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Automobile gear carburizing process

The transmission gear of automobile rear axle is an important part of automobile transmission mechanism, and carburizing is an extremely important production process of automobile gear. In addition to pressure resistance, wear resistance, impact resistance, plastic deformation resistance, surface contact fatigue resistance and bending fatigue resistance, automobile gears also have strict requirements on shape and dimensional accuracy in transmission. Gear accuracy includes geometric accuracy and contact accuracy. And the contact precision includes running stability, contact area and noise, so the contact precision directly affects the life of the gear, especially the rear axle gear in the automobile transmission is particularly important. If the contact area of a pair of gears in transmission is not good, it will not only cause high noise, but also easily cause early wear of the tooth surface and shorten the service life of the gear. Gear heat treatment distortion is an important factor affecting gear quality. Because there are many factors affecting gear distortion, and many of them interact with each other, super standard distortion has always occurred, and it has become an inevitable problem in gear manufacturing. This paper briefly introduces the heat treatment distortion of automobile rear axle carburized gear, discusses the cleaning and the interaction of related factors, and provides a basis for reducing the heat treatment distortion of automobile carburized gear.

1.Factors Affecting Carburized Gear Distortion

The heat treatment distortion of carburized gear includes distortion caused by volume change and distortion caused by stress. The distortion caused by volume change is inevitable, but as long as the machined steel and preparatory heat treatment of the gear are OK, this change is regular. Although the changes caused by stress concentration are complex, they can also be reduced and controlled. Because the microstructure of gears before and after carburizing and quenching is different, the surface carbon content is also different, which leads to the difference in volume change.

Due to the volume change of gear after carburizing and quenching, and the change of shape and size caused by the stress generated during processing, the heat treatment distortion of gear is inevitable. However, for the mass-produced automobile gears, especially the front and rear axle gears, as long as all the factors affecting gear distortion and their influencing data are mastered and become a stable law, it is possible to control the deformation characteristic values and errors of the gear tooth profile and tooth direction through the distribution of machining allowance, so that the heat treatment distortion of the gear is controlled within the range of technical requirements.

There are many factors that affect heat treatment distortion of automobile carburized gears, including gear design, material characteristics, forging, normalizing (or annealing), cutting, carburizing, quenching, tempering and many other factors in all gear manufacturing processes. Recently, it has been found that the cleaning of workpieces before carburizing is also the most important process that affects gear deformation. If the cleaning is improper or the selection of process and cleaning agent is inappropriate, the gear deformation will be affected, and the gear may also be scrapped due to failure.

(1) Influence of Cleaning Process on Gear Distortion

Generally, workpieces need to be cleaned before carburizing. Some manufacturers do not clean workpieces before carburizing in order to save costs if carburizing quality requirements are not strict. In this way, because there is oil on the surface of the workpiece, it is easy to produce large lampblack when the workpiece enters the carburizing furnace. For carburizing of automobile gears, most of them are carburized in continuous carburizing furnace or sealed box multi-purpose furnace at present. If the workpieces are not cleaned before carburizing, the atmosphere in the furnace will be destroyed if the oil smoke in the furnace is too large. Especially for continuous furnace, since the five or four zones are connected, the atmosphere in the second and third zones is most vulnerable to be destroyed, affecting the quality of carburizing. At the same time, carbon black will also be produced in the furnace to shorten the carbon burning cycle. Therefore, for automobile carburized gear, it is necessary to clean before heat treatment and carburizing before carburizing and quenching. However, if the cleaning process is not well implemented and the amount of cleaning agent can not be well controlled, the quality of the workpiece after carburization will also be affected, especially the quality of the contact area.

At present, most cleaning machines include conveying system, spray cleaning system, rinsing system, mist exhaust system, drying system, oil skimming system and cleaning liquid heating system. In the process, the workpieces to be cleaned are generally installed on the conveyor belt of the cleaning machine, and then cleaned by the conveyor through the aqueous solution spray cleaning system containing cleaning agent, rinsed by the aqueous solution containing anti rust agent, and finally blown dry. Some continuous carburizing furnaces are equipped with a cleaning machine before pretreatment to clean the workpieces before carburizing. For the continuous furnace equipped with a cleaning machine before carburizing, clean water can be used for rinsing, because the workpiece will enter the furnace for carburizing in a short time after cleaning. The cleaning process has strict requirements on the temperature of cleaning solution and the ratio of cleaning agent to water at each stage. If the water temperature does not meet the process requirements and the water temperature is too low, the detergent cannot be evenly dispersed in the detergent because most of the detergent is solid; If the cleaning agent is not mixed with water according to the proportion specified in the process, and if there is too much cleaning agent, the cleaning agent cannot be evenly dispersed in the cleaning solution, so that after cleaning, the workpiece will have many small drops like traces on the surface, and many solid cleaning agents that are not completely dissolved in water will adhere to the gear surface. Because most cleaning agents are chemical substances, some of them are easy to react with metals and affect the surface carbon content of carburized workpieces. As a result, the gear surface changes inconsistently after carburizing and quenching, resulting in distortion (see Figure 1). The gears with spots on the gear surface after carburizing will have poor contact after tooth grinding (see Figure 2). Many gears are often scrapped due to the poor contact area.

