Austenitic stainless steel by solid solution treatment to soften, generally heated stainless steel to 950 ~ 1150 ℃, holding a period of time, so that carbide and various alloying elements fully and uniformly dissolved in austenite, and then quickly quenched water cooling, carbon and other alloying elements to precipitate, to obtain pure austenite organization, called solid solution treatment.
The role of solid solution treatment has three points.
One. Make the steel pipe organization and composition uniform, which is particularly important for raw materials, because the hot-rolled wire sections of the rolling temperature and cooling rate is not the same, resulting in inconsistent organizational structure.
Atomic activity intensifies at high temperatures, σ phase dissolution, the chemical composition tends to be uniform, rapid cooling to obtain a uniform single-phase organization.
Two. Elimination of work hardening to facilitate continued cold working.
Through solid solution treatment, the distorted lattice recovery, elongation and broken grains recrystallization, internal stress elimination, steel pipe tensile strength decreases, elongation rises.
Three. Restore the inherent corrosion resistance of stainless steel.
As a result of cold processing caused by carbide precipitation, lattice defects, so that the corrosion resistance of stainless steel decreased. Solid solution treatment after the steel pipe corrosion resistance to the best state.
For stainless steel tubes, the three elements of solution treatment are temperature, holding time and cooling rate. The solid solution temperature is determined mainly according to the chemical composition.
Generally speaking, the variety of alloying elements, high content of the grade, the solid solution temperature should be increased accordingly. In particular, manganese, molybdenum, nickel, silicon content of high steel, only to improve the solid solution temperature, so that it is fully dissolved, in order to achieve the softening effect.
However, stabilised steel, such as 1Cr18Ni9Ti, solid solution temperature is high when the carbide of the stabilising element is fully dissolved in austenite, in the subsequent cooling will be in the form of Cr23C6 precipitation in the grain boundary, resulting in intergranular corrosion. In order to stabilise the carbides of the elements (TiC and Nbc) without decomposition and solid solution, the lower limit of solid solution temperature is generally used.
Stainless steel is commonly known as the steel that does not rust easily, in fact there is a part of the stainless steel, containing both stainless and acid resistance (corrosion resistance). Stainless steel's stainless and corrosion resistance is due to the formation of a chromium-rich oxide film (passivation film) on its surface. Where stainless and corrosion resistance are relative.
Experimentally proven, steel in the atmosphere, water and other weak media and nitric acid and other oxidizing media, its corrosion resistance will increase with the increase in chromium content in steel, then is proportional to . When the chromium content reaches a certain percentage, the corrosion resistance of steel will be a sudden change, that is, from easy to rust to not easy to rust, from corrosion resistance to corrosion resistance.
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September 16, 2022
