Annealing is a metal heat treatment process that refers to slowly heating a metal to a certain temperature, holding it for a sufficient time, and then cooling it at an appropriate rate.
The purpose of annealing:
(1) Reduce hardness and improve machinability;
(2) Reduce residual stress, stabilize dimensions, and minimize deformation and crack tendency;
(3) Refine grain size, adjust structure, and eliminate structural defects;
(4) Uniform material organization and composition can improve material properties or prepare the organization for future heat treatment. Annealing processes include: complete annealing, spheroidization annealing, isothermal annealing, graphite annealing, diffusion annealing, stress relief annealing, incomplete annealing, post weld annealing, etc.
The specific processes of three commonly used annealing methods:
1. Fully annealed
Objective: To refine grain size, achieve uniform microstructure, eliminate internal stress and processing defects, reduce hardness, improve cutting performance and cold plastic deformation ability.
Used to refine the coarse overheated structure of medium and low carbon steel with poor mechanical properties after casting, forging, and welding. 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. During the cooling process, austenite undergoes another transformation, which can make the structure of the steel finer.
2. Stress relief annealing
Used to eliminate internal stress in steel castings and welded parts. For steel products that begin to form austenite at temperatures below 100-200 ℃ after heating, internal stress can be eliminated by cooling in air after insulation.
3. Incomplete annealing
The heating temperature is between Ac1~Accm, and the cooling rate is above 500-600 ℃. Carbon steel is 100-200 ℃/h, alloy steel is 50-100 ℃/h, and high alloy steel is 20-60 ℃/h. It is mainly used for hypereutectoid steel.
Quality inspection standards for annealing:
Inspection scope: 1. Annealing temperature, heating time, and holding time. 2. Surface quality and hardness testing of annealed products.