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Process

Technology


Low pressure carburizing
Gas nitriding

Oxidation treatment

Oxidation treatment

In a broad sense, the process of losing electrons in a substance is called oxidation, while the process of obtaining electrons is called reduction. From a narrow perspective, people tend to refer to the process of substances reacting with oxygen to form oxides as oxidation, and the process of substances losing oxygen as reduction. In the field of metal heat treatment, oxidation process mainly refers to the surface treatment process where metals react with oxidants to generate metal oxides. Usually refers to the process of oxygen atoms reacting with Fe on the metal surface to generate Fe2O3 and Fe3O4. Among them, Fe2O3 generated by oxidation is generally orange red in color, with a loose texture and poor corrosion resistance, which is commonly referred to as "red rust" or "iron rust"; Fe3O4 is blue black in color, with a dense texture and excellent corrosion resistance. Therefore, if we can control the formation of only a dense Fe3O4 oxide layer on the metal surface without a loose Fe2O3 oxide layer, not only can we obtain a uniform blue black appearance, but also achieve excellent corrosion resistance on the metal surface. After further polishing and immersion lubrication, the friction coefficient of the metal surface will be smaller, resulting in better resistance to friction and wear.


There are many types of oxidation treatment for metal materials, and the most commonly used method is to treat the metal surface with steam in the temperature range of 450 ℃ to 550 ℃, hence it is also known as "Steam Treatment". The oxidation of iron based materials is also known as the "bluening" or "blackening" process because the Fe3O4 oxide film generated is mainly blue black. Some oxidation treatments are also carried out in salt baths, typically using the "QPQ" process, which is actually a composite process of salt bath nitriding and oxidation.


After gas nitriding, it is also possible to follow the oxidation process, which is commonly referred to as "post oxidation", meaning oxidation after nitriding. The post oxidation process can be carried out immediately in a nitriding furnace or in a specialized oxidation furnace. The main medium used for post oxidation is pure water, which can be vaporized into water vapor before being introduced into the furnace, or directly dropped into the furnace for high-temperature vaporization. The temperature of post oxidation is generally controlled below 550 ℃, beyond which unstable FeO will be generated, affecting the density of the oxide film. The time of post oxidation depends on different steels and the required thickness of the oxide film. Generally, the thickness of the oxide film is controlled between 1um and 3um. Beyond this thickness, the Fe2O3 content in the oxide film will increase, which will actually affect its corrosion resistance.


The process combination of nitriding and post oxidation has been successfully applied to many automotive components that require wear and corrosion resistance, such as ball joints, pins, bearing rings, fasteners, gaskets, and so on in steering mechanisms. The perfect combination of the super strong surface hardness after nitriding and the corrosion resistance after oxidation minimizes the wear of the parts, greatly extending their service life. The nitriding and post oxidation treatment process is also increasingly applied in the mold industry, such as the hot working steel H13 used as an aluminum extrusion mold. After surface treatment with nitriding and post oxidation, the heat resistance and melting loss performance of the mold can be greatly improved, reducing the adhesion of aluminum liquid on the surface of the mold, and doubling the service life of the mold.