Heat Treatment Process of Die Casting Mold

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1. Die Casting Mold Manufacturing Process Route

General die-casting mold: forging → annealing → machining → stabilization treatment → finishing forming → quenching and tempering → fitter repair → bluing treatment.
Die-casting molds with complex shapes and high precision requirements: forging → annealing → rough machining → quenching and tempering → finishing forming → fitter repair → nitriding (or nitrocarburizing) → grinding and polishing.

2. Heat Treatment Characteristics of Die Casting Mold

Die-casting mold cavity is more complicated, and large internal stress will be generated during rough machining and semi-finish machining. In order to reduce the quenching deformation, stress relief annealing should be carried out after rough machining, the process is 650-680℃, and the holding time is 3-5h. After the heat preservation is over, the mold with a simple cavity can be directly out of the furnace and cooled evenly and slowly in still air. Die-casting molds with complex shapes need to be furnace-cooled to 400°C and air-cooled. The mold cavity processed by EDM will have a brittle metamorphic layer on the surface, which will cause tensile stress and easily cause mold cracks. The method to eliminate the metamorphic layer is to use grinding or polishing, and then perform stress relief annealing.

The preliminary heat treatment of the die-casting mold generally adopts spheroidizing annealing, the purpose of which is to obtain a uniform structure and dispersed carbides before the final heat treatment to improve the strength and toughness of the steel.

Die-casting mold steel is mostly high-alloy steel, which must be heated slowly due to its poor thermal conductivity. Usually preheating measures are used, and the number of preheating times depends on the composition of the steel and the requirements for mold deformation. For molds with low deformation requirements, the number of preheating can be less without cracking, but molds with high deformation requirements must be preheated multiple times. Preheating at a lower temperature (400~650℃) is generally carried out in an air furnace. The preheating time can be calculated as 1min/mm. For preheating at a higher temperature, a salt bath furnace should be used, and the preheating time is still 1min/ mm meter.

Quenching and heating. For typical die-casting die steels, high quenching heating temperature is conducive to improving thermal stability and resistance to softening, and reducing thermal fatigue tendency, but it will cause grain growth and carbide formation at grain boundaries, which will reduce toughness and plasticity, resulting in Severe cracking. Therefore, for die-casting molds with higher toughness requirements, low temperature tempering is often used, and higher temperature quenching is used when higher high temperature strength is required.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 molds is relatively long. Generally, the heating coefficient in the salt bath furnace is 0.8~1min/mm.

Quench and cool. For die-casting die steel, the oil quenching speed is fast and good performance can be obtained, but the tendency of deformation and cracking is high. Therefore, oil-cooled die-casting molds with simple shapes and low deformation requirements are used; while die-casting molds with complex shapes and high deformation requirements are stepped quenching. 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℃ and tempered immediately after soaking for a certain period of time. The soaking time can be calculated by reference to 0.6min/mm.

Tempering. Die casting molds must be fully tempered, generally 3 times. The first tempering temperature is selected in the temperature range of the second hardening, the second tempering temperature is selected to make the mold reach the required hardness value, and the third tempering temperature is lower than the second time by 10~20℃. After tempering, oil cooling or air cooling is adopted, and the tempering time is not less than 2h.

Surface strengthening treatment. In order to prevent molten metal adhesion and corrosion, and to improve the corrosion resistance and wear resistance of the formed part of the die-casting mold, the die-casting mold often adopts surface strengthening treatment. Commonly used surface strengthening treatment methods include nitriding, nitrocarburizing, chromizing, aluminizing, boronizing and so on.

3. Requirements For Die Casting Mold Materials

Requirements for the materials of parts in contact with liquid metal. Good forgeability and machinability, with high high temperature strength, high temperature hardness, tempering resistance and impact toughness. For high melting point alloys, there is little change in strength and hardness. Good thermal conductivity and thermal fatigue resistance. Has sufficient high temperature oxidation resistance. The coefficient of thermal expansion is small to ensure the dimensional accuracy of the casting and the accuracy of the matching parts of the mold. High wear resistance and good heat treatment deformation rate.

Requirements for sliding fitting parts. Good abrasion resistance and proper strength. Appropriate hardenability and small heat treatment deformation rate.

Requirements for mold sleeves and fastening parts. The die sleeve and fastening parts should have sufficient strength.

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