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20CrMo(Carburizing, Quenching, Low-temperature Tempering) Metallographic Analysis
20CrMo is a medium-carbon steel alloy containing chromium and molybdenum, which is mostly known for its strength and wear resistance properties. Its carbon content ranges from 0.17-0.23%, chromium content from 0.8-1.2%, and molybdenum content from 0.15-0.25%. These elements give the alloy high compression strength and strength at elevated temperatures, yielding to its range of common applications, such as engine components, shafts, and gears.
In metallography, 20CrMo can be analyzed through the use of induction heat treatments. In particular, it is exposed to carburizing, quenching, and low-temperature tempering. During carburizing, the alloy is subjected to a high temperature in a low-oxygen atmosphere. This causes more carbon to enter the surface of the metal, increasing the concentration of that element. This is then followed by quenching, wherein the metal is suddenly cooled after being at a very high temperature. During the process, the metal undergoes martensite crystallization, which greatly increases the hardness of the metal’s surface as well as its wear resistance. Finally, the metal undergoes low-temperature tempering, during which a low temperature is used to induce softening in the metal’s crystal lattice. This improves its resistance to corrosion and increases impact resistance.
By analyzing the metal under an optical microscope after the induction heat treatments, the effect of the heat treatments can be seen. In the images, the 20CrMo metal can be seen to take on a distinctive carbon distribution on the surface. This varies depending on the thickness of the metal, with thicker sections appearing more homogeneously darkened as compared to thinner sections. Additionally, the quenching and tempering treatment can be seen to create a white portion on the surface of the metal, most prevalent at the center of the grain boundaries, which indicates the presence of martensite crystal structure.
Overall, carburizing, quenching, and low-temperature tempering are essential inductive heat treatments to improve the mechanical properties of 20CrMo steel. Metallographic analysis under an optical microscope can give an indication of the effect of the heat treatments on the metal, yielding an image of the carbon distribution as well as the effect of the quenching process. Through such analysis, the effectiveness of the heat treatments can be evaluated and parameters adjusted to yield the desired results.
