Three new and improved heat treatment processes for die-casting molds, very practical!

Metal die-casting has the characteristics of high production efficiency, saving raw materials, reducing production costs, good product performance, and high precision, and is widely used in production.

The working surface of die-casting molds is directly in contact with liquid metal, enduring the erosion and heating of high pressure and high-speed flowing liquid metal. After the workpiece is demolded, it is rapidly cooled. Therefore, thermal fatigue cracking, thermal wear, and hot melt corrosion are common failure forms of die-casting molds. Therefore, die-casting molds are required to have cold and hot fatigue resistance, strength and toughness at high temperatures, and resistance to liquid metal erosion High heat resistance, high thermal conductivity, good oxidation resistance, high hardenability, and wear resistance.

Introduction to Heat Treatment Process of Die Casting Mold

Heat treatment is an important step in improving the service life of die-casting molds. The investigation shows that mold fracture failure caused by improper heat treatment process or operation accounts for about 60% of the total number of failures. Therefore, in the production of die-casting molds, it is necessary to carry out the correct heat treatment process operation.

1、 Manufacturing process route of die-casting molds

1. General die-casting mold

Forging spheroidizing annealing mechanical rough machining stabilization treatment precision machining forming quenching and tempering fitter assembly.

2. Die casting molds with complex shapes and high precision requirements

Forging spheroidizing annealing (or quenching and tempering treatment) - rough machining - quenching and tempering - electric machining or precision machining forming - fitter grinding - nitriding (or nitrocarburizing) - grinding and polishing.

2、 Conventional heat treatment process for die-casting molds

The heat treatment process is widely used in the manufacturing of die-casting molds, which can improve the performance of mold parts and extend the service life of molds. In addition, heat treatment can also improve the processing performance of die-casting dies, improve processing quality, and reduce tool wear. Therefore, it plays a very important role in die manufacturing.

Die casting molds are mainly made of steel, and the conventional heat treatment in their manufacturing process includes spheroidizing annealing, stabilization treatment, quenching and tempering. Through these heat treatment processes, the microstructure of the steel is changed to obtain the required microstructure and properties for the die casting mold.

1. Pre processing

The forged die casting mold blank must be subjected to spheroidizing annealing or quenching and tempering heat treatment. On the one hand, it can eliminate stress and reduce hardness, facilitate cutting processing, and at the same time, 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 mold steel. Due to the superior effect of quenching and tempering treatment over spheroidizing annealing, molds with high requirements for strength and toughness often use quenching and tempering instead of spheroidizing annealing.

2. Stabilization treatment

Generally speaking, die-casting molds have complex cavities, which generate significant internal stress during rough machining and deformation during quenching. In order to eliminate stress, stress relief annealing, or stabilization treatment, should generally be carried out after rough machining.

The process is as follows: heating temperature 650 ℃ -680 ℃, insulation for 2-4 hours, and then air cooling after being discharged from the furnace. Die casting molds with complex shapes require furnace cooling to below 400 ℃ and air cooling after discharge. After quenching and tempering of the mold, electric discharge machining will produce a metamorphic layer on the machined surface, which can easily cause wire cutting cracks. Therefore, lower temperature stress relieving annealing should also be carried out.

3. Quenching preheating

The steel used for die-casting molds is mostly high alloy steel. Due to its poor thermal conductivity, quenching and heating must be carried out slowly, and preheating measures are often taken. For molds with low anti deformation requirements, the preheating frequency can be reduced without cracking, but molds with high anti deformation requirements must be preheated multiple times. Preheating at lower temperatures (400 ℃ -650 ℃) is generally carried out in an air furnace; For preheating at higher temperatures, a salt bath furnace should be used, and the preheating time should still be calculated at 1 minute/mm.

4. Quenching heating

For typical die-casting die steel, a high quenching and heating temperature is beneficial for improving thermal stability and resistance to softening, reducing the tendency to thermal fatigue, but it can cause grain growth and the formation of carbides at grain boundaries, resulting in a decrease in toughness and plasticity, leading to severe cracking. Therefore, when die-casting molds require high toughness, low temperature quenching is often used, while when high temperature strength is required, higher temperature quenching is used.

