Annealing process of seamless steel pipe
1. Fully annealed. It is used to refine the coarse and superheated structure with poor mechanical properties in medium and low carbon steels after casting, forging and welding. The workpiece is heated to a temperature between 30 and 50 ° C above the temperature at which ferrite is completely transformed into austenite, and is held for a period of time, and then slowly cooled with the furnace. During the cooling process, the austenite is transformed again, which can make the steel structure fine .
2. Spheroidizing annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to a temperature between 20 and 40 ° C above the temperature at which the steel begins to form austenite, 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 sheets during cold drawing and cold rolling. The heating temperature is generally 50 to 150 ° C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be 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. Used to homogenize the chemical composition of alloy castings and improve their 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 steel castings and weldments.
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