Study On The Surface Alloying Of 30CrMnSi Steel By Scanning Electron Beam

30CrMnSi steel is widely used to manufacture shaft and gear parts,with the characteristics of high strength,high toughness,good hardenability and resistance to excessive stability.The traditional heat treatment process for 30CrMnSi steel has the disadvantages of poor surface hardness,poor impact toughness,and easy deformation,which is difficult to meet the using conditions.Electron beam surface alloying technology has the advantages of high energy conversion efficiency,small deformation and so forth,which can be used to treat 30CrMnSi steel.Plasma spraying technology is used to prepare Ni-Al and Fe-Al composite coatings with different powder ratios on 30CrMnSi steel surface respectively in this paper,sprayed specimens surface are then alloyed by scanning electron beam technology.Optimal powder ratio and beam parameters are optimized by orthogonal experiment,influence of powder ratio and beam parameters on cross-section microstructure and element distribution and mechanical properties are discussed.Quanta FEG 450 field emission scanning electron microscope,D8 Advance X-ray diffractometer,HDX-1000TM microhardness tester,HSR-2M vertical universal friction-abrasion testing machine are used to analyze and measure microstructure of cross section,surface phase composition and mechanical properties.Analyzing the specific gravity segregation in microstructure of alloyed layer after treatment;discussing the influence of powder ratio on microstructure specific gravity segregation of alloyed layer and the influence of specific gravity segregation on mechanical properties;discussing the influence of E-beam processing parameters on microstructure specific gravity segregation of alloyed layer and the influence of specific gravity segregation on mechanical properties.The results show that microstructure of cross section after treatment is composed of alloyed layer,heat affected zone and substrate,microstructure of alloyed layer is composed of Ni-Al intermetallic compound and Ni solid solution after Ni-Al alloying,the average microhardness is 750HV;microstructure of alloyed layer is composed of Fe-Al intermetallic compound and martensite after Fe-Al alloying,the average microhardness is 695HV;microstructure of heat affected zone is mainly composed of martensite,the microhardness is 654HV;microstructure of substrate is sorbite,the average microhardness is 275.7HV.Influence of Ni-Al powder ratio and beam parameters are analyzed by orthogonal experiments,surface microhardness and wear resistance increase before decreasing with the rising Al powder content;surface microhardness and wear resistance decrease with the increasing beam current;surface microhardness and wear resistance increase with the rising specimen moving velocity.The optimal scheme obtained are Ni-Al powder ratio 50:50,beam current 8mA,specimen moving velocity 360mm/min.Microstructure of alloyed layer is composed of Ni-Al intermetallic compound and Ni solid solution.Cross section microhardness of the specimen obtained by optimal parameters gradient distributes from 748HV to 265.7HV with the rising depth,surface hardness is nearly 3 times higher than substrate.Influence of Fe-Al powder ratio and beam parameters on mechanical properties are analyzed with orthogonal experiment,microhardness and wear resistance decrease before increasing with the increasing Al powder content,microhardness and wear resistance decrease before increasing with the increasing beam current,surface hardness and wear resistance decrease before increasing with the rising specimen moving velocity.The optimal scheme gained are Fe-Al powder ratio 25:75,beam current 8mA,specimen moving velocity 360mm/min.Microstructure of alloyed layer is composed of needle-like FeAl and dump-like Fe2Al5 intermetallic compound and martensite,microstructure of cross section decreases from 695HV to 270.3HV,surface hardness is 2.6 times higher than substrate.Microstructure of alloyed layer after treatment are Fe-Al intermetallic compounds distributed on surface and subsurface at 75%Al or 8mA beam current or 360mm/min specimen moving velocity at 50%Al,microhardness of cross section increase before decreasing with the rising depth;specific gravity segregation are induced in alloyed layer microstructure,Fe-Al compound distribute on subsurface at 25%Al or when beam current is larger than 12mA or moving velocity is less than 300mm/min at 50%Al,microhardness of cross section decrease before increasing and then decrease with the rising depth.Additionally,Fe-Al intermetallic compounds distribute randomly on martensite in surface and subsurface at 35%-75%Al,without specific gravity segregation;specific gravity segregation are induced in microstructure of alloyed layer at 25%Al,Fe-Al intermetallic compound distributes on subsurface,and martensite distributes on surface.The maximal surface hardness is 703HV and the minimum wear loss is 0.00014g at 75%Al;the minimum surface hardness is 585HV and the maximal wear loss is 0.00136g at 35%Al.