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Stainless Steel refers to steel that is resistant to corrosion by weak corrosive agents such as air, steam, and water, as well as by chemically aggressive substances such as acids, alkalis, and salts; it is also known as rust-resistant and acid-resistant steel. Steel grades that are resistant to weak corrosive agents such as air, steam, and water, or that possess rust resistance, are known as stainless steel ； Steel grades that can resist chemical corrosive agents such as acids, alkalis, and salts are called acid-resistant steel. Due to the differences in their chemical composition, the former is not necessarily resistant to corrosion by chemical agents, whereas the latter generally possesses rust resistance. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Generally, based on their microstructure, ordinary stainless steels are divided into three categories: austenitic stainless steels, ferritic stainless steels, and martensitic stainless steels. Based on these three basic microstructural types, dual-phase steels, precipitation-hardening stainless steels, and high-alloy steels with an iron content of less than 50% have been developed to meet specific requirements and purposes. Classified by microstructure: 01 Austenitic stainless steel. A stainless steel in which the matrix is primarily composed of austenite structure (CY phase) with a face-centered cubic crystal structure, is non-magnetic; it is strengthened mainly through cold working (which may also result in some magnetism). The American Iron and Steel Institute uses numbers from the 200 and 300 series to identify them, such as 304. 02 Ferritic stainless steel. It is a stainless steel in which the matrix is primarily composed of ferrite structure (phase a) with a body-centered cubic crystal structure; it is magnetic and generally cannot be hardened through heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Institute uses 430 and 446 as designations. 03 Maraging stainless steel. Stainless steel with a martensitic structure (body-centered cubic or cubic), which is magnetic and whose mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute designates them with the numbers 410, 420, and 440. Martensite has an austenitic structure at high temperatures; when cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (i.e., hardening). 04 Austenite-ferrite (duplex) stainless steels. The matrix consists of both austenite and ferrite phases; in materials with a smaller proportion of one of these phases, the content of the matrix is generally greater than 15%. It is a stainless steel that is magnetic and can be strengthened through cold working, and 329 is a typical example of a duplex stainless steel. Compared to austenitic stainless steels, duplex steels have higher strength, as well as significantly improved resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion. 05 Precipitation-hardening stainless steel. Stainless steel with an austenitic or martensitic structure that can be hardened through precipitation hardening. The American Iron and Steel Institute uses numbers from the 600 series to identify them, such as 630, which corresponds to 17-4PH. Generally speaking, except for alloys, austenitic stainless steels exhibit relatively excellent corrosion resistance. In environments with low corrosivity, ferritic stainless steels can be used. In mildly corrosive environments, if the material is required to have high strength or hardness, martensitic stainless steels and precipitation-hardening stainless steels can be utilized. Properties and uses, surface treatment, thickness variations: 1. During the rolling process in steel mills, the rollers heat up and undergo slight deformation, which results in variations in the thickness of the sheets produced; generally, the thickness is greater in the middle and smaller on the sides. When measuring the thickness of the board, it is specified that the middle part of the board’s head should be measured. 2. Tolerances arise from market demands and are generally divided into large tolerances and small tolerances. For example: What kind of stainless steel is less prone to rusting? There are three main factors that affect the corrosion of stainless steel: 1. The content of alloying elements. Generally speaking, steel with a chromium content of 10.5% is less prone to rusting. The higher the content of chromium and nickel, the better the corrosion resistance. For example, in 304 stainless steel, the nickel content should be between 8% and 10%, while the chromium content should be between 18% and 20%; such stainless steel generally does not rust. 2. The smelting process used by manufacturing enterprises also affects the corrosion resistance of stainless steel. Large stainless steel manufacturers with advanced smelting techniques, sophisticated equipment, and modern processing methods can ensure effective control over alloy elements, removal of impurities, and regulation of the cooling temperature of steel billets. As a result, their products have stable and reliable quality, excellent internal properties, and are less prone to rusting. On the other hand, some small steel mills have outdated equipment and processes; during the smelting process, impurities cannot be removed, which inevitably leads to the rusting of the products produced. 3. It does not rust easily in conditions of dry climate and good ventilation. Rusting is likely to occur in areas with high air humidity, continuous rainy weather, or environments with a high acidity or alkalinity in the air. If the surrounding environment is too harsh, stainless steel of grade 304 can also rust. How to deal with rust spots on stainless steel? 1. Chemical method: Acid cleaning paste or spray is used to help re-passivate the rusted areas, forming a chromium oxide film that restores their corrosion resistance. After acid cleaning, it is important to rinse thoroughly with clean water in order to remove all contaminants and residual acids. After all processing, re-polish with polishing equipment and seal with polishing wax. For areas with slight rust spots, a 1:1 mixture of gasoline and engine oil can be used; the rust spots can be removed by wiping them away with a clean cloth. 2. Mechanical methods: sandblasting for cleaning, shot blasting using glass or ceramic particles, abrading, brushing, and polishing. It is possible to mechanically remove contamination caused by previously removed materials, polishing materials, or obliterating materials. Various types of pollution, especially foreign iron particles, can be a source of corrosion, particularly in humid environments. Therefore, mechanical surface cleaning should preferably be carried out as a proper cleaning process under dry conditions. Mechanical methods can only clean the surface; they cannot alter the material’s inherent corrosion resistance. Therefore, it is recommended to repolish with polishing equipment after mechanical cleaning, and seal it with polishing wax. Common stainless steel grades and their properties. 1. 