18
2025
-
02
Changes in the metal heating process: oxidation and decarbonization
Oxidation and decarbonization are two important concepts in materials science, especially significant in metal processing and heat treatment.
Oxidation and decarbonization are two important concepts in materials science, especially significant in metal processing and heat treatment..
1. Oxidation
Definition: Oxidation is the reaction in which reactants lose electrons during a chemical reaction, as well as the chemical reaction between a substance and oxygen. In organic reactions, the introduction of oxygen to organic substances or the removal of hydrogen is also called oxidation. Depending on the type of reaction, oxidation can be divided into slow oxidation and rapid oxidation (such as combustion).
Characteristics:
Metals react chemically with oxygen during heating, forming oxides (i.e., oxide scale). This oxide scale not only causes discoloration and loss of luster on the metal surface but may also affect the mechanical properties of the metal.
During oxidation, the oxidation state of the substance increases, losing electrons.
Slow oxidation, such as metal rusting and biological respiration, occurs slowly and is exothermic but does not emit light.
Rapid oxidation, such as combustion, occurs quickly and emits light and heat.
Effects:
For steel subjected to cold working (such as cold rolling, drawing, etc.) and steel plates used for coating, the presence of oxide scale (i.e., metal oxides) affects the surface quality of the products and increases tool wear.
In chemical industrial production, oxidation reactions play an important role in the preparation of many compounds, such as the oxidation of iron sulfide to sulfur dioxide and then the preparation of sulfuric acid.
2. Decarbonization
Definition: Decarbonization refers to the phenomenon where carbon is completely or partially lost from the surface layer of materials or components made of steel and iron-based alloys, which is a defect characterized by a reduction in carbon content on the surface of steel.
Causes of Formation:
Decarbonization is likely to occur when the heating temperature of steel is too high or when it stays at high temperatures for too long.
The carbon in steel reacts with oxygen, carbon dioxide, and hydrogen in the air, producing carbon monoxide or methane, leading to the escape of carbon from the surface metal.
Effects:
Decarbonized steel loses strength and softens, adversely affecting the material's performance.
For steel that requires quenching, decarbonization reduces the carbon content in the surface layer, preventing the martensitic transformation or resulting in incomplete transformation after quenching, thus failing to achieve the desired hardness.
Decarbonization of bearing steel surfaces can cause soft spots after quenching, making them prone to contact fatigue damage; decarbonization of high-speed tool steel surfaces can reduce red hardness.
Characteristics of Decarbonization Layer:
For carbon-containing metals (such as steel), decarbonization may occur during the heating process. That is, the carbon in the metal reacts with gases such as oxygen and carbon dioxide, generating gases like carbon monoxide or methane that escape from the metal surface, leading to a reduction in carbon content in the metal's surface layer. Decarbonization affects the mechanical properties and heat treatment of the metal.
Due to the oxidation of carbon, the carbon content of the decarbonization layer is lower than that of the normal structure.
Reflected in the metallographic structure, the number of cementite (Fe3C) is less than that in the normal structure.
Reflected in mechanical properties, its strength or hardness is lower than that of the normal structure.
Summary:
Oxidation and decarbonization are phenomena that require close attention during the processing and heat treatment of materials. By controlling heating temperature, heating time, and adopting appropriate protective measures (such as gas protection), the occurrence of oxidation and decarbonization can be effectively reduced, ensuring the performance and quality of the materials.
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