1.1 chemical composition and metallographic structure
The main chemical compositions of some main high alloy austenitic stainless steels are given in Table 1. Among them, Al - 6 x and 254 SMO are typical The 6 mo super austenitic stainless steel, while the 654 SMO is a typical 7 mo super austenitic stainless steel. The basic metallographic structure of super austenitic stainless steel is a typical one hundred percent austenite. But because of the high content of chromium and molybdenum, There are likely to be some metal mesophase, such as Chi and sigma. These metal mesophase often appear in the center of the plate. However, if the heat treatment is correct, the formation of these mesophase can be avoided and nearly one hundred percent austenite can be obtained. 254 SMO The metallographic organization does not have any other metal mesophase. The structure was obtained after heat treatment at temperatures of 1150 ~ 1200C. If there is a small amount of metal mesophase, they will not have corrosion resistance to mechanical properties and surfaces. It has a great influence. But try to avoid temperatures ranging from 600 to 1000C, especially during welding and hot working.
1.2 mechanical properties
The austenitic structure generally has moderate strength and high Forgability. After adding a certain amount of nitrogen, the corrosion resistance is improved. In addition, while maintaining austenitic stainless steel forgings and toughness, high nitrogen super austenitic stainless steel also has very high mechanical strength. his The yield strength is 50 to 100% higher than that of ordinary austenitic stainless steel. The effects of nitrogen on mechanical properties were observed at room temperature and higher temperature respectively. Table 2 and table 3 are shown. Mechanical properties of high alloy austenitic stainless steel at temperatures of 2 + 20
Table 3 yield strength of high alloy austenitic stainless steel at high temperature (Rp0.2MPa) As shown in Table 2 and table 3, the mechanical strength increases with the increase of nitrogen content at all temperatures. Although the strength has increased a lot, but The elongation of super austenitic stainless steel is still very high. It is even higher than the elongation of many low alloy steels. This is mainly due to its higher content. Another characteristic of nitrogen and its high work hardening rate is shown in figures 2 and 3. Therefore, the parts formed by cold processing can be obtained. The strength is very high. The use of this characteristic includes pipes and bolts in deeper wells. As with ordinary austenitic stainless steel, The low temperature performance of super austenitic stainless steel is also good. The impact resistance and fracture resistance of super austenitic stainless steel are very high. And only when the temperature is as low as - 196 C will it decrease slightly.
1.3 physical properties
The physical properties mainly depend on the austenite structure and partly depend on the chemical composition of the material. That is to say the super austenite is not Stainless steel is much different from ordinary austenitic stainless steel, such as 304 or 316, in terms of physical properties. Table 4 lists the different combinations Some of the typical physical properties of gold. Table 4 physical properties of some stainless steels and a nickel based alloy
The thermal expansion of 6 stainless steel super austenitic stainless steel is larger than that of duplex stainless steel 2205, so it is possible to weld at the bonding site. There will be some deformation. Although the thermal expansion of nickel based alloys is generally low, the poor thermal conductivity of the nickel alloys is offset by this advantage. These physical properties are of great importance in the design of stainless steel parts or stainless steel connections with other alloys. Corrosion resistance of 2 super austenitic stainless steel To a large extent, the development of austenitic stainless steel is to meet the requirements for corrosion resistance in various environments. Many alloys have been Designed to be used in a particular environment, the scope of its application is growing more and more widely. Therefore, the selection of super austenitic stainless steel Its corrosion resistance is a very important basis. The uniform corrosion, pitting corrosion, crevice corrosion and stress corrosion cracking are mainly introduced here.
3.1 uniform corrosion
The most important alloy elements to improve the stability of stainless steel are chromium and molybdenum. The content of these components in super austenitic stainless steel is higher. Therefore, it has good corrosion resistance in various solutions. In some environments, the addition of elements such as silicon, copper and tungsten can be further improved. The corrosion resistance of the material. Figure 1 shows the equal corrosion velocity diagram of some austenitic stainless steels in pure sulfuric acid. As you can see, Stainless steels with high alloy content, such as 904L, 254 SMO and 654 SMO, are larger than normal Ordovician in larger concentration and temperature range. Stainless steel, such as 304 and 316, has better corrosion resistance. The diagram also shows that the high silicon stainless steel SX is very strong. The ability to resist concentrated sulfuric acid. Fig. 1 the equal corrosion velocity curve of some austenitic stainless steel in pure sulfuric acid, the corrosion rate is 0.1 mm / year.
