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annealing classification and selection
1. The difference between incomplete annealing and complete annealing
(1) Incomplete annealing is also called incomplete crystallization annealing. The temperature is the temperature between AC1 and AC3 or AC1 and ACM in the heating channel of steel, which is cooled slowly after heat preservation, resulting in incomplete recrystallization in the steel structure.
(2) Incomplete annealing is to heat the workpiece to semi austenitizing for annealing, while complete annealing is to heat the workpiece to fully austenitizing for annealing.
(3) Hypoeutectoid and hypereutectoid steels usually use incomplete annealing, while hypoeutectoid steels usually only use complete annealing.
(4) Generally, lamellar pearlite is obtained by complete annealing, while spherical pearlite is obtained by incomplete annealing.
2. Application scope of incomplete annealing
Incomplete annealing can be used in hypoeutectoid and hypereutectic steels. It is characterized in that after annealing, the cementite of pearlite becomes spherical, which is also called spheroidizing annealing.
For hypoeutectoid steels, incomplete annealing is usually caused by the following conditions:
(1) This kind of annealing is mainly used for forging structural parts, especially for structural parts with high carbon content. Because the pearlite after forging is too fine, so the hardness is too high to cut.
(2) This annealing is used for hypoeutectoid steels requiring cold deformation.
3. Isothermal annealing and ordinary annealing
There are some differences between isothermal annealing and ordinary annealing in process and cooling method. Common annealing usually refers to the slow cooling of the workpiece in the furnace after the heat preservation process. When the workpiece is cooled to below 500 ℃, it can be discharged into the furnace for air cooling. Isothermal annealing is to cool austenite to a specific temperature lower than AC1 for a period of time at a faster cooling rate after heating and holding the workpiece, so as to decompose austenite into pearlite under isothermal condition, and then cool to room temperature at a faster cooling rate (air cooling). The characteristics of isothermal annealing are short time and good quality.
4. Cooling method of isothermal annealing
(1) The cooling rate of workpiece from annealing temperature to isothermal temperature can be set arbitrarily, which is usually carried out in two furnaces in production.
(2) The isothermal temperature is generally 10-30 ℃ lower than that of AR1.
(3) The cooling after transformation can be carried out at will
5. How to return the ball
(1) Ordinary ball annealing: the steel parts are heated to a slightly higher temperature and AC1 (usually AC1 + 20-30 ℃), slowly cooled to below 500, kept at 20-50 ℃ / h, and cooled out of the furnace with air.
(2) Isothermal ball annealing: after maintaining the same heating temperature as ordinary ball annealing, the furnace is cooled to a temperature slightly lower than AR1 temperature for isothermal treatment, and the furnace is air-cooled after isothermal treatment.
(3) Regular ball taking: alternately heating and cooling on AC1, equivalent to cold multiple isothermal ball taking.