Microstructural Evolution And Wear Resistance Of Fe-Cr-C-X Hardfacing Alloy

With high efficiency and low price, hardfacing technology is one of the coretechnology of Green Remanufacturing. By hardfacing technology, the work-pieces withlarge size, high additional value and special performance requirements, such as wearresistance, heat resistance and corrosion resistance, can be restored and remanufactured,which in turn effectively prolong the service life of the work-pieces. Therefore, it issignificant for the wide application of hardfacing technology. Fe-Cr-C alloy, with excellentwear resistance, contains a large number of M （M=Cr, Fe）7C3carbides. However, fortraditional Fe-Cr-C alloy, it is the large and block M₇C₃carbide, which causes the carbidesdesquamated. Therefore, the application of the alloy has been limited widely in hardfacingfield.Flux-cored wires of Fe-Cr-C alloy with the self-shield ability were prepared in thiswork. On the basis of analyzing the microstructure-properties of the Fe-Cr-C hardfacingalloy, the composition of the alloy was designed and the recipe of the flux-cored wires wasoptimized. By adding strong carbide forming elements Ti, Nb and V as well as rare earth（RE） oxides La2O3and CeO2, the shape, size and distribution of M₇C₃carbide in Fe-Cr-Calloy were investigated. Meanwhile, by the aid of Bramfitt two-dimensional lattice misfitand first-principles calculation, the heterogeneous nucleation of M₇C₃carbide wastheoretical analyzed, the doped phase inducing heterogeneous nucleation of M₇C₃carbidewas discussed and the refinement mechanism of M₇C₃carbide was explained.The microstructure of Fe-Cr-C hardfacing alloy consists of M₇C₃carbide, martensite（α-Fe） and austenite （γ-Fe）. With increased C content, the microstructure can be changedfrom hypoeutecitc to eutectic, and even hypereutectic ones. The eutectic reaction ofFe-27Cr-[1.5-5.5]C （wt.%） occurs at3.1wt.%C. When C content in the alloy is larger than3.1wt.%, the hypereutectic reaction occurrs at the initial stage of solidification. Withincreased C content, the amount and dimension of M₇C₃carbide are increased, meanwhile,the hardness and adhesive wear resistance of the hardfacing alloy can be improved.However, the plastic deformation of the hardfacing alloy in plough area of the wear scratch becomes weaker, which causes that the M₇C₃carbides easily desquamate from thematrix.During hardfacing process, M₇C₃carbides show the preferred orientationcharacteristic, and grow along the direction of welding heat flux density. The shape ofprimary M₇C₃carbide is polygonal rod, while that of eutectic M₇C₃carbide is strip orneedle. The hardness and Young modulus of M₇C₃carbide in preferred orientation sectionare21.2±0.3GPa and291±3GPa, and those in non-preferred orientation section are20.1±0.3GPa and267±3GPa.When alloy elements M（M=Ti, Nb, V） were added into Fe-Cr-C hardfacing alloy, theprimary TiC, NbC and secondary VC carbides can be formed, which in turn refine themicrostructure of Fe-Cr-C hardfacing alloy. According to Bramfitt’s two-dimensionallattice misfit theory, the misfit of （110）TiCand （010）Cr₇C₃is δ=9.3%, so the effectiveness ismiddle. Therefore, the TiC carbide can be as heterogeneous nuclei of the Cr₇C₃, and refinethe primary Cr₇C₃carbide. Moreover, the concentration of C atom in the molten pool isconsumed by the formed MC carbide, which accelerates the refinement of Cr₇C₃carbidein turn. However, when the excessive alloy elements M were added, the microstructure ofthe hardfacing alloy changes from hypereutectic one to hypoeutectic one, and the wearresistance of the alloy can be decreased. The optimum amount of Ti in Fe-16Cr-3.8Chardfacing alloy is0.63wt.%.When RE oxides La2O3and CeO2were added, they participate in the metallurgicalreaction of Fe-Cr-C hardfacing alloy. The formed RE compounds remain in M₇C₃carbideor at the boundary of M₇C₃carbide and austenite, which in turn play a role in deoxidationand desulfurization to the molten pool. Moreover, the primary M₇C₃carbide can be refinedand the wear resistance of the hardfacing alloy can be improved by the added RE oxides.However, when the excessive RE oxides were added, the refinement of M₇C₃carbide issignificantly weakened, and even coarsened. The optimum amounts of RE oxides inFe-25Cr-5C hardfacing alloy are4.0wt.%La2O3, and2.0wt.%CeO2.With the same condition, the doped Cr atoms contribute to the stability of Fe7-xCrxC3multiple carbides. The orthorhombic Cr₇C₃carbide can be preferentially formed at theinitial stage of solidification, for its formation energy is lower than that of hexagonal one. With increased Cr content, the hardness of Fe_7-xCr_xC₃multiple carbides increases, and thatof Fe₄Cr₃C₃is the maximum. Cr-C-Ti covalent chains can be observed at the two kind ofTiC/Cr₇C₃interface. TiC（100） interface promotes the heterogeneous nucleation of Cr₇C₃carbide, which shows a stronger grain refinement ability than that of TiC（110） interface.The interface of Fe₃Cr₄C₃/LaAlO₃can be divided into LaO and AlO₂terminations.LaO-terminated interface, with larger work of adhesion, smaller interfacial separation andinterfacial energy, is favorable for primary carbide to carry out heterogeneous nucleationon LaAlO3particle surface.