Classification and selection of annealing

Classification and selection of annealing

1. The difference between incomplete annealing and complete annealing

(1) Incomplete annealing is also called incomplete crystallization annealing. It is to heat the steel to a certain temperature between Ac1 and Ac3 or Ac1 and Acm, and then slowly cool down after heat preservation, so that the steel structure is incompletely recrystallized.

(2) Incomplete annealing is to heat the workpiece to semi-austenitized for annealing, while complete annealing is to heat the workpiece to fully austenitized for annealing.

(3) Incomplete annealing is generally used for hypereutectoid steels, and hypoeutectoid steels can also be used, while complete annealing is generally used for hypoeutectoid steels only.

(4) Complete annealing generally obtains flake pearlite, while incomplete annealing obtains spherical pearlite.

2. Application range of incomplete annealing

Incomplete annealing can be used for hypoeutectoid steel and hypereutectic steel. The characteristic is that the cementite of pearlite becomes spherical after annealing. This incomplete annealing is also called spheroidizing annealing.

For hypoeutectoid steels, incomplete annealing is often due to the following conditions:

(1) Improve cutting performance. This type of annealing is mainly used for forging structural parts, especially structural parts with high carbon content. Because the pearlite is too fine after forging, the hardness is too high and it is difficult to cut.

(2) Ball retreat to improve cold deformation performance. This type of annealing is used for hypoeutectoid steels that require cold deformation.

3. The difference between isothermal annealing and ordinary annealing

There is a certain difference between isothermal annealing and ordinary annealing in the process and cooling method. Ordinary annealing generally involves slow cooling of the workpiece with the furnace after the heat preservation process is completed. When the workpiece is cooled to below 500 degrees Celsius, it can be air cooled. Isothermal annealing is to cool the workpiece to a temperature below Ac1 for a period of time after heating and holding the workpiece at a faster cooling rate to isothermally decompose austenite into pearlite, and then cool it to room temperature at a faster cooling rate (air cooling). Isothermal annealing is short in time and good in quality.

4. Cooling method of isothermal annealing

(1) The cooling rate of the workpiece from annealing temperature to isothermal temperature can be arbitrary, and it is usually carried out in two furnaces in production.

(2) The isothermal temperature is generally 10-30 degrees Celsius below Ar1, which is the pearlite transition temperature.

(3) The cooling after the transformation is optional.

5. Ball annealing method

(1) Ordinary spherical annealing: heat the steel to a slightly higher temperature than Ac1 (generally Ac1 + 20-30 degrees Celsius), and then slowly cool it at 20-50 degrees Celsius/h to below 500 degrees Celsius after heat preservation, and then air-cool it out of the furnace.

(2) Isothermal ball annealing: After heat preservation at the same heating temperature as the ordinary ball retreat, the furnace is cooled to slightly lower than the Ar1 temperature for isothermal, and after the isothermal is completed, the furnace is air cooled.

(3) Periodic spherical annealing: it is heating and cooling alternately above and below Ac1, which is equivalent to multiple isothermal spherical annealing of cold.