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Defects in the mold manufacturing process and preventive measures

update:22-10-2020
abstract:

1. Forging processing    High carbon, high alloy steels […]

1. Forging processing
   High carbon, high alloy steels, such as Cr12MoV, W18Cr4V, etc., are widely used to make molds. However, this type of steel has various defects such as composition segregation, coarse and uneven carbides, and uneven structure. When using high-carbon and high-alloy steel to make molds, a reasonable forging process must be used to form the module blanks. This way, on the one hand, the steel can reach the size and specifications of the module blanks, and on the other hand, the structure and properties of the steel can be improved. In addition, high-carbon, high-alloy die steel has poor thermal conductivity, and the heating speed should not be too fast, and the heating should be uniform. Within the forging temperature range, a reasonable forging ratio should be used.
  2, cutting
  The cutting process of the mold should strictly ensure the fillet radius at the size transition, and the arc and the straight line should be smooth. If the cutting quality of the mold is poor, it may cause mold damage in the following three aspects: 1) Due to improper cutting, sharp corners or too small fillet radius will cause severe stress concentration during mold work . 2) If the surface after cutting is too rough, there may be defects such as knife marks, cracks, cuts, etc. They are not only stress concentration points, but also the initiation of cracks, fatigue cracks or thermal fatigue cracks. 3) The cutting process fails to completely and uniformly remove the decarburized layer generated during rolling or forging of mold burrs, which may produce uneven hardened layer during mold heat treatment, resulting in decreased wear resistance.
   3. Grinding
   Molds generally need to be ground after being fired and tempered to reduce the surface roughness value. Due to the influence of factors such as too high grinding speed, too fine grinding wheel particle size or poor cooling conditions, local overheating of the mold surface caused by local microstructure changes, or surface softening, reduced hardness, or high residual tensile stress Such phenomena will reduce the service life of the mold. Select appropriate grinding process parameters to reduce local heating. After grinding, stress relief treatment under possible conditions can effectively prevent grinding cracks. There are many measures to prevent grinding overheating and grinding cracks, such as: using a coarse-grained grinding wheel with strong cutting force or a grinding wheel with poor adhesion to reduce the grinding feed rate of the mold; selecting a suitable coolant; grinding processing After tempering at 250-300℃, the grinding stress is eliminated.
  4, electrical discharge machining
When the EDM process is used to process the mold, the current density in the discharge zone is very large, and a lot of heat is generated. The temperature of the processed area of ​​the mold is as high as about 10,000 ℃. It melts due to high temperature, then cools rapidly, and solidifies quickly to form a resolidified layer. It can be seen under the microscope that the resolidified layer is white and bright with many micro-cracks inside. In order to prolong the life of the mold, the following measures can be adopted: adjust the EDM parameters and grind the surface after the EDM by electrolytic or mechanical grinding to remove the white layer in the abnormal layer, especially to remove the microcracks. Arrange a low-temperature tempering after EDM to stabilize the abnormal layer and prevent the propagation of microcracks.