What is the reason for the fast speed of multi-purpose carburizing furnace?

What is the reason for the fast speed of multi-purpose carburizing furnace?

The carburizing multipurpose furnace is carried out under the bombardment of ions. The average energy of ions in the potential drop region is about tens of electron volts. When the electric pressure is 800 volts, the energy of nitrogen ions is 3000 times higher than the energy of nitrogen atoms obtained during the decomposition of gaseous nitriding ammonia. High-energy particle bombardment produces sputtering, and at the same time as iron atoms are separated from the surface, carbon, oxygen and alloying elements are also bombarded out, so that the oxides and carbides on the surface of the parts are reduced. If there is hydrogen, it can not only prevent the oxidation of residual oxygen in the atmosphere, but also reduce the oxide on the surface of the part, so as to obtain an active clean surface, so that the nitriding reaction is quite active.

The migration of nitrogen during nitriding is mainly achieved by the sputtering of iron atoms and the deposition of iron nitride. Therefore, at the beginning of nitriding, the nitrogen-rich phase is formed in direct contact with the α-Fe on the surface of the part. This high rate of nitrogen supply causes α-Fe to quickly become saturated with nitrogen. After a few minutes, the corresponding compound layer is in equilibrium with the nitrogen-saturated α-Fe, and the nitride layer on the surface of the gas nitriding part generally takes 1 to 2 hours.

During nitriding, high energy particles collide with atoms in the metal surface lattice, resulting in high density dislocation. Electron microscope observation of pure iron foils subjected to ion bombardment shows that the increase of dislocation density will increase the permeability of the material and thus accelerate the diffusion of nitrogen.

At the initial stage of gas nitriding, nitrogen diffusion mainly occurs along grain boundaries. Nitrogen comes into contact with carbides at grain boundaries to form carbon and nitrogen compounds. In this way, a considerable amount of nitrogen is consumed unusefully, and the resulting carbon and nitrogen compounds also strongly hinder grain boundary diffusion, slowing down the inward advance of the nitriding layer, and mainly along the dislocation. In addition, carbon is sputtered from the surface, leaving the grain boundary in a decarbonized state, which significantly hinders the formation of carbon and nitrogen compounds on the Austenite crystal boundary. In this way, nitrogen atoms can also be smoothly diffused along the grain boundaries of carbon-free nitrogen compounds, so the diffusion rate is significantly accelerated.

Nitriding temperature and holding time are the same, the nitriding layer is deeper than gas nitriding, especially when the nitriding layer is below 0.2mm, nitriding can more effectively shorten the nitriding time, taking 38CrMoAlA steel as an example, if the required depth of the nitriding layer is 0.5mm, nitriding takes 20 to 30 hours, gas nitriding takes 40 to 50 hours, in the case of shallow nitriding layer, Nitriding to obtain 0.2mm nitriding layer takes only 2 to 4 hours, and gas nitriding takes 10 hours.