Duplex stainless steels [1] [2] [3] [4] [5] are a family of stainless steels. These are called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) 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 A2/304 or A4/316. 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 (enabling thinner sections to be used) 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 duplex stainless steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280 MPa (41 ksi), a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450 MPa (65 ksi) and a superduplex grade a minimum of 550 MPa (80 ksi). [6]
Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N. [7]
Chemicals composition of grades from EN 10088-1 (2014) Standard are given in the table below: [9]
ISO Steel designation | EN Number | UNS equiv [10] | C, max. | Si | Mn | P, max. | S, max. | N | Cr | Cu | Mo | Ni | Other |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
X2CrNiN22-2 | 1.4062 | S32202 | 0.03 | ≤1.00 | ≤2.00 | 0.04 | 0.010 | 0.16 to 0.28 | 21.5 to 24.0 | - | ≤0.45 | 1.00 to 2.90 | - |
X2CrCuNiN23-2-2 | 1.4669 | 0.045 | ≤1.00 | 1.00 to 3.00 | 0.04 | 0.030 | 0.12 to 0.20 | 21.5 to 24.0 | 1.60 to 3.00 | ≤0.50 | 1.00 to 3.00 | - | |
X2CrNiMoSi18-5-3 | 1.4424 | S31500 | 0.03 | 1.40 to 2.00 | 1.20 to 2.00 | 0.035 | 0.015 | 0.05 to 0.10 | 18.0 to 19.0 | - | 2.5 to 3.0 | 4.5 to 5.2 | - |
X2CrNiN23-4 | 1.4362 | S32304 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.5 | 0.10 to 0.60 | 0.10 to 0.60 | 3.5 to 5.5 | - |
X2CrMnNiN21-5-1 | 1.4162 | S32101 | 0.04 | ≤1.00 | 4.0 to 6.0 | 0.040 | 0.015 | 0.20 to 0.25 | 21.0 to 22.0 | 0.10 to 0.80 | 0.10 to 0.80 | 1.35 to 1.90 | - |
X2CrMnNiMoN21-5-3 | 1.4482 | 0.03 | ≤1.00 | 4.0 to 6.0 | 0.035 | 0.030 | 0.05 to 0.20 | 19.5 to 21.5 | ≤1.00 | 0.10 to 0.60 | 1.50 to 3.50 | - | |
X2CrNiMoN22-5-3 | 1.4462 | S31803,
S32205 |
0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.10 to 0.22 | 21.0 to 23.0 | - | 2.50 to 3.50 | 4.5 to 6.5 | - |
X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 0.03 | ≤0.70 | 2.5 to 4.0 | 0.035 | 0.005 | 0.20 to 0.30 | 23.0 to 25.0 | 0.10 to 0.80 | 1.00 to 2.00 | 3.0 to 4.5 | ||
X2CrNiMoCuN25-6-3 | 1.4507 | S32520 | 0.03 | ≤0.70 | ≤2.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 1.00 to 2.50 | 3.0 to 4.0 | 6.0 to 8.0 | - |
X3CrNiMoN27-5-2 | 1.4460 | S31200 | 0.05 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 25.0 to 28.0 | - | 1.30 to 2.00 | 4.5 to 6.5 | - |
X2CrNiMoN25-7-4 | 1.4410 | S32750 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.24 to 0.35 | 24.0 to 26.0 | - | 3.0 to 4.5 | 6.0 to 8.0 | - |
X2CrNiMoCuWN25-7-4 | 1.4501 | S32760 | 0.03 | ≤1.00 | ≤1.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 0.50 to 1.00 | 3.0 to 4.0 | 6.0 to 8.0 | W 0.50 to 1.00 |
X2CrNiMoN29-7-2 | 1.4477 | S32906 | 0.03 | ≤0.50 | 0.80 to 1.50 | 0.030 | 0.015 | 0.30 to 0.40 | 28.0 to 30.0 | ≤0.80 | 1.50 to 2.60 | 5.8 to 7.5 | - |
X2CrNiMoCoN28-8-5-1 | 1.4658 | S32707 | 0.03 | ≤0.50 | ≤1.50 | 0.035 | 0.010 | 0.30 to 0.50 | 26.0 to 29.0 | ≤1.00 | 4.0 to 5.0 | 5.5 to 9.5 | Co 0.50 to 2.00 |
X2CrNiCuN23-4 | 1.4655 | S32304 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.0 | 1.00 to 3.00 | 0.10 to 0.60 | 3.5 to 5.5 | - |
