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Super stainless steel and nickel-based alloys are special types of stainless steel; they differ from ordinary stainless steel in terms of their chemical composition, as they are high-alloy stainless steels containing high levels of nickel, chromium, and molybdenum. Based on the microstructural characteristics of stainless steel materials, super stainless steels are classified into several types, including super ferritic stainless steels, super austenitic stainless steels, super martensitic stainless steels, and super duplex stainless steels. Super austenitic stainless steels are based on conventional austenitic stainless steels; by increasing the purity of the alloy, raising the amount of beneficial elements, and reducing the C content to prevent the formation of Cr23C6 which could cause intergranular corrosion, these materials achieve excellent mechanical properties, processability, and resistance to local corrosion. They have thus replaced Ti-stabilized stainless steels. Super ferritic stainless steels retain the advantages of conventional ferritic stainless steels, such as high strength, good oxidation resistance, and excellent resistance to stress corrosion. At the same time, they overcome the limitations of these steels, including the ductile-to-brittle transition, sensitivity to intergranular corrosion, and low toughness in the welded state. By employing refining techniques to reduce the C and N contents and adding elements that stabilize the material and enhance the toughness of the weld metal, it is possible to obtain super ferritic stainless steels with high levels of Cr and Mo as well as extremely low levels of C and N. This advancement brings ferritic stainless steels to a new level in terms of their performance in applications requiring corrosion resistance, as well as resistance to pitting and crevice corrosion caused by chlorides. Super duplex stainless steels: These types of steel were developed in the late 1980s. The main grades include SAF2507, UR52N, Zeron100, etc. They are characterized by a low carbon content, as well as high levels of Molybdenum and nitrogen. The ferrite phase makes up 40% to 45% of the steel’s composition, giving them excellent corrosion resistance. Super martensitic stainless steel is a type of hardenable stainless steel that possesses high hardness, strength, and wear resistance, but it has poor toughness and weldability. Ordinary martensitic stainless steels lack sufficient ductility, are highly sensitive to stress during deformation, and are difficult to form through cold working. Supermartensitic stainless steel can be obtained by reducing the carbon content and increasing the nickel content. In recent years, countries have invested heavily in developing low-carbon, low-nitrogen supermartensitic steels, and a number of such steels for various applications have been developed. Supermartensitic steel has been widely used in oil and gas exploration, storage and transportation equipment, hydroelectric power generation, the chemical industry, and equipment for high-temperature pulp production. Functional stainless steel: In response to changing market demands, various types of stainless steel with special uses and functions continue to emerge. New types of medical Ni-free austenitic stainless steel materials are mainly Cr-Ni austenitic stainless steels, which possess excellent biocompatibility and contain 13% to 15% Ni. Ni is a sensitizing agent, and it poses hazards to organisms such as causing birth defects and cancer. Stainless steel containing Ni implants, when used in the body for an extended period, will gradually degrade and release Ni ions. When Ni ions accumulate in tissues surrounding the implant site in the human body, they can induce toxic effects, leading to adverse reactions such as cell damage and inflammation. The Cr-Mn-N type nickel-free medical austenitic stainless steel developed by the Institute of Metallurgy, Chinese Academy of Sciences, has demonstrated, through biocompatibility tests, superior performance compared to the Cr-Ni austenitic stainless steels currently in use in clinical settings. Another example is antibacterial stainless steel: as people’s living standards improve, there is an increasing emphasis on the environment they live in and their own health, which has spurred research and development in antibacterial materials. Since 1980, developed countries represented by Japan have begun to research and apply antibacterial materials in areas such as home appliances, food packaging, daily necessities, and bathroom equipment. Nisshin Steel Co., Ltd. and Kawasaki Steel Corporation have developed Cu-containing and Ag-containing antibacterial stainless steels, respectively. The Cu-containing antibacterial stainless steel involves adding 0.5% to 1.0% Cu to the stainless steel, and through special heat treatment, ε-Cu precipitates are evenly dispersed from the surface to the interior of the stainless steel, thereby providing antibacterial effects. This Cu-containing antibacterial stainless steel is suitable for use in high-end kitchen utensils and other related products, as