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Vacuum carburizing process of transmission gear

The vacuum carburizing process that carries out carburizing treatment under reduced pressure has many advantages such as no grain boundary oxidation, no abnormal carburized layer on the surface of the workpiece, shortened heat treatment time, less exhaust gas, and energy saving. It is the most suitable for variable speed. A typical process for box transmission gears to achieve high fatigue strength and wear resistance of tooth surfaces. Heat treatment is an indispensable process technology to improve the mechanical properties of steel (workpiece). However, a large amount of energy is required to heat the workpiece to the proper (target) processing temperature. Under the background that the whole society demands to protect the earth’s environment, “reducing energy consumption and improving production efficiency” is the eternal theme of heat treatment workers.

The future development trend of gear vacuum carburizing and vacuum quenching

Traditional heat treatment is divided into pre- and post-processes at the beginning, and any parts must be treated with the same heat treatment equipment. However, this hinders the synchronization of the front and rear processes, resulting in a large inventory of workpieces, which becomes an obstacle to improving the productivity of all parts. The development trend of heat treatment in the future should be to develop in series (straight-through) treatment. It is important to find the best form of heat treatment production line and related technologies suitable for the processed parts.

How to reduce the tact difference between the front and rear processes and shorten the carburizing time is a crucial issue for the serialization (straight-through) of heat treatment. In this regard, high-temperature carburizing is an effective method (in terms of production, vacuum carburizing can form high temperature through vacuum insulation to shorten the processing time). As far as heat treatment equipment is concerned, it should have a high-temperature resistant structure and be easy to vacuum carburize favorable conditions.

The purpose and advantages and disadvantages of introducing vacuum carburizing furnace

In terms of capital construction investment, the cost of the vacuum carburizing furnace is relatively high, but since there is no need to provide security guards on holidays including weekends, it is judged that the vacuum carburizing furnace has an advantage based on the comparison of the total expenditure over the past 25 years. The biggest advantage in the quality of heat treatment parts is that no grain boundary oxide layer is produced, and the strength of vacuum carburized parts can be increased by 10% compared with gas carburized parts (plane). Since there is no Budeo reaction of gas carburizing (referring to the oxidation reaction of carbon, that is, the Bay-Wave reaction), the acute angle parts of vacuum carburized products are easy to form over carburization, which is subject to the setting of carburizing conditions and the shape of the product. The problem of constraints.

Application and effect of vacuum carburizing

Vacuum carburizing equipment introduction schedule: Compared with the continuous gas carburizing furnace, the vacuum carburizing furnace introduced this time can greatly shorten the time for introducing equipment. The reason is that a dedicated test furnace is installed in the equipment manufacturer, and various tests can be carried out during the equipment manufacturing (ordering) process. This method can ensure that the quality of the test furnace and the purchased furnace do not change, and can reconfirm the degree of product quality reproducibility.

Schedule for quality compliance (mature): The key to vacuum carburizing is how to introduce appropriate carburizing gas for the surface area of the product. If the amount of carburizing gas is lower than the required amount, the atmosphere in the furnace will lose uniformity, which will cause problems such as uneven carburizing of the workpiece. On the contrary, if the carburizing gas is too much, it will produce carbon black that is not conducive to carburizing ( soot), leading to blockage of the exhaust system and deterioration of process performance. Therefore, the setting of the optimal amount of carburizing gas is very important (the usual method is to heat the workpiece to a specified temperature, after soaking, direct the carburizing gas into the furnace for carburizing, and then stop the gas supply. Diffusion treatment, adjust the high carbon density to an appropriate carbon mass density, and set the carburizing time and diffusion time reasonably).

However, various parts are produced, and it takes a lot of manpower to determine the vacuum heat treatment conditions suitable for each part. The vacuum carburizing furnace introduced this time comes with a simulation software for setting vacuum heat treatment conditions. By inputting the necessary information for the product, the necessary vacuum heat treatment parameters such as the amount of carburizing gas introduced, carburizing time, and number of times can be calculated.

Since the input product information is the material and the total surface area of the product (the surface area of a single workpiece × the number of workpieces processed in a batch), it is most important to correctly grasp the surface area of the product.

