Hardened gears have the advantages of high bearing capacity, good wear resistance and small volume. They have been widely used in mechanical transmission. Studying the fatigue damage of hardened gears has important guiding significance for production.
1 contact fatigue failure of hardened gears 1.1 The form of contact fatigue failures are flank gray spots and pitting failure.
(1) Tooth surface gray spots. Regardless of the carburized quenching gear or the nitriding gear, after a loading sequence of approximately 106 cycles, a slight gray band appears between the pitch line and the lowest line of the single tooth engagement on most tooth surfaces, with the number of runs With the increase, the gray spot becomes more and more serious, and its width gradually develops toward the pitch line. In the area where gray spots appear, the roughness increases and the gloss becomes dark. It can be found by scanning electron microscopy that the flank gray spot is composed of a large number of micropitting and microcracks. Micropitting is developed by microcracks.
(2) Pitting failure. For the carburized and quenched gears, when the number of cycles increases to a certain value, a large pitting pit suddenly appears on a tooth surface, and then gradually spreads after a long period of operation until failure.
For the nitriding gear, as the number of cycles increases further, a large number of micropittings in the gray spot area increase unevenly and deepen, and similar wear dents appear below the pitch line, and continue to operate, and a large pitting pit appears in the area. , approaching or exceeding the failure assessment criteria. Scanning electron microscopy was used to observe the cross section of the damaged tooth. It was found that there were cracks originating from the tooth surface and extending downward with the tooth surface at about 30°. These cracks were the secondary cracks generated by the individual micropitting pits on the tooth surface. The result of development. It can be seen from the fatigue crack propagation strips in the lower part of some large pitting pits that the crack originates from the tooth surface. When the crack develops to a certain depth, the secondary crack in the direction of the vertical tooth surface causes the whole piece to fall off and form a pitting pit.
1.2 Failure Analysis (1) The effect of Hertzian stress. When two infinitely long cylinders are in contact, the stress in the contact zone is σH=1π×FNb×1R1-μ21E1 1-μ2E2, where: σH—the contact surface Hertz stress; b—the tooth surface width; FN— Normal force; E1, E2 --- Young cylinder elastic modulus; μ1, μ2 -- Poisson ratio of two cylinders; R -- equivalent radius, 1R = 1R1 1R2; R1, R2― -- Two cylinder radii.
During the spur gear meshing process, FN, R1, and R2 all change with the meshing point. When R1=R2, 1R reaches the minimum value, and the larger the difference between R1 and R2, the larger the 1R.
Generally, the coefficient of coincidence of the spur gear changes between 1 and 2. When the double tooth meshes into the single tooth mesh, the load on the tooth surface suddenly increases.
From the above analysis, the Hertz stress maximum is at the single tooth meshing starting point.
(2) The influence of the friction of the tooth surface. The root height portion of the driving wheel and the high tooth portion of the tooth top are opposite in direction, and are far away from the pitch line, and the farther away from the pitch line, the larger the sliding coefficient. The direction of the tooth surface friction is the same as the sliding direction. It can be seen that the tip of the tooth surface microcrack is exactly opposite to the tooth surface friction. The tooth surface friction is the largest at the single tooth meshing starting point, which will make the maximum shear stress under the tooth surface of the region close to the tooth surface, and the secondary crack caused by microcrack and micropitting will expand into the tooth surface.
(3) The running condition of the hardened gear is poor. The hardened gears wear little during operation. Even if pitting occurs, the machining marks of the tooth faces still exist. These tool marks form many peaks and troughs.
Since the peaks are not eliminated during the running, when the boundary is lubricated, a large contact stress is generated at these peaks, resulting in the generation of microcracks and gray spots.
2 Bending fatigue failure of hard-toothed gears Bending fatigue breaks basically start from the 30° tangent to the root of the tensioned side, and then extend to full-tooth fracture. Observed by scanning electron microscopy, the rigid fracture of the hardened gear can be divided into three areas—the crack origin, the fatigue extension and the fast termination. Cracks are generally generated on the surface of the root and break completely in this region in a strictly crystalline manner. The cracks in the hardened layer below are expanded by cleavage transgranulation and fine crystal mixing. In the next matrix, the fatigue expansion of the periodic joint can be observed, and the fatigue crack can be observed very little. Then, the ductile fatigue expansion zone is entered, and the obvious fatigue crack and secondary crack can be seen in this area, and then enter the fast. In the termination zone, this zone is a brittle fracture zone and a large number of dimples can be observed. The final hardened layer fracture zone is a mixture of quasi-decrystallized and severely crystallized. For the nitriding gear, the ductile fatigue expansion zone is large, and the shear lip is high and obvious.
3 Improve the fatigue strength of the hardened gears. 3.1 Select the appropriate lubricating oil (1) In the boundary lubrication state, the lubricating oil containing the extreme pressure anti-wear additive should be used. In the boundary lubrication state, since the oil film is thicker than λ<1, the tooth surface has a convex peak colliding when the gear is working. At this time, the viscosity of the lubricating oil does not play any role, and the task of reducing friction and avoiding wear is caused by the pole. The additive is used to form a physical, chemical adsorption film or chemical reaction film on the surface of the metal to protect the tooth surface.
(2) In the mixed state, a gear oil containing a small amount of extreme pressure anti-wear additive should be selected. In the mixed state, 1<λ<3, the oil film thickness is relatively increased, and sometimes the tooth surface is collided. The friction force is composed of the friction between the peaks and the friction inside the lubricating oil. The tooth surface load is composed of the oil film. Shared with the peaks.
(3) Under full-film lubrication, the oil film is thicker than λ>3. That is, the oil film thickness is much larger than the surface roughness, and the 2 moving surface is completely separated by the oil film. Therefore, the viscosity of the lubricant plays a leading role, and the additive does not play any role.
3.2 For the important gears, vacuum furnace carburizing and quenching carburizing and quenching the tooth surface can generate residual compressive stress, which is very beneficial to improve the bending fatigue strength of the gear. The residual compressive stress is generated due to the higher carbon content of the tooth surface layer after carburizing and the lower carbon content of the inner layer. During the quenching process, the temperature at which martensite begins to transform varies with the carbon content. The gear teeth produce residual compressive stress from the different order of microstructure transitions between the layers. Surface decarburization affects the microstructure of the tooth flanks and thus affects residual stress.
For the more important gears, the heat treatment process of the carburizing and quenching of the vacuum furnace can be adopted. The low-grade carburized steel gear is directly quenched after carburizing, and there is no secondary heating and heat quenching process, and the decarburization phenomenon is obviously reduced.
3.3 Hard shot peening to improve the fatigue strength of carburized gears For carburized gears, the more the retained austenite in the steel, the more the amount of retained austenite transformed into martensite by hard shot peening, the martensite The micro-substructure is refined, and the amount of phase change expansion is larger. At the same time, the dislocation density increases, the subgrain boundary is more refined, the lattice distortion is intensified, and the resulting increase in residual compressive stress and hardness is increased, and the fatigue life is correspondingly increased. The aging treatment of the gear after shot peening can further increase the strength. For the gear of 20CrMnTi material, the microstructure before shot blasting is high-carbon martensite fine-grained carbide with more retained austenite, and after shot peening, more fine-grained martensite is formed, carbide The number is also increased, and the retained austenite is significantly reduced. After the low temperature aging treatment, fine alloy carbides are precipitated from martensite and austenite. In addition, low temperature tempering can effectively relax the high stress field generated after shot peening, prevent the stress from causing fatigue cracks, and correspondingly improve the fatigue life of the gear.
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