Stainless steel has a small elongation, a large elastic modulus E, a high hardening index, and a very small thickness anisotropy index r value (0.9-0.11 for stainless steel and 1.3-2.0 for mild steel). The plastic deformation stage from yield to rupture of stainless steel is short, especially the weighted value R of its plastic strain ratio is large. Tensile cracking of stainless steel plates sometimes occurs after deep drawing, sometimes immediately when the drawn part is withdrawn from the die; sometimes it occurs when it is hit or vibrated after stretching deformation; sometimes it occurs after a period of storage or during use after stretching deformation. The causes of defects in stainless steel plates during stretching are analyzed and solutions are proposed.

Analysis of common problems in stainless steel drawing stamping parts

1 Reason of cracking

The cold work hardening index of austenitic stainless steel is high (0.34 for stainless steel). Austenitic stainless steel is metastable and undergoes phase transformation during deformation, inducing martensite phase. Martensite phase is relatively brittle and therefore prone to cracking. During plastic deformation, as the deformation increases, the induced martensite content will also increase with the deformation, and the residual stress will also increase.

Figure 1 Relationship between residual stress and martensite content The higher the induced martensite phase content, the greater the residual stress caused, and the easier it is to crack during processing.

2 Reason of surface scratches

The scratches on the surface of stainless steel tensile parts are mainly due to the relative movement between the workpiece and the mold surface. Under the action of a certain pressure, the blank and the local surface of the mold directly produce friction. In addition, the deformation heat of the blank causes the blank and metal chips to melt on the mold surface, causing scratches on the workpiece surface. Follow the WeChat public account, Stamping Helper.

Preventive measures for common stainless steel forming defects

1 Choose the right stainless steel material

The commonly used materials in austenitic stainless steel are 1Cr18Ni9Ti and 0Cr18Ni9Ti, and the mechanical properties are shown in Table 1.

Table 1 Mechanical properties of austenitic stainless steel

During the deep drawing process, 1Cr18Ni9Ti is more stable than 0Cr18Ni9Ti and has better crack resistance. Therefore, 1Cr18Ni9Ti material should be selected as much as possible.

2 Reasonable selection of mold material

Stainless steel hardens significantly during deep drawing, producing many hard metal points, causing adhesion, making the workpiece and mold surface easy to scratch and wear, so general mold tool steel cannot be used.

Practice has proved that the selection of copper-based alloy molds can eliminate scratches and scratches on the surface of stainless steel parts and reduce the breakage rate. Another material is high-aluminum copper-based alloy mold material (containing aluminum 13wt%~16wt%). This material has low solubility with SUS304 stainless steel, there is no adhesion between the deep drawing part and the mold, and it is not easy to produce scratches and scratches on the surface of the deep drawing part. The product polishing cost is low and it has been successfully applied in the field of stainless steel deep drawing. However, due to the low hardness of this mold (40HRC ~ 45HRC), it is often used to produce products with a relatively small relative thickness t/D. Generally, after drawing 1500 to 2000 pieces, radial drawing edges starting from the fillet R are likely to appear on the surface of the die. Silicon nitride ceramics (Si3N4) have become an important engineering material, especially reaction-sintered silicon nitride ceramics, which have good high and low temperature mechanical properties, heat shock resistance and chemical stability, and can be easily made into parts with complex shapes. The high hardness, high wear resistance and high chemical stability of ceramic materials can be used to replace metal molds with reaction-sintered silicon nitride material molds to draw SUS304 stainless steel.

3 Choose reasonable convex and concave die radius

The die radius has a great relationship with the size and distribution of stress. Follow the WeChat public account, Stamping Helper. If the fillet radius is large, the blank holder ring will have insufficient pressing area, which is prone to instability and wrinkling; if the fillet is too small, the resistance of the material entering the die during deformation will increase, and the material will not flow and transfer inward easily, thereby increasing the maximum tensile stress in the force transmission area, which may cause cracking. Therefore, it is crucial to choose a reasonable fillet radius for the punch and die. Figure 2 Relationship between the relative fillet radius RP/T of the punch and the limit drawing force f

The relationship between the relative fillet radius RP/T of the punch and the limit drawing force f is shown in Figure 2, and the relationship between the relative fillet radius RD/T of the die and the drawing coefficient m of the cylindrical part is shown in Figure 3. It can be seen from Figure 2 that when the relative fillet radius RP/T of the punch is approximately equal to 4, it is most conducive to preventing cracking. It can be seen from Figure 3 that as the relative fillet radius of the die and the punch increases, the extreme deformation degree will increase, and it is beneficial to prevent cracking when the relative fillet radius of the die is 5mm to 8mm. Figure 3 Relationship between relative fillet radius rd/t of concave die and limit coefficient m

4 Drawing with tape thinning

Previous researchers have also proved through experiments that drawing with tape thinning can significantly reduce the maximum value of tangential residual stress of drawn parts and effectively prevent longitudinal cracking. According to different deformation degrees and original sheet thickness, select appropriate thinning coefficient Ψn (generally 0.9t～0.95t). If the value of Ψn is too small, the deformation stress will increase sharply, resulting in the bottom of the drawn part to break.

5 Adding intermediate annealing process in the drawing process

An intermediate annealing process should be carried out after multiple deep drawing, which can completely eliminate residual stress and restore the structure of austenitic stainless steel. For high-hardness stainless steel, intermediate annealing is generally required after 1 to 2 stretching processes. For example, 1Cr18Ni9Ti, the heating temperature is usually 1150℃～1170℃, the heating time is 30min, and it is cooled in air flow or water. Moreover, whether it is heat treatment between processes or heat treatment of the final product, it should be carried out as soon as possible after stretching to avoid deformation or cracking of the workpiece due to internal stress due to long-term storage. However, annealing and cleaning after annealing will increase the production cycle and affect the surface quality.

6 Use appropriate lubricants

Using appropriate lubricants has a significant effect on the stretching of stainless steel. Lubricants can form a film with certain toughness and elongation between the convex and concave dies, which is beneficial to the deep drawing of stainless steel.

For stainless steel stretch parts with large stretching deformation and difficult forming, polyvinyl fluoride film can be used as a lubricant in actual production. Polyvinyl fluoride film has excellent tear strength, certain toughness and elongation and is easy to clean. Pay attention to the WeChat public account, Stamping Helper After applying the dry film, the dry film can deform with the blank during the stretching process, and can always separate the blank from the mold. In addition, the film itself has a certain porosity and a large number of fiber cracks, so it can also store a certain amount of lubricating oil, so the film is equivalent to a layer of dry film lubricant. This lubrication method can effectively isolate the deformed stainless steel plate from the mold surface, and has a good lubrication effect, which is beneficial to improving the mold service life and product qualification rate.

7 Other preventive measures

The following preventive measures can also be taken during the stretching of stainless steel:

(1) Since white cast iron has good oil storage performance and is easy to form a lubricating oil film, the material of the blank holder can be white cast iron;

(2) Use a conical blank holder;

(3) The edge processing is smooth and there should be no microcracks or notches.