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Vacuum heat treatment of die casting parts

For the production of small gasoline engine parts, die casting molds are very important and key components. The life of a die casting mold can significantly affect a factory’s production. According to statistics, in the cost of the mold, the material cost accounts for 15%, the processing cost accounts for 80%, and the heat treatment cost accounts for 5%. However, by improving the heat treatment process, the service life of the mold can be effectively increased. Compared with the traditional heat treatment process, the vacuum heat treatment process can increase the life of the die casting mold from about 20,000 times to 100,000 times. Thereby greatly reducing production costs and improving production efficiency.

Process flow:

The heat treatment of the mold can be divided into the following stages: raw material → spheroidizing annealing → processing → stress relief annealing → quenching → tempering → nitriding.

1 Annealing

It includes two parts: spheroidizing annealing after forging and stress relief annealing during mold making.

Its main purpose: to improve the crystalline structure at the raw material stage; to facilitate processing and reduce hardness to prevent deformation and quenching cracks after processing to remove internal stress.

(1) Spheroidizing annealing.

After the die steel is forged, the internal structure of the steel becomes an unstable crystal, and it is difficult to cut with high hardness. In addition, the steel in this state has a large internal stress, is easy to deform and crack after processing, and has poor mechanical properties. In order to crystallize carbides To become a spheroidized stable structure, spheroidization annealing is required.

(2) Stress relief annealing.

Machining the die steel with residual stress will cause deformation after machining. If there is still stress after machining, large deformation or quenching cracks will occur during quenching. To prevent these problems, stress relief annealing is necessary. The stress relief annealing process is an indispensable and important process in the mold manufacturing process, and three stress relief annealing is required in the mold manufacturing process.

(1) When cutting off the shape of more than 1/3 of the volume of the raw material or processing the depth of 1/2 of the thickness of the raw material, leave a machining allowance of 5~10mm, and perform the first stress relief annealing.

(2) When there is a margin (2~5mm) in the finishing process, carry out the second stress relief annealing.

(3) After the mold test, the third stress relief annealing is performed before quenching.

2 Vacuum quenching

The vacuum furnace quenching process is as follows:

(1) Before quenching: use the heat balance method to improve the overall consistency of mold heating and cooling. All thin-walled holes, grooves, cavities, etc. that affect this point must be filled and blocked, and the mold can be heated and cooled evenly as much as possible. resulting deformation.

(2) Heating of the mold: During the heating process, it should be heated slowly (with a temperature of 200°C/h), and a two-stage preheating method should be adopted to prevent the rapid temperature rise from causing excessive temperature difference between the inside and outside of the mold, causing excessive thermal stress. At the same time, the transformation stress is reduced.

(3) Quenching temperature and holding time: The lower limit of quenching heating temperature should be used, and the soaking time should not be too short or too long. Generally, the soaking time is determined by the wall thickness and hardness. The relationship between quenching hardness and holding time of H13 steel is shown in Figure 2.

(4) Quenching cooling: The pre-cooling method is adopted, and the cooling speed is effectively controlled by adjusting the air pressure and wind speed, so as to maximize the ideal cooling. That is: after pre-cooling to 850℃, increase the cooling rate and quickly pass through the nose of the “C” curve, gradually reduce the cooling rate when the mold temperature is below 500℃, and adopt the cooling method of approximate isothermal transformation when the mold temperature is below the Ms point. Minimize quenching deformation. When the mold is cooled to about 150°C, turn off the cooling fan and let the mold cool naturally.

3 Tempering

When the quenched mold is cooled to about 100 °C, it should be tempered immediately to prevent continued deformation or even cracking. The tempering temperature is determined by the working hardness, and generally three temperings are required.

4 Nitriding treatment

Generally, the die-casting mold can be used after quenching and tempering (45~47HRC), but in order to improve the wear resistance, corrosion resistance and oxidation resistance of the mold, prevent mold sticking, and prolong the life of the mold, nitriding treatment must be carried out. The depth of the nitride layer is generally 0.15~0.2mm.

After nitriding, it needs to be polished to remove the white bright layer (about 0.01mm thick).

5 Points to note:

(1) The heat treatment deformation of the mold is caused by the combined action of phase transformation stress and thermal stress, and is affected by many factors. Therefore, under the premise of correct material selection, attention should also be paid to the forging of the blank, and the method of six-sided forging should be used for repeated upsetting.

At the same time, in the design stage of the mold, attention must be paid to make the wall thickness as uniform as possible (process holes should be opened when the wall thickness is uneven): for molds with complex shapes, the mosaic structure should be used instead of the overall structure; , For sharp-cornered molds, fillet transitions should be used and the fillet radius should be increased.

During heat treatment, data records should be made, the amount of deformation in each direction of length, width and thickness, heat treatment conditions (furnace loading method, heating temperature, cooling rate, hardness, etc.), to accumulate experience for future heat treatment of molds.

Selection of vacuum heat treatment equipment: SIMUWU produces various types of vacuum furnace products, including vacuum annealing furnace, vacuum air quenching furnace, vacuum oil quenching furnace, vacuum nitriding furnace, vacuum carburizing furnace, etc. The rich product lines can meet the needs of major manufacturers, and are exported to developed countries such as Europe and the United States and developing countries such as Vietnam, and are widely praised.