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“What exactly are \"steel\" and \"iron\", what are their properties, and what is the relationship between them? How did the terms 304, 304L, 316, and 316L come about, and what are the differences among them? Steel: A material whose main element is iron, with a carbon content generally below 2%, and which also contains other elements. ——GB/T 13304－91 \"Classification of steels\" Iron: a metal element with atomic number 26. Iron materials possess strong ferromagnetism, as well as good plasticity and thermal conductivity. Stainless steel: A type of steel that is resistant to mild corrosive agents such as air, steam, and water, or that possesses rust-resistant properties. The commonly used steel grades are 304, 304L, 316, and 316L, which are 300 series steels of austenitic stainless steel. What are the differences between stainless steels 304, 304L, 316, and 316L? Performance and applications of stainless steel valves; Introduction to the properties of 304 stainless steel. 304 stainless steel is the most widely used type of steel; as a steel with broad applications, it possesses good corrosion resistance, heat resistance, strength at low temperatures, and mechanical properties ； It has good hot workability for stamping, bending, etc., and shows no hardening due to heat treatment (non-magnetic, usable temperature range: -196°C to 800°C). Applications of 304 stainless steel: Household items (tableware of types 1 and 2, cabinets, indoor piping, water heaters, boilers, bathtubs); automotive parts (windshield wipers, mufflers, molded products); medical devices, building materials, the chemical industry, the food industry, agriculture, ship components. Overview of the properties of 304L stainless steel (where ‘L’ denotes low carbon content). What are the differences between stainless steels 304, 304L, 316, and 316L? Performance and application range of stainless steel valves: As a low-carbon 304 steel, under normal conditions its corrosion resistance is similar to that of 304 steel; however, after welding or stress relief, it exhibits excellent resistance to intergranular corrosion ； It maintains good corrosion resistance even without heat treatment, and can be used at temperatures ranging from -196°C to 800°C. Applications of 304L stainless steel: It is used in outdoor machinery in the chemical, coal, and petroleum industries where high resistance to intergranular corrosion is required, as well as in building materials for heat-resistant components and in parts for which heat treatment is difficult. Introduction to the properties of 316 stainless steel: Due to the addition of molybdenum, 316 stainless steel exhibits excellent corrosion resistance, resistance to atmospheric corrosion, and high-temperature strength, allowing it to be used in harsh conditions ； Excellent work hardening property (non-magnetic). Applications of 316 stainless steel: Equipment used in seawater, as well as production equipment in the chemical, dye, papermaking, oxalic acid, fertilizer industries ； Photography, food industry, facilities in coastal areas, ropes, CD rods, bolts, nuts. Introduction to the properties of 316L stainless steel (L stands for low carbon): As a low-carbon variant of the 316 steel series, it possesses the same characteristics as 316 steel, in addition to having an excellent resistance to intergranular corrosion. Application range of 316L stainless steel: Products with special requirements regarding resistance to intergranular corrosion. Comparison of the properties of 316 and 316L stainless steels: Chemical composition. Comparison of the properties of 316 and 316L stainless steels – what are the differences between stainless steels 304, 304L, 316, and 316L? Performance and applications of stainless steel valves: 316 and 316L stainless steels are molybdenum-containing stainless steel grades. The molybdenum content in 316L stainless steel is slightly higher than that in 316 stainless steel. Due to the molybdenum in the steel, its overall performance is superior to that of 310 and 304 stainless steels. Under high-temperature conditions, when the concentration of sulfuric acid is below 15% or above 85%, 316 stainless steel has a wide range of applications. 316 stainless steel also has good resistance to chloride corrosion, which is why it is commonly used in marine environments. The maximum carbon content of 316L stainless steel is 0.03; it can be used in applications where annealing is not possible after welding and where maximum corrosion resistance is required. Corrosion resistance of 316 and 316L stainless steels: The corrosion resistance of 316 stainless steel is superior to that of 304 stainless steel, and it exhibits good corrosion resistance during the pulp and paper production process. Moreover, 316 stainless steel is also resistant to corrosion by marine and corrosive industrial atmospheres. Generally speaking, 304 stainless steel and 316 stainless steel differ little in terms of chemical corrosion resistance, although there are differences in certain specific media. The first stainless steel to be developed was 304, and under certain conditions, this material is relatively sensitive to pitting corrosion. Adding an additional 2-3% molybdenum can reduce this sensitivity, which led to the creation of 316. Furthermore, this additional molybdenum can also reduce the corrosion caused by certain thermal organic acids. 