The Role of Solution Treatment in Stainless Steel Seamless Pipes

The solution treatment temperature is primarily determined by the chemical composition of the steel. Generally, grades with a higher variety and content of alloying elements require higher solution treatment temperatures. For steels with high amounts of manganese, molybdenum, nickel, and silicon, increasing the solution temperature ensures these elements dissolve completely, achieving a softening effect.

However, in stabilized steels like 321 stainless steel pipes, a high solution treatment temperature causes the stabilizing elements' carbides to dissolve into the austenite. During subsequent cooling, these carbides precipitate at grain boundaries as Cr23C6, leading to intergranular corrosion. To prevent the decomposition and dissolution of the stabilizing elements' carbides (TiC and NbC), the lower limit of the solution treatment temperature is usually adopted.

For stainless steel seamless pipes, the three key factors of solution treatment are temperature, holding time, and cooling rate. What are the effects of solution treatment on stainless steel seamless pipes?

Uniformity in Structure and Composition: Solution treatment ensures uniform structure and composition, which is especially crucial for raw materials. This uniformity is important because the rolling temperature and cooling rate during hot rolling vary across different sections of the pipe, leading to inconsistent structures. At high temperatures, atomic activity increases, the σ phase dissolves, and the chemical composition becomes uniform. Rapid cooling results in a uniform single-phase structure.

Elimination of Work Hardening: Solution treatment eliminates work hardening, facilitating further cold processing. The treatment helps restore the distorted lattice, recrystallize the elongated and fragmented grains, relieve internal stresses, reduce tensile strength, and increase elongation.

Restoration of Corrosion Resistance: Cold working can cause carbide precipitation and lattice defects, reducing the corrosion resistance of stainless steel pipes. After solution treatment, the corrosion resistance of stainless steel seamless pipes is restored to its optimal state.