Zhaoqing Die Casting Plant Knowledge Sharing: Research and Application of Die Casting Die Heat Treat
Category: Industry Information
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Metal die casting has the characteristics of high production efficiency, saving raw materials, reducing production costs, good product performance and high accuracy
The working surface of die-casting die is directly in contact with liquid metal, which is subjected to erosion and heating of high-pressure and high-speed liquid metal. After demoulding, the die-casting die is rapidly cooled. Thermal fatigue cracking, thermal wear and hot melting are common failure forms of die-casting die. Therefore, cold and heat resistance of die-casting die is required. Fatigue property, strength and toughness at high temperature, erosion resistance of liquid metal, high heat resistance and thermal conductivity, good oxidation resistance and high hardenability and wear resistance.
Heat treatment is an important link to improve the service life of casting die. The investigation shows that the failure rate of die fracture caused by improper heat treatment process or operation accounts for about 60% of the total failure rate. Therefore, in the production of die casting die, correct heat treatment process operation is needed.
I. Manufacturing Technology Route of Die Casting Die
1. General Die Casting Die
Forging - spheroidizing annealing - Mechanical roughing - stabilization treatment - finishing forming - quenching and tempering - fitter assembly.
2. Die Casting Die with Complex Shape and High Precision Requirements
Forging - spheroidizing annealing (or tempering) - Roughing - tempering - electroforming or finishing - bench grinding - nitriding (or nitrocarburizing) - grinding and polishing.
2. Conventional heat treatment process for die casting dies
Heat treatment process is widely used in die casting die manufacturing. It can improve the performance of die parts and prolong the service life of the die. In addition, heat treatment can also improve the processing performance of die-casting die, improve the processing quality and reduce tool wear, so it plays an important role in die-casting die manufacturing.
Die-casting die is mainly made of steel. The conventional heat treatment in the manufacturing process is spheroidizing annealing, stabilization, quenching and tempering. Through these heat treatment processes, the structure of steel is changed, so that the required structure and properties of die casting die can be obtained.
The forged die-casting billet must be treated by spheroidizing annealing or quenching and tempering heat treatment. On the one hand, it can eliminate stress, reduce hardness, facilitate cutting, and prepare the structure for final heat treatment. After annealing, uniform structure and dispersed carbides can be obtained to improve the strength and toughness of die steel. Because the effect of quenching and tempering treatment is better than spheroidizing annealing, the mould with high strength and toughness requirement often replaces spheroidizing annealing with quenching and tempering.
2. Stabilization treatment
Generally speaking, the die-casting die cavity is more complex, and there will be greater internal stress during roughing and deformation during quenching. In order to eliminate stress, stress relief annealing, i. e. stabilization, should be carried out after roughing.
The process is as follows: heating temperature 650 - 680 C, holding for 2 - 4 hours, then discharging air-cooling. Die-casting dies with complex shapes need to be cooled to air-cooled below 400 C. After quenching and tempering, EDM will produce deteriorating layer on the machined surface, which is easy to cause wire cutting cracks, and stress relief annealing at lower temperature should also be carried out.
3. Quenching preheating
Most of the steels used in die casting die are high alloy steels. Because of their poor thermal conductivity, they must be slowly quenched and heated, and preheating measures are often taken. For the die with low requirement of anti-deformation, the number of preheating can be less without cracking, but the die with high requirement of anti-deformation must be preheated many times. Preheating at lower temperatures (400-650 C) is usually carried out in air furnace; preheating at higher temperatures should be carried out in salt bath furnace, and the preheating time is still 1 min/mm.
4. Quenching heating
For typical die-casting die steels, high quenching heating temperature is beneficial to improving thermal stability and softening resistance and reducing thermal fatigue tendency, but it will cause grain growth and carbide formation at grain boundaries, which will decrease toughness and plasticity and lead to serious cracking. Therefore, when the die casting die requires high toughness, low temperature quenching is often used, while higher temperature quenching is used when the die casting die requires high temperature strength.
In order to obtain good high temperature performance, ensure that carbides can be fully dissolved, and obtain austenite with uniform composition, the quenching and holding time of die casting die is relatively long. Generally, the heating and holding coefficient is 0.8-1.0 min/mm in salt bath furnace.
