Research on the deformation and deformation causes of sophisticated and complex molds, and explore measures to reduce and control the deformation of sophisticated molds to improve the quality and service life of mold products.
one. Mold material influence
1. Mold selection
A mold company considers the selection of materials and heat treatment as simple as possible. The T10A steel is made of a complex mold with a large difference in cross-section dimensions and requires less deformation after quenching. The hardness requirement is 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 company was made of micro-deformed steel Cr12 steel, and the hardness and deformation of the mold after heat treatment met the requirements. Precautionary measures: To manufacture precision and complex molds requiring less deformation, it is necessary to use micro-deformed steel, such as air-quenched steel.
2. Mold material influence
A certain batch of Cr12MoV steel was sent from a factory to manufacture more complex molds. The molds all had Φ60mm round holes. After the heat treatment of the mold, some of the mold holes appeared elliptical, causing the mold to be scrapped. In general, Cr12MoV steel is a micro-deformed steel and should not exhibit large deformation. We carried out metallographic analysis of the severely deformed mold 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 because of the presence of uneven carbides distributed in a certain direction in the die steel. The coefficient of expansion of the carbide is about 30% smaller than that of the steel. When heated, it prevents the inner hole of the mold from expanding, and prevents the inner hole of the mold from shrinking when cooled. The unevenness of the inner hole of the mold is caused, 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. Don't use cheap steel.
2 For mold steels with serious segregation of carbides, reasonable forging is required to break up the carbide ingots, reduce the grade of uneven distribution of carbides, and eliminate the anisotropy of properties.
3 For the forged die steel, heat treatment should be carried out 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, the solid solution double refining treatment can be used to make the carbides refine and evenly distributed, and the edges and corners are rounded, which can reduce the heat treatment deformation of the mold.
two. 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. Cause of deformation
Due to the 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, which results in different volume expansion of each part, which causes deformation of the mold after quenching.
2. Precaution
When designing the mold, in order to meet the actual production needs, the mold thickness and the structural asymmetry should be minimized. At the thick and thin junction of the mold, the structural design such as smooth transition should be adopted as much as possible. According to the deformation rule 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 combined structure can be employed.
three. Mold manufacturing process and 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 the mechanical processing and final heat treatment stages.
1. Cause of 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. Precaution
(1) After roughing and semi-finishing, a stress relief annealing shall be carried out, that is, (630-680) °C × (3-4) h furnace cooling to below 500 °C, air cooling may be used, and 400 °C × (2- 3) h stress relief treatment.
(2) Reduce the quenching temperature and reduce the residual stress after quenching.
(3) Oil-cooled (graded quenching) with 170oC 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.
four. Effect of heat treatment heating process
Effect of heating rate
The deformation of the mold after heat treatment is generally considered to be caused by cooling, which is not true. 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 inconsistency of the expansion of the various parts in the mold, resulting in heating. Uneven internal stress. 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 center portion, the greater the stress, and the greater the deformation caused by the heat treatment of the mold.
(2) Preventive measures
Slow heating should be performed on complex molds that are heated below the phase transition point. In general, mold vacuum heat treatment deformation is much smaller than salt bath furnace heat quenching. With preheating, one preheating (550-620oC) can be used for low alloy steel molds; secondary preheating (550-620oC and 800-850oC) should be used for high alloy steel molds.
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Label: How to reduce and control sophisticated mold deformation
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