Deposit And Metallurgical Behavior Of Nitrogen-alloying Hardfacing Alloy

As a gap-atom element, nitrogen can improve many properties of steels throughcombining with other alloy elements, including hardness, toughness, creep resistance,wearing resistance and corrosion resistance. Using the nitrogen instead of the carbon, thewelding hot-crack resistance, wear resistance, corrosion resistance and high temperaturestability of hardfacing materials can be greatly improved. The hardfacing materials arealloyed through nitrogen replacing part of carbon, which has been used as a new way toimprove the comprehensive properties of hardfacing alloy.The behavior of nitrogen and the factors affecting nitrogen content in the martensitestainless hardfacing alloy had been studied systematically. The reasonable nitrogen contentof hardfacing alloy was obtained, and the alloying composition of the nitrogen-alloyhardfacing alloy was designed. The dissolution and release of nitrogen during welding ofhardfacing alloy was studied by submerged-arc welding using nitrogen-alloyinghardfacing flux-cored wire with addition of CrN. The maximal nitrogen content in Crl3steel is approximately 0.10%, which can up to 0.20% and has no porosity in it throughadding extremely a little quantities of Nb, V and Ti elements. Additions of Nb, V and Tican increase nitrogen solubility in the hardfacing alloy, which has two means: Firstly, theycan reduce activity coefficient of nitrogen in molten metal and then increase its solidsolubility. Secondly, these alloying elements as the most effective carbonitride formingelements added in harfacing alloy to enlarge nitrogen content. When the nitrogen contentof nitrogen-alloying hardfacing alloy is controlled within reasonable range from 0.10% to0.13%, the carbon content can be reduced to less than 0.20%.The self-shielded and submerged-arc nitrogen-alloying hardfacing flux-cored wireswith good welding performance had been developed successfully. The welding process ofself-shield hardfacing flux-cored wire was gone deep into analysis, and a new preparingapproach of nitrogen-alloying harfacing alloy had been created by making full use ofnatural nitrogen in air. The hardfacing alloy of self-shielded hardfacing flux-cored wirecould get 0.08% nitrogen content through adjusting slag system and effectivenitride-forming elements, which had no porosity in it. And a low cost of nitrogen-alloying self-shielded nitrogen-alloying hardfacing flux-cored wire was developed. Thenitrogen-alloying submerged-arc hardfacing flux-cored wire was made by addition ofnitride, and a kind of sintered flux which is adaptive to the hardfacing flux-cored wire wasalso developed. It has the slag series for MgO-CaF₂-Al₂O₃-SiO₂, and the alkalinity is1.8～2.0.The precipitation behavior of carbonitride in hardfacing alloy was systematicallyanalyzed, and the strengthening mechanism of it was also studied. It was revealed thatcarbonitride particles in the hardfacing alloy are complex M(C,N) distributing on thegrain boundary or matrix of the hardfacing alloy. These carbonitrides have dissimilar size,morphology and composition in different formation process of it. In as-welded condition,most of carbonitride are (Nb,V,Ti)(C,N) precipitate with characteristic cuboidal shape,which can readily precipitate from the hardfacing alloy with large size(1～3.5μm) as theywere formed already during solidification. At the temper temperature of 450～650℃, alarge number of carbonitride Nb(C,N) can be precipitated out with the size range from 5to 200nm. These fine carbonitride has two forms of bar and globular, which have a greatsecondary hardening effect on the matrix. The carbonitride in hardfaced alloy caneffectively refine the microstructure during welding and hinder the crystal boundarymigration by fixing the crystal boundary, which had a grain refinement and precipitationstrengthening effect on the hardfacing alloy. The carbonitride precipitates prevented theformation of chromium-rich phase at grain boundaries and intergranular chromiumdepletion, so the hardfacing alloy had good stability and corrosion resistance. The effect ofcarbonitride precipitates on the abrasive wear behaviour of hardfacing alloy was studied.The results showed that the carbonitride had an effectively protecting effect on thehardfacing alloy to keep the hardfacing alloy from wearing by the abrasion particles. Thehomogeneous distribution of very fine carbonitride could precipitate out during the tempertemperature of 500～600℃and improve markedly the wear resistance of hardfacing alloy.The microstructure of nitrogen-alloying hardfacing alloy was deeply analyzed. Theresult shows that the hardfacing alloy has three main phases of lath martensite,carbonitride and the residual austenite. It is found that the temper temperature has asignificant effect on the microstructure of hardfacing alloy, and the temperature of carbonitride precipitation was above that of the carbide precipitation. The carbonitride canprecipitate out during the temperature of 500～650℃and has a good stability at the hightemperature. Meanwhile, the carbide precipitated out below the temper temperature of 500℃and decompounded above 600℃. The hardness of hardfacing alloy decreased sharplywhen the carbide decompounded. The homogeneous distribution of very fine carbonitridecould improve markedly the hardness of hardfacing alloy, so the nitrogen-alloyinghardfacing alloy had a good softening resistance at high temperature tempering. The Ms ofnitrogen-alloying hardfacing alloy was established, and the calculation formula is listed:Ms/℃=550-330(w_C+0.86w_N-0.05)-35w_Mn-17w_Ni-12w_Cr-21w_Mo-10 w_Si-35(w_Nb+w_V+w_Ti).The high temperature wear resistance of nitrogen-alloying hardfacing alloy wastested under a high load, and compared with those of conventional carbon-alloyinghardfacing alloy. It is found that the high temperature wear mechanism of hardfacing alloyand 42CrMo steel was abrasive wear due to the effect of scale formation on the surface of42CrMo steel. The scale was sticking on the surface of hardfacing alloys and increasingthe abrasive wear damage. And the crack occured on the grain-boundary because of thehigh stress effect on the wear surface of metals, which decreased the metal-metal wearresistance of hardfacing alloy. The results showed that the nitrogen-alloying hardfacingalloy had more excellent high temperature wear property than that of carbon-alloyinghardfacing alloy due to the fine microstructure, good tenacious property and the effect of alarge number carbonitride homogeneously precipitated out during high temperaturewearing.