Study On Formation Mechanisim Of Surface Nanocrystalline Layer During Plasma Nitriding Of Steel

During the past decade,we have successfully realized the grain nanocrystallization of surface layer for the various Cr equivalent quenched steels such as 17-4PH,1Cr13,2Cr13,304,316,M50NiL,38CrMoAl,40CrNiMo,30CrMnSi,etc,treated by plasma nitriding/nitrocarburizing at low temperature.But the existing nanocrystallization mechanism at present,can not explain the reason about the appearing of low nitrogen compounds after nanocrystallization.In this paper,spinodal decomposition of Fe-Cr-N system was studied in order to reveal the mechanism of nanocrystallization after plasma nitriding.The Gibbs free energy of Fe-Cr-N system was calculated by two thermodynamic models.The component free energy curve for the ?' N phase are calculated employing pseudo binary model and has a double potential well function feature.It is shown that spinodal decomposition may occur in the Fe-Cr-N system under certain conditions.The relationship between the Gibbs free energy of nitrogen-containing martensite and the nitrogen content was calculated by the double sub-lattice model in different Cr equivalent steel.The nitriding temperature should be chosen to be around 800K,and the level of nitrogen potential is related to the Cr content.In this paper,the nano-mechanism of Fe-Cr alloy steel is that with the increase of N content,the original large-size ?' N occurrs spinodal decomposition and form nano-sized low-nitrogen compounds FeNz and high nitrogen martensite ?' N,reasulating the infiltration layer refinement.The structures of different Cr equivalent nitrogen-containing martensites and the structures of low nitrogen compounds are simulated by the first principle.It was found that the total energy of the nitrogen-containing martensite and low nitrogen compounds was less than 0,indicating that these structures could be present at OK.When the Cr equivalent is 1/11 and 1/7,the nitrogen-containing martensite binding energies and formation energies have a tendency to a double potential well function feature.,which further confirmed the thermodynamics conclusion about that the ?' N transition mechanism during nitriding process is spinodal decomposition.Due to the strong hybridization of Fe-3d and N-2p,the pseudo-energy gap appears in the vicinity of the Fermi level of nitrogen-containing martensite,which shows that these structures are relatively stable at OK again,and provides the conditions for the occurrence of the spinodal decomposition in the nitriding temperature.Through the first principle calculation,fcc structure of low nitrogen compounds is more stable than bcc structure,from the energy and electronic structure,which explains why the low nitrogen compounds detected during the experiment are fcc structure.