Parts whose alloy steel surface is hardened by carburizing, nitriding, etc. are more susceptible to surface grinding damage during grinding. Grinding damage mainly refers to grinding burns and grinding cracks. The existence of grinding burns and grinding cracks seriously reduces the service life of parts and is a defect that is not allowed to exist.
Grinding cracks are caused by grinding tensile stress exceeding the fracture strength of the material. The cutting of the grinding wheel uses many polygonal sand grains in the grinding wheel to form many knives, which are friction ally cut when in contact with the work piece. During the cutting process, the metal in front of the blade (sand grains) is squeezed and torn, and the intense friction between the metal behind the blade and the grinding wheel causes the grains to be pulled and slipped, thereby causing elastic and plastic deformation of the metal. When the cutting action stops, the metal surface strives to return to its original position under the action of elastic deformation force, so tensile stress parallel to the grinding track appears, but the formation of grinding cracks is not only related to grinding conditions and processes, but also closely related to factors such as material and poor heat treatment.
The following measures can effectively eliminate serious grinding cracks:
1. First, reduce the quenching temperature of carburized parts. Carburize at 930℃, quench directly after carburizing, and the quenching temperature is reduced from 860℃ to 830℃.
2. Second, adjust the tempering temperature and tempering holding time. The tempering time in spring, autumn and summer is extended from 2 hours to 3 hours; in winter, it is extended to 4 hours, and the lower limit of the tempering temperature is increased from 180℃ to 200℃. Gears with displacements above 80 are tempered for 2 hours, or aged in hot oil at 160-180℃ for 12 hours.
3. Third, control the surface carbon concentration, and the carbon concentration is controlled at 0.65% to 0.8%. The carbon concentration distribution gradient should be gentle to ensure good surface strength and stress distribution. The carbon content of heavy-duty gears should be controlled at the lower limit to facilitate the control of the size and shape of carbides. When the carbon content is controlled at the upper limit, the tendency to form retained austenite will be enhanced, and there will be a tendency to increase carbides, surface oxidation and reduce tooth root strength. According to relevant data, the carbon concentration on the surface of heavy-duty gears in the United States has been controlled at around 0.65%.
4. Fourth, control the amount of retained austenite to prevent the gears from undergoing structural transformation during grinding and generating large structural stress. Strictly control the retained austenite within 25%, and for heavy-duty gears, it should be controlled within 20%. Control the size, quantity, morphology and distribution of carbides to obtain dispersed fine-grained carbides, thereby increasing the fracture strength of the material and reducing brittleness. Control the level of martensite to obtain cryptocrystalline and fine needle-shaped martensite, avoid the generation of coarse needle-shaped martensite, thereby reducing crack sources and increasing the fracture strength of the material. Level 3 of martensite is the best.
5. Fifth, choose a sharper grinding wheel PA36-46J, and replace the original grinding wheel with a hardness of K grade to J grade; reduce the amount of cutting, increase the number of cutting (grinding), and increase the grinding wheel speed; keep the sharpness of the diamond of the grinding wheel dresser, because its sharp working state is the premise and guarantee for repairing the grinding wheel.
6. Sixth, increase the flow rate of coolant and the intensity of jet flushing, regularly clean the cooling oil tank, check the filter, and replace new oil to ensure the cooling quality and effect.
The fundamental reason for the generation of grinding cracks is that the heat generated by the metal surface during the grinding process cannot be taken away by the cooling medium in time, and the surface material is re-tempered or forms a new quenched martensite, which causes the surface to generate high organizational stress and thermal stress to form tensile stress. Poor organizational state and grinding process are the main causes of grinding cracks.
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