Conventional overall quenching is no longer sufficient 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 die-casting molds, but also maintain sufficient strength and toughness of the substrate, while preventing molten metal from sticking to the mold and corroding. This is very effective in improving the comprehensive performance of die-casting molds, saving alloy elements, significantly reducing costs, fully tapping 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 for die-casting molds include carburizing, nitriding, nitrogen carbon co carburizing, boronizing, chromizing, and aluminum infiltration.
1. Carbonization
Carbonization is currently a widely used chemical heat treatment method in the mechanical industry. The process characteristics are as follows: low alloy die steel with medium to low carbon content and high alloy die steel with medium to high carbon content are heated to 900 ℃ -930 ℃ in an active medium (carburizing agent) for carbonization, allowing carbon atoms to penetrate the surface layer of the die. Then, quenching and low-temperature tempering are carried out to give the surface and core of the die different compositions, structures, and properties.
Carbonization is divided into solid carburizing, liquid carburizing, and gas carburizing. Recently, it has also developed into controlled atmosphere carburizing, vacuum carburizing, and benzene ion carburizing.
2. Nitriding
The process of nitrogen infiltration into the surface of steel is called nitriding of steel. Nitriding can provide mold parts with higher surface hardness, wear resistance, fatigue resistance, red hardness, and corrosion resistance than carburizing. Due to the low nitriding temperature (500-570 ℃), the deformation of the mold parts after nitriding is relatively small.
The 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 applied, which shorten the nitriding time and obtain high-quality nitriding layers.
3. Nitrogen carbon co infiltration
Nitrogen carbon co infiltration is a low-temperature nitrogen carbon co infiltration process (530 ℃ -580 ℃) that simultaneously infiltrates nitrogen and carbon into a medium containing activated carbon and nitrogen atoms, with nitrogen infiltration as the main method. The brittleness of the nitrogen carbon co infiltration layer is small, and the co infiltration time is much shorter than the nitriding time. After nitrogen carbon co diffusion, the thermal fatigue performance of die-casting molds can be significantly improved.
The harsh working conditions require die-casting molds to have good high-temperature mechanical properties, resistance to cold and hot fatigue, resistance to liquid metal erosion, oxidation resistance, and 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 steel, so that the surface of the mold obtains high hardness and wear resistance, while the core still has sufficient strength and toughness, and effectively prevents molten metal from sticking to the mold and corroding. Choosing appropriate heat treatment processes can reduce waste and significantly improve the service life of molds.
