Metallographic diagram of 30CrNi3Mo (oil quenched at 860°C)

The microstructure of 20CrNi3Mo (860°C quenching oil)is considered to be a ferritic microstructure. As seen in the micrograph, the structure consists of a ferrite matrix with a few intergranular theta needle networks, which are the eutectic structures formed by the precipitation of low-melting-point interlayers in the ferrite matrix during high semiferrite transformation.

The micrograph clearly shows that the shape of the ferritic matrix is mostly band-like and uneven in shape. This indicates that the effect of heat treatment on the microstructure of 20CrNi3Mo (860°C quenching oil) is fairly good. It can be speculated from the shape of the ferrite matrix that the austenitizing temperature was not too high, and it was guided by plate-shaped ferrite during the high semiferrite transformation.

The network of theta needles, as the main chromium-rich interstitial formation, is distributed in the ferrite matrix. The needle-like eutectic structure appears black in the micrograph, which indicates that the Cr content of the interlayer is relatively high, and it also has a pronounced role in increasing the hardness of the material.

In terms of carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide carbide inside the 20CrNi3Mo (860°C quenching oil) material, the microstructure mainly consists of M3C and M6C types of carbides. The M3C type of carbide is more evenly distributed in the matrix, and it forms a fine blocky shape of acicular or fringed shape, indicating that it is the result of a moderate microalloying element and a proper tempering process. The M6C type of carbide is elongated and distributed in the ferrite matrix, indicating that it has the structure of eutectic or metastable chromium-rich carbide, which is mainly formed by the precipitation of elements during the cooling process.

Overall, the microstructure of 20CrNi3Mo (860°C quenching oil) exhibits good hardness after heat treatment, with fine ferrite and a small amount of theta needle networks distributed in the matrix. The presence of M3C and M6C carbides also contributes to the overall hardness of the material.

30CrNi3Mo steel is a kind of alloy steel. It is mainly composed of iron, chromium, nickel and molybdenum. It has the characteristics of high strength, high toughness, good plasticity and excellent weldability. It is mainly used in the production of parts that require high strength and impact resistance under high stress or at low temperature, such as fasteners, gears, shafts, valve bodies and flanges.

Metallographic examination of the 30CrNi3Mo steel shows that the microstructure consists mainly of a matrix of martensite. The martensite is fine-grained and homogeneous with a grain size of less than 10μm. In the martensite matrix, a small amount of carbides is present, mainly as molybdenum carbides. This precision of the structure is due to its quenching process in oil at 860°C.

In conclusion, the 30CrNi3Mo steel is a high strength and tough alloy steel with excellent weldability, mainly used in the production of parts such as shafts, gears, valves and fasteners. Through metallographic examination, the microstructure of 30CrNi3Mo steel is mainly composed of homogeneous and fine-grained martensite. This fine structure is due to its quenching process in oil at 860°C.