5. Quenching and Cooling
Oil-cooled die-casting die with simple shape and low anti-deformation requirement is adopted, while step quenching is used for die-casting die with complex shape and high anti-deformation requirement. In order to prevent deformation and cracking, no matter what cooling method is adopted, it is not allowed to cool to room temperature. Generally, it should be cooled to 150-180 C and tempered immediately after soaking for a certain time. The soaking time can be calculated as 0.6 min/mm.
Die casting die must be tempered sufficiently, generally three times. The first tempering temperature is chosen in the temperature range of secondary hardening, the second tempering temperature is chosen to make the die meet the required hardness, and the third tempering temperature is lower than the second tempering temperature of l0 - 20. Oil-cooled or air-cooled are used after tempering, and tempering time is not less than 2 hours.
3. Surface Strengthening Technology of Die Casting Die
Conventional overall quenching has been difficult to meet the requirements of high surface wear resistance and matrix strength and toughness of die casting dies.
Surface hardening can not only improve the wear resistance and other properties of die-casting die surface, but also keep the matrix strong and tough enough, and prevent the molten metal from sticking and etching. It can improve the comprehensive performance of die-casting die, save alloy elements, reduce costs substantially, give full play to the potential of materials, and better. The use of new materials is very effective.
Production practice shows that surface strengthening treatment is an important measure to improve the quality of casting die and prolong the service life of die. The surface strengthening treatment processes commonly used in die casting die are carburizing, nitriding, nitrocarburizing, boronizing, chromizing and aluminizing.
Carburizing is one of the most widely used chemical heat treatment methods in machinery industry. The process features are as follows: low alloy die steels with medium and low carbon and high alloy steels with medium and high carbon are heated to 900-930 C in the active medium (carburizing agent), so that carbon atoms penetrate into the surface layer of the die, and then quenched and tempered at low temperature, so that the surface and core of the die have different composition, structure and properties.
Carburizing can be divided into solid carburizing, liquid carburizing and gas carburizing. Recently, controlled atmosphere carburizing, vacuum carburizing and benzene ion carburizing have been developed.
The process of nitriding into steel surface is called nitriding of steel. Nitriding can make die parts obtain higher surface hardness, wear resistance, fatigue performance, red hardness and corrosion resistance than carburizing. Because the nitriding temperature is lower (500-570 C), the deformation of the die parts after nitriding is smaller.
Nitriding methods include solid nitriding, liquid nitriding and gas nitriding. At present, new technologies such as ion nitriding, vacuum nitriding, electrolytic catalytic nitriding and high frequency nitriding are widely used, which can shorten the nitriding time and obtain high quality nitriding layer.
Nitrocarburizing is a low temperature nitrocarburizing process (530 - 580 C) in which nitrogen and carbon are simultaneously infiltrated into the medium containing active carbon and nitrogen atoms. Nitrocarburizing has less brittleness and shorter time than nitriding. The thermal fatigue properties of die casting die can be significantly improved by nitrocarburizing.
The bad working conditions require die casting die to have good high temperature mechanical properties, cold and thermal fatigue resistance, liquid metal erosion resistance, oxidation resistance, high hardenability and wear resistance. Heat treatment is the main manufacturing process to determine these properties.
The heat treatment of die casting die is to change the structure of steel, so that the surface of die can obtain high hardness and wear resistance, while the core still has enough strength and toughness, and at the same time effectively prevent the molten metal from sticking and etching. Choosing appropriate heat treatment process can reduce waste products and significantly improve the service life of the die.
Zhaoqing Xin Hi-tech Co., Ltd. was founded in 2010 (registration time 2012). The company is located in the north of Jintao Industrial Park, Jinli Town, Zhaoqing City, Guangdong Province. The plant covers an area of 30 mu, with a total investment of 30 million yuan and an annual output value of 150 million yuan. Main business: security accessories, automobile year matching, communication accessories, lighting accessories, furniture accessories. It has 400T-1, 280T-4, 200T-5, 180T-5 horizontal injection moulding machines, 5 vertical injection moulding machines, 40 CNC numerical control lathes, a batch of post-casting processing equipment and 2 large spraying lines. The company has a mold R&D and manufacturing department, and can develop die-casting molds for customers. The company has passed ISO 9001 (2015) certification of quality management system and ISO 1400 of environmental management system.
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