Effect Of Plasma Enhanced Diffusion Treatment At Low Temperature On The Microstructure And Properties Of H13 Steel

The service environment of hot-work mould steel is harsh,and it needs to bear high temperature and pressure,thermal fatigue,erosion of high temperature melt etc.The harsh service environment leads to a series of failures,such as deformation,erosion,wear,thermal fatigue cracking,etc.To improve the service life of hot-work mould steel is an important issue in the industrial field.In this paper,low temperature plasma nitriding and nitrocarburizing of H13 steel were carried out in N₂-H₂,N₂-CH₄,N₂-H₂-CH₄ atmosphere respectively by hollow cathode plasma source assisted nitriding equipment.Optical microscope,Scanning electron microscope（SEM）,X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）,Electron probe（EPMA）were used to characterize the microstructure,distribution of elements and phase of the treated samples;Surface hardness,cross-section hardness,rebound resilience,brittleness and wear resistance of the layer were characterized by nano-indentation,Vickers hardness tester,surface optical profilometer and pin-disc friction and wear tester.Conclusions were drawn as following:（1）With the aid of hollow cathode plasma source,rapid nitriding achieves on the surface of H13 steel.In N₂-H₂ atmosphere,the thickness of the nitrided layers reach more than 90μm in 2 h;When the concentration of N₂ decreases to 3%,the compound layer disappears,and the diffusion layer maintains a thickness of 91μm.In N₂-CH₄ atmosphere,the thickness of the nitrocarburized layers reach more than 100μm in 2 h;The addition of CH₄ increases theε-phase content in the compound layer,and a very small amount of cementite forms.In N₂-H₂-CH₄atmosphere,the thickness of the nitrocarburized layers reach 40.6-126μm within 2 h;When N₂concentration decreases to 3%,the compound layer disappears,the diffusion layer thickness is40.6μm,more Fe-N compounds forming on the surface comparing with the sample of compound layer free in N₂-H₂ group,and quite a few of cementite forming.（2）The vein-like structures consist of Fe-N and Fe-C compounds in the low temperature plasma nitriding and nitrocarburizing of H13 steel;When the concentration of N₂ was 3%in N₂-H₂,and when the concentration of N₂ and CH₄ are 3%and 2%respectively in N₂-CH₄-H₂,no vein-like structure forms;Vein-like structures disappear with the disappearance of the compound layer,the number and size of which increase with the increases of theεphase content in the compound layer.In N₂-H₂ and N₂-CH₄ groups,the width of vein-like structures ranges from 0.3 to 0.7μm.In N₂-H₂-CH₄ group,perhaps due to the formation of cementite,the width of vein-like structures reach 1.13μm when the concentration of N₂ is 23%;The thickness of vein-like structures cover whole diffusion layer when the concentration of N₂ is 13%;Reducing the concentration of N₂ in N₂-H₂ and N₂-H₂-CH₄ and decreasing the concentration of CH₄ in N₂-CH₄ can inhibit theε-phase content in the compound layer,which has positive effect on inhibiting the formation of vein-like structures.（3）The brittleness of treated samples are Grade 1;There is no crack around the indentation in the samples of compound layer free below 30 kg.With N₂:H₂ changing from 1:1 to 1:9,the apparent fracture toughness of treated sample increases from 7.65 to 9.03 MPa·m^1/2.With N₂:CH₄ changing from 98:2 to 90:10,the apparent fracture toughness decreases from 7.71 to7.32 MPa·m^1/2.When N₂:H₂:CH₄ is 13:85:2,the apparent fracture toughness is 9.19 MPa·m^1/2.H ₂ is effective in reducing the content ofεphase and brittleness of layers because it can enhance sputtering effect.（4）Compared with substrate,the wear resistance of the treated samples improve by 4-11times.Possibly due to more graphite C on the surface,the friction coefficient of N₂-CH₄ group is significantly lower than that of the other two groups,and the wear resistance of N₂-CH₄ group is higher than that of the other two groups.The samples with the highest wear resistance in each group have common characteristics:they all have compound layer with low thickness,which is dense and tough.Therefore,under 20 N load,inhibiting compound layer thickness,increasing layer density andγ’-phase content have positive effects on the improvement of wear resistance.