Study On Carburizing and Quenching Process Of 20Cr2Ni4 Steel Gear

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Shanghai Gehang Vacuum Technology Co., Ltd. uses 150KW drop pit type carburizing furnace to carburize and quench the gear. Its effective size is Φ1200×1200. The furnace is sealed with the furnace pot by the weight of the furnace cover itself (about 1 ton) and the asbestos tray root, eliminating the continuous heavy manual labor of the bolts and nuts of the furnace cover and furnace pot.After the cold and hot debugging, the furnace temperature is uniform, the equipment sealing performance is good, no air leakage.The carbon potential of the furnace gas in the furnace tank is uniform, the fluctuation is small, and the carbon is fixed by steel foil at 920℃.When the oxygen potential E=1140mV, the carbon potential difference between the upper and lower parts of the furnace tank is 0.02%C.When the oxygen potential E=1125mV, the carbon potential difference between the upper and lower parts of the furnace tank is 0.03%C.

We use thermocouple as the sensing element of furnace temperature and oxygen probe as the sensing element of oxygen potential (furnace gas carbon potential).Solenoid valve and SCR as carbon potential and temperature actuators.Carburizing medium using industrial methanol and isopropyl alcohol.When used, it is directly dropped into the gas carburizing furnace, which produces active carbon atoms after cracking, and the carbon concentration on the surface of the gear is controlled by adjusting the drip amount.We use Honeywell well furnace carburizing microcomputer control system, which has drip type controlled atmosphere carburizing and quenching process control program.

The carburizing process adopts an improved two-stage process of strong infiltration and diffusion.The two-stage method has the advantages of fast carburizing speed and good quality control of carburizing layer concentration.Further improvement is made on this basis.Give full play to the control function of computer fast calculation, adjust the opening frequency and pulse width of solenoid valve timely according to the working condition, adjust the amount of methanol and isopropyl alcohol, and maintain the proportion relationship between the two.Thus gradually adjust the furnace gas carbon potential, so that the carburizing speed is faster, the tooth surface carbon concentration is gentle, the tooth surface hardness gradient is gentle after quenching, and the carburizing quality is better, which is called the self-adaptive carburizing method.

Practice has proved that the application of microcomputer control system, the worker operation is simple, the worker wants to input steel code only, the hardening layer depth and the surface carbon content of the requirement, its everything is completed automatically by the computer control, when the hardening layer depth and the surface carbon content reach the predetermined target value, the automatic bell hints the operator to come out of the oven.In the whole process of carburizing each technological parameter with digital tube display, and can print records.Compared with the set value of 0.80%C, the carbon concentration on tooth surface can be precisely controlled within ±0.05%C by the control system.The surface carbon concentration and quenching temperature of the gear are directly related to the thickness of martensitic needle and the amount of retained austenite after quenching.When the carbon concentration of tooth surface and quenching temperature are too high, the content of carbon dissolved into austenite will increase during quenching and heating, which will reduce the transformation of martensite and increase the residual austenite in steel.Too low quenching temperature causes low hardness of the tooth core and reduces the mechanical properties of the tooth core.After multi – furnace test to determine such as carburizing quenching process.The carbon concentration on the tooth surface was controlled at 0.80%C and the carburizing temperature was 920℃.After the end of carburizing, the quenching temperature was reduced to 840℃, and the oil was cooled for 30 minutes.Quenching oil adopts fast quenching oil to ensure the full transformation of the structure.

20Cr2Ni4 gear carburizing quenching, as long as strict control of the internal quality of gear forged body, austenite grain size is better than or equal to 6 grade.Carefully implement normalizing and tempering process, do a good job of carburizing and quenching before the organization preparation.The direct quenching process can be realized by using advanced quenching and tempering equipment and microcomputer control system to precisely control the carbon concentration and quenching and tempering temperature on tooth surface.

Editing by Frank Lee

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