The elongation ability of stainless steel is small, the elastic modulus E is larger, and the hardening index is higher. The cracking of the SS material sometimes occurs in drawing process e.g in course of deformation, sometimes it happens immediately after the part is withdrawn from the dies. Sometimes it occurs when the drawing parts are impacted or vibrated in the deformation process. And sometimes it happens after a period of time storage of the finished tooling or in the course of using of the dies.

Analysis of Common Problems in Stainless Steel Deep Drawing Process

1 Cause of cracking

Austenitic stainless steel has a high cold hardening index (0.34% for SS). Austenitic stainless steel is a meta-stable type, which will undergo transformation during deformation thus induce martensitic phase. The martensite phase is brittle, and easy to crack. In the course of plasticity deformation, with the increase of extension, the martensitic content will then increase, the residual stress will all be increased. The relationship between residual stress and martensite content is, the higher the content of induced martensite phase, the greater the residual stress, and the easier cracking is ocurring during processing.

Causes for surface scratches

The main reason for all scratches on the surface of stainless steel drawing parts is due to the relative movement of the workpiece and the surface of the die, which, under certain pressure, results in the direct friction between the material and the surface of the die. In addition, the heat produced during the process is also a reason, it makes the raw material and deposited metal chips cling to the surface of the die, resulting in scratches on the surface of the workpiece.

Preventive Measures of the Common Defects in Stainless Steel
(1) choose the most appropriate stainless steel material: in austenitic stainless steel, the commonly used materials are 1Cr18Ni9Ti and 0Cr18Ni9Ti. During drawing, 1Cr18Ni9Ti is more stable than 0Cr18Ni9Ti and has good cracking resistance. Therefore, 1Cr18Ni9Ti materials should be selected as far as possible.

(2) Choose the correct die material. In the deep drawing process, Stainless steel will be hardened greatly, producing lots of hard metal spots, resulting in adhesion, so it will induce scratching and wear between workpiece and die surface, so the SS should not be chosen for the dies. It has been proved that copper based alloy die can eliminate surface wearing, scratch and reduce a damage rate of SS parts. Another kind of material is high aluminum copper based alloy die material (containing aluminum 13Wtt to 16W t), this kind of material has little solubility with SUS304 stainless steel, the drawing parts do not stick the dies, it is easy not o produce scratches on the parts surface, so the polishing cost of products is low. It has been successfully applied in the field of stainless steel drawing. However, due to the low hardness of the mould, 40HRC (45HRC) is often used to produce products with relatively low thickness t / D. Generally, after 1500 pieces, it is prompted to produce radial edges at the corner R on the surface of the die. Silicon nitride ceramics (Si3N4) has become an important engineering material, especially the reactive sintering silicon nitride ceramics. It has good mechanical properties at high and low temperature, heat resistance impact 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 utilized. So, instead of metal dies, the reactive sintering silicon nitride mould is used to produce SUS304 stainless steel parts.

(3) Select reasonable cavity (Concave) and core (Convex) Radius Degree, the cavity die radius has a great deal to do with the size and distribution of stress. The larger radius of the fillet, the more insufficient area of the blank holder, it will be easy to cause instability and wrinkle. If the fillet is too small, the resistance of the material to enter the die or cavity will increase during in the forming process, so the material is not easy to flow and transfer inward. Thus, the tensile stress in the transmission zone is increased, which may lead to tensile cracking. Therefore, to select a reasonable convex, cavity radius is crucial. When the relative fillet radius rp/t of the punch is about 4, it is most advantageous to prevent the cracking. The relative fillet radius of die and punch increases, and the degree of polar deformation will increase. When the relative radius of die is 5mm~8mm, the cracking may not be likely happen.

(4) it is also proved by experiments that the maximum tangential residual stress can be greatly reduced by using the thinning drawing, which can effectively prevent the longitudinal cracking. According to the different degree of deformation and the thickness of the original sheet metal, the appropriate thinning coefficient ψ n (0.9t ~ 0.95 t) is chosen. ButiIf the value of ψ n is too small, the deformation stress will increase sharply, which will lead to the fracture of the bottom of the drawing piece.

(5) Add an intermediate annealing process in the drawing process should deep drawing be carried out several times, which can completely eliminate the residual stress and restore the microstructure of austenitic stainless steel. For stainless steel with high hardness, intermediate annealing is required after 1 or 2 drawing processes. For example, 1Crl8Ni9Ti is usually heated at 1150 ℃ or 1170 ℃ for 30 mins and cooled in air or water. Moreover, heat treatment between processes or final products should be carried out as soon as possible after being drawn so as not to cause deformation or cracking due to internal stress in the workpiece from long-time storage. However, annealing and cleaning after annealing will increase the production cycle and affect the surface quality.

