The annealing process mainly includes the following types, each with its specific purpose and application scenarios:
Complete annealing:
Objective: To refine grain size, achieve uniform microstructure, eliminate internal stress and processing defects, reduce hardness, improve cutting performance and cold plastic deformation ability.
Operation: Heat the workpiece to 30-50 ℃ above the temperature at which all ferrite transforms into austenite, hold for a period of time, and then slowly cool with the furnace.
Spheroidization annealing:
Purpose: To reduce the high hardness of tool steel and bearing steel after forging.
Operation: Heat the workpiece to 20-40 ℃ above the temperature at which the steel begins to form austenite. After insulation, slowly cool it to transform the lamellar carbides in the pearlite into spherical shapes.
There are various specific processes for spheroidization annealing, such as ordinary (slow cooling) spheroidization annealing, isothermal spheroidization annealing, periodic spheroidization annealing, low-temperature spheroidization annealing, and deformation spheroidization annealing. Each process is suitable for different steel grades and processing requirements.
Isothermal annealing:
Purpose: To reduce the high hardness of certain alloy structural steels with high nickel and chromium content for cutting processing.
Operation: Generally, cool at a fast rate to the most unstable temperature of austenite, hold for an appropriate period of time, and austenite will transform into martensite or martensite, resulting in a decrease in hardness.
Graphite annealing:
Purpose: To transform cast iron containing a large amount of cementite into malleable cast iron with good plasticity.
Operation: Heat the casting to around 950 ℃, keep it warm for a certain period of time, and then cool it appropriately to decompose the cementite into clustered graphite.
Diffusion annealing:
Purpose: To eliminate dendritic segregation and regional segregation generated during the solidification process of ingots, and to achieve uniformity in composition and structure.
Operation: Heat the steel ingot, casting or forging blank to a temperature slightly below the solidus line for a long time, and then slowly cool it down. The heating temperature is usually 100200 ℃ above Ac3 or Accm, and the holding time is generally 1015 hours.
Stress relief annealing:
Purpose: To eliminate residual internal stress.
Operation: Heat the steel component to a temperature lower than Ac1 (usually 500-650 ℃), keep it warm, and then cool it in the furnace.
Recrystallization annealing:
Purpose: To transform the internal structure of metals into fine equiaxed grains, eliminate deformation hardening, and restore the plasticity and deformation ability of metals or alloys.
Operation: Heat the cold deformed metal above the recrystallization temperature and maintain it for an appropriate period of time.
Incomplete annealing, post weld annealing, and other annealing processes also have their specific applications and operating methods.
Overall, the choice of annealing process depends on the type of material, processing requirements, and expected material properties.