The role of solution treatment has three points.
1. Make the structure and composition of the steel pipe uniform, which is especially important for the raw materials, because the rolling temperature and cooling rate of each section of the hot-rolled wire rod are different, resulting in inconsistent structure.
At high temperatures, the atomic activity intensifies, the σ phase dissolves, the chemical composition tends to be uniform, and a uniform single-phase structure is obtained after rapid cooling.
2. Eliminate work hardening to facilitate continued cold working.
Through the 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 pipe is reduced, and the elongation rate is increased.
3. Restore the inherent corrosion resistance of stainless steel.
Due to the precipitation of carbides and lattice defects due to cold working, the corrosion resistance of stainless steel is reduced. After the solution treatment, the corrosion resistance of the steel pipe is restored to the state.
For stainless steel pipes, the three elements of solution treatment are temperature, holding time, and cooling rate. The solid solution temperature is mainly determined according to the chemical composition.
Generally speaking, the solid solution temperature should be increased correspondingly for the grades with many kinds of alloying elements and high content. Especially for steels with high manganese, molybdenum, nickel, and silicon content, the softening effect can only be achieved by increasing the solid solution temperature and making it fully dissolved.
However, stabilized steel, such as 1Cr18Ni9Ti, when the solid solution temperature is high, the carbides of the stabilizing elements are fully dissolved in austenite and will precipitate in the grain boundaries in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. To prevent the carbides of stabilizing elements (TiC and NBC) from decomposing and solid solution, the lower limit solid solution temperature is generally adopted.
As the saying goes, stainless steel is steel that is not easy to rust. Some stainless steel contains both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. Among them, rust resistance and corrosion resistance are relative.