Study On The Effect Of Rare Earths On Plasma Electrolytic Carburized Layer Of Stainless Steel

Carburizing technology is widely used in the surface modification of key equipment parts such as turbines and aerospace.However,conventional gas carburizing technology has problems of high treatment temperature and long treatment time,the normal plasma carburizing possesses the boundary effect,which will lead to non-uniform carburizing layer.To overcome the problems of conventional carburizing,using AISI 630 stainless steel as the object,by using liquid-phase plasma electrolytic carburizing technology,the process and the properties of carburized layer with and without rare earth addition were investigated.The evolution law of microstructure and the influence of rare earth were analyzed,the influence law of process parameters and rare earth addition on the properties of carburized layer were studied,and the electronic structure and mechanical properties of the typical formed phases in plasma electrolytic carburized layer were investigated by first-principles calculation,the strengthening mechanism of liquid-phase plasma electrolytic carburized layer were explored.The experimental results show that by using liquid-phase plasma electrolytic carburizing technology,a carburized layer with a certain thickness and uniform microstructure can be quickly formed on the surface of stainless steel,and the carburizing thickness can reach to 22.1μm after 30 min treatment.The carburized layer consists mainly of the carbon-expandedαphase（α_C）,Fe₃C,Cr₃C₂,and a small amount of Fe₃O₄ phase.The addition of rare earth can increase the thickness of the carburized layer and the thickness of the carburized layer can reach to 25.7μm after 30 min treatment,but it has little effect on the phase structure of the carburized layer.The rare earths of La in the carburized layer exist mainly in the form of atomic state and La³⁺.Hardness and wear resistance tests have shown that the hardness of the liquid phase plasma electrolytic carburized layer can reach to 560 HV_0.1（nanohardness 6.50 GPa）,which is more than one times higher than that of the untreated stainless steel.Rare earth addition increases the hardness of the carburized layer to 740 HV_0.1（nanohardness 7.30 GPa）.The corrosion resistance test shows that the corrosion resistance of the stainless steel does not change much after the liquid phase plasma electrolytic carburization,but the addition of rare earth can improve the corrosion resistance of the carburized layer.First principles calculations show that both Fe₃C and Cr₃C₂ carbide phases are easily formed and can exist stably,which is consistent with the experimental results.For theα_C phase,a structural model of Fe_13-xCr₃M_xC_y（x=0,1;y=0,1,2,3,4;M=Ni,Ti,V,Mn）was constructed.The results show that the alloying element M affects the stability and properties of the structure,and the constructedα_C phase structure is easily formed and satisfies the energy stability when the carbon content is lower than 11.11 at.%.The hardnesses of Fe₃C and Cr₃C₂ phases with orthogonal structures are 10.67 GPa and 18.02 GPa,respectively,for theα_C phase structure with carbon content,the hardnesses are between 11.32 GPa and 17.88 GPa.The electronic structure calculations show that the metallic bonds,covalent bonds and ionic bonds exist in the carbide andα_C phase structures,and the combined effect of all three kinds of bonds is the reason for the high hardness of the carbides andα_C phase with different carbon content.