The century-long development of super austenitic stainless steels

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Super austenitic stainless steels are austenitic stainless steels with high levels of Cr, Mo, and N, and a pitting resistance index of not less than 40. Compared to austenitic stainless steels, super austenitic stainless steels exhibit excellent resistance to uniform corrosion, pitting corrosion, crevice corrosion, and stress corrosion. Thanks to nitrogen alloying, they also possess high strength and ductility. They are widely used in fields such as municipal waste treatment and flue gas desulfurization for waste gas treatment, as well as in demanding environments like the chemical industry and pulp and paper manufacturing. They are also utilized in resource extraction sectors such as oil and gas drilling and seawater desalination. The development of super austenitic stainless steels has been driven by increasing material demands and advancements in industrial technology, with an aim to achieve excellent corrosion resistance. Over several decades of development, these materials have been refined to meet the requirements of harsh environments. However, with the progress of time and technological advancements, as industrialization accelerates, super austenitic stainless steels are being used in an increasingly wide range of applications under more demanding conditions. Therefore, studying their development process and technical advancements plays an important role in developing super austenitic stainless steels with even better corrosion resistance. Image 1: The evolution of super austenitic stainless steels. The concept of super austenitic stainless steels emerged in the 1980s, alongside super ferritic stainless steels and super duplex stainless steels; it is similar to the concept of nickel-based superalloys used for high-nickel alloys. Looking at the development history of super austenitic stainless steels, it can be said that their evolution has gone through three stages: increasing the Mo content, adding N element, and raising both the Mo and N contents. 01 Increasing Mo content: The first phase was in the 1930s, when industries related to sulfuric acid developed rapidly, which in turn led to higher requirements for the stainless steels used. To address the corrosion problem of stainless steel in sulfuric acid environments, the United States developed alloy 20 (20Cr-30Ni-2.5Mo-3.5Cu), while France and Sweden developed alloys with similar compositions (20Cr-25Ni-4.5Mo-1.5Cu), known as Uranus B6 alloy and 2RK65 respectively. Although alloy 20 was developed to improve resistance to corrosion in sulfuric acid environments, it is also widely used in processing and manufacturing equipment in industries such as solvents, **, plastics, synthetic fibers, organic chemicals, pharmaceuticals, and food. After the 1970s, the Uranus B6 and 2RK65 alloys were generally referred to as 904L. 904L exhibits good resistance to uniform corrosion in sulfuric acid and phosphoric acid environments, and is commonly used in industries such as petrochemicals, pulp and paper manufacturing, mineral processing, and the food industry. It can also replace heat exchangers made of 304 and 316 stainless steel in high-temperature water media containing chlorides. The development of alloy 20 and 904L was the main cornerstone in the advancement of super austenitic stainless steels. With the rapid development of industry, the medium environments in which stainless steel is used are becoming increasingly complex and demanding; especially in chloride-containing environments, the corrosion resistance of stainless steel is somewhat insufficient. The addition of Mo element can enhance the corrosion resistance of stainless steel to chloride-containing media by raising the corrosion potential and the stability and density of the passivation film, promoting re-passivation, and reducing the number and size of pitting nucleation sites and metastable pitting pits. In the 1950s, the Swedish company Avesta developed a steel grade with a composition of 16.5Cr-30Ni-6Mo by increasing the Mo content; this grade was the precursor to 254 SMO (S31254, 20Cr-18Ni-6Mo-0.2N). In the 1960s, the European company Ugine developed an NSCD alloy resistant to seawater corrosion, with a Mo mass fraction of over 5%. The development of super austenitic stainless steels has taken a step forward. In 1967, International Nickel Company filed a patent for alloys with a chromium content of 14%~21%, a nickel content of 20%~40%, and a molybdenum content of 6%~12%. In the same year, the American company Allegheny produced AL-6X (20Cr-24Ni-6Mo), which is primarily used for thin condenser tubes in power plants cooled by seawater. However, due to its high alloy content, it is difficult to cool thick sections of this steel rapidly during manufacturing; as a result, intermetallic compounds (such as the σ phase, χ phase, and Laves phase) tend to form during slow cooling at high temperatures. Intermetallic compounds are rich in Cr and Mo elements, leading to local depletion of these elements; this not only results in a decrease in mechanical properties but also reduces corrosion resistance. Therefore, the thickness of commercial AL-6X is limited to below 2.5 mm. 02 Addition of nitrogen element: In 1976, Avesta Company filed a new patent for stainless steel and introduced 254 SMO, which is a typical representative of the second stage in the development of super austenitic stainless steels. It utilizes off-furnace refining technology along with nitrogen alloying to control the mass fraction of N in the steel at around 0.2% and the mass fraction of Mo at around 6%. Subsequently, other similar grades were developed through nitrogen alloying; for example, in the 1970s, the American company Allegheny produced AL-6XN (N08367, 21Cr-24Ni-6Mo-0.2N) and AL-6XN Plus on the basis of AL-6X to address the issue of seawater corrosion. In the 1980s, the German company VDM developed Cronifer 1925hMo (N08926, 20Cr-25Ni-6Mo-0.2N) by increasing the Mo content and adding 0.2% N to 904L. AL-6XN has a similar composition range to Cronifer 1925hMo, with 7% more Ni compared to 254 SMO, resulting in a more stable austenite phase. 