Common defects in heat treatment of carburized gears and preventive measures
Carburized gears are widely used in mechanical engineering. Carburized gears are used in many machinery such as transmissions, reducers, and rear axles. Practical use shows that most of the gear failures are related to the defects of the carburized gear during the heat treatment process. These defects seriously affect the life of the gear and even the use of the entire assembly. Therefore, discussing the heat treatment defects of carburized gears and their preventive measures has important guiding significance for improving the mechanical performance.
Common defects in heat treatment of carburized gears
1. Transitional carburizing of gear surface
2. The surface hardness after quenching is low
3. The insufficient hardness of the gear core
4. The gear hardened layer is shallow
5. Uneven depth of the carburized layer
1. Transitional carburizing of gear surface
After the carburized gear is improperly handled and excessively carburized, the surface layer will appear blocky and networked carbides. A small amount of granular carbide can improve the wear resistance and contact fatigue strength of the gear. If there are too many blocky and networked carbides It will increase the brittleness of the gear surface layer, easy to fall off, reduce the plastic deformation ability of the gear during use, reduce the impact resistance, reduce the bending fatigue performance of the tooth root, and the tooth tip angle will become brittle and easy to crack. After quenching, the carburized gear is in the grinding process. When it is easy to crack.
Cause Analysis
During gas carburizing, if the carbon potential in the carburizing furnace is too high and the infiltration time is too long, the degree of hypereutectoid (pearlite + secondary cementite) of the surface layer will be greater, and excessive carburization of the gear surface will occur. Especially for carburized steels containing strong carbide forming elements such as Mo and W, the diffusion rate of carbon elements is slow, and the surface carbon concentration of the gear carburized layer is high, reaching the carburized layer of hypereutectoid composition. Cementite is precipitated at the grain boundaries of the stenite to form a massive and networked distribution.
Precaution
(1) When gas carburizing, in order to prevent excessive carburization of the surface layer, the diffusion stage is arranged in the later stage of the strong infiltration, and the reasonable arrangement of the time of the strong infiltration and diffusion stage has a great relationship to control the depth of the infiltration layer.
(2) For gears that have already produced excessive surface carburization, diffusion treatment should be carried out in a low carbon potential carburizing furnace, or carbide spheroidizing annealing treatment (to obtain a granular pearlite structure and prepare the structure for quenching). Perform re-quenching.
2. The surface hardness after quenching is low
The low surface hardness of carburized gears will result in the reduction of wear resistance and fatigue resistance of the gears, and will adversely affect the friction and wear resistance of the tooth surface.
Cause Analysis
(1) Surface decarburization, metallographic examination showed carbon-lean phenomenon, and a soft layer was found on the surface of the test piece of the file, which was caused by insufficient protective atmosphere during the quenching process after carburizing diffusion.
(2) Due to equipment failure (such as chuck, furnace opening, and maintenance, etc.), oxidation occurs in the high-temperature stage, and the carbon on the surface is oxidized into gas and burned away.
(2) The cooling rate is too low. Under the microscope, the surface structure is not martensite, but sorbite. In metallographic observation, the acicular martensite has obvious corrosion resistance, while the sorbite is darker (easy to corrode), and the hardness of the microhardness tester varies greatly.
(4) The high carburizing temperature and quenching temperature of the gear cause too much residual austenitic mass on the surface after quenching.
(5) The gear material has poor hardenability and insufficient cooling capacity of the quenching cooling medium.
(6) The tempering temperature is too high after quenching, and the tempered martensite structure is not obtained.
Precaution
(1) Take appropriate carbon increase treatment for gears that have caused low carbon content on the gear surface.
(2) Select materials with appropriate hardenability and cooling medium with appropriate cooling capacity, and quench and cool.
(3) Take measures in advance to reduce the amount of retained austenite after quenching. For carburized gears containing too much-retained austenite, perform a high-temperature tempering at 650~670℃ for more than 3h to precipitate a part of alloy carbides, thereby reducing the stability of austenite during reheating and quenching, and promote austenite The body transforms to martensite.
(3) Gear carburization cooling or reheating and quenching should be carried out in a protective atmosphere. For gears that have already undergone oxidation, the oxide scale should be removed (not affecting the thermal post-processing size of the gear), and the surface layer should be carburized before quenching.
5) If the low hardness of the gear surface is caused by too high tempering temperature, it should be re-quenched and a lower temperature should be selected for tempering.
(6) The operator and related personnel should regularly inspect the equipment, especially during the production cycle, to avoid the failure of the quenching of the workpiece due to the equipment, and the martensitic structure is not obtained.
