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Second generation super austenitic stainless steel

The beneficial role of nitrogen in austenitic stainless steel has been recognized for a long time. At that time, the most important role of nitrogen was to improve the mechanical strength of the material and replace the expensive nickel element. Since the 1960s, the content range of nitrogen has been stipulated in many material standards in Sweden. Later, it was found that nitrogen has a great positive effect on improving the local corrosion resistance and delaying the precipitation of carbides. This result has been used to develop a series of super austenitic and duplex stainless steels, such as 6 mo super austenitic stainless steel and 25 CR super duplex stainless steel. Nowadays, nitrogen is more and more widely used in stainless steel. Nitrogen content is higher in many modern austenitic and duplex stainless steels. Together with molybdenum, the resistance to pitting corrosion is greatly improved. For austenite, nitrogen can also improve the strength of materials through solution strengthening. At the same time, the solid austenite has higher nitrogen solubility than liquid austenite. All of these laid a foundation for the more extensive application of nitrogen in austenitic stainless steel and opened up a broader prospect.
In the early development of super austenitic stainless steel, not only nitrogen was used, but also the content of molybdenum was increased. In the 1970s, a super austenitic stainless steel containing 20 Cr, 15 Ni, 4.5 Mo, 8 Mn and 0.45 n appeared in Sweden, which has good corrosion resistance. In the first generation of super high austenitic stainless steel, there is up to 6% molybdenum. Starting with this, a series of 6-mo super austenitic stainless steels have been developed, which have good application performance. However, these first generation super high austenitic stainless steels still have corrosion problems in seawater environment with certain temperature. Because of its high manganese content, the smelting process and subsequent processing caused greater difficulties, and the risk of metal intermediate phase precipitation is also greatly increased.
At the same time, great progress has been made in the development of thermodynamic calculation tool thermo calc. The theoretical calculation results show that the combination of chromium and molybdenum has great help to the solubility of nitrogen, so it is no longer necessary to have high manganese content. Especially in austenite, chromium and molybdenum can improve the solubility of nitrogen. Therefore, up to 0.5% nitrogen can be successfully added into austenitic stainless steel without excessive manganese. This result directly led to the appearance of the second generation super austenitic stainless steel ultra654 SMO (24Cr, 22ni, 7.3mo, 3MN, 0.5N). It is characterized by extremely high chromium, high molybdenum and high nitrogen content.
In the process of modern industrial society development, sustainable development and more stringent environmental protection requirements are put in the first place. This is not only for the production process, but also for the discharge and utilization of industrial waste, waste gas and by-products produced in the production process, which puts forward newer and stricter requirements and regulations. Many industrial emissions are treated in closed-circuit production processes or in special treatment plants, such as industrial waste incineration plants. These changes greatly increase the risk of local corrosion and stress corrosion cracking of stainless steel, and put forward higher requirements for materials. High strength, high corrosion resistance and better processability are not only the basic requirements of new materials, but also one of the driving forces for the development of the second generation super austenitic stainless steel.
The chemical composition of ultra 654 SMO is given in Table 1. It should be noted that 7mo and 0.5N are the highest levels of all stainless steels at present. For comparison, the chemical compositions of the first generation of super stainless steel and nickel base alloy are also given. Although the alloying degree of ultra 654 SMO is very high, the elements are relatively balanced and the microstructure is relatively stable. The tendency of intermetallic precipitation is similar to that of other super high austenitic stainless steels. At the same time, the weldability is the same as other super austenitic stainless steels. With the continuous development of its special welding materials, the welding performance of the alloy is greatly improved.