Another way to explain the ability to resist uniform corrosion in specific environments is to measure the corrosion rate of 0.1 millimeters per year (or 0.5 milliliters per year). The temperature of the degree. Table 5 shows a series of chemical solutions with different concentrations. These solutions are more common in chemical production, but also at the same time. The critical temperature of corrosion of different steels in these solutions at 0.1 mm / year is given. It can be seen that the critical temperature is along with the combination The increase in gold content. In all solutions, the critical temperature of super austenitic stainless steel, such as 254 SMO and 654 SMO, is the most. High, fully demonstrated its excellent uniform corrosion resistance. Table 5 also includes two common wet process phosphoric acids, WPA 1 and WPA 2, and their main components are shown in Table 6. Table 5 the critical temperature c of 0.1 mm / year corrosion rate in different chemicals.
Table 6 main chemical compositions and weight percentages of WPA 1 and WPA 2
The ordering between different alloys varies with the operating conditions. Type 2205 duplex stainless steel is a good example. This The properties of the steel in some environments are even better than some high-alloy austenitic stainless steels. But in some environments, it doesn’t perform very well. it is good. Another example is the 904L stainless steel. In pure phosphoric acid, this stainless steel performs best in all steels but in the wet process industry. In phosphoric acid, it is inferior to the other two super austenitic stainless steels. In a mixed solution WPA 2, its corrosion resistance is the most Poor, see Table 5. Therefore, be very careful when recommending the most suitable stainless steel for equipment in manufacturing, such as reactors, pipes and storage tanks. It is best to have specific data on working conditions.
2.2 Pitting corrosion and crevice corrosion
Pitting corrosion and crevice corrosion are two closely related types of corrosion, all of which are localized. Its main production conditions are chlorine-containing Sub-environment. However, temperature and pH (pH) also play an important role. When the stainless steel is in a chlorine-containing environment, it must be Pitting corrosion occurs at temperature. It is well known that an increase in the chromium and molybdenum content helps to enhance the resistance of the stainless steel to localized corrosion. chromium, The combined effects of molybdenum and nitrogen on resistance to localized corrosion are often expressed in the empirical formula WS (Wirksumme). WS (PRE) = % chromium + 3.3 × % molybdenum + 16 × % nitrogen The WS value in the formula is generally referred to as the Pitting Resistance Index (PRE). Therefore, it is often expressed by PRE. Given by the formula The nitrogen factor of 16 is the most frequently used. However, other factors have been used in the literature, such as the Mannesmann Institute. Dr. Herbsleb suggested using 30. Other components such as tungsten also have a positive effect on corrosion resistance. Weight percent algorithm Calculated, the effect is about half of molybdenum. For comparison, use 16 and 30 as the coefficients of nitrogen in the PRE formula as shown in Table 1. Some steel grades calculate the PRE value. The results are given in Table 7. Table 7 PRE value and critical pitting temperature and critical crevice corrosion temperature of some high alloy stainless steel
* European uniform standard, ** in 1 mole of NaCl solution, *** in 3.5% NaCl solution, corrosion potential is 700mVSCE It can be seen that PRE(16) and PRE(30) are not very different for many steel grades. The most important thing is the alignment of two coefficients. Different stainless steels have no effect. Table 7 also shows the critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT) for some stainless steels. These two critical temperatures Degree is often used to measure the ability of stainless steel to resist localized corrosion. A lot of research work and practical experience show that PRE value and stainless steel resistance The ability of localized corrosion, such as CPT and CCT values, is proportional. 317LMN , 904L two austenitic stainless steels and 2205 The PRE values of duplex stainless steels are approximately the same, and their resistance to pitting and crevice corrosion should be the same. Recorded usage data It shows that the anti-pitting ability of 904L stainless steel is slightly better than other steel grades, and the resistance to crevice corrosion of 2205 is stronger. The actual usage is consistent. Super austenitic stainless steels containing 6% molybdenum and 7% molybdenum, such as 254 SMO and 654 SMO, have higher PRE values and CPT/CCT Value, see Table 7. Indicates its superior resistance to localized corrosion. Therefore, the super austenitic stainless steel family has also been widely used. For applications with high resistance to pitting, such as seawater treatment equipment, pulp bleaching and components in flue gas desulfurization equipment. In a The critical chloride ion concentration that causes crevice corrosion is measured in a test used to evaluate the materials used in flue gas desulfurization equipment. Material is soaked In a solution containing sulfur dioxide and containing an acidic (pH 1) chloride at a temperature of 80 °C. Testing of some candidate materials The results are shown in Table 8. Table 8 Critical chlorine content that can cause crevice corrosion in a simulated desulfurization tower environment at 80 °C.