3. Insufficient hardness of the gear core
The core of carburized gear requires a certain degree of hardness and low hardness, and the yield point of the gear material is reduced, which is prone to plastic deformation of the core, which reduces the peeling resistance of the hardened layer of the gear and the bending fatigue performance of the tooth root.
Cause Analysis
(1) The gear material is poor in hardenability, the gear material is poor, and the internal band structure of the steel is serious.
(2) After the gear is carburized, the pre-cooling temperature before quenching is too low, so that the temperature difference between the pre-cooling temperature and the quenching temperature is small, and the cooling rate is insufficient.
(3) But the speed is not enough. The metallographic structure observation is not a low-carbon martensite structure, but a mixed structure of sorbite and martensite.
(4) A large amount of undissolved ferrite remains in the core, which is caused by low heating temperature or insufficient heating time (quenching treatment is performed before complete austenitization).
Precaution
(1) Choose steel with good hardenability and good material as carburized gear material.
(2) Control the temperature of the diffusion zone and the pre-cooling quenching zone to ensure that the cooling rate can meet the quenching requirements
(3) Select a cooling medium with good cooling performance for quenching to obtain low-carbon martensite structure in the core.
(4) Choose an appropriate quenching temperature and heating time to obtain uniform austenite in the core, so as to obtain after quenching
4. The gear hardened layer is shallow
The surface hardness of the carburized gear is not deep enough, which reduces the anti-stripping performance of the hardened surface layer and also reduces the service life.
Cause Analysis
(1) During the carburizing process, the carburizing time is too short, the carburizing temperature is low, and the carburized layer is shallow.
(2) The temperature distribution of the effective heating zone in the furnace is uneven.
(3) Improper control of the carbon potential in the strong infiltration stage and the diffusion stage in the carburizing process.
(4) The gears are not cleaned of grease before the furnace is installed, the amount of the furnace is too much, and the remaining pores are too small.
(5) The selected gear steel material and hardenability are poor, and the cooling performance of the quenching medium is insufficient, which causes the hardened layer to be shallow after normal carburizing and quenching.
Precaution
(1) Reasonably select steel with suitable hardenability as the carburized gear material, and strictly control the quality of gear steel. Before entering the factory, the chemical composition and organization of the steel must be checked for acceptance according to quality standards.
(2) Strictly control the surface quality of the gear before carburizing, the amount of furnace installed, the temperature in the furnace, the carbon potential atmosphere in the furnace, the infiltration and diffusion time, the quenching temperature after carbon, the temperature of the cooling medium and the cooling medium, etc.
(3) Carburize the gears with insufficient carburization according to the current depth to prepare a reasonable repair process.
5. Uneven depth of carburized layer
Under normal circumstances, during the carburizing process of the gear, the tooth root is slightly shallower than other parts due to the geometric shape and the radius of curvature. Geometric factors cause uneven carburized layer to be unavoidable. However, due to other factors, the seepage layer is more uneven than normal, which will cause discontinuity in the performance of different parts of the gear. The weak area is destroyed first, and then the entire gear is damaged, which seriously affects the life of the gear.
Cause Analysis
(1) During gas carburizing, the temperature in the furnace is uneven, the carbon potential is uneven, and the atmosphere in the furnace is not well circulated.
(2) The gears are not cleaned before the furnace is installed, and impurities such as oil stains and carbon black remain on the gear surface, and the degreasing temperature is unreasonable. Coking occurs on the tooth surface during carburizing.
(4) The furnace loading is too large, the furnace loading method is unreasonable, and the workpieces contact and collide with each other.
(4) In areas where carburization is not required, uneven and unreasonable application of the anti-seepage agent may cause uneven depth of the carburized layer
Precaution
(1) When carburizing, pay attention to the full circulation of the furnace atmosphere, the temperature of all parts in the furnace should be uniform, and the fans in each zone should be checked regularly in accordance with relevant regulations. In addition, a reasonable pressure ratio between the front and rear of the furnace must be selected.
(2) Do the preparatory work before heat treatment and carburization, remove the tooth surface oil, carbon black and other substances. Practice has proved that after cleaning, the degreasing temperature of 400-500℃ is ideal for removing materials on the gear surface.
(3) Reasonable design of tooling fixtures, the amount of furnace installation should not be too much, and the furnace installation method is reasonable.
(4) For workpieces that need anti-seepage, the anti-seepage agent should be evenly brushed.
(5) The sealing performance of the carbon furnace should be good, and the carburizing furnace should be overhauled regularly
