Austenitic stainless steel, as the name suggests, its structure is austenite, and the heat treatment of austenitic stainless steel is very important, because the important task of austenitic stainless steel is corrosion resistance, improper heat treatment, its corrosion resistance will be greatly reduced, this issue mainly to everyone Describes the heat treatment of austenitic stainless steels.
Austenitic stainless steel is a common stainless steel (18-8 steel). For example, many tableware in the kitchen are made of austenitic stainless steel. Austenitic stainless steel, as its name implies, is austenite, which has no magnetic properties and no hardenability.
Austenitic stainless steel has very strong corrosion resistance in an oxidizing environment. The so-called oxidizing environment can be simply understood as an environment containing more oxygen. Austenitic stainless steel has good toughness and is easy to be processed and formed, so it is widely used.
Austenitic stainless steel is mainly used for corrosion resistance purposes, and heat treatment has a great influence on it. The corrosion resistance and acid resistance of austenitic stainless steel mainly depends on surface passivation, if the surface passivation is not maintained, it will corrode. Therefore, austenitic stainless steel is not completely stainless, it is only for oxidizing and acidic environments. For special ions, it does not have strong resistance. The heat treatment of austenitic stainless steel mainly affects the passivation ability of the surface layer, thus affecting its corrosion performance.
Uniform corrosion is the most common corrosion phenomenon, and uniform corrosion depends on the uniformity of the distribution of chromium elements. Heat treatment affects the distribution of chromium elements, which naturally affects the uniform corrosion resistance of austenitic stainless steel.
Intergranular corrosion is also one of the important corrosion properties for evaluating austenitic stainless steels. Generally speaking, if austenitic stainless steel is sensitized and a large number of beaded carbides are precipitated at the grain boundary, the intergranular corrosion performance will be greatly reduced. If austenitic stainless steel is sensitized, severe intergranular corrosion will occur even in a very common electrochemical environment.
Stress corrosion cracking is the most common form of failure in austenitic stainless steels. It should be noted that stress corrosion cracking depends on two main factors: first, there must be stress, which may be external stress or residual stress; second, stress corrosion cracking sensitive ions, such as halogen element ions, especially Chloride ions are the most common. Where austenitic stainless steel is applied, its ability to withstand stress is often not used, so special attention should be paid to residual stress, which can cause stress corrosion cracking in an environment containing chloride ions. The method of removing residual stress is stress relief annealing.
Pitting corrosion is the most dreaded form of corrosion. There are two main reasons for pitting corrosion: 1. The material composition is not uniform, such as sensitization, and austenitic stainless steel is particularly prone to pitting corrosion. 2. The concentration of the environmental corrosive medium is uneven, which is also the cause of pitting corrosion. Once pitting corrosion occurs, the local passivation film is destroyed, so there will be competition between the two states of active and passivation. Once passivation cannot occur, pitting corrosion will continue until the component is perforated.
Austenitic stainless steel has no solid-state transformation point from room temperature to high temperature. The purpose of heat treatment is to dissolve carbides generated during processing into the matrix, so that the distribution of alloying elements is more uniform. The austenitic stainless steel is heated to a high temperature to dissolve the carbides into the matrix, and then rapidly cooled to room temperature. In this process, the austenitic stainless steel will not harden, because there is no phase transformation, and the austenitic state remains at room temperature. This process It is called solution treatment. In solution treatment, the purpose of rapid cooling is only to make the distribution of carbon atoms and alloying elements more uniform.
During solution treatment of austenitic stainless steel, if the cooling rate is too slow, as the temperature drops, the solubility of carbon atoms in the matrix decreases, and carbides will precipitate. And carbon atoms are particularly easy to combine with chromium to form M23C6 carbides, which are distributed on the grain boundaries, and the phenomenon of chromium depletion occurs at the grain boundaries, resulting in sensitization. After the austenitic stainless steel is sensitized, it should be heated above 850oC, the carbides will be solid solution, and then rapid cooling can solve the sensitization problem.