2. Factors affecting mold failure: (1) mold structure (2) mold material (3) hot and cold processing manufacturing process A, forging B, heat treatment C, cutting processing D, grinding processing, electrical processing 3. mold working conditions The effect of grinding on the life of the mold has not attracted people's attention. Due to the incorrect grinding process, the surface of the workpiece is burned, cracked, ground and grinding stress. This is the subsequent process and the mold during service. Mechanical fatigue, hot and cold fatigue, the origin of cracks, seriously affecting its working life.
(1) During grinding, the grinding wheel and the workpiece are in surface contact, the workpiece is plastically deformed during cutting and the frictional friction between the grinding wheel and the workpiece is sharp, so that equal and opposite grinding forces are formed between the grinding wheel and the workpiece. Due to the plastic deformation of the surface material, the metal molecules in the workpiece material are moved relative to each other to form internal friction and generate heat. The external friction between the grinding wheel and the workpiece also generates heat. This grinding heat generates local transients in the grinding zone. 1000 ° C high temperature, and the grinding wheel is not easy to transfer heat, so 80% of the heat is transferred into the workpiece and wear debris, and the metal changes in the solid state changes from temperature to another crystal lattice, metallurgy occurs The transformation of the structure, when grinding the hardened steel, the cooling is sufficient, the surface layer is subjected to secondary quenching, part of the retained austenite is transformed into martensite, and the specific volume of martensite is larger, the specific volume is increased, and the surface is subjected to compressive stress. If the grinding is not well cooled, or the coolant is not used, the surface will be tempered, martensite transformation will occur, and tensile stress will be generated on the surface (for example, the volume of iron will expand by 1% when γ-Fe is converted to α-Fe). (Residual Stress It can reach 500-1000MPa or 500-1000kg/mm2). If it exceeds the yield limit of the material, it will produce grinding cracks. In addition, after the heat treatment, the mold will not be tempered immediately, the quenching temperature is too high, and there is reticulated carbonization. Excessive tempered martensite or residual austenite after fire, phase change occurs during grinding, and stress occurs to cause cracks in the surface layer of the workpiece. Grinding cracks are very fine surface cracks, which are mostly perpendicular to the grinding direction and sometimes mesh, with a depth of 0.03 mm or less.
(2) Incorrect grinding and heat treatment processes during grinding, after grinding, cause surface annealing, burns, grinding cracks and residual stress on the workpiece surface, resulting in deformation of the workpiece. The main causes of mold fatigue failure are stress concentration and cyclic loading. The mold is normally served in a high-strength and low-plastic state, and the mold is subjected to cyclic loading, and the microcrack is elongated to finally cause fatigue failure.
(3) Measures to reduce grinding defects. There are many factors that cause grinding cracks and residual stresses, mainly from the process system.
The first is to choose the grinding amount reasonably, and the grinding depth is the main factor affecting the grinding heat. Increasing the speed of the workpiece and the grinding wheel can also reduce the occurrence of cutting heat. Secondly, it is reasonable to select and dress the grinding wheel, and the grinding wheel with white corundum is compared. Well, its performance is hard and brittle, and it is easy to produce new cutting edges. Therefore, the grinding force is small, the grinding heat is small, and the medium grain size is used in the particle size. For example, No. 46 No. 60 is better, and the hardness of the grinding wheel is medium soft and soft. (ZR1, ZR2 and R1, R2), that is, the coarse-grained, low-hardness grinding wheel has good self-excitation to reduce the cutting heat. Also pay attention to dressing the grinding wheel during grinding to keep the sharpening edge sharp. Then there is the rational use of the cooling lubricating fluid, which plays the three functions of cooling, washing and lubricating, keeps the cooling and lubrication clean, and thus controls the increase of the grinding heat, so that the grinding heat is within the allowable range, thus preventing the workpiece from being thermally deformed. In the fourth aspect, the quenching stress after heat treatment is reduced to a minimum, because the quenching stress and the reticulated carbonized structure under the action of the grinding force, the phase change of the structure is extremely easy to cause cracks in the workpiece. For high-precision molds, in order to eliminate the residual stress of grinding, after grinding, low-temperature aging treatment should be carried out to improve the toughness.
In short, the mold should pay full attention to the grinding process when manufacturing, and reduce the grinding microcracks and residual stress to a minimum to improve the service life of the mold.
Effect of grinding on die life
In order to improve the life of the mold, the molds that have failed are analyzed. The forms of mold damage mainly include: plastic deformation, wear, fatigue and hot and cold fatigue, fracture and cracking, and corrosion. In the process of service, the mold may have multiple forms of damage at the same time. The various damages penetrate and mutually promote each other, and each develops until the product produced by the mold is waste, and the mold fails. In order to improve the life of the mold (service life), it is necessary to carefully analyze the causes of the damage to the mold and various influencing factors, and formulate methods and measures for overcoming.