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Heat Treatment Process of Hot Extrusion Die

The manufacturing process of the hot extrusion die is generally: blanking→forging→preparatory heat treatment→machining→quenching and tempering→finishing. The following are the technological characteristics of each thermal processing procedure:

1. Forging process, hot extrusion die steel is mostly high alloy steel, so die blanks need to be forged well, especially hot work die steel containing molybdenum. Pay attention to the control of forging heating temperature and holding time to avoid serious Decarburization leads to early failure of the mold.

2. Preliminary heat treatment

Annealing. The annealing process of the hot extrusion die mainly lies in selecting the annealing temperature, ensuring sufficient holding time, and cooling at an appropriate cooling rate. In addition, in order to ensure good wear resistance, a certain amount of carbides must be retained after quenching. Since the shape of carbides has a great influence on the toughness of steel, attention should also be paid to the shape of carbides after annealing. It is generally desirable to obtain round and fine carbides.

High temperature quenching and tempering. In order to improve the mechanical properties (especially the fracture toughness) of the blank after forging, the pre-treatment of the blank is often carried out by quenching and tempering after forging. This heat treatment method is to heat the forged die blank to a high temperature for quenching, and then subject it to high temperature tempering. After this treatment, the carbides can be uniformly distributed, and the shape is round and small, which not only improves the performance of the steel, but also shortens the pretreatment cycle. The quenching heating temperature of quenching and tempering treatment can be determined according to different steel grades. For example, HM1 steel is 1200℃, which is similar to the conventional quenching temperature. The high temperature tempering temperature is generally between 700~750℃.

Normalizing after forging. After forging, there is a blank with carbides along the crystal chain, which needs to be eliminated by normalizing and then spheroidizing annealing. Because this kind of chain carbide is difficult to eliminate by annealing.

Quenching and tempering When selecting the quenching temperature of commonly used hot extrusion die steel, the main consideration is the size of the austenite grain size and the impact toughness. Secondly, the working conditions, structure and shape of the die should be considered. The performance requirements of the failure form.

For the selection of the quenching holding time, the main consideration is to determine the completion of the structural transformation, so that the carbon and alloy elements are fully dissolved to ensure high tempering resistance and thermal hardness. The quenching holding time coefficient is generally 0.5~1mm/min in the salt furnace. The smaller the size, the larger the coefficient.

Since the hot extrusion die steel is a high-alloy steel with good hardenability, oil cooling or air cooling can be used for quenching and cooling. Austempering or graded quenching can also be used for molds that require small deformation.

The correctness of the tempering process plays an important role in the failure mode of the mold. The principle of choosing the tempering temperature is to increase the hardness of the mold as much as possible without affecting the brittle fracture resistance of the mold. This requires determining the tempering parameters according to the specific failure mode of the mold.

The quenched molds should be tempered as soon as possible, especially for molds with complex shapes. When the mold surface temperature is lower than 80°C, tempering must be performed. In order to avoid the generation of residual stress. The tempering heating and cooling should be carried out slowly.

Tempering is generally carried out twice, and the tempering time can be calculated as 3 min/mm, but it should not be less than 2h. The second tempering temperature can be 10~20℃ lower than the first time.