| At present,a new research direction for the development of cast iron hardfacing materials is to develop slag-free self-shielded flux cored wires.Hardfacing layers are protected by the alloy elements and gas-forming agents in the slag-free self-shielded flux cored wires without slag-forming agents and shielding gas.Hardfacing efficiency is improved and the cost is reduced.Hypereutectic high chromium cast iron hardfacing materials have high abrasion resistance because of a large amount of primary M7C3 carbides.But,large primary carbides caused many crack initiation sites.Besides,the large primary carbides were easy to spall during the wear process.These factors are detrimental to the tribological performances of hardfacing alloys.On the other hand,the non-uniform distribution of primary carbides is also detrimental to tribological performances of hard facing alloys.In this paper,high carbon ferro-chrome,graphite,ferro-boron,aluminum-magnesium alloy,electrolytic manganese,ferro-silicon and low carbon steel strip were used to prepare self-shielded flux cored wires.Five types of self-shielded flux cored wires were prepared by adding different contents of CeO2 nano-additives into the core powders.Five types of hardfacing alloys were prepared by depositing the above mentioned wires under the same hardfacing parameters.Effect of nano-additives on the microstructure,mechanical properties,tribological properties and corrosion resistance of the hardfacing alloys were analyzed.X-ray diffraction results showed that the hardfacing alloys were composed of M7C3 carbide,martensite and retained austenite.The position and intensity of a few diffraction peaks of M7C3 changed with the increase of CeO2 nano-additives,implying that nano-additives influenced the growth orientation of carbide.Nano-additives increased the volume fraction of primary carbide.The volume fraction of primary carbide in the hardfacing alloy with 0.288 wt.% nano-additives was 22.2%,which increased 5.1% compared to that in the hardfacing alloy without nano-additives.Statistical analysis of primary carbides indicated that nano-additives refined the grain size of primary carbide.Nano-additives also improved the distribution of primary carbide.The area of the primary carbide in the hardfacing alloy without nano-additives was 828.75?m2.But the areas of the primary carbides in the hardfacing alloys with 0.288 wt.% and 0.432 wt.% nano-additives were 406.25?m2 and 506.58?m2,which decreased by 51% and 38.9% compared to that in the hardfacing alloy without nano-additives.The analysis of mechanical properties of hardfacing alloys indicated that Ce O 2 nano-additives increased their macro-hardness and bending strength.The hardness of the hardfacing alloy without nano-additives was HRC61.5.But the hardness of the hardfacing alloy with 0.288% nano-additives reached HRC64.5.The bending strength of the hardfacing alloy without nano-additives was 589.77 MPa.The bending strengths of the hardfacing alloys with 0.144 wt.%,0.288 wt.%,0.432 wt.% and 0.576 wt.% nano-additives increased by 37.2%,46.4%,32.4% and 39.7% compared to that of the hardfacing alloy without nano-additives.Rare earth compounds including Ce3NS3,CeAl11O18 and Ce10OS14 formed in the hardfacing alloys with nano-additives accompanied by reduction of Mn S,leading to the improvement of bending strength.Nano-additives purified the grain boundaries.The fracture toughness of the primary carbide in the hardfacing alloy without nano-additives was 6.11MPa?m.But the fracture toughnesses of the primary carbides in the hardfacing alloys with 0.288 wt.% and 0.432 wt.% nano-additives increased by 12% and 52% compared to that in the hardfacing alloy without nano-additives.The wear test results showed that CeO2 nano-additives improved the tribological properties of hardfacing alloys.The dry sliding wear tests showed that the wear rates of hardfacing alloys decreased firstly and then increased with the increase of nano-additives under fixed normal loads when the counterparts were Si 3N4 and YG13 cemented carbide balls.The wear rates of hardfacing alloys decreased and tended to be steady with the increase of normal loads when the counterparts were Si3N4 balls.And the main wear mechanisms were micro-cutting and oxidative wear when the normal loads were 5N and 30 N.The wear rates of the hardfacing alloys sliding against YG13 cemented carbide balls were lower than those of the hardfacing alloys sliding against Si3N4 balls.And the main wear mechanisms were adhesion wear and oxidative wear when the normal loads were 10 N and 30 N.The erosion volume decreased firstly and then increased with the increase of nano-additives under normal impingement angle.The erosion volume of the hardfacing alloy with 0.432 wt.% nano-additives was the minimum.The main erosion mechanism was deformation damage.The high volume fraction and fracture toughness of primary carbide were favorable to the improvement of erosion resistance.CeO2 nano-additives increased the overall work function of hardfacing alloys.The overall work function of the hardfacing alloy without nano-additives was about 4.5e V.But the overall work functions of the hardfacing alloys with nano-additives exceeded 4.6e V.When the content of nano-additives was less than 0.576 wt.%,it changed the microstructure of hardfacing alloys obviously,which influenced the overall work function and mechanical properties simultaneously.The overall work function and mechanical properties were correlated with each other.The overall work function could reflect the change in tribological properties of hardfacing alloys.When the content of nano-additives was 0.576 wt.%,the main determinant factors of overall work function and mechanical properties were different.The overall work function could not reflect the variation of mechanical properties and tribological properties.The analysis of the polarization curves showed that nano-additives increased the corrosion potential of hardfacing alloys in near-neutral,acid and alkaline 3.5wt.% NaCl solutions.The hardfacing alloy with 0.288 wt.% nano-additives had the highest corrosion potential.The increase in volume fraction of primary and eutectic carbides increased the corrosion potential.The analysis of impedance spectra indicated that the hardfacing alloy with 0.288 wt.% nano-additives had better corrosion resistance in acid 3.5wt.% NaCl solution.But the hardfacing alloy with 0.576 wt.% nano-additives had better corrosion resistance in the near-neutral and alkaline 3.5wt.% NaCl solutions. |
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