Effect Of Argon Protection On Laser Alloying Ni-based Cr₃C₂ Powder Coating

Low carbon steel Q235 which is widely used has good toughness and weldability,but its hardness and wear resistance are very poor.In this paper,a Ni-based Cr₃C₂ alloy layer with high bonding strength was prepared on the surface of Q235 steel by laser alloying,the effects of laser alloying process parameters,powder ratio and atmosphere protection on the alloy layer were studied.Thereby,an alloy layer superior in surface properties and quality is obtained,and the surface hardness and wear resistance of the substrate are improved.The surface morphology and microhardness of the alloy layer were studied by single-channel and multi-channel laser alloying.The results showed that the alloy layer obtained the best surface quality and performance when the laser power was 2800W,the scanning speed was 240 mm/min,the thickness of the prefabricated layer was 300 um,the powder ratio?Ni60:NiCr-Cr₃C₂?was 7:3,the flow rate of argon was 15L/min,the angle to the horizontal plane was 45 degrees,and the distance between the nozzle and the molten pool was 20 mm.The free surface curvature of the molten pool decreases with the increase of argon flow rate and the angle between nozzle and horizontal surface.The microstructures,elements and phases of the multichannel alloyed layer were analyzed respectively by metallographic microscope and so on.The results show that The Ni-based Cr₃C₂ alloy layer is mainly composed of"cross-shaped"equiaxed dendrites and a small amount of cellular and columnar dendrites.As the content of Cr₃C₂ increases,the number of"cross-shaped"dendrites increases and the grains are refined.Argon gas protection also has the effect of refining grains and making the grain size more uniform.The main element types in the alloy layer are Fe,Ni,Cr and C.No O element is found,which indicates that the argon gas protection has a good anti-oxidation effect;The phase is solid solution such as?-?Fe,Ni?,?-Fe,Ni-Cr-Fe,CrFe₄,and M₇C₃?M is Fe,Cr?type carbide,and no Cr₃C₂ phase is found.This is roughly the same as the phase simulated by the thermodynamic software JMatPro.In addition,the main defect forms of the alloy layer are compositional inhomogeneity and stress concentration,and improvement measures are proposed respectively.The properties of the alloy layer were studied by a microhardness tester and so on.The results show that within a certain range,the microhardness and wear resistance of the alloy layer enhance gradually with the increase of the NiCr-Cr₃C₂ content.When the NiCr-Cr₃C₂ is 30%,the microhardness of the alloy layer reaches the best,which is 2.45 times of the matrix,and the wear is also the lowest,which is 1/6 of the matrix.The variation of friction coefficient is small,and the wear type of the alloy layer is slight adhesive wear and abrasive wear.The surface hardness of the alloy layer under argon protection is slightly lower than that of the alloy layer in the air,but the former alloy layer has a more uniform hardness and a higher average hardness,which indicates that the gas protection has a more uniform effect on the alloy layer composition.Argon protection has no effect on the wear resistance of the alloy layer,but it is beneficial to improve the tensile strength and yield strength of the alloy layer.