Metal forming is the process of plastic deformation of metal materials by applying external force to obtain the desired shape and size. Metal deep-drawing process is an important method of metal forming, it is mainly for plate processing, through the deep-drawing mold so that the metal plate to produce local plastic deformation, the formation of open hollow parts. Stainless steel deep drawing refers to the stainless steel material through the deep drawing process into a variety of shapes, due to the stainless steel has good corrosion resistance and molding properties, so it occupies an important position in the metal deep drawing process. The relationship between the three is that the metal deep drawing process is a means of metal molding, and stainless steel deep drawing is a metal deep drawing process in the application of specific materials.
Analysis of common problems in the process of stainless steel deep drawing.
1, the reasons for the formation of cracking
Austenitic stainless steel cold work hardening index is high (0.34 for stainless steel). Austenitic stainless steel is sub-stable, in the deformation of the phase change will occur, inducing the martensitic phase. The martensitic phase is 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, the greater the residual stress. Residual stress and martensite content of the relationship: the higher the content of induced martensite phase, caused by the residual stress is also greater, in the process of processing is also more prone to cracking.
2, the reasons for the formation of surface scratches
Stainless steel deep-drawing surface scratches mainly due to the workpiece and mold surface relative movement, under the action of a certain pressure, resulting in billet and mold local surface friction directly, coupled with the billet's deformation of the heat so that the billet and the metal chips fused on the surface of the mold, so that the surface of the workpiece abrasions produced scratches.
Stainless steel common molding defects in preventive measures
1, choose the right stainless steel material: in the austenitic stainless steel commonly used materials are 1Cr18Ni9Ti and 0Cr18Ni9Ti. 1Cr18Ni9Ti is more stable than 0Cr18Ni9Ti in the deep-drawing process, and has good crack resistance. Therefore, 1Cr18Ni9Ti material should be selected as far as possible.
2, a reasonable choice of mold materials stainless steel in the deep drawing process of hardening significantly, resulting in many hard metal points, resulting in adhesion, so that the surface of the workpiece and the mold is easy to scratch, wear and tear, and therefore can not be used in general mold tool steel. Practice has proved: the choice of copper-based alloy molds can eliminate the stainless steel parts surface scratches, scratches, reduce the breakage rate. Another material for high aluminum copper-based alloy mold materials (containing aluminum 13Wt% ~ 16Wt%), this material and SUS304 stainless steel mutual solubility is small, deep-drawing parts and molds do not adhere to the surface of the deep-drawing parts is not easy to produce scratches scratches, product polishing cost is low, in the field of stainless steel deep-drawing forming has been successfully applied. However, due to the low hardness of this mold (40HRC ~ 45HRC), commonly used in the production of relative thickness t / D smaller products. General deep-drawing 1500 pieces ~ 2000 pieces after the surface of the concave mold is easy to produce from the rounded corner R at the radial deep-drawing prism. Silicon nitride ceramics (Si3N4) has become an important engineering materials, especially reaction sintered silicon nitride ceramics, with good mechanical properties at high and low temperatures, thermal shock resistance and chemical stability, but also can be very easy to make complex shaped parts. Ceramic materials can be used to take advantage of the high hardness, high wear resistance and high chemical stability, reaction sintered silicon nitride material mold instead of metal mold deep drawing SUS304 stainless steel.
3, choose a reasonable convex and concave mold angle concave mold angle and stress size and distribution has a great relationship. The radius of the round corner is large, the pressure area of the crimping circle is insufficient, and it is easy to produce instability and wrinkle; and if the round corner is too small, the resistance of the material to enter the concave mold in the deformation process will increase, and the material is not easy to flow inward and transfer, which increases the maximum tensile stress in the force transmission area, and it may lead to tensile cracking. Therefore, it is crucial to choose reasonable radius of the rounding angle of the convex and concave molds. The relative fillet radius of the convex mold, rp/t, is most conducive to preventing cracking when it is approximately equal to 4. Convex and convex mold relative radius increases, the degree of extreme deformation will increase, the relative radius of the concave mold to take 5mm ~ 8mm is conducive to prevent cracking.
4, the use of band thinning deep-drawing previous explorers through the test also proved that the use of band thinning deep-drawing can greatly reduce the maximum tangential residual stress of the deep-drawing parts, can effectively prevent the occurrence of longitudinal cracking. According to different degrees of deformation and the original sheet thickness, select the appropriate thinning coefficient Ψn (generally 0.9t ~ 0.95t), if the value of Ψn is too small will make the deformation stress increases sharply, which will lead to the bottom of the deep-drawing parts rupture.
5, in the deep drawing process to join the intermediate annealing process can completely eliminate residual stress, and restore the organization of austenitic stainless steel. For high hardness stainless steel, intermediate annealing is required. Such as 1Crl8Ni9Ti, usually heated at 1150 ℃ ~ 1170 ℃, heating time of 30min, and cooled in the airflow or water. And, whether it is inter-process heat treatment, or the final product heat treatment, should be carried out as far as possible immediately after deep-drawing, so as to avoid deformation or cracking due to long-term storage, the workpiece due to the role of internal stress.
6, the use of appropriate lubricants using appropriate lubricants for stainless steel deep drawing has obvious effects. The lubricant can form a layer of toughness and elongation of the film between the convex and concave molds, which is conducive to the deep-drawing forming of stainless steel. For deep-drawing deformation degree is large, forming difficult stainless steel deep-drawing parts, in the actual production can be used in polyvinyl fluoride film to act as a lubricant. Polyfluoroethylene film has excellent tear strength, a certain toughness and elongation and easy to clean. Coated with dry film, in the deep-drawing process, the dry film can be deformed with the blank, can always be separated from the blank and the mold, coupled with the film itself has a certain porosity and a large number of fiber cracks, it can also store a certain amount of lubricant, so the film is equivalent to a layer of dry film lubricant. This lubrication can effectively deform the stainless steel plate and mold surface isolation, lubrication effect is good, is conducive to improving the service life of the mold and product qualification rate.
7, other preventive measures
In the deep drawing of stainless steel can also take the following precautions: (1) due to the good oil storage performance of white cast iron, easy to form a lubricant film, the material of the crimping ring can be used white cast iron; (2) the use of conical crimping ring; (3) smooth edge processing, there can be no micro-crack notch.