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The 904L alloy steel has the following characteristics: 904L is a highly alloyed austenitic stainless steel with a very low carbon content. This steel is designed for environments with severe corrosion conditions. Originally, this alloy was developed to resist corrosion in dilute sulfuric acid. This feature has been proven to be very successful through years of practical use. Now, 904L has been standardized in many ** and approved for use in manufacturing pressure vessels. Like other commonly used CrNi austenitic steels, the 904L alloy exhibits good resistance to pitting and crevice corrosion, high resistance to stress corrosion cracking, good resistance to intergranular corrosion, as well as good machinability and weldability. During hot forging, the maximum heating temperature can reach 1180 degrees Celsius, while the minimum temperature at which forging stops must be no less than 900 degrees Celsius. The hot forming of this steel can be carried out at 1000–1150 degrees Celsius. The heat treatment process for this steel involves heating to 1100–1150 degrees Celsius, followed by rapid cooling. Although this steel can be welded using conventional welding processes, the most appropriate methods are shielded metal arc welding and tungsten inert gas welding. When using manual arc welding to weld sheets no thicker than 6 millimeters, the diameter of the electrode shall not exceed 2.5 millimeters ； When the plate thickness is greater than 6 mm, the electrode diameter is less than 3.2 mm. When heat treatment is required after welding, it can be carried out by heating to 1075–1125 degrees Celsius followed by rapid cooling. For TIG welding, the filler metal can be the same material as the electrode; after welding, the weld seam must be pickled and passivated. Microstructural structure of 904L: 904L has a fully austenitic structure. Compared to other austenitic stainless steels with high molybdenum content, 904L is less sensitive to the precipitation of ferrite and alpha phases. Corrosion resistance of 904L: Due to its very low carbon content (up to 0.020%), no carbides precipitate under normal heat treatment and welding conditions. This eliminates the risk of intergranular corrosion that occurs after conventional heat treatment and welding. Due to its high chromium, nickel, and molybdenum content, as well as the addition of copper, 904L can be passivated even in reducing environments, such as in sulfuric acid and formic acid. Its high nickel content results in a low corrosion rate even when it is in an active state. In pure sulfuric acid within a concentration range of 0–98%, the operating temperature for 904L can reach up to 40 degrees Celsius. Pure phosphoric acid exhibits excellent corrosion resistance in the concentration range of 0–85%. In industrial phosphoric acid produced by wet-process methods, impurities have a strong impact on corrosion resistance. Among all types of phosphates, 904L has better corrosion resistance than ordinary stainless steel. In strongly oxidizing nitric acid, 904L has lower corrosion resistance compared to highly alloyed steel grades without molybdenum. In hydrochloric acid, the use of 904L is limited to lower concentrations of 1-2%. Within this concentration range. The corrosion resistance of 904L is better than that of conventional stainless steels. 904L steel has a high resistance to pitting corrosion. It also exhibits excellent resistance to crevice corrosion in chloride solutions. The high nickel content of 904L reduces the corrosion rate in pits and crevices. Ordinary austenitic stainless steels can be sensitive to stress corrosion in a chloride-rich environment at temperatures above 60 degrees Celsius; increasing the nickel content of the stainless steel can reduce this susceptibility. Due to its high nickel content, 904L exhibits excellent resistance to stress corrosion cracking in chloride solutions, concentrated hydroxide solutions, and environments rich in hydrogen sulfide. Application areas of 904L: The 904L alloy is a versatile material that can be used in many industrial fields: 1. Petroleum and petrochemical equipment, such as reactors in such facilities. 2. Equipment for the storage and transportation of sulfuric acid, such as heat exchangers, etc. 3. Flue gas desulfurization units in power plants are mainly used in components such as the tower body of the absorption tower, flues, damper plates, internal components, and spraying systems. 4. Washers and fans in the organic acid treatment system. Similar grades: GB/T, UNS, AISI/ASTM, ID, W.Nr. 00Cr20Ni25Mo4.5Cu, N08904, 904L, F904L, 1.4539. Chemical composition: C: 0.02, Si: 1, Mn: 2, P: 0.045, S: 0.035, Cr: 19–23, Ni: 23–28, Mo: 4–5, Cu: 1–2; remainder is iron. Mechanical properties: Density: 8.0 g/cm³; Melting point: 1300–1390°C. Tensile strength Rm: N/mm²; Yield strength Rp0.2: N/mm²; Elongation A5%: unspecified℃