Study On Microstructure And Properties Of Cobalt-based Composite Powder Plasma-arc Surfacing Layers Reinforcement By SiC

The sealing surface of the valve is easily damaged because it often works in bad working conditions,which brings great economic losses to the state or enterprises.Cobalt based alloys belong to cemented carbides good performance of corrosion resistance and high temperature performance.Considering this unique advantage,they are often overlaying welded on the surface of workpieces and form the hardfacing layer to enhance wear resistance and corrosion resisting property.Some strengthening elements are added to the cobalt based alloy surfacing layer in order to produce cobalt based alloys with better comprehensive properties.The principle is through the form of reinforcing phase or the change the pool crystallization mode for the improvement on the microstructure and meachanical properties.Since plasma arc surfacing has the characteristics of high efficiency,good appearance and easy to achieve automation,the research attention has gradually shifted from laser cladding Co base alloy to plasma surfacing technology in recent years.At present,a lot of research and remarkable achievements have been made in plasma surfacing cobalt based alloys in China.Meanwhile,the main adding elements and ceramic particles include Ti,W,Hf,Te,etc.,and VN,WC,SiC,etc.respectivly.SiC has the advantages of low price,easy to prepare and so on.It is an ideal strengthening material for cobalt based alloy surfacing.In this paper,the 304 stainless steel was selected as the matrix materia.Different fractions of SiC were added to the series of Co-Cr-W(Stellite 6),Co-Cr-Ni(Stellite F),Co-Mo-Cr(T-400)alloy powder.Then,surfacing layer produced by plasma arc surfacing were investigated.The addition of SiC affects the growth of the crystal and changes the microstructure and properties of the surfacing layer.After adding SiC to Stellite F,the area of surfacing layer can be roughly divided into planar grain zone,cellular grain zone,columnar dendritic zone,chaotic dendritic zone and surface fine grain zone.The shapes of each zone are relatively visible.When 6%SiC was added in alloy powder,the microstructure was regular and the dendrites can be found.While 12%SiC cause the coarsening of whole grains,the hot cracks even appeared on the surfacing layer.The surfacing layer consists of y-Co solid solution and the eutectic microstructure formed by the solid solution of y-Co and and carbides(M=Cr,W,Fe).After SiC particles were added to Stellite 6 alloy,fine grain appeared on the microstructure of the surfacing layer and the fusion zone is also visible.The fusion zone is roughly divided into the cellular grain zone,the columnar grain zone and the near-surface fine equiaxed grain zone.In the surfacing layer with 9%SiC,the diffused and vermicular shape was found in the near-surface surfacing layer and the matrix interface.This mircostructure may have an effect on the wear resistance of the surfacing layer.Moreover,it will enhance the impact resistance of the surfacing layer.When the SiC content was increased to 12%,the mircostructure has been coarse-grained seiously,and the surfacing layer is composed mainly of y-Co,Cr3CO5Si2 and several kinds of carbides.When 3%SiC was added to the T-400 alloy,the cellular grain region of the white band(the fusion zone)is relatively narrow and the grain growth directions mainly point towards to the outward surface though they are interlaced in some extent.A large number of massive structures appeared after the 6%SiC was added into surfacing layer.It was worth pointing out that the growth direction is not distinct any more.When the content of SiC increased to 9%,the surface of the interface is roughly the planar grain zone of the fusion zone,the cellular grain zone,the elongated columnar dendritic zone and the near surface fine crystal region.When 12%SiC is added,the range of the surface fine grain zone is narrow and the corrosion resistance of the area was not seriously reduced and the grain coarsening was not found in the heat affected zone.The surfacing layer appears the eutectic mircostructure consists of main phase of y-Co solid solution and a variety of carbides and silicides,which plays an important role in the strengthening of the surfacing layer.T-400 alloy shows high hardness and high temperature wear resistance when 6%SiC added to the sufacing layer and formed massive structure.The addition of SiC into Stellite 6 has no significant effect on its properties.When 6%SiC was added to Stellite F powder,it also exhibits excellent normal temperature wear resistance.It is found that when SiC content reached 12%,which causes the coarse grain size due to the inhibition effect to the heat flow diffusion.Therefore,microstructure became uneven and mechanical properties were deteriorated compared with its counterparts.Among three kinds of cobalt base alloy,the effect of SiC on T-400 surfacing layer is significant,the hardness and wear resistance were improved more significant than that of other two groups.The surfacing layer using Stellite F powder with 6%SiC also exhibits excellent wear resistance at room temperature though Stellite F shows the worst wear resistance and hardness of three kinds of alloy powder.