Study On Industrial Pure Iron Subjected To Surface Severe Plastic Deformation And Nitriding

By means of surface mechanical attrition treatment(SMAT)a severe plastic deformation layer which consists of nanocrystalline and ultrafine-grained microstructure can be fabricated.The grain refinement provides a fast atomic diffusion channel for nitrogen atom in nitriding process.This has led to reduction of the nitriding temperature,and has helped to accelerate the nitriding procedure.This paper investigates the severe plastic deformation behavior of industrial pure iron induced by SMAT.A nanostructured surface layer can be produced by this processing.Subsequently nitriding at different temperature was conducted for the coarse-grained Fe and SMAT Fe.SMAT Fe and nitrided SMAT Fe were annealed at different temperature to investigate their thermal stability.The microstructure of the processed materials was characterized via optical microscope,laser scanning confocal microscope(LSCM)?X-ray diffraction(XRD)?scanning electron microscope(SEM)and transmission electron microscope(TEM)etc.Mechanical properties of these processed samples are investigated by means of tensile test and hardness measurement.Strengthening and toughening mechanisms of the severe plastic deformation layer with nanostructure were analyzed.The main results are as follows:(1)Plastic deformation microstructure with nanostructure was formed on the surface layer of industrial pure iron via surface severe plastic deformation.On the top surface layer of pure iron equiaxed grains with nanocrystalline size was formed.Yield strength and tensile strength increase with the degree of plastic deformation,whereas work hardening rates decreases with plastic deformation.Compound layer and diffusion layer of nitrogen can be formed when the SMAT Fe was nitrided.The thickness of the compound layer and the diffusion layer of the SMAT Fe nitrided at 500? was larger than that of the SMAT Fe nitrided at 420?(2)Significantly increased hardness in the surface layer of the SMAT Fe and the nitrided SMAT Fe is obtained.The uniform elongation decreases with the degree of plastic deformation.The strength of the SMAT Fe nitrided at 420? is higher than that of the SMAT Fe nitrided at 500?,whereas elongation of the former decreases.(3)Nanostructure can be maintained in the top surface layer when the SMAT Fe was annealed at 400? or below.When the SMAT Fe was annealed between 200 ??300 ? yield strength,tensile strength and plasticity increase significantly in comparison with that of the SMAT Fe.Toughness of the severe plastic deformation layer increase significantly when the SMAT Fe was annealed at relatively low temperature.(4)Yield strength and tensile strength can be maintained when the nitrided SMAT Fe was annealed at 600? indicating that its thermal stability can be maintained.Maintaining of the strengthening originates from the plastic deformation layer and its nitride.