What causes cracking of stainless steel products?

Time：2024-06-04 Source： Hits：515

The elongation of stainless steel is small, the elastic modulus E is large, and the hardening index is high. The cracking of stainless steel plate sometimes occurs after the drawing deformation, and sometimes occurs immediately when the drawing part exits from the die. Sometimes it occurs when it is hit or vibrated after deep drawing deformation; It also sometimes occurs after a period of storage or during use after deep drawing deformation.

Austenitic stainless steel has a high cold work hardening index (0.34 for stainless steel). Austenitic stainless steel is a metastable type, which will undergo phase transformation during deformation and induce martensitic phase. Martensite is relatively brittle and therefore prone to cracking. In plastic deformation, with the increase of deformation, the induced martensite content will also increase with the increase of deformation, and the residual stress will be larger. The relationship between residual stress and martensitic content: the higher the induced martensitic phase content, the greater the residual stress caused, and the easier it is to crack during processing. The scratches on the surface of stainless steel drawing parts are mainly due to the relative movement of the workpiece and the surface of the mold, under the action of a certain pressure, resulting in direct friction between the billet and the local surface of the mold, and the deformation heat of the billet makes the billet and metal chips melt on the surface of the mold, so that the workpiece surface scratches.

Stainless steel hardening significantly in the deep drawing process, resulting in many hard metal points, resulting in adhesion, so that the workpiece and the surface of the mold is easy to scratch, wear, so can not use the general mold tool steel. Practice has proved that the selection of copper alloy mold can eliminate scratches and scratches on the surface of stainless steel parts and reduce the breakage rate. The other material is high aluminum copper base alloy mold material (containing aluminum 13Wt% ~ 16Wt%), this material and SUS304 stainless steel mismissibility is small, the drawing part and the mold is not sticking, the drawing part surface is not easy to produce scratches, the product polishing cost is low, in the field of stainless steel drawing forming has been successfully applied.

The selection of reasonable convex and concave die fillet has a great relationship with the stress size and distribution. Large radius of rounded corner and insufficient area of pressing ring make it easy to produce buckling and wrinkling. If the rounded corner is too small, the resistance of the material entering the die during deformation will increase, and the material is not easy to flow and transfer inward, thus increasing the maximum tensile stress in the force transfer zone, which may lead to tensile cracking.

An intermediate annealing process should be carried out after multiple drawing, which can completely eliminate the residual stress and restore the structure of austenitic stainless steel. And, whether it is the heat treatment between processes or the final heat treatment of the finished product, it should be carried out as soon as possible after deep drawing, so as to avoid deformation or cracking of the workpiece due to internal stress due to long-term storage. However, annealing and cleaning after annealing will lead to an increase in the production cycle and affect the surface quality.

The use of appropriate lubricants The use of appropriate lubricants on the drawing of stainless steel has obvious effects. The lubricant can form a film with a certain toughness and elongation between the convex and concave dies, which is conducive to the deep drawing of stainless steel. This lubrication method can effectively isolate the deformed stainless steel plate from the mold surface, and the lubrication effect is good, which is conducive to improving the service life of the mold and the pass rate of the product.

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