Method of applying protective coating to graphite electrode

Abstract

FIELD: coatings.SUBSTANCE: invention relates to a method for applying a protective coating to a graphite electrode. The surface of the graphitized electrode is cleaned and a hexagonal relief is formed on the surface of said electrode. Then the formed surface is heated to 150-170 °C using the arc metallization method with composite wire, a coating with a thickness of 0.25-0.4 mm is applied. The sheath of the composite wire consists of aluminium, and the core is made of nitrided ferrosilicon powder with a content of 3-7% of the total mass of the wire.EFFECT: consumption of graphite electrodes is reduced by forming a uniform, tightly bonded protective coating on their side surface, stabilizing the thermal operating mode of the side surface of the electrode.1 cl, 3 ex

Description

[0001] The invention relates to ferrous and non-ferrous metallurgy, namely to methods for protecting electrodes of electric arc furnaces and ladle furnace units from oxidation.

[0002] The metallurgical industry requires reliable and high-temperature-resistant materials. Graphite electrodes are indispensable in metallurgical plants, as they provide electrical energy input for processes involving extremely high temperatures.

[0003] The main cause of electrode failure is surface oxidation during melting of the charge and between melts. Graphite, a structural form of carbon, begins to actively interact with oxygen at temperatures as low as 700°C.

[0004] When using graphite electrodes in electric arc furnaces, intense erosive destruction of the side surfaces of the electrode is observed due to intense oxidation of graphite as a result of heating at high temperatures, which leads to a decrease in its diameter in the working part and an increase in the current density in this section of the electrode [Yachikov I.M., Kolokoltsev V.M. Reducing the consumption of graphite electrodes in arc furnaces with their forced cooling / Electrometallurgy, 2008, No. 8, pp. 23-26].

[0005] A protective coating is applied to graphite electrodes to protect against oxidation, stabilize the arc, and ensure the quality of the steel produced. It forms a slag layer that prevents the metal from coming into contact with air.

[0006] Methods are known for protecting graphite and carbon electrodes from oxidation by impregnating their surfaces with solutions of various components. For example, one method involves impregnating the electrodes with an aqueous solution of sodium polyphosphate with an average degree of polymerization of 4-6 at a concentration of 18-20% and maleic acid at a concentration of 0.05-0.1 wt.% at 90-100°C. (Author's Certificate No. 1699909, IPC С01В 31 / 02, published 23.12.1991). Using this solution increases the electrode's electrical resistance, which is especially unacceptable in the contact zone with current leads.

[0007] Another group includes methods in which an antioxidant protective coating is applied to the side surface of the electrodes by spraying, immersion or brushing, for example by spraying in the form of aqueous solutions of a nitride-borophosphate composition (Patent of the Republic of Belarus No. 11708, IPC C09K 15 / 00, C01B 31 / 00, published on 2009.04.03) or by brushing with the consistency of a liquid slurry of a composition based on high-alumina material with additives of carbon powder and orthophosphoric acid (Patent of the Russian Federation No. 2006189, IPC H05B 7 / 08 (1990.01), C25C 7 / 02, C25B 11 / 12, published on 1994.01.15).

[0008] There are known methods of strengthening the surface of electrodes by metallization - applying a coating by spraying aluminum (Kostareva T.V. et al. Electrodes for melting cast iron. Foundry production. - 1981. - 7. - P. 29) or ferrosilicon (Kablukovsky A.F., Molchanov A.E., Kablukovskaya M.A. Brief Handbook of an Electric Steelmaker. Moscow: Metallurgy, 1994, P. 80).

[0009] A known method for applying a heat-resistant coating based on iron aluminide to the surface of products operating under conditions of high-temperature gas corrosion, adopted as the closest analogue. The method involves cleaning the surface of the product, heating it to 200-250°C and applying the coating. The coating, 0.1-1.6 mm thick, is applied by arc metallization using a flux-cored wire consisting of a steel sheath and a core made of a charge containing, by weight: 15-20% aluminum, iron - the rest. (Patent of the Russian Federation No. 2772342)

[0010] The main disadvantages of this method are the insufficient heat resistance of the proposed protective coating and the low electrical conductivity of the coating materials. Furthermore, the lack of an additional relief along which the applied coating droplets can mechanically adhere to the graphite surface results in low adhesion between the protective coating and the graphitized electrode surface.

