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Stainless steel is generally a general term for stainless steels and acid-resistant steels. Stainless steel refers to steel that is resistant to corrosion by weak agents such as the atmosphere, steam, and water, whereas acid-resistant steel refers to steel that is resistant to corrosion by chemically aggressive agents such as acids, alkalis, and salts. The difference between wrought iron and stainless steel lies in the level of nickel content. Stainless iron is produced by reprocessing recycled scrap iron, lead, steel, and other materials through secondary melting and magnet removal; it is a type of stainless steel. Its grades include 409, 410, 430, and 444, and it belongs to the martensitic and ferritic stainless steel categories – it becomes magnetic when exposed to a magnet. Austenitic stainless steels include grades such as 201, 202, 304, 321, and 316L. Stainless steel (also known as corrosion-resistant steel) refers to steel that can resist corrosion by atmospheric agents or chemical media such as acids. Stainless steel isn’t immune to rust; it simply exhibits different corrosion behaviors in various environments. There are many types of stainless steels, each with different properties; over the course of their development, they have gradually been categorized into several main groups. Classified by organizational structure, they are divided into four main categories: martensitic stainless steels (including precipitation-hardening stainless steels), ferritic stainless steels, austenitic stainless steels, and austenitic-ferritic duplex stainless steels ； Based on the main chemical components or characteristic elements in the steel, they can be classified into chromium stainless steels, chromium-nickel stainless steels, chromium-nickel-molybdenum stainless steels, as well as low-carbon stainless steels, high-molybdenum stainless steels, and high-purity stainless steels, etc ； Based on the performance characteristics and applications of steel, they can be classified into nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel, pitting corrosion-resistant stainless steel, stress corrosion-resistant stainless steel, high-strength stainless steel, etc ； Based on their functional characteristics, steels can be classified into low-temperature stainless steels, non-magnetic stainless steels, free-cutting stainless steels, superplastic stainless steels, etc. The commonly used classification methods at present are those based on the structural characteristics of the steel, its chemical composition, and a combination of both. They are generally classified into martensitic stainless steels, ferritic stainless steels, austenitic stainless steels, duplex stainless steels, and precipitation-hardening stainless steels, or into two main categories: chromium-based stainless steels and nickel-based stainless steels. a. Martensitic stainless steel: The carbon content in commonly used martensitic stainless steels is 0.1–0.45%, while the chromium content is 12–14%. These steels belong to the category of chromium stainless steels; they generally refer to Cr13-type stainless steels. Typical steel grades include 1Cr13, 2Cr13, 3Cr13, 4Cr13, etc. This type of steel is generally used to manufacture various valves, pumps, and other components that need to withstand loads as well as have corrosion resistance, as well as some stainless steel tools. To improve corrosion resistance, the carbon content in martensitic stainless steels is kept within a very low range, generally not exceeding 0.4%. The lower the carbon content, the better the corrosion resistance of the steel; whereas the higher the carbon content, the greater the strength and hardness of the steel ； The higher the carbon content, the more chromium carbides are formed, and thus its corrosion resistance becomes poorer. It is evident from this that the strength and hardness indicators of 4Cr13 are superior to those of 1Cr13; however, its corrosion resistance is inferior to that of 1Cr13. 1Cr13 and 2Cr13 have resistance to corrosion by media such as the atmosphere and steam, and are commonly used as corrosion-resistant structural steels. To achieve good overall performance, quenching followed by high-temperature tempering (600–700°C) to produce tempered sorbite is commonly used in the manufacture of turbine blades, boiler tube fittings, and similar components. In contrast, 3Cr13 and 4Cr13 steels have a relatively lower corrosion resistance due to their higher carbon content. Through quenching followed by low-temperature tempering (200–300°C), tempered martensite is formed, which exhibits high strength and hardness (HRC up to 50). Therefore, these steels are commonly used as tool steels for manufacturing medical instruments, cutting tools, hot oil pump shafts, etc. b. Ferritic stainless steels: The carbon content in commonly used ferritic stainless steels is less than 0.15%, while the chromium content ranges from 12% to 30%. These also fall under the category of chromium stainless steels. Typical steel grades include 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28, etc. Due