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

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

The carburizing multipurpose furnace is carried out under the bombardment of ions. The average energy of the ions in the drop zone is about tens of electron volts. When the voltage is 800 volts, the energy of nitrogen ions is 3000 times higher than that of gaseous nitriding decomposition. The bombardment of high-energy particles produces sputtering, separating the sputtering from the surface with iron atoms, while carbon, oxygen and alloying elements are also bombarded, so that the oxides and carbides on the surface of the parts can be 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 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 in direct contact with the α-Fe on the surface of the part. Such a high rate of nitrogen supply causes α-Fe to become rapidly saturated with nitrogen. After a few minutes, the corresponding compound layer establishes an equilibrium with the nitrogen-saturated α-Fe, and the nitride layer on the surface of the gas nitriding component usually takes 1 to 2 hours.

During nitriding, the energetic particles collide with atoms in the lattice of the metal surface, resulting in a high density dislocation. Electron microscope observation shows that the increase of dislocation density will increase the permeability of the material and thus accelerate the diffusion of nitrogen.

Nitrogen diffusion at the initial stage of gas nitriding is mainly along grain boundaries. Nitrogen and grain boundary carbide contact to form carbon nitrogen compounds. In this way, a considerable part of nitrogen is consumed, and the formation of carbon and nitrogen compounds strongly hinders the diffusion of grain boundaries, slowing down the inward advance of the nitriding layer, mainly along the wrong diffusion. In addition, carbon sputters from the surface, leaving the grain boundary in a decarbonized state, which obviously hinders the formation of carbon and nitrogen compounds in the Austenitic crystal boundary. As a result, nitrogen atoms can also spread smoothly along the grain boundaries of carbon-free nitrogen compounds, so the diffusion rate is significantly accelerated.

When the nitriding temperature and holding time are the same, the permeating layer is deeper than the gas. Especially when the infiltration layer is below 0.2mm, nitriding can shorten the nitriding time more effectively. Taking 38crMoAlA steel as an example, if the required nitriding depth is 0.5mm, nitriding takes 20 to 30 hours, and gas nitriding takes 40 to 50 hours. When the nitriding layer is shallow, it takes only 2 to 4 hours to get a 0.2mm nitriding layer, while gas nitriding takes 10 hours.