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Vacuum heat treatment of large molds

Die-casting mold is a tool for casting metal parts, a tool for completing the die-casting process on a special die-casting die forging machine, which is widely used in the field of alloy casting. Die-casting molds work in harsh environments, require high performance, and are always heat-treated when the cavity shape is close to completion and close to dimensional tolerances.

Large die-casting molds are an essential tool for casting large metal parts, and large-scale casting alloy parts are widely used in the fields of aviation, shipbuilding, automobiles, and industrial equipment. Generally, a die-casting die with a width greater than 450mm and a thickness greater than 150mm is called a large die-casting die. Large die-casting mold cavity has complex shape and large size, and is easily deformed during heat treatment.

For large mold blanks, deformation can be reduced by selecting high-quality mold materials and vacuum heat treatment technology. The surface of the vacuum heat treatment process has no oxidation, no decarburization, and small deformation. Heat treatment effect is good. The mold after vacuum heat treatment can be put into use without subsequent processing, which can greatly save the cost for subsequent machining.

Manufacturing Process of Large Die Casting Die

Mold structure design – mold material selection – cold and hot processing – heat treatment is the main step in the production of die-casting molds. Here are the main points of each:

1) In terms of mold design, sharp corners and excessive section changes should be avoided as much as possible. Sharp corners are easy to cause stress concentration, and excessive cross-sectional changes are easy to generate thermal stress. The size of the section should be uniform, and the shape should be symmetrical and simple to reduce heat treatment deformation. For complex and extra-large molds, a combined type can be used, and process holes should be opened in some parts if necessary.

2) The mold material strives to have a uniform structure, otherwise, during the heat treatment, phase transformation stress will be generated in different periods of structure transformation, resulting in deformation.

3) In terms of machining, after rough machining, it is best to perform a stress relief treatment in order to eliminate machining stress in time.

Vacuum Quenching of Large Die Casting Moulds

Quenching and cooling is an important stage to ensure the quality and service life of the mold, and it is also a key step to control the deformation. After quenching, a microstructure capable of working in a die-casting mold must be obtained. In order to maintain the requirements of impact toughness and dimensional stability, it is necessary to adopt graded quenching cooling, that is, keep the temperature slightly higher than the martensite transformation temperature for a period of time to reduce the temperature difference between the inside and outside of the mold, thereby reducing thermal stress and reducing deformation.

The RVGQ-8812 high-pressure gas quenching vacuum furnace of SIMUWU is selected for vacuum heat treatment. The ultimate vacuum degree is 0.67×10-3Pa, and the gas quenching pressure is 6bar.

Process flow:

Heat treatment process 600 ℃, 800 ℃ secondary preheating, 1020 ℃ heating and quenching, nitrogen gas cooling to the workpiece surface temperature of 400 ± 50 ℃, graded quenching, by adjusting the fan speed, the workpiece surface temperature stays within this range for 30min to reduce the temperature. The temperature difference between the surface and the core of the small workpiece achieves the purpose of reducing thermal stress.

For the deformation of large die-casting molds, the main factor affecting the deformation is thermal stress. This is because large molds have large dimensions and are prone to large temperature differences during quenching. The appearance of the temperature difference makes the shrinkage rate different, resulting in thermal stress. Therefore, the influence of gas quenching medium and pressure needs to be considered when gas quenching.

The cooling capacity of various gas quenching media is different. In general, the slower the cooling rate, the smaller the deformation during quenching and cooling. The cooling rate of nitrogen in the vacuum furnace is relatively slow, so the gas quenching deformation is small. At the same time, pressure is also a crucial factor, and it is necessary to formulate appropriate process parameters through the characteristics of the die-casting mold.

When the vacuum furnace is air-cooled, the way the gas is blown to the workpiece has a great influence on the deformation during quenching. If the gas is not blown to the workpiece in the correct way, the cooling will not be uniform in each part, resulting in deformation. The vacuum air quenching furnace produced by SIMUWU is equipped with gas nozzles evenly distributed in the heating chamber, which is circulated and cooled by fans and motors, thereby minimizing heat treatment deformation.

For large die-casting molds, the warpage of the bottom plane can be controlled within 0.5mm after the above process. The total cost of the mold from material – machining – heat treatment is about ten times the material cost. Through vacuum heat treatment, the problem of heat treatment deformation of large die-casting molds is solved, the subsequent machining costs after heat treatment are saved, and significant economic benefits are produced. It can be seen that the advantages of vacuum heat treatment in the treatment of such workpiece products.

Vacuum heat treatment equipment selection: The RVGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of die-casting molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.