From the test and analysis, the cracking of this batch of gears occurs in the cooling process after carburizing. The main reason for cracking is improper cooling of various parts. The timing and type of subsurface and surface layer transformation of carburized layer are not. In the same manner, the tensile stress formed on the surface of the gear at a low temperature exceeds the strength at that portion, and breakage occurs to form a crack.
Process Analysis According to the above analysis, such cracks can be avoided as long as the carburized gear is transformed from the same structure in the surface. There are two solutions: (1) rapid cooling after carburizing, that is, oil quenching, which causes martensite transformation in both the surface layer and the subsurface layer. (2) Slow cooling after carburizing. When the cooling rate after carburizing is sufficiently slow, pearlite transformation occurs in both the surface layer and the subsurface layer, and no significant internal stress is generated in the layer. Due to the slow cooling of the traditional slow cooling pit, the cooling speed is not easy to control. In the case of low ambient temperature (such as winter), cracks occur from time to time, so the furnace is slowly cooled to ensure that the cooling rate is slow enough in the range of 600-690 °C. To ensure the transformation of austenite to pearlite in the subsurface.
Process formulation (1) Determination of the preheating temperature of the slow cooling furnace. In the initial stage of the test, the preheating temperature was determined to be 400 to 450 °C. The results show that the preheating temperature is high, the workpiece heats up after entering the furnace, and the furnace temperature is overburdened. The highest temperature has reached 650 °C, which causes the furnace temperature to drop slowly, and the workpiece stays in the high temperature zone for too long, resulting in serious surface oxidative decarburization. Therefore, under the premise of ensuring the slow cooling rate, the preheating temperature is appropriately lowered. According to the test, the appropriate preheating temperature is 200 to 300 ° C, and the specific temperature is determined depending on the amount of the furnace. (2) The conditions for the furnace after slow cooling. When the furnace temperature is lower than 300 °C, the surface temperature of the workpiece is 450-550 °C. At this time, the transformation of the secondary surface austenite to pearlite has been completed, so that the air cooling can be performed.