Mechanical properties from European Standard EN 10088-3 (2014) [9] (for product thickness below 160 mm):
ISO desig. | EN num. | 0.2% proof stress, min | Ultimate tensile strength | Elongation, min (%) |
---|---|---|---|---|
X2CrNiN23-4 | 1.4362 | 400 MPa (58 ksi) | 600 to 830 MPa (87 to 120 ksi) | 25 |
X2CrNiMoN22-5-3 | 1.4462 | 450 MPa (65 ksi) | 650 to 880 MPa (94 to 128 ksi) | 25 |
X3CrNiMoN27-5-2 | 1.4460 | 450 MPa (65 ksi) | 620 to 680 MPa (90 to 99 ksi) | 20 |
X2CrNiN22-2 | 1.4062 | 380 MPa (55 ksi) | 650 to 900 MPa (94 to 131 ksi) | 30 |
X2CrCuNiN23-2-2 | 1.4669 | 400 MPa (58 ksi) | 650 to 900 MPa (94 to 131 ksi) | 25 |
X2CrNiMoSi18-5-3 | 1.4424 | 400 MPa (58 ksi) | 680 to 900 MPa (99 to 131 ksi) | 25 |
X2CrMnNiN21-5-1 | 1.4162 | 400 MPa (58 ksi) | 650 to 900 MPa (94 to 131 ksi) | 25 |
X2CrMnNiMoN21-5-3 | 1.4482 | 400 MPa (58 ksi) | 650 to 900 MPa (94 to 131 ksi) | 25 |
X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 450 MPa (65 ksi) | 650 to 900 MPa (94 to 131 ksi) | 25 |
X2CrNiMoCuN25-6-3 | 1.4507 | 500 MPa (73 ksi) | 700 to 900 MPa (100 to 130 ksi) | 25 |
X2CrNiMoN25-7-4 | 1.4410 | 530 MPa (77 ksi) | 730 to 930 MPa (106 to 135 ksi) | 25 |
X2CrNiMoCuWN25-7-4 | 1.4501 | 530 MPa (77 ksi) | 730 to 930 MPa (106 to 135 ksi) | 25 |
X2CrNiMoN29-7-2 | 1.4477 | 550 MPa (80 ksi) | 750 to 1,000 MPa (109 to 145 ksi) | 25 |
X2CrNiMoCoN28-8-5-1* | 1.4658 | 650 MPa (94 ksi) | 800 to 1,000 MPa (120 to 150 ksi) | 25 |
*for thickness ≤ 5 mm (0.20 in)
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.
EBSD map with austenite grains excluded (white). The scale bar is 500 µm. Colours denote the crystal orientation and are taken from the inverse pole figure at the lower right corner.Duplex stainless is widely used in the industry because it possesses excellent oxidation resistance but can have limited toughness due to its large ferritic grain size, and they have hardened, and embrittlement tendencies at temperatures ranging from 280 to 500 °C, especially at 475 °C, where spinodal decomposition of the supersaturated solid ferrite solution into Fe-rich nanophase () and Cr-rich nanophase (), accompanied by G-phase precipitation, occurs, [13] [14] [15] which makes the ferrite phase a preferential initiation site for micro-cracks. [16]
UNS No. Grade | EN No. | Hot forming temperature range | Minimum soaking temperature |
---|---|---|---|
S32304 | 1.4362 | 1,150 to 950 °C (2,100 to 1,740 °F) | 980 °C (1,800 °F) |
S32205 | 1.4462 | 1,230 to 950 °C (2,250 to 1,740 °F) | 1,040 °C (1,900 °F) |
S32750 | 1.4410 | 1,235 to 1,025 °C (2,255 to 1,877 °F) | 1,050 °C (1,920 °F) |
S32520 | 1.4507 | 1,230 to 1,000 °C (2,250 to 1,830 °F) | 1,080 °C (1,980 °F) |
S32760 | 1.4501 | 1,230 to 1,000 °C (2,250 to 1,830 °F) | 1,100 °C (2,010 °F) |
Duplex stainless steel grades must be cooled as quickly as possible to room temperature after hot forming to avoid the precipitation of intermetallic phases (Sigma phase in particular) which drastically reduce the impact resistance at room temperature as well as the corrosion resistance. [17]
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.
Duplex stainless steels are usually selected for their high mechanical properties and good to very high corrosion resistance (particularly to stress corrosion cracking).