Study On Plasma Post-oxidation After Nitriding And Plasma Oxynitriding For AISI 4140 Steel

Plasma nitriding is also called glow ion nitriding, and it is a kind of chemical heat treatment technology, which uses glow discharge phenomenon to drive reactive nitrogen atoms into material interior. Research and development of combined processes on the basis of ion nitriding are focused in this area. Post-oxidation after plasma nitriding and plasma oxynitriding were investigated in this research, which can effectively improve the performance of the compound layer.In this research, AISI 4140 steel was selected as the material. Both post-oxidation after plasma nitriding and plasma oxynitriding using plain air were conducted innovatively in the same pulsed plasma-ion nitriding system for AISI 4140 steel. Compound layer, cross-sectional microstructures, phase structure, micro-hardness, corrosion resistance of composite layer were characterized by optical microscopy, x-ray diffraction, scanning electron microscopy and microhardness tester, electrochemical workstations, respectively. Additionally, AISI 4140 steel was plasma post-oxidized by different air flow under the same conditions of processing temperature, time and pressure.The results showed that the post-oxidation using plain air in the same pulsed plasma-ion nitriding system after plasma nitriding. A thin dense iron oxide layer of 1~2 μm thickness is formed on the compound layer during the post-oxidizing process, which is composed of magnetite(Fe3O4) and hematite(Fe2O3). The thin iron oxide layer brings out significant improvement of corrosion resistance, especially at high ratio of Fe3O4 to Fe2O3 post-oxidized at 400℃ and 60 min due to lower standard Gibbs free energy and appropriate forming rate for the formation of Fe3O4 at this temperature. Surface corroded specimens after polarization images test shows that there exists no corrosion pit for the specimen post-oxidized at 400℃ for 60 min. Meanwhile, the thickness of the compound layer formed during nitriding process tends to decrease due to spallation by air plasma bombardment during post-oxidizing process with the increase of treating temperature.It is found that plain air can be used as oxygen bearing gas in plasma oxynitiding for AISI 4140 steel, and plasma oxynitriding enhances the nitriding efficiency, makes the compound layer deeper under the same conditions of temperature and time. Surface hardness can reach 900HV0.05, which is 60 HV0.05 higher than that of conventional ion nitriding processing, and the gradient of micro-hardness is smoother. At the same time, the phase constituent in nitriding layer changes with the air flow. Meanwhile, the trace of Fe3O4 which formed in the nitriding layer improves its corrosion resistance. And air flow rate of 0.3L/min is the best process parameters.