duplex stainless steel grades must be cooled as quickly as possible to room temperature after hot forming to avoid the precipitation of intermetallic phases which drastically reduce the impact resistance at room temperature as well as the corrosion resistance..
Alloying elements Cr, Mo, W, Si increase the stability and the formation of intermetallic phases. Therefore super duplex grades have a higher hot working temperature range and require faster cooling rates than the lean duplex grades.

Applications of duplex stainless steels

Duplex stainless steels are usually selected for their high mechanical properties and good to very high corrosion resistance.