What causes the uneven hardness of induction quenching? Commonly used current frequencies

During the heat treatment of steel, there will be some poor product results. The following Kehua editor will share with you about the uneven effect of high frequency quenching hardness. What is the reason? The following will introduce you one by one. High-frequency quenching uneven hardness About high-frequency quenching High-frequency quenching is mostly used for surface quenching of industrial metal parts. Induction heating equipment, that is, equipment for induction heating of workpieces for surface quenching. The principle of induction heating: the workpiece is placed in the inductor, and the inductor is generally a hollow copper tube inputting intermediate frequency or high frequency alternating current (1000-300000Hz or higher). The alternating magnetic field generates an induced current of the same frequency in the workpiece. The distribution of this induced current in the workpiece is uneven, it is strong on the surface, but very weak in the interior, and it is close to 0 in the center. This skin effect is used. , the surface of the workpiece can be heated rapidly, the surface temperature rises to 800-1000 ℃ in a few seconds, and the temperature of the core part is very small. Frequency selection The relationship between the depth δ (mm) of the induced current flowing into the surface of the workpiece at room temperature and the current frequency f (HZ) is that the frequency increases, the current penetration depth decreases, and the hardening layer decreases. The commonly used current frequencies are: 1. High-frequency heating: 100-500KHZ, commonly used 200-300KHZ, it is a tube type high-frequency heating, the hardened layer depth is 0.5-2.5mm, suitable for small and medium-sized parts. 2. Intermediate frequency heating: The current frequency is 500~10000HZ, commonly used 2500~8000HZ, and the power supply equipment is a mechanical intermediate frequency heating device or a thyristor intermediate frequency generator. The depth of the hardened layer is 2 to 10 mm. Suitable for larger diameter shafts, medium and large gears, etc. 3. Power frequency heating: the current frequency is 50HZ. Using mechanical power frequency heating power supply equipment, the depth of the hardened layer can reach 10-20mm, which is suitable for surface quenching of large-diameter workpieces. Uneven hardness of induction quenching Uneven hardness of induction quenching Generally similar to the situation of insufficient hardness, there are two main reasons, insufficient cooling or excessive cooling. For specific operations, first determine whether the cooling is insufficient, and confirm whether the workpiece after quenching has more residual heat. The method of judgment is to check whether the temperature of the workpiece after quenching is too high. The principle is generally the temperature of the workpiece. You can also check the temperature. If the temperature is high, it means that the residual heat is large, and the hardness of the workpiece after tempering is low. There is also too much cooling, that is, the temperature does not fully reach the required quenching temperature, or part of the quenching temperature is reached, and the cooling is extremely rapid, which will also lead to low hardness in some parts. You can also indirectly find out the reason for insufficient hardness by checking the microscopic metallographic structure of the workpiece to see the structure. Why is it said that high-frequency quenching is better than the ancient quenching hardness? Compared with ordinary heating and quenching, high-frequency quenching is mainly due to the extremely fast heating speed, short transformation time, and refinement of austenite grains. In this way, the surface of the workpiece can be obtained after quenching. Very fine cryptocrystalline martensite, under the same conditions, fine grains have higher strength and hardness than coarse grains, so the hardness is higher. Secondly, due to the short heating time of the high-frequency quenched workpiece, the surface is not easy to be oxidized and decarburized, and its original carbon content is maintained, and the carbon content will not be reduced due to decarburization, which is also a reason. To sum up, Kehua Xiaobian shared the relevant knowledge about quenching for everyone, and also analyzed the reasons for the uneven hardness of high-frequency quenching for everyone. You can make improvements according to the situation.