duplex stainless steel

Duplex stainless steel: Duplex stainless steel is a type in which, within its solid solution structure, the ferrite phase and the austenite phase each make up half of it; generally, the content of either phase should also be at least 30%. At low carbon levels, the Cr content ranges from 18% to 28%, while the Ni content ranges from 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N. This type of steel possesses the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic steels, it offers higher plasticity and toughness, lacks room-temperature brittleness, and exhibits significantly improved resistance to intergranular corrosion as well as weldability. At the same time, it retains the brittleness at 475°C and high thermal conductivity associated with ferritic stainless steels, as well as superplasticity. Compared to austenitic stainless steels, it has higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel possesses excellent pitting resistance and is also a nickel-saving stainless steel. 　　Performance characteristics of duplex stainless steel Due to its two-phase structure, and through proper control of chemical composition and heat treatment processes, duplex stainless steel combines the advantages of both ferritic and austenitic stainless steels. It merges the excellent toughness and weldability of austenitic stainless steels with the higher strength and resistance to chloride stress corrosion of ferritic stainless steels. It is these superior properties that have led to the rapid development of duplex stainless steel as a weldable structural material; since the 1980s, it has become one of the major types of stainless steel, alongside martensitic, austenitic, and ferritic stainless steels. Dual-phase stainless steel has the following performance characteristics: (1) Molybdenum-containing dual-phase stainless steel exhibits good resistance to chloride stress corrosion at low stresses. Generally, 18-8 austenitic stainless steels are prone to stress corrosion cracking in neutral chloride solutions at temperatures above 60°C. Equipment such as heat exchangers and evaporators manufactured from these steels and used in industrial environments containing trace amounts of chlorides and hydrogen sulfide is susceptible to stress corrosion cracking, whereas duplex stainless steels exhibit good resistance to this phenomenon. 　　(2) Mo-containing duplex stainless steels exhibit good pitting resistance. When they have the same pitting resistance equivalent value (PRE=Cr%+3.3Mo%+16N%), the critical pitting potentials of duplex stainless steels and austenitic stainless steels are similar. The pitting resistance of duplex stainless steels and austenitic stainless steels is comparable to that of AISI 316L. The pitting and crevice corrosion resistance of 25%Cr stainless steels, especially nitrogen-containing high-chromium duplex stainless steels, exceeds that of AISI 316L. 　　(3) It has good resistance to corrosion fatigue and wear corrosion. Under certain corrosive medium conditions, it is suitable for manufacturing power equipment such as pumps and valves. 　　(4) It has good comprehensive mechanical properties. It has high strength and fatigue resistance, with a yield strength that is twice that of 18-8 austenitic stainless steel. The elongation rate in the solid solution state reaches 25%, and the toughness value AK (V-notch) is above 100 J. 　　(5) It has good weldability and a low tendency to thermal cracking; generally, no preheating is required before welding, nor is heat treatment necessary after welding. It can be welded to different materials such as 18-8 austenitic stainless steel or carbon steel. 　　(6) The hot working temperature range for duplex stainless steels with low chromium content (18%Cr) is wider than that of 18-8 austenitic stainless steels; they exhibit less resistance, allowing steel plates to be produced directly through rolling without the need for forging. Thermal processing of duplex stainless steels with high chromium content (25% Cr) is slightly more difficult than that of austenitic stainless steels, but products such as sheets, tubes, and wires can be manufactured. 　　(7) During cold working, the work hardening effect is greater than that of 18-8 austenitic stainless steel; at the initial stage of deformation in tubes and plates, a higher stress must be applied to cause deformation. 　　(8) Compared with austenitic stainless steels, it has a higher thermal conductivity and a lower linear expansion coefficient, making it suitable for use as linings in equipment and for producing composite sheets. It is also suitable for use in heat exchanger wafers, offering a higher heat exchange efficiency than austenitic stainless steel. 　　(9) It still exhibits various brittle tendencies typical of high-chromium ferritic stainless steels, and is not suitable for use in operating conditions above 300°C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phases such as σ become. 　　Duplex Stainless Steel, abbreviated as DSS, refers to a type of stainless steel in which ferrite and austenite each make up approximately 50%; generally, the proportion of the minority phase should be at least 30%. 　　Since its inception in the United States in the 1940s, duplex stainless steel has evolved to its third generation. Its main feature is that its yield strength can reach 400–550 MPa, which is twice that of ordinary stainless steel; thus, it allows for reduced material usage and lower costs in equipment manufacturing. In terms of corrosion resistance, especially in harsh medium environments such as seawater with high chloride content, duplex stainless steels exhibit significantly better performance against pitting corrosion, crevice corrosion, stress corrosion, and corrosion fatigue compared to ordinary austenitic stainless steels, and can rival highly alloyed austenitic stainless steels. 　　