Solution treatment is the most common heat treatment method after stainless steel casting process. The carbide phase is completely or substantially dissolved after the austenitic stainless steel is heated to about 1100 °C . The carbon is dissolved in the austenite and then it is cooled quickly to room temperature, which makes the carbon reach a supersaturated state. This kind of method is called solution heat treatment. The rapid cooling in solid solution heat treatment seems like the quenching of ordinary steel, but the ‘quenching’ in this situation is different from that of ordinary steel, because the former is softening while the latter is hardening. To obtain different hardness, the latter method used different heating temperatures , but did not reach 1100 °C.
With the method of solution treatment, stainless steel castings are softened. Generally, stainless steel castings are heated to around 950-1150 °C and kept for a period of time for the purpose of making carbides and various alloy elements to be fully and uniformly dissolved in austenite, and then it is quickly quenched and cooled. The carbon and other alloy elements are too late to precipitate, and a pure austenite structure is obtained, which is named solid solution treatment.
Why Solution Treatment for Stainless Steel Castings?
1. To make the structure and composition of stainless steel castings uniform, which is especially important for raw materials, because the casting temperature and cooling rate are not the same, resulting in inconsistent microstructure. With high temperatures, the atomic activity intensifies and the σ phase dissolves, and its chemical composition tends to be uniform. A uniform single-phase structure is obtained after cooling quickly .
2. To eliminate hardening so as to facilitate the following machining. With the solution treatment, the twisted lattice is restored, the elongated and broken grains are recrystallized and the internal stress is eliminated. The tensile strength of the stainless steel casting is reduced and the elongation is increased.
3. To recover the inherent corrosion resistance of stainless steel castings. The corrosion resistance of stainless steel decreases after investment casting. The corrosion resistance of stainless steel castings recovered to the optimum state after solution treatment.
As for stainless steel castings, the main factors of solution treatment are: temperature, heat preservation time and cooling speed.