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The solution temperature is mainly determined by chemical composition. In general, the solution temperature should increase for those containing many kinds of alloy elements or with high alloy. Especially for those that contains high steel, like manganese, molybdenum, nickel, silicon. Only by increasing the solution temperature can we dissolve and soften them. But for stabilized steels such as 321 stainless steel pipe, when the solution temperature is high, the carbide of the stabilizing elements are fully dissolved in the austenite and precipitate in the grain boundary with the form of Cr23C6 after cooling. That can cause intergranular corrosion. We often adopt the lowest solution temperature for carbide(Tic and NbC) of the stabilizing elements which can't be dissolved or treated by solid solution.
For seamless stainless steel pipe, the three elements of the solution treatment are temperature, holding time and cooling rate. What is the role of solution treatment of seamless stainless steel pipe?
1. Keep uniformity for organization and composition of the stainless steel seamless tube are particularly important for raw materials. For the rolling temperature and cooling rate of the different sections of the hot rolling pipe are not the same, which can cause organizational structure inconformity. Under the high temperature, the atomic activity increases, σ dissolves, the chemical composition tends to be uniform, and the uniform single-phase structure is obtained after rapid cooling.
2. Eliminate the work hardening in order to facilitate the cold processing. Through the solution treatment, the crooked lattice will recover and elongate, and the broken grain recrystallize, inner stress eliminate, wire tensile strength decrease and elongation increase.
3. Restore the inherent corrosion resistance of stainless steel tube. The cold processing has caused precipitation of the carbide and lattice defect, which decreases the corrosion resistance of stainless steel pipe. After the solution treatment, the corrosion resistance of stainless steel seamless pipe is restored to the best condition.