Microstructure And Properties Of YG20/42CrMo Diffusion Welding Layer

Cemented carbide roll ring has the properties of high strength,high thermal fatigue resistance and high red heat resistance.Using it as the working surface of rolling steel roll can greatly improve the working efficiency of rolling high wire and bar.If steel is used to replace part of cemented carbide as the inner lining of the roll ring,the cost can be effectively reduced.The thermal expansion coefficients of cemented carbide and steel do not match.If the cemented carbide/steel is directly diffusion welded,thermal cracks are easily generated at the weld,resulting in desoldering.Therefore,an intermediate layer needs to be introduced to reduce residual stress and the formation of deleterious brittle phases.In this thesis,based on the thermal and kinetic analysis of element diffusion and phase formation,as well as the thermal expansion coefficient of the material system,Ni/Nb/Ni,Ni/V/Ni foil composite interlayers and Ni-Cr alloy interlayers are scientifically designed.The phase diffusion method was used to weld YG20 and 42CrMo steel,and the joint morphology,diffusion layer element distribution,phase formation mechanism and tensile properties were systematically studied.The main research contents of this thesis are as follows:（1）The Ni/Nb/Ni composite interlayer is used to connect YG20 and42CrMo steel by diffusion.Combined with the thermodynamic calculation and the microstructure analysis of the diffusion layer,the study shows that:（Ni,Co）solid solution and（γFe,Ni）solid solution are formed at the interface of YG20/Ni and Ni/42CrMo,and Nb Ni₃+Nb C,Nb₂C and Nb₇Ni₆phases.The large difference in the thermal expansion coefficients of Nb Ni3and Nb C leads to the easy peeling of Nb C from the Nb Ni₃ matrix,and as the connection temperature increases from 1150℃to 1200℃,more Nb C precipitation leads to the increase of voids,and the tensile strength of the joint decreases from 40 MPa up to 24 MPa,and all fail in the brittle mode in the Nb Ni₃+Nb C layer.（2）Ni/V/Ni composite interlayer was used to connect YG20 and42CrMo steel by diffusion.Microstructure analysis showed that（Ni,Co）and（γFe,Ni）solid solutions were formed at the interface of YG20/Ni and Ni/42CrMo.Ni₂V,Ni₂V₃ V₂C and V₃C₂ phases were formed at the Ni/V interface in turn.In the connection temperature range of 1000～1150℃,the strength of the joint increases first and then decreases with the increase of temperature.The maximum tensile strength of 240 MPa is obtained at1050℃,and the joint fails in the mixed fracture mode.The thicknesses of Ni₂V,Ni₂V₃ V₂C and V₃C₂ phases formed at the Ni/V/Ni interface gradually increased with the increase of temperature.In particular,the thermal expansion coefficients of V₂C and Ni₂V₃ are quite different,and cracks are easily formed in the V₂C and Ni₂V₃ layers during the cooling stage,resulting in the reduction of the joint strength and the joint failure at the Ni/V/Ni interface in a brittle mode.（3）The prepared Ni-Cr alloy intermediate layer is used to connect YG20 and 42CrMo steel.Combined with thermodynamic calculation and diffusion layer microstructure analysis,it is shown that（Ni,Co）and（γFe,Ni）solid solution are formed at the interface of YG20/Ni and Ni/42CrMo.A small amount of Cr₂₃C₆ phase was formed in the intermediate layer.In the connection temperature range of 1000℃～1100℃,the tensile strength of the joint increases gradually with the temperature,and the tensile strength of the joint increases from 156 MPa to 297 MPa.The Dahl effect caused the joints to fail in ductile mode at the Ni-Cr/42CrMo steel interface.