Why should stainless steel tubes be solution annealed?

Austenitic stainless steel is softened by solution treatment. Generally, the stainless steel tube is heated to about 950-1150 °C, and kept for a period of time, so that the carbides and various alloy elements are fully and uniformly dissolved in the austenite, and then rapidly quenched and cooled by water. , carbon and other alloying elements are too late to precipitate, and a pure austenite structure is obtained, which is called solid solution treatment.

There are three functions of solution treatment of stainless steel pipes.

1. Make the structure and composition of the steel pipe uniform, which is especially important for raw materials, because the rolling temperature and cooling speed of each section of the hot-rolled wire rod are different, resulting in inconsistent structure.

At high temperature, the atomic activity intensifies, the σ phase dissolves, and the chemical composition tends to be uniform, and a uniform single-phase structure is obtained after rapid cooling.

2. Eliminate work hardening to facilitate continued cold working.

Through solution treatment, the distorted lattice is restored, the elongation and broken grains are recrystallized, the internal stress is eliminated, the tensile strength of the steel pipe decreases, and the elongation increases.

3. Restore the inherent corrosion resistance of stainless steel.

Due to the cold working of stainless steel pipes, carbides are precipitated and lattice defects are caused, which reduces the corrosion resistance of stainless steel. After the solution treatment, the corrosion resistance of the steel pipe is restored to the best state.

For stainless steel pipes, the three elements of solution treatment are temperature, holding time and cooling rate. The solution temperature is mainly determined according to the chemical composition.

Generally speaking, for the grades with many kinds of alloy elements and high content, the solution temperature should be increased accordingly. Especially for steel with high content of manganese, molybdenum, nickel and silicon, the softening effect can only be achieved by increasing the solution temperature and making it fully dissolved.

However, in stabilized steel, such as 1Cr18Ni9Ti, when the solution temperature is high, the carbides of the stabilized elements are fully dissolved in the austenite, and in the subsequent cooling, they will precipitate at the grain boundaries in the form of Cr23C6, causing intergranular corrosion. In order to prevent the carbides (TiC and Nbc) of the stabilizing elements from decomposing and solid solution, the lower limit solution temperature is generally used.

As the saying goes, stainless steel is steel that is not easy to rust. In fact, some stainless steels contain both rust resistance and acid resistance (corrosion resistance). Among them, rust resistance and corrosion resistance are relative. The rust and corrosion resistance of stainless steel is due to the formation of a chromium-rich oxide film (passivation film) on its surface.

Experiments have shown that the corrosion resistance of steel in weak media such as atmosphere and water and in oxidizing media such as nitric acid will increase with the increase of chromium water content in steel, which is proportional. When the chromium content reaches a certain percentage When the corrosion resistance of steel changes suddenly, that is, from easy to rust to not easy to rust, from no corrosion resistance to corrosion resistance.