[0011] The problem that the invention is aimed at solving is to increase the adhesion strength of the applied protective coating to the surface of the graphitized electrode, as well as to increase the resistance of graphitized electrodes to high-temperature oxidation under the operating conditions of electric arc furnaces and to reduce their consumption during the smelting of metals and alloys in arc furnaces and ladle furnace units while maintaining contact resistance at the points where the electrodes are attached in the units.

[0012] The technical result consists in reducing the consumption of graphite electrodes due to the formation of a uniform, tightly adhered protective coating on their side surface, stabilizing the thermal operating mode of the side surface of the electrode.

[0013] The technical problem is solved in that in the method of applying a protective coating to a graphite electrode, which includes cleaning the surface of the product, heating it and applying the coating by arc metallization using a wire consisting of a shell and a core, unlike the closest analogue, after cleaning the surface, a hexagonal relief is applied to the surface of the said electrode, the surface is heated to 150-170 ° C, a coating with a thickness of 0.25-0.4 mm is applied using a composite wire, the shell of which consists of aluminum, and the core is made of nitrided ferrosilicon powder with a content of 3-7% in the total mass of the wire.

[0014] The heat resistance of graphite electrodes for electric arc furnaces (EAF) and ladle furnaces (LAF) is the material's ability to withstand high temperatures without deformation or failure. This is important, as the electrodes supply electric current to the furnace chamber, where the arc heats the metal to its melting point.

[0015] In the proposed method for applying a protective coating to a graphite electrode, after cleaning its surface, a hexagonal relief is applied to the surface of said electrode, which is a system of intersecting longitudinal and circumferential grooves to form hexagonal cells.

[0016] The formation of a hexagonal relief on the surface of the graphite electrode ensures mechanical interlocking of the particles of the sprayed material in the relief cells, increases the contact area of ​​the coating with the graphite and promotes a more uniform distribution of the coating on the surface of the electrode.

[0017] This ensures high adhesion strength of the applied coating to the surface of the graphitized electrode, along which the mechanical engagement of the drops of the applied coating with the graphite surface occurs, which ultimately leads to an improvement in its protective properties when used in conditions of high-temperature gas corrosion and a reduction in their consumption when smelting metals and alloys in arc furnaces.

[0018] The coating is applied more uniformly on the surface of the hexagonal structure, which provides better protection of the welding arc and weld pool from oxygen and other gases.

[0019] Heating the surface of the graphite electrode before applying the protective coating to 150-170°C improves the adhesion of the coating to the surface, reduces the likelihood of pores and cracks in the coating, and also improves the electrical properties of the process.

[0020] Heating helps remove moisture and contaminants from the surface, which is critical to the quality of the coating, as the presence of moisture can lead to increased hydrogen evolution and brittleness of the coating.

[0021] The hot surface of the graphite electrode better adheres to the coating components. This ensures a tighter and stronger bond between the coating and the graphite rod.

[0022] Heating helps prevent cracks and other defects in the coating. The hot surface prevents rapid cooling of the coating, allowing it to crystallize properly and ensuring its uniformity.

[0023] The lower heating limit of 150°C corresponds to the temperature of particles during arc metallization.

[0024] Electric arc coating (EAD) is a process for applying a protective anti-corrosion coating by melting two conductive wires with an electric arc and spraying the droplets onto the surface of a part using compressed gas (nitrogen). The arc temperature reaches 5000-6000°C, but due to rapid spraying and particle transfer, the temperature of the part itself does not exceed 150°C, preventing deformation.

