1. Among austenitic stainless steels, the commonly used materials are 1Cr18Ni9Ti and 0Cr18Ni9Ti. The tensile properties of 1Cr18Ni9Ti are more stable than those of 0Cr18Ni9Ti, and it has better crack resistance. Therefore, whenever possible, 1Cr18Ni9Ti should be selected.
2. For stainless steel deep drawing, select the die material reasonably: During the drawing process, stainless steel generates a large number of hard metal particles, leading to adhesion, which easily scratches the workpiece and die surfaces and causes wear. Therefore, ordinary tool steels cannot be used for dies. Results show that selecting copper-based alloy dies can eliminate surface scratches on stainless steel parts, reduce scratching, and lower the wear rate. Another material is a high-aluminum copper-based alloy die material (aluminum 13 wt% to 16 wt%). This material has low solubility with SUS304 stainless steel, so there is no adhesion between the drawn part and the die head, the surface of the drawn part is less prone to scratches, and the polishing cost of the product is low. It has been successfully applied in the field of stainless steel deep drawing. However, due to the low hardness of the die (40 HRC to 45 HRC), it is usually used to produce products with relatively small thickness-to-diameter ratios (t/D). After drawing about 15,000 to 20,000 pieces, the die surface tends to develop radial drawing ridges starting from the fillet radius R. Silicon nitride ceramics (Si3N4) have become an important engineering material, especially reaction-bonded silicon nitride ceramics, which have good mechanical properties at high and low temperatures, thermal shock resistance, chemical stability, and can be used to make parts of various complex shapes. Taking advantage of the high hardness, high wear resistance, and high chemical stability of ceramic materials, reaction-bonded silicon nitride dies can replace metal dies for deep drawing SUS304 stainless steel.
3. For stainless steel deep drawing, the appropriate punch and die fillet radii should be selected, as they are closely related to the magnitude and distribution of stress. If the fillet radius is large, the blank holder area is insufficient, which can easily cause instability and wrinkling. If the fillet radius is too small, it increases the resistance of the material entering the die during deformation, making it difficult for the material to flow inward and transfer, thereby increasing the force transmission zone and potentially leading to cracking. Therefore, selecting reasonable punch and die fillet radii is very important. Under the condition of preventing cracks, the relative punch fillet radius rp/t is approximately 4. The degree of polar deformation of the die and punch will increase, and the relative die fillet radius should be increased by 5 mm to 8 mm, which helps prevent cracking.
4. For stainless steel deep drawing, use the thin-strip drawing method: Previous testers have also confirmed that the thin-strip drawing method can significantly reduce the maximum tangential residual stress of the drawn part and effectively prevent longitudinal cracking. According to the degree of deformation and the original sheet thickness, select an appropriate thinning coefficient (usually 0.9t to 0.95t). If the value is too small, the deformation stress of the drawn part will increase sharply, leading to bottom cracking.
5. For stainless steel deep drawing, add an intermediate annealing process in the drawing method: After multiple drawing operations, an intermediate annealing process should be performed to completely eliminate residual stress and restore the austenitic stainless steel structure. For high-strength steels, generally one to two drawing operations are required before intermediate annealing. For example, for 1Cr18Ni9Ti, the typical heating temperature is 1150 to 1170°C, heating for 30 minutes, followed by cooling in an air stream or in water. Moreover, whether it is heat treatment between processes or the final heat treatment of the finished product, it should be carried out as soon as possible after drawing to avoid deformation or cracking of the workpiece due to internal stress caused by long-term storage. However, annealing and cleaning after annealing can extend the product cycle and affect surface quality.
6. For stainless steel deep drawing, use an appropriate lubricant: Using an appropriate lubricant has a significant effect on stainless steel deep drawing. The lubricant can form a film with certain toughness and ductility between the punch and die, which is beneficial for the deep drawing of stainless steel. Since stainless steel drawn parts undergo large deformation and are difficult to form, polytetrafluoroethylene (PTFE) film can be used as a lubricant in actual production. PTFE film has good tear resistance, certain toughness and ductility, and is easy to clean. After coating and drying the film, during the drawing process, the dry film can deform together with the blank and always keep the blank separated from the die. The film itself has a certain porosity and a large number of fiber cracks, so it can store a certain amount of lubricating oil.
The above common defects and preventive measures for stainless steel drawn parts. In actual production, the drawing die is prone to scratching. To solve this problem, the characteristics of the contact pair should be changed to reduce adhesive wear. I hope the above can be helpful to the readers.