Common heat treatment defects of gears and their prevention methods

1. Cracks

1) In the carburizing diffusion stage, the carbon potential is high, the carburizing layer carbon concentration is too high and the gradient is large, and the tooth surface peels and falls off during grinding. Solution: control the carbon potential between 1.0 ~ 1.20% C in the strong carburizing stage and about 0.80% C in the diffusion stage, and keep it warm for 90 minutes, which can effectively prevent the peeling and falling off of the gear carburizing layer during grinding.

2) When the carburizing temperature of the gear is lower than or equal to 840 ℃, the gear is discharged and piled up, and the cooling speed is slow. The carburizing layer produces network carbide and large ferrite. These two kinds of structures are extremely brittle, and the tooth surface is bound to crack during grinding. Solution: the well type gas carburizing furnace is discharged at 850 ± 10 ℃, and the gears are scattered for air cooling; Multi purpose furnace 860 ± 10 ℃ front chamber fan cooling.

3) When the carburized gear is normalized and air cooled, it gathers, the cooling speed is slow, the carburized layer produces network carbide and large ferrite, and the tooth surface cracks during grinding. Solution: when carburizing gear is normalized and air cooled, it must be dispersed, and the normalizing fan of multi-functional furnace shall be cooled properly. The carburized layer of gear has produced network carbide and large ferrite, which can be eliminated by re normalizing at 920 ± 10 ℃. If not handled properly, the above situation will occur again.

4) There was no timely tempering and the gear cracked. Solution: the quenching gear has great organizational stress and thermal stress, and there is a risk of cracking at any time. It needs to be tempered in time and the tempering time is sufficient.

2. Deformation

The deformation is arranged according to the quenching mode: overall quenching > surface quenching; The deformation is arranged according to the heating equipment: multipurpose furnace > salt bath furnace > high frequency and medium frequency furnace.

1) High heating speed, high quenching temperature, fast cooling speed and large temperature difference between the surface and the center will inevitably increase the internal organizational stress and thermal stress of the gear, resulting in the out of tolerance of the gear deformation. Solution: adopt lower quenching temperature and control the heating rate at 150 ~ 180 ℃ / 1 hour, or adopt sectional heating method to reduce the temperature difference between the surface and the heart, reduce the thermal stress, and effectively control the gear deformation. Induction heating can adopt high-power and short-time rapid heating. The heating layer on the gear surface is shallow, and the tissue stress and thermal stress are small, which can reduce the deformation.

2) During induction heating, the gear is not concentric with the inductor, and the uneven heating of the outer circle will cause deformation. Solution: the gear is heated in the sensor, and the rotation method can reduce the deformation.

3) Gear quenching furnace is placed improperly, heated unevenly, and the gear will be deformed if the cooling method is improper. Solution: the gears shall be placed evenly and flatly, the shaft gears shall be placed vertically, and the gears with small thickness can be overlapped. During quenching, the gear shall be subject to the end face and the gear shaft shall be subject to the axis. It shall be vertically quenched into 30 ~ 60 ℃ hot oil and move up and down. It shall be cooled to 120 ~ 180 ℃ (it is best if the gear surface emits blue smoke but does not catch fire) and the oil shall be air cooled with small deformation. The multi-purpose furnace adopts stirring cooling. When the oil temperature is cold to 120 ~ 180 ℃, stop stirring. The workpiece is cooled in hot oil, or the pinion with modulus ≤ 5 adopts non stirring cooling, which can reduce the deformation.

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