Table.1 Chemical composition of super stainless steel and nickel base alloy

20200709032300 38164 - Second generation super austenitic stainless steel

Table.2 Mechanical properties, microstructure types and pre values of super stainless steel and nickel base alloy

20200709034233 73397 - Second generation super austenitic stainless steel

The mechanical properties and pre values of these materials are listed in Table 2. The pre value reflects the contribution of alloy elements to local corrosion resistance. With the increase of pre value, the corrosion resistance also increases. When pre > 40, it is generally called super stainless steel. Common stainless steels with pre values less than 40, such as 316L and 2205, and the first generation of super stainless steels 2507 and 254 SMO were selected as reference materials. It can be seen from table 2 that the strength of ultra654 SMO is the highest among austenitic stainless steels, which is close to the level of duplex stainless steels. At the same time, it also has a high elongation, which shows that its processing and forming performance is also very superior.
Due to the high alloying degree of ultra 654 SMO, all corrosion tests are carried out in extremely harsh environments, such as very high chloride content, high temperature, long time and low pH value. Only in such an environment can the true capabilities of ultra 654 SMO be fully reflected. At the same time, different welding methods were used to test the corrosion resistance of weld metal by adding welding filler material and surface treatment after welding. The main results of these tests are introduced in this paper. Finally, some practical application cases are listed.

Super austenitic stainless steel in seawater environment

The first generation of super stainless steel has been used in seawater environment since it came into being. It has successfully replaced many ordinary stainless steels and solved a lot of material corrosion problems. But there are also some cases of failure. It is mainly due to local corrosion caused by harsh environment. In terms of corrosion resistance, although these first generation of super stainless steels have higher ability, there is still a certain gap compared with nickel based alloys. The second generation super austenitic stainless steel represented by ultra 654 SMO fills the gap between the two kinds of materials.
When natural seawater is used in industry, chlorine is often added to remove biofilm or other pollutants. However, chlorine is a strong oxidant, which can greatly increase the corrosion potential of stainless steel, thus increasing the risk of point corrosion and crevice corrosion. Therefore, seawater is a corrosive environment, which is one of the more severe environments. In many industries, plate heat exchangers are used and seawater is used for cooling. In the traditional plate heat exchanger, there are many close gaps between the metal plate and the rubber pad as well as between the two metal plates. Experience shows that there are many failures of 6 mo super austenitic stainless steel in such environment, and crevice corrosion caused by chlorine treated natural seawater is the main reason.
In order to test the performance of the second generation super austenitic stainless steel in this respect, many simulation tests have been carried out. When the corrosion potential is high, the risk of local corrosion is also increased. At a relatively high corrosion potential (+ 500 MV), the crevice corrosion test was carried out in simulated seawater for 30 days, and the chloride concentration was controlled at a high level, about 25000 ppm. The experimental results show that crevice corrosion occurs at about 300C for super austenitic stainless steel and nickel base alloy 625 with 6 mo, and the corrosion depth is still large. However, the crevice corrosion critical temperature of the second generation super austenitic stainless steel (SMO) and nickel base alloy C-276 is almost twice as high. The specific results are shown in Table 3.
Table.3 Critical temperature of crevice corrosion resistance (CCT, ℃)
20200709035154 36860 - Second generation super austenitic stainless steel

Because the test environment is very harsh, the corrosion potential is high, the gap between the sample and rubber ring is very close, so the specific crevice corrosion critical temperature obtained is lower than that measured in other environments. But the process of material alignment is very clear: ultra654 SMO and alloy c-276 have the highest corrosion resistance, which is basically at the same level; while the corrosion resistance of super austenitic stainless steel with 6 molybdenum and alloy 625 is relatively lower by a grade.
Seawater often contains a large number of microorganisms, including corrosive microorganisms. Some microorganisms are easy to adhere to the surface of steel and form biofilm. In the circulating cooling water system, these microorganisms not only produce corrosive substances, but also adsorb on the metal surface with extracellular polymeric substances (EPS) to reduce the heat transfer efficiency. Biofilm usually contains aerobic and anaerobic bacteria. With the increase of immersion time, the main body of biofilm on metal surface will change from aerobic microorganism to anaerobic microorganism. The corrosion of stainless steel is related to the anaerobic biofilm on the surface. Among them, sulfate reducing bacteria (SRB) is the most common and corrosive anaerobic bacteria. Sulfate reducing bacteria (SRB) are very active in oxygen containing seawater systems. It is generally believed that they can metabolize sulfate in seawater to form hydrogen sulfide and reduce pH value. Hydrogen sulfide corrodes stainless steel and causes local corrosion. Especially when SRB covers a small area, it will cause local polarization and even lead to corrosion perforation.
In order to simulate such environment, a special corrosion experiment was designed. The pH value was controlled at the level of 4.0 ~ 4.8 and the content of hydrogen sulfide was about 2000 ~ 3000 ppm. The observation results show that only all four test samples of ultra654 SMO do not have any crevice corrosion, while all four samples of the other three materials have crevice corrosion, as shown in Table 4. The corrosion depth of 6 mo super austenitic stainless steel is 0.75 mm.
Table.4 Crevice corrosion in environment containing H2S
20200709035511 10140 - Second generation super austenitic stainless steel

