Study On Microstructure And Properties Of Super Thick Compound Layer Produced By Low Temperature Gas Multicomponent Thermochemical And Its Application

The compound layer formed on nitrided surface shows an extensive application due to its excellent mechanical properties and corrosion resistance.But the thickness of the compound layer is very thin,thus its properties cannot be tested exactly.In order to characterize comprehensive properties of the compound layer systematically,a thick compound layer of 120μm was produced by the low temperature gas multicomponent thermochemical treatment of 650℃×3h+450℃×2.5h+400℃×2h.The microstructure,phase,hardness,damping capacity,magnetic property,neutral salt spray corrosion and polarization performance of the compound layer were measured in this paper.After low temperature gas multicomponent thermochemical treatment,an oxide layer was formed on the treated surface followed with a compound layer and diffusion layer underneath.The hardness was improved significantly.The main phase of compound layer is Fe2N with a trace of Fe3N.With the increase of proportion of the compound layer,damping capacity was promoted while saturation magnetization exhibits an opposite tendency.The time of neutral salt spray corrosion is more than 900h under the maximum protection level of 10.By comparing the corrosion potential and the anode Tafel slope,the thicker the compound layer is,the larger the corrosion resistance obtains.The magnetic moment and property of Fe2N,Fe3N and α-Fe phase was calculated by first principles method.The results shows that the magnetic moment of Fe atoms increased with the decrease of N concentration.The electronic structure of those phases was also calculated.The magnetic properties were analyzed by density of state and the calculated results agreed well with the experimental data.Through comparing the tensile data and fracture mode of different samples,the tensile strength of compound layer with the value of 990MPa was calculated by theoretical analysis.This paper studied the application of low temperature gas multicomponent thermochemical treatment in crack healing.The crack whose width is 125μm can be healed by this treatment method.The difference of hardness between healing area and matrix is very small,which indicates the minimum stress concentration.After healing treatment,the tensile strength and plasticity increased significantly.Meanwhile,the healing mechanism was clarified.