In the industrial production of annealing heat treatment, the annealing process is widely used. According to the different purposes of workpiece annealing, there are some differences in the annealing process, such as complete annealing, spheroidizing annealing, stress relief annealing and so on.
1. Complete annealing
It is used to refine the coarse overheating arrangement with poor mechanical function of medium and low carbon steel after forging, forging and welding. Heat the workpiece to 30 ~ 50 ℃ above the temperature at which all ferrite changes to austenite, keep it for a period of time, and then cool it slowly with the furnace. In the cooling process, austenite changes again, which can make the arrangement of steel thinner.
2. Spheroidizing annealing
Use the following to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20 ~ 40 ℃ above the temperature at which the steel begins to form austenite, and then cooled slowly after holding. During the cooling process, the lamellar cementite in pearlite becomes spherical, and then the hardness decreases.
3. Isothermal annealing. Use the following to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting. Usually, it is cooled to the unstable temperature of austenite at a faster speed. At the appropriate time of heat preservation, the austenite changes to troostite or sorbite, and the hardness can be reduced.
4. Recrystallization annealing
It is used to eliminate the hardening phenomenon of metal wire and sheet in the process of cold drawing and cold rolling (hardness increase and plasticity decrease). The heating temperature is usually 50 ~ 150 ℃ below the temperature at which the steel begins to form austenite. Only in this way can metallization be achieved in addition to the work hardening effect.
5. Graphitization annealing
It is used to make cast iron containing a lot of cementite into malleable cast iron with excellent plasticity. The technical operation is to heat the casting to 950 ℃ and properly cool it after holding for a certain time, so as to divide the cementite into flocculent graphite.
6. Dispersion annealing
It is used to homogenize the chemical composition of alloy castings and improve their use function. The method is to heat the casting to the highest temperature possible without melting, keep it warm for a long time, and cool it slowly after various elements in the alloy disperse and tend to be evenly distributed.
7. Stress relief annealing
It is used to the internal stress of steel castings and weldments. The temperature of austenite formed at the beginning of steel products after heating is 100 ~ 200 ℃ below the temperature, and the internal stress can be eliminated by cooling in the air after heat preservation.
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