Super austenitic stainless steel in brine environment

The chloride content in seawater is usually between 20000 ppm and 300000ppm. However, the content of chloride ion in brine may be higher, often close to saturation state. The higher content of chloride ion has bactericidal effect in seawater and can prevent the formation of biofilm. So it is often used in seawater systems. But on the other hand, too high chloride content will also increase the risk of stainless steel corrosion. The corrosion in brine environment is related to chloride content, temperature, pH value and oxygen content. A lot of stainless steel corrosion tests have been carried out in high salt environment. The corrosion resistance of super austenitic stainless steel needs to be tested by higher chloride ion content and temperature. The work of this paper includes a series of long time (> 1300 hours) corrosion tests in NaCl solution at 90 ℃. Since crevice corrosion is the most severe corrosion phenomenon of stainless steel, the influence of chloride ion content, pH value and oxygen content on crevice corrosion is observed. The main results are given in Table 5.
When the chloride ion content and pH (pH = 8, 100 000 ppm Cl -) remain unchanged, the oxygen environment is much more severe than the anaerobic environment, even causing crevice corrosion of 6Mo super austenitic stainless steel. Due to the high chloride content, ordinary stainless steels, 316L and 2205, were corroded in both environments, while the second generation super austenitic stainless steel (ultra 654 SMO) did not corrode in all environments including aerobic conditions.
Although the chloride content was reduced to 20 000 ppm, crevice corrosion occurred in all steels except ultra 654 SMO, as shown in Table 5. This result clearly shows the effect of pH value and oxygen content, and also shows the excellent corrosion resistance of ultra 654 SMO. In actual working conditions, there are similar environments where the content of chloride ion is not particularly high but causes corrosion of stainless steel.
Table.5 Crevice corrosion test in NaCl solution: 90 ℃
20200709040046 55143 - Second generation super austenitic stainless steel

In order to further determine the specific corrosion resistance of each steel in high chlorine environment, the critical temperature of pitting corrosion resistance (CPT, ℃) was determined in NaCl solution according to astmg150 standard. In this harsh environment (100000 ppm Cl -, + 700 MV), the critical point corrosion resistance temperature of 316L is very low, only a dozen degrees, as shown in Figure 1. With the increase of alloy content, the critical temperature of pitting corrosion resistance also increases. The ultra 654 SMO has the highest temperature over 90 ℃.

20200709040218 60707 - Second generation super austenitic stainless steel

Figure.1 Critical temperature of point corrosion resistance (CPT, ℃)
Test in NaCl solution according to ASTM G150; 100000 ppm Cl -, + 700 MV