254 SMO, AL-6XN, and Cronifer 1925hMo are three 6Mo steels that are currently well-developed and easy to manufacture. They are commonly used in marine environments such as offshore oil and gas platforms, desalination plants, seawater heat exchangers, and condenser tubes. They are also employed in environments with high concentrations of chlorides, such as flue gas desulfurization systems, pulp and paper industries, bleaching units, nuclear power plants, and crude oil distillation facilities. Additionally, they find use in reaction vessels and piping for chemical processing applications. In addition to the three types of 6Mo steel that are widely used in industry as mentioned above, various countries have also developed high-performance versions of this steel. For example, in 1984, South Korea filed a patent for SR50A (S32050, 23Cr-21Ni-6Mo-0.25N). Compared to 254 SMO, AL-6XN, and Cronifer 1925hMo, S32050 has higher levels of Cr and N, which gives it excellent corrosion resistance as well as strength levels far superior to those of conventional austenitic stainless steels. It also possesses local corrosion resistance similar to that of titanium alloys, making it suitable for applications where high corrosion resistance is required, such as nuclear power plants, flue gas desulfurization systems, rocket engine components, and biomaterials. The NIROSTA4565S (S34565, 24Cr-17Ni-5Mo-6Mn-0.5N), developed by the Finnish company Outokumpu in 1988, increases the solubility of N primarily by raising the manganese content. Although Mn is a weak austenite-forming element, it is also a strong austenite-stabilizing element. Mn and N can replace or reduce the amount of expensive Ni in stainless steels, enabling these steels to maintain good corrosion resistance while offering cost advantages. Moreover, when the mass fraction of N in the steel reaches 0.5%, the yield strength of the steel increases by 40% compared to 254 SMO. It is now widely used in fields such as pulp and paper manufacturing, storage and transportation of chemicals, hydrometallurgy, flue gas desulfurization, and seawater desalination. In the 1990s, the Japanese company Yakin developed NAS 254N (S32053, 23Cr-25Ni-5Mo-0.2N) by building on 254 SMO and making appropriate adjustments to the chromium and molybdenum levels as well as optimizing the contents of other elements. This approach not only ensured the corrosion resistance of the super-austenitic stainless steel but also reduced the risk of intermetallic compound formation. NAS 254N is now widely used in marine engineering, the chemical industry, pulp and paper manufacturing, and pollution control systems. 03 Increasing the Mo and N contents: Although alloys such as 254 SMO, AL-6XN, Cronifer 1925hMo, and S34565 possess good corrosion resistance, their performance in environments with gaps or high temperatures is limited; in such cases, nickel-based or titanium alloys are more advantageous, though they are expensive. To bridge this gap, in 1992 Avesta developed 654 SMO (S32654, 24Cr-22Ni-8Mo-3Mn-0.5N) containing 7% Mo. This represented the third generation of super austenitic stainless steels; it featured significantly increased levels of Cr, Mo, and N compared to 6Mo steels, with a Mo content of 7% and a N content of 0.5%. An appropriate amount of Mn was also added, enabling its production through conventional AOD refining and continuous casting, without the need to worry about nitrogen escaping from the steel during subsequent processing. 654 SMO represents a milestone in the development of super austenitic stainless steels; through the synergistic effect of Cr, Mo, and N, it enables these steels to exhibit excellent corrosion resistance in halide environments, while also significantly increasing their strength without compromising their forgeability and toughness. 654 SMO is widely used in applications such as seawater desalination, pulp bleaching, and flue gas desulfurization, where high requirements are placed on the corrosion resistance of materials, and it has gradually become a substitute for nickel-based alloys and titanium alloys. In 1994, France developed B66 (S31266, 24Cr-22Ni-6Mo-3Mn-2W-0.5N) by replacing part of the Mo with W. Compared to most austenitic stainless steels and nickel-based alloys, it has relatively higher levels of Ni and N, which reduces the risk of intermetallic compound formation. It was designed to enhance structural stability and mechanical properties, and is a super-austenitic stainless steel suitable for large components with thick walls. In the cable industry used for oil and gas extraction, 316 has low corrosion resistance; while highly alloyed nickel-based and cobalt-based alloys possess excellent strength and corrosion resistance, they are expensive. Although 6%Mo steel has closed this gap, it still cannot match the corrosion resistance and strength of nickel-based and cobalt-based alloys. To narrow the difference between 6%Mo steel and these alloys, Special Metals Corporation in the United States developed a new type of super-austenitic stainless steel, Incoloy 27-7Mo (S31277, 22Cr-27Ni-8Mo-0.35N), in the year 2000.