Using normalizing big gear and quenching and tempering small gear, the tooth surface after use can be extruded into mirror shape, which can also make the tooth surface produce cold work hardening effect and improve the hardness of tooth surface and subsurface. For example, the gear stress of the reducer used in a cement plant is 4600kgf / cm2. After several months of use, the gear surface is mirror like. After 20 years of use, there is no pitting defect on the gear surface. The composition of the steel sampled from the gear is 0.39% C and 1.6% Si. The metallographic analysis is shown in Fig. 2. The large gear is normalized with hardness of hb210, and the small gear is quenched and tempered with hardness of hb250. After 27 years of use, the cold work hardening layer has been formed on the tooth surface. The hardness of the pinion tooth surface is Hv350, hv290 at 0.15mm of the subsurface layer, and hv270 near the ferrite at 0.25mm from the surface.
By using the micro plastic deformation of the tooth surface, the partial load of the tooth surface can be eliminated quickly at the initial stage of use, and the full tooth contact can be achieved. The thin (0.1-0.3 mm) uniform retained austenite layer, decarburization layer and copper plating layer are retained on the surface of the sea far unloading moon, which makes the working face produce micro plastic deformation and wear, increases the contact surface and reduces the stress eccentric load, so as to prevent early fatigue pitting.
Of course, this method can only be effective under a certain contact stress. Excessive contact stress will still cause a lot of wear and pitting.
Under the condition that the machining accuracy of gears is basically guaranteed, the load-bearing capacity of large low-speed and heavy-duty gears can be significantly improved by using medium hardness tooth surface (hb320-360) quenching and tempering gears, but the finish of tooth surface should be improved at the same time. This is because the load balancing effect brought by ferrite micro plastic deformation and industrial running in is not as good as the benefit of greatly improving the hardness. The medium hardness gear can be used in the stress range of 5000-7000 kgf / cm2. The stress values of more than ten large low speed and heavy load reducers with medium hard tooth surface produced by sugar mill and cement plant are about 6000 kgf / cm2. After more than one year’s use, it shows that the effect is good.
Although the allowable stress of soft tooth surface gear is related to the hardness of tooth surface, it is not a simple and absolute corresponding relationship. For some gears, it is difficult to produce micro plastic deformation in the process of industrial running in, which is not conducive to uniform load and leads to overload fatigue pitting. The calculation of gear allowable stress published by ISO gear organization adopts block diagram, that is, the allowable stress corresponding to hardness value fluctuates in a range, which depends on manufacturing level, material and other factors. It can be seen from the block diagram that the maximum allowable stress of normalizing may be larger than that of quenching and tempering with high hardness.