(6)Use appropriate lubricants

Lubricants have obvious effect on the deep drawing of stainless steel. The lubricant can form a film layer between the convex and concave die with certain toughness and elongation, thus it is beneficial for the deep drawing of stainless steel. For the deep drawn, difficult stainless steel drawing parts, Polyvinylidene film can be used as a lubricant in actual production. PVDF film has excellent tearing strength, certain toughness and elongation and easy to wash. After coating dry film, the dry film can be formed with the blank material in the process of drawing, and the part can always be separated from the mold. The film itself has a certain porosity and a large amount of fiber, so it can store certain amount of lubricant, which acts a layer of dry film lubricants. This kind of lubrication can effectively separate the formed stainless steel plate from the mold surface, and its effect is good and is beneficial to improve the service life of the die and the rate of the good parts.

(7)Other Preventive Measures

The following precautions can also be taken into consideration during the process

1.because white cast iron has good oil storage properties, it is easy to form a lubricating oil film, so the material of the holder ring can be made of white cast iron

2.the edge of holder ring can be conical shape

3.The edge should be smooth, there should be no dents or fissures.

A corporate provided a solution to the drawing defect of Auto muffler shell for FORD motor company (the material is stainless steel), cracking often occurs in both pre-draw and draw processes. In the actual factory production, the probability of deep-draw cracking is about 5% or 8%. For this reason, annealing and pickling are added between pre-draw process and forming process, but the effect is not obvious.

1. The reason of the part cracking is analyzed as follows. The workpiece is difficult to draw because of a deep depth of the workpiece (the drawn depth is 94 mm ~ 95 mm), the head section is small, the wall is thin and so on. The cracking position of the workpiece appears at the radius of the punch and the radius of the die. Through analysis, it is concluded that the causes of workpiece deep draw cracking are as follows, (1) the reason for cracking at the corner or radius of the punch (where the thinnest area) is the tensile stress exceeds the strength limit of the material. (2) Unreasonable flange area, thus causing the material forming resistance to grow bigger, thus causing cracking.

As stated above, the corner radius of the die has a great relationship with the distribution of the internal stresses. Because of the small corner of the die, the forming resistance of the material and the maximum tensile stress in the transit zone are increased. As a result, the stress in the forming zone is thus enlarged, causing the part to crack at the corner of the die. Measures Taken only, the usual lubrications. The friction on the surface of the same die increases the forming resistance when the material cures, so tensile stress is also increased accordingly — This hinders the flow of materials during forming process.

(2) According to the analysis above for the causes of the cracking, the following improvement measures are adopted to reduce the cracking rate of the deep drawing parts.

(1)Use lubricant dry coating materials on the surface of die (main ingredients include nitride cotton, oleic alkyd resin and plasticizer, pressure agent material, etc.) to reduce the friction resistance between the material and the surface of the die during the forming process, it makes the material flow more easily from the deformation zone to the transit zone. The dry film of the lubricant can separate the die from the blank materials, thus preventing the surface from scratching the parts and bonding of the mould, this way it improves the surface quality and the yield of good parts. At the same time, the dry film itself has a certain toughness, good for the forming process.

(2)It is propitious to optimize the diameter of the flange, reduce the maximum tensile stress and reduce the forming resistance, the transverse dimension of the die is increased for the first step, and the radius of the die for the first drawing process is increased.

(3)After comprehensive analysis, it was decided to add an edge cutting process after the end of the pre-draw process to remove the excess material as much as possible. Reducing the material forming resistance, effectively preventing the material from cracking in the process of further deformation, (4) Merge the final two processes of the original process (that is, the forming drawing, The flanging process integrates it into a single process (forming and flanging, so that the processing steps of the part are not increased, thus effectively controlling the processing cost.

Through the above series of improvement measures, the cracking of this part in the process of pre-drawing and final drawing is well solved. Conclusion, although as often as there is a cracking problem to stainless steel products, and also other defects, such as wrinkling and surface scratching, it is by selecting stainless steel material with good formability, reasonable convex and concave die fillet, etc. that the success rate of SS deep drawing products can be greatly improved! And often high quality stainless steel products can be reached by adding proper heat treatment process and reasonable preventive measures such as coating lubricant on the surface of die. At the same time, using this research results, the cracking problem of automobile muffler shell in pre-drawing process and forming process can be solved successfully, which has been praised the factory and achieved much better economic and social benefits.