Effect Of Nitrogen On Microstructure Evolution And Wear Resistance Of Typical Fe-Cr-C Hardfacing Alloy

As one of the key technologies on surface engineering,hardfacing technology has many advantages such as high efficiency and good economy.By hardfacing technology,the surface of the mechanical components can be strengthened and remanufactured,which not only improve the surface properties,but also can restore the initial shape and size of the components.Many hardfacing materials applied for different fields have developed very quickly with the development of hardfacing technology.In addition,as one of the most advanced hardfacing materials,flux-core wire is widely applied in modern industry production.In this work,self-shield flux cored wires of typical Fe-Cr-C hardfacing alloys were designed.By optimizing composition of the core powders,typical hardfacing alloys of Fe-13Cr-0.2C and Fe-25Cr-4C with different nitrogen contents were prepared.The chemical composition of the hardfacing alloys were calculated and optimized by ThermoCalc software.The microstructure and phase structure of the hardfacing alloys were observed and analyzed by optical microsope?OM?,field emission scanning electron microscope?FESEM?,X-ray diffraction?XRD?,energy dispersive X-ray spectrometry?EDS?,transmission electron microscope?TEM?and so on.The properties of hardfacing alloys with different nitrogen contents were tested by electromechanical universal testing machine,rockwell hardness tester,microhardness tester,nanoindentation tester,abrasion tester and so on.Moreover,the surface and interface behaviors between different phases of the hardfacing alloys were calculated by first-principle method,and the effect of nitrogen alloying on the microstructure and properties of different hardfacing alloys were investigated.The nitrogen instead of the carbon in hardfacing alloy Fe-13Cr-0.2C can reduce the fraction of austenite that is transformed from the reaction of peritectic,and then prior austenite grain can be refined.Moreover,the transformation from austenite to martensite is influenced by the refinement of prior austenite,and fine lath matensite is obtained.Compared with hardfacing alloy Fe-13Cr-0.2C without nitrogen content,the martensite lath width of the hardfacing alloy with nitrogen content is decreased from 0.35 ?m to 0.20 ?m.In addition,a computing method about calculation of the yield strength of hardfacing alloy Fe-13Cr-0.2C by modified Hall-Petch relationship was proposed,and the calculated results are matched with the experimental results very well.By analyzing the microstructure of hardfacing alloy Fe-25Cr-4C with different nitrogen content,it is found that the crystal structure of M₇C₃ carbide and the caronitride with titanium and niobium are orthorhombic structure and face centered cubic structure respectively.Furthermore,M23C6 carbide,high density dislocations and martensite lath were also found around the M₇C₃ carbides,which are benifical to reduce the hardness gradient between M₇C₃ carbides and ?-Fe,and also reduce the tendency of carbide spalled from the matrix.Meanwhile,stacking faults were also formed in M₇C₃ carbides and ?-Fe,which is important to improve the strength of matrix and the toughness of M₇C₃ carbides of the hardfacing alloys.According to the theory of lattice misfit proposed by Bramfitt,the lattice misfit ? between?110??Ti,Nb??C,N?and?100?M₇C₃ is 6.15 %,which prove that the?Ti,Nb??C,N?serve as heterogeneous nucleation sites for the primary M₇C₃ carbide is effective and the primary M₇C₃ carbide can be refined.Therefore,the phenomenon that the primary M₇C₃ carbide is refined with the increase of nitrogen in hardfacing alloy Fe-25Cr-4C-Ti-Nb is well explained by the theory of lattice misfit.In addition,the lattice misfit between LaAlO₃ and M₇C₃,TiC,TiN and?Ti,Nb??C,N?were also calculated respectively,which also prove that LaAlO₃ served as heterogeneous nucleation sites for the primary M₇C₃ carbide,TiC,TiN and?Ti,Nb??C,N?are also effective.During wear test,wear behaviors were different with the vary of carbide size.Large bulk carbides are easily cracked and fractured,and spalled from matrix during the abrasion process.According to the roughness of the worn surface and the continuous probability distribution of the surface roughness data,a proportional relationship between the carbide size and the shape parameter of ? and ? was proposed.Besides,the characteristic of carbides bended and spalled layer by layer were observed in the hypereutectic hardfacing alloy,which are matched well with the microstructure of stacking faults and lath martensite and so on.Stability and bounding among the interface atoms of M?C,N??111?/Fe3Cr4C3?0001?,La AlO3?100?/TiC?100?and LaAlO₃?111?/MX?111?were decipted by the behavior of adhesion work,surface energy,electronic structure and bounding and so on.Therefore,the efficiency of M?C,N?and LaAlO₃ served as heterogeneous nucleation sites for the primary M₇C₃ carbide,and LaAlO₃ serve as M?C,N?has been proved from many aspects.