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Vacuum high pressure gas nitriding process

Gas nitriding process is widely used in the production of mechanical parts, it has many advantages, but its process time is long. For this reason, many methods have been adopted to increase the nitriding speed, and high-pressure nitriding is one of them.

Compared with liquid nitriding and ion nitriding, the heating, cooling, surface adsorption process and interface reaction rate of gas nitriding are slower. In order to explore ways to improve the nitriding speed and the direction of equipment improvement, an analysis and comparison are made here.

1 The basic principle of nitriding process

There are three nitriding process methods widely used at present, namely gas nitriding, salt bath nitriding (liquid nitriding) and ion nitriding. The main difference between them is the difference in surface adsorption and interfacial reaction rates. Once the Fe[N] solid solution or nitride is formed on the metal surface, the diffusion principle and speed of nitrogen atoms into the interior can be considered to be the same.

The basic principle of nitriding process, it is divided into two main processes, the first is the physical and chemical action of the metal surface, the formation of metal nitride or interstitial solid solution. Then, driven by the diffusion activation energy, nitrogen atoms diffuse into the metal to form metal nitrides. Obviously, the first stage has an important influence on the nitriding process.

1.1 Liquid nitriding

The parts are directly immersed in molten salt, and the heating time is short. The formation power and speed of the nitrided layer are larger and faster than those in the gas phase. After the heat preservation is over, the parts can be air-cooled or oil-cooled immediately. Therefore, the nitriding process is much faster than gas nitriding.

1.2 Ion nitriding

As the cathode, the parts are bombarded by nitrogen ions in a high-voltage electric field. The energy of nitrogen ions is more than 3000 times that of nitrogen atoms in ammonia decomposed under general gas nitriding conditions. Iron atoms ionize with nitrogen in the excited state to form iron nitrides and are adsorbed on the metal surface where the passivation layer has been removed by bombardment. These regions have strong adsorption capacity due to the high density of dislocation defects in the metal lattice, so that the first-stage compound reaction can be completed quickly. The heating and cooling rate of ion nitriding is also higher than that of gas nitriding.

1.3 Gas nitriding

Most of the gas nitriding is to pass ammonia gas into a heat-resistant steel tank, evacuate the air and then heat it. Due to the large heat capacity of the tank and the slow heat conduction of external heating, the heating and cooling are very slow. And because the atmospheric pressure is low, the rate of adsorption and interface reaction is low, so the infiltration rate is slow.

2 Parameters affecting gas nitriding process

Among the main parameters of gas nitriding control, people are more clear about the effects of temperature, time, flow rate and ammonia decomposition rate on the process, but they do not pay enough attention to the effect of furnace pressure on the nitriding process. The effect of furnace pressure on gas nitriding has the following aspects:

2.1 Increasing the air pressure in the furnace can increase the amount of adsorption on the surface of the parts

The metal surface is uneven when viewed under a microscope. From a chemical point of view, most of the atoms or ions on these sharp corners and protruding edges have not reached saturation, and their chemical properties are the most active and easy to combine with external molecules. These sites are called active centers. The adsorption of gas molecules on metal surfaces by unsaturated valences is called chemisorption. In addition, the adsorption of gas molecules by gravitational force (van der Waals force) is called physical adsorption. According to physical and chemical principles, the amount of gas adsorbed on the metal surface is determined by temperature and pressure. When the temperature is constant, it depends on the gas pressure. In addition, the increase of furnace pressure means that the density of ammonia gas in the furnace increases. The increase of gas density can provide more opportunities for surface adsorption.

2.2 Increase the furnace pressure and increase the kinetic energy of gas molecules

Gas pressure is a macroscopic manifestation of molecular collision force. When the furnace pressure is increased, the kinetic energy of the ammonia molecules is greatly increased, which is helpful for overcoming various obstacles and resistances on the metal surface, increasing the surface adsorption and promoting the interface reaction.

