Why do stainless steel pipes need solution annealing

Austenitic stainless steel is softened by solid solution treatment. Generally, the stainless steel pipe is heated to about 950~1150℃, and kept for some time, so that the carbides and various alloying elements are fully and evenly dissolved in the austenite, and then quickly quenched to cool. , carbon and other alloying elements have no time to precipitate, and a pure austenite structure is obtained, which is called solid solution treatment.

The role of solid solution treatment is threefold.
1. Make the structure and composition of the steel pipe uniform, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of the hot-rolled wire rod are different, resulting in inconsistent organizational structure. 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 pipe decreases, and the elongation rate increases.
3. Restore the inherent corrosion resistance of stainless steel. Due to the precipitation of carbides and lattice defects caused by cold working, the corrosion resistance of stainless steel decreases. After solution treatment, the corrosion resistance of the steel pipe returns to its original 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 based on the chemical composition. Generally speaking, for grades with many types of alloying elements and high content, the solid solution temperature should be increased accordingly. Especially for steel with a 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, when the solid solution temperature of stabilized steel, such as 1Cr18Ni9Ti, 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. To prevent the carbides (TiC and Nbc) of stabilizing elements from decomposing or solid solution, the lower limit solid solution temperature is generally adopted.

As the saying goes, stainless steel is steel that does not rust easily. Some stainless steels have both stainless properties and acid resistance (corrosion resistance). The stainless steel and corrosion resistance of stainless steel is due to the formation of a chromium-rich oxide film (passivation film) on its surface. Among them, stainlessness and corrosion resistance are relative.
Experiments have shown that the corrosion resistance of steel in weak media such as the atmosphere, or water, and oxidizing media such as nitric acid will increase with the increase in the chromium water content in the steel, which is proportional to the increase. When the chromium content reaches a certain percentage At this time, the corrosion resistance of steel undergoes a sudden change, that is, from easy to rust to not easy to rust, from not resistant to corrosion to corrosion resistant.