The stainless steel seamless tube is softened by solution treatment. Generally, the stainless steel seamless tube is heated to about 950 to 1150 ° C for a period of time, so that the carbide and various alloy elements are sufficiently uniformly dissolved in the austenite, and then rapidly quenched. Water cooling, carbon and other alloying elements are too late to precipitate, to obtain pure austenite structure, called solid solution treatment.
There are three points in the role of solution treatment:
- Make the uniformity and composition of the stainless steel seamless pipe uniform, which is especially important for the 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 temperatures, atomic activity is intensified, the σ phase dissolves, the chemical composition tends to be uniform, and a uniform single-phase structure is obtained after rapid cooling.
- Eliminate work hardening to facilitate further cold work. Through the solution treatment, the lattice recovery of the twist, the elongated and broken crystal grains recrystallize, the internal stress is eliminated, the tensile strength of the steel wire is lowered, and the elongation is increased.
- Restore the inherent corrosion resistance of stainless steel tubes. Due to the precipitation of carbides caused by cold working, the lattice defects cause the corrosion resistance of the stainless steel tube to decrease. After the solution treatment, the corrosion resistance of the stainless steel seamless tube is restored to a good state.
For stainless steel seamless pipes, the three elements of solution treatment are temperature, holding time and cooling rate.
The solution temperature is mainly determined based on the chemical composition. Generally speaking, the number of alloying elements is high, and the solid solution temperature is correspondingly increased. In particular, steels with high manganese, molybdenum, nickel, and silicon content can only achieve a softening effect by increasing the solid solution temperature and allowing them to be fully dissolved. However, stabilized steel, such as 321 stainless steel tube, has a high solid solution temperature and the carbide of the stabilizing element is sufficiently dissolved in the austenite, and precipitates at the grain boundary in the form of Cr23C6 in the subsequent cooling, causing intergranular corrosion. In order to prevent the carbides (TiC and NbC) of the stabilizing element from decomposing or being solid-solved, the lower limit solid solution temperature is generally employed.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)