Study on carburizing process of low speed and heavy gear

The traditional gas carburizing method uses a single liquid (such as kerosene) carburizing to control the surface carbon content and the depth of carburized layer by controlling the amount and time of dropping liquid. Therefore, the control accuracy is poor and the surface carbon content fluctuates greatly. Now the commonly used controlled carburizing method is carrier gas carburizing method, that is, the controlled atmosphere with stable composition and medium carbon potential is used as the carrier gas, which is introduced into the carburizing furnace at a large flow rate Then, the carbon potential of furnace gas and the quality of carburized layer can be accurately controlled by adjusting the flow rate of rich gas. The fluctuation of surface carbon content and layer depth can be controlled within 0.05% and + 0.1 mm respectively.

Because the air flow ratio is usually adjusted to adjust the fuel quantity. On the one hand, the change of atmosphere is sensitive to the air flow rate; on the other hand, when the fuel flow rate is adjusted, the risk of carbon black is high. In this experiment, the direct air flow rate (acetone flow rate is basically fixed) is used to realize the dynamic carbon potential control.

The carbon potential of the furnace gas is the characterization of the carburizing ability of the furnace gas, which is related to the composition and temperature of the furnace gas. Its height reflects the strength of the carburizing ability of the furnace gas. The carbon potential control is to control the relative amount of the composition of the furnace gas. Under the temperature required by the process, the composition of the furnace gas is adjusted to reach the process set value with the carbon content on the surface of the workpiece In the international production test, we strengthen the monitoring of carbon potential, because of C02, HZO, CO, CH. 、H。 The actual carbon potential in the furnace will be affected by the partial pressure, furnace temperature, total pressure in the furnace, the flow rate of rich gas, the accuracy of CO analyzer and the quality of oxygen probe. However, at a certain temperature, the furnace gas consists of C0: 、 HZ0 、 C0 、 OZ 、 CH. There are seven kinds of reactants and products in the system, which are composed of C and H. According to the calculation formula of the degree of freedom of the system, only three gases are independent variables in the carburizing atmosphere of the seven substances, so the accurate control can be achieved by controlling the three gas components.

Four kinds of carburizing and quenching heating temperatures were selected in the experiment. The higher temperature is 980 ℃ carburizing + 860 ℃ quenching, but the hardness is lower. This is because of the high heating temperature, the high degree of austenite homogenization, the alloy elements fully dissolved in austenite, resulting in the decrease of temperature and the increase of retained austenite. The selection of carburizing and quenching temperature should take into account the microstructure changes of the core and the surface. If the temperature is too low, the microstructure transformation of the core is not complete, which is unfavorable to the properties of the core. The core temperature of the steel is about 810 ℃, so the best heating temperature for carburizing and quenching is 940 ℃ carburizing + 850 ℃ quenching. After quenching, the oil is cooled to 180 ~ 240 ℃, then air cooled for 5 h, and then tempered at 150 “–” 180 ℃ for 4 h. the surface hardness is hrc59.1, and the core hardness is hrc42.9, which can meet the requirements of use. Although there is still 17.4% retained austenite after final quenching and tempering, they have no adverse effect on the properties of the steel.

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