1、 During mold processing and manufacturing
1. Quality problem of blank forging
Some molds have cracks after only a few hundred pieces have been produced, and the cracks develop rapidly. It is possible that only the external dimension is guaranteed during forging, while the loose defects such as dendritic crystal, carbide inclusion, shrinkage cavity and bubble in the steel are extended and elongated along the processing method to form a streamline, which has a great impact on the final quenching deformation, cracking, embrittlement and failure tendency in the use process.
2. The cutting stress generated during final machining such as turning, milling and planing can be eliminated by intermediate annealing.
3. Grinding stress is generated during grinding of quenched steel, and friction heat is generated during grinding, resulting in softening layer and decarburization layer, which reduces the thermal fatigue strength and easily leads to hot crack and early crack. After finishing grinding, the H13 steel can be heated to 510-570 ℃, and the stress relief annealing can be carried out by holding for one hour every 25mm in thickness.
4. Electric spark, machining produces stress. A layer of white bright layer rich in electrode elements and dielectric elements is produced on the surface of the mold, which is hard and brittle. This layer itself will have cracks and stress. High frequency shall be adopted during EDM to minimize the white bright layer, which must be removed by polishing and tempered at the third tempering temperature.
2、 During mold processing
Improper heat treatment will lead to die cracking and premature scrapping, especially if only quenching and tempering, no quenching, and surface nitriding process are used, surface cracking and cracking will occur after several thousand die casting times. The stress produced during steel quenching is the result of the superposition of the thermal stress during cooling and the structural stress during phase transformation. The quenching stress is the cause of deformation and cracking. Therefore, tempering must be carried out to eliminate the stress.
3、 During die casting production
1. Mold temperature
The mold should be preheated to a certain temperature before production, otherwise when the high temperature metal liquid fills the mold, it will cause chilling, resulting in the increase of the temperature gradient of the inner and outer layers of the mold, forming thermal stress, causing the mold surface to crack or even crack. During the production process, the mold temperature keeps rising. When the mold temperature is overheated, it is easy to cause mold sticking, and the failure of moving parts will lead to mold surface damage. A cooling temperature control system should be set to keep the working temperature of the mold within a certain range.
2. Filling
Molten metal filling with high pressure and high speed will inevitably produce severe impact and erosion on the mold, thus generating mechanical stress and thermal stress. During the impact process, liquid metal, impurities and gases will also have complex chemical interaction with the mold surface, and accelerate the formation of corrosion and cracks. When the liquid metal is wrapped with gas, it will expand first in the low pressure area of the cavity. When the gas pressure rises, it will cause inward explosion, pull out the metal particles on the surface of the cavity and cause damage, and cause cracks due to cavitation.
3. Production process
In the production process of each die casting, due to the heat exchange between the mold and the molten metal, the mold surface has periodic temperature changes, causing periodic thermal expansion and contraction, and generating periodic thermal stress. For example, the mold surface is subjected to compressive stress due to temperature rise during pouring, and the mold surface is subjected to tensile stress due to temperature drop after the mold is opened and ejected from the casting. When the alternating stress circulates repeatedly, the accumulated stress in the mold becomes larger and larger. When the stress exceeds the fatigue limit of the material, the mold surface cracks.