Since the carburizing process of vacuum carburizing is different from gas carburizing, it can solve the problem of carbon concentration easily caused by the sharp corner of the workpiece. The solution is to use the pulse carburizing method. When performing simulation calculations, not only the plane part of the workpiece (such as the middle part of the gear tooth) is simulated, but also the change of carbon mass concentration at the acute angle part of the tooth surface is also used as the simulation object. , to set the vacuum heat treatment conditions. In gas carburizing, the carbon mass concentration on the entire surface of the product is roughly the same, while vacuum carburizing is theoretically impossible. Therefore, it is stipulated that in the vacuum carburizing process, the carbon mass concentration of the flat part of the workpiece is set lower than that in the gas carburizing process.

Normally, the hardness of the workpiece remains the same as long as the carbon mass concentration is above 0.6%, and it has been confirmed that even if the carbon mass concentration is above 0.6%, the product quality will not be affected if the carbon mass concentration is lowered.

The impact on the subsequent process: the biggest highlight of vacuum carburizing is different from gas carburizing is: the surface of the workpiece will not cause grain boundary oxidation, and will not produce an abnormal quenching layer. As for the difference in the appearance of the workpiece, after the carburizing and quenching treatment, after the tempering treatment, the vacuum carburized parts exhibit the characteristics of high brightness. At first glance, the workpiece treated by vacuum carburizing shows an aesthetic feeling of extremely high brightness, but it is sometimes greatly affected by it when it is processed in the subsequent processing process. Especially in the case of ordinary cutting and grinding processing, I was worried about the impact on tool life and processing time, but on the contrary, good results were obtained. However, vacuum carburizing does not produce a soft quenching abnormal layer. Therefore, when the machining allowance is small, such as inner hole honing, it is easy to suffer from the increase of machining time when using a machining method that does not increase the cutting speed. Influence. Through grinding tools (grinding wheels) and improving processing conditions, processing time close to that of gas carburized workpieces can also be achieved. However, this requires the understanding and support of finishing technicians, and is not a problem that can be solved only by heat treatment. The influence on the follow-up process has been discussed above, but for combined parts, it is sometimes restricted. Since the outer surface of vacuum carburized parts is relatively hard; therefore, when the matching parts are designed based on gas carburizing; sometimes, the hardness of the matching parts will also be set lower. This will increase the amount of wear on the mating parts. Therefore, the parts that are in contact with the finished surface such as the grinding surface are fine, but the parts that are in direct contact with the carburized and quenched surface need to be fully confirmed during application.

Corrosion resistance (toughness): Since gas carburizing is carburizing in reducing gas, generally speaking, the surface of the product has strong corrosion resistance. It is also pointed out in the literature that, on the contrary, when there is an oxide film on the surface, the depth of the hardened layer is deeper under the same conditions.

Then, what is the effect of vacuum carburizing on corrosion? Red rust was generated on the half cycle of the same product, and the effect of corrosion on the depth of hardened layer in this case was verified. The verification results are consistent with those described in the relevant literature. The oxidized part on the surface has good carbon adsorption, and the hardened layer is deeper than the part without red rust.

As imagined, the adsorption rate of parts with red rust is higher, and the parts with red rust on the surface of the product can be seen to have obvious carbon black adhesion. Vacuum carburizing also obtains the same quality, and its corrosion resistance is not bad. However, compared with gas carburizing and vacuum carburizing, the carbon black on the surface of the workpiece is more attached. Under the condition of introducing the optimal amount of carburizing gas in vacuum carburizing, if there is a lot of serious corrosion on the workpiece, the hardened layer Depth also becomes shallower.

It is speculated that there is no need to deal with a large number of rusted products at the mass production site. When dealing with such parts, it is necessary to increase the number of inspections in daily inspections (increase the workload). The time when the vacuum carburizing process is effectively applied to mass production and Not long, and even in Japan, there are not many practical examples.

In short, there are still some aspects of the heat treatment process that cannot be mastered, and there is a tendency among technicians not to adapt to technological development. However, the gas carburizing technology has stagnated due to the bottleneck in the expected condition management in ordinary gas carburizing. Vacuum carburizing needs to reduce the carburizing gas to the limit. In order to solve many problems arising from this, it is necessary to brainstorm and overcome difficulties.