Heat treatment process and application of die casting die
Metal die casting has the characteristics of high production efficiency, saving raw materials, reducing production cost, good performance and high accuracy, which has been widely used in production.
Die casting mould of the surface of the work, direct contact with the liquid metal, under high pressure and high speed flow of liquid metal erosion and heating, after the workpiece demoulding, and rapid cooling, therefore, thermal fatigue cracking, thermal wear and hot melt erosion is a common failure forms of die casting mould, so, for die casting die with cold and hot fatigue resistance, high temperature strength and toughness, liquid metal erosion resistance, high heat resistance and high thermal conductivity, good oxidation resistance and high hardenability and wear resistance, etc.
Heat treatment is an important step to improve die life. The investigation shows that the failure of the mold due to heat treatment or improper operation accounts for about 60% of the total failure. Therefore, in die casting die production, the correct heat treatment process operation is required.
The manufacturing process of die casting die
1. General die casting mold
Forging - spheroidization annealing - mechanical roughing - stabilization processing - finishing forming - quenching and tempering - fitter assembly.
2. Die-casting mold with complex shapes and high precision requirements
Forging - spheroidizing annealing (or tempering) - roughing - tempering - electroforming or finishing - locksmith grinding - nitriding (or nitrocarburizing) - grinding and polishing.
Die casting die conventional heat treatment process
Heat treatment technology is widely used in die casting die manufacturing, which can improve the performance of die parts and extend the service life of die. In addition, heat treatment can also improve the process performance of die casting mold, improve the quality of processing, reduce tool wear, therefore, in the mold manufacturing occupy a very important position.
Die casting die is mainly made of steel. The normal heat treatment in the manufacturing process is spheroidizing annealing, stabilizing treatment, tempering, quenching and tempering. Through these heat treatment processes, the microstructure of the steel is changed so that the required microstructure and properties can be obtained.
The die casting mould blank after forging press must adopt spheroidizing annealing or tempering heat treatment, on the one hand to eliminate the stress and reduce the hardness, easy for cutting processing, and at the same time to prepare the structure for the final heat treatment. After annealing, uniform microstructure and dispersed carbides can be obtained to improve the strength and toughness of die steel. As the effect of tempering treatment is better than that of spheroidizing annealing, the mold with high demand for strength and toughness is often replaced by tempering by spheroidizing annealing.
2. Stabilization treatment
Die casting die cavity generally more complex, in rough processing will produce greater internal stress, in quenching will produce deformation. In order to eliminate stress, stress annealing should be carried out after rough machining, that is, stabilization treatment.
The following process: heating temperature 650 ℃ to 680 ℃, the heat preservation from air cooling after 2-4 h. Die casting furnace cooling to shape more complex under 400 ℃ from air cooling. After quenching and tempering, the surface of the mold will produce metamorphic layer, which is easy to cause line cutting crack, and should be de-stressed at a lower temperature.
The steel used for die casting is mostly high alloy steel. Due to its poor thermal conductivity, it must be carried out slowly during quenching and heating, and preheating measures are often adopted. For molds that require less deformation, the preheating times can be reduced without cracking, but molds that require more deformation must be preheated multiple times. Low temperature (400 ℃ -- 400 ℃) of preheating, general in air furnace; Salt bath furnace should be used for preheating at higher temperature, and the preheating time should still be calculated according to 1 min/mm.
4. Quench heating
For typical die casting steel, high quenching and heating temperature is conducive to improving thermal stability and anti-softening ability and reducing thermal fatigue tendency, but it will cause grain growth and grain boundary formation of carbides, decrease toughness and plasticity and lead to serious cracking. Therefore, when the die-casting mold requires high toughness, it is often hardened at low temperature, while when it is required to have high strength at high temperature, it is quenched at high temperature.
In order to obtain good high-temperature performance, ensure that the carbide can be fully dissolved, and obtain the austenite with uniform composition, the quenching and heat preservation time of the die-casting mold is relatively long. Generally, the heat preservation coefficient of the salt bath furnace is 0.8-1.0min /mm.
5. Quench cooling
The die casting mould with simple shape and less need for deformation prevention is cooled by oil. The die - casting mould with complicated shape and high requirement of anti-deformation adopts step quenching. In order to prevent deformation and cracking, no matter use what method of cooling, are not allowed to cool to room temperature, the general should be cold to the 150 ℃ to 180 ℃, the soaking time immediately after tempering, soaking time can be calculated at 0.6 mm/min.
Die casting mold must be fully tempered, generally tempered three times. The first tempering temperature is selected in the temperature range of secondary hardening. The selection of the second tempering temperature shall make the mold meet the required hardness. The third tempering is lower than the second l0 ℃ ~ 20 ℃. After tempering, both oil cooling and air cooling are adopted, and the tempering time shall not be less than 2 h.
Die - casting die surface strengthening process
Conventional general quenching has been difficult to meet the high surface wear resistance and matrix toughness requirements of die casting die.
Surface strengthening treatment can not only improve the surface wear resistance and other properties of die casting mould, and can keep the matrix enough strong toughness, at the same time to prevent the molten metal stick mold, etching, which to improve the comprehensive performance of die casting mould, save alloying elements, reduce the cost, give full play to the potential of materials, and better use of new materials, are very effective.
The production practice shows that surface treatment is an important measure to improve die casting quality and prolong die service life. The surface strengthening process used in die casting die is carburizing, nitrifying, carburizing, boronizing, chromizing and aluminum carburizing.
1. The carburizing
Carburization is the most widely used chemical heat treatment method in the mechanical industry. Its technological characteristics are: the low and medium high carbon low alloy steel and high carbon high alloy steel mold in the active medium carbon (carburizing agent), heated to 900 ℃ to 930 ℃, the carbon atoms into mold surface layer, followed by quenching and low temperature tempering, the mold surface and center with different composition, microstructure and performance.
Carburizing can be divided into solid carburizing, liquid carburizing and gas carburizing. Recently, controllable atmosphere carburization, vacuum carburization and benzene ion carburization have been developed.
2. The nitriding
The process of seeping nitrogen into the surface of steel is called nitrification of steel. The surface hardness, wear resistance, fatigue, red hardness and corrosion resistance of the mold parts are higher than carburization. Because of lower nitriding temperature (500-570 ℃), mold parts after nitriding deformation is small.
Nitriding methods include solid nitriding, liquid nitriding and gas nitriding. At present, new technologies such as ion nitriding, vacuum nitriding, electrolytic nitriding and high frequency nitriding are widely applied, which shortens the nitriding time and can obtain high quality nitriding layer.
Carbon and nitrogen altogether permeability is the medium containing activated carbon, nitrogen and infiltration of nitrogen and carbon, and low temperature is given priority to with nitriding carbon nitrogen altogether permeability process (530 ℃ -- 530 ℃). The nitrification layer is less brittle, and the time of nitrification is much shorter than that of nitrification. After nitrocarburizing, the thermal fatigue property of the die casting can be improved.
Poor working conditions require good mechanical properties at high temperature, cold and hot fatigue resistance, liquid metal erosion resistance, oxidation resistance, high hardenability and wear resistance of die casting molds. Heat treatment is the main manufacturing process to determine these properties.
The heat treatment of die casting mold is to change the structure of the steel to obtain high hardness and wear resistance of the mold surface, while the core still has sufficient strength and toughness, at the same time effectively prevent the molten metal from sticking and erosion. Selecting proper heat treatment technology can reduce waste and improve the die life.