to the corresponding decrease in carbon content and increase in chromium content, when the steel is heated from room temperature to high temperatures (960–1100°C), its microstructure remains a single-phase ferritic structure. Its corrosion resistance, plasticity, and weldability are all superior to those of martensitic stainless steel. High-chromium ferritic stainless steels exhibit strong resistance to corrosion in oxidizing media; as the chromium content increases, their corrosion resistance is further improved. Adding titanium to steel can refine the grain structure, stabilize carbon and nitrogen, and improve the toughness and weldability of the steel. Ferritic stainless steels cannot be strengthened by heat treatment, as no phase transformation occurs during heating and cooling. If grain coarsening occurs during heating, only cold plastic deformation and recrystallization can be applied to improve the microstructure and properties. If this type of steel is held at 450–550°C, it becomes brittle; this phenomenon is known as “475°C brittleness”. By heating to about 600°C and then quenching, embrittlement can be eliminated. It should also be noted that when such steels are heated at temperatures between 600 and 800°C for an extended period, a hard and brittle σ-phase is formed, resulting in σ-phase brittleness in the material. Additionally, when rapidly cooled above 9250°C, there is a tendency for intergranular corrosion and brittleness caused by significant grain coarsening. These phenomena are serious problems for the welded joints. The former can be eliminated through short-term tempering at 650~815°C. The strength of this type of steel is evidently lower than that of martensitic stainless steel. It is mainly used for manufacturing corrosion-resistant parts and is widely applied in the nitric acid and nitrogen fertilizer industries. c. Austenitic stainless steel: Austenitic stainless steel is obtained by adding 8–11% Ni to steel containing 18% Cr. For example, 1Cr18Ni9 is the most typical steel grade. Due to the addition of nickel, this type of steel expands the austenite region, thereby allowing a metastable single-phase austenite structure to be obtained at room temperature. Due to its high content of chromium and nickel and a single-phase austenitic structure, it possesses greater chemical stability than chromium stainless steels and better corrosion resistance; it is therefore the most widely used type of stainless steel today. Stainless steel of type 18-8 exhibits an austenite + carbide microstructure in its annealed state. The presence of carbides significantly reduces the steel’s corrosion resistance; therefore, a solution treatment process is usually employed. In this process, the steel is heated to 1100°C and then quickly cooled in water, causing the carbides to dissolve into the austenite formed at high temperatures. Subsequent rapid cooling results in a single-phase austenite structure at room temperature. The commonly referred to stainless iron refers to ferritic stainless steel and martensitic stainless steel. It is used to distinguish itself from austenitic stainless steels, which have excellent rust resistance and are the most widely used. d. Austenite-ferrite duplex stainless steel: It is a type of stainless steel in which austenite and ferrite structures each make up about half of it. At low carbon levels, the Cr content ranges from 18% to 28%, while the Ni content ranges from 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel possesses the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic steels, it has higher plasticity and toughness, no room-temperature brittleness, and significantly improved resistance to intergranular corrosion as well as weldability. At the same time, it retains the 475°C brittleness and high thermal conductivity associated with ferritic stainless steels, as well as superplasticity. Compared to austenitic stainless steels, it has higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting resistance and is also a nickel-saving stainless steel. How can one distinguish between stainless steel and \"rust-proof iron\" products? Identification from markings: Many stainless steel products have stamps on their surfaces, such as “13-0”, “18-8”, etc. The number before the hyphen indicates the chromium content in the product, while the number after the hyphen represents the nickel content. A notation like 13-0 indicates that it contains only chromium and no nickel; it is commonly known as \"stainless iron\"” ； The designation 18-8 indicates that the product contains both chromium and nickel; this is what makes it stainless steel. Negotiating by sound: tapping on stainless steel or \"rust-proof iron\" products can also be used as a method for making judgments. Attract with a permanent magnet: Genuine stainless steel is not attracted to magnets, while \"stainless iron\" is attracted to them. Although there are differences between “stainless iron” and stainless steel in terms of their properties, both are significantly superior to wrought iron and cast iron cookware when it comes to resistance to corrosion.