Metallographic diagram of 20CrNi2Mo (pit cooling after carbonitriding, reheating quenching and low temperature tempering)

Metallography of 20CrNi2Mo after Carbon Nitride Diffusion and Low Temperature Annealing

Metallography is the study of metal structures, primarily at the microscopic level, to understand the behavior of metals in various conditions. In this experiment, the metallography of 20CrNi2Mo after carbon nitride diffusion and low temperature annealing was investigated. The metal was heated to 860-900oC and then held at this temperature for one hour. After this treatment, the metal was quickly cooled below 500oC and heated to 600-620oC for one hour. This low temperature annealing was conducted to improve the mechanical properties and toughness of the metal.

The first step in the metallographic examination was to prepare the metal specimen. The specimen was cleaned with a standard solution to remove any oxides and other surface contaminants. Then the specimen was cut into thin slices and mounted in an epoxy resin to prepare it for viewing. After mounting, the specimen was ground, polished and etched with a reagent to reveal the microstructural features of the metal.

The results of the examination showed that the carbon nitride diffusion treatment caused a significant increase in the grain size of the metal. The grain sizes observed were much larger than before the treatment. This was due to the decomposition of the nitride scale and the subsequent grain growth that occurred during the diffusion in the metal. Additionally, the low temperature annealing had an additional effect of reducing the grain size of the metal.

In the metallographic examination of the metal, a two-phase microstructure was observed. The two-phase microstructure consisted mainly of an α-ferrite phase and a martensite phase. The α-ferrite phase consisted of fine grains, while the martensite phase had larger grains with a dendritic structure. The martensitic phase was more prevalent towards the surface of the metal where it was exposed to the heat treatment.

The micrographs also revealed a number of features associated with the grain structure of the metal. There were areas of discontinuity in the grain structure and a number of twins were observed in the ferrite phase. Additionally, some grain boundary sliding and deformation of boundaries was observed as a result of the diffusion process. This was due to the high diffusion rate of the carbon nitrogen species during the low temperature annealing process.

Overall, the metallographic examination of the 20CrNi2Mo after carbon nitride diffusion and low temperature annealing revealed a two-phase microstructure composed of α-ferrite and martensite grains. The diffusion process caused a significant increase in the grain size of the metal, with the martensite phase being the most prevalent phase at the surface of the metal. Additionally, the low temperature annealing process had a further effect on the grain structure of the metal, causing grain boundary deformation and discontinuities in the grain structure.

The 20CrNi2Mo is a carbon and nitrogen-diffused after quenching cold and then re-heated and tempered in low temperature tempering. The purpose is to improve its mechanical properties. According to its composition, 20CrNi2Mo steel is mainly composed of 20Cr, Ni, and Mo. The mechanical properties of the 20CrNi2Mo steel mainly depend on its hardness, which can be characterized by its yield strength, ultimate tensile strength, elongation and intensity. Its properties are generally for 20CrNi2Mo can be seen from the microstructure.

20CrNi2Mo microstructure mainly consists of martensite and ferrite, which provides higher strength while maintaining ductility, energy absorption and environmental resistance. Generally, 20CrNi2Mo steel has good machining performance, corrosion resistance and good wear resistance after hardening and tempering. The microstructure of the 20CrNi2Mo steel is mainly ferrite and martensite. The size, amount and distribution of these ferrites and martensites are affected by the cooling rate during hardenin g and tempering processes.

The hardness of 20CrNi2Mo steel is determined by the homogenization of phase composition. The homogenous microstructure and phase composition of the 20CrNi2Mo steel are necessary to obtain its desired mechanical properties, so the heat treatment is the most important process in the production process. Generally, the homogenization of phase composition is achieved by homogenization, tempering, and quenching and tempering processes.

Quenching and tempering can form high hardness martensite and at the same time keep some ferrite, which will improve the wear resistance, fatigue strength and high temperature

The microstructure of the 20CrNi2Mo steel is mainly martensite and ferrite. The martensite is a dispersion phase composed of Cr, Ni and Mo atoms. And the ferrite is composed of Fe, Cr and Ni atoms. Under the conditions of quenching and tempering, most of the martensite and at the same time some ferrite can be obtained. And the tempering process can make the martensite more dispersed and improve the mechanical properties of the 20CrNi2Mo steel. Therefore, it is necessary to properly control the cooling rate and tempering temperature, which can make 20CrNi2Mo steel obtain the best comprehensive performance.