Since the 1930s, in order to improve resistance to acid corrosion, especially sulfuric acid corrosion, the content of molybdenum and chromium in austenitic stainless steel has been continuously increased, and super austenitic stainless steel has been gradually developed. In France and Sweden, an alloy containing 20%Cr-25%Ni-4.5%Mo-1.5%Cu was developed and named 904L. In the United States, a No. 20 alloy containing 20%Cr-30% Ni-2.5%Mo-3.5%Cu was developed in a similar way. The development of No. 20 alloy and 904L laid the foundation for the development of super austenitic stainless steel.
In the 1950s, Avista Steel in Sweden produced the first stainless steel containing 6% molybdenum for use in special environments. The main alloy content is: 16.5%Cr-30%Ni-6%Mo, which is the embryonic form of 254SMO later. The United States also developed the AL-6X in the early 1970s. The main alloy content is: 20%Cr-25%Ni-6%Mo. The main use of this steel is thin-walled condensate pipes cooled by sea water in power plants. The high alloy content makes this stainless steel easy to produce the precipitation of the metal intermediate phase, thus hindering the manufacture of thick-walled sections or pipes.
In the early 1970s, the AOD technology innovation enabled better control of the addition process of alloying elements. The addition and control of special nitrogen and the process of removing harmful trace elements have also been greatly improved. These are all for manufacturing higher alloying. The super austenitic stainless steel has laid the foundation.
In 1976, Sweden Avista Steel Co., Ltd. developed a new type of 6% Mo stainless steel, namely Avesta254SMO, which also obtained a patent. Since the addition of nitrogen makes the precipitation of the metal mesophase slower, it is beneficial to the production of thicker materials, such as medium-thick plates and thick-walled pipes. At the same time, its corrosion resistance and mechanical properties have also been greatly improved. The common feature of super austenitic stainless steels containing 6% Mo is that they all have very high resistance to pitting and crevice corrosion. Therefore, it has been widely used in offshore and desalination industry, seawater treatment, flue gas desulfurization and other devices. The emergence of Avesta 254SMO marks the beginning of the industrialization and commercialization of 6Mo super austenitic stainless steel.
In the 1980s, Sweden and Germany both developed alloys containing more chromium, manganese and molybdenum and high nitrogen content. The typical steel grade is 654SMO, which contains about 3% Mn, 0.5% N, and 7% Mo. 654SMO is a super austenitic stainless steel containing 7% molybdenum, and its corrosion resistance is comparable to the best nickel-based alloys. The emergence of Avesta654SMO is a milestone in the development history of austenitic stainless steel.
At present, due to its high corrosion resistance, high strength, high toughness, excellent processing performance and cost advantages, super austenitic stainless steel has been widely used in the fields of chemical engineering and marine engineering, and even replaced nickel-based corrosion resistance in some fields. Alloys and titanium alloys will become a very important class of special materials.