On 2018-08-08 14:48:54
Annealing heat treatment process
Annealing heat treatment is a metal heat treatment process that heats the workpiece to a predetermined temperature and cools it slowly after holding for a certain time.
The purpose of annealing is to:
1. Improve or eliminate all kinds of structural defects and residual stress caused by steel in the process of casting, forging, rolling and welding to prevent deformation and cracking of workpiece.
2. Soften the workpiece for cutting.
3. Refine the grain and improve the structure to improve the mechanical properties of the workpiece.
4. Make organization preparation for final heat treatment (quenching and tempering).
The commonly used annealing and heat treatment processes are:
1. Fully annealed. It is used to refine the coarse and overheated structure of medium and low carbon steel with poor mechanical properties after casting, forging and welding. When the workpiece is heated to a temperature of 30-50 ℃ above the temperature of ferritic transformation to austenite, it is kept warm for a period of time, and then cooled slowly with the furnace. During the cooling process, the austenite transforms again, which can make the structure of steel finer.
2. Spheroidizing annealing. 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. After heat preservation, it is cooled slowly. During the cooling process, the lamellar cementite in pearlite becomes spherical, thus reducing the hardness.
3. Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with high content of nickel and chromium for cutting. Generally, the hardness of austenite can be reduced by cooling to the most unstable temperature of austenite at a fast speed, holding for a proper time, and transformation of austenite into troostite or sorbite.
4. Recrystallization annealing. It is used to eliminate the hardening phenomenon (increase of hardness and decrease of plasticity) in the process of cold drawing and cold rolling. The heating temperature is generally 50-150 ℃ below the temperature when the steel begins to form austenite. Only in this way can the work hardening effect be eliminated to soften the metal.
5. Graphitization annealing. It is used to make the cast iron with a large amount of cementite into malleable cast iron with good plasticity. The process is to heat the casting to about 950 ℃, cool it properly after holding for a certain time, and decompose the cementite to form flocculent graphite.
6. Diffusion annealing. It is used to homogenize the chemical composition of alloy casting and improve its service performance. The method is to heat the casting to as high temperature as possible without melting, keep it warm for a long time, and cool it slowly after the diffusion of various elements in the alloy tends to uniform distribution.
7. Stress relief annealing. To eliminate the internal stress of steel castings and weldments. For iron and steel products, the temperature below which austenite begins to form after heating is 100-200 ℃, and the internal stress can be eliminated by cooling in air after heat preservation.
Annealing in order to eliminate the internal stress of plastic products or control the crystallization process, the products are heated to a proper temperature and kept for a certain time, and then cooled slowly.