Duplex stainless steel exhibits good weldability. Compared with ferritic and austenitic stainless steels, it does not suffer from a significantly reduced ductility and toughness in its weld heat-affected zone due to severe grain coarsening, as is the case with ferritic stainless steels; nor is it as susceptible to weld hot cracks as austenitic stainless steels. 　　Due to its unique advantages, duplex stainless steel is widely used in various industrial fields such as petrochemical equipment, seawater and wastewater treatment systems, oil and gas pipelines, and papermaking machinery. In recent years, it has also been explored for use in bridge load-bearing structures, showing great potential for further development. The grades of duplex stainless steel that I have seen include: 2205, 2506, 2507, 2520, 329J1, 329J1L ； The following information is hoped to be useful to everyone: Bipolar stainless steels are designed from a metallurgical perspective so that their microstructure in the annealed state contains roughly equal proportions of austenite and ferrite, which is achieved by keeping the nickel content at a moderately low level and increasing the chromium content to 22%–26%. The molybdenum content remains roughly the same as that of 317L stainless steel. Therefore, if the PRE value is higher than 30, it is mainly due to high chromium and nitrogen contents. The biphasic structure exhibits characteristics in which each phase holds dominance. Most importantly, these steel grades possess very high strength as well as usable plasticity and toughness. Because their nickel content is not very high, they offer an excellent combination of strength, resistance to uniform corrosion, and resistance to stress corrosion cracking at a moderate price. Similar to austenitic stainless steels, heat treatment does not harden these duplex steel grades. When processing duplex stainless steel, care must be taken to avoid damage to the secondary phases and to maintain a balance in which the austenite phase and the ferrite phase are present in roughly equal amounts. In this regard, they have stricter requirements than austenitic stainless steels. Table 3 lists the main forged and rolled high-performance duplex stainless steels. Table 3 Chemical composition* of rolled high-performance duplex stainless steels, weight%** Steel grade UNS Subclass C N Cr Ni Mo Cu Others Pitting resistance Grade equivalent number Alloy 329 S32900 D-1 0.08 – 23.0–28.0 2.5–5.0 1.00–2.00 – – 26 3RE60 S31500 0.03 0.05–0.10 18.0–19.0 4.25–5.25 2.50–3.00 – Si:1.40–2.00 27 Mn:1.20–2.00 2304 S32304 0.03 0.05–0.20 21.5–24.5 3.0–5.5 0.05–0.60 – – 22 45M*** D-2 0.03 0.15 24.3 5 1.5 1 – 32 44LN S31200 0.03 0.14–0.20 24.0–26.0 5.5–6.5 1.20–2.00 – – 30 2205 S31803 0.03 0.08–0.20 21.0–23.0 4.5–6.5 2.5–3.5 – – 31 2205 S32205 D-3 0.03 0.14–0.20 22.0–23.0 4.5–6.5 3.0–3.5 – – 34 7-MoPLUS S32950 0.03 0.15–0.35 26.0–29.0 3.5–5.2 1.00–2.50 – – 32 DP3 S31260 0.03 0.10–0.30 24.0–26.0 5.5–7.5 2.5–3.5 0.20–0.80 W:0.10–0.50 34 UR47N 0.03 0.14–0.20 24.0–26.0 5.5–7.5 2.5–3.5 – – 34 64*** D-4 0.03 0.14 25 6.4 3.5 – – 39 255 S32550 0.04 0.10–0.25 24.0–27.0 4.5–6.5 2.9–3.9 1.50–2.50 – 35 DP3W S39274 0.03 0.24–0.32 24.0–26.0 6.0–8.0 2.5–3.5 0.20–0.80 W:1.50–2.50 36 100 S32760 0.03 0.20–0.30 24.0–26.0 6.0–8.0 3.0–4.0 0.50–1.00 W:0.50–1.00 37 2507 S32750 0.03 0.24–0.32 24.0–26.0 6.0–8.0 3.0–5.0 –0.5 – 38 52N+ S32520 0.03 0.20–0.35 24.0–26.0 5.5–8.0 3.0–5.0 0.50–3.00 – 37 *Taken from ASTM sheet and strip specifications or manufacturer’s data sheets ； **Maximum, unless a range is specified or it is specified as minimum ； ***Nominal chemical composition. ①D–l category of duplex stainless steels: This category includes only one steel grade, namely 2304. Although 2304 does not have superior corrosion resistance compared to standard austenitic stainless steels, it is still classified as a high-performance stainless steel; as with all second-generation duplex stainless steels, the low-carbon, high-nitrogen composition of 2304 allows it to offer a better balance between machinability and corrosion resistance than earlier duplex steel grades. It is easy to weld, and compared to 316L or 317L, it has higher strength as well as better resistance to stress corrosion cracking. ②D–2 category of duplex stainless steels: The duplex stainless steels in this category, particularly 2205, are the most useful among duplex stainless steels, as they offer advantages in terms of corrosion resistance, workability, and cost. They possess various advantages in terms of machinability and corrosion resistance, and their resistance to stress corrosion cracking is superior to that of 316 stainless steel. ③D–3 category duplex stainless steels: These 25Cr duplex stainless steels, such as Ferralium 255, have a higher chromium content than the steels in the D–2 category, and therefore possess greater resistance to localized corrosion. However, it is generally believed that it cannot withstand seawater corrosion in harsh environments. Chromium provides good resistance to corrosion by oxidizing acids. These steels require a higher content of nickel in order to balance the high chromium content, which in turn improves their resistance to corrosion by reducing acids. The disadvantage of high chromium content is that it accelerates the kinetics of the precipitation of harmful phases; therefore, processing involving heat treatment requires precise control of temperature conditions. In some cases, rapid precipitation