[0025] The result is a layer with high adhesion and low porosity, which significantly increases the service life of products by protecting them from corrosion.

[0026] To apply the protective coating, special materials are used that provide high adhesion to graphite.

[0027] Nitrided ferrosilicon (Fe-Si3N4) is known as an additive to improve strength and heat resistance.

[0028] It is impossible to obtain an alloy of aluminum and nitrided ferrosilicon, therefore, in the proposed invention, protective coatings based on composite wire with a sheath of aluminum and a core of nitrided ferrosilicon (Fe-Si3N4) with a content of 3-7% of the total mass of the wire are used to protect the electrodes.

[0029] During arc metallization of an aluminum alloy containing nitrided ferrosilicon, the following processes occur in the melt and at the boundary with the gas phase:

[0030] • oxidation of aluminum with the formation of a dense oxide film Al2O3;

[0031] • partial decomposition and preservation of the nitride phase Si3N4 from nitrided ferrosilicon, which forms dispersed nitride inclusions in the coating that are stable at high temperatures and prevent the growth of the oxide layer and the diffusion of oxygen.

[0032] Due to this combined structure, the coating has increased resistance to high-temperature oxidation compared to purely aluminum or ferrosilicon coatings, while significantly increasing the service life of graphite electrodes, protecting them from corrosion.

[0033] Producing a protective coating using a composite wire with an aluminum sheath and a nitrided ferrosilicon core, with a 3-7% ferrosilicon content in the total wire mass, also produces a high-quality protective coating. The presence of silicon nitride in the aluminum wire used for coating ensures the electrode operates at operating temperatures. Silicon nitride wets graphite well, resulting in a gas-impermeable film that increases the coating density and quality.

[0034] When the content of nitrided ferrosilicon in aluminum wire increases above 7%, excess silicon nitride is formed in the coating, which leads to partial destruction and cracking of the coating surface, and when the content of nitrided ferrosilicon is less than 3%, the resulting amount of silicon nitride does not provide the necessary gas impermeability of the electrode surface.

[0035] Experimental studies have shown that the optimal protective coating thickness is 0.25-0.4 mm. A coating of 0.25-0.4 mm thickness forms an electrically conductive layer, which improves electrode properties such as electrical conductivity and heat resistance. This occurs when using wire with the proposed composition: stable silicon nitrides form at the coating-base interface, providing high electrical conductivity and reducing the consumption of graphite electrodes.

[0036] Coatings thinner than 0.25 mm are highly porous and gas permeable, resulting in low heat resistance due to intense internal oxidation during high-temperature exposure, which is associated with active oxygen diffusion through the pores down to the protected substrate. At the same time, increasing the coating thickness beyond 0.4 mm leads to a decrease in adhesion strength and cracking due to increased residual stress in the coating, which also negatively impacts its heat resistance.

[0037] Example of the method implementation.

[0038] Graphite electrodes with a protective coating applied according to the proposed invention were tested in electric arc steel-smelting furnaces and ladle furnace units of one of the leading ferrous metallurgy enterprises in the Russian Federation.

[0039] The proposed method for preparing the electrode surface involved mechanical treatment to remove contaminants using a pneumatic nozzle head.

[0040] As a method for mechanical processing of the electrode surface to remove contaminants that can lead to local peeling of the coating, the proposed method is centerless processing of a graphite electrode on a roller rotator using a grinding attachment head mounted on an automated machine, equipped with diamond cups, while other abrasive and CBN tools, abrasive-polymer, metal brushes, etc. are allowed to be used to remove contaminants.

[0041] Then a hexagonal relief is formed, which is a system of intersecting longitudinal and circumferential grooves to form hexagonal cells, while the relief depth is 0.2-3.0 mm.

[0042] Strengthening deformation shaping of the surfaces of graphite electrodes is carried out using a rolling tool in the form of rollers, on the periphery of which a closed system of wedge-shaped indenters of a hexagonal structure is located.

