Why should the stainless steel welded pipe be treated with solid solution
In the production of stainless steel welded pipe, there is a process is very important, that is, solid solution treatment.Solid solution treatment is a heat treatment process in which the alloy is heated to a high temperature and maintained constant temperature in the single-phase region, and the excess phase is fully dissolved into the solid solution and then cooled rapidly to obtain the susulate solid solution.So why to stainless steel welded tubes for solid solution treatment, it has any role?
The stainless steel welded pipe is softened by solid solution treatment. Generally, the stainless steel welded pipe is heated to about 950 ~ 1150℃ and kept warm for a period of time, so that the carbide and various alloying elements can be fully and uniformly dissolved in austenite. Then, the carbon and other alloying elements can be quickly quenched and cooled.It has the following three functions:
(1) uniform structure and composition of welded steel pipe, which is especially important for raw materials, because the rolling temperature and cooling rate of each section of hot-rolled wire are different, resulting in inconsistent structure.At high temperatures, the activity of the atoms increases, the phase dissolves, and the chemical composition tends to be uniform. After rapid cooling, uniform single-phase tissue is obtained.
(2) eliminate work hardening in favor of continued cold work.Through the solution treatment, the twisted lattice was restored, the elongation and the broken grains were recrystallized, the internal stress was eliminated, the tensile strength of the steel tube decreased, and the elongation rate increased.
(3) restore the inherent corrosion resistance of stainless steel welding.The corrosion resistance of stainless steel is reduced due to the carbide precipitation and lattice defects caused by cold processing, and the corrosion resistance of stainless steel welded tubes is restored to a good state after the solid solution treatment.
After the solid solution treatment of stainless steel welded pipe, its performance in all aspects can reach the state, so the solid solution treatment of stainless steel welded pipe is very important.
What are the disadvantages of the stainless steel welded pipe solution treatment
All cold working of stainless steel actually like welding, will bring inevitable damage to the performance of stainless steel tube, especially corrosion resistance or heat resistance.Stainless steel, especially austenitic stainless steel, has excellent plasticity, making it easy to realize cold processing methods such as cold drawing, cold rolling, cold rolling, cold bending, cold expansion, cold twisting, etc., without solid solution treatment, the following specific situations will occur.
1. It will lead to the increase of microscopic defects such as lattice dislocation and surface roughness, and induce martensitic phase transition and carbide precipitation.Such as the cold processing of austenitic steel showed an increase in magnetic phenomenon.
2. If the lattice dislocation or phase transition of the material occurs on the surface, it will become the starting point of local corrosion such as pore erosion.This phenomenon will have a direct adverse effect when the deformation degree reaches 20% minus the section rate.
3, after cold processing will leave residual stress in the material, residual stress on the material's resistance to stress corrosion cracking (SCC) is extremely adverse.Any degree of cold processing will increase the sensitivity of the material to SCC.
4, the degree of cold processing of austenitic stainless steel has a negative impact on the high temperature durable strength.Generally, the higher the working temperature is or the higher the fracture life requirement is, the lower the allowable cold working degree is.
5, for the application of stainless steel tube under alternating load, cold machining will increase its cracking and expansion rate due to the reduction of elongation and residual elongation, resulting in adverse effects.
To sum up, the stainless steel welded pipe must be soldered and polished to ensure corrosion resistance and prevent the precipitation of austenitic carbide.