Microstructure And Properties Of Cobalt-based Alloy Plasma-arc Surfacing Layers Reinforcement By TiC

The surface of valves,drill pipe joints and continuous casting rollers is often damaged by various loads when they work under high temperature,high pressure or high corrosivity.Surfacing technology is to melt metals and place them on the surface of workpieces by electric welding or gas welding in order to achieve the purpose of surface strengthening.Cobalt-based alloys have good high temperature resistance,creep resistance and corrosion resistance,and can work in various working environments,so they have been widely used.but sometimes it is difficult to meet the worse working conditions by using cobalt-based alloy to strengthen the workpiece surface.TiC has the characteristics of high strength,high modulus and good heat resistance.In this paper,TiC and cobalt-based alloy are mixed as surfacing materials,and metal-based composite surfacing layer is prepared by plasma surfacing technology to improve the surface properties of the workpiece.Surface properties.Tribaloy 400,Stellite 6 and tellite F cobalt-based alloy powders were selected as cobalt-based alloy powders,which were mixed with TiC powders proportionally and surfacing on 304 stainless steel surface.By optimizing process parameters,a well-formed composite surfacing layer was obtained.The structure and properties of the surfacing layer were analyzed and tested.The effect of TiC addition on the structure and properties of the cobalt-based alloy surfacing layer was studied,and the best formula TiC addition was selected..The optimum technological parameters were determined by orthogonal test:T400 cobalt-based alloy surfacing parameters were ion gas 2L/min,welding current 105A and scanning speed 50mm/min;ST-6 cobalt-based alloy surfacing parameters were ion gas 2L/min,welding current 105A and scanning speed 60mm/min;SF cobalt-based alloy surfacing parameters were ion gas 2L/min,welding current 110A and scanning speed 65mm/min.Composite surfacing layers are well fused without defects such as pore and crack.With the increase of TiC content,the columnar crystals in T400 Co-based alloy composite surfacing layer gradually become coarse and disordered,and there are more unmelted carbides.For ST6+TiC series surfacing layer,when 25%and 30%of TiC content is added,TiC phase begins to appear in the surfacing layer,in which the unmelted TiC particles are round and the precipitated TiC is cross-shaped;when the amount of SF+TiC surfacing layer is small,the surfacing layer will be welded.The dendrites in the layer are more orderly and orderly.With the increase of TiC content,the dendrites become disorderly,and there are dispersed black unmelted or semi-melted TiC particles.The unmelted TiC particles in the surfacing layer hinder the grain growth,and the effect of arc stirring can refine the grain.The structure of composite surfacing layer is based on gamma-Co,and Cr2Fe14C,Co7Mo6,Co6Mo6Cx,Co2TiSi,M6C and other phases are formed.When TiC is added more,TiC particles are not melted or semi-melted in the surfacing layer.In addition,because Ti element is a strong carbide forming element,after adding TiC,supersaturated Ti prevents the combination of C and other elements,thus greatly reducing the types of carbides in the surfacing layer.The addition of TiC can improve the hardness of Co-based alloy composite surfacing layer.When TiC content is 30%,the Rockwell hardness of T400 and ST6 composite surfacing layer is the highest,which is 17.2 HRC and 25 HRC higher than that without TiC addition.When TiC content is 25%.the Rockwell hardness of SF cobalt-based alloy surfacing layer is 12 HRC higher than that without TiC addition.The effect of TiC addition on the normal temperature wear resistance of different series of Co-based alloy composite surfacing layers is different.Among the three series of surfacing layers,the best wear resistance is T400 + 10%TiC,ST6 + 20%TiC and SF + 25%TiC,which are 2.56 times,4.6 times and 9.31 times higher than that without adding.The high temperature wear test of ST6+TiC surfacing layer shows that the wear resistance of TiC-reinforced composite surfacing layer is much stronger than that of pure cobalt-based alloy surfacing layer.The high temperature wear resistance of ST6+20%TiC composite surfacing layer is the best,and its main wear modes are adhesion wear and oxidation wear.The corrosion resistance test shows that the corrosion resistance of T400+10%TiC composite surfacing layer is the best in the same series;the corrosion resistance of SF+25%TiC composite surfacing layer is the best in the same series;while for ST6 cobalt-based alloy surfacing layer,adding TiC makes its corrosion resistance worse.