I. Overview 1. Status and development of automobile carburized steel parts Important parts such as automobile gears and drive shafts are generally made of low-alloy carburized steel. This type of steel is one of the most widely used and largest steel grades in automotive alloy structural steels. Generally, it needs to be forged, pre-heat treated and cut. Processing, carburizing, quenching, tempering and other hot and cold processing steps to obtain high surface hardness and good core toughness, so that the workpiece has good characteristics such as wear resistance, fatigue resistance and pitting resistance. Due to the different experiences of resources and industrial development in various countries, the carburized steel grades used by major factories in various countries are not identical. At present, there are many carburized steel grades listed in national standards. Carburizing steel for automobile gears is mainly low carbon alloy steel, among which Cr steel, Mn steel and Mo steel are used for secondary and small gears, Cr - Ni Steel, Cr-Mn steel, Cr-Mo steel, Mn-Mo steel and Cr-Ni-Mo steel are used for important parts with large dimensions. With the increasing technical level of automobile products and the increasingly intensified market competition, the steel for automobile gears is in the transitional stage from the experience of various enterprises to the scientific and international development, with the unique characteristics of Cr-Ni steel. , Cr-Ni-Mo steel, Ni-Mo steel transition to low-cost, general-purpose Cr-Mo steel. Therefore, it is necessary to use a correspondingly reasonable new heat treatment process to match it. 2. Current status and existing problems of pre-heat treatment of forged blanks of carburized steel parts The pre-heat treatment of the forged blank not only has a great influence on the machinability, but also has a significant influence on the final heat treatment deformation. In order to improve the machinability of the tooth blank, eliminate the forging stress, and homogenize the structure, the domestic carburizing steel tooth blank is generally subjected to normalizing treatment. Normalizing is the heating of steel or steel to a suitable temperature above the critical point Ac3 or Ac m, and after cooling for a certain period of time, it is cooled in air to obtain a heat treatment process of pearlite-like structure. Normalizing is a traditional old process. Because of its simple equipment and process requirements, and low energy consumption, it has been widely used, but it is not perfect. With the development of the automobile industry and the improvement of product quality requirements, especially the diversification of steel materials for the introduction of models, ordinary normalizing treatment has not achieved the purpose of pre-heat treatment of the tooth blank. In view of the normal normalizing treatment, the steel is heated to a high temperature austenitizing and then cooled to room temperature in the air (sometimes blowing), which belongs to the heat treatment of the blank, and the hardness range required for normalizing steel parts is wide (156～207HBS). Generally, the microstructure is not inspected, and in addition, it is carried out in a forging factory (workshop), so it is usually easily overlooked. For the microstructural morphology and hardness index that needs to be obtained after forging a forging, there is neither a deep theoretical study nor a lack of productive practice. In fact, there are widespread problems such as rough surface after cutting of these parts, low service life of cutting tools, large deformation fluctuation before and after carburizing and quenching, etc. These are closely related to the normal microstructure and poor hardness. In actual production, after the normalized treatment of the forged steel of the carburized steel, since the cooling rate of the normalizing cannot be controlled, the austenite decomposition phase transformation cannot be controlled, and it must be continuously carried out in a temperature range, thus obtaining the microstructure. And hardness may also be different. Some steel parts may have partial or even complete non-equilibrium structure (α-Fe Wei's structure, bainite, etc.) due to the high cooling rate, which not only affects the cutting performance, but also changes the carburizing and quenching of steel parts. The deformation law will be scrapped due to excessive deformation, which is more likely to occur in steels with large fluctuations in hardenability. For steel parts with a small cooling rate, since the hardness of the steel is too low, plastic deformation is likely to occur during cutting, and a cutting tumor is formed, and a "sticking knife" or a "burning knife" phenomenon occurs. Generally, the amount of deformation of the heat treatment increases with the increase of the deformation amount of the random machining, due to improper machining process, such as excessive broaching speed, residual stress caused by cutting of the tool, unevenness of the reference surface during broaching, and presence of iron filings. When the foreign matter, the gear pulls the spline hole, the direction of the hole is improper, etc., the heat treatment deformation amount can be increased. By improving the machining process, the amount of machining deformation can be effectively controlled. In recent years, with the introduction of models to achieve a variety of gear materials and high standards for gear quality, the use of ordinary normalizing has been difficult to meet the requirements of automotive production. The normalizing treatment of forgings not only requires the hardness to be within a narrow range (the steel parts are easy to be broken during the cutting process, the surface is smooth), but also requires a stable microstructure (the coarser ferrite grains are finer) Pearlite) to improve the cutting performance and stabilize the deformation law after carburizing and quenching. One of the goals of the American Institute of Metals' International R&D Program submitted to the Department of Energy is "to strive to achieve zero deformation and maximum uniformity of heat treated parts." In order to meet the above requirements, the normalizing process needs to be improved to obtain the microstructure and hardness range required for normalizing. Second, the normalizing process of carburized steel Normalizing is the main means of preheating the carburized steel forgings. The purpose is to eliminate or improve various tissue defects caused by the preparation of the blank, obtain the structure and hardness which are most favorable for cutting, improve the morphology and distribution of the phase in the structure, refine the crystal grains, and prepare the structure for the final heat treatment. 1. Normalizing structure, hardness and machinability of steel The machinability of a material refers to the ease with which a material can be machined. The cutting performance of steel parts depends mainly on its mechanical properties and microstructure, and the mechanical properties are affected by the microstructure. The higher the hardness and strength of the steel to be processed, the greater the resistance of the blade to insert and open the surface layer, the higher the cutting heat, the faster the tool wear; when the hardness and strength of the steel are too low, the plasticity tends to increase. It is not easy to break the chip during cutting, and the steel piece is easy to bond with the blade. The tool is prone to cold welding wear, which reduces the service life of the tool, and it is easy to produce built-up edge to deteriorate the quality of the machined surface and increase the surface roughness of the steel. Next page Lamp Air Purifier,Germicidal Uv Light For Hvac,Air Sterilization Uv Lights,Uv Duct Light Guangdong Kingrate Optoelectronic Technology Co., Ltd. , https://www.kingrateuv.com