The microstructure before carburizing and quenching is ferrite + pearlite. Pearlite is distributed on the ferrite matrix in lamellar form. The metallographic structure level is grade 1, which meets the requirements of grade 1 ~ 4 in the national standard GB / T 13320-2007. After carburizing and quenching, before grinding, the fan tooth surface is a typical “tempered martensite structure”, the carbide is dispersed, and its grade meets the relevant technical requirements. After grinding, a typical “crescent white bright area” was found in the microstructure analysis near the grinding crack, indicating that the surface of the gear was secondary quenched during grinding to form “secondary quenched martensite”, and the dark black microstructure near the secondary quenched martensite was “tempered troostite”, indicating that “secondary quenched burn” occurred during grinding And “grinding burn”.
In the process of gear grinding, under high grinding speed, the surface grinding area will produce instantaneous high temperature (650 ~ 1500 ℃). If the temperature cannot be effectively reduced, the microstructure of the workpiece surface will be changed and the so-called “grinding burn” will be formed. The instantaneous grinding heat has exceeded the phase transformation temperature of the steel. Under the quench of the coolant, the surface structure is “secondary quenched” to form the so-called “secondary quenched layer”. The hardness of the secondary quenching layer is very high. After corrosion, it presents a typical “white bright” zone. The adjacent “grinding burn” area is tempered troostite structure, and the two-phase interface properties are quite different. The sharp change of the hardness gradient of the gear surface and the tissue stress generated in the process of phase transformation make the gear surface present a “tensile stress” state. The stress has been accumulated in a short time and can not be effectively relaxed. Therefore, micro grinding cracks appear at the tooth root first. With the extension of time, they gradually expand to the involute tooth surface, and their overall expansion tends to be perpendicular to the grinding direction, It is distributed on the tooth surface in an irregular network.
The generation of grinding cracks is closely related to the residual stress on the grinding surface. When the local stress in the grinding process exceeds the tensile strength of the material, grinding cracks will appear on the grinding surface. Grinding cracks on the tooth root will greatly reduce the bending fatigue strength of the gear, and it is easy to break the teeth in the meshing process; Grinding cracks on the tooth surface will greatly reduce the contact fatigue strength of the gear, and will become the grinding crack source of “pitting corrosion” and “falling block” in the meshing process. Therefore, in order to ensure the effective service life of the gear, it is necessary to prohibit the generation of grinding cracks in the grinding process.