316 stainless steel has almost become the standard material in the food and beverage industry. Due to the global shortage of molybdenum and the higher nickel content in 316 stainless steel, 316 stainless steel is more expensive than 304 stainless steel. Pitting corrosion is a phenomenon primarily caused by corrosion deposits on the surface of stainless steel, as a lack of oxygen prevents the formation of a chromium oxide protective layer. Especially in small valves, the possibility of deposits forming on the valve plate is very low; therefore, pitting corrosion also occurs rarely. In various types of water media (distilled water, drinking water, river water, boiler water, seawater, etc.), the corrosion resistance of 304 stainless steel and 316 stainless steel is almost the same, unless the chloride content in the medium is very high; in such cases, 316 stainless steel is more suitable. In most cases, there is not much difference in corrosion resistance between 304 stainless steel and 316 stainless steel, but in some situations the difference can be significant, and it is necessary to analyze each case on its own. Generally speaking, valve users should be aware of this, as they choose the material for containers and pipes based on the properties of the medium; it is not advisable to recommend materials to users. Heat resistance of 316L and 316 stainless steel: 316 stainless steel exhibits good oxidation resistance when used intermittently at temperatures below 1600 degrees, and when used continuously at temperatures below 1700 degrees. Within the range of 800–1575 degrees, it is not advisable to expose 316 stainless steel to such conditions continuously; however, when used continuously outside this temperature range, 316 stainless steel exhibits good heat resistance. 316L stainless steel has better resistance to carbide precipitation than 316 stainless steel, and can be used within the aforementioned temperature range. Stainless steel heat treatment involves annealing at temperatures ranging from 1850 to 2050 degrees, followed by rapid cooling. 316 stainless steel cannot be hardened by heat treatment. Stainless steel welding: 316 stainless steel has good weldability. All standard welding methods can be used for welding. During welding, stainless steel filler rods or electrodes such as 316Cb, 316L, or 309Cb can be used depending on the application. To achieve the best corrosion resistance, the welded joint of 316 stainless steel requires post-weld annealing. If 316L stainless steel is used, welding annealing is not required. Mechanical properties of stainless steel: What are the differences between stainless steel 304, 304L, 316, and 316L? Performance and application range of stainless steel valves: Among all types of steel, austenitic stainless steel has the lowest yield strength. Therefore, from the perspective of mechanical properties, austenitic stainless steel is not the best material for valve stems, as increasing the diameter of the valve stem is required to ensure a certain level of strength. The yield strength cannot be increased through heat treatment, but it can be improved through cold forming. Magnetism of stainless steel: The widespread use of austenitic stainless steel has led to the mistaken impression that all stainless steels are non-magnetic. For austenitic stainless steels, it can generally be considered non-magnetic, and the same is true for quenched forged steel. However, 304 that has undergone cold forming will have some magnetism. For cast steel, 100% austenitic stainless steel is non-magnetic. Low-carbon stainless steels: The corrosion resistance of austenitic stainless steels comes from a chromium oxide protective layer formed on the metal surface. If the material is heated to high temperatures ranging from 450°C to 900°C, its structure changes, and chromium carbide forms along the crystal edges. As a result, a chromium oxide protective layer cannot form at the edges of the crystal, leading to a decrease in corrosion resistance. This type of corrosion is called “intergranular corrosion”. As a result, 304L stainless steel and 316L stainless steel were developed to counter this corrosion. Both 304L stainless steel and 316L stainless steel have low carbon contents. Due to this reduced carbon content, chromium carbide does not form, and thus intergranular corrosion does not occur. It should be noted that a higher susceptibility to intergranular corrosion does not mean that non-low-carbon materials are more prone to corrosion. In high-chloride environments, this sensitivity is even higher. Please note that this phenomenon is caused by high temperatures (450°C–900°C). Usually, welding is the direct cause of reaching this temperature. For conventional butterfly valves with soft seat valves, since we do not perform welding on the valve disc, it is not very useful to use low-carbon stainless steel; however, most specification sheets require 304L or 316L stainless steel. Why does stainless steel rust too? Why does stainless steel also rust? When brown rust spots appear on the surface of stainless steel pipes, people are very surprised; they think, \"Stainless steel doesn’t rust, so if it rusts, it’s no longer stainless steel – there must be a problem with the steel.\" In fact, this is a one-sided and incorrect view resulting from a lack of understanding of stainless steel. Stainless steel can also rust under certain conditions. Stainless steel has the ability to resist oxidation in the atmosphere – that is, its rust resistance – as well as the ability to resist corrosion in media containing acids, alkalis, and salts – that is, its corrosion resistance. However, its corrosion resistance varies depending on the chemical composition of the steel itself, its protective condition, operating conditions, and the type of environmental medium. For example, 304 steel pipes have excellent corrosion resistance in a dry and clean atmosphere, but when placed in coastal areas exposed to sea fog containing high levels of salt, they rust very quickly ； The 316 steel pipe, on the other hand, performed well. Therefore, not every type of stainless steel is resistant to corrosion and rusting in any environment.