What is duplex stainless steel?
What is duplex stainless steel?
Duplex stainless steel (DSS for short) refers to stainless steel with ferrite and austenite accounting for about 50% respectively, and the content of less phase generally needs to reach 30% at least. In the case of low C content, Cr content is 18% ~ 28%, Ni content is 3% ~ 10%. Some steels also contain Mo, Cu, Nb, Ti, N and other alloy elements.
This kind of steel has the characteristics of austenite and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, and maintained the 475 ℃ brittleness, high thermal conductivity and Superplasticity of ferritic stainless steel. Compared with austenitic stainless steel, it has higher strength, intergranular corrosion resistance and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel saving stainless steel.
Duplex stainless steel has developed to the third generation since it was born in the United States in 1940s. Its main characteristic is that its yield strength can reach 400-550mpa, which is twice that of ordinary stainless steel, so it can save materials and reduce equipment manufacturing cost. In the aspect of corrosion resistance, especially in the condition of bad medium environment (such as seawater, high chloride ion content), the pitting corrosion resistance, crevice corrosion resistance, stress corrosion resistance and corrosion fatigue performance of duplex stainless steel are obviously superior to that of ordinary austenitic stainless steel, which can be compared with high alloy austenitic stainless steel.
Due to the characteristics of two-phase structure, through the correct control of chemical composition and heat treatment process, duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel. It combines the excellent toughness and weldability of austenitic stainless steel with the high strength and resistance of ferritic stainless steel
Duplex stainless steel
Combined with chloride stress corrosion properties, it is these excellent properties that make duplex stainless steel develop rapidly as a weldable structural material. Since 1980s, it has become a kind of steel juxtaposed with martensitic, austenite and ferritic stainless steel. Duplex stainless steel has the following properties:
- (1) Molybdenum containing duplex stainless steel has good chloride stress corrosion resistance under low stress. Generally, 18-8 austenitic stainless steel is prone to stress corrosion fracture in neutral chloride solution above 60 ° C. heat exchanger, evaporator and other equipment made of this kind of stainless steel in the industrial medium of trace chloride and hydrogen sulfide have the tendency of stress corrosion fracture, while duplex stainless steel has good resistance.
- (2) Molybdenum containing duplex stainless steel has good pitting corrosion resistance. With the same equivalent value of pitting resistance (pre = CR% + 3.3mo% + 16N%), the critical pitting potential of duplex stainless steel is similar to that of austenitic stainless steel. The pitting corrosion resistance of duplex stainless steel and austenitic stainless steel is equivalent to AISI 316L. The pitting and crevice corrosion resistance of high chromium duplex stainless steel containing 25% Cr, especially nitrogen, is better than AISI 316L.
- (3) It has good corrosion fatigue and wear corrosion resistance. Under the condition of some corrosive medium, it is suitable for making pump, valve and other power equipment.
- (4) Good comprehensive mechanical properties. The yield strength of 18-8 austenitic stainless steel is twice as high as that of 18-8 austenitic stainless steel. The elongation of solid solution is 25%, and the toughness value AK (V-notch) is more than 100J.
- (5) It can be welded with 18-8 austenitic stainless steel or carbon steel without preheating before welding and heat treatment after welding.
- (6) The hot working temperature range of duplex stainless steel containing low chromium (18% Cr) is wider than that of 18-8 austenitic stainless steel, and the resistance is smaller. It can be rolled directly to produce steel plate without forging. Compared with austenitic stainless steel, duplex stainless steel with high chromium (25% Cr) is a little more difficult in hot working, which can produce plate, tube, wire and other products.
- (7) Compared with 18-8 type austenitic stainless steel, the effect of work hardening is greater in cold working. In the initial stage of tube and plate bearing deformation, large stress is required to deform.
- (8) Compared with austenitic stainless steel, it has large thermal conductivity and small linear expansion coefficient, and is suitable for lining equipment and production of composite plate. It is also suitable for making the core of heat exchanger. The heat transfer efficiency is higher than that of austenitic stainless steel.
- (9) There are still various brittleness tendencies of high chromium ferritic stainless steel, so it is not suitable for working conditions higher than 300 ° C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phase such as σ is.