Welding of super austenitic stainless steel

Since the advent of the second generation of super austenitic stainless steel, there has been a lot of research on its welding mechanism. The general consensus is that the use of superalloyed nickel based alloy as filler material can obtain better point corrosion resistance. Due to the high nitrogen content of super austenitic stainless steel, different degrees of nitrogen dilution will occur when different welding methods are used. Moreover, too high nitrogen content will not only lead to the loss of nitrogen in the weld metal, but also produce bubbles or shrinkage cavity. In order to have the required corrosion resistance in harsh environment, in addition to welding methods, how to conduct post weld treatment is also very important. In order to make the weld metal have the corrosion resistance close to the base metal itself, it is necessary to carry out sufficient and effective post weld treatment. Although a lot of research work has been done on the welding of 7-mo super austenitic stainless steel, further research is needed on the practical welding parameters and their influence on the corrosion resistance of weld metal. This paper discusses how to weld the second generation super austenitic stainless steel, including the effects of welding method, dilution degree and post weld treatment on the microstructure and properties, especially the pitting corrosion resistance.
Gas tungsten arc welding (GTAW / TIG) and plasma arc welding (PAW) are used as welding methods. Several kinds of welding gases were tested: pure argon was used for GTAW / TIG, and 5% and 10% nitrogen were added respectively. The plasma gas of paw is pure argon, mixed with 5% nitrogen and 10% nitrogen + 5% hydrogen respectively. The welding material is nickel base alloy with high Cr and Mo contents (23% Cr and 16% Mo), but no nitrogen element. After welding, the spot corrosion test was carried out in ferric chloride solution according to ASTM g48 method a for 24 hours.
Welding gas has great influence on the corrosion resistance of weld. When gas shielded tungsten arc welding (GTAW / TIG) is used, if pure argon is used, the critical temperature of pitting corrosion is relatively low, less than 70 ℃. The critical temperature of pitting corrosion will increase when nitrogen is added into welding gas. When nitrogen is 10%, the critical temperature of pitting corrosion has reached 87.5 ℃. The specific results are shown in Figure 2. The lower corrosion resistance of pure argon is related to the loss of nitrogen in weld metal. The measurement shows that the nitrogen content is only 0.36%, which is far lower than that of the base metal.
The pitting corrosion resistance of weld metal can be greatly improved by using superalloyed nickel base alloy as welding filler material. Figure 2 clearly shows that even pure argon + filling material can greatly increase the critical temperature of pitting corrosion. From less than 70 ℃ to 85 ℃, it even exceeds the level of Ar + 5% N2 + unfilled material, which shows its great effect. The critical temperature of pitting corrosion can be further increased to 90 ℃ if the filler material and the mixed gun gas containing 5% N2 are used simultaneously.

20200709040357 81165 - Second generation super austenitic stainless steel

Fig.2 Effect of welding gas and filler material on critical temperature of pitting corrosion (CPT, ℃) of weld metal

  • Welding parameters: gas shielded tungsten arc welding (GTAW / TIG)
  • Welding gas: AR; Ar + 5% N2; Ar + 10% N2, the linear energy is about 1.0 kJ / mm

20200709040600 22543 - Second generation super austenitic stainless steel

Fig.3 Effect of welding method and gas on critical temperature of pitting corrosion (CPT, ℃) of weld metal

  • Welding parameters: gas shielded tungsten arc welding (GTAW / TIG)
  • Welding gas: AR; Ar + 5% N2; Ar + 10% N2
  • The welding gas of plasma arc welding (PAW) is ar; Ar + 5% N2; Ar + 10% N2 + 5% H2

The welding method also has a great influence on the corrosion resistance of super austenitic stainless steel weld metal. The test results show that plasma arc welding can give a higher critical temperature of pitting corrosion even if pure argon is used without filler material, which exceeds 95 ℃, as shown in Fig. 3. After adding 5% N2 into the shielding gas, the critical temperature of pitting corrosion is further increased to boiling temperature, which is close to the level of base metal. This result is related to the nitrogen content in the weld metal. The results show that the nitrogen content is the same as that of the base metal, only a small amount of nitrogen is lost, even no loss.
The high corrosion resistance of super austenitic stainless steel is due to its high alloying degree. After welding, the chemical composition of the weld metal, especially the chemical composition of the surface, determines the corrosion resistance of this part. The surface oxide formed after welding can change the content of some important alloy elements, such as chromium, molybdenum or nitrogen. Therefore, the surface treatment after welding is very important. Whether the oxide can be completely removed plays a decisive role. Because of its high corrosion resistance, the removal of these oxides is very difficult. The results show that the corrosion resistance of the surface without any treatment is lower than 60 ℃. After preliminary cleaning, such as shot peening, the critical temperature of pitting corrosion has been greatly increased, which is close to 80 ℃. The best results need to be processed many times. After shot peening and pickling in a certain temperature pickling bath, the best pitting corrosion resistance can be obtained, which is close to 90 ℃, as shown in Fig. 4. After mechanical grinding and pickling with pickling paste, satisfactory results can also be obtained.
20200709040744 79022 - Second generation super austenitic stainless steel

