The austenitic stainless steel is softened by solid solution treatment. Generally, the stainless steel pipe tubing is heated to about 950~1150℃ and kept for a period of time so that the carbides and various alloying elements are fully and uniformly dissolved in the austenite. Then it is quickly quenched and cooled, and carbon and other alloying elements are too late to precipitate to obtain a pure austenite structure, which is called solution treatment.
1) Make the structure and composition of the stainless steel pipe tubing 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 dissolution, 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 crystal grains are recrystallized, the internal stress is eliminated, the tensile strength of the stainless steel tube is reduced, and the elongation rate is increased.
3) Restore the inherent corrosion resistance of stainless steel pipes and tubes.
Due to the precipitation of carbides and lattice defects due to cold working, the corrosion resistance of stainless steel pipes and tubes is reduced. After solution treatment, the corrosion resistance of the steel pipe is restored to the best state.
For stainless steel pipes and tubes, 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, for stabilized steel, if the solid solution temperature is high, the carbide of the stabilizing element is fully dissolved in austenite, and will precipitate in the grain boundary in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. In order 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 pipe is a steel pipe that is not easy to rust. In fact, some stainless steel contains both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel pipes and tubes are due to the formation of chromium-rich oxide film (passivation film) on its surface. Among them, rust resistance and corrosion resistance are relative.
Experiments have proved that the corrosion resistance of steel in weak media such as the atmosphere and water and oxidizing media such as nitric acid will increase with the increase of the water content of chromium in the steel, which is directly proportional. When the chromium content reaches a certain percentage, the corrosion resistance of the steel changes suddenly, that is, from easy to rust to not easy to rust, and from no corrosion resistance to corrosion resistance.
