The key points of isothermal spheroidizing annealing furnace are as follows: ① austenitizing temperature as low as possible (slightly higher than AC1); ② isothermal temperature as high as possible; ③ sufficient isothermal time to complete transformation spheroidization
Isothermal annealing process of die steel the isothermal annealing process of die steel is to heat the die steel to be annealed above the critical temperature (hypoeutectic steel is heated to above AC3, eutectoid steel hypereutectoid steel is heated to above AC1) to maintain appropriate manufacturing time. Austenite austenite are homogeneous. Then it is put into another furnace slightly lower than AC1, placed in the original heating furnace, the die steel is rapidly cooled to a temperature slightly lower than AR1 for constant temperature. In the isothermal process, austenite will be transformed into pearlite spheroid with appropriate thickness according to the isothermal temperature used. After the conversion, the air cooling device can be removed the furnace. Isothermal annealing process of die steel includes three stages: austenite heating holding: rapid cooling to isothermal temperature holding time; air cooling after release furnace. The selection of austenitizing temperature is only related to die steel, but also adjusted according to technical requirements original structure of die steel. For example, higher austenitizing temperature can form layered structure. Spheroidization can be easily obtained at lower austenitizing temperature. The isothermal temperature of austenitized die steel should be determined according to the characteristics described later the Austenite Isothermal Transformation of steel grade. For example, the closer the isothermal temperature is to A1, the thicker the pearlite layer is obtained (the lower the hardness of die steel). The farther away A1, the thinner the pearlite layer (the higher the hardness of steel). Therefore, in order to obtain soft tissue, lower austenitizing temperature higher isothermal temperature can be used. However, it should be noted that in the selection of isothermal temperature, the time for filtering austenite to complete pearlite transformation should also be considered, that is, the isothermal temperature requiring a shorter time obtaining the required hardness should be selected. The more, the better. In addition, the holding time of steel at isothermal temperature should be longer than that indicated on isothermal transformation diagram to ensure transformation of undercooled austenite. This is especially true for steels with larger cross-sections, the austenitizing temperature takes a period of time to cool before the core is cooled to the isothermal temperature. After isothermal transformation, the structure has been completed. At this point, the steel is removed the furnace. No matter which cooling method is used, its structure will change. However, if the cooling rate is too high, the steel may deform due to force. Therefore, it is usually cooled in air. During annealing. There are high requirements for temperature control accuracy.
Isothermal annealing can also be used to prevent the formation of white spots. The alloy carburizing steel used in mould should also be annealed at isothermal temperature. The austenitizing temperature is 930-940 ℃, which is slightly higher than the subsequent carburizing temperature. The purpose of this treatment is to reduce any deformation that occurs during carburizing. ~ 4H conversion completed. It's made of iron pearlite. Suitable for most cutting processes. Isothermal annealing can be used at all stages of steel production, such as high alloy ingots hot rolled billets, which are prone to fracture when freely cooled to room temperature. The temperature is maintained in an isothermal annealing furnace at about 700 ° C, which is equal to the pearlite transformation temperature of the steel, after the transformation, it is freely cooled to room temperature. Many steels will spheroidize cementite during isothermal annealing, so isothermal annealing spheroidizing annealing are sometimes confused, which is usually called isothermal spheroidizing annealing. The key points of isothermal spheroidizing annealing are as follows: ① the austenitizing temperature as low as possible (slightly higher than AC1); ② the isothermal temperature as high as possible; ③ long enough isothermal time to complete the transformation spheroidization.
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