kinetics may limit the effective cross-sectional size. ④D–4 category of duplex stainless steels: This category of stainless steels represents the group with the highest degree of alloying among duplex stainless steels. The high contents of chromium, molybdenum, nickel, and nitrogen confer the best corrosion resistance of all duplex stainless steels, and their strength is greater than that of any other high-performance stainless steel. For this reason, this group of steels is sometimes referred to as super duplex stainless steel. Their resistance to pitting and crevice corrosion in seawater at room temperature is similar to that of 6% molybdenum austenitic stainless steels of category A-4, with a yield strength exceeding 550 MPa (80 ksi). However, due to their high alloy content, the limitations on their hot working processes are even stricter than those of D–3 category steels. Austenitic stainless steel: Austenitic stainless steel – Stainless steel that has an austenitic structure at room temperature. When steel contains about 18% Cr, 8%~10% Ni, and about 0.1% C, it has a stable austenitic structure. Austenitic chromium-nickel stainless steels include the well-known 18Cr-8Ni steel, as well as high Cr-Ni series steels that were developed by increasing the amounts of Cr and Ni and adding elements such as Mo, Cu, Si, Nb, and Ti. Austenitic stainless steels are non-magnetic and possess high toughness and ductility, but they have low strength; they cannot be strengthened through phase transformation and can only be strengthened by cold working. If elements such as S, Ca, Se, and Te are added, it exhibits good machinability. In addition to resisting corrosion in oxidizing acidic media, such steels can also withstand corrosion by sulfuric acid, phosphoric acid, as well as formic acid, acetic acid, urea, etc., if they contain elements such as Mo and Cu. If the carbon content in such steels is below 0.03% or if they contain Ti and Ni, their resistance to intergranular corrosion can be significantly improved. Austenitic stainless steels with high silicon content exhibit good corrosion resistance in concentrated nitric acid. Due to their comprehensive and excellent overall properties, austenitic stainless steels are widely used in various industries. 　　Austenitic steel (1)1Cr17Mn6Ni15N ； (2)1Cr18Mn8Ni5N ； (3)1Cr18Ni9 ； (4)1Cr18Ni9Si3 ； (5)0Cr18Ni9 ； (6)00Cr19Ni10 ； (7)0Cr19Ni9N ； (8)0Cr19Ni10NbN ； (9)00Cr18Ni10N ； (10)1Cr18Ni12 ； (11) 0Cr23Ni13 ； (12)0Cr25Ni20 ； (13) 0Cr17Ni12Mo2 ； (14) 00Cr17Ni14Mo2 ； (15) 0Cr17Ni12Mo2N ； (16) 00Cr17Ni13Mo2N ； (17) 1Cr18Ni12Mo2Ti ； (18) 0Cr18Ni12Mo2Ti ； (19) 1Cr18Ni12Mo3Ti ； (20) 0Cr18Ni12Mo3Ti ； (21) 0Cr18Ni12Mo2Cu2 ； (22) 00Cr18Ni14Mo2Cu2 ； (23) 0Cr19Ni13Mo3 ； (24) 00Cr19Ni13Mo3 ； (25) 0Cr18Ni16Mo5 ； (26) 1Cr18Ni9Ti ； (27) 0Cr18Ni10Ti ； (28) 0Cr18Ni11Nb ； (29) 0Cr18Ni13Si4 　　1. Overview 　　Austenitic stainless steel was introduced in Germany in 1913, and it has always played a crucial role among stainless steels; its production and consumption account for approximately 70% of the total production and usage of stainless steels. There are also the most various steel grades; in China today, there are over 40 grades of austenitic stainless steel in common use, with the 18-8 type being the most common. 　　Definition: Stainless steel that has an austenitic structure at room temperature. 　　Classification: Fe-Cr-Ni (main type), Fe-Cr-Mn. Comparison of domestic and international grades: GB (China), ASTM (USA), JIS (Japan), DIN (Germany). 1Cr17Ni7, 301, SUS301, X12CrNi177; 1Cr18Ni9, 302, SUS302, X12CrNi188; 1Cr18Ni10, 303, SUS303, X12CrNiS188; 0Cr18Ni9, 304, SUS304, X5CrNi189; 0Cr19Ni10, 304L, SUS304L, X2CrNi189; 0Cr17Ni12Mo2, 316, SUS316, X5CrNiMo1810; 00Cr17Ni14Mo2, 316L, SUS316L, X2CrNiMo1810; 0Cr18Ni10Ti, 321, SUS321, X10CrNiTi189; 0Cr19Ni13Mo3, 317, SUS317, X2CrNiMo1816. 2. Composition of austenitic stainless steels: Evolving from the composition of 18-8 type stainless steels, there have been several key developments, including: 1) Addition of Mo to improve pitting and crevice corrosion resistance; 2) Reduction of C or addition of Ti and Nb to decrease the tendency for intergranular corrosion; 3) Addition of Ni and Cr to enhance high-temperature oxidation resistance and strength; 4) Addition of Ni to improve stress corrosion resistance. (Source: “I Want Stainless Steel”); 5) Addition of S and Se to improve machinability and surface precision of components. Diagram of the composition system for austenitic stainless steels. 3. Microstructure of austenitic stainless steels. 3.1 Formation of ferrite phase. 3.1.1 Influence of ferrite phase on the properties of austenitic stainless steels: The presence of the F phase generally has an adverse effect on the properties of austenitic stainless steels, such as increasing the tendency to develop cracks during hot processing ； The pitting resistance of steel decreases, and its corrosion resistance deteriorates in various corrosive environments (such as urea production) ； When heated at high temperatures for an extended period, phase F transforms into phase σ, causing the steel to become brittle, among other effects. 　　3.1.2 Formation of the ferrite phase and rough estimation of its content Rough estimation of content: Creq=%Cr+1.5×%Si+%Mo, Nieq=%Ni+30×(%C+%N)+0.5×%Mn 3.1.3 Elimination of the ferrite phase The fundamental approach is to increase the content of elements that promote austenite formation in the steel. Ni is the preferred element, but from an economic perspective, Mn and N are also given attention. In particular, N has a ferrite-inhibiting ability 30 times that of Ni, and it also improves corrosion resistance and strength.