It is used in heat exchanger, cold shower and device of seawater, high temperature and concentrated nitric acid resistant in oil refining, chemical fertilizer, papermaking, petroleum and chemical industry.
Structure and type
Duplex stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel due to its dual phase structure of austenite and ferrite, and the content of the two phase structures is basically the same. The yield strength can reach 400MPa ~ 550MPa, which is twice of that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittleness transition temperature, significantly improved intergranular corrosion resistance and welding performance, while retaining some characteristics of ferritic stainless steel, such as brittleness at 475 ℃, high thermal conductivity, small linear expansion coefficient, superplasticity and magnetism, etc. Compared with austenitic stainless steel, the strength of duplex stainless steel is higher, especially the yield strength is significantly improved, and the pitting corrosion resistance, stress corrosion resistance and corrosion fatigue resistance are also significantly improved.
According to its chemical composition, duplex stainless steel can be divided into four types: Cr18, cr23 (excluding Mo), Cr22 and Cr25. For Cr25 duplex stainless steel, it can be divided into ordinary and super duplex stainless steel, among which Cr22 and Cr25 are widely used. Most of the duplex stainless steels used in China are made in Sweden. The specific brands are: 3re60 (Cr18), saf2304 (cr23), SAF2205 (Cr22), SAF2507 (Cr25).
Biphase refers to two kinds of metallographic structure, not the “direction” of direction.
Chemical composition of duplex stainless steel
Mechanical properties of duplex stainless steel
Main application fields and applications of duplex stainless steel
Neutral chloride environment
Duplex stainless steel is widely used in neutral chloride environment. In the process of processing and manufacturing, a small amount of solution containing chloride ions is usually used as cooling water, which leads to the general austenitic stainless steel (such as AISI 304 / 316) tends to produce stress corrosion, while duplex stainless steel can solve this problem well, especially for the stress corrosion cracking caused by pitting corrosion Environmental Science. The pitting equivalent index pre = CR% + 3.3mo% + 16N% is usually used to evaluate the corrosion resistance. The pre value of duplex stainless steel is greater than 24, while that of austenite is less than 20.
Oil and gas industry
Since the 1980s, the amount of duplex stainless steel used in the production of acid gas and oil has increased gradually, mainly used for the production of pipe lining, heat exchanger and onshore and offshore pipeline systems. Especially in the production of oil and natural gas, most of the duplex stainless steel faces the acid environment, that is, the environment containing a lot of chloride ions, carbon dioxide and some sulfur dioxide. In the malignant environment of wet carbon dioxide containing chloride ions, duplex stainless steel is the first choice of ideal materials, which can resist the wear and corrosion of high flow rate. Compared with carbon steel and stainless steel with corrosion inhibitor, duplex stainless steel can resist the wear and corrosion of high flow rate, and it can be used in well pipe system, which can reduce a large amount of material weight.
Seawater is one of the most corrosive media in the natural environment, especially when the microbial film is adhered on the metal surface, the corrosion potential will increase, and the tendency of pitting and crevice corrosion will also increase. In terms of hot seawater, at present, most of the super duplex stainless steel is used, for example, the seawater exchanger made of SAF2507 super duplex stainless steel has not been found to be corroded in 3 years, while the titanium tube can only be used for 3 months due to its non corrosion resistance.
Pulp and paper industry
The development and application of duplex stainless steel in pulp and paper industry has a history of more than 40 years. 3re60 duplex stainless steel was first applied in this field. In addition to 3re60 steel, other duplex stainless steels such as ur45n (SAF2205), ur47n (00cr25ni6.5mo3n), ur52n + (00cr25ni6.5mo3.5cun) are used in various industrial fields. As duplex stainless steel has excellent mechanical properties, as well as wear and corrosion resistance, stress corrosion resistance and good fatigue corrosion resistance, it has achieved good application results in the manufacture of pulp and industrial paper-making sawdust pre steamer, paper-making pressure roller, continuous and intermittent pulp digester and recycling equipment.
