Temperature uniformity control of ion nitriding furnace

Second, the gas composition is not uniform. Glow discharge characteristics are related to gas composition. When ammonia gas is directly used as the gas source, the composition of the gas phase in the furnace gradually changes from the top to the bottom in the furnace due to the gradual ionization decomposition during the flow in the furnace. Near the air inlet is mainly fresh ammonia gas, and near the air suction port, the decomposition gas, nitrogen and hydrogen, is mostly present. When the anode-anode voltage is constant and the ammonia content in the gas phase is large, the cathode current density is small, and the current density of the decomposition gas is large. Therefore, the temperature of the part near the air inlet is the lowest, and the temperature of the part near the air suction port is the highest.

Third, the grooves and small holes on the parts are overlapped due to the glow, and the current density increases, causing the so-called glow concentration. These places heat up quickly, and the temperature will be higher than the normal glow.

(2) Effect of the “shape effect” of the part on the temperature The surface area to weight ratio of different parts or different parts of a part is different on the parts with evenly distributed glow. The ratio of surface area to weight is large, which means that it has more heating energy. These places heat up faster, and finally the stable equilibrium temperature is higher than the part with smaller surface area to weight ratio.

(3) The effect of the furnace loading method on the temperature The components in the ion nitriding furnace are in a cold environment, and the hot workpiece is cooled to the cold furnace. The workpiece is in a vacuum environment, the heat dissipation is mainly radiation, the conduction is second, and the convection is negligible.

When the parts are stacked on the bell-type ion nitriding furnace on the cathode chassis, the temperature of the upper nitriding parts tends to be low due to the large heat dissipation space at the upper part and the low furnace temperature. Well-type ion nitriding furnaces with hanging clamping parts are just the opposite, and the temperature at the lower part of the parts tends to be low.

For parts surrounded by other nitriding parts, the actual heat loss is greatly reduced and the temperature is high due to the heat radiation of other parts. If the parts are too close to each other, the local temperature of the adjacent portions is high due to the influence of heat radiation.

In summary, there are many factors affecting the temperature uniformity of the ion nitriding furnace, and the interaction is complicated. Any distribution of the surface temperature of an ion nitriding part is the result of a combination of factors affecting the heat balance under that particular condition. Therefore, it is unrealistic to achieve absolute temperature uniformity during ion nitriding, but by appropriately adjusting certain process parameters and improving the ion nitriding furnace and tooling structure, the temperature uniformity is improved, and the temperature difference between parts is reduced as much as possible. Small, to meet the requirements of the nitriding process, this can be done.

3. Control of furnace temperature uniformity

First of all, it is a strict requirement for the furnace loading method. All the grooved holes on the nitriding parts that are capable of generating concentrated glow without nitriding requirements are uniformly shielded. This point is clearly stated in the "Ion Nitriding Operation Guide". Generally, the pores with pores of 3 to 8 mm must be protected to avoid excessive local temperature due to concentration of glow.

For the batch processing of the workpiece, the distance between each part and the anode should be approximately equal, and the parts should be evenly spaced from each other. The general principle is to make the heat absorption and heat dissipation conditions of each part as uniform as possible. According to this criterion, in the actual furnace loading process, our factory puts the parts along the anode in a circle or staggered into several circles, which sacrifices the precious effective space in the middle of the furnace, but ensures the uniform temperature and good effect.

For small batches of workpieces that need to be furnace-treated, the surface area to weight ratio of these parts should be as close as possible. Of course, in actual production, in order to ensure production efficiency, it is impossible to deliberately pursue this perfection, but for both the set parts and the solid parts, if the two are in the same furnace, because of the "shape effect" of the parts, the set parts are better than the solid parts. The heating temperature is fast, and the heat loss of the inner wall of the sleeve parts is small, which causes the temperature to be much higher than that of the solid parts. Therefore, it is forbidden to treat the parts and the solid parts in the same furnace.

For more complex parts, local temperature unevenness may occur in one individual, and the temperature defects may be reduced or even eliminated by adjusting the placement position. Here, one principle can be followed, that is, one end of the part whose temperature may be higher, such as the larger diameter end or the end with the nitriding inner hole, is placed in the heat-dissipating part of the furnace (the upper part of the hood furnace, the lower part of the pit furnace) ), the opposite influence factors are partially offset, thereby reducing the temperature difference of the parts. Or place one end that is not required for nitriding in these areas to ensure a more uniform temperature at the part where the part requires nitriding.

Second, it is to control the temperature rise. In the temperature rising phase, since the ratio of surface area to weight is different between parts, the heating rate is different. If the heating rate is too fast, the concentrated heat is not conducted, and the temperature of the thin section is too high. Slow heating is beneficial for uniform temperature.

Again, the auxiliary anode and the auxiliary cathode are set. The auxiliary anode is often used in a hood furnace, and a flat plate with an adjustable upper and lower position is generally provided as an auxiliary anode at the top, and also has a function of a heat shield.

The auxiliary cathode is one or several steel parts at or near the temperature of the part, and it illuminates together with the processing part. The auxiliary cathode acts as a thermal barrier, reducing local heat loss from the part.

In the well type ion nitriding furnace of our factory, when dealing with long shaft parts, the hardness of the upper and lower parts and the depth of the nitriding layer are often severely uneven. This is because in the well type ion nitriding furnace, the temperature of the upper part of the long-axis workpiece is higher than that of the lower part. At the same time, due to the unreasonable spreader, the use of the combination hook increases the glow area of ​​the upper part of the workpiece, causing the temperature to rise sharply. High, causing severe unevenness in the temperature of the upper and lower parts of the workpiece. In response to this situation, the technicians of our factory redesigned the spreader according to the needs of production and the rationality of the mounting, which increased the space utilization of the furnace and reduced the increase of the glow area caused by the combination hook. At the same time, for the lower part of the long axis, the temperature is obviously low, and the booster plate (ie auxiliary cathode) is designed. The effect is exactly the opposite of the above-mentioned spreader, and it is required to increase the surface area as much as possible. Nowadays, when dealing with long-axis parts, the temperature uniformity of the upper and lower parts of the workpiece is greatly improved.

Finally, it is the transformation of the ion nitriding furnace. Adjust the direction of the air inlet. In order to avoid direct injection of the airflow, the direction of the air inlet nozzle is slightly adjusted so that the air inlet nozzle faces up to the top of the bell jar. This improvement effect is obvious. The heat shield and insulation layer are provided. The heat shield and the insulation layer can increase the temperature inside the ion nitriding furnace and greatly reduce the temperature gradient of the workpiece to the furnace wall, which not only can greatly save power, but also reduce heat dissipation throughout the parts. The difference in conditions, so that the temperature uniformity is improved.

4. Conclusion

The above is a little experience of the author's control of the temperature uniformity of the ion nitriding furnace. By controlling the uniformity of the furnace temperature, the one-time pass rate of the ion nitriding parts is greatly improved.

About the author: Tang Fusheng, Hangzhou Qianjin Gearbox Group Co., Ltd.