Study On Process Of Active Screen Plasma Nitriding And Gas Phase Transportation Mechanism Of Nitrogen

Active screen plasma nitriding is a preferable plasma nitriding method, which has been applied already. Yet, there has been no public consensus on transportation mechanism that nitrogen atoms, in the process of plasma nitriding of active screen, from the gas phase to surface of workpiece, thereby, to a certain degree, handicapping application of the technology. In the thesis, further experimental study has been conducted on the transport mechanism that nitrogen atoms from the gas phase to surface of workpiece so as to lay the new experimental function for further study of the mechanism of screen plasma nitriding.In the thesis, at first, copper active screen have been adopted to nitridize plasma of 45 steel by using the features that the copper active screen does not absorb and react with nitrogen at pure nitrogen and to conduct an intensive study on tissue of metallographic phase of the nitriding layer, nitriding depth, components of the nitriding layer and microhardness of the nitriding layer by using detection means for instance, metalloscope, scanning electron microscope, X-ray diffracting apparatus, microhardness tester and the like. The experimental result represents that plasma nitriding under pure nitrogen atmosphere by using the copper active screen can still obtain nitriding layer in higher concentration, or even generate gamma phase (Fe4N) in the absence of ferro-nitrogen compound particles and ions of hydronitrogen. In the process of active screen plasma nitriding, the active nitrogen atoms, which are transported, apart from ferro-nitrogen compound particles, are further transported in other important, unneglectable ways. Nitrogen-containing plasmas generated by ionization move toward cathode increasingly under effect of electric field and they are possible to capture the electrons emitted by the cathode so as to convert to electrically neutral active nitrogen containing particles. The plasmas that maintain kinetic energy of nitrogen containing particles can move forward continuously, further to penetrate through the active screen possibly to reach the surface of workpiece, thereby finishing the task to transport active nitrogen atoms; secondly, it is possible that the electric plasmas collide with the nitrogen molecules non-elastically to directly generate active nitrogen atoms which may reach the surface of the workpiece by way of shifting and diffusion to transport certain active nitrogens.In the thesis, 38CrMoAl steel have been nitridized by using the active screen plasma nitriding, influences on nitriding due to nitriding temperature, voltage, air pressure and time under ammonia atmosphere have been studied and effects of active screen plasma nitriding and impulse plasma nitriding in same process parameters have been compared. The experimental result represents that along with raise of nitriding temperature, thickness of the nitriding layer and surface hardness are increased and that it is of little influence of nitriding air pressure and voltage on surface hardness of samples of the active screen plasma nitriding. In same process parameters, the active screen plasma nitriding can obtain the effect same with that of impulse plasma nitriding. The experiment determines that preferable process parameters of the active screen plasma nitriding at ammonia atmosphere is 550 degrees centigrade of nitriding temperature, 4h of nitriding time, 400Pa of air pressure and 900V of voltage.