What is the difference between stainless steel and stainless iron? How to tell?

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Abstract: Stainless steel generally refers to the collective term for stainless steels and acid-resistant steels. Stainless steel refers to steel that is resistant to corrosion by weak media such as air, steam, and water, while acid-resistant steel refers to steel that can withstand corrosion by chemically corrosive media such as acids, alkalis, and salts. The difference between rust-resistant iron and stainless steel lies in the level of nickel content. How to distinguish between stainless steel and stainless iron? Stainless iron is a type of stainless steel; its grades include 409, 410, 430, and 444. It belongs to the martensitic and ferritic stainless steels, and it is magnetic when exposed to a magnet. Austenitic stainless steels include grades such as 201, 202, 304, 321, and 316L. Stainless steel (also known as rust-resistant acid-resistant steel) refers to steel that can resist corrosion by chemical agents such as the atmosphere or acids. Stainless steel does not remain rust-free; it simply exhibits different corrosion behaviors in various environments. Common stainless steels can be classified into three types based on their microstructural characteristics: martensitic stainless steels, ferritic stainless steels, and austenitic stainless steels. a. Martensitic stainless steel: The carbon content in commonly used martensitic stainless steels ranges from 0.1 to 0.45%, while the chromium content is between 12 and 14%. They belong to the category of chromium stainless steels, and are typically referred to as 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 certain stainless steel tools. To improve corrosion resistance, the carbon content in martensitic stainless steels is kept at a very low level, 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 its corrosion resistance becomes somewhat lower. It is easy to see that 4Cr13 has better strength and hardness properties than 1Cr13, but its corrosion resistance is inferior to that of 1Cr13. 1Cr13 and 2Cr13 possess resistance to corrosion by atmospheric conditions, steam, and other media, and are often used as corrosion-resistant structural steels. To achieve good overall properties, 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. As for 3Cr13 and 4Cr13 steels, their carbon content being higher results in relatively poorer corrosion resistance. By undergoing quenching followed by low-temperature tempering (at 200–300°C), tempered martensite is formed, which confers high strength and hardness (up to HRC 50); therefore, these steels are often used as tool steels for manufacturing medical devices, cutting tools, hot oil pump shafts, and similar items. b. Ferritic stainless steels: The carbon content in commonly used ferritic stainless steels is below 0.15%, while the chromium content ranges from 12 to 30%. They also belong to the category of chromium stainless steels; typical grades include 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28, etc. Due to the corresponding decrease in carbon content and the corresponding 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 steels. High-chromium ferritic stainless steels possess strong resistance to oxidative and medium-corrosive environments, and their corrosion resistance increases further as the chromium content rises. 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 employed to improve the microstructure and thus the properties. If such steel is held at 450–550°C, it will become brittle, a phenomenon known as \"475°C brittleness\". Brittleness can be eliminated by heating to about 600°C and then rapid cooling. 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. Furthermore, when quenched rapidly at temperatures above 9250C, intergranular corrosion tendency and brittleness resulting from significant grain coarsening occur. These phenomena are serious problems for the welded areas. The former can be eliminated through short-term tempering at 650~815°C. The strength of this type of steel is clearly lower than that of martensitic stainless steels; it is mainly used to manufacture corrosion-resistant parts and is widely employed 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 enabling a metastable single-phase austenite structure at room temperature. Due to its high content of chromium and nickel and its 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 typically employed. In this process, the steel is heated to 1100°C and then quickly cooled in water, causing the carbides to dissolve within 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.