The metallographic microstructure of the finished automobile connecting rod shall be uniform fine grain structure, and no sheet ferrite is allowed.

The microstructure of automobile connecting rod of general quenched and tempered steel is tempered sorbite, while the microstructure of automobile connecting rod of non quenched and tempered steel is normal normalized structure, that is, it is composed of ferrite, pearlite and bainite. However, they are different from the normalized structure of ordinary carbon steel. The difference is that the ferrite in the non quenched and tempered steel is not reticular but massive, and the fine flake or flake precipitation of 100 ~ 400A can be observed under the electron microscope. Therefore, the properties of non quenched and tempered steel are not only affected by ferrite and pearlite structure, but also related to the size, quantity and distribution of precipitates. These factors are closely related to the chemical composition of the steel, especially the precipitated elements and processing conditions (heating temperature, forging or rolling temperature, cooling rate, etc.).

In the metallographic structure of automobile connecting rod of non quenched and tempered steel, there are more ferrite in the core structure, more pearlite in the sub surface structure, and the spacing of pearlite in the sub surface layer is smaller than that in the core. The mechanical properties of the automobile connecting rod need to be checked for hardness, impact toughness, reduction of area and tensile strength. For the requirements that meet the requirements of the automobile connecting rod for hardness uniformity, the Brinell hardness values at all parts of the automobile connecting rod are evenly distributed and 100% checked.