Aluminum alloy die-casting mold vacuum heat treatment

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Aluminum alloy die-casting mold cavity, core, slider, ejector, gate and other core parts are often subjected to plastic deformation, wear, cracking and fatigue due to the high temperature, erosion, erosion, rapid cooling and rapid heating of liquid aluminum alloy during work. It is required that the selected steel has sufficiently high high temperature yield strength, fracture toughness, thermal wear resistance, thermal fatigue resistance and impact toughness.

According to the importance and life requirements of the parts, the mold manufacturer selects rolling materials or forgings of different quality grades and price levels from ordinary electric furnaces, out-of-furnace refining, electroslag remelting, and vacuum electroslag. In the process of heat treatment of parts, decarburization, carbon increase and depletion of alloy elements on the surface will lead to early cracks in the use of parts, so vacuum heat treatment is required, which is especially important for parts that have been finished and formed. The harmful gas and some non-metallic inclusions remaining in the steel are first diffused from the inside to the outside under the vacuum heating condition, and then separated from the workpiece and discharged out of the furnace, and the workpiece is further vacuum purified under the solid state transformation.

Selection of vacuum furnace equipment: RVGQ-669 vacuum heat treatment furnace produced by SIMUWU company, with an effective area of ​​600*600*900mm. RVGQ series vacuum furnace is a high-quality product for the vacuum heat treatment process of tools and 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.

Vacuum heat treatment process:

The vacuum heat treatment furnace adopts a full computer automatic control and recording system with man-machine dialogue. The parameter settings of each stage of the process curve are described below.

1.Warm up

The main purpose of the first preheating is to eliminate the machining stress generated in the machining of the parts before quenching, prevent and reduce the quenching distortion caused by the heating process, and the high preheating temperature has a significant effect of eliminating stress. The preheating temperature is selected as 580℃, the heating speed is 8℃/min and the holding time is twice that of the quenching heating and holding time, and the vacuum degree is set at 2.66Pa.

The second preheating adopts 850℃, the heating rate is 12℃/min, the vacuum degree is set to 26.66Pa, and the holding time is one third of the total heating time.

2.Quenching heating and cooling

The quenching heating and subsequent heat preservation is to fully dissolve the alloying elements and complete the austenite homogenization. When quenched and heated at 1000 ~ 1100 ℃, the hardness after quenching can exceed 55HRC. Considering that the austenite grains are not coarse and fine needle-like quenched martensite is obtained, the quenching heating temperature is selected to be 1030-1040°C. The holding time is affected by factors such as the power of the electric furnace, the amount of furnace loading, the furnace loading method and the size of the workpiece. The holding time should not be too long, not only to save electricity and reduce costs, but also to prevent coarse grains. The holding time is calculated according to 30min + number of inches of workpiece * 10min, and the setting time of this process is 50 minutes.

The heating speed is V=12℃/min. In order to prevent the evaporation of carbon and alloy elements, the vacuum degree is controlled at 26.66Pa. After the heat preservation, it is moved to the cooling room. According to the size of the workpiece and the amount of furnace loading, select (0.20~0.54MP) high-purity nitrogen (99.999%). ) for high pressure gas quenching. The cooling rate is also related to the heat exchange capacity of the cooling chamber in addition to the nitrogen pressure. Due to the large power of the quenching cooling fan and the water flow of the heat exchanger, it is possible to ensure that the larger size parts have a sufficiently high quench hardness and hardened layer depth. During the cooling process, no isothermal stay was performed, and the furnace was continuously cooled to 80°C.

3.Vacuum tempering

The use of vacuum insulation, rapid cooling at low pressure, and release at room temperature is the key to ensuring no oxidation and decarburization on the surface of parts tempered at high temperature. Recharging high-purity nitrogen is mainly to make the furnace temperature uniform, increase the heating speed, and adjust the vacuum degree. 4C5MoSiV1 steel is tempered at high temperature for three times, and the heating rate is 8℃/min each time. According to the specific requirements of each heat, the cooling rate is 005~0.12MPa for fast cooling, or 300℃ is released and continues to be cooled by fans, or cold. Bake to room temperature.

The main purpose of the first high temperature tempering is to make the part have a high enough hardness. 4C5MoSN1 steel belongs to the tempering brittle zone at about 500℃, so choose 580℃ for the first tempering, which not only avoids the brittle zone but also helps to obtain the highest tempering hardness as possible. The main purpose of the second high temperature tempering is to stabilize the hardness. 4C5MoSiV1 steel has the best combination of strength, toughness, wear resistance and thermal fatigue resistance when tempered at 590℃ to 49HRC. Tempering at 580°C should be used when 50HRC or above is required, and about 600°C should be used when 46-48HRC is required. The main purpose of the third tempering is to fully eliminate the thermal stress and structural stress generated during the quenching process. The temperature is 5 to 10°C lower than the second tempering. The vacuum degree control during the tempering process is determined according to the requirements of the parts. Tempering under high vacuum, and some pulse recharged high-purity krypton are kept under low vacuum.

The specific process curve is as follows:

Vacuum stress relief treatment

In order to improve the service life of the die-casting mold, the mold structure including preheating and oil circulation cooling system should be reasonable and correct. From the perspective of part load and material strength, the cavity, guiding, core pulling, heating, cooling and other systems are reasonably arranged to avoid dangerous sections and reduce stress concentration. The designer’s concept and intention should be fully implemented during parts processing, and attention should be paid to processing details (such as removing residual iron filings in inner holes and blind holes) to prevent grinding cracks, grinding and tempering, and grinding and quenching to remove wire cutting. , Metamorphic layers, solidification pits, and fine cracks produced by EDM. The mold should be strictly preheated, not beaten cold, not overheated, and it is best to operate at a constant temperature of about 350 °C; to eliminate the need for direct use without tempering after repair welding. deal with.

The stress relief treatment is mainly used in the following three aspects:

(1) Stress relief of parts before quenching If the customer requires treatment after rough machining and before finishing, the shape of the part is complex and prone to quenching distortion, and the residual stress of cold working is large, and a stress relief treatment is required before quenching.

(2) Parts that have reached a certain service life after being put into production are subjected to stress relief treatment during the maintenance process, which can delay the formation of primary cracks. There are also data recommendations, the workpiece before assembly by EDM, the workpiece after the new mold produces about 1500 molds, and the parts after each batch production of 10,000 molds need stress relief treatment.

(3) Parts after repair welding

After repair welding of 4C5MoSiV1 steel, under the condition of air cooling, quenched martensite structure will be formed in the welded part. If it is directly put into production without stress relief tempering, it will easily cause early cracking. The stress relief treatment is heated at 550-560℃, and the vacuum degree is set to 2.66Pa first.

The aluminum alloy die-casting mold with the above treatment has high precision and long service life, can maintain the quality in long-term use, and perfectly adapt to the processing needs.

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