Defects in the mold manufacturing process and preventive measures

1. Forging processing

High carbon and 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, so that on the one hand, the steel can reach the size and specifications of the module blanks, and on the other hand, the structure and performance 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) If the cutting process fails to completely and uniformly remove the decarburized layer generated during rolling or forging of the mold burrs, an uneven hardened layer may be produced during the heat treatment of the mold, resulting in a decrease in wear resistance.

3. Grinding

Moulds are generally grinded after quenching and tempering 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. Choose appropriate grinding process parameters to reduce local heating. After grinding, stress relief treatment under possible conditions can effectively prevent the generation of 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 can be eliminated.

4, EDM

When the EDM process is used to process the mold, the current density in the discharge area is very large, and a lot of heat is generated. The temperature of the processed area of u200bu200bthe mold is as high as about 10000℃. Due to the high temperature, the metallographic structure of the heat-affected zone will inevitably change , The surface layer of the mold melts due to high temperature, and then is quenched and solidified quickly to form a resolidified layer. It can be seen under the microscope that the resolidified layer is bright and white with many micro-cracks inside. In order to prolong the life of the mold, the following measures can be used: Adjust the EDM parameters to grind the surface after EDM by electrolytic or mechanical grinding to remove the white layer in the abnormal layer, especially to remove the microcracks. After EDM. Arrange a low-temperature tempering to stabilize the abnormal layer and prevent the propagation of microcracks.