Isothermal annealing greatly reduces time

There may be many people who don't know the concept of isothermal annealing. After all, we have not come into contact with these things in our lives. They are generally used in industry, so it is understandable. Let me introduce the relevant information together with Kehua. . Isothermal annealing isothermal annealing isothermal annealing is a new annealing method used to replace complete annealing and incomplete annealing. Air cooling, the structure and properties obtained after treatment are also very similar to each other, but the time required for treatment can often be greatly shortened. Due to the difference in the composition of the steel, the purpose of annealing is different. The treatment methods are also often different. Commonly used are so-called complete annealing, incomplete annealing, diffusion annealing, spheroidizing annealing, stress relief annealing, etc. Points to note In order to obtain the expected effect of the steel after isothermal annealing, the following points should be paid attention to during operation: (1) The heating temperature of isothermal annealing should be determined according to the composition, technical requirements and original state of the steel. Hypo-eutectoid steels use fully annealed heating temperatures (above the heating temperature), and hypereutectoid steels use incomplete annealed heating temperatures (above the heating temperature). The temperature required to improve the microstructure is high, so that it can obtain uniform austenite, which is beneficial to obtain uniform transformation products. The temperature that is purely required to reduce the hardness is low, so that it can obtain uneven austenite, which is conducive to accelerated transformation, which is particularly important for alloy steel and high alloy tool steel. The original structure is fine and uniform, because austenite is easy to form, so a lower temperature can be used. (2) Generally speaking, the heating rate has little effect on the result of steel after annealing, but it should not be too fast. When heating, it can be calculated at a thickness of 1.5~1.8 minutes/mm for carbon steel, 1.8~2.0 minutes/mm for alloy steel, and 100~200℃/hour for high alloy steel. The holding time of steel is usually calculated by one-third or one-half of the heating time of steel. For alloy steels requiring reduced hardness, it is better to keep the holding time shorter, so that the isothermal holding time during cooling can be shortened. (3) The amount of furnace charging for steel annealing is often large. In order to ensure uniform heating of the workpiece and avoid insufficient heating or overheating, when charging, select workpieces with the same size and thickness as possible, and carry out in the same furnace. annealing. When the workpiece size is large and the furnace capacity is large, after entering the furnace, when the temperature rises to 500~650℃, it should be kept for a certain time (carbon steel and low alloy steel can be 1~2 hours, high alloy steel can be 3 to 4 hours), so that the workpieces in the furnace are heated evenly. Isothermal annealing (4) The isothermal holding temperature of the workpiece is mainly selected according to the requirements of the workpiece. The lower the hardness is, the higher the isothermal holding temperature is, and the temperature of 20~30℃ is usually selected. The specific temperature should be based on the austenite isothermal transformation curve of the processed steel, and the temperature range in which the supercooled austenite transformation is completed with the shortest time and the required hardness can be obtained. The isothermal holding time should be based on the complete transformation of supercooled austenite into transformation products at this temperature as the lower limit, which can be found from the austenite isothermal transformation curve. However, considering that it takes a certain amount of time to cool the workpiece to the temperature of isothermal maintenance, and in order to complete the transformation of each workpiece inside and outside the furnace, the isothermal holding time should be longer than the time found above. Usually, the isothermal holding time of carbon steel can be 1~2 hours, and that of alloy steel is 3~4 hours. For high alloy steels, a longer isothermal hold is required. (5) After the workpiece has been transformed by isothermal maintenance, the cooling method after being released has no effect on the properties of the steel. However, it is considered that water cooling or oil cooling may cause large thermal stress to the workpiece to bend and deform the workpiece, and its operation process is quite heavy. If furnace cooling is used, the productivity of the furnace will be greatly reduced, so air cooling is usually used. In this way, the productivity of the furnace will not be reduced, and the workpiece will not be deformed due to excessive internal stress, and the operation process is also very simple. However, it should be pointed out that for isothermal annealing to prevent the formation of white spots, after isothermal maintenance, the furnace must be cooled, and it must be slowly cooled to about 100 °C before it can be released from the furnace. There are many operations of isothermal annealing, and each step must not be wrong, otherwise it will affect the quality of your product, so we need to know some precautions to better grasp its temperature.