The solution temperature is mainly determined according to the chemical composition. Generally speaking, the solution temperature of the grades with many kinds and high content of alloy elements should be increased accordingly. Especially for steel with high content of manganese, molybdenum, nickel and silicon, the softening effect can be achieved only by increasing the solution temperature and making it fully dissolved. However, for stabilized steels, such as 321 stainless steel tubes, when the solution temperature is high, the carbides of stabilized elements are fully dissolved in austenite, which will precipitate at the grain boundary in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion. In order to prevent the carbides (TiC and NbC) of stabilized elements from decomposition and solution, the lower limit solution temperature is generally adopted.
For stainless steel seamless tubes, the three elements of solution treatment are temperature, holding time and cooling rate. What is the effect of solution treatment on stainless steel seamless tubes?
1. It is particularly important for raw materials to make the structure and composition of stainless steel seamless pipe uniform, because the rolling temperature and cooling speed of each section of hot rolled pipe are different, resulting in inconsistent structure. Atomic activity intensifies at high temperatures, σ Phase dissolution, chemical composition tends to be uniform, and uniform single-phase structure is obtained after rapid cooling.
2. Eliminate work hardening to facilitate continuous 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 pipe. Due to carbide precipitation and lattice defects caused by cold working, the corrosion resistance of stainless steel tubes is reduced. After solution treatment, the corrosion resistance of stainless steel seamless pipe recovers to the best state.