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Research on Vacuum Nitriding Technology

1.Vacuum nitriding process

During the experiment, the vacuum degree in the furnace was reduced to 1.33Pa, and then the temperature was raised to the nitriding temperature. After reaching the temperature, the temperature was maintained for 30 minutes to purify and degas the sample surface. The maximum pressure does not exceed (0.4~0.6)×10Pa. After holding for a certain period of time, it will be cooled to 300 ℃ with the furnace. The schematic diagram of the pressure in the vacuum furnace during vacuum nitriding is shown in the figure below.

2.Influence of process parameters of vacuum nitriding

The effect of temperature on the infiltration layer: the vacuum pulse nitriding temperature is too high, the compound of the infiltration layer is coarse; the nitriding temperature is too low, the infiltration layer is shallow, the formation of the infiltration layer compound is small, and the hardness is low. The temperature of vacuum pulse nitriding is in the range of 510℃~570℃, which has obvious influence on the hardness of the infiltrated layer.

The influence of different furnace pressures is that the higher the upper furnace pressure, the better the depth and hardness of the infiltrated layer; the effect of the lower furnace pressure on the infiltrated layer is that the higher the vacuum degree, the better the hardness and thickness of the infiltrated layer.

The effect of vacuum nitriding time increases with the increase of time, the hardness increases, and there is a compound layer, the hardness increases more obviously, and the nitriding layer also deepens. The higher the flow rate of NH, the higher the hardness and the deeper the infiltration layer. For example, 1600 grids are much better than 1000 grids. If the pulse time is too long, the seepage layer will become thinner, and if the time is too short, the surface brittleness will increase.

Vacuum nitriding is to first pump the helium furnace to make the vacuum degree in the furnace reach -0.1MPa, and then introduce ammonia gas for nitriding. By changing the process parameters of vacuum nitriding, the phase composition and structure of the nitriding layer can be controlled. Vacuum nitriding has a very good effect on nitriding slits and micropores, which is beyond the reach of gas nitriding and glow ion nitriding. . Vacuum nitriding adopts pulsed ammonia supply, which enables nitriding of blind holes and compressed planes on the workpiece. After inspection, the helium layer of the workpiece in each part of the furnace is relatively uniform, and the vacuum helium infiltration and ammonia gas The consumption is 1/5 of that of traditional nitriding, and another advantage is that by controlling the type and amount of the compound gas containing activated substances sent into the furnace, almost no compound layer (white layer) can be obtained, but only diffusion layer organization.

3. Characteristics of vacuum nitriding

Compared with ordinary gas nitriding, vacuum nitriding has the following characteristics:

Fast speed, 38CrMoAl steel can get 0.3mm nitriding layer after vacuum nitriding at 530℃×10h, while ordinary gas nitriding needs more than 20h. Therefore, the nitriding speed can be doubled;

The hardness of the nitriding layer is high. Due to the high nitrogen potential in the atmosphere, the nitrogen concentration of the steel surface layer and the hardness of the nitriding layer are relatively high; The liquid ammonia required for the nitriding layer of ~0.5mm is less than 1kg, while the consumption of ordinary gas nitriding is more than 2kg;

Vacuum nitriding can use higher temperature, which is not advisable for ordinary gas nitriding;

Activates and decontaminates metal surfaces. During the whole heat treatment process of heating, heat preservation and cooling, the impure trace gas is discharged, and the pure composite gas containing active substances is fed in, which makes it possible to adjust and control the surface layer phase structure, improve the quality and improve the efficiency.

The vacuum nitriding layer has similar structure and performance characteristics to other nitriding layers. Moreover, due to the purification effect of vacuum and the high nitrogen potential of the furnace gas, the speed of vacuum nitriding is more than double that of ordinary gas nitriding. Vacuum nitriding overcomes the shortcomings of ordinary gas nitriding and ion ammonia nitriding, and provides a new way to achieve rapid and high-quality nitriding.

Selection of vacuum nitriding equipment: The RVN series vacuum nitriding furnace produced by SIMUWU is an excellent choice for completing this type of process. Its process performance can fully meet the needs of such hot processing, with good temperature control accuracy, temperature uniformity and gas quenching uniformity.