Chemical fertilizer industry
The urea industry is also one of the first departments to use duplex stainless steel. The heat exchange equipment containing chloride ion water is widely used in the unit. For example, 304L Austenitic stainless steel tube bundle was originally used in the three-stage cooler of carbon dioxide compressor in the urea unit, and the leakage was caused by stress corrosion cracking one month later. Duplex stainless steel can be used for more than 5 years, and then the first and second stage coolers are also used Both of them are made of 18-5Mo or SAF2205 duplex stainless steel.
Because duplex stainless steel has good anti-corrosion fatigue performance in urea medium, it is very suitable for manufacturing the key equipment of urea production, methylammonia pump body. Domestic 00Cr25Ni6Mo2N duplex stainless steel can pass the test of intergranular corrosion tendency by Huey method, and has been used in Hongsheng special steel nitrogen fertilizer plant, Heilongjiang chemical fertilizer plant and other large-scale chemical fertilizer plants. In some small chemical fertilizer plants in China, the pump body of ammonia pump is basically made of 18-5Mo steel, and dozens of them are made of high chromium and lead containing duplex stainless steel. In addition, the forging of this steel has passed the inspection of JIS g0573, g0591 nitric acid method and sulfuric acid method in Japan. It is exported to Japan in batches, and the price is more favorable than that of local production in Japan.
In addition, the domestic 0cr25ni6mo3cun aging strengthening duplex stainless steel is used to process the internals of the high pressure stop valve of the main process pipeline of the urea plant with various specifications by virtue of its excellent wear resistance and corrosion resistance.
In recent years, the marine chemical carrier industry is the largest two-phase stainless steel user in foreign countries, and the consumption accounts for about 50% of the hot-rolled plate. There are many kinds of liquid cargoes loaded on chemical ships, including chemical and petrochemical products. The cabin materials are required to be corrosion-resistant, with high strength and excellent plasticity. At present, SAF2205 duplex stainless steel has replaced 316L and 317L austenitic stainless steel and become the preferred material for offshore chemical ships. China started late in this respect. Qingshan shipyard of China Yangtze River Shipping Group adopted the European construction standard and imported SAF2205 stainless steel plate. The first 18500 ton chemical ship was manufactured by itself. The consumption of steel plate is about 1200 tons. It has been exported to Belgium, realizing the breakthrough of building chemical ship with duplex stainless steel. The factory has formed a large-scale production capacity.
Compared with ferritic stainless steel and austenitic stainless steel, duplex stainless steel has good weldability. It is neither like the HAZ of ferritic stainless steel, nor like austenitic stainless steel, it is more sensitive to weld hot crack because of the grain coarsening.
Due to its special advantages, duplex stainless steel is widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, paper-making machinery and other industrial fields. In recent years, it has also been used in the field of bridge bearing structure, with a good development prospect.
The problem of welding performance often occurs in economical dual phase steel. However, welding standard dual phase steel is not a problem, and no matter what process is adopted, there are suitable welding materials for these applications. From the perspective of metallography, there is no problem in welding 2101 (1.4162). In fact, it is even easier to weld than standard grade dual phase steel, because this material can be welded by acetylene welding process in fact, and for standard dual phase steel material, this process must be avoided at the beginning and the end. The practical problem of welding 2101 is that the viscosity of the molten pool is different, so the wettability is a little bit poor. This forces operators to use more arc welding in the welding process, which is the problem. Although it can be made up by selecting superalloyed welding materials, we often want to select matching welding materials.
In 2101, there is also a heat affected zone interaction between the microstructures in the low temperature heat affected zone and the high temperature heat affected zone, which is more favorable than 2304, 2205 or 2507. In 2101 test, it has also been found that due to the low nickel content, different types of “tempering color” containing more nitrogen and manganese have been produced, which affects the corrosion performance. This loss of composition in the arc and bath is due to the evaporation and deposition of nitrogen and manganese, which is a new problem for dual phase steel grade materials, so it will be described more in this lecture.
The welding characteristics of duplex stainless steel are as follows:
After normal solution treatment (1020 ℃ ~ 1100 ℃ heating and water cooling), the duplex stainless steel contains about 50% ~ 60% austenite and 50% ~ 40% ferrite. With the increase of heating temperature, the ratio of two phases does not change obviously.
