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 carbides and various alloy elements are completely and uniformly dissolved in austenite, and then quenched and cooled quickly. Carbon and other alloying elements are too late to precipitate and a pure austenite structure is obtained, which is called a solution treatment.
Solution treatment has three effects.
1. Due to the different rolling temperature and cooling rate of each section of the hot-rolled wire rod, the structure is inconsistent. Therefore, it is particularly important for the raw materials to make the structure and composition of the stainless steel pipe uniform.
At high temperatures, the atomic activity is enhanced, the polymer phase is dissolved, and the chemical composition tends to be uniform. After cooling, a uniform single-phase structure is obtained.
2. Eliminate work hardening to promote continuous cold work.
After the solution treatment, the deformed crystal lattice is restored, the elongated and fractured grains are recrystallized, the internal stress is eliminated, the tensile strength of the stainless steel pipe is reduced, and the elongation is increased.
3. Repair the inherent corrosion resistance of stainless steel pipe fittings.
Due to carbide precipitation caused by cold working, lattice defects lead to reduced corrosion resistance of stainless steel pipe fittings. After solution treatment, the corrosion resistance of stainless steel pipe fittings is restored to a better state.
The three elements of solution treatment of stainless steel pipe fittings are temperature, holding time and cooling rate. The solution temperature is mainly determined by the chemical composition.
In general, for grades with multiple alloying elements and high contents, the solution temperature should be increased accordingly. In particular, steels with high Mn, Mo, Ni, and Si content can only be softened by increasing the solution temperature and sufficiently dissolving.
However, in stable steels such as 1Cr18Ni9Ti, when the solid solution temperature is high, the carbides of the stabilizing element are sufficiently dissolved in austenite, and then precipitated at the grain boundaries in the form of Cr23C6 during subsequent cooling, causing intergranular corrosion.
In order to prevent decomposition or solid solution of the carbides (TiC and Nbc) of the stabilizing element, a lower limit solid solution temperature is usually used.
As the saying goes, stainless steel pipe is not easy to rust. In fact, part of the stainless steel pipe contains both stainless steel and acid resistance (corrosion resistance). The rust and corrosion resistance of stainless steel pipe fittings is caused by the formation of a chromium-rich oxide film (passivation film) on the surface. Stainless steel tubing and corrosion resistance are relative.
The experimental results show that the corrosion resistance of steel in weak media such as atmosphere and water and the oxidation medium such as nitric acid will increase with the increase of chromium water content in steel, and it is proportional to the corrosion resistance of stainless steel pipe fittings.
When the chromium content reaches a certain percentage, the corrosion resistance of stainless steel pipe fittings changes suddenly, that is, from rust to non-rust, and from corrosion to corrosion resistance.