Annealing of cs seamless pipe refers to a heat treatment process in which the steel pipe is heated to the annealing temperature and kept for a certain period of time, and then slowly cooled to a certain temperature with the furnace and then cooled. The steel pipe annealing process can be divided into: spheroidizing annealing, complete annealing, stress relief annealing, etc.

The main functions of cs seamless pipe annealing:

1) Reduce the hardness of the steel pipe and improve its plasticity to facilitate subsequent cutting or cold deformation processing;

2) Refine grains, eliminate structural defects, uniform internal structure and composition, and improve the performance of steel pipes Or prepare for subsequent procedures;

3) Eliminate the internal stress of the steel pipe to prevent deformation or cracking.

General principles of annealing

1) When the cs seamless pipe is heated, the recovery and recrystallization process will occur, and the driving force is the cold deformation energy storage, that is, the free energy increment after cold deformation. When heated, the structure of the metal will transition to an equilibrium state. The heat treatment that transforms the cold-deformed metal to an equilibrium state is called annealing.

2) When the annealing temperature is low and the annealing time is short, the main process of cold deformation metal is recovery.

3) From a certain temperature, the microstructure of cold deformation changes significantly, and new grains can also be observed under an optical microscope with a small magnification. This phenomenon is called recrystallization.

4) After the recrystallized grains are formed, if you continue to prolong the holding time or increase the heating temperature, the recrystallized grains will be coarsened.

The annealing process parameters mainly include heating rate, annealing temperature, holding time and cooling rate. Its influence on product organization and performance is as follows:

1. Heating speed
When the cs seamless pipe is annealed, rapid heating should be used as much as possible. This is because the steel pipe has good thermal conductivity and has the conditions for rapid heating. The more important reason is that when heating rapidly, generally finer grains can be obtained. When heating slowly, the grains are easy to be coarse, and the coarse grains will reduce the deep drawing performance and surface quality of the product.

2. Annealing temperature
The annealing temperature of CS seamless pipe has the most obvious effect on the microstructure and properties. The work hardened metal is produced by cold deformation. According to the different heating temperature, the change process of the structure and properties of the steel pipe can be divided into three stages: recovery, recrystallization and grain growth.

1) Recovery stage: When observed with an optical microscope, there is no change in the internal structure of the steel pipe. At this time, the strength of the steel pipe is slightly reduced, the plasticity is slightly increased, and the internal stress is significantly reduced.

2) Recrystallization stage: Recrystallization is: when the annealing temperature of the steel pipe is high enough and the time is long enough, in the microstructure of the deformed metal or alloy, new strain-free grains—recrystallization cores are generated. The new grains continue to grow until the original deformed structure completely disappears, and the properties of the metal or alloy also change significantly. This process is called recrystallization; when the steel pipe is heated to the starting recrystallization temperature, the cold deformation of the metal and alloy occurs. On the substrate, new grains began to form. With the increase of the heating temperature of the steel pipe or the extension of the holding time, the number of new grains increases continuously until all new recrystallized grains are formed. At this stage, the fibrous structure (elongated grains) is transformed into a recrystallized structure (equiaxed recrystallized grains). At this time, the phenomenon of work hardening is completely eliminated, the strength of the metal drops sharply, and the plasticity increases significantly.

3) Grain growth: After the cold-deformed metal is completely recrystallized, uniform and fine equiaxed grains can generally be obtained. However, if the heating temperature is too high or the heating time is too long, the new grains after recrystallization will merge and grow, making the grains coarser and the mechanical properties of the metal correspondingly deteriorated.

3. Holding time
When the heating temperature is not high, that is, when the metal is in the recovery stage, the effect of the holding time on the tissue properties is not obvious. However, when the metal is in the recrystallization stage, if the holding time is too long, the grains will become coarse and the mechanical properties will also deteriorate accordingly.

4. Cooling speed
For pure aluminum heat-treated non-strengthened alloys, the cooling rate has little effect on the microstructure and properties. Therefore, the method of cooling with the furnace or slow cooling can be adopted during annealing.