Fig.4 Effect of post weld treatment on critical pitting corrosion temperature (CPT, ℃) of weld metal
Different post weld surface treatment methods:

  1. Post weld condition
  2. Shot peening
  3. 3: Shot peening + pickling paste treatment
  4. Shot peening + pickling bath treatment, Welding method: gas tungsten arc welding

Application cases of super austenitic stainless steel

Seawater system

Offshore oil platforms use seawater to cool crude oil. The first generation of super austenitic stainless steel has been widely used in many parts without any problems. However, when the temperature exceeds 35 ℃, the degree of chlorination is high and there are cracks, the 6Mo super austenitic stainless steel will fail due to crevice corrosion. Many similar cases have been found on the North Sea oil platform. Due to the process requirements, the temperature of some parts of the crude oil cooling system can exceed 70 ℃, and the continuous time of staying at the sub temperature for more than 24 hours each time. Under such working conditions, crevice corrosion occurs not only in 6Mo super austenitic stainless steel, but also in alloy 625. After the second generation super austenitic stainless steel (ultra 654 SMO) was selected, the problem was completely solved and no crevice corrosion occurred. Since then, ultra 654 SMO has been used in all parts in the same environment.

Flue gas desulfurization system

The corrosivity of flue gas desulfurization system is directly related to the combustor. Coal fired boiler system is related to the quality of raw coal, especially the sulfur content. But in the waste incineration plant flue gas, often contains halogen element, corrosivity is very strong. Field coupon test shows that ordinary stainless steel, including the first generation of super stainless steel, has very serious corrosion, while ultra654 SMO, alloy 276 and 22 have almost no corrosion, and ultra 654 SMO has the best performance. The test also shows that the titanium plate has a serious uniform corrosion due to the presence of hydrochloric acid. The actual flue gas of a refuse incineration plant contains 35 000 ppm chloride ions, pH value is 0.5 and temperature is about 80 ℃. After screening a variety of materials and many tests, ultra 654 SMO was finally selected as the main material of the scrubber, which was applied to the tower body and other key parts. After many years of service, no corrosion was found and the washing tower was in good condition.

Conclusion

The local corrosion resistance of the second generation super austenitic stainless steel (ultra 654 SMO) is superior to that of the first generation super stainless steel, and is the same as that of nickel based alloys such as UNS N10276.
All welding methods can be used to weld the second generation super austenitic stainless steel (ultra 654 SMO). But there will be different results:

  • Gas shielded tungsten arc welding (GTAW / TIG) can cause the loss of nitrogen in the weld metal, thus reducing the corrosion resistance, and the corresponding CPT value is also low. When 5% or 10% nitrogen is added into welding gas, the corrosion resistance of weld metal will be greatly improved. Therefore, it is necessary to use proper amount of nitrogen.
  • Plasma arc welding (PAW) weld metal corrosion resistance is very good, welding gas to add appropriate amount of nitrogen, the effect will be better.
  • If super alloyed and nickel based filling materials are used, the corrosion resistance will be greatly enhanced.
  • Post weld treatment has a great influence on the corrosion resistance of weld metal. The best corrosion resistance can be obtained by shot peening and pickling.

The second generation super austenitic stainless steel (ultra 654 SMO) has been successfully used in many harsh environments, such as seawater treatment system, flue gas desulfurization, pulp production and petrochemical industry.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.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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