Seamless pipe annealing process

Seamless pipe annealing process

One of the most important process parameters for annealing seamless steel pipes is the maximum heating temperature (annealing temperature). The selection of annealing heating temperature for most alloys is based on the phase diagram of the alloy system. For example, the iron-carbon balance diagram for carbon steel is Base. The annealing temperatures of various steels (including carbon steel and alloy steel) vary depending on the specific annealing purpose. The annealing temperature of various non-ferrous alloys is a temperature below the solidus temperature of the alloy and above or below the solid solubility line temperature.

General principles of annealing

When the steel pipe is heated, recovery and recrystallization processes will occur. The driving force is the cold deformation energy storage, that is, the free energy increase after cold deformation. When heated, the metal's organizational structure will transform to an equilibrium state. The heat treatment that transforms a cold-deformed metal into an equilibrium state is called annealing.

When the annealing temperature is low and the annealing time is short, the main process that occurs in cold deformed metal is recovery.

Starting from a certain temperature, the cold deformation microstructure changes significantly, and new grains can be observed under an optical microscope with a moderate magnification. This phenomenon is called recrystallization.

After the recrystallized grains are formed, if the holding time is continued to be extended or the heating temperature is increased, the recrystallized grains will become coarser.

Effect of annealing process parameters on product structure and properties

Annealing process parameters mainly include heating rate, annealing temperature, holding time and cooling rate. Its impact on the product structure and performance is as follows:

1. Heating speed

When annealing steel pipes, rapid heating should be used as much as possible. This is because steel pipes have good thermal conductivity and are capable of rapid heating. The more important reason is that when heated rapidly, smaller grains can generally be obtained. When heated slowly, the grains tend to become coarse, and coarse grains will reduce the deep drawing performance and surface quality of the product.

2. Annealing temperature

The annealing temperature of steel pipe has the most obvious impact on the structure and properties. Work-hardened metal is produced after cold deformation. Depending on the heating temperature, the change process of the steel pipe structure and properties can be divided into three stages: recovery, recrystallization and grain growth.

① Recovery stage: When observed with an optical microscope, no changes in the internal structure of the steel pipe can be seen. At this time, the strength of the steel pipe is slightly reduced, the plasticity is slightly improved, and the internal stress is significantly reduced.

② Recrystallization stage: Recrystallization means: when the annealing temperature of the steel pipe is high enough and the time is long enough, new strain-free grains—recrystallization cores—are produced in the microstructure of the deformed metal or alloy. 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, new grains begin to form on the matrix of cold deformed metal and alloy. As the heating temperature of the steel pipe increases or the holding time is extended, the number of new grains continues to increase until all new recrystallized grains are formed. At this stage, the fibrous structure (elongated grains) transforms into a recrystallized structure (equiaxed recrystallized grains). At this time, the work hardening phenomenon is completely eliminated, the strength of the metal drops sharply, and the plasticity is significantly improved.

③Grain growth: After complete recrystallization of cold deformed metal, 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, causing the grains to become coarser and the mechanical properties of the metal to deteriorate accordingly.

3. Keeping time

When the heating temperature is not high, that is, when the metal is in the recovery stage, the holding time has little effect on the tissue properties. 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 accordingly deteriorate.

4. Cooling speed

For alloys that cannot be strengthened by heat treatment of pure aluminum, the cooling rate has little effect on the structural properties. Therefore, the method of cooling with the furnace or slow cooling after exiting the furnace can be used during annealing.

Article source: https://article-realm.com/article/Business/61856-Seamless-pipe-annealing-process.html