The microscopic morphology of the fracture surface of the gear [36] observed by the scanning electron microscope to observe the fracture surface of the gear: 1) There are three regions inward from the root, namely: carburized layer; crack source region (along the crystal fracture); fatigue crack growth region The beach pattern is presented. As shown in a, the spacing of the beach patterns is gradually increased, indicating that the tooth root fatigue load is increasing. 2) The color of the carburized layer is gray, flat and fine, and there are no defects such as cracks and inclusions. As shown by b, c is the junction morphology. 3) The source region of the crack is a sugar-like fracture with a reflective facet, which belongs to the typical morphological fracture morphology, as shown in a; there is an oxide film at the edge of the crack source region, indicating that micro-cracks occur when the gear is carburized and quenched, such as b Show.
Relevant data show that after carburizing of 20CrMnMo steel, the austenite stability of each part of the carburized layer is different due to different carbon concentration. If the heat treatment process is improper, cracks are likely to occur. 4) The fatigue crack growth zone is quasi-cleavage morphology and there are typical fatigue stripes, as shown by b and c. a is the overall shape of the fatigue crack growth zone. 5) The instantaneous fault zone morphology is a quasi-cleavage dimple, and there is a local tear, as shown.
Tooth shape and hardness test analysis It was found by tooth profile inspection that the gear toothing surface adjacent to the fracture surface was severely worn, and the tooth profile on one side of the meshing surface was severely thinned (the position of the tooth tip was particularly serious). There is a clear rib at the lower limit meshing position, the rib is below the rib, and the meshing surface above the rib is severely worn.
Through the hardness test, it is found that the hardness of the worn tooth surface is low, and the hardness of the rest of the gear is qualified. The remaining meshing surfaces of the driving gear (Z1=14) are all intact, and the meshing surfaces of the driven gears (Z2=63) meshing with them are all intact, and the transmission ratio i=Z2/Z1=4.5 integer. The analysis shows that the severe wear of the toothed mesh surface is caused by the insufficient hardness of the tooth surface, which is independent of the driven gear. Insufficient local tooth surface hardness is related to improper heat treatment process. When the gear is solid carburized, the spacing of the parts is too small or placed vertically, which will affect the carburization uniformity and the carburizing dead angle, resulting in insufficient local tooth surface hardness after quenching [7].
Analysis of the root stress state [5] The tooth root dangerous section stress distribution is shown. It can be obtained by looking up the table: the gear transmission coincidence degree = 2.1, and the number of meshing teeth is small during the transmission process. During the use of the gear, due to the low hardness of the tooth surface, the meshing surface is gradually worn until it completely fails. At this time, the gear transmission coincidence degree is reduced, and the adjacent gear is subjected to the transmission load, and the stress distribution of the tooth root of the gear tooth is large. The smoothness of the transmission is reduced and an impact is generated. When the load acts on the top of the tooth, the root is subjected to the maximum tensile stress, and the tooth is in a dangerous state. At this time, the maximum tensile stress of the root is F=(Ft/bm)YFYsY(1) where Ft is the circumferential force and b is the tooth. Width, m is the modulus, YF is the tooth profile coefficient, Ys is the root stress concentration factor, Y is the transmission coincidence coefficient, Y=0.25 0.75/.
Conclusion 1) Gear fracture is a one-sided bending low stress fatigue fracture. 2) The main cause of gear fatigue fracture is improper gear heat treatment process, resulting in quenching cracks at the junction of carburized layer and core and insufficient hardness of local tooth surface, resulting in gear wear failure, increased load of adjacent gear teeth and impact. The tooth root is subjected to a sharp increase in tensile stress, and the gear is fatigue-fractured. 3) In the production of gears, the placement and position of the parts during carburizing and quenching should be strictly checked to avoid uneven carburization and ensure the hardness of the tooth surface; in the process of carburizing and quenching, the gear heat treatment process should be strictly controlled. Avoid quenching cracks.
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