Cause analysis and preventive measures of common defects in gear carburizing process

1 Excessive carburizing of gear surface

Carburizing gear due to improper treatment of excessive carburizing, surface will appear block, mesh carbide, plastic deformation ability of the gear is reduced when using, impact resistance is weakened, tooth root bending fatigue performance is reduced, tooth tip Angle becomes brittle, easy to crack, after quenching carburizing gear in grinding process easy to crack.

1.1 Cause Analysis

1) When the gear is carburized in solid medium, the carbon potential in the carburizing box is too high, and the carbon potential can not be arbitrarily adjusted, so the higher the carburizing temperature, the longer the time, the greater the degree of surface eutectoid. Especially for the carburizing steel containing strong carbon compounds forming elements Cr and Mo, the diffusion of carbon is slower, the carbon concentration on the surface of the gear carburizing layer is higher, and the carburizing layer of the eutectoid component is reached. When it is cooled, the cementite is precipitated from the austenite grain boundary to form a network distribution.

2) When carburizing in the gas medium, if the carbon potential in the carburizing furnace is too high and the strong carburizing time is too long, excessive carburizing will also occur on the surface of the gear.

1.2 Preventive Measures

1) solid carburizing, in order to prevent excessive carbon potential caused by excessive carburizing, can use a lower carburizing temperature or the use of weak carburizing agent.

2) When gas carburizing, in order to prevent excessive surface carburizing, the diffusion stage is arranged in the late stage of carburizing, and the time of strong infiltration and diffusion stage can be operated according to the heat treatment process.

3) For the gears that have produced excessive surface carburizing, diffusion treatment should be carried out in the low carbon potential carburizing furnace, or after spheroidizing annealing treatment of carbide, quenching should be carried out.

The hardening layer of gear is shallow

The surface hardness of carburized gear is too shallow, which leads to the decrease of anti-peeling performance of surface hardening layer and the decrease of service life.

2.1 Cause Analysis

1) in the process of carburizing, carburizing time too short, carburizing temperature is low, shallow carburizing layer, effective heating area uneven distribution of temperature in the furnace, strong permeability in the process of carburizing stage, improper control of carbon potential and diffusion stages, before the furnace charging gear not clean-up and furnace charging amount is too much, left by factors such as pore is too small and cause partial shallow carburized gear hardening layer.

2) The steel quality and hardenability of the selected gear are poor, and the cooling performance of the quenching medium is insufficient, resulting in a shallow hardening layer after normal carburizing.

2.2 Preventive Measures

1) Steel with suitable hardenability is selected as carburizing gear material, the quality of gear steel is strictly controlled, and the quality standard of steel must be checked before entering the factory.

2) Strictly control the surface quality of gear before carburizing, furnace load, furnace temperature, furnace carbon potential atmosphere, strong infiltration and diffusion time, quenching temperature after carburizing, cooling medium, etc.

3) Carburizing should be carried out for the gear with insufficient carburizing.

3. The depth of carburizing layer is not uniform

The depth of carburizing layer on the surface of the gear is not uniform, resulting in discontinuous performance of different parts. The weak area is destroyed first, and then the whole gear is damaged, which seriously affects the service life of the gear.

3.1 Cause Analysis

1) solid carburizing, the temperature difference in each part of the carburizing box is large, the uneven carburizing agent causes a great difference in the depth of carburizing, and the size of the carburizing box, the amount of furnace, furnace, heating rate, the charcoal thermal conductivity of the carburizing agent is too low, will affect the depth of the carburizing layer.

2) When the gas is carburized, the temperature in the furnace is not uniform, the atmosphere in the furnace is not circulated well, the oil is not removed from the gear before the furnace, and the carbon black deposition on the surface of the gear may cause the uneven depth of the carburizing layer.

3.2 Preventive Measures

1) batch production of gear should try to avoid the use of solid carburizing, must be solid carburizing, should strictly implement the operation process, the amount of furnace is appropriate, infiltration agent, charcoal to mix evenly. Carburizing box in the middle of the furnace temperature uniformity, carburizing process in the middle of the appropriate exchange of carburizing box position.

2) when gas carburizing, pay attention to the full circulation of the atmosphere in the furnace, the furnace temperature should be uniform, remove oil stains on the tooth surface, the amount of furnace loading should not be too much, the sealing performance of the carburizing furnace is good, the leakage muffle tank should be replaced in time, and the carburizing furnace should be repaired regularly.

4 The surface hardness after quenching is low

Low surface hardness of carburized gear will reduce the wear resistance and fatigue resistance of gear, and adversely affect the friction resistance and wear resistance of tooth surface.

4.1 Cause Analysis

1) surface decarburization, metallographic examination has decarburization layer, is due to poor protection in the process of normalizing or quenching after carburization.

2) The cooling rate is too low. Under the microscope, the surface tissue is not martensite tissue but soxtensite tissue. During metallographic observation, acicular martensite is obviously corrosion-resistant, while sortensite is darker (easy to corrode). Microhardness tester testing hardness difference is large.

3) Gear carburizing temperature and quenching temperature are too high, resulting in too much residual austenite volume on quenched surface.

4) Poor hardenability of gear materials and insufficient cooling capacity of quenching cooling medium.

5) After quenching, the tempering temperature is too high and the insulation time is too long.

4.2 Preventive Measures

1) Appropriate carburization treatment should be taken for the gear with low carbon content on the surface.

2) Select materials with appropriate hardenability and cooling medium with appropriate cooling capacity for quenching and cooling.

3) Take measures in advance to reduce the residual austenite volume after quenching. For the carburizing gear containing excessive residual austenite, a high temperature tempering at 650-670 ℃ for more than 3h was carried out to precipitate part of the alloy carbide, thus reducing the austenite stability during reheating and quenching, and promoting the transformation of austenite to martensite.

4) Gear carburizing cooling or reheating quenching should be carried out in a protective atmosphere, the oxidation phenomenon of gear should be removed, surface carburizing and then quenching.

5) If the surface hardness of gear is low and the tempering temperature is too high, it should be re-quenched and the appropriate temperature should be selected for tempering.

The hardness of gear core is insufficient

The core of carburized gear requires a certain hardness. When hardness is low, the yield point of gear material decreases, and the plastic deformation of the core is easy to occur, which reduces the anti-peeling performance of gear surface hardening layer and the bending fatigue performance of tooth root.

5.1 Cause Analysis

1) Poor hardenability of gear material, poor gear material, serious banded structure inside steel.

2) After carburizing, the pre-cooling temperature before direct quenching is too low or the quenching temperature is too low after carburizing and re-quenching.

3) The cooling rate is not enough, and the metallographic structure is not low carbon martensite, but soxtensite.

4) A large amount of undissolved ferrite exists in the core, which is caused by low heating temperature or insufficient heating time.

5.2 Preventive Measures

1) Choose the cooling medium with good cooling performance to quench, so that the core can obtain low carbon martensite structure.

2) Select appropriate quenching temperature and heating time to obtain uniform austenite in the heart, so that martensite structure can be obtained after quenching.

3) Steel with good hardenability and good material is selected as carburizing gear material.