2.3 Increasing the furnace pressure can increase the interface reaction rate

The nitriding process can be regarded as a catalytic reaction. Increasing the blast furnace pressure can increase the interfacial reaction rate and the amount of nitride in the surface layer, creating conditions for diffusion.

2.4 Increasing the blast furnace pressure can increase the activity of the ammoniated atmosphere

When the furnace pressure is increased, the ammonia decomposition rate decreases, and the ammonia gas can be saved by 20% to 30%.

2.5 Increasing the furnace pressure can improve the nitriding ability of slits, deep holes, blind holes, small holes, etc.

Due to the small ventilation cross-section in these parts, the gas flow rate and flow rate are reduced due to the obstruction of gas flow, and the gas near the pipe wall hardly flows, forming a “viscous flow”. When the pressure increases, the gas density increases, and the pressure gradient also increases greatly, thereby promoting the increase of ammonia flow in these parts and improving the nitriding ability. Practice has proved that after increasing the pressure, the seepage layer in these parts is excellent.

3 Measures to improve the nitriding speed of gas nitriding furnace

3.1 Internal heating structure

At present, many gas nitriding furnaces are all external heating structures, that is, the electric heating element is outside the metal tank, and the parts are installed in the tank. The electric heating element first heats the tank body to above the nitriding temperature, and then heats the parts in the tank to the nitriding temperature by the thermal energy of the tank body. With this structure, not only the heat loss is large, but also the heat conduction lags behind, making it difficult to control the temperature, so that the temperature rises slowly and the temperature falls slowly.

Vacuum – high pressure nitriding furnace adopts internal heating structure, which greatly improves the state of slow heating and cooling. The temperature control sensitivity and accuracy are greatly improved.

3.2 Vacuum system

The configuration of the vacuum system has the following functions: First, the air in the pre-extraction furnace can shorten the time for introducing ammonia gas and prevent the parts from being oxidized; the second is to purify the surface of the parts and remove the oxide film layer in a vacuum state, which is conducive to adsorption and interface reaction; Third, diffusion treatment can be performed to reduce the brittleness of the high-concentration surface layer.

3.3 Fast cooling system

Equipped with an air cooling system composed of a ventilation circuit, a heat exchanger and a centrifugal fan, the cooling after nitriding can be carried out quickly, the performance of the infiltrated layer can be improved, the brittle structure of the infiltrated layer can be reduced, and the phenomenon of temper brittleness can be avoided.

3.4 Pulsation inflation method

According to a certain period, the atmosphere pressure in the furnace fluctuates within a certain range, which can improve the quality of the seepage layer. This goal is achieved by controlling the charging and discharging of the valve.

The ammonia gas fed into the furnace is directly input from the liquid ammonia tank, and the tank pressure is maintained at 0.8~1.2MPa, which meets the requirements of the furnace pressure of 0.1~1MPa.

The pressure control valve opens only when the ammonia pressure in the furnace exceeds the upper limit set by the pressure valve. Once the furnace pressure is lower than the control value, the valve closes immediately. Therefore, regardless of the size of the furnace, the consumption of ammonia gas is not large. Due to the small emissions, the heat loss is not large. The exhausted gas is discharged into the water tank (turned into ammonia water), which greatly reduces the pollution to the environment.

4 The effect of vacuum high pressure nitriding

(1) Fast penetration. Taking 38CrMoAl as an example, the heating and cooling times are greatly reduced. The average infiltration rate during the heat preservation period can reach more than 0.03~0.04mm/h, and the 40Cr steel can reach more than 0.06~0.08mm/h.

(2) High hardness. 38CrMoAl can reach HV1000; 40Cr can reach HV600 or more.

(3) The infiltration layer is uniform. Small holes, deep holes and blind holes can get a uniform infiltration layer, and the depth and hardness are almost the same as the outer surface.

(4) Tempering and nitriding can be combined, because of the rapid cooling system, the tempering brittleness and poor infiltration layer structure are completely eliminated.

(5) Easy to operate. There is no special requirement for furnace loading, and it can be stacked without affecting the quality of the seepage layer.

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.