Introduction to heat treatment skills of mechanical knowledge points

Introduction to the heat treatment skills of mechanical knowledge points

The heat treatment of steel refers to the different heating, heat preservation, and cooling of the steel in the 'solid state' to modify the internal arrangement of the steel, and then obtain the required arrangement And function of a technical approach.　　 In mechanical manufacturing, all important tools and parts must be heat treated, and some parts must be processed more than twice in the entire technical process. In addition to alloying, heat treatment is the main way to change the function of metal materials.　　 Heat treatment is different from other processing procedures. Its intention is not to change the shape and dimensions of the part, but only to change its internal arrangement and function. It is an important process to ensure the internal quality of parts. According to the different requirements and technical methods of heat treatment, the heat treatment of steel is classified as follows: (1) General heat treatment annealing heats steel to a certain temperature, keeps it for a certain time, and then slowly cools it (following furnace cooling, pit cooling, sand cooling, ash Cold) heat treatment technique is called annealing. 'Normalizing' A heat treatment technology that heats steel to 30-50 degrees above the phase transition temperature, keeps it warm for a certain time, and then air-cools it. 'Quenching' The steel is heated to a temperature above the phase transition temperature, and then quickly cooled to room temperature (oil-cooled or water-cooled), and then the hardness and wear resistance of the steel are improved.　 Tempering 　 The quenched workpiece is heated to a temperature lower than the phase transition point for a certain period of time, and then cooled, so that the arrangement becomes a more stable condition, which is called tempering. Comparison between annealing and normalizing: 　　 common point: mainly used for the heat treatment of casting, forging, and welding blanks to improve the machining function of the blanks, remove internal stress, prepare for the final heat treatment, and can also be used for machinery with low functional requirements Final heat treatment of the part.　　The difference: the cooling rate is different: the normalizing cooling rate is higher, and the strength and hardness are also higher. Normalizing has a shorter production cycle than annealing and a higher utilization rate of equipment, which is more economical. [Note] If the size of the part is large or the shape is messy, normalizing may cause large internal stress and deformation, and even cracking.　　The connection between tempering and quenching: 　　After quenching, although the hardness of steel is high, it is too brittle and the arrangement is unstable, so it is not suitable for direct use. The hardness is reduced by tempering, but the durability is greatly improved. The purpose of tempering is to reduce the brittleness of quenched steel, reduce or eliminate internal stress, and avoid deformation and cracking of the workpiece. The unstable arrangement is stabilized, so as to maintain the shape and scale accuracy of the workpiece, and obtain the required arrangement and function.　　 [Attention] Quenching and tempering must be used in cooperation, it is meaningless to perform any operation alone.　　 (2) Surface heat treatment and surface quenching　 Surface quenching is the use of rapid heating to austenitize the outer surface layer without changing the chemical composition and core arrangement of the steel and then quench it to strengthen the heat treatment method for the surface of the part. Currently, induction heating surface quenching and flame heating surface quenching are widely used in production. 'Chemical heat treatment' A heat treatment technique that places steel parts in a specific medium at a certain temperature to keep the active atoms in the medium into the surface of the steel parts, and then changes the chemical composition and arrangement of the surface to change its function.　Chemical heat treatment methods include: carburizing, nitriding, carbonitriding, chromizing, aluminizing, boronizing, etc. The most commonly used in current production are carburizing, nitriding and carbonitriding.