Austenitic stainless steel is softened by solution treatment. Generally, the stainless steel pipe is heated to about 950-1150℃, kept warm for a period of time, so that carbides and various alloy elements are fully and evenly dissolved in austenite, and then quickly quenched and cooled. Carbon and other alloy elements have no time to precipitate, and pure austenite structure is obtained, which is called solution treatment.
Austenitic stainless steel is softened by solution treatment. Generally, the
stainless steel pipe is heated to about 950-1150℃, kept warm for a period of time, so that carbides and various alloy elements are fully and evenly dissolved in austenite, and then quickly quenched and cooled. Carbon and other alloy elements have no time to precipitate, and pure austenite structure is obtained, which is called solution treatment.
Solution treatment has three functions.
1. Make the steel pipe structure and composition uniform, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of hot-rolled wire are different, resulting in inconsistent structure.
At high temperature, atomic activity intensifies, σ phase dissolves, chemical composition tends to be uniform, and uniform single-phase structure is obtained after rapid cooling.
2. Eliminate work hardening to facilitate continued cold processing.
Through solution treatment, the distorted 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 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 the best state.
For stainless steel pipes, the three elements of solution treatment are temperature, holding time and cooling rate. The solution temperature is mainly determined by the chemical composition.
Generally speaking, the solution temperature should be increased accordingly for grades with many types and high content of alloy elements. In particular, for steels with high content of manganese, molybdenum, nickel and silicon, the softening effect can only be achieved by increasing the solution temperature and fully dissolving them.
However, for stabilized steels, such as 1Cr18Ni9Ti, the carbides of the stabilizing elements are fully dissolved in austenite when the solution temperature is high, and will precipitate at the grain boundary in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. In order to prevent the carbides of the stabilizing elements (TiC and Nbc) from decomposing and dissolving, the lower limit solution temperature is generally used.
Stainless steel is commonly known as steel that is not easy to rust. In fact, some stainless steels are both stainless and acid-resistant (corrosion-resistant). The stainlessness and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. Stainlessness and corrosion resistance are relative.
Experiments have shown that the corrosion resistance of steel in weak media such as the atmosphere and water and in oxidizing media such as nitric acid will increase with the increase of the chromium content in the steel, which is directly proportional. When the chromium content reaches a certain percentage, the corrosion resistance of the steel will suddenly change from easy to rust to not easy to rust, and from not corrosion-resistant to corrosion-resistant.
