Effect Of Processing Parameters On Microstructure And Properties Of Ni60 Coatings By Induction Remelting And Cooling

There are some problems in Ni60 spray coating,such as the microstructures of interface fusion zone are not uniform.unbalanced distribution of elements and insufficient bonding strength.In order to improve the inherent defect of the spray coating,supersonic atmospheric plasma spraying(SAPS)is used to prefabricate the Ni60 coating on the 45 steel matrix.The samples are subjected to induction remelting and forced cooling treatment under natural heat dissipation and surface insulation conditions.The microstructures,elements diffusion and properties of Ni60 remelt cooling coatings were analyzed using optical metallographic microscope,SEM(EDS),XRD,EPMA,macrohardness and microhardness test instrument and pin disc type friction and wear tester.Phase types and proportion of micro-interface strengthening phase of coating were analyzed by Jade 6.5、Origin 8.0、Image-pro plus 6.0 software.The results show that there are many holes and defects in the Ni60 spray coating.The microstructures of the Ni60 remelt coating are fine and uniform.It is interlaced by needle,rod and block,and formed metallurgical bonding zone with matrix.Under the condition of natural heat dissipation,the Ni60 remelt cooling coating is a dense directionally solidified structure,the pores disappeared basically,and it is composed of columnar dendrites and dark grey structures between dendrites.The main phases are λ-Ni,λ-(Fe,Ni),λ-(Fe,C),Cr2B,Cr7C3 and FeB;The element of Fe is uniformly distributed in the coating,and the forced cooling of one-way heat dissipation inhibits the remote diffusion of the element of Fe in the matrix.The average macrohardness of the coating surface is 33.4 HRC and the average microhardness of the cross section is 566.1 HV0.2.Under the condition of surface insulation,the structure of Ni60 remelt cooling coating has the tendency of transition to isometric crystal,massive crystal and needle,rod and bar.The structure is fine,and has the characteristics of casting and fusion bonding.The bonding interface between the coating and the matrix is close to the white band with planar characteristics on one side of the coating,and the light bright color compound layer which grows intermittently on the other side of the matrix.The main phases are λ-Ni,λ-(Fe,Ni),λ-(Fe,C),Cr2B,Cr23C6,and a small number of FeB phases were detected.Compared with the matrix,the content of Fe in the upper layer of the coating increased and fluctuated greatly,and the thermal insulation layer made the tendency of migration and aggregation for the element of Fe to the coating surface.The average macrohardness of the coating surface is 54.4 HRC and the average microhardness of the cross section is 659.2 HV0.2.Under the natural heat dissipation condition,when the power is 0.7kW,the two oriented columnar dendrites of Ni60 remelt cooling coating are in the form of divergent and convergent competitive growth with different growth ability.When the power is 2kW,there is a deviation angle between the columnar dendrite and the direction of heat flow of Ni60 remelt cooling coating,and there is a phenomenon of transgranular growth.When the power is 2.5kW,the oriented grains in the coating grew in the direction of heat flow,and the structure of the coating is a cellular dendrite which keeps well in the direction of heat flow.When the power is 3.3kW,the structure of the coating is the short cellular grain structure with larger grain size,and the coarse cellular crystals showed directional growth characteristics.When the power is 4.5kW,the coating structure consists of cellular crystal without regular orientation.Under the natural heat dissipation conditions,columnar dendrites in the Ni60 remelt cooling coating show convergent and divergent competitive growth form when the cooling water flow is 60L/h,and columnar crystal-isometric crystal transformation occurs in the upper part of the coating.The phenomenon of dendrite arm fusing is obvious in the coating when the cooling water flow is 70L/h.When the cooling water flow is 80L/h,the coating comprises equiaxed crystal region,columnar crystal region,dendritic crystal region and elongate cellular crystal region,the structure is more diversified.When the cooling water flow is 90L/h.the solidified grains of the coating are good columnar grains,and the grain size is smaller.When the cooling water flow is 10OL/h,the columnar crystal orientation of the coating is irregular,there are disordered short rod crystals,and the grains are finer.Under the same heating power and heating time,the dendrite structure is refined with the increase of cooling rate,and dendrite spacing decreases.The results of friction and wear experiments show that when the load is the same,the wear of the non-adiabatic Ni60 remelt cooling coating on the surface layer is lower than that of the adiabatic Ni60 remelt cooling coating on the surface layer at room temperature,and the directionally solidified coating has better wear resistance.The wear mechanism of surface unadiabatic Ni60 remelt cooling coating is slight scratch wear,slight abrasive wear and adhesive wear with smudging properties.The wear mechanism of surface adiabatic Ni60 remelt cooling coating is slight scratch wear.serious adhesive wear,and local area appear fatigue wear characteristics.