Metallographic diagram of 45 steel (940℃ solid boronizing for 5.5h)

The microstructure of G45 steel after solid solution treatment and quench tempering is mainly the structure of tempered martensite. As shown in Figure 1, it can be seen from the microscope that the microstructure of the G45 steel is composed of tempered martensite matrix and a small amount of bainite. After 5.5h of quenching at 940°C, the microstructure of G45 steel mainly consists of tempered martensite, which accounts for about 85%, and a small amount of bainite, which accounts for about 15%.The length of the tempered martensite lath varies due to different quenching rate, ranging from 1 to 10 microns. A small amount of bainite matrix with round non-equiaxed structure exists in the microstructure of the G45 steel.It can be seen from the microstructure of G45 steel that the grain size of tempered martensite is finer, the grain boundaries are obvious, the number of carbide particles is large, and the carbide particles are distributed along the grain boundaries. The carbide particulates mainly include coarse and fine carbide particulates. The coarse carbide particles are mainly distributed in the grain boundary and grain boundary triple points, and their average grain size is relatively coarse. While there are a lot of fine carbide particles distributed in the grain boundary, and their grain size is finer. In addition, a small amount of undissolved borax particles can be observed in the polished cross section of Φ50 specimen. It can be seen from the micrograph that the size of the undissolved borax particles is inhomogeneous, mainly widely distributed in some metallographic fragments, and the shape of the undissolved borax particles is irregular.Through microstructure observation of G45 steel, it can be seen that after solid solution treatment and quenching tempering, the microstructure of G45 steel is mainly composed of tempered martensite matrix, a small amount of bainite and a small amount of undissolved boron particles. The tempered martensite laths of G45 steel at different quenching rate are distinct. The grain boundary of tempered martensite of G45 steel is more obvious, and the number of carbon particles is large, distributed along the grain boundary. In addition, there are also coarse and fine carbon particles, and a small amount of undissolved borax particles.

45 steel is a low-carbon martensitic stainless steel. It has low sulfur and phosphorus content and high strength. It also has good hardenability, formability and weldability. In terms of metallographic structure, the alloy has a homogeneous quenching and tempering microstructure consisting of martensite and dispersed precipitated carbides.

The grain structure of 45 steel after quenching and tempering treatment is quite fine, and the martensitic transformation is uniform. After quenching, 440℃ tempering can significantly reduce the hardness of martensite and make the structure more stable and reliable. After quenching and annealing, the hardness is reduced to HRC 33-37, which is much lower than prior quenching and tempering.

The hardness of 45 steel after 940℃ solid solubilization state quenching 5.5 hours is HRC45-53. The microstructure observation of 45 steel in this state shows that it consists of the original eutectic carbide and the newly formed, austenite-diffused, hexagonal-shaped MC carbide. The MC carbide has a higher hardness and is distributed around the grain boundary and angle, so that the grain boundary strength of the material is effectively improved.