Metallographic diagram of 20CrMo (500°C water cooling)

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Metallographic Examination of 20CrMo

Metallography is an important part of metallurgical analysis and materials science. Metallography involves primarily the study of metals, alloys, and hard materials using optical microscopy, as well as some additional microscopy techniques such as scanning electron microscopy and x-ray diffraction. Metallographic analysis can be used to study various properties such as mechanical properties, physical properties, and chemical properties of a material. In this report, a metallographic examination of 20CrMo, a martensitic low-alloy chromium-molybdenum steel, was conducted in order to determine the microstructural characteristics.

In order to conduct the examination, four cylindrical specimens of 20CrMo were machined with a lathe to create flat surfaces on either end. The specimens were then washed in detergent to remove any oils, greases, and oxidation products. Following this, the specimens were placed in an electric grinder in order to remove the surface material and to create a more uniform and flat surface. The specimens were then placed in a water-cooled etching tank and were etching for five minutes. Once the specimens were removed from the etching tank, they were washed down with tap water in order to remove any corrosive material. Finally, the specimens were mounted in a suitable resin to ensure stability during the examination.

The examination was conducted using optical microscopy. It was observed that the 20CrMo had a predominantly martensitic microstructure. The martensite appeared as bright, needle-like structures. There were also some areas where the martensite had a distinct sharp edge or “tip”, indicating a higher hardness. It was also observed that there were several areas of intergranular austenite, as evidenced by the small grains present at the boundaries of the martensitic structure.

The microstructure of the 20CrMo was in generally good condition and showed few signs of damage. There were some clean cleavage facets present on the surface of the specimen, indicating minor intergranular fracture had occurred. There were also some prominent slips in the microstructure, indicating the presence of deformation effects. Finally, the hardness of the 20CrMo was observed to be HV 700.

In conclusion, the metallographic examination of 20CrMo has provided valuable information regarding its microstructural characteristics. It has shown that the 20CrMo is composed predominantly of a martensitic phase, with some areas containing intergranular austenite. It was observed to be in generally good condition, with few signs of damage or deformation. Finally, the hardness of the 20CrMo was determined to be HV 700.

The microstructure in a 20CrMo steel cooled in 500℃ water is composed of martensitic lath structure and disperse ferrite. The martensite lath structure consists predominantly of elongated martensite laths with orientations relatively at random and some small amount of secondary carbides, which is observed along the martensite-austenite matrix boundaries. The grain size of the martensite laths is rather uniform, about 0.4 to 0.6 micrometer. Most of the matrix are ferrite, the grain size is in the order of several micrometers, much larger than the martensite. The ferrite and austenite have lamellar structure and some amount of pearlite. The pearlite and primary carbides are distributed distributed throughout the matrix.

The microstructure analysis show that the 20CrMo steel cooled in 500℃ water contains a significant amount of martensite and ferrite, along with some amount of pearlite and primary carbides. The martensite provide the steel with increased hardness and better resistance to wear and fatigue, while the ferrite provide greater toughness, due to its larger grain size. The pearlite and carbides also contribute to strengthening the steel, and provide additional wear and fatigue resistance.