The heat treatment process is widely used in the manufacture of die-casting molds , which can improve the performance of mold parts and prolong the service life of the mold. In addition, heat treatment can also improve the processing performance of die-casting molds, improve processing quality, and reduce tool wear, so it occupies a very important position in mold manufacturing. Die casting molds are mainly made of steel, and the conventional heat treatment in the manufacturing process is: spheroidizing annealing, stabilization treatment, quenching and tempering, quenching, and tempering. Through these heat treatment processes, the structure of the steel is changed, so that the die-casting mold can obtain the required structure and properties.
After forging, the die-casting die blank must be spheroidized annealed or quenched and tempered. On the one hand, stress is eliminated to reduce the hardness, which is convenient for cutting, and at the same time prepares the structure for the final heat treatment. After annealing, uniform structure and dispersed carbides can be obtained to improve the strength and toughness of die steel. Since the effect of quenching and tempering is better than that of spheroidizing annealing, spheroidizing annealing is usually replaced by quenching and tempering for molds with high toughness and toughness requirements.
Generally speaking, the cavity of die-casting mold is more complex, which will generate large internal stress during rough machining and deformation during quenching. In order to eliminate stress, stress relief annealing, that is, stabilization treatment, should generally be performed after rough machining. The process is as follows: the heating temperature is 650°C-680°C, and the furnace is air-cooled after being kept for 2-4 hours. Die-casting molds with complex shapes need to be furnace-cooled below 400°C and air-cooled. After the die-casting mold is quenched and tempered, EDM is performed, and the machined surface will produce a metamorphic layer, which is easy to cause wire cutting cracks, and should also be subjected to stress relief annealing at a lower temperature.
Most of the steel used in die-casting molds is high-alloy steel. Because of its poor thermal conductivity, it must be carried out slowly during quenching and heating, and preheating measures are often taken. For molds with low anti-deformation requirements, the preheating times can be less without cracking, but molds with high anti-deformation requirements must be preheated multiple times. Preheating at a lower temperature is generally carried out in an air furnace; preheating at a higher temperature, a salt bath furnace should be used.
For typical die-casting die steel, high quenching heating temperature is beneficial to improve thermal stability and softening resistance, and reduce thermal fatigue tendency, but it will cause grain growth and grain boundary to form carbides, which will reduce toughness and plasticity. lead to severe cracking. Therefore, when the die casting mold requires higher toughness, low temperature quenching is often used, and when higher high temperature strength is required, higher temperature quenching is used. In order to obtain good high temperature performance, ensure that the carbides can be fully dissolved, and obtain austenite with uniform composition, the quenching and holding time of the die casting mold is relatively long.
Oil cooling is used for die-casting molds with simple shapes and low anti-deformation requirements, while graded quenching is used for die-casting molds with complex shapes and high anti-deformation requirements. In order to prevent deformation and cracking, no matter what cooling method is used, it is not allowed to cool to room temperature. Generally, it should be cooled to 150℃-180℃, soaked for a certain period of time and then tempered immediately.
The die-casting mold must be fully tempered, generally three times. The temperature of the first tempering is selected in the temperature range of the secondary hardening; the temperature of the second tempering should be selected to make the mold reach the required hardness; the third tempering should be lower than the second 10℃-20℃. After tempering, oil cooling or air cooling is adopted, and the tempering time is not less than 2h.
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