During the processing of 304 stainless steel mechanical kettles, surface scratches can directly impact the product's appearance quality and market competitiveness. Therefore, a systematic protection plan must be developed across multiple dimensions, encompassing material properties, equipment status, process parameters, and operational specifications.
The surface oxide film of 304 stainless steel is the core of its corrosion resistance, but this film is fragile in mechanical contact and is particularly susceptible to damage from friction with hard objects, uneven polishing wheel grit, or excessive force. For example, during lifting, transportation, or structural processing, if the mechanical kettle casing is not properly protected, human factors such as bumps, dragging, or hammering can cause linear scratches on the surface. Therefore, prior to processing, raw materials must be rigorously inspected to ensure the integrity of the surface oxide film. Soft packaging materials should be used to isolate the workpiece from the equipment throughout the entire processing process.
The operating status of the processing equipment directly impacts surface quality. Die wear, reduced roller accuracy, or abnormal clearance in the transmission mechanism can lead to unbalanced contact stresses on the processed surface, which in turn can cause scratches. For example, when stamping die cutting edges wear, the contact surface with the stainless steel sheet becomes rough, easily scratching the material surface during high-speed stamping. Therefore, a regular die maintenance system should be established, with mandatory polishing after every 5,000 stamping cycles to restore the die surface finish to Ra ≤ 0.4μm. Nano-coating technology should also be used to reduce the coefficient of friction.
Process parameters must be precisely matched to the material's characteristics. Excessive stamping speeds can result in excessive instantaneous impact forces, causing abnormal friction between the material and the die. Bend angle deviations can cause material misalignment and increase the risk of surface damage. For the processing of 304 stainless steel mechanical kettles, it is recommended to control the lower die clearance within 8%-12% of the sheet thickness, using a progressive stamping rate of 0.8-1.2 mm/s, and dynamically adjust the pressure ratio to reduce stress concentration. During the bending process, the upper and lower die clearance tolerance must be monitored closely, and calibration procedures should be initiated immediately if the deviation exceeds 8% of the sheet thickness.
Standardized operating procedures are key to controlling scratches. Inaccurate workpiece positioning, failure to use protective gaskets, or rough handling can significantly increase the likelihood of scratches. For example, in CNC punching operations, if feed accuracy is not verified using both positioning pins and an optical alignment system, the workpiece may shift, leading to scraping against the die edge during punching. Therefore, strict adherence to standardized operating procedures and adjusting the press foot pressure to a range of 0.15-0.25 MPa are essential to prevent material slippage and surface indentations caused by overpressure.
The choice of surface treatment process is crucial for scratch repair. For existing minor scratches, either pickling and passivation or electropolishing can be used. Pickling and passivation involves immersion at room temperature, using a chemical solution to remove weld marks and conceal scratches, while also forming a passivation film to enhance corrosion resistance. Electropolishing, on the other hand, uses an electric current to treat the metal surface, achieving a mirror, glossy, or matte finish. Regardless of the process used, the workpiece must be thoroughly cleaned before treatment to remove oil, impurities, and other substances.
The establishment of a comprehensive inspection mechanism throughout the entire process enables early detection and intervention of scratch defects. During incoming material inspection, a spectrum analyzer measures the composition of stainless steel sheets, and a surface roughness meter assesses their initial condition. During processing, intelligent sensing equipment collects real-time data on key parameters such as stamping speed and die clearance. At the finished product stage, AQL sampling inspection procedures are implemented, establishing grading criteria based on scratch length, depth, and distribution density.
Continuous improvement and closed-loop management are the long-term guarantee for improving surface quality. The MES system enables quality data traceability, linking defect types to processing equipment, operators, and process versions, providing a basis for decision-making in process optimization. For example, if scratches on a batch of mechanical kettle casings are found to be concentrated in the bending process, the lubricant ratio can be adjusted or the die radius design can be optimized.
