Annealing is a metal heat treatment process in which a weathering steel plate is slowly heated to a certain temperature, maintained for a sufficient time, and then cooled at an appropriate rate.
Annealing heat treatment can be divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of the annealed material can be tested by tensile tests or hardness tests. Many weathering steel plates are supplied in an annealed heat treatment state. The hardness testing of weathering steel plates can use Rockwell hardness testers to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, surface Rockwell hardness testing can be used. HRT hardness.
The common annealing process of weathering steel plate is as follows:1. Complete annealing. It is used to refine the coarse superheated structure with poor mechanical properties after casting, forging and welding of medium and low carbon steel. The weathering steel plate
SPA- H is heated to a temperature of 30-50 ° C above the temperature at which ferrite is completely transformed into austenite. After being held for a period of time, it is slowly cooled with the furnace. During the cooling process, the austenite is transformed again, which can change the structure of the steel.
2. Spheroidizing annealing. To reduce the high hardness of tool steel and bearing steel after forging. The weathering steel sheet is heated to a temperature of 20 to 60 ° C above the temperature at which austenite starts to form, and is slowly cooled after the heat preservation. During the cooling process, the lamellar cementite in pearlite becomes spherical, thereby reducing the hardness.
3. Isothermal annealing. Used to reduce the high hardness of some alloy structural steels with higher nickel and chromium content for cutting. Generally, it is first cooled to the most unstable temperature of austenite at a relatively rapid rate, and after holding for an appropriate time, the austenite is transformed into toustenite or sorbite, and the hardness can be reduced.
4. Recrystallization annealing. It is used to eliminate the hardening phenomenon (increased hardness and decreased plasticity) of metal wires and weathering steel plates during cold drawing and cold rolling. The heating temperature is generally 50 to 180 ° C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.
5. Graphitizing annealing. Used to make cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The technical operation is to heat the casting to about 950 ° C, and then cool it properly after holding for a certain period of time to decompose the cementite to form flocculent graphite.
6. Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve its performance. The method is to heat the casting to the highest possible temperature without melting, and keep it warm for a long time, and then slowly cool after the various elements in the alloy tend to be uniformly distributed.
7. Stress relief annealing. Used to eliminate the internal stress of weathering steel plates and weldments. For steel products, the temperature at which the austenite starts to form is 100 to 300 ° C below the temperature. After cooling, it is cooled in the air to eliminate internal stress.
What is the purpose of weathering steel sheet annealing ?
1. Improve or eliminate various structural defects and residual stresses caused by weathering steel plates during casting, forging, rolling and welding to prevent workpiece deformation and cracking
2. Soften the workpiece for cutting.
3. Refine the grains and improve the structure to improve the mechanical properties of the workpiece.
4. Prepare the organization for the final heat treatment (quenching, tempering).
