Stress includes thermal stress and mechanical stress, w […]
Stress includes thermal stress and mechanical stress, which are mainly generated from mechanical, chemical, operational shock, and heat. Explains several aspects after stress generation.
I. In the process of die-casting
1. A cooling temperature control system should be set to keep the working temperature of the mould within a certain range.
2.During the production process, the mould temperature keeps rising. When the mould temperature is too hot, it is easy to produce sticky moulds, and the moving parts fail to cause damage to the mould surface.
3. The mould should be pre-heated to a certain temperature before production, otherwise, when the high temperature metal liquid is filled, it will cause chilling, which will cause the temperature gradient of the inner and outer layers of the mould to increase, forming thermal stress, cracking and even cracking the mould surface.
Second, the mould processing process.
1. The stress generated during quenching is the result of the superposition of the thermal stress during cooling and the structural stress during the phase transition. The quenching stress is the cause of deformation and cracking, and it must be tempered to eliminate the stress.
2. Improper heat treatment will lead to cracking and premature scrapping of the mould, especially using only quenching and tempering, and then surface nitriding, which will cause surface cracking and cracking after thousands of die casting.
Third, in the mould processing and manufacturing process
1. EDM produces stress. A bright white layer rich in electrode elements and dielectric elements is produced on the surface of the mould, which is hard and brittle. This layer itself has cracks and stress. EDM should use high frequency to minimize the white bright layer. It must be polished and removed, and tempered. Tempering is performed at tempering temperature.
2. Quality problems of blank forging.
Some moulds cracked after only a few hundred pieces were produced, and cracks developed rapidly. It may be that only the external dimensions are guaranteed during forging, and the loose defects such as dendritic crystals, inclusion carbides, shrinkage pores, and bubbles in the steel are extended and elongated along the processing to form a streamline. Hardening deformation, cracking, brittle cracking during use, and failure tendency have a great impact.
3. Grinding stress is generated during grinding of quenched steel, frictional heat is generated during grinding, and a softened layer and a decarburized layer are generated, which reduces the thermal fatigue strength and easily causes hot cracking and early cracking. After finishing the h13 steel, it can be heated to 510-570 ℃, and the thickness is maintained for one hour every 25mm for stress relief annealing.
4. Cutting stress generated during final processing such as turning, milling, planing, etc. This stress can be eliminated by intermediate annealing.