20Cr2Ni4 Steel Gear Carburizing and Quenching Process Study

The gear of reducer of coal mill has spiral bevel gear and cylindrical gear. Its maximum dimension is 982mm in diameter, and the maximum modulus is m=14. It is made of S17Cr2Ni2Mo, 20Cr2Ni2Mo, J20CrNi2Mo chromium-nickel cementized steel.Also made a batch of 20Cr2Ni4 steel heavy duty gear.

Its process route: forging – normalizing and tempering – rough car – eliminating stress – semi – finishing – carburizing & quenching and low temperature tempering – finishing.

Carburizing and quenching of gear, the use of a quenching method, that is, after the gear carburizing for high temperature tempering and heating to quenching temperature.

1.Technical requirements for alloy carburized steel gears

In order to meet the requirements of use, alloy carburized steel gear technical requirements:

1) Hardness of gear forging blank after normalizing is HB≤ 229.

2) After carburizing and quenching hardness: tooth surface 58 ~ 62HRC, core 40 ~ 45HRC.

3) The depth of tooth surface carburizing layer: according to the different modulus of the gear, generally 1.5 ~ 2.5mm.

4) The austenite grain size of gear forging blank is above grade 4, and banded ferrite is below grade 2.

5) After the tooth surface carburizing and quenching, the microstructure is cryptocrystalline or fine acicular martensite structure, and there is no acicular and netlike carbide and a large number of residual austenite.Carbide grade 3 or below qualified, martensite plus residual austenite grade 3 or below qualified.The core is a low carbon martensite structure after quenching, and the core ferrite is not allowed to exceed grade 2.

6) After tooth surface carburizing and quenching, the hardness gradient and the carbon concentration are all uniform.

7) Gear precision grade 6.

2.Strictly control the quality of gear forging blank

In order to ensure the quality of the forging, the forging should be re-inspected, including the surface quality, mechanical properties, non-metallic inclusions, low-fold structure, austenite grain size, banded structure, etc., which must meet the qualified range of the technical requirements.The tooth shape (single tooth) carburizing and quenching test block with the same modulus as the gear is made from the forging blank of the same melting furnace number, the same material and the same forging process.

3.Carefully prepare the heat treatment

The quality of preparation heat treatment of gear forging blank is directly related to the quality of carburizing and quenching.This includes the thickness of martensitic needles.Strictly implement the normalizing and tempering process, we choose the normalizing temperature is higher than the carburizing temperature of 20℃, that is, 3 hours 940℃ keep warm and air cooling, 4 hours 650 ~ 700℃ keep warm and air cooling. Strictly check the hardness, metallographic structure and grain size of gear forging blank after normalizing.The structure is fine pearlite and the grain size is better than grade 6.

4.Equipment and process of gear carburizing and quenching

Hangzhou Ruiqin Mechanical Equipment Co., Ltd. uses the model RGC-P0618 drip type 150KW pit type carburizing furnace to carburize and quench the gear. Its effective size is Φ600×1800. 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.

5. Gear tempering equipment and process

We use 100KW well type tempering furnace, the effective size Φ1200×1200, the use of microcomputer control system, by using 6 thermocouples in the furnace tank on the top, middle and lower points to measure the uniformity of furnace temperature.When the instrument indicates 180℃, the maximum temperature difference is 5℃.

After carburizing and quenching, gears must be tempered in time.The purpose of tempering is to reduce quenching stress, reduce brittleness, try to maintain the high hardness and high wear resistance of steel.The hardness of tooth surface is maintained at 58 ~ 62HRC.

The decomposition rate of martensite is closely related to tempering temperature and tempering time.The higher the tempering temperature is, the faster the martensite decomposes and the more carbides are precipitated, so the lower the carbon concentration and square degree of martensite are.Therefore, it is necessary to find an appropriate tempering temperature to ensure the high hardness of tooth surface, and to fully decompose martensite and reduce the supersaturation of martensite.Tempering to heat preservation – fixed time, the purpose is to make the surface of the workpiece and the core of the temperature uniform, to ensure the full transformation of the organization.After testing to make the tooth surface hardness and tempering temperature of the relationship curve.

We adopt the two times tempering process of 180℃ heat preservation for 10-12 hours and space cooling.

Carburized quenched gear after tempering check tooth surface hardness can ensure 58 ~ 62HRC, tooth surface carbide control in 1 ~ 3 level, martensite plus residual austenite control in 1 ~ 3 level.The heart ferrite is controlled at level 1.

6. Conclusion

The austenite grain size of chromium-nickel steel gear is better than or equal to 6 grade as long as the internal quality of forged gear is strictly controlled.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 Francis Lee

Controlled Atmosphere Pit Type Carburizing Furnace

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