The material composition of the shearer gear meets the technical requirements, the hardness of the oxidation zone at the tooth root decreases obviously, the direction of the gear grinding crack is basically perpendicular to the gear grinding direction, and there is a black oxidation zone at the gear grinding crack, which occurs only at a higher temperature, and there is obvious spheroidization precipitation of cementite in the microstructure at this position, It shows that the local metallographic structure has experienced high-temperature tempering, combined with the distribution of gear grinding cracks, it is certain that this is the gear grinding crack generated in the process of gear grinding, and the main reasons are:
(1) firstly, according to the gear grinding law of the grinder, if the feed rate of the grinding wheel is too large, there is too much one-time gear grinding, or the coolant is not enough to take away the heat generated in the gear grinding process in time, the temperature at the gear grinding position of the workpiece will rise sharply; The heating rate is up to 6000 ℃ / s, and the heat can make the workpiece surface reach 820 ~ 840 ℃, which is enough to make the surface material at the gear grinding position reach the high-temperature austenitizing state. Then, due to rapid cooling, this part of the material will be re quenched to produce martensitic structure, and the volume will increase and deform when austenite transforms into martensitic structure, This will produce additional tissue transformation stress, which may produce gear grinding cracks.
(2) Secondly, due to the rapid heating speed during gear grinding, the sharp change of material temperature will also lead to large thermal stress. The above-mentioned microstructure transformation stress and thermal stress are superimposed with each other, and the stress increases. At this time, the hardness of the material surface decreases due to the increase of temperature, which will cause gear grinding cracks on the workpiece surface.
(3) as the heat treatment process of gear is quenching after carburizing + low temperature tempering, there will be a certain amount of residual austenite and carbide in the structure after quenching, as well as corresponding surface compressive stress. During gear grinding, the increase of workpiece temperature will lead to the decomposition of residual austenite in the structure and transformation into martensite, resulting in local expansion of part volume, This will lead to the redistribution of tissue stress. When this stress exceeds the tensile strength limit of the material, it will lead to the generation of gear grinding cracks.
This is the third cause of gear grinding cracks. It is known from the above analysis that gear grinding cracks are generated. Through process tracing, it is found that there are large amount of gear grinding and unreasonable grinding wheel. The following measures are taken: ① replace brown corundum sand with white corundum wheel with low calorific value; ② Reduce the gear grinding depth and reduce the single gear grinding amount from 0.08 mm to 0.04 mm; ③ Before the gear grinding process, the gear shall be subjected to 160 ℃ × 72 h manual failure treatment to reduce the internal stress of the gear. After improvement, the problem of gear grinding crack is completely solved.