304 stainless steel. It is one of the austenitic stainless steels with the highest application volume and widest range of uses. It is suitable for manufacturing deep-drawn components, acid transport pipelines, containers, structural parts, and various instrument housings; it can also be used to produce non-magnetic equipment and components for low-temperature applications. 2. 304L stainless steel. Ultra-low carbon austenitic stainless steels, developed to address the severe intergranular corrosion tendency of 304 stainless steel under certain conditions due to the precipitation of Cr23C6, exhibit significantly superior intergranular corrosion resistance in their sensitized state compared to 304 stainless steel. Except for its slightly lower strength, its other properties are the same as those of 321 stainless steel. It is primarily used in corrosion-resistant equipment and components that require welding but cannot undergo solution treatment, and can be used to manufacture various instrument housings. 3. 304H stainless steel. The internal variants of 304 stainless steel have a carbon content of 0.04% to 0.10%, and their high-temperature performance is superior to that of 304 stainless steel. 4. 316 stainless steel. Adding molybdenum to 10Cr18Ni12 steel endows the steel with good resistance to reducing media and pitting corrosion. In seawater and various other media, its corrosion resistance is superior to that of 304 stainless steel; it is mainly used as a pitting-resistant material. 5. 316L stainless steel. Ultra-low carbon steel, which exhibits excellent resistance to intergranular corrosion in the sensitized state, is suitable for manufacturing welded components and equipment with large cross-sectional dimensions, such as corrosion-resistant materials used in petrochemical equipment. 6. 316H stainless steel. The internal variants of 316 stainless steel have a carbon content of 0.04%–0.10%, and their high-temperature performance is superior to that of 316 stainless steel. 7. 317 stainless steel. It has better pitting resistance and creep resistance than 316L stainless steel, and is used to manufacture equipment for the petrochemical industry and resistant to organic acid corrosion. 8. 321 stainless steel. Titanium-stabilized austenitic stainless steel, in which titanium is added to improve resistance to intergranular corrosion, and possesses good high-temperature mechanical properties; it can be replaced by ultra-low carbon austenitic stainless steel. It is not recommended for general use, except in specialized applications such as high-temperature or hydrogen corrosion resistance. 9. 347 stainless steel. Niobium-stabilized austenitic stainless steel, in which niobium is added to improve resistance to intergranular corrosion; it possesses the same corrosion resistance as 321 stainless steel in corrosive media such as acids, alkalis, and salts. It has good weldability, and can be used both as a corrosion-resistant material and as a heat-resistant steel. It is primarily utilized in the power generation and petrochemical industries, for applications such as manufacturing containers, pipes, heat exchangers, shafts, furnace tubes in industrial furnaces, and furnace tube thermometers. 10. 904L stainless steel. Super austenitic stainless steel is a type of super austenitic stainless steel invented by the Finnish company OUTOKUMPU. It has a nickel content of 24%–26%, with a carbon content of less than 0.02%. It possesses excellent corrosion resistance; it performs well in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, and phosphoric acid. Additionally, it offers good resistance to crevice corrosion and stress corrosion. It is suitable for sulfuric acids of various concentrations at temperatures below 70°C. At normal pressure, it exhibits excellent corrosion resistance in acetic acid and formic acid of any concentration and at any temperature, as well as in mixtures of acetic acid and formic acid. The original standard ASMESB-625 classified it as a nickel-based alloy, while the new standard classifies it as a stainless steel. In China, only steel with a grade similar to 015Cr19Ni26Mo5Cu2 is available; a few European instrument manufacturers use 904L stainless steel for critical components. For example, the measuring tubes in E+H’s mass flow meters are made of 904L stainless steel, and the casings of Rolex watches are also constructed from this material. 11. 440C stainless steel. Martensitic stainless steel has the highest hardness among hardenable stainless steels and stainless metals, with a hardness of HRC 57. It is mainly used to manufacture nozzles, bearings, as well as valve cores, valve seats, sleeves, valve stems, and other components for valves. 12, 17-4PH stainless steel. Martensitic precipitation-hardening stainless steel with a hardness of HRC 44, featuring high strength, hardness, and corrosion resistance; it cannot be used at temperatures above 300°C. It exhibits excellent corrosion resistance to both atmospheric conditions and diluted acids or salts. Its corrosion resistance is on par with that of 304 and 430 stainless steels, and it is used in the manufacture of offshore platforms, turbine blades, as well as valve cores, seatings, sleeves, and valve stems. 13. 300 series – Chromium-nickel austenitic stainless steels; 301 – Good ductility, used for shaped products. It can also be rapidly hardened through machining, and has good weldability. Its wear resistance and fatigue strength are superior to those of 304 stainless steel. 301 stainless steel exhibits significant work hardening upon deformation, and is used in various applications that require high strength. 302 – is essentially a variant of 304 stainless steel with a higher carbon content; it can achieve greater strength through cold rolling. 302B is a stainless steel with a high silicon content, and it possesses excellent resistance to high-temperature oxidation. 303 and 303Se are free-cutting stainless steels containing sulfur and selenium respectively, used in applications where free-cutting properties and high surface finish are primarily required. 303Se stainless steel is also used to manufacture components that require hot heading, as it exhibits good hot workability under such conditions. 304N is a nitrogen-containing stainless steel; nitrogen is added to enhance the strength of the steel. 305 and 384 – Stainless steels with a higher nickel content exhibit a lower rate of work hardening, making them suitable for various applications where high cold formability is required. 308——Used for making welding electrodes. Stainless steels 309, 310, 314, and 330 have relatively high nickel and chromium contents, which is intended to enhance the steel’s oxidation resistance and creep strength at high temperatures. 30S5 and 310S are variants of 309 and 310 stainless steels; the only difference is their lower carbon content, which is intended to minimize the formation of carbides in the area near the weld. 330 stainless steel possesses particularly high resistance to carburization and thermal shock.