Quenching and tempering of steel shot: improve the strength and toughness of steel shot

　　Quenching and tempering of steel shot is a common heat treatment process used to improve the strength and toughness of steel shot. Below I will explain to you the process and effects of these two techniques.

　　Quenching refers to the process of rapidly cooling steel shot heated to a high temperature state so that it undergoes phase transformation and obtains high hardness. Quenching fixes the austenite crystal structure in the steel through rapid cooling and turns it into martensite, thereby increasing the hardness of the steel shot. Specifically, the quenching steps include three stages: heating, holding and cooling.

　　First, the steel shot is heated to an appropriate temperature, generally between 800°C and 950°C, to completely dissolve the carbides in it. Then, after maintaining this temperature for a period of time, the carbon element in the steel shot will diffuse and form a solid solution. If the temperature is maintained long enough, the carbon elements in the steel shot will be evenly distributed in the crystal structure.

　　Then, the heated steel shot is quickly cooled. The quenching cooling rate is very important. The faster the cooling rate, the higher the hardness of the steel shot. Commonly used quenching media include water, oil and brine, etc. The specific choice depends on the composition of the steel shot and the required hardness.

　　After quenching, martensite is formed on the surface of the steel shot and has high hardness. However, martensite is highly brittle, so tempering is required to improve the toughness of the steel shot.

　　Tempering refers to heating the quenched steel shot to a relatively low temperature, then keeping it for a period of time and then cooling it. Tempering temperature and time are determined based on the required toughness. Tempering temperatures are generally between 150°C and 600°C, and tempering times are usually shorter.

　　The purpose of tempering is to reduce the hardness of martensite, transform it into a more ductile structure, and at the same time reduce the residual stress in the steel shot. During the tempering process, part of the martensite will transform into bainite, retained austenite or cementite. These structures are relatively soft and can improve the toughness of the steel shot.

　　Through the combined use of quenching and tempering, steel shot can achieve higher strength and toughness. Quenching increases the hardness of the steel shot, while tempering increases the toughness, giving the steel shot higher overall performance.

　　In summary, quenching and tempering are common heat treatment processes used to improve the strength and toughness of steel shot. Quenching makes the steel shot hard and brittle, while tempering improves the toughness of the steel shot by reducing the hardness and transforming the crystal structure. The combination of the two can give steel shot high strength and toughness, making it suitable for various mechanical and engineering applications.