zero flow process in vacuum nitriding furnace

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Nitriding, also known as nitriding, refers to a chemical heat treatment process (at a certain temperature, when the atmosphere in the furnace reaches a certain requirement) to infiltrate nitrogen atoms into the surface of metal workpiece to form a kind of surface that can improve the surface hardness and wear resistance of parts. Nitriding can be divided into gas nitriding, liquid nitriding and ion nitriding. This paper mainly introduces a new gas nitriding process zero flow gas nitriding.

According to different process carriers, gas nitriding can be divided into traditional gas nitriding (100% NH3), controlled gas nitriding (NH3 + dissnh3 or NH3 + N2) and zero flow nitriding (100% NH3).

1. Traditional gas nitriding process

The traditional gas nitriding process was published by Germany afry in 1923. The workpiece is placed in the furnace, and ammonia gas is introduced into the furnace at 500 ~ 550 ℃ for 20 ~ 100h to decompose ammonia gas into (n) and (H) in atomic state. Chemical reaction between (n) and metal can realize nitriding treatment, so as to obtain high hardness of workpiece surface, good fatigue resistance and improve corrosion resistance. Generally, the thickness of nitriding layer is 0.02-0.2mm, and the surface hardness can reach 1000-1200hv. The decomposition rate of NH3 depends on the flow rate and temperature. The higher the flow rate is, the lower the decomposition degree is. The lower the flow rate is, the higher the decomposition rate is. The higher the temperature is, the higher the decomposition rate is, and the lower the temperature is. The thermal decomposition of NH3 at 570 ℃ is as follows: NH3 → [n] Fe + 2 / 3h2

The decomposed n diffuses into the surface of the steel. The traditional gas nitriding process has very limited control on the depth of nitriding layer, and the nitriding layer is formed by the superposition of E + G + a phase because 100% ammonia is introduced. Through many years of industrial practice, the e + G ﹣ phase (usually extremely thick and brittle) of nitriding surface is usually polished off after heat treatment, which increases the extra production cost.

2. Controlled gas nitriding process

Controlled gas nitriding was introduced 50-60 years ago, showing many advantages compared with the traditional process of 100% ammonia. The controlled gas nitriding process uses ammonia + pyrolysis ammonia or ammonia + N2. The desired nitriding layer can be obtained by introducing the gas into the reaction chamber through appropriate proportion composition and adjustment, including e + G ﹣ a phase, G ﹣ a phase or a phase, and the desired nitriding layer thickness can be obtained in a certain area. The composition of nitriding layer is controlled by two parameters: temperature T and nitrogen potential kn. Because the nitriding layer can be controlled, the defects of nitriding layer can be avoided. Therefore, the grinding process of E + G ^ + a phase nitriding layer after heat treatment is omitted, and the production cost is reduced. However, this kind of nitriding process using two kinds of gases consumes a large amount of gas just like the traditional process.

In addition, this process has other disadvantages. The nitriding process of ammonia NH3 + nitrogen N2 has the characteristics of unbalanced nitriding, which makes it less accurate than the nitriding process of ammonia + pyrolysis ammonia. However, the use of ammonia + cracking ammonia requires the installation of ammonia cracker, which will also increase the additional cost.

3. Zero flow nitriding process

Zero flow nitriding process is a new technology created by many years of practice. The so-called zero flow nitriding process only uses ammonia gas in the process of controlled nitriding. Compared with the process of ammonia + pyrolysis ammonia or ammonia + nitrogen, zero flow is simpler. The nitrogen potential Kn in nitriding process is controlled by adjusting the ammonia flow rate into the nitration chamber. The unique advanced nature of the zero flow process is that the ammonia flow rate is zero at a certain time when entering the nitrided chamber during nitration process, which enlarges the decomposition range of the atmosphere components in the nitrided chamber and increases the nitrogen potential Kn.

The simple control principle of zero flow is to control the periodic switch of ammonia injection valve through the target set by gas analyzer to maintain the required gas composition in nitriding chamber, that is, the set value of nitrogen potential kn.

The concept of zero flow is based on practice and theory, which is tested by using quartz tube nitriding test equipment in laboratory and vacuum nitriding furnace with metal shell nitriding chamber and circulating fan in factory. Ammonia (zero flow), ammonia + pyrolysis ammonia and ammonia + nitrogen were used to nitride the samples in two nitriding environments.

The data obtained from the experiments in these two kinds of test equipment show that the growth rate of nitrided layer only depends on the gas composition or nitrogen potential kn in the nitriding chamber, which has nothing to do with the process gas (ammonia, ammonia + pyrolysis ammonia or ammonia + nitrogen) injected into the nitriding chamber, and also has nothing to do with the nitriding environment. This experiment also proves that in the process of zero flow nitriding in vacuum nitriding furnace, the periodic cut-off of ammonia injection will not affect the growth of nitriding layer.

There are two stages in the process of zero flow nitriding

In the first stage, a certain amount of ammonia gas is introduced into the vacuum nitriding furnace to make the atmosphere composition in the nitriding chamber reach a set value, so that the nitrogen potential will increase rapidly.

In the second stage, it can be seen that the flow rate of ammonia in the nitriding process periodically changes to zero, because when the atmosphere composition in the nitriding chamber reaches the set value, the ammonia control valve will automatically close periodically. At this stage, the injection amount of ammonia decreases rapidly, resulting in the decrease of nitrogen potential kn, which is what we hope. Because the nitrogen potential value kn is usually controlled by sections, high nitrogen potential is used in the initial stage to increase the nitrogen content on the surface, and the nitrogen potential is reduced in the middle and later stage to make the high nitrogen on the surface diffuse to the center, so as to reduce the thickness of the high nitrogen white layer on the surface and avoid the formation of network nitrides.

Compared with other controlled atmosphere nitriding processes, the zero flow process is simpler. It only needs to install an ammonia injection control valve and gas detection instrument on the equipment. The hydrogen partial pressure in the nitriding chamber is detected by the detection instrument to control the ammonia injection flow, so as to realize the control of nitrogen potential kn and nitriding quality of nitriding parts surface, and obtain the desired metallographic.

Compared with controlled atmosphere nitriding technology (NH3 + nh3diss. Or NH3 + N2 two-component atmosphere), zero flow process nitriding technology can obtain the same quality nitriding layer. In the process, the atmosphere composition can be achieved by adjusting the ammonia flow into the nitriding chamber, and the flow can be closed periodically by pulse mode to achieve the same control precision as NH3 + nh3diss. Atmosphere, and zero flow nitriding technology is more effective Compared with controlled atmosphere nitriding technology, this technology has the advantages of low gas consumption, simplified equipment and lower cost of control system. It is easier to control the formation of nitriding layer than controlled atmosphere nitriding.

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