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The role of solution treatment of high-precision cold drawn tubes
For high-precision cold drawn tubes, the three elements of solution treatment are temperature, holding time and cooling rate. The functions of solid solution treatment of precision cold drawn tubes are as follows:
1: Make the structure and composition of high-precision cold-drawn pipes uniform, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of hot-rolled pipes are different, resulting in inconsistent organizational structures. At high temperatures, atomic activity intensifies, the σ phase dissolves, and the chemical composition becomes uniform. After rapid cooling, a uniform single-phase structure is obtained.
2: Eliminate work hardening to facilitate continued cold working. Through solid solution treatment, the distorted crystal lattice is restored, the elongated and broken grains are recrystallized, the internal stress is eliminated, the tensile strength of the steel wire decreases, and the elongation rate increases.
3: Restore the inherent corrosion resistance of high-precision cold drawn tubes. Due to the precipitation of carbides and lattice defects caused by cold working, the corrosion resistance of precision cold drawn tubes is reduced. After solution treatment, the corrosion resistance of precision cold drawn pipes is restored to its best state.
The solid solution temperature is mainly determined based on the chemical composition. Generally speaking, for grades with many types and high content of alloying elements, the solid solution temperature should be increased accordingly. Especially for steel with high content of manganese, molybdenum, nickel and silicon, the softening effect can only be achieved by raising the solid solution temperature and fully dissolving it. However, in stabilized steel, such as high-precision cold-drawn pipes, when the solid solution temperature is high, the carbides of the stabilizing elements are fully dissolved in the austenite, and will precipitate in the form of Cr23C6 at the grain boundaries during subsequent cooling, causing intergranular corrosion. In order to prevent the carbides (TiC and NbC) of stabilizing elements from decomposing or solid solution, the lower limit solid solution temperature is generally adopted.
1: Make the structure and composition of high-precision cold-drawn pipes uniform, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of hot-rolled pipes are different, resulting in inconsistent organizational structures. At high temperatures, atomic activity intensifies, the σ phase dissolves, and the chemical composition becomes uniform. After rapid cooling, a uniform single-phase structure is obtained.
2: Eliminate work hardening to facilitate continued cold working. Through solid solution treatment, the distorted crystal lattice is restored, the elongated and broken grains are recrystallized, the internal stress is eliminated, the tensile strength of the steel wire decreases, and the elongation rate increases.
3: Restore the inherent corrosion resistance of high-precision cold drawn tubes. Due to the precipitation of carbides and lattice defects caused by cold working, the corrosion resistance of precision cold drawn tubes is reduced. After solution treatment, the corrosion resistance of precision cold drawn pipes is restored to its best state.
The solid solution temperature is mainly determined based on the chemical composition. Generally speaking, for grades with many types and high content of alloying elements, the solid solution temperature should be increased accordingly. Especially for steel with high content of manganese, molybdenum, nickel and silicon, the softening effect can only be achieved by raising the solid solution temperature and fully dissolving it. However, in stabilized steel, such as high-precision cold-drawn pipes, when the solid solution temperature is high, the carbides of the stabilizing elements are fully dissolved in the austenite, and will precipitate in the form of Cr23C6 at the grain boundaries during subsequent cooling, causing intergranular corrosion. In order to prevent the carbides (TiC and NbC) of stabilizing elements from decomposing or solid solution, the lower limit solid solution temperature is generally adopted.