1. Excessive carburization on the surface of the gear
If carburized gears are over-carburized due to improper treatment, blocky or network-like carbides will appear on the surface. During use, the gears' plastic deformation ability will be reduced, their impact resistance will be weakened, the bending fatigue performance at the tooth root will decrease, the tooth tip angle will become brittle and prone to cracking, and the carburized gears after quenching will be prone to cracking during grinding.
1.1 Cause Analysis
1) When gears are carburized in a solid medium, the carbon potential in the carburizing chamber is too high, and the carbon potential cannot be adjusted arbitrarily. Therefore, the higher the carburizing temperature and the longer the time, the greater the degree of hypereutectoid formation on the surface. Especially for carburizing steel containing strong carbon compound forming elements such as Cr and Mo, carbon diffusion is slower, and the carbon concentration on the surface of the gear carburized layer is even higher, reaching the hypereutectoid composition. During cooling, cementite precipitates from the austenite grain boundaries to form a network distribution.
2) During carburizing in a gaseous medium, if the carbon potential in the carburizing furnace is too high and the strong carburizing time is too long, excessive carburizing of the gear surface will also occur.
1.2 Preventive Measures
1) In solid carburizing, in order to prevent excessive carburizing caused by excessively high carbon potential, a lower carburizing temperature or a weaker carburizing agent can be used.
2) During gas carburizing, in order to prevent excessive carburizing of the surface layer, a diffusion stage is arranged in the later stage of carburizing. The duration of the strong carburizing and diffusion stages can be operated according to the heat treatment process.
3) For gears that have already undergone excessive surface carburization, diffusion treatment should be carried out in a low-carbon potential carburizing furnace, or quenching should be performed after carbide spheroidizing annealing.
2. The hardened layer on the gear is too shallow.
The surface hardness of carburized gears is relatively shallow, which reduces the anti-peeling performance of the surface hardened layer and also leads to a reduction in service life.
2.1 Cause Analysis
1) During the carburizing process, factors such as short carburizing time, low carburizing temperature, shallow carburized layer, uneven temperature distribution in the effective heating zone of the furnace, improper control of carbon potential during the strong carburizing and diffusion stages, failure to remove oil stains from gears before loading into the furnace, excessive loading, and too small pores can all cause the hardened layer of the carburized gears to be too shallow.
2) The selected gear steel material and hardenability are poor, and the cooling performance of the quenching medium is insufficient, resulting in a shallow hardened layer after normal carburizing and quenching.
2.2 Preventive Measures
1) Select steel with suitable hardenability as the material for carburized gears, strictly control the quality of gear steel, and conduct quality standard inspections on the steel before it enters the factory.
2) Strictly control the surface quality of the gears before carburizing, the amount of fuel in the furnace, the temperature inside the furnace, the carbon potential atmosphere inside the furnace, the strong carburizing and diffusion time, the quenching temperature after carburizing, and the cooling medium.
3) Gears with insufficient carburization should be supplemented with carburization.
3. Uneven depth of carburized layer
Uneven depth of carburized layer on gear surface causes discontinuous performance in different parts. Weak areas are damaged first, followed by damage to the entire gear, which seriously affects the service life of the gear.
3.1 Cause Analysis
1) During solid carburizing, the temperature difference between different parts of the carburizing box is large, and the uneven distribution of the carburizing agent causes a large difference in the carburizing depth. In addition, the size of the carburizing box, the amount of furnace charge, the charging method, the heating rate, and the low thermal conductivity of the charcoal in the carburizing agent will all affect the depth of the carburized layer.
2) During gas carburizing, uneven furnace temperature, poor furnace atmosphere circulation, failure to remove oil stains from gears before loading, and carbon black deposition on gear surfaces can all cause uneven carburizing layer depth.
3.2 Preventive Measures
1) Solid carburizing should be avoided as much as possible in the mass production of gears. If solid carburizing is necessary, the operating procedures should be strictly followed, the furnace loading should be appropriate, and the carburizing agent and charcoal should be mixed evenly. The carburizing box should be placed in the middle of the furnace where the temperature is uniform, and the position of the carburizing box should be changed appropriately during the carburizing process.
2) When carburizing with gas, pay attention to the full circulation of the atmosphere in the furnace, the uniform furnace temperature, remove oil stains from the tooth surface, do not overload the furnace, ensure good sealing performance of the carburizing furnace, replace leaking muffles in time, and regularly inspect the carburizing furnace.
4. The surface hardness is relatively low after quenching.
Low surface hardness of carburized gears will lead to reduced wear resistance and fatigue resistance, which will have an adverse effect on the friction and wear resistance of the gear surface.
4.1 Cause Analysis
1) Surface decarburization, with a decarburized layer observed during metallographic examination, is caused by inadequate protection during normalizing or quenching after carburizing.
2) The cooling rate was too low. Under a microscope, the surface structure appeared to be sorbite rather than martensite. Metallographic observation revealed that the acicular martensite exhibited significant corrosion resistance, while the sorbite appeared darker (and more easily corroded). The difference in hardness was also observed with a microhardness tester.
3) Excessive carburizing and quenching temperatures in gears result in excessive residual austenite on the surface after quenching.
4) The gear material has poor hardenability and the cooling capacity of the quenching medium is insufficient.
5) The tempering temperature after quenching is too high and the holding time is too long.
4.2 Preventive Measures
1) Take appropriate carbon enrichment treatment for gears whose surface carbon content is low.
2) Select materials with suitable hardenability and cooling media with appropriate cooling capacity for quenching and cooling.
3) Take preventative measures to reduce the amount of retained austenite after quenching. For carburized gears containing excessive retained austenite, perform a high-temperature tempering at 650~670℃ for more than 3 hours to precipitate some of the alloy carbides, thereby reducing the stability of austenite during reheating and quenching, and promoting the transformation of austenite into martensite.
4) Gear carburizing and cooling or reheating and quenching should be carried out under a protective atmosphere. For gears that have already oxidized, the oxide scale should be removed, and surface carburizing should be performed before quenching.
5) If the low surface hardness of the gear is caused by excessive tempering temperature, it should be re-quenched and tempered at a suitable temperature.
5. Insufficient hardness of the gear core
The core of carburized gears requires a certain degree of hardness. If the hardness is too low, the yield point of the gear material is lowered, making the core more prone to plastic deformation, which reduces the resistance of the hardened layer on the gear surface to spalling and the bending fatigue resistance of the tooth root.
5.1 Cause Analysis
1) The gear material has poor hardenability, poor material quality, and severe banded structure inside the steel.
2) After carburizing the gear, if the pre-cooling temperature before direct quenching is too low or the quenching temperature is too low when re-quenching after carburizing.
3) The cooling rate was insufficient. Metallographic observation showed that it was not low-carbon martensite, but sorbite.
4) The presence of a large amount of undissolved ferrite in the core is caused by low heating temperature or insufficient heating time.
5.2 Preventive Measures
1) Use a cooling medium with good cooling performance for quenching to obtain a low-carbon martensite structure in the core.
2) Select appropriate quenching temperature and heating time to obtain uniform austenite in the core so that martensitic structure can be obtained after quenching.
3) Select steel with good hardenability and good material quality as the material for carburized gears.
