Classification, grades, and standards of duplex steel

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Classification, grades, and standards of duplex stainless steels. Duplex stainless steels can generally be divided into four categories: low-alloy type, medium-alloy type, high-alloy type, and super duplex stainless steel type. The common grades of duplex stainless steel and their approximate equivalents in different countries are shown in the table below: Properties of common duplex stainless steel: 1. Chemical composition (%); 2. Mechanical properties: Figure 3. The continuous service temperature range for duplex stainless steel is -50°C to 60°C. The hot working temperature should be no less than 950°C. Introduction to duplex stainless steel: Duplex stainless steel is a material whose microstructure consists of two phases – ferrite and austenite – with each phase accounting for approximately 50%. In practical use, a value around 40-60% is more appropriate. Taking into account the characteristics of the two-phase structure, by properly controlling the chemical composition and heat treatment processes, the excellent toughness and weldability of austenitic stainless steels are combined with the high strength and resistance to chloride stress corrosion of ferritic stainless steels, thereby making duplex stainless steels a type of steel that possesses many outstanding properties such as excellent corrosion resistance, high strength, and ease of processing. Their physical properties lie between austenitic stainless steels and ferritic stainless steels, but are closer to ferritic stainless steels and carbon steel. The resistance of duplex stainless steel to chloride pitting and crevice corrosion is related to the contents of chromium, molybdenum, and nitrogen; its resistance to pitting and crevice corrosion can be similar to that of 316 stainless steel, or even higher than that of stainless steels used in seawater applications such as 6% Mo austenitic stainless steel. All duplex stainless steels exhibit significantly greater resistance to chloride stress corrosion cracking compared to 300-series austenitic stainless steels. Their strength is also **higher than that of austenitic stainless steels, while they maintain good plasticity and toughness. Double Feng Stainless Steel offers a variety of product forms: sheets and strips, pipes – welded and seamless, forged products, fittings and flanges, as well as rods and wires. Bipolar stainless steels can generally be divided into four categories: the first category is the low-alloy type, represented by the grade UNSS32304; this steel does not contain molybdenum, with a PREN value of 24-25. It can be used as a substitute for AISI 304 or 316 in applications requiring resistance to stress corrosion. In the second category, the alloy type is represented by the grade UNSS31803; its PREN value is 32–33, and its corrosion resistance lies between that of AISI316L and 6%Mo+N austenitic stainless steels. The third category is the high-alloy type, which generally contains 25% Cr, as well as molybdenum and nitrogen; some of these materials also contain copper and tungsten. The standard grade is UNSS32550, with a PREN value of 38–39. Its corrosion resistance is higher than that of 22% Cr duplex stainless steels. The fourth category consists of super duplex stainless steels that contain high levels of molybdenum and nitrogen; the standard grade is UNSS32750. Some of these steels also contain tungsten and copper. With a PREN value of >40, they can be used in harsh environmental conditions, offering excellent corrosion resistance as well as mechanical properties, and can compete with super austenitic stainless steels. (Note: PREN: Equivalent value of pitting resistance) Chemical composition: The main alloying elements in duplex steel are Cr, Ni, Mo, and N. Among them, Cr and Mo increase the ferrite content, while Ni and N are austenite-stabilizing elements. Some steel grades also contain elements such as Mn, Cu, and W. Cr, Ni, and Mo can improve corrosion resistance. In chloride-containing environments, it exhibits excellent resistance to pitting and crevice corrosion. 1. Chemical composition (%): 2. Mechanical properties. The mechanical properties of duplex steel depend on the product form and the final heat treatment; the specified limits are listed in the table below. Within the temperature range of -50°C to 280°C, duplex stainless steel exhibits excellent mechanical properties. When duplex steel is exposed to high temperatures above 300°C for an extended period, its microstructure changes, resulting in a decrease in toughness. However, this reduction in toughness does not necessarily affect the material’s properties at operating temperatures. Corrosion resistance: Compared with austenitic steel grades with similar alloy contents, duplex and superduplex steel matrix materials exhibit pitting and crack corrosion resistance similar to that of heart-centered structures, but they generally possess excellent resistance to stress corrosion and organic acid corrosion. It is well known in the industry that the pitting resistance grade is expressed using the equivalent value of pitting resistance, PREN. Physical properties: The thermal conductivity of duplex steel is listed in the table below, and it is compared with 316L. Thermal conductivity in W/M·°C: The thermal expansion of duplex stainless steel is similar to that of carbon steel, which gives duplex steel a clear advantage over austenitic stainless steel. The microstructure of the UNSS31803 duplex stainless steel used by our company in the West-East Gas Pipeline Project is shown below; its ferrite content is 54%. Advantages of duplex stainless steel: 1. Compared with austenitic stainless steels, 1) it has a yield strength more than twice that of ordinary austenitic stainless steels, along with sufficient plasticity and toughness required for shaping. Using duplex stainless steel to manufacture storage tanks or pressure vessels reduces the required thickness by 30-50% compared to conventional austenitic stainless steels, which helps to lower costs. 2) It possesses excellent resistance to stress corrosion cracking, especially in environments containing chloride ions. Even the duplex stainless steels with the lowest alloy content exhibit higher resistance to stress corrosion cracking than austenitic stainless steels; stress corrosion is a significant problem that is difficult to address in conventional austenitic stainless steels. 3) The corrosion resistance of 2205 duplex stainless steel, which is the most commonly used in many media, is superior to that of conventional 316L austenitic stainless steel. Super duplex stainless steels possess extremely high corrosion resistance; in some media such as acetic acid and formic acid, they can even replace highly alloyed austenitic stainless steels as well as corrosion-resistant alloys. 4) It possesses good resistance to local corrosion; compared with austenitic stainless steels with the same alloy content, it exhibits superior performance in terms of wear corrosion and corrosion fatigue resistance. 5) Austenitic stainless steel has a lower linear expansion coefficient compared to carbon steel, and it is close to that of carbon steel; this makes it suitable for joining with carbon steel, holding significant engineering importance such as in the production of composite sheets or linings. 2. Compared with ferritic stainless steels, the advantages of duplex stainless steels are as follows: 1) Their overall mechanical properties are higher than those of ferritic stainless steels, especially in terms of plasticity and toughness. It is not as sensitive to brittleness as ferritic stainless steels. 2) Except for its resistance to stress corrosion, its resistance to other forms of local corrosion is superior to that of ferritic stainless steels. 3) Its cold working process properties and cold forming properties are far superior to those of ferritic stainless steels. 4) Its weldability is far superior to that of ferritic stainless steels; generally, no preheating is required before welding, nor is heat treatment necessary after welding. 5) It has a wider range of applications than ferritic stainless steels. Application: Due to its high strength, duplex steel can often help save materials, such as by reducing the wall thickness of pipes. Applications using SAF2205 and SAF2507W as examples. SAF2205 is suitable for chlorinated environments; it can be used in refining or other process media that contain chlorides. SAF2205 is particularly suitable for heat exchangers that use chlorinated aqueous solutions or brackish water as cooling media. This material is also suitable for diluted sulfuric acid solutions, pure organic acids, and their mixtures. For example: oil pipes in the oil and gas industry; crude oil desalination in refineries, purification of sulfur-containing gases, wastewater treatment equipment ； Cooling systems using brackish water or chlorinated solutions. Applications of SAF2205 duplex stainless steel: SAF2507 was developed specifically for harsh chlorine-containing environments, and it also has many important applications in the processing industry.