Study On Microstructure Nanocrystallization And Deep Diffusion By Spinodal Decompostion Of Surface Layer Produced During Low Temperature Plasma Nitriding

In this paper, we have researched on the carbonized steel’s surface spinodal-decomposition (S.D) by plasma nitriding. In order to find the evidence of S.D, anew thermodynamic method which called pseudo-binary model has been used.Based on the Gibbs free energy curve of α-Fe, we can predict the volume-fractionof FeN_zdecomposed by S.D using phase-fied simulation. And then, some plasmanitriding experiments have been started within the calculated temperature andcomponent ranges. Therefore, the different performance between S.D plasmanitriding and conventional nitriding can be received by micro-hardness andwear resistance testing.we believe that S.D is the key of nanocrystalized micro-structure and deep diffusion by plasma nitriding after pretreatment solution.The pseudo-binary model not only can make it clear that FeN_zand Nappearance is because of the uphill-diffusion by S.D, but also help us to gain thehighest temperature and the optimal composition which is1969K andN=wt.1.85%. With increasing temperature, S.D is more easily to generate, butcan not exceed the austenitizing temperature of863K. Phase field simulationshows that the spinodal decompositon generate a low volume fraction of FeN_z.Its shape is round and granular, Its volume fraction range is between0.91%and39.5%get by theoretical calculation which has certain difference compare withthe less than3%get by phase field simulation.In subsequent trials, we not only confirmed the S.D occurred in plasmanitriding, but also obtained pure nitrogen martensite and FeN_z. For40CrNi steelwhich have the S.D at520C, we make the surface TEM oberation. It indicatedthat the nanoscale and FeN_zalong with amorphous are on the surface, thesize are10-30nm.And we also found a similar phenomenon in other steel like2Cr13,17-4PH. Finally, the micro-hardness and wear resistance testing indicatedthat S.D not only can lead to deep diffusion and layer hardness gradient is alsovery gentle. This is mainly because on the one hand, spinodal decompostionmicrosturcture refinement lead to the increase in diffusion paths of N atoms, onthe other hand, there has no rigid nitrogen compound(-Fe2-3N and γ′-Fe4N) onthe spinodal decompositon surface.