Defects such as underheating, overheating and overburning in the heating process of steel are mainly caused by inaccurate or malfunctioning furnace temperature instruments, improper furnace installation methods, and uneven furnace temperature.
The so-called underheating refers to the appearance of ferrite in the quenched structure of hypoeutectoid steel, resulting in insufficient response of the quenched steel; for hypereutectoid steel, there are more undissolved carbides in the quenched structure. The so-called overheating refers to the coarsening of austenite grains when the steel is heated. After quenching, coarse martensite is obtained, which makes the workpiece brittle. The over-firing is not only the violent coarsening of the austenite grains, but also the oxidation of the grain boundaries, and even the melting of the grain boundaries, causing the workpiece to be scrapped.
In order to prevent these defects, temperature measuring instruments should be checked frequently, and correct heating specifications and furnace installation methods should be adopted. If the steel is underheated or overheated during heating, the steel can be annealed or normalized once, and then quenched again.
Oxidation refers to the process in which the surface of steel interacts with oxygen, oxidizing gas, and oxidizing impurities in the heating medium to form iron oxide. Due to the formation of iron oxide scale, the size of the workpiece will be reduced, the surface finish will be reduced, and the cooling rate during quenching will be seriously affected, resulting in insufficient soft spots or hardness. Although the oxidation of steel is a chemical reaction, after an oxide film is formed on the surface of the steel, the rate of oxidation mainly depends on the diffusion rate of oxygen and iron atoms through the oxide film. As the temperature increases, the diffusion rate of atoms increases, and the oxidation rate of steel increases sharply, especially when the temperature is above 600℃, the oxide film formed is mainly FeO, which is not dense, and oxygen and Iron atoms easily penetrate through this oxide film, and the oxide film is getting thicker and thicker; while below 600℃, the oxide film is composed of relatively dense Fe3O4, so the oxidation rate is relatively slow.
Decarburization means that the carbon in the surface layer of steel is oxidized and the carbon content of the surface layer is reduced. The higher the heating temperature, the higher the carbon content of the steel (especially when it contains more elements such as silicon, molybdenum and aluminum), the steel is easier to decarburize, because the diffusion rate of carbon is faster, so the decarburization rate of steel It is always greater than its oxidation rate, and there is usually a decarburized layer of a certain thickness under the steel oxide layer. Because decarburization is a decrease in the carbon content of the surface of the steel, the surface hardness of the workpiece after quenching is insufficient, the fatigue strength is reduced, and it is easy to form surface cracks on the surface of the steel.
In order to prevent oxidation and decarburization, according to the requirements and actual conditions of the workpiece, protective atmosphere heating, vacuum heating, and surface coating packaging heating methods can be used; when heating in a salt bath, a standardized deoxidation system should be established, and deoxidation should be added regularly Agent.
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