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What are the issues to pay attention to during gas nitriding?

人气: Article Source: Unknown Popularity: Issuing time: 2019-03-23 20:03
Ordinary gas nitriding is a surface strengthening process widely used in mechanical manufacturing, but the cycle is long, the operation is complex, and the quality of nitriding is not easy to stabilize. Therefore, how to stabilize the process, shorten the production cycle and reduce the brittleness of the injection must control the process parameters of the production process.
1. Heating method
** Use step heating. Because ordinary box-type or well-type electric furnaces heat up very fast below 500 ~ 600 ℃, if the temperature of the nitriding parts is not controlled with the furnace, the furnace temperature generally rises to the nitriding temperature in about 2 ~ 5h. There is still a difference between the workpiece temperature and the furnace temperature. The larger the workpiece cross-section or the larger the furnace load, the greater the difference between the two, so it is difficult to accurately calculate the nitriding time. The step heating method can significantly reduce the temperature difference between the furnace temperature and the workpiece, and can also reduce the deformation of the workpiece with complex shapes. The step heating specification is determined by the size of the workpiece, the amount of furnace installed, and the complexity of the shape of the workpiece. Generally, it is maintained for 20 ~ 40min with each rise of 50 ~ 100 ℃.
2. Ammonia decomposition rate
Ammonia must be dried before entering the nitriding furnace. Common desiccants include anhydrous calcium chloride, silica gel, and quicklime. From the effect of water absorption, anhydrous calcium chloride is taken as **. The desiccant must have a sufficient volume, which is mainly selected empirically based on the amount of ammonia gas.
The ammonia decomposition rate is often controlled in a relatively wide range in actual operation. The following table (1) is the commonly used control range of the decomposition rate at different temperatures, and the table (2) is the influence of the ammonia decomposition rate on the depth and hardness of the penetration layer.
Table (1) Effect of ammonia decomposition rate on depth of seepage layer
Nitriding temperature ℃
Decomposition rate (%)
Nitriding
Nitriding
510
18 ~ 35
18 ~ 25
530
25 ~ 45
30 ~ 40
550
35 ~ 55
35 ~ 55
Table (2) Effect of ammonia decomposition rate on hardness
Ammonia fraction (%)
Nitriding layer depth / mm
Surface hardness (HV)
20
0.60
850
40
0.60
915
60
0.60
1000
80
0.58
850
In order to reduce the brittleness of the permeation layer, it is easy to operate and save ammonia gas (510 nitriding, the decomposition rate is increased from 30 to 50, the ammonia consumption can be reduced by 25), and the ammonia decomposition rate is increased as much as possible.
3. Cooling method
Most nitriding workpieces are slowly cooled to 150 ~ 200 ° C with ammonia through the furnace. The average cooling rate of this cooling method is about 15 ~ 25 ° C / h, so the cooling time is very long, even tens of hours. Generally, slow cooling is considered. The brittleness of the nitrided layer is increased, and the slow cooling also reduces the corrosion resistance of the workpiece. Therefore, in order to shorten the nitriding cycle and improve the performance of the nitriding layer, it is advisable to use quick cooling after nitriding, or even to release oil cooling from the furnace. In order to reduce deformation, it is more appropriate to control the average cooling rate at 50 ~ 80 ° C / h. That is, the nitriding tank is removed from the furnace or is cooled by compressed air between the nitriding tank and the heating body. For workpieces with complex shapes and strict deformation requirements, the nitriding tank can be removed from the furnace and air-cooled after the furnace is cooled to 400-500 ° C.
4. Stove capacity
The furnace loading is 1/2 ~ 1/3 of the theoretical furnace loading calculated from the effective size of the furnace tank and the furnace loading method.

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