Research On New Process Of Ion Nitriding At Pure Nitrogen Atmosphere And Ion Nitriding Mechanism

When the pure NH3 or N2-H2 mixture has been adopted for the conventional ion nitriding, there have been some hidden troubles in nitriding heat process because of the ammonia decomposed incompletely and the hydrogen gas burning and exploding easily. The advantages that ion nitriding process may improve surface performance of metal materials evidently are affirmed at the same time, the environmental protection and safe problems of media in ion nitriding process are deserved concern particularly. That safe and environmental protection nitriding media replacing pure NH3 or N2-H2 mixture will be explored to bring into play its potential on metal materials surface strengthen as best as it can, have become one of hotspot problems solved urgently on development of ion nitriding process at present.Based on the theories of plasma discharge and the principles of the chemistry heat treatment at low temperature, the dissociation behavior and energy condition about the nitriding media such as pure N2, N2-H2 mixture and N2-NH3 mixture have been studied in this dissertation. The energy of the positive ions N2+, H2+, NH3+among the nitriding media such as pure N2, N2-H2 mixture and N2-NH3 mixture being from the electric field and the critical dissociation energy of N2 molecules for different nitriding media have been also calculated quantitatively by the determination of the electric, temperature and gas pressure parameters related to ion nitriding and put forward the new process of ion nitriding at pure nitrogen atmosphere. The microstructure, hardness gradient distribution, phase composition and phase comparative content of nitriding layers of alloys which were ion nitrided via furnace shut and gas provided intermittently or conventional process respectively have been researched with some test methods such as Microhardness apparatus, Microcopy and X-ray diffractometer etc. Not only the Spi-nodal structures of and e phases have been analyzed and the surface nitrogen concentrations of ion nitriding samples have been calculated farther, but also the influence of ion nitriding process parameters on producing and diffusion of active nitrogen atoms and ion nitriding mechanism has been discussed thoroughly.The theoretical analysis and experimental results show that:(l) The new process of ion nitriding at pure nitrogen atmosphere which is put forward in this paper, only the cathode voltage which is higher than the cathode voltage threshold value of 650 V is adopted to ion nitriding, then can make N2+ ions have its critical energy 48.64 eV ofdissociating the neutral N2 molecules and the active nitrogen atoms are produced from the non-elastically collision dissociating between the N2+ ions and the neutral N2 molecules. Besides the new process of ion nitriding at pure nitrogen atmosphere is required to reach to a certainty work electric voltage, that the ion nitriding under furnace shut and N2 provided intermittently is an essential condition, or else the ion nitriding can't be carried out. (2) Theoretically, the ion critical energy of dissociation nitrogen molecules in nitriding media has been put forward to describe the extent if they are dissociated easily. The cathode voltage threshold corresponding with the ion critical energy of dissociation nitrogen molecules has been also put forward to control ion nitriding process parameters conveniently. (3) There are similarity microstructures and phase compositions in the ion nitrided layers of pure NH3, pure N2, N2-H2 mixture and N2-NH3 mixture, which mainly relate to gas source category, cathode voltage and mixture ratio that decide the active nitrogen atom quantity from the nitriding media dissociated. The parameter matches of cathode voltage and mixture ratio that benefiting to ion nitriding media dissociated to yield active nitrogen atoms will increase the volume percent of Fe3N phase with high nitrogen content in the nitrided layers, enhance their surface nitrogen concentration and hardness, at meantime which make the superficial layer, diffusion zone and core micro structure in ion nitrided...