* European uniform standard, ** For samples with poor metallographic structure, there have been problems with chloride ion concentrations as low as 4000 ppm. It can be seen that in this very harsh environment, the corrosion resistance of super austenitic stainless steel is on a level with that of nickel-based alloys.
2.3 Stress corrosion cracking
Ordinary austenitic stainless steel is more susceptible to stress corrosion cracking caused by chloride than ferritic stainless steel and duplex stainless steel. However, super austenitic stainless steels have a very high resistance to stress corrosion cracking, and in many cases are superior to the ability of duplex stainless steels to resist stress corrosion cracking. Table 9 shows the critical stresses that cause stress corrosion cracking in the case of evaporation (determined by the drip test). The test time is 500 hours. It can be clearly seen that super austenitic stainless steel has excellent resistance to stress corrosion cracking compared to ordinary stainless steel. Table 9 Critical stresses leading to cracks
* European standard The presence of hydrogen sulphide (often found in oil and gas wells) increases the risk of stress corrosion cracking. Because of the hydrogen embrittlement of the ferrite phase, duplex stainless steels, especially those that have been deeply machined, are prone to cracking. In the presence of both hydrogen sulfide and chloride ions, the risk of stress corrosion cracking of stainless steel is greater. Super austenitic stainless steels are highly resistant to stress corrosion cracking in such "acidic" environments. NACE MR0175 -95 is a standard specifically designed for the selection of materials for vulcanization stress corrosion cracking in oil and gas production. This standard includes 254 SMO and also includes both annealing and cold working conditions. The maximum allowable hardness value (35 HRC) is also much higher than that of normal austenitic stainless steel (22 HRC). From this point of view, super austenitic stainless steel is the best material choice in the most dangerous oil and gas environment with a large amount of hydrogen sulfide.
2.4 Corrosion in seawater
The most common environment that causes pitting, crevice corrosion and stress corrosion cracking in stainless steel is in water, especially in seawater. Because the chloride ion content of sea water is very high. Since the critical point corrosion temperature and critical crevice corrosion temperature of super austenitic stainless steel are very high, see Table 7, which shows that its ability to resist local corrosion in seawater is also very strong. Therefore, super austenitic stainless steel containing 6% molybdenum and 7% molybdenum has been widely used in seawater like nickel-based alloys. As the actual situation is very different, the reported results are also very different. Some have been in good condition for a few years, and some have had serious corrosion problems within a year. As with all stainless steels in contact with chloride-containing water, the decisive factor is the oxides and tiny gaps created by welding, and the residual chlorine content is also a very important factor. Chlorine added to seawater to kill marine microorganisms is a strong oxidant that can easily cause the corrosion potential of stainless steel to exceed its critical pitting and crevice corrosion potentials. At less than 50 ° C, there should be no pitting problems on the surface of a clean 6 molybdenum super austenitic stainless steel. However, in some practical applications, there are also examples of 6 molybdenum super austenitic stainless steels having better performance at higher operating temperatures. The most restrictive factor is crevice corrosion. If the gap is severe, corrosion will occur even at temperatures of 20 to 30 ° C). However, this type of stainless steel is generally acceptable, at least at temperatures up to 30 ° C and residual chlorine content of about 0.5 parts per million. When the gap is very serious (as is the case with some types of plate heat exchangers), even if the temperature is kept below 25 °C, 6 molybdenum super austenitic stainless steel is generally not used for this type. use. In applications where the gap is severe but no chlorine is added, the use of 6 molybdenum super austenitic stainless steel has been very successful at least at a temperature of 35 °C. Source: China Super Duplex Stainless Steel Fittings Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)