The process performance of the crack-free surfacing welding rod uses Ni80Cr20 type electric furnace wire as the core wire, and the outer coating is a metal-graphite type flux. Although no ore powder (such as marble, rutile, and mica) is added to the flux to stabilize the arc, slag, and gas generation, the addition of a large amount of flake graphite (C) to the flux plays a lubricating and plasticizing role during the pressing of the welding rod, and during welding, most of the C is oxidized to generate CO (or CO2) gas, which displaces the air in the arc column area and protects the molten metal. In addition, the flux of this welding rod also contains a certain amount of ferrosilicon, ferrobore, and other metal alloy powders. Under the action of the welding arc heat, B and Si in the flux have a high affinity for oxygen and undergo strong oxidation-reduction reactions during droplet transition or in the molten pool to generate B and Si oxides, which react with other metal oxides to form a low-melting-point borosilicate (glassy) complex compound that encapsulates the molten metal droplets, protects the smooth transition of the alloy, and floats to the surface of the molten pool metal to form an extremely thin and uniform protective film under the strong stirring action of the molten pool, preventing the infiltration of harmful gases in the air.

Because the flux formula does not contain any ore powder, the welding smoke and dust is small, the smoke and dust contains very few gases harmful to the human body, and there is almost no slag on the weld after welding, so there is no problem of difficult slag removal after welding, as is the case with general (slagged) welding rods for surfacing nickel-chromium-boron-silicon alloys. The welding rod flux is almost entirely composed of ferroalloy powder, without the volume percentage of ore powder, and there is no loss of alloy transition to the slag after welding. Therefore, it can be designed to be much thinner than that of ordinary conventional (slagged) surfacing welding rods (core wire diameter 4.0 mm, flux outer diameter 7.8 mm), while the melting factor of the welding rod is larger than that of ordinary conventional (slagged) surfacing welding rods, thus solving to a certain extent the problem of the tail of the welding rod turning red and the flux cracking due to the use of a larger welding current when making nickel-chromium-boron-silicon alloys into ordinary conventional (slagged) surfacing welding rods. Compared with surfacing welding rods with ore powder, since the welding rod flux is composed of metal powder, less melting heat is required to melt the ore powder during welding. At the same time, B and Si elements release a large amount of reaction heat during high-temperature reactions with other alloy elements, so the melting coefficient of the welding rod is increased, and except for a small part of the loss due to evaporation, oxidation and spatter during welding, there is no loss of alloy penetration into the slag, and the melting coefficient of the welding rod is also larger than that of ordinary conventional surfacing welding rods. Chemical composition analysis of surfacing metal Continuous surfacing of 8 layers on a 80mm60mm Q235 steel part. After aging treatment to reduce hardness, the surface oxides are removed with a grinding wheel and polished, then clamped on a CA6140 lathe. The speed is set to a low gear at first, and a cemented carbide cutter is used to remove the surface layer of the sample. Then, a clean piece of paper is placed under the tool, and the sample is cut. After repeated adjustments, the optimal chemical composition of the surfacing metal is determined. The effect of various alloy elements on the hardness of the surfacing metal was studied. Initially, it was attempted to improve the performance of the surfacing metal by adding a small amount of various alloy elements. The experimental results showed that as the types of alloys increased in the surfacing metal, the hardness of the surfacing metal initially showed an upward trend. After the total content and types of metals reached a certain value, the hardness of the surfacing metal decreased, and the addition of some alloys also affected the crack resistance of the surfacing metal. To investigate the influence of various alloy elements on the performance of surfacing metal, the study preliminarily explored the influence of the types and amounts of various alloy elements in the welding rod flux formula on the hardness and other properties of the surfacing metal by increasing or decreasing them. The results show that as the Ni content in the welding rod increases, the hardness of the surfacing metal shows a downward trend, and the tendency for metal cracking increases; when the C content increases, the hardness initially increases, when the C content in the alloy exceeds about 1.2 (mass fraction), the crack resistance of the surfacing metal Crack resistance is an important indicator of the wear resistance of surfacing metal. Due to the austenite matrix of the surfacing metal, the large coefficient of linear expansion of the alloy, and its poor plasticity, the crack resistance is poor, often manifested as high-temperature plastic deformation cracks. To improve the crack resistance of the surfacing metal, the content of elements such as C, B, and Si in the alloy should be controlled, while trace rare earth elements and a certain amount of Mo are beneficial to the crack resistance