Micro-wrinkles on the inner wall of stainless steel electrolytic tubes are a core issue affecting pipe quality, and their formation is closely related to raw material condition, cold rolling process, annealing, and electrolytic polishing. To avoid these defects, a comprehensive process control approach is needed, optimizing raw material selection, rolling parameters, annealing processes, and electrolytic conditions to establish a closed-loop quality control system.
Raw material quality is fundamental to preventing wrinkles. If the raw material for stainless steel electrolytic tubes has issues such as central porosity, segregation, or excessive non-metallic inclusions, these defects will be amplified during subsequent processing. For example, shrinkage cavities or inclusions in the tube blank will be elongated during cold rolling, forming micro-cracks or wrinkles along the rolling direction. Therefore, strict raw material inspection standards must be established, using ultrasonic testing or metallographic inspection to remove unqualified tube blanks. Simultaneously, suppliers must be required to optimize smelting processes, reducing the content of harmful elements such as sulfur and phosphorus, and minimizing the segregation of low-melting-point compounds at grain boundaries, thus reducing the risk of wrinkles at the source.
The rationality of the cold rolling process directly affects the smoothness of the inner wall. During cold rolling, excessive reduction in a single pass or excessively high rolling speed can lead to uneven metal flow, causing micro-wrinkles on the inner wall due to stress concentration. For example, one company uses a multi-pass, small-deformation rolling process to control the reduction in a single pass within a reasonable range. By adjusting the roll speed and tension, the metal flow within the deformation zone is made more uniform, effectively reducing the formation of wrinkles on the inner wall. Furthermore, the surface quality of the rolls is crucial; a rough roll surface can scratch the tube wall, forming microscopic defects. Regular grinding of the rolls and inspection of their surface roughness are necessary.
The annealing process is a key step in eliminating rolling stress. If cold-rolled stainless steel tubes are not sufficiently annealed, residual stress will promote abnormal grain growth, leading to wrinkles during subsequent processing. If the annealing temperature is too low or the time is insufficient, carbides cannot dissolve completely, reducing grain boundary strength and making slip lines more likely to form during bending or electrolysis, resulting in wrinkles. Therefore, a precise annealing process must be developed based on the pipe specifications. For example, bright annealing technology can be used, rapidly heating to above the critical temperature in a protective atmosphere and controlling the holding time to ensure uniform and refined grains while preventing oxide scale formation and reducing corrosion of the pipe wall by subsequent pickling.
The parameter control of the electropolishing process directly affects the smoothness of the inner wall. Electrolyte composition, current density, and polishing time are the core factors determining surface quality. If the current density is too high, the electrolytic reaction will be too vigorous, causing localized overheating on the pipe wall, leading to uneven metal dissolution and an uneven surface. If the polishing time is insufficient, minor defects generated during cold rolling or annealing cannot be completely eliminated. For example, one company optimized the electrolyte formula, using a mixture of phosphoric acid and sulfuric acid, and controlled the current density within a reasonable range, enabling the pipe wall to form a uniform passivation film during electrolysis, which improved corrosion resistance and prevented wrinkles.
The precision and maintenance of processing equipment are equally important. The wear condition of the cold rolling mill's roll gap, tension control system, and guide devices directly affects the forming quality of the pipe. Inconsistent roll gaps or loose guide devices can cause pipe misalignment during rolling, resulting in uneven stress on the inner wall and wrinkles. Therefore, regular equipment calibration, replacement of worn parts, and the establishment of equipment maintenance records are necessary to ensure the stability of the rolling process.
A comprehensive quality traceability system is crucial to preventing wrinkles. By recording the raw material batch, process parameters, and test results for each pipe through the MES system, a unique QR code is generated, enabling quality traceability to specific workstations and operators. For example, one company analyzed the metallographic structure of returned pipes and found that excessive heat input led to grain coarsening. Adjusting the welding current improved the weld impact toughness, reducing the risk of wrinkles from the process end.
Avoiding micro-wrinkles on the inner wall of stainless steel electrolytic tubes requires a closed-loop control system encompassing raw material selection, cold rolling, annealing, electrolytic polishing, equipment maintenance, and quality traceability. Optimizing process parameters, improving equipment precision, and strengthening quality traceability can significantly reduce the wrinkle defect rate and improve the surface quality and reliability of the pipes.
