Research On Microstructure And Properties Of Stellite6 Coating By Laser Cladding On 17-4PH Stainless Steel

The final stage blade of the steam turbine occur forms of failures,such as corrosion,crack,wear and fatigue fracture due to the effects of water droplets erosion and alternating loads.To solve these problems,the Stellite6 alloy coating was prepared on the surface of 17-4PH stainless steel by laser cladding,and then the cladded plates were heat treated at510℃and shot peened.The microstructures and properties of laser cladded Stellite6 coating were studied.In addition,the effects of heat treatment and shot peening were investigated.The microstructure,phases and elements distribution of Stellite6coating were studied by modern material analysis methods.The results show that 2800W,8mm/s and 13.56g/min are the optimal parameters for laser cladding.The microstructures in stellite6 coating are plane,cellular and columnar,dendritic,equiaxed crystals.The cellular and dendritic crystals are formed ofγ-Co solid solution with FCC structure,and inter-dendritic is the eutectic structure including some carbides such as M₂₃C₆,Cr₇C₃,CoC_x and W₂C.The heat treatment and shot peening almost have no effects on the phases and elements distribution.With unreasonable parameters,the coating layer is prone to occur crack defects.The hardness distribution,wear resistance and tensile properties of Stellite6 coating were studied.The results show that the microhardness of coating is about 1.5 times of the substrate.The softening phenomenon occurs in the overlapping regions between the layers and channels and HAZ areas due to the effect of laser tempering.The heat treatment reduces the hardness,however shot peening has hardening effect.The friction coefficient of Stellite6 coating is lower and more stable,its weight reduction is just about 10.5%of the substrate.The coating and substrate show abrasive wear mechanism and adhesive wear mechanism.The tensile strength of the substrate is 950MPa,belongs to the ductile fracture mode.The tensile strength of cladded specimen is 721MPa with poor plasticity.The heat treatment and shot peening can improve the strength and plasticity.The coating side belongs to the along dendrites and quasi-cleavage fracture mode,the substrate side belongs to the ductile fracture mode.The corrosion resistance properties,stress states and high cycle fatigue properties of Stellite6 coatings were analyzed.The results show that the corrosion current density of the coating and substrate are0.834nA/cm² and 8.534nA/cm² respectively.The corrosion resistance property of the coating is about 10 times higher than the substrate in3.5wt%NaCl solution.The residual stresses of the longitudinal and transverse directions in the HAZ are both tensile stress in the cladded condition.The heat treatment can reduce the residual stresses by one third,and shot peening can make the surface in a high compressive condition.The fatigue strength of cladded specimen is 380MPa,which can be increased to 400MPa and 420MPa with the heat treatment and shot peening.The sources of fatigue crack in cladded specimens usually occurred at the corner or surface of the substrate side.The source region has the characteristics of ductile and quasi-cleavage fracture.In crack propagation region,typical fatigue characteristics such as fatigue striation,secondary crack,fatigue steps and type pattern can be observed.The instantaneous fracture region shows ductile fracture characteristics in the substrate side,and along dendritic ductile fracture and quasi-cleavage brittle fracture characteristics in the coating side.