The dual phase stainless steel has good low temperature impact toughness, such as the impact absorption energy of 20 mm thick plate transverse specimen at – 80 ℃ can reach more than 100 J. In most media, the resistance to uniform corrosion and pitting corrosion is better, but it should be noted that the stress corrosion resistance of this kind of steel will be significantly deteriorated due to the precipitation of σ phase when it is heat treated below 950 ℃. Due to the proper ratio of Cr equivalent to Ni equivalent, a large amount of primary austenite remains after high temperature heating, and secondary austenite can be formed during cooling. As a result, the total amount of austenite phase in the steel is not less than 30% – 40%, so the steel has good intergranular corrosion resistance.
In addition, as mentioned before, the tendency of crack is very low when welding this kind of steel, so preheating and post weld heat treatment are not necessary. Due to the high content of N in the base metal, the single-phase ferrite area will not be formed near the weld seam, and the austenite content is generally no less than 30%. The suitable welding methods are TIG welding and electrode arc welding. Generally, in order to prevent grain coarsening near the seam, low linear energy welding should be used as much as possible.
The factors affecting the welding quality of duplex stainless steel are mainly reflected in the following aspects:
N content effect
G ó mez de Salazar JM et al. Studied the effect of different content of N2 in shielding gas on the properties of duplex stainless steel. The results show that with the increase of N2 partial pressure pN2 in the mixed gas, the nitrogen content ω (n) in the weld begins to increase rapidly, and then changes little. The ferrite content φ (α) in the weld decreases linearly with the increase of ω (n), but the effect of φ (α) on tensile strength and elongation is just opposite to that of ω (n). With the same ferrite content φ (α), the tensile strength and elongation of the base metal are higher than those of the weld. This is due to the different microstructure. The increase of N content in the weld metal of duplex stainless steel can improve the impact toughness of the joint, which is due to the increase of γ content in the weld metal and the decrease of Cr2N precipitation.
Heat input effect
Different from the weld zone, the ω (n) of the heat affected zone will not change during welding, which is the ω (n) of the base metal, so the main factor affecting the structure and performance at this time is the heat input during welding. According to the literature, the proper line energy should be selected during welding. If the heat input is too large during welding, the heat affected zone of the weld increases, and the metallographic structure tends to be coarse and disordered, resulting in embrittlement, which is mainly manifested in the decrease of the plasticity index of the welded joint. If the heat input of welding is too small, the quenched structure will be formed and cracks will be produced easily, which is also unfavorable to the impact toughness of HAZ. In addition, the impact toughness of HAZ will be affected by the factors that affect the cooling rate, such as plate thickness, joint form, etc.
Sigma phase embrittlement
The σ phase embrittlement of duplex stainless steel and its weld metal caused by reheating is introduced in foreign literature. In the reheating process of base metal and weld metal, the fine secondary austenite γ * is formed from α phase, and then σ phase is precipitated. The results show that the brittle fracture occurs at the interface of σ phase and matrix and σ phase. The fracture observation of the base metal shows that there are dimples in the region around σ phase. Because of the wide α phase region, a large amount of σ phase will reduce the toughness. However, the α phase region in the weld is small, and the fracture still shows brittle fracture. As long as a small amount of σ phase is generated, it is enough to cause the toughness of the weld metal Therefore, the embrittlement tendency of σ phase in weld metal is much greater than that of base metal.
Hydrogen induced cracking
The hydrogen embrittlement of duplex stainless steel welding joint usually occurs in α phase, and the sensitivity of hydrogen embrittlement increases with the increase of peak temperature. The microstructure changes as follows: the peak temperature increases, the content of γ phase decreases, the content of α phase increases, and the amount of Cr2N precipitated from α phase boundary and inside increases, so hydrogen embrittlement is very easy to occur.
Stress corrosion cracking
The cracks in both the base metal and the weld metal start at the α phase side of the α / γ interface and propagate in the α phase. Austenite (γ) can prevent crack growth because of its inherent low hydrogen embrittlement sensitivity. Because DSS contains a certain amount of austenite, the tendency of stress corrosion cracking is small.
