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First, the impact of mold materials
1. Selection of mold: A mechanical factory chooses T10A steel for simple consideration of the difference in cross-section size and requires less deformation after quenching. The hardness requires 56-60HRC. After the heat treatment, the hardness of the mold meets the technical requirements, but the deformation of the mold is large and cannot be used, resulting in the scrapping of the mold. Later, the plant was made of micro-deformed steel Cr12 steel, and the hardness and deformation of the mold after heat treatment met the requirements. Precautionary measures: Therefore, to manufacture precision and complex molds requiring less deformation, it is necessary to use micro-deformed steel, such as air-quenched steel.
2. The influence of mold material: a factory sent a batch of Cr12MoV steel more complex molds, the molds have ¢60m m round holes, after the mold heat treatment, some mold round holes appear elliptical, resulting in mold scrap. In general, Cr12MoV steel is a micro-deformed steel and should not exhibit large deformation. We have carried out metallographic analysis of the severely deformed molds and found that the mold steel contains a large amount of eutectic carbides and is distributed in a strip shape and a block shape.
(1) Causes of mold ellipse (deformation): This is due to the presence of uneven carbides distributed in a certain direction in the die steel. The expansion coefficient of carbide is about 30% smaller than that of steel. When it is heated, it prevents the mold. The inner hole expands, and when the cooling is performed, the inner hole of the mold is prevented from contracting, and the inner hole of the mold is deformed unevenly, so that the round hole of the mold is elliptical.
(2) Preventive measures: 1 When manufacturing sophisticated molds, try to choose mold steel with less segregation of carbides. Do not use cheap steel. 2 For the die steel with serious segregation of carbides, reasonable forging should be carried out to break up the carbide ingots, reduce the grade of uneven distribution of carbides, and eliminate the anisotropy of properties. 3 The forged die steel shall be subjected to quenching and tempering heat treatment to obtain a uniform, fine and dispersed carbide structure of the carbide, thereby reducing the deformation of the precision complex mold after heat treatment. 4 For molds with large size or forging, it can be treated by solid solution double refining, which makes the carbide refinement, uniform distribution and rounded corners, which can reduce the heat treatment deformation of the mold.
Second, the impact of mold structure design
Some mold materials and steel materials are very good, often because of the unreasonable design of the mold structure, such as thin edges, sharp corners, grooves, abrupt steps, thickness and other disparities, resulting in large deformation of the mold after heat treatment.
1. Reasons for deformation: Due to uneven thickness or sharp rounded corners of the mold, different thermal stress and tissue stress between the various parts of the mold are caused during quenching, resulting in different volume expansion of each part, so that after quenching of the mold Deformation occurs.
2. Precautionary measures: When designing the mold, in the case of meeting the actual production needs, the thickness of the mold should be reduced as much as possible, and the structure is asymmetrical. At the thick and thin junction of the mold, a smooth transition and other structural design should be adopted as much as possible. According to the deformation law of the mold, the machining allowance is reserved, and after the quenching, the mold is not scrapped due to the deformation of the mold. For molds with particularly complicated shapes, in order to make the cooling uniform during quenching, a joint structure can be employed.
Third, the mold manufacturing process and the impact of residual stress
It is often found in the factory that some molds with complicated shapes and high precision require large deformation after heat treatment. After careful investigation, it is found that the mold has not undergone any pre-heat treatment in mechanical processing and final heat treatment.
1. Reasons for deformation: The residual stress in the machining process and the stress after quenching are superimposed, which increases the deformation of the mold after heat treatment.
2. Precautionary measures: (1) After roughing and semi-finishing, a stress-relieving annealing should be carried out, that is, (630-680) °C × (3-4) h furnace cooling to below 500 ° C, air cooling, 400 can also be used. °C × (2-3) h stress relief treatment. (2) Reduce the quenching temperature and reduce the residual stress after quenching. (3) Air-cooling (classified quenching) with 170# C of quenching oil. (4) The quenching residual stress can be reduced by the austempering process. By adopting the above measures, the residual stress after the quenching of the mold is reduced, and the deformation of the mold is small.
Fourth, the impact of heat treatment heating process
1. Effect of heating rate: The deformation of the mold after heat treatment is generally considered to be caused by cooling, which is not correct. Molds, especially complex molds, have a large impact on the deformation of the mold. The comparison of some mold heating processes shows that the heating speed is faster and tends to produce larger deformation.
(1) Reasons for deformation: Any metal should be expanded when heated. Because the steel is heated, the temperature of each part in the same mold is uneven (that is, the uneven heating) will inevitably cause the expansion of the various parts in the mold. Sex, thereby forming internal stresses due to uneven heating. At temperatures below the phase transition point of steel, uneven heating mainly produces thermal stress, which is unevenly heated beyond the phase transition temperature, and also causes unequality of tissue transformation, which causes both tissue stress. Therefore, the faster the heating rate, the greater the difference in temperature between the surface of the mold and the core, the greater the stress, and the greater the deformation caused by the heat treatment of the mold.
(2) Preventive measures: When the complex mold is heated below the phase change point, it should be slowly heated. Generally speaking, the mold vacuum heat treatment deformation is much smaller than the salt bath furnace heating quenching. ? Preheating, one preheating (550-620 ° C) for low alloy steel molds; secondary preheating (550-620 ° C and 800-850 ° C) for high alloy steel molds.
Analysis and Treatment of Deformation Causes of Precision Die Heat Treatment
Reasons for the heat treatment of precision molds and their prevention methods: To study the deformation and deformation causes of sophisticated molds, and to explore measures to reduce and control the deformation of sophisticated molds to improve the quality and service life of mold products.