On the basis of traditional oxidation (blackening), electroplating, nitriding and other processes, with the continuous development of technology, many new surface strengthening and protection methods have been proposed. A variety of traditional surface strengthening and protective heat treatment methods are used in metal mold steel. According to the nature of the processing technology and the bonding properties of the surface modification layer and the substrate, they are divided into 4 categories: penetration, coating, conversion coating formation, surface hardening and hardening.
1) Infiltration method. Inhomogeneous element atoms are transported through the medium to the surface of the workpiece to be absorbed. Because the infiltrated element has a certain solubility in the iron matrix, there is a concentration gradient on the iron surface and inside, forming a diffusion channel gradually saturated by the extravasated element. A transition layer appears with a continuous change in composition and concentration in the unchanged region. The combination of the surface modification layer and the substrate is called "metallurgical combination", which is the strongest combination in various surface treatment methods such as nitriding and carburizing.
2) Coating method. Using vacuum evaporation, ion plating, spray painting and other methods, the surface is coated with a high-performance coating. Mainly use TiN, TiC ion plating and diamond coating, they have excellent performance in hardness, wear resistance, wear resistance, wear resistance, and chemical protection. Similar to the conventional coating, the combination of the coating and the substrate adopts a physical method, and the combination of the coating below 560 ℃ is basically a non-metallurgical combination. If the surface of the workpiece is not cleaned before electroplating, or is in another inert state, the coating is too thick and easily peeled off. Even with the corresponding process improvements, the bond strength is far less than the penetration method, so generally only a very thin coating is used.
3) Chemical conversion coating method. The surface of the composite layer is formed by the chemical interaction of its elements with the elements in the steel (usually iron), and the solubility of the elements participating in the ferrous medium is very small, so the structure and composition of the metamorphic layer and the substrate are There is no gradient, and the bond to the substrate is non-metallurgical, and no solid solution concentration can be established at the surface. The name of this method is usually defined as "chemical", such as oxidation and phosphorylation.
Due to historical reasons, the naming of permeation method and the formation method of chemical conversion film are also inconsistent with the understanding of its mechanism, classification and naming, and sometimes cause confusion. The method of forming the FeS layer is often referred to as "sulfurization".
4) Superficial hardening of the surface. In recent years, this is a category in which laser hardening has been used for high-speed steel tools. For high-speed steel tools, deep hardening (flame, induction heating) is rarely used. Using a high-energy micro-spot, a selected part is scanned on a tool that has already been quenched and tempered, and a shallow surface quench layer with controlled location and secondary quench depth control can be obtained. This process can ensure that the surface of the tool is free of oxidation, and the surface of the tool will not undergo secondary hardening after secondary quenching. The hardness of the second hardened layer after tempering is significantly higher than that of the substrate.
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