of the surfacing metal. In addition, in the specific surfacing process, except for some large workpieces, complex-shaped workpieces, or thick-plate workpieces that need to be preheated to 200-300, general workpieces can be directly welded or slightly preheated to control crack generation. The effect of heat treatment process on the microstructure of surfacing metal The relationship between the surfacing metal aging treatment at different temperatures and times and the hardness. It can be seen that within a certain temperature and time range, the coarse austenite matrix gradually disappears, and a large number of intermediate phases precipitate, making the hardness increase (the increase is generally between 25HRC), and after reaching a certain temperature or time, the precipitates or two-phase particles will decompose and dissolve into the matrix (austenite), and the hardness decreases again, and as the aging temperature increases, the required time for aging shortens. From the change in the microstructure shown, it can be seen that the as-welded microstructure of the surfacing metal is extremely uneven (a). After aging treatment at a higher temperature (880°C), as the time increases, the microstructure of the surfacing metal becomes uniform, and the grains are refined within a certain time (b), and the hardness is improved, but there is a coarsening trend as the aging time increases, and the metal microstructure undergoes over-aging (c), and its hardness decreases. At the same time, from (b) and (d), it can be seen that after the developed surfacing metal undergoes aging stabilization treatment and then 680°C isothermal treatment for 24h, its microstructure is almost unchanged before and after the isothermal treatment, which indirectly reflects that the microstructure of the surfacing metal is stable when it is working in a high-temperature environment below 680°C for a long time. In addition, after aging treatment, due to the diffusion and recrystallization of alloy elements in the base metal side of the fusion zone, a certain transition appears in its microstructure, which greatly improves the performance of the fusion zone, enhances the bonding performance between the surfacing metal and the base metal, and effectively reduces the tendency of the surfacing metal to peel off during operation, greatly increasing the service life of the workpiece.

Adhesion wear data shows that the developed weld metal, in its as-welded state (sample B), has a slightly lower wear resistance at 600°C than the cobalt-based alloy (sample A) due to its lower hardness. However, after aging treatment (sample C), when the hardness of the weld metal approaches that of the cobalt-based alloy, its high-temperature wear resistance at 600°C is also quite close to that of the cobalt-based alloy. This indicates that wear resistance is closely related to hardness, and it also reflects the excellent high-temperature wear resistance of the weld metal. In addition, from the observation of the sample surface after high-temperature adhesion wear, the entire surface of the weld metal exhibits a colored oxide film (with an oxidation degree similar to that of the cobalt-based alloy sample), while the base metal portion is a black oxide film, reflecting the excellent high-temperature oxidation resistance of the weld metal, and its sample surface morphology (SEM) after high-temperature adhesion wear. The surface morphology (SEM) (SEM) of the weld metal after wear. Conclusion: Although the welding rod is not protected by slag during the welding process, the welding rod has good process performance and the weld quality is guaranteed due to the protection of self-generated gas and borosilicate film, etc. The influence of alloying elements on the performance of the weld metal is multifaceted. As the number of alloying element types in the weld metal increases, the relative amount of each element in the weld metal decreases, which will have an adverse effect on the hardness and crack resistance of the weld metal, but it may be beneficial for post-weld aging treatment. Ni and Cr are important elements for ensuring the high-temperature oxidation resistance and adhesion wear resistance of the weld metal, while B, Ti, V, and C are indispensable elements for ensuring the strength, red hardness, and wear resistance of the alloy. The combined effect of these alloying elements makes the weld metal possess similar high-temperature wear resistance and other comprehensive high-temperature properties to cobalt-based weld metals (cobalt-chromium-tungsten alloy). The as-welded microstructure of the weld metal is austenitic, containing a large number of strengthening phases such as carbides, borides, and intermetallic compounds. Its microstructure has poor high-temperature stability, and the weld metal should be subjected to 880°C high-temperature aging treatment after welding. After aging treatment, due to the precipitation of two-phase particles and recrystallization, the stability of the microstructure is significantly improved compared to the as-welded microstructure. The aged microstructure can remain stable under long-term operation at 680°C. After aging treatment, not only is the bonding strength between the weld metal and the substrate increased, and the tendency of weld metal spalling is reduced, but also its hardness is increased by 25 HRC, and the wear resistance is improved, increasing the average service life of the workpiece by about 1.3 times.