1. Influence of parts on cyanidation process
Due to the need of infiltration layer, the gear parts of this type of gear pump have to undergo twice cyanidation treatment, which will affect the structure and surface stress state of the parts. 12cr2ni4a gear is carburized at 830 ℃ and cooled to room temperature, and then tempered at 650 ℃. The tempering temperature is high and the grinding crack tendency is large. This is because the increase of carbon content in coarse acicular martensite caused by grain growth at high temperature reduces the thermal conductivity and increases the friction heat, so it also increases the thermal stress. In addition, the surface hardness is significantly improved (HRC ≥ 61), and the semi precision grinding will produce great stress due to more allowance removal.
2. Influence of grinding parameters
In the process of semi precision grinding and precision grinding, due to the mutual movement of gear and grinding wheel and the influence of grinding heat, large residual stress will be generated in the gear, that is, axial residual compressive stress and circumferential residual tensile stress. The circumferential tensile stress plays a real role in the initiation of gear crack. Through the research on the influence of grinding wheel speed, grinding amount and feed amount on residual stress, the result is that the grinding amount has less influence on circumferential residual tensile stress and greater influence on axial residual compressive stress. With the increase of grinding amount, the axial residual compressive stress will increase and the distribution range will deepen gradually. With the increase of grinding speed and feed rate, the circumferential residual tensile stress increases linearly. When the setting of grinding parameters is unreasonable, the heat generated in the grinding process will rapidly increase the temperature around the gear contact part. Under the action of coolant, the gear surface will undergo secondary quenching. At the same time, due to the time difference between grinding temperature rise and cooling, the expansion and contraction during grinding are different, and the interaction between gear and grinding wheel, These will be the factors leading to large residual stress in the process of grinding gears.
Through the physical and chemical analysis of the gear crack, it is considered that the local slight crack of the gear crack is caused by the release of grinding stress, which is directly related to the selection of equipment, grinding wheel and cutting parameters in the machining process. Therefore, the unreasonable selection of grinding wheel and the adjustment of processing parameters and the uneven heat dissipation of parts will also lead to the increase of surface stress and gear burn cracks.
3. Influence of coolant
During grinding, the coolant not only cools the part surface, but also prevents the pore blockage of the grinding wheel and washes the grinding metal foam. In this way, it can prevent the temperature rise, reduce the friction between the abrasive particles and the parts, so as to ensure the accuracy of the parts and avoid grinding defects. Insufficient cooling during grinding, the grinding heat generated by grinding will make the surface of the part Austrian to a great extent, and then quench again to form quenched martensite, resulting in additional structural stress on the surface of the part. The superposition of structural stress and thermal stress caused by grinding leads to gear grinding cracks. At the corner steps and tooth roots of the gear shaft, the coolant cooling supplement during grinding, and the heat dissipation conditions are relatively poor, resulting in defects such as gear grinding burns and gear grinding cracks.