Knowledge

Carburizing treatment refers to the process of heating and holding steel parts in a carburizing medium to increase the carbon content of the surface layer and form a certain carbon concentration gradient, allowing carbon atoms to penetrate into the surface layer of the steel. This can give low-carbon steel parts a high-carbon steel surface layer. After quenching and low-temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still retains the toughness and plasticity of low-carbon steel.

The characteristics and evaluation methods of carburized layers mainly include the following aspects:

1) Organizational Characteristics: Metal components that have undergone carburizing and quenching treatment have a surface structure mainly composed of acicular tempered martensite and fine secondary cementite. This organizational structure endows the surface with extremely high hardness, typically ranging from HRC58 to 65 (the specific value may vary depending on different testing methods and standards). The core structure varies according to the type of steel used and may include multiple organizational forms such as low-carbon martensite, troostite, and sorbite, with a hardness range roughly between HRC20 and 45. Particularly for parts subjected to heavy loads, the core hardness is usually no less than HRC30 to ensure they possess good comprehensive mechanical properties.

2) Depth of Carburized Layer: The depth of the carburized layer can generally reach 4 to 10 millimeters. This depth range not only meets the requirements of most engineering applications but also ensures that the parts maintain stable performance during long-term use. The hardness distribution curve of the carburized layer is relatively gentle, which means that it is less likely to peel off when subjected to external force impacts, thereby significantly enhancing the service life and reliability of the parts.

3) Mechanical Properties: Metal components after carburizing treatment exhibit excellent mechanical properties, featuring high bending fatigue strength. Their surface wear resistance is significantly enhanced, while the core retains good impact resistance. Compared with medium carbon steel surface quenched parts, carburized parts have obvious advantages in both wear resistance and impact resistance. Moreover, carburizing treatment can achieve a uniform and depth-controllable hardened layer. This characteristic makes the carburizing process almost unrestricted by the complexity of the part shape, and it is widely used in the manufacturing of various precision mechanical parts.

When evaluating the quality of the carburized layer, the following key aspects usually need to be comprehensively considered:

1) Depth of carburized layer: The accurate measurement of the depth of the carburized layer is one of the important indicators for evaluating its quality. Commonly used measurement methods include chemical analysis, metallographic method, and hardness method, etc. The chemical analysis method determines the depth by analyzing the carbon concentration distribution in the carburized layer; the metallographic method determines the depth by observing the microstructure changes of the carburized layer under a microscope; the hardness method calculates the depth of the carburized layer by measuring the hardness values at different depths.

2) Hardness Distribution: The hardness distribution of the carburized layer should be uniform and meet the design requirements. Hardness testing is typically conducted using a Vickers hardness tester or a Rockwell hardness tester. By taking multiple measurements at different depth positions and plotting the hardness distribution curve, the uniformity and hardness gradient of The Carburizing layer can be evaluated

3) Metallographic Structure: The observation of metallographic structure is one of the important means to evaluate the quality of the carburized layer. Through a metallographic microscope, the microstructure of the carburized layer can be clearly observed, such as the needle-like structure of martensite and the distribution of cementite. The ideal carburized layer structure should be uniform and fine, without obvious defects such as cracks and holes, as these defects can significantly affect the performance of the parts in use.

4). Mechanical Properties: The testing of mechanical properties is a key step in evaluating the comprehensive performance of the carburized layer. Tensile tests can determine the tensile strength and elongation of the parts; impact tests can assess the impact toughness of the parts. These mechanical property tests can provide detailed data on the key mechanical properties such as strength and toughness of the parts, thereby comprehensively evaluating the effect of carburizing treatment on the performance of metal parts.

In conclusion, the characteristics and evaluation of the carburized layer of metal parts is a multi-dimensional and comprehensive process. It requires a comprehensive consideration from multiple aspects such as microstructure characteristics, carburized layer depth, and mechanical properties, and should be combined with specific testing methods and standards to ensure that the quality of the carburized layer meets the design and usage requirements.

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