Why should stainless steel pipe be a solid solution annealed

Austenitic stainless steel is softened by solid solution treatment. Generally, the stainless steel tube is heated to about 950-1150°C and kept for some time, so that carbides and various alloying elements are fully and evenly dissolved in austenite, and then quickly quenched and cooled. , carbon and other alloying elements are too late to precipitate, and a pure austenite structure is obtained, which is called solid solution treatment.

The effect of solid solution treatment has three points.

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 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 solution treatment, the distorted lattice recovers, the elongated and broken grains recrystallize, the internal stress is eliminated, the tensile strength of the steel pipe decreases, and the elongation 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 is reduced. 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 by the chemical composition.

Generally speaking, for brands with many types of alloying elements and high content, the solid solution temperature should be increased accordingly. 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 fully dissolving them.

However, for stabilized steels, such as 1Cr18Ni9Ti, when the solid solution temperature is high, the carbides of the stabilizing elements are fully dissolved in the austenite and will precipitate at the grain boundaries in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. To prevent the carbides (TiC and NBC) of stabilizing elements 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 has both stainless and acid resistance (corrosion resistance). The stainless and corrosion resistance of stainless steel is due to the formation of a chromium-rich oxide film (passive 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 atmosphere and water and oxidizing media such as nitric acid will increase with the increase of chromium water content in steel, which is proportional. When the chromium content reaches a certain percentage At this time, the corrosion resistance of steel will change suddenly, that is, from easy to rust to not easy to rust, from non-corrosion resistant to corrosion resistant.