Fig.1 Surface spots of workpiece after improper cleaning and heat treatment

Fig.2 Contact area of surface spot workpiece after grinding

(2) Effect of cleaning agent on distortion of carburized gear

The selection of cleaning agents and the quality of cleaning agents are very important for the cleaning effect and quality of workpieces. The cleaning agent selected by most manufacturers is sodium nitrite (NaNO2). As a water-soluble corrosion inhibitor for ferrous metals, sodium nitrite has unique characteristics such as low price, good effect and convenient use. Low concentration sodium nitrite solution has corrosion inhibition and rust prevention effects, so it is widely used. As sodium nitrite is an industrial salt with strong oxidizing and reducing properties, it will gradually oxidize in the air, and the surface will become sodium nitrate, which can also be oxidized by oxidants. Therefore, its solution can passivate the metal surface, form a layer of oxide film on the surface of the workpiece, and play a role in rust prevention of the workpiece.

At present, most carburized gear materials are 20CrMnTiH and 20CrNiMo (8620). As 20CrNiMo (8620) materials contain Ni, reducing the content of C or increasing Ni and Cr can improve the high-temperature oxidation resistance of stainless steel and increase the strength of the workpiece, as well as the stress and corrosion resistance of the workpiece. Therefore, the oxidation resistance of 20CrNiMo (8620) material is stronger than that of 20CrMnTiH, and this material is not easy to appear spots as shown in Figure 1 after cleaning with sodium nitrite solution. However, after the 20CrMnTi material is cleaned with sodium nitrite solution, if the concentration is too high or uneven, the thickness of the oxide film formed on the surface of the workpiece after cleaning is inconsistent, and in severe cases, some metals may be oxidized by sodium nitrite, resulting in uneven surface of the workpiece. This kind of oxide film with inconsistent thickness or uneven height may cause inconsistent carbon concentration on the surface of the tooth surface in the carburizing furnace, This may cause uneven distortion of the gear. After quenching, the spots as shown in Fig. 1 will appear, and after grinding, the contact area as shown in Fig. 2 will appear.

2.Measures taken after such adverse contact

After carburizing and quenching of gears, if the alloy elements of gear materials, material manufacturers and production processes have not changed, and the produced gears appear spots as shown in Figure 1, first grind the typical workpieces to see the effect of tooth grinding. After tooth grinding, if the quality indicators of gears, especially the contact accuracy, fail to meet the technical requirements, it is necessary to find out the reasons for cleaning, Find out the causes and take measures mainly from the following aspects:

(1) Check whether the operation process of the cleaning process is in strict accordance with the operating procedures, mainly including the water temperature, the amount of detergent added each time, and whether the replacement cycle of the cleaning solution is in strict accordance with the process. If not, correct it immediately, and operate in strict accordance with the process operating procedures. Especially the replacement of cleaning agent, if the cleaning time is too long without replacement, it will directly affect the cleaning quality and effect of the workpiece, and thus affect the carburizing quality of the workpiece.

(2) If such bad contact area occurs, it is recommended to replace the cleaning solution in the cleaning machine. For the cleaning of carburized gears before heat treatment, it is better not to use sodium nitrite solution. It is recommended that the detergent should be alkaline and free of nitro, water-soluble industrial detergent, which has temporary rust protection (for storage and transportation), Regularly check the concentration of cleaning agent (generally controlled between 3% and 5%). This is not only conducive to the oil cleaning of the workpiece surface, but also does not affect the carburizing quality.

(3) If the carburized and quenched workpiece has a contact area as shown in Figure 1, it will also have a bad contact as shown in Figure 2 after gear lapping. When lapping the workpiece, it is necessary to increase the gear lapping cycle (increase the lapping time) to remove more metal, so that the bad contact area can be removed.

3.Conclusion

The cleaning process can be said to be the simplest process in the gear production process. It is precisely because we look at it too simply and do not pay attention to this process in thinking, so it is most easily ignored by us. Sooner or later, it must be To a quality problem. In the various processes of gear production, it is not said which process is important and which process is not important. If there is a difference, it is only relative. The negligence of any process will make a good set of gears become defective or waste products. Therefore, in the process of gear production, whether it is a simple or complex process, we must pay attention to every detail, and only in this way can we produce high-quality products.