Study On The Process And Microstructure And Properties Of Gradient Coating Prepared By Double Glow Plasma Surface Alloying On 40Cr Surface

Gear materials work under a variety of stress loads where the situation of force is more complicated,and require more comprehensive performance.Fatigue failure of the tooth surface is main failure mode.In order to solve the fatigue failure of the tooth surface,the gear surface needs high hardness.Gear is widely used in a variety of complex environments,such as aerospace,marine,and petrochemical.In addition to high hardness,the gear material needs to have good corrosion resistance.It is well known that Cr and Mo have high hardness and excellent corrosion resistance,and preparing alloy coating of the two elements in the gear material can effectively improve the hardness and corrosion resistance of the gear surface.The double glow plasma surface alloying?DGPSA?technology is a novel surface-strengthening metal that can not only prepare non-metallic coatings,but also can prepare various alloy layers,thereby broadening the application field of surface strengthening technology.Compared with other surface strengthening technologies,such as thermal spraying,laser cladding,chemical vapor deposition and magnetron sputtering,DGPSA can prepare gradient layers with reasonable distribution,stable interface structure,high hardness,and good corrosion resistance performance.In this paper,40Cr gear steel was used as the starting material,and a gradient Cr layer and a gradient CrMo layer were prepared on the surface of 40Cr steel by DGPSA.The influence of DGPSA process parameters on the microstructure of the layer was carried out.Backscattered electronic imaging and energy spectrometer were used to characterize the microstructure of the layer and matrix.At the same time,the hardness and corrosion resistance of the layer were also investigated.The main work of the thesis is as follows:?1?The effects of DGPSA process parameters?chromium process time,source voltage,cathode voltage,nitrogen gas pressure and pole spacing?on the microstructure of the Cr layer were analyzed and compared.?2?Cr-coating were fabricated at 930^oC for 3h?6h and 10h,respectively.And CrMo-coating was fabricated at 920?for 6h.The phase composition of Cr-coating and CrMo-coating was analyzed by X-ray diffraction?XRD?.In addition,the hardness of the Cr-coating and CrMo-coating and the corrosion behavior in a 3.5%NaCl aqueous solution were investigated.?3?Annealing treatment of 6-hour and 10-hour Chromizing samples and 6-hour CrMo sample,and quenching and tempering of 10-hour Cr sample and 6-hour CrMo sample were carried out to study the effects of annealing,quenching and tempering on the microstructure and properties of the Cr-coating and CrMo-coating.In addition,the hardness and polarization curve of the annealed,quenched and tempered samples were measured.?4?The original state sample,the annealed sample and the quenched and tempered sample were characterized by field emission electron microscopy,respectively.The microstructure evolution and formation mechanism of the layered structure before and after heat treatment are described.By further analyzing the experimental results,the following conclusions are obtained:?1?DGPSA process parameters?chromizing process time,source voltage,cathode voltage,nitrogen gas pressure and pole spacing?not only have a significant impact on the microstructure and phase of the layer,but also seriously affect the thickness and compactness of the layer.?2?The chromized samples composed of a deposited-layer,a diffusion-layer,an affected-layer,and normal matrix.The difference is that as the DGPSA processing time increases,the relative thicknesses of the four layers are quite different.For the Cr coating of sample chromized for 3 hours consists mainly of pure Cr and carbon chromium compounds(Cr₂₃C₆),ferrochrome carbon compound?FCC?of?Cr,Fe?₂₃C₆(Cr_22.23Fe_0.77C₆ and Cr_21.34Fe_1.66C₆).For sample chromized for 6 hours,Cr₂N is observed in Cr-coating.For sample chromized for 10 hours,the proportion of pure Cr increases but the proportion of chromium carbide decreases.The Cr-Fe intermetallics?Cr-Fe IMs?layer appeared in all of the original Cr samples,while the Cr-Fe solid solution?Cr-Fe SS?layer only appeared in the samples chromizied for 6 hours and 10hours.?3?After annealing,rod-shaped second phase particles parallel to each other or at an angle of 69 degrees were observed in the sample chromized for 6 hours,indicating that they have a specific orientation relationship with the pure Cr matrix.In addition,Cr-Fe IMs disappeared and formed a new Cr-Fe SS layer in the samples chromized for 6 hours and 10 hours.After quenching and tempering,the Cr-Fe SS grains of the sample chromizied for 10 hours became smaller,and the structure of affected layer was transformed into tempered sorbite.?4?Cr-Fe SS layer has a high hardness,where the hardness of the quenched and tempered sample is the highest,reaching about 1600 HV_0.2.Compared to 304 stainless steel and 40Cr steel,all Cr layers have better corrosion resistance in 3.5%aqueous solution,especially quenched and tempered sample chromizied for 10 hours.?5?CrMo samples are composed of a diffusion layer,an influence layer and a normal matrix.The diffusion layer is divided into columnar crystals composed of coarse grains and transition layers composed of Cr and Mo carbides.Columnar crystal grains are composed of an?-Fe solid solution containing Cr and Mo??-Fe-Cr-Mo SS?,and a Fe3Mo3N phase is formed in the structure.After annealing treatment,the structure of diffusion layer in CrMo-6-A sample is more dense,and the granular carbide is distributed in?-Fe-Cr-Mo SS.Granular or short rod-like carbides distributed on?-Fe-Cr-Mo SS in the transition layer,and the affected layer composed of ferrite.After the quenching and tempering treatment,granular carbides and nitrides are dispersed on?-Fe-Cr-Mo SS.The intermediate layer consists of nanoparticles.?7?The micro-hardness of?-Fe-Cr-Mo SS is about 350 HV_0.2,and the hardness of the intermediate layer is about 525 HV_0.2.Annealing treatment generally reduced the hardness of sample.However,after the quenching and tempering treatment,the hardness of the intermediate layer is increased to about 575 HV_0.2 due to the formation of the nanoparticles.Compared to 40Cr steel and 304 stainless steel,CrMo sample have better corrosion resistance in 3.5%NaCl aqueous solution.Compared to the diffusion sample,the corrosion resistance of the annealed and tempered samples was increased by 22 times and 43 times,respectively.