Stainless steel stamping presents unique challenges and considerations compared to ordinary carbon steel stamping. Due to its high strength, hardness, toughness, and stringent surface quality requirements, targeted process measures are essential. The following are key considerations for stainless steel stamping, covering materials, molds, processes, and post-processing:
I. Material Preparation Stage
Material Selection: Different grades of stainless steel (e.g., 304, 304L, 316, 430, etc.) exhibit significant differences in stamping performance. Austenitic stainless steel (304 series) has good toughness but is prone to work hardening; martensitic stainless steel (430 series) has high hardness but relatively poor toughness. Appropriate materials should be selected based on the product's intended use.
Surface Finish: Common finishes include 2B (cold-rolled, bright annealed), BA (bright annealed), and No.4 (brushed). The product's surface quality requirements must be clearly defined, and an appropriate initial surface finish should be selected to avoid scrap due to scratches or indentations.
Thickness Tolerance: Stainless steel stamping is sensitive to material thickness fluctuations. Strict thickness tolerance is essential for ensuring stable product quality and long die life.
Material Processing:
Cleaning: Ensure the sheet metal surface is clean, free of oil, dust, and other impurities. These impurities can damage the die or workpiece surface during stamping.
Lubrication: This is paramount in stainless steel stamping.
II. Die Design and Manufacturing
Die Material: High-strength, high-wear-resistant, and high-toughness high-quality die steel must be selected. Powdered high-speed steel and cemented carbide are recommended. For high-volume, high-precision products, cemented carbide is the preferred choice. Heat treatment hardness is typically required to be above HRC 58-62, and proper heat treatment processes must be ensured to avoid cracking and deformation.
Die Clearance: The die clearance for stainless steel stamping should be larger than that for ordinary carbon steel. This is because stainless steel is harder, has greater springback, and is prone to work hardening.
Recommended Clearance: Typically 12% - 15% of the material thickness (one side). Specific values need to be adjusted according to the material thickness, grade, and stamping process (such as blanking, punching, and drawing). Insufficient clearance will lead to accelerated die wear and even chipping; excessive clearance will increase burrs and result in poor cross-sectional quality.
Die Structure Design:
Rounded Corner Design: The rounded corner radii of all punches, dies, and drawing dies should be appropriately increased to avoid sharp corners. This effectively reduces stress concentration and prevents material tearing and die cracking.
Unloading and Blank Holding: Use powerful elastic or pneumatic unloading devices to ensure the sheet metal is effectively clamped during stamping, preventing material movement and wrinkling. For the drawing process, the blank holder force needs precise control.
Ventilation Design: Design vents on the drawing and forming dies to prevent product deformation or inability to demold due to trapped air.
III. Stamping Process Control
Lubrication: Special stainless steel stamping oil must be used. This oil should possess excellent extreme pressure properties, lubrication, and cooling performance, effectively reducing friction and adhesion between the mold and the material, dissipating heat, and preventing "roughening," "scratching," and "sintering." Oil application should be even and sufficient, especially during stretching and forming processes.
Press Selection and Adjustment: Choose a press with good rigidity to ensure a smooth, vibration-free stamping process. Adjust the appropriate slide stroke and speed. For stainless steel, it is recommended to use a lower stamping speed to reduce impact force and heat accumulation.
Avoid Work Hardening: Stainless steel (especially austenitic stainless steel) undergoes significant work hardening during stamping deformation, leading to increased hardness, decreased plasticity, difficulty in subsequent processing, and even cracking.
Optimize Process Arrangement: For complex parts, use multiple processes, with annealing treatment between processes to eliminate work hardening and restore material plasticity.
Reduce Deformation: The deformation amount in a single stamping should not be too large.
Timely Die Sharpening: Sharp die edges can reduce the degree of work hardening.
IV. Post-processing and Quality Inspection
**Product Deburring:** Burrs will appear on the edges of the stamped product and must be removed. Methods such as vibratory grinding, magnetic grinding, and polishing can be used, taking care to protect the product surface from scratches.
**Cleaning:** Remove stamping oil from the product surface. If subsequent surface treatments such as electroplating or spraying are required, thorough degreasing and cleaning are necessary.
**Rust Prevention:** Although stainless steel is corrosion-resistant, it may still rust under certain environments (such as those containing chloride ions) or after surface damage. After cleaning, keep the product dry, or apply rust-preventive oil as needed.
**Quality Inspection:**
**Dimensional Accuracy:** Check that key dimensions meet the drawing requirements.
**Surface Quality:** Focus on checking for defects such as scratches, indentations, roughening, and pits.
**Burr Height:** Check that burrs are within acceptable limits.
**Cracks:** For stretched, bent, or other formed parts, use a magnifying glass or penetrant testing to check for microcracks.