The solution temperature is mainly determined according to the chemical composition. Generally speaking, the variety and content of alloying elements are high, and the solution temperature will increase accordingly. Especially for steels with high content of manganese, molybdenum, nickel and silicon, the softening effect can only be achieved by increasing the solution temperature and making it fully dissolved.
However, for stabilized steel, such as 321 stainless steel, when the solution temperature is high, the carbides of the stabilized elements are fully dissolved in the austenite, and in the subsequent cooling, they will precipitate at the grain boundaries in the form of Cr23C6, causing intergranular corrosion. In order to prevent the carbides (TiC and NbC) of the stabilizing elements from decomposing and solid solution, the lower limit solution temperature is generally used. For stainless steel seamless pipes, the three elements of solution treatment are temperature, holding time and cooling rate.
The role of solution treatment of stainless steel seamless pipes
1. Make the structure and composition of the stainless steel seamless pipe uniform, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of the hot-rolled pipe are different, resulting in inconsistent structure. At high temperature, the atomic activity intensifies, the σ phase dissolves, and 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 is restored, the elongated and broken grains are recrystallized, the internal stress is eliminated, the tensile strength of the steel wire decreases, and the elongation increases.
3: Restore the inherent corrosion resistance of stainless steel pipes. Due to the precipitation of carbides and lattice defects caused by cold working, the corrosion resistance of stainless steel pipes is reduced. After solution treatment, the corrosion resistance of stainless steel seamless pipe is restored to the best state.