In order to improve the performance of stainless steel, certain cold processing methods are often used, such as cold drawing, cold rolling, cold-rolling, cold bending, cold expansion, cold twisting, etc. These methods can achieve certain results, but they also cause certain damage. In fact, all these cold processing methods, like welding, will inevitably cause damage to the performance of stainless tubes, especially their corrosion resistance or heat resistance.
If material lattice dislocations or phase changes occur on the surface, they will become the starting point for local corrosion, such as pitting corrosion. This phenomenon will have a direct adverse effect when the deformation degree reaches a 20% reduction rate.
Cold processing will increase material micro defects such as lattice dislocations and surface roughness, and induce martensite phase changes and carbide precipitation. For example, austenitic steel exhibits an increase in magnetism after cold working.
Cold working will leave residual stresses in the material, which is extremely unfavorable for the material's resistance to stress corrosion cracking. Any degree of cold working will greatly increase the SCC sensitivity of the material.
The degree of cold working also has an adverse effect on the high-temperature resistance and strength of austenitic stainless tubes. The lower the allowable cold working degree, the higher the working temperature or fracture life required.
For stainless tubes subjected to alternating loads, cold working will cause unfavorable effects due to the decrease in elongation rate and residual elongation rate, which will lead to an increase in crack propagation rate.
Most national standards for stainless tubes, especially the unified European stainless tube standards, stipulate that all seamless stainless tubes must be supplied in a solid solution or annealed state to eliminate the performance damage caused by cold working and welding.
Solid solution treatment should pay attention to three key parameters: heating temperature, rapid cooling method, and high-temperature dwell time. Excessive solid solution treatment temperature or dwell time will be detrimental to the material's corrosion resistance. To determine whether the solid solution is in place, hardness testing, expansion, curling, flattening, and tensile data can be used to judge, with hardness testing being simpler.
Since solid solution or annealing increases manufacturing costs and production cycles due to high-temperature heating and acid pickling, as well as emissions of acid mist and wastewater, some companies omit this process. The use of such products can easily cause accidents in production and personnel.
For some products or application conditions that may be difficult to implement solid solution or annealing, controlling the degree of cold working (amount of cold deformation) and locally carrying out low-temperature stress relief annealing are practical methods to reduce harmful effects.
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