Analysis of the causes and preventive measures of common defects in carburized gear quenching

Carburizing is a process operation in which the workpiece is placed in a carburizing atmosphere, heated and kept at a temperature of 900-950°C to carburize its surface layer. The purpose of carburizing the workpiece is to make the surface of the workpiece have high hardness and wear resistance after continuing to undergo corresponding heat treatment, while the core still maintains a certain strength and high toughness. The gears are often carburized. It can be seen that carburization is an important process in gear processing. Defects generated during the process have a lot of impact on the quality of gears. How to prevent the occurrence of these defects has become a problem that people are more concerned about.

There are many types of defects in carburized gears. In order to solve or prevent these defects, we must find the cause of such defects.

Defect 1. The hardness of the blank is too high, the normalizing temperature is too low or the holding time is insufficient, leaving a small amount of Widmanstatten structure with higher hardness (≥250HV) remaining in the structure. Normalizing temperature > the temperature at which the steel grains grow significantly, which leads to the hardness of the blank. The reason for the high level. Therefore, we should re-formulate the normalizing process or check the temperature control instrument, calibrate the temperature, control the normalizing cooling rate, etc. to solve the problem.

Defect 2. The surface martensite needles are thick. This is caused by the high quenching temperature. Just lower the quenching temperature.

Defect 3: Insufficient depth of penetration layer. Low carbon potential or low temperature and insufficient permeability period may cause this problem. For the reason of low carbon potential, we should increase the carbon potential.

Defect 4. The reason for surface decarburization is that the temperature of the furnace after infiltration is too high. The method is to lower the temperature of the furnace.

Defect 5: The surface carbon content is too high to form a large carbide network, the carbon potential of the furnace gas is too high, and the intensive penetration time process may cause this defect. The solution is to reduce the carbon potential or shorten the intensive penetration time based on the cause.

Defect 6. Banded segregation. The segregation of alloy elements and impurities in steel, which is difficult to eliminate by normalizing, are factors that may cause band segregation. Faced with this problem, we can only replace the material.

Defect 7. The hardness of the blank is low, which is caused by slow normalizing and cooling. Re-normalizing and strengthening cooling is a good solution.

Defect 8. The surface carbon content is too low. The carbon potential of the furnace gas is too low, the furnace temperature is too high, and the diffusion time is too long, which may cause the surface carbon content to be too low. For these reasons, the solution is to increase the carbon potential, check the furnace temperature, and adjust the ratio of strong penetration and diffusion time.

Defect 9. Excessive residual austenite on the surface. There are two factors causing this defect:

1. The carbon content is too high. We should adjust the carburizing process to control the carbon content.

2. The cooling after carburizing is too fast, the amount of carbon precipitated is not enough, and the quenching temperature is too high. We can solve the problem by lowering the quenching temperature by not taking the temperature out of the carburizing furnace or precooling furnace too high.

Defect 10. Low core hardness

1. The quenching temperature is too low, increase the quenching temperature.

2. Improper cooling rate, strengthen quenching cooling.

3. Improper material selection, the only choice is to replace the material.

Defect 11. The penetration layer is too deep. There are two reasons for this defect:

1. The carbon potential is too high, we should reduce the carbon potential.

2. If the seepage period is too long, we can shorten the cycle and adjust the process to solve the problem.

Defect 12. Distortion. This is a relatively common defect and should be paid special attention to.

1. The quenching temperature is too high, adjust the quenching process.

2. Improper cooling method, improve cooling conditions.

3. The fixture design is unreasonable. Design the fixture reasonably.

4. Improper material selection, replace steel.

If defects occur during heat treatment, we must find ways to prevent them. This is something that will not change. There are some other defects in the heat treatment of carburized gears. Let’s briefly talk about these today. You might as well try it and see what the effect is? I believe you won't be disappointed.