Stress includes thermal stress and mechanical stress, mainly generated by mechanical, chemical, operational impact, and thermal factors. Specifically, it is generated in the following aspects:
1、 During the die-casting production process
1. A cooling and temperature control system should be installed to maintain the working temperature of the mold within a certain range.
During the production process, the mold temperature continues to rise. When the mold temperature overheats, it is easy to cause mold sticking and malfunction of moving parts, resulting in surface damage to the mold.
3. The mold should be preheated to a certain temperature before production, otherwise, when the high-temperature metal liquid is filled, it will produce rapid cooling, causing an increase in the temperature gradient between the inner and outer layers of the mold, forming thermal stress, and causing the surface of the mold to crack or even crack.
2、 During mold processing
1. The stress generated during steel quenching is the result of the superposition of thermal stress during the cooling process and structural stress during phase transformation. Quenching stress is the cause of deformation and cracking, and tempering must be carried out to eliminate stress.
2. Improper heat treatment can lead to mold cracking and premature scrapping, especially when only quenching and tempering are used without quenching, followed by surface nitriding process. After thousands of die casting cycles, surface cracking and cracking will occur.
3、 In the process of mold processing and manufacturing
1. Electrical discharge machining generates stress. A white bright layer enriched with electrode and dielectric elements is formed on the surface of the mold, which is hard and brittle. This layer itself will have cracks and stress. High frequency should be used during electrical discharge machining to minimize the white bright layer, which must be removed by polishing and tempered at the tempering temperature.
2. Quality issues with rough forging.
Some molds only produce a few hundred pieces before cracking occurs, and the cracks develop quickly. It is possible that during forging, only the external dimensions were guaranteed, while loose defects such as dendritic crystals, inclusions of carbides, shrinkage cavities, and bubbles in the steel were extended and elongated along the processing, forming streamline lines. These streamline lines have a significant impact on the final quenching deformation, cracking, brittle fracture, and failure tendency during use in the future.
During the grinding of quenched steel, grinding stress is generated, which generates frictional heat, resulting in softening and decarburization layers, reducing thermal fatigue strength and easily leading to hot cracking and early cracking. After precision grinding, H13 steel can be heated to 510-570 ℃ and subjected to stress relief annealing with a thickness of every 25mm held for one hour.
4. The cutting stress generated during final processing such as turning, milling, and planing can be eliminated through intermediate annealing.
I believe that by following the above three processes step by step and carefully inspecting, the losses caused by stress can be reduced, and the output of die-casting molds will also be improved to a certain extent.
