The austenitic stainless steel is softened by solution treatment. Generally, the stainless steel pipe is heated to about 950 to 1150 ° C for a period of time, so that the carbide and various alloying elements are sufficiently uniformly dissolved in the austenite, and then rapidly quenched and cooled. Carbon and other alloying elements are too late to precipitate, and a pure austenite structure is obtained, which is called solution treatment.
There are three points in the role of solution treatment:
First, the steel pipe structure and composition are uniform, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of the hot-rolled wire are different, resulting in inconsistent structure.
At high temperatures, atomic activity increases, the σ phase dissolves, the chemical composition tends to be uniform, and a uniform single-phase structure is obtained after rapid cooling.
Second, eliminate work hardening, in order to facilitate the continued cold processing.
Through the solution treatment, the lattice recovery of the twist, the elongated and broken grains recrystallize, the internal stress is eliminated, the tensile strength of the steel pipe is lowered, and the elongation is increased.
Third, restore the inherent corrosion resistance of stainless steel.
Due to the precipitation of carbides caused by cold working, the lattice defects cause the corrosion resistance of the stainless steel to decrease. After the solution treatment, the corrosion resistance of the steel pipe is restored to the optimum state.
For stainless steel 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, in stabilized steel, such as 1Cr18Ni9Ti, the carbide of the stabilizing element is sufficiently dissolved in the austenite when the solid solution temperature is high, 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.
Stainless steel is a steel that is not easy to rust. In fact, there is a part of stainless steel that contains both rust and acid resistance (corrosion resistance). The stainless steel’s rust and corrosion resistance is due to the formation of a chromium-rich oxide film (passivation film) on its surface. Among them, the rust and corrosion resistance are relative.
Experiments have shown that in the weak medium such as atmosphere and water and the oxidizing medium such as nitric acid, the corrosion resistance of steel increases with the increase of the water content of chromium in steel, which is proportional. When the chromium content reaches a certain percentage, the corrosion resistance of the steel is abrupt, that is, from rust to rust, from corrosion resistance to corrosion resistance.