Austenitic stainless steel becomes soft after solution treatment. Generally, heating stainless steel fittings to about 950 to 1150 ℃ for a period of time promotes the complete and uniform dissolution of carbides and various alloy elements in austenite, followed by rapid heat treatment and cooling. Carbon and other alloy elements are too late to deposit and obtain pure austenite structure, which is called solid solution treatment. Solid solution treatment has three practical effects.
- Due to the different rolling temperatures and cooling rates of each section of hot-rolled wire rods, their structure is inconsistent. Therefore, it is particularly important for raw materials to ensure the uniform structure and composition of stainless steel pipe fittings.
At high temperatures, the atomic activity increases, the polymer phase dissolves, and the chemical composition tends to be uniform. After rapid cooling, a uniform single-phase structure is obtained.
- Eliminate work hardening to promote continuous cold working.
After solid solution treatment, the deformed lattice is restored, and the elongated and fractured grains are recrystallized, eliminating internal stress. The tensile strength of stainless steel fittings decreases and the elongation increases.
- Repair the inherent corrosion resistance of stainless steel fittings.
Due to the precipitation of carbides and lattice defects caused by cold working, the corrosion resistance of stainless steel fittings is reduced. After solid solution treatment, the corrosion resistance of stainless steel pipe fittings returned to a relatively good state.
The three elements of solid solution treatment for stainless steel pipe fittings are temperature, insulation time, and cooling rate. The temperature of the solution is mainly determined by its chemical composition.
Generally speaking, for grades with multiple alloying elements and high content, the solution temperature should be correspondingly increased. Especially, steel with high Mn, Mo, Ni, and Si contents can only be softened by increasing the solid solution temperature and fully dissolving.
However, in stable steels such as 1Cr18Ni9Ti, when the solid solution temperature is high, the carbides of the stabilizing elements are fully dissolved in austenite, and precipitate in the form of Cr23C6 at grain boundaries during subsequent cooling, causing intergranular corrosion. In order to prevent the carbide (TiC and Nbc) of stabilized elements from decomposition or solid solution, the lower limit solid solution temperature is usually used.