In order to achieve good high-temperature performance, ensure that carbides can be fully dissolved, and obtain austenite with uniform composition, the quenching and insulation time of die-casting molds is relatively long. Generally, the insulation coefficient is taken as 0.8-1.0 min/mm when heated in a salt bath furnace.

5. Quenching cooling

For die-casting molds with simple shapes and low requirements for deformation prevention, oil cooling is used; The die-casting molds with complex shapes and high requirements for deformation prevention use graded 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 immediately tempered after soaking for a certain time. The soaking time can be calculated as 0.6 min/mm.

6. Tempering

The die-casting mold must be fully tempered, usually three times. The first tempering temperature is selected within the temperature range of the second hardening; The selection of the second tempering temperature should ensure that the mold reaches the required hardness; The third tempering should be lower than the second tempering at l0 ℃ -20 ℃. After tempering, oil cooling or air cooling shall be used, and the tempering time shall not be less than 2 hours.

3、 Surface Strengthening Treatment Process for Die Casting Dies

Conventional overall quenching is difficult to meet the high surface wear resistance and matrix strength and toughness requirements of die-casting molds.

Surface strengthening treatment can not only improve the wear resistance and other properties of the surface of die-casting molds, but also maintain sufficient strength and toughness of the matrix, while preventing molten metal from sticking and etching. This is very effective in improving the comprehensive performance of die-casting molds, saving alloy elements, significantly reducing costs, fully utilizing the potential of materials, and better utilizing new materials.

Production practice has shown that surface strengthening treatment is an important measure to improve the quality of die casting molds and extend their service life. The surface strengthening treatment processes commonly used in die casting molds include carburizing, nitriding, nitrocarburizing, boronizing, chromizing, and aluminizing.

1. Carburization

Carburization is currently the most widely used chemical heat treatment method in the mechanical industry. The process feature is that the low alloy die steel with medium to low and high carbon and the high alloy steel die with medium to high carbon are heated to 900 ℃ -930 ℃ in a carburizing active medium (carburizing agent), allowing carbon atoms to penetrate the surface layer of the die, followed by quenching and low-temperature tempering, resulting in different compositions, structures, and properties on the surface and core of the die.

Carburization can be further divided into solid carburization, liquid carburization, and gas carburization. Recently, it has developed into controllable atmosphere carburization, vacuum carburization, and benzene ion carburization.

2. Nitriding

The process of penetrating nitrogen into the surface of steel is called nitridation of steel. Nitriding can enable mold parts to achieve higher surface hardness, wear resistance, fatigue performance, red hardness, and corrosion resistance than carburization. Due to the lower nitriding temperature (500-570 ℃), the deformation of the mold parts after nitriding is relatively 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 used, which shorten the nitriding time and can obtain high-quality nitriding layers.

3. Nitrocarburizing

Nitrogen carbon co infiltration is a low-temperature nitrogen carbon co infiltration process (530 ℃ -580 ℃) where nitrogen and carbon are simultaneously infiltrated into a medium containing activated carbon and nitrogen atoms, and nitrogen is the main method. The brittleness of the nitrocarburizing layer is small, and the co carburizing time is much shorter than the nitriding time. After nitrocarburization, the thermal fatigue performance of die-casting molds can be significantly improved.

Poor working conditions require die-casting molds to have good high-temperature mechanical properties, cold and hot fatigue resistance, liquid metal erosion resistance, oxidation resistance, high hardenability, and wear resistance. Heat treatment is the main manufacturing process that determines these properties.

The heat treatment of die-casting molds is to change the microstructure of the steel to achieve high hardness and wear resistance on the surface of the mold, while the core still has sufficient strength and toughness, while effectively preventing molten metal from sticking and etching. Choosing appropriate heat treatment processes can reduce waste and significantly improve the service life of the mold.