well as in other products that require high workability and antibacterial properties. Ag-added antibacterial stainless steel exhibits a high level of antibacterial efficacy against bacteria such as E. coli and Staphylococcus aureus; in particular, it maintains its strong antibacterial properties even when subjected to processing, grinding, or surface wear. Nitrogen-alloyed stainless steel: When added to stainless steel as an alloying element, nitrogen can enhance the stability of austenite, balance the proportions of the phases in duplex steel, increase the strength and corrosion resistance of the steel without affecting its plasticity and toughness, and can partially replace Ni in stainless steel. In duplex steel, N delays the dispersed precipitation of intermetallic compounds ； In martensitic steels, nitrogen forms nitrides with other elements that are distributed along the grain boundaries; this enhances the hardenability and prevents the growth of austenite and ferrite grains during high-temperature tempering. The high-N-content austenitic stainless steels developed in recent years, namely high-strength non-magnetic austenitic stainless steels, possess high-temperature strength. They will be widely used as low-temperature superconducting materials, as well as materials with high corrosion resistance and non-magnetic properties. High-purity stainless steel: At present, domestic stainless steel manufacturers face a product rejection rate of over 20% due to impurities. Therefore, high-purity purification focused on impurity control in the stainless steel smelting process is attracting increasing attention. During the stainless steel smelting process, endogenous inclusions are generated during deoxidation, alloying, and solidification of the molten steel. The foreign inclusions in the finished product are generated during the smelting, casting, and transportation of the molten steel. To obtain stainless steel with high purity, attention must be paid to the raw materials fed into the furnace, deoxidizers, deoxidation procedures, refining processes, and continuous casting procedures. Among them, the steel containing 6% Mo (254SMo) is quite well-known. This type of steel exhibits excellent resistance to localized corrosion; under conditions involving seawater, aeration, crevices, and low-velocity flow, it demonstrates good resistance to pitting corrosion (PI ≥ 40) as well as decent resistance to stress corrosion. It can be used as a substitute material for nickel-based alloys and titanium alloys. Secondly, in terms of high-temperature resistance or corrosion resistance, it exhibits superior performance in these areas, making it irreplaceable compared to 304 stainless steel. Furthermore, in terms of the classification of stainless steels, the microstructure of special stainless steels is a stable austenitic microstructure. Since this special type of stainless steel is a highly alloyed material, its manufacturing process is quite complex; generally, people can only rely on traditional methods to produce it, such as casting, forging, rolling, and so on. In many fields, such as: 1. Oceans: marine structures in marine environments, seawater desalination, marine aquaculture, seawater heat exchange, etc. 2. Environmental protection sector: flue gas desulfurization devices for thermal power generation, wastewater treatment, etc. 3. Energy sector: nuclear power generation, comprehensive utilization of coal, tidal power generation, etc. 4. Petrochemical industry: oil refining, chemical and petrochemical equipment, etc. 5. Food industry: salt production, soy sauce brewing, etc. In many of the aforementioned fields, ordinary stainless steel 304 simply cannot suffice. In these specialized areas, special-grade stainless steels are indispensable and irreplaceable. In recent years, with the rapid economic development and the continuous improvement of the standards in the industrial sector, an increasing number of projects require higher-grade stainless steels – specialty stainless steels (super stainless steels, nickel-based alloys). Some representative special stainless steels include: 1. Super stainless steel, which is a type of special stainless steel containing about 6% molybdenum; there are around a dozen such steel grades in the world. It is also known as 6 molybdenum stainless steel. For example, the main components are ； 25Ni-23Cr-5.5Mo-0.2N. 2. The Incoloy series of alloys, such as Incoloy800, have main components of 32Ni-21Cr-Ti, Al. 3. The Inconel series of alloys, such as Inconel600, have main components of 73Ni-15Cr-Ti, Al. 4. Hastelloy alloys, such as C-276, have main components of 59Ni-15Cr-16Mo-4W. 5. Monel alloys, such as Monel400, have main components of 65Ni-32Cu. Based on these examples, if ordinary stainless steel (such as 304) is used instead of special stainless steels, then ordinary stainless steel (304) is not suitable for such high-temperature or highly corrosive environments; it will corrode immediately or undergo high-temperature oxidation. Therefore, in many environments where resistance to high temperatures and corrosion is required, special stainless steels are the best choice.