Generally speaking, people identify engineering materials in practice, on the one hand, they rely on various instruments and equipment, and on the other hand, they perform visual inspections based on the experience accumulated in practice. Accurate data can be obtained with the aid of instruments and equipment, but it is also possible to obtain valuable results by identifying steel section, hardness, spark burst and other phenomena based on experience under the conditions of inconvenient use of instruments and equipment. Moreover, the latter is simple and easy to implement, which is more suitable for on-site construction. Through the empirical identification method, the general steps for determining the element content and characteristics of steel and the identification of grades are: section identification, test hardness, and spark identification.

 

 

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1. Section identification

 

Approximate carbon content. Steel with low carbon content has fine cross-section grains and grayish-white color; steel with high carbon content has thicker cross-section grains and slightly white color. The fracture of alloy steel generally has fine grains. The section of high-speed steel has finer grains, similar to broken fine porcelain. The section grains of the steel castings are finer, the color is silver gray, and the edges and corners are sharp, while the section grains of the gray cast iron are very large, the color is gray, and there are small silver flash points. The fine powder filed with a fine file can be used in paper. Draw black lines on it. The thickness of the carburized layer can also be judged by the identification of the cross section: the fracture of the high-carbon structure of the surface layer is brittle fracture, showing bright color; the fracture of the low-carbon structure of the center layer is plastic fracture, not showing the bright color.

 

 

2. Hardness identification

 

Hardness can reflect the carbon content of steel to a certain extent. Hardness can be measured by various hardness testing instruments, and files can be used to roughly identify the hardness of ordinary steel. It is difficult to file with high hardness, and it is easy to file with low hardness, so that the carbon content of steel can be roughly judged. Low-hardness steel can be filed with any file, medium-hard steel can only be filed with a fine file or polished file, and high-hardness steel can only be filed with a polished file.

 

3. Spark identification

 

Spark identification is to observe the spark phenomenon to identify the chemical composition of steel. The steel is polished on the grinding wheel, and it is cut into fine hot powder by the grinding wheel and shot into the air at high speed. It oxidizes rapidly in the air and generates high heat, even reaching the melting point of the steel, in a molten state. The carbon in the powder combines with oxidation to generate carbon monoxide gas due to volume expansion, which produces a large internal pressure stress. When the internal pressure exceeds the surface tension of the molten liquid, it will burst and produce sparks. After one burst, if there are residual carbon elements in the pulverized powder, the second or more bursts will occur.

 

Different grades of steel have different shapes and colors of sparks. The elements contained in steel also have a great influence on the characteristics of sparks. Carbon is the main element that causes spark burst. As the carbon content increases, pollen increases and brightness increases. When other elements coexist with carbon, the effect of promoting the explosion of carbon and preventing the explosion of carbon is different. Among them, elements such as manganese and chromium contribute to the explosion of carbon, and elements such as tungsten, silicon, nickel, and steel prevent the explosion of carbon. This is the basis for judging the chemical composition of steel through spark identification.