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An ingot of 2507 duplex stainless steelDuplex 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]
Grades of duplex stainless steels
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Microstructures of four kinds of duplex stainless steel in each directionDuplex 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]
Chemical compositions
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Chemicals composition of grades from EN 10088-1 (2014) Standard are given in the table below:[9]
Composition by weight (%) 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
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Mechanical properties from European Standard EN 10088-3 (2014)[9] (for product thickness below 160 mm):
Mechanical properties at room temperature of solution-annealed austenitic–ferritic stainless steels 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.
475 °C embrittlement
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[11]Electron backscatter diffraction map of 128 hrs age hardened duplex stainless steel with the ferrite phase forming the matrix and austenite grains sporadically spread. The ferrite phase volume fraction is 58%.[12]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.
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 ( a ´ {\displaystyle {\acute {a}}} ) and Cr-rich nanophase ( a ´ ´ {\displaystyle {\acute {a}}{\acute {}}} ), accompanied by G-phase precipitation, occurs,[13][14][15] which makes the ferrite phase a preferential initiation site for micro-cracks.[16]
Heat treatment
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Recommended hot forming and annealing/soaking temperatures 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.
Applications of duplex stainless steels
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Duplex stainless steels are usually selected for their high mechanical properties and good to very high corrosion resistance (particularly to stress corrosion cracking).
Further reading
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See also
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References
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The origin of duplex stainless steels can be traced back to the 1930s. Aggressive process environments in pulp and papermaking mills in Sweden spurred the development of more corrosion-resistant grades by local manufacturer Avesta. Higher process temperatures and pressures, together with aggressive sulphite solutions proved to be an onerous environment.
Now that PREN values (Pitting Corrosion Resistance Equivalent Number) are an accepted indication of general pitting corrosion resistance, it is obvious that increasing chromium content will improve general performance. Duplex stainless steels were established as a product family around a chromium content of c. 22%, compared with 16-18% for standard austenitic stainless steels.
As suggested by their name, duplex grades exhibit two distinct microstructural phases, that of both austenitic and ferritic. Each microstructural phase is a particular type of crystal structure that informs its properties. In duplex stainless steel, ‘islands’ of ferrite form within a matrix of austenite. Consequently, they take the best parts of both structures properties, namely; higher strength; good toughness; relative ease of fabrication; good corrosion resistance and resistance to stress corrosion cracking. The key to maintaining this combination of favourable properties is to maintain a near 50:50 balance in the microstructure, which is achieved through the composition and process conditions.
The Schaeffler diagram is an interesting chart even for non-metallurgists. A number of elements are no known to act in a similar way to chromium, encouraging the formation of a particular microstructure (ferrite). In the same way, other elements are known to act in the same way as nickel and encourage an austenitic microstructure. By calculating the chromium and nickel ‘equivalents’, it is possible to predict the final microstructure. As shown in the chart below, the progression from duplex to super duplex, to hyper duplex makes a little more sense. Chromium (and chromium equivalent) content increases, providing increased pitting corrosion resistance. However, nickel (and nickel equivalent) content must also increase to maintain the right balance of microstructure, physical and mechanical properties.
Whilst duplex stainless steels have existed in one form or another since the 1930s, it wasn’t until the 1970’s when their uptake improved. Better steelmaking technology allowed tighter control of composition, particularly that of nitrogen. Higher levels of nitrogen improve the pitting corrosion resistance in a cost-effective manner compared with more expensive alloying elements. Also, improved knowledge of weldability, maintaining the duplex microstructure through the heat-affected zone of the weld region, meant that it could be used without risk of premature failure due to poor fabrication.
At around the same time, super duplex stainless steels were invented. Led initially by Langley Alloys unique Ferralium 255 product in 1969, it was followed by Zeron 100 (UNS S32760) in 1980 and then SAF2507 (UNS S32750) in 1988. All three super duplex alloys are based upon a 25% chromium content and are now formulated to achieve a minimum PREN of 40 as this is a requirement of the Norsok standard (M-001).
Hyper duplex alloys are now being developed as potential alternatives to more expensive nickel alloys. Sandvik has developed their Sandvik SAF 2707 HD grade, pushing the chromium content beyond 27% and the PREN to c. 48. This has been exceeded by Sandvik SAF 3207 HD grade, with a chromium level of 32% and a PREN of c.50. These compositional changes also result in an equivalent increase in yield strength of >100ksi (compared with 80ksi for S32750/32760 and 85ksi for Ferralium 255-SD50).
To date, these highly-alloyed grades have only been made available commercially as extruded seamless tubes, typically for use in umbilicals, heat exchangers, condensers and seawater coolers. Producing forged bars and rolled plates has proven more problematical – difficult processing, lower yield and manufacturing defects have precluded their launch. Matching the corrosion performance of certain nickel alloys is definitely possible, and offering a cost-effective alternative is also viable, particularly if nickel prices increased in level or volatility.
Duplex Stainless Steels
Super Duplex Stainless Steels
Hyper Duplex Stainless Steels
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