[0043] Immediately before applying the protective coating, the surface of the graphite electrode is heated to 150-170°C, ensuring the removal of moisture and gases from the surface layer and improving the crystallization conditions of the sprayed material.

[0044] Heating is carried out by local gas heating with a head located on an automated unit.

[0045] A protective coating with a thickness of 0.25-0.4 mm is applied by electric arc metallization using a composite wire, the shell of which consists of aluminum, and the core of nitrided ferrosilicon powder with a content of 3-7% of the total mass of the wire.

[0046] The thickness of the adhesion sublayer and the applied coating was measured using a Constant K5 electromagnetic thickness gauge, and the depth of the relief irregularities was measured using a GM-250.01 MIC micrometer depth gauge.

[0047] The durability of electrodes without protective coatings and in accordance with various variants of the developed method was determined by the consumption of electrodes per ton of smelted metal.

[0048] According to the proposed method, the coating was applied to the prepared surface of the electrodes using several options, which are described below.

[0049] Option 1.

[0050] A graphite electrode 2.7 m long and 610 mm in diameter, after preliminary mechanical cleaning with a relief applied to a depth of 0.2-1.0 mm and heated to 150°C, is placed in an arc metallization unit.

[0051] For arc metallization, a composite wire with a diameter of 1.6 mm is used, consisting of an aluminum shell and a core of nitrided ferrosilicon in an amount of 5% of the total mass of the wire.

[0052] Spraying is carried out at an arc voltage of 300 V, a current of 150 A, a supply current frequency of 50 Hz, and a compressed air consumption of 0.4-0.6 m 3 / min with a pressure of 8 bar

[0053] The thickness of the resulting protective coating is 0.3 mm.

[0054] The actual reduction in electrode consumption was 22%

[0055] Option 2.

[0056] A graphite electrode 2.1 m long and 508 mm in diameter, after preliminary mechanical cleaning with a relief applied to a depth of 0.2-1.0 mm and heated to 170°C, is placed in an arc metallization unit.

[0057] For arc metallization, a composite wire with a diameter of 2.0 mm is used, consisting of an aluminum shell and a core of nitrided ferrosilicon in an amount of 7% of the total mass of the wire.

[0058] Spraying is carried out at an arc voltage of 320 V, a current of 100 A, a supply current frequency of 50 Hz, and a compressed air consumption of 0.4-0.6 m 3 / min.

[0059] The thickness of the resulting protective coating is 0.45 mm.

[0060] The actual reduction in electrode consumption was 13%

[0061] Option 3.

[0062] A graphite electrode 1.8 m long and 406 mm in diameter, after preliminary mechanical cleaning with a relief applied to a depth of 0.2-2.0 mm and heated to 150°C, is placed in an arc metallization unit.

[0063] For arc metallization, a composite wire with a diameter of 1.6 mm is used, consisting of an aluminum shell and a core of nitrided ferrosilicon in an amount of 3% of the total mass of the wire.

[0064] Spraying is carried out at an arc voltage of 280 V, a current of 200 A, a supply current frequency of 50 Hz, and a compressed air consumption of 0.4-0.6 m 3 / min.

[0065] The thickness of the resulting protective coating is 0.3 mm.

[0066] The actual reduction in electrode consumption was 7.5%.

[0067] The technical result of the proposed invention is a reduction in the consumption of graphite electrodes with a protective coating obtained by applying it to their surface using the developed method, compared to electrodes without a protective coating during metal smelting in electric arc steel-making furnaces and ladle furnace units.

Claims

A method for applying a protective coating to a graphite electrode, which includes cleaning the surface of the graphite electrode, heating it and applying the coating by arc metallization using a composite wire consisting of a shell and a core, characterized in that after cleaning said surface, a hexagonal relief is formed on the cleaned surface of said electrode, then the formed surface is heated to 150-170°C and said coating is applied with a thickness of 0.25-0.4 mm using a composite wire, the shell of which consists of aluminum, and the core of nitrided ferrosilicon powder with a content of 3-7% in the total mass of the wire.