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1 Reasons for Heat Treatment Deformation It is of great significance to study the causes of heat treatment deformation of workpieces, to grasp its regularity, and to find out the technical measures to reduce deformation. The deformation of the workpiece includes both dimensional changes and geometric changes. Regardless of the deformation, it is mainly caused by internal stress generated inside the workpiece during heat treatment. According to different reasons for the formation of internal stress, it can be divided into thermal stress and tissue stress. The deformation of the workpiece is the result of the combined effects of these two stresses. When the stress is greater than the yield limit of the material, permanent deformation occurs, and if it is greater than the strength limit of the material, the workpiece will crack. The two stress-induced deformations are studied separately.
1.1 Thermal stress and its induced deformation Due to the uneven heating or cooling, there is a temperature difference in the surface of the part, which causes inconsistency in thermal expansion and contraction, thereby generating thermal stress. Practice has proved that the thermal stress causes the deformation of the workpiece to be similar to the result of the high hydrostatic pressure inside the object. It makes the plane become convex, the right angle becomes obtuse, the long direction becomes shorter, and the short direction becomes longer.
In a word, the parts tend to be spherical. As shown in 1.
1.2 Tissue stress and its induced internal stress The internal stress caused by the different specific volume of austenite and its transformation products and the time of tissue transition between the surface, the core or parts of the workpiece is called the tissue stress. The austenite has the smallest specific volume and the martensite has the largest specific volume, so the volume expansion occurs when the steel is quenched, resulting in tissue stress.
The tissue stress has two characteristics: the workpiece surface is subjected to tensile stress and the core is subjected to compressive stress; near the surface layer, the tangential tensile stress is greater than the axial tensile stress. The deformation of the workpiece caused by the tissue stress is opposite to the thermal stress, which makes the plane concave, the right angle becomes sharp, the long direction becomes longer, and the short direction becomes shorter. In a word, make the sharp corners stand out as shown. In particular, the tangential tensile stress on the surface of the specimen is large, and the large tangential tensile stress is often the main cause of longitudinal cracking of the part.
The deformation of the quenched part is the result of a combination of thermal stress and tissue stress, as shown. In addition to the internal stress, the deformation of the part is affected by the composition of the raw material, the shape of the workpiece and the cooling rate of the medium. The actual situation is much more complicated. Therefore, when solving the actual problem, it is necessary to comprehensively analyze and find out the leading role. Thermal stress is also a tissue stress in order to determine the tendency of deformation or the possibility of cracking, and to take various measures to control or prevent it.
2 Factors affecting heat treatment It is well known that the quality of the gear is not only affected by the steel itself but also by the heat treatment process. The heat treatment process mentioned here refers not only to the carburizing and quenching process of the gear, but also includes gear forging. The heat treatment process of the blank, generally speaking, the heat treatment process of the gear forging blank has two purposes: one is to ensure good cutting performance, which requires obtaining a uniform equiaxed grain of ferrite-pearlite structure after heat treatment; It is to ensure that the gear blank obtains uniform and uniform structure, which is beneficial to reduce the irregular deformation which is difficult to avoid during carburizing and quenching, and is important for ensuring good contact surface of the gear and reducing noise. There are many factors affecting the deformation performance of heat-treated gear steel, mainly in the following aspects.
2.1 Hardenability of raw materials The deformation of the quenched workpiece is closely related to the depth of the hardened layer. Different materials have different hardenability, and the microstructure after carburizing and quenching will be different, and the deformation will be different. In the case of complete hardening, the deformation is mainly based on the structure stress, so the size increases after quenching. If it is not hardened due to its large size, its size is shortened. The hardening curve of the same grade of steel smelted by different heats varies within a certain range. We refer to the range of hardness change at a certain distance from the quenching end as the hardenability bandwidth of the steel.
If the hardenability of the steel is different for each batch, and the fluctuation is very large, that is, the hardenability band is too wide, it will inevitably lead to irregular heat treatment of the gear. Foreign gear manufacturers have long recognized the effect of hardenability on the heat treatment deformation of gears. Therefore, strict hardenability bandwidth requirements are imposed on gear materials. The general condition is controlled at around 5HRC. For gears with high machining precision, hardenability The belt requirements are narrower. China's gear materials, the hardenability band is relatively wider, and some even exceed 10HRC, which has a great impact on the deformation.
2.2 The original microstructure of the material and the segregation of carbides and the presence of carbide nets can increase the deformation. Due to the different microstructures of the parts before carburizing and quenching, different microstructures have different specific volumes, so the parts are different. The volume after carburizing and quenching is inevitably different from that before carburizing and quenching. In addition, the granular pearlite has a smaller deformation than the flaky pearlite after quenching.
2.3 Final heat treatment The surface carbon concentration and the depth of the layer of the carburized gear will affect the expansion coefficient of the layered structure. If the carbon potential and process of the furnace gas are not strictly controlled during carburizing, the surface carbon concentration of each furnace part, The carburizing depth is different, and the fluctuation is large, which will cause the gear deformation to be irregular.
3 Control method According to the analysis of the influencing factors of gear steel heat treatment, the related control methods are mainly reflected in the following points.
(1) Due to the different hardenability, the length of the common normal line of the gear and the deformation of the outer tooth across the rod are different. Therefore, in order to reduce the fluctuation of heat treatment deformation of the gear, it is necessary to control the hardenability bandwidth of the raw materials, so that the hardenability of each batch of incoming steel is as close as possible. Of course, the specific range of hardenability bandwidth is determined by factors such as gear modulus, part size, machining accuracy, and steel price that meets the hardenability bandwidth requirements.
(2) For alloy carburized steel parts, isothermal annealing can be used to obtain the microstructure and hardness required for pre-heat treatment reliably and stably, and to effectively control heat treatment deformation.
(3) The advanced carbon potential automatic control technology can be used to achieve gear reversal with good reproducibility, and the furnace carbon potential and carburizing process are strictly controlled to ensure the intrinsic quality of each carburizing gear. In addition, the quenching oil tank should be equipped with a heater and a cooling system to keep the oil temperature at the temperature specified by the process, so that the cooling conditions are constant.
Analysis of heating deformation and deformation of auto parts gear materials