Study On The Preparation And Toughening Mechanism Of Bainite Surface Modification By Arc Hardfacing And Laser Remelting

An excellent surface modified layer on the surface of the work piece by new techniques of surface modification is widely used in the high abrasion fields,such as mechanical manufacturing,and mine metallurgy.Bainite steel has attracted attention for its high strength,high toughness and good dimensional stability.However,up to now,it is reported rarely to prepare the bainite surface modified layer.Therefore,in this paper,the bainite surface layers in the mid-carbon and low-carbon steels were prepared by arc hardfacing and laser surface modification technology,and the strengthening and toughening mechanism was studied,which can provide the theory foundation for the application widely of the workpiece with bainite surface modified layer.The composition of the surface modified layers was designed by Jmatpro and Thermocale thermodynamics software.The microstructure and grain size of the surface modified layer were observed and measured by optical microsope?OM?;Phase structure and crystal structure were analyzed by X-ray diffraction?XRD?and transmission electron microscope?TEM?;The mechanical properties of the surface modification layer were tested by Rockwell apparatus,Microhardness tester,Nanoindentor,Tensile testing machine and Friction-abrasion testing machine;The temperature field and stress field of the surface modified layer were analyzed by numerical simulation method.The composition of the arc hardfacing layer was designed,and the granular bainite microstructure can be obtained by air cooling after welding.The granular bainite steel with excellent mechanical properties are consisted of bainite ferrite and island,and the island are composed of matrtensite and austenite.The hardness of the hardfacing layer can be increased with the increase of C content,however,high C content increase the susceptibility to weld cracking.The bainite can be obtained by the isothermal transformation at 300?after the arc hardfacing,and it can be formed in the micro segregation between the dendrites.The crack resistance of the arc hardfacing layer can be inproved significantly.With the Y₂O₃ additive,the hardness and tensile strength of the arc hardfacing layer were increased from HV272±13 to HV312±8 and from 764MPa to 885MPa.The coefficient of friction is decreased from 1.22 to 1.10 and the height loss is decreased from 0.180 mm to0.135 mm.According to Bramfitt two-dimensional lattice misfit theory,the lattice misfit between the face?100?of Y₂O₃ and face?001?of austenite is 3.67%,which indicates that Y₂O₃ as heterogeneous nuclei of austenite is most effective.The low-carbon bainite was modified by laser surface modification.The microstructure of the modified layer is refined by the laser surface modification,and the hardness is increased from HV375 to HV445.The M_s temperature of the steel can be declined by laser surface modification due to the decrease of the retained austenite fraction.Laser surface modification improves the solid solubility of C atom,and the lattice constant of bainite ferrite is increased from 0.28692 nm to 0.28704 nm.Carbide-free bainite surface modificated layer with high hardness are formed on the surface of mid-carbon alloy steel by laser surface modification.The hardness of the laser surface modified layer is increased by the carbon supersaturation in bainite ferrite and the shear band and?'-martensite is induced by laser stress in mid-carbon alloy steel layer to further improve the hardness.The hardness of the modificated layer is increased from6.66GPa to 7.76GPa,and the analysis of the nano-indentation load-depth curves proves that the hard points?8.81GPa?in surface modificated laye are related with the shear band.The wear results show that the nanobanite layer is equipped with good wear resistance at 20oC,100oC and 200oC.The embrittlement is relieved by the ductile phases,such as retained austenite and shear bands,and the cracking and spalling can be avoided when the tungsten carbide ball pass through the sample surface.The formation of nano-twin,?-martensite,and the consistency of orientation between stress-induced martensite and stress demonstrates that the shear band and stress-induced martensite can be induced by thermal stress.According to the temperature field and stress field simulation of arc hardfacing and laser surface modified layer,the residual stress on the surface is significantly reduced by the isothermal transformation after the surface modification.Under the same conditions of arc welding voltage,the maximum temperature of the melting zone is increased 140?with the ambient temperature increased from 25?to 300?;under the same ambient temperature,the maximum temperature of the melting zone is increased 96?with the welding voltage increased 2V.The residual stress are reduced 85MPa by increasing the ambient temperature.The maximum temperature of the laser surface modification layer decreased from 1956?to 1428?with the laser moving speed increased from 7.5mm/s to 40 mm/s,the equivalent stress of the modified layer can be reduced from 0.94GPa to 0.39GPa by the laser surface modification and following isothermal transformation.