Based on the above test results of the effective hardened layer depth and microstructure at several positions of the tooth-shaped sample, such as the uncracked tooth surface (concave), the cracked tooth surface (convex) away from the crack pitch circle, the convex surface close to the root of the crack and the crack, it can be seen that the surface hardness, effective hardened layer depth and microstructure at each position of the tooth-shaped sample present different characteristics, The surface hardness, effective hardened layer depth and microstructure at the pitch circle of the uncracked tooth surface (concave) and the crack tooth surface (convex) far away from the crack pitch circle conform to the drawing technology, without grinding burn characteristics; However, the root of the uncracked tooth surface (concave surface), the root of the cracked tooth surface (convex surface) near the crack and the crack all show different degrees of grinding burn characteristics, especially in the root of the cracked tooth surface near the crack and the crack, the obvious crescent-shaped white and black grinding burn zone can be observed, the depth of the deepest white zone at the crack is about 0.3 mm, the depth of the black zone is about 0.6 mm, and the white zone is re-quenched due to grinding heat, The maximum hardness is about 820 HV0.1. The black area is tempered and softened due to grinding heat, and the minimum hardness is only 486 HV. Therefore, after carburizing, quenching and grinding, the spiral bevel gear has grinding burns at the tooth root and crack. The tooth surface crack is caused by secondary quenching caused by grinding burns. The circular arc crack on the tooth surface has a sense of lifting on the touch, and the crack is not flat, which also means that the circular arc crack is generated after grinding.
The tooth surface hardness and strength of carburized and quenched spiral bevel gears are relatively high. In the subsequent grinding process, the abrasive particles play the role of cutting, scoring and sliding. The generation and release of grinding heat is the most important reason that affects the degree of grinding burn. The grinding wheel and grinding parameters play a critical role in the generation of grinding heat. The cutting fluid and carburized layer carbon concentration have a decisive role in the release of grinding heat. Therefore, In the process of grinding, once the surface temperature of spiral bevel gear caused by grinding heat exceeds 350 ℃ and is lower than Ac1 point, the martensite on the carburized surface will be tempered and softened and transformed into tempered troostite or tempered sorbite, which is generally called tempering burn; If the surface temperature exceeds Ac1 or even above Accm, under the cooling effect of grinding fluid, secondary quenching martensite will be directly formed on the carburized surface. Generally, this kind of burn is called quenching burn. When observed under the optical microscope, its typical feature is that the obvious crescent white and black structure morphology can be observed at the burn area. The secondary quenching area corresponding to the white area has high hardness, and the tempering softening area corresponding to the black area, The hardness decreased significantly. When grinding spiral bevel gears, improper selection of grinding wheel and large amount of feed will cause sharp increase of friction stress. The combined action of friction stress, thermal stress and tissue stress will crack the material surface.
Grinding burn will weaken the compressive stress on the surface of spiral bevel gear, and even cause tensile stress. The change of tensile stress in the transition layer is easy to produce microcracks. Under the subsequent cyclic load, the fatigue strength and service life of spiral bevel gear will be greatly affected. The tooth surface will be worn and pitted too early, resulting in the early failure of spiral bevel gear.