Pitting resistance is an important characteristic of duplex stainless steel, which is closely related to its chemical composition and microstructure. Pitting generally occurs at the α / γ interface, so it is considered to occur at the γ * phase between the γ phase and α phase. This means that the Cr content in the γ – phase is lower than that in the γ – phase. The composition of γ phase is different from that of γ phase because the content of Cr and Mo in γ phase is lower than that in the initial γ phase. Further study shows that the pitting potential of steel with low N content is sensitive to cooling rate. Therefore, when welding duplex stainless steel with low N content, the control of cooling rate is more strict. In the welding process of duplex stainless steel, reasonable control of welding line energy is the key to obtain high quality duplex stainless steel joints. If the linear energy is too small, the cooling speed of the weld metal and the heat affected zone is too fast, and the austenite is too late to precipitate, the ferrite content in the structure will increase; if the linear energy is too large, although enough austenite can be formed in the structure, it will also cause the growth of ferrite grains in the heat affected zone and the precipitation of σ equivalent harmful phases. In general, welding methods such as shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), flux cored wire arc welding (FCAW) and plasma arc welding (PAW) can be used for the welding of duplex stainless steel, and preheating measures are generally not required before welding, and also after welding No heat treatment is required.
Limitation of duplex stainless steel
- 1. It is necessary to control the phase proportion. The most appropriate proportion is that the ferrite phase and austenite phase account for about half each, and the number of a certain phase cannot exceed 65% at most, so as to ensure the best comprehensive performance. If the ratio of two phases is out of balance, for example, too many ferrite phases, it is easy to form single-phase ferrite in welding HAZ, which is sensitive to stress corrosion cracking in some media.
- 2. It is necessary to master the structure transformation rule of duplex stainless steel and be familiar with the TTT and CCT transformation curve of each steel type, which is the key to correctly guide the formulation of heat treatment, thermoforming and other processes of duplex stainless steel. The precipitation of brittle phase of duplex stainless steel is more sensitive than that of austenitic stainless steel.
- 3. The continuous service temperature range of duplex stainless steel is – 50 ~ 250 ℃, the lower limit depends on the brittle transition temperature of the steel, the upper limit is limited by 475 ℃, and the upper limit cannot exceed 300 ℃.
- 4. Fast cooling is needed after solution treatment of duplex stainless steel. Slow cooling will cause the precipitation of brittle phase, which will lead to the decrease of toughness, especially local corrosion resistance of steel.
- 5. The lower limit temperature of hot working and hot forming of high chromium molybdenum duplex stainless steel shall not be lower than 950 ℃, and that of super duplex stainless steel shall not be lower than 980 ℃, and that of low chromium molybdenum duplex stainless steel shall not be lower than 900 ℃, so as to avoid the surface crack caused by the precipitation of brittle phase during the processing
- 6. The stress relief treatment of 650-800 ℃ commonly used for austenitic stainless steel cannot be used, and the solution annealing treatment is generally used. After surfacing of duplex stainless steel on the surface of low alloy steel, the toughness and corrosion resistance caused by the precipitation of brittle phase, especially the decrease of local corrosion resistance, must be taken into account when the overall stress relief treatment at 600-650 ℃ is needed, so as to shorten the heating time in this temperature range as much as possible. The heat treatment of low alloy steel and duplex stainless steel composite plates should also be considered.
- 7. It is necessary to be familiar with the welding rules of duplex stainless steel, not all of which can be welded with austenitic stainless steel. Whether the equipment of duplex stainless steel can be used safely or not has a lot to do with correctly mastering the welding process of steel. The failure of some equipment is often related to welding. The key lies in the control of line energy and interpass temperature. It is also very important to select welding materials correctly. The two-phase ratio of the welded joint (weld metal and welding HAZ), especially the amount of austenite in the welding HAZ, is very important to ensure that the welded joint has the same performance as the base metal.
- 8. When choosing duplex stainless steel in different corrosion environment, it should be noted that the corrosion resistance of steel is always relative. Although duplex stainless steel has better local corrosion resistance, for a certain duplex stainless steel, it also has a suitable range of medium conditions, including temperature, pressure, medium concentration, pH value, etc., which should be carefully selected. Many of the data obtained from the literature and manual are laboratory corrosion test results, which are often different from the actual conditions of the project. Therefore, attention should be paid to the selection of materials. If necessary, corrosion test in the actual medium or coupon test under the field conditions, or even the test of simulation device should be carried out.
Source: China Duplex Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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