Duplex stainless steel is a type of stainless steel in which austenite and ferrite coexist; the most commonly used grades include 2205 and CD4MCu

The toughness of duplex steel is much lower than that of austenitic stainless steel. The commonly used grades are S31803, S32304, and S32750; the only duplex steel specified in GB150 is actually used very rarely

The explanation on the 2nd floor is quite detailed. Simply put, duplex stainless steel is a type of stainless steel in which austenite and ferrite coexist; the element compositions must differ in such materials. Common grades are 2205 and 2506. Welding double-phase stainless steel requires high precision, as it is 100% ferritic at high temperatures; austenite forms during cooling. During welding, it is not possible to maintain a high temperature for an extended period in order to ensure the formation of austenite, as this would lead to coarse ferrite grains in the heat-affected zone, which affects corrosion resistance and mechanical properties. Nor can the temperature be too low, as this would prevent sufficient time for austenite to form.

As the name implies, duplex stainless steel has two phases, and it possesses certain corrosion resistance properties that ordinary austenitic or ferritic steels do not have! Of course, many things have been explained in detail upstairs; even welding technology is a mature technology – you can find answers just by doing a quick search!

Duplex stainless steel combines the properties of austenitic and ferritic stainless steels; it has a low carbon content, with a Cr content of 18%–28% and a Ni content of 3%–10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N. Austenitic stainless steel is a type of stainless steel with an austenitic structure; it maintains a stable austenitic structure when it contains approximately 18% Cr, 8%~10% Ni, and about 0.1% C. Compared to austenitic stainless steels, duplex stainless steels have higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. In addition, they exhibit excellent resistance to pitting corrosion, and are also a type of nickel-saving stainless steel. The commonly used grades of duplex stainless steel today are: S32304, S31803, S32750, etc.

What is the difference between duplex stainless steel and austenitic stainless steel? The difference lies in their microstructural composition: austenitic stainless steel has an austenitic crystal structure, while duplex stainless steel contains both austenitic and ferritic crystal structures. What are the commonly used duplex stainless steels today? In China, they include 2205 and 2507; abroad, they include S31803 (S32205), S32750, and S32760

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