Austenitic stainless steel, as the name suggests, its structure is austenite, 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 article mainly tells you 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 the name suggests, its structure is austenite, it has no magnetism and no hardenability.

Austenitic stainless steel has very strong corrosion resistance in oxidative environments. The so-called oxidative environment can be simply understood as an environment with more oxygen. Austenitic stainless steel has good toughness and is easy to process and shape, so it is very versatile.

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 depend on surface passivation. If this behavior of surface passivation cannot be maintained, it will corrode. Therefore, austenitic stainless steel is not completely stainless, it is only for oxidizing environment and acid environment. 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.

Polarization curve of 304 stainless steel, anodic passivation zone appears

Uniform corrosion is the most common corrosion phenomenon, and uniform corrosion depends on the uniform distribution of chromium. Heat treatment affects the distribution of chromium, which naturally affects the uniform corrosion resistance of austenitic stainless steel.

Intergranular corrosion is also one of the important corrosion properties of austenitic stainless steel. Generally speaking, if the austenitic stainless steel is sensitized and a large number of beaded carbides are precipitated at the grain boundaries, its intergranular corrosion performance will be greatly reduced. If austenitic stainless steel is sensitized, severe intergranular corrosion will occur even in a common electrochemical environment.

                                                                                                                                  Intergranular corrosion cracking

Stress corrosion cracking is the most common form of failure in austenitic stainless steels. Everyone needs to pay attention 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 used, its ability to withstand stress is often not used, so special attention should be paid to residual stress, because residual stress can cause stress corrosion cracking in environments containing chloride ions. The method of removing residual stress is stress relief annealing.

Pitting corrosion is the most dreaded corrosion. To say that it is the most terrible corrosion, it is more appropriate to use an ancient saying to describe this problem: “A dike of a thousand miles collapses in an ant’s nest”. There are two main reasons for pitting corrosion: one is that the composition of the material is uneven, such as sensitization, and austenitic stainless steel is particularly prone to pitting corrosion; the other is that the concentration of the corrosive medium in the environment is uneven, which is also the cause of pitting corrosion. reason. Once pitting occurs, the local passivation film layer is destroyed, so it will compete between the two states of activity and passivation. Once passivation cannot occur, pitting will continue until the component is perforated.

                                                                                                   

                                                                                                                  Pitting corrosion of austenitic stainless steel

Austenitic stainless steel has no solid phase transition point at room temperature to high temperature. The purpose of its heat treatment is mainly to dissolve the carbides generated during processing into the matrix, so that the distribution of alloying elements is more uniform. Heating austenitic stainless steel to a high temperature to dissolve carbides into the matrix, and then rapidly cooling to room temperature, in this process, austenitic stainless steel will not harden, because there is no phase transformation, and the austenitic state is still maintained at room temperature. 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.

                                                                                         Schematic diagram of sensitization of austenitic stainless steel

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. Moreover, carbon atoms are particularly easy to combine with chromium to form M23C6 carbides, which are distributed on the grain boundaries, and the chromium-depleted phenomenon occurs at the grain boundaries, resulting in sensitization. After the austenitic stainless steel is sensitized, it should be heated above 850°C, the carbides will be in a solid solution, and the sensitization problem can be solved by rapid cooling.

                                                                                                   Metallographic structure of austenitic stainless steel