On 2018-08-08 14:48:54
Distortion of gear during heat treatment
In heat treatment, the distortion control of heat treatment is usually focused on "quenching distortion". The reason is that the quenching cooling takes place in the instantaneous sharp temperature reduction and structure transformation, accompanied by a large volume expansion and stress generation; while the heating process is much slower, and with the temperature rising, all kinds of stresses in the metal material are released. In addition, the distortion of the workpiece in the heating process can not be measured, so the distortion of the heating process is little known and easy to be ignored. In fact, the distortion is not only caused by thermal stress, but also by the release of internal stress itself. In the process of manufacturing high-precision gear products, the distortion of gear heat treatment in the heating process should also be paid attention to.
In the process of gear cutting, due to the plastic deformation of the tool on the metal extrusion, the work hardening occurs, and at the same time, the residual stress occurs. It is generally believed that the stress relief annealing after machining can eliminate the residual stress and therefore the mechanical conditions causing distortion. However, the experiment shows that during the stress relief annealing at 650 ℃, with the recovery of distorted lattice and the release of internal stress, the result is "the stress is eliminated and the distortion is left behind". In the process of heating, the normalized structure changes to austenite, because the specific volume of ferrite + pearlite is different from that of austenite. Therefore, if the original structure of normalized gear is not uniform, the volume change will be uneven due to the different speed of transformation, which will lead to distortion. In addition, due to the influence of structural shape, clamping, placement and furnace loading, the heating of gears is uneven. Especially at 925-950 ℃ high temperature carburizing temperature, some parts will be mainly heated by radiation, while others will be mainly heated by convection. Because the heating speed of radiation heating is much faster than that of convection heating, the gears will be heated by convection A large temperature difference is formed between various parts of the workpiece, resulting in thermal stress.
How to control the distortion of gear during heat treatment?
1、 Gear blank isothermal normalizing. The dispersion of structure and hardness of gear blank after normal normalizing is large. Isothermal normalizing can first improve the uniformity of structure and hardness, and at the same time reduce the distortion caused by the uneven processing stress. In recent years, the application of isothermal normalizing of gear blanks has played a good role in controlling the distortion of gear heat treatment in China.
2、 Preheating, step heating. On the one hand, it aims to make the temperature of the surface, inside and all parts of the workpiece more uniform, and reduce the temperature difference with the target temperature; the most fundamental is to reduce the temperature difference of all parts of the workpiece. Based on the effect of preheating, the American military standard of "heat treatment process of steel" stipulates that the final machined parts shall be preheated at 540 ℃ or 650 ℃ before being heated to 740 ℃.
3、 After stress relief annealing, high temperature heat treatment such as carburizing is carried out. It is equivalent to dividing the original workpiece heated directly at high temperature into two sections for heating. The first section is heated far below the carburizing temperature. Due to the low heating temperature, the thermal stress is small. At the same time, the heating speed is low, and the temperature gradient is also small, which is not only conducive to reducing the thermal stress, but also conducive to the slow processing of stress release and distorted lattice recovery. Then high temperature heat treatment such as carburizing is carried out. Therefore, it is better to put the workpiece into the high temperature carburizing furnace after stress relief annealing, so as to get the best effect. In production, the existing tempering furnace can be used for this operation