IRON BORIDE (Fe 2B) ALLOY POWDER AS AN ALTERNATIVE TO THERMOCHEMICAL BORIDING

Abstract

This invention relates to a coating alloy, based on Iron Boride (Fe2B), a powder developed for bonding the surfaces of steels. Coatings made with the powder of this invention, using methods such as fusion or thermal spraying, provide an increase in surface hardness, wear resistance, and corrosion resistance of metallic materials, especially steels, and therefore represent a significant alternative to bonding through thermochemical methods. The alloy powder developed in this invention essentially contains iron (Fe) and boron (B) elements, with a boron (B) content between 7.1 % and 10% by weight.

Description

[0001] IRON BORIDE (Fe2B) ALLOY POWDER AS AN ALTERNATIVE TO THERMOCHEMICAL BORIDING

[0002] TECHNICAL FIELD

[0003] This invention relates to a coating alloy based on Iron Boride (Fe2B) powder, developed for boron coating the surfaces of steels. Coatings made with the powder of the invention by methods based on melting or thermal spraying provide an important alternative to thermochemical boron coating by increasing the surface hardness, wear, and corrosion resistance of metallic materials, particularly steels. The alloy powder realized with this invention primarily contains iron (Fe) and boron (B) elements, with boron (B) content ranging from 7.1% to 10% by weight.

[0004] PRIOR ART

[0005] Surface treatments are employed to enhance the mechanical and tribological properties of materials - including hardness, corrosion, fatigue, wear, thermal, and impact resistance - and to decrease production expenses. Boosting resistance to wear and corrosion is crucial, as these issues lead to significant yearly financial losses. Furthermore, products damaged by corrosion and wear are often not recyclable. Therefore, applying coatings that withstand both environmental and operational conditions is imperative.

[0006] Interconnected machine components often suffer surface damage due to wear caused by the interaction between contacting surfaces. The extent of this surface damage is influenced by material mechanical properties, applied loads, surface roughness, interfacial chemistry of contacting surfaces, and other factors. In numerous applications, wear serves as a life-limiting factor for the components involved, making wear reduction essential for their prolonged operation. Wear can be reduced by enhancing the hardness of the contacting surfaces

[0007] Corrosion is known as the loss of material from metallic surfaces due to chemical or electrochemical reactions with their environment. Corrosion poses a significant problem in many different sectors. Major material losses occur due to corrosion in industries such as construction, automotive, maritime, energy production, and chemical processing. Preventing corrosion is essential to extend the lifespan of materials, ensure safety, and reduce economic losses. One important method of preventing corrosion is coating the surface with corrosion-resistant materials. In practical applications, various coating alloys are available, and these are applied to surfaces using different methods

[0008] Thermochemical processing refers to a range of treatments used to modify the surface properties of a material and increase its durability. These treatments create chemical and structural changes on the surface through the interaction of heat and chemicals. The fundamental principle of thermochemical treatments is the diffusion of a chemical applied to the surface at high temperatures into the substrate material. This diffusion process forms new chemical compounds and alters the microstructural properties on the surface of the substrate material. The processing temperature determines the rate of diffusion and the depth of the thermochemical treatment. Higher temperatures result in faster diffusion and a deeper treated layer. The processing atmosphere influences the chemical being diffused and the resulting surface properties. The atmosphere typically consists of protective gases, active gases, or salt baths. Thermochemical processing has a wide range of applications, including the automotive industry (for wear-prone parts such as gears, shafts, bearings, molds, and cutting tools), aerospace (for high-temperature and wearresistant parts such as aircraft engine components, aircraft frames, and rocket engines), the energy industry (for parts exposed to high-temperature and wear conditions such as turbines, pumps, and valves), and medical devices (for parts such as surgical instruments, implants, and prostheses).

[0009] Thermochemical bonding is a thermochemical treatment employed to create highly wear-resistant surfaces, characterized by high hardness and a low coefficient of friction. It can be applied to a wide variety of materials, including most ferrous alloys and also non-ferrous metals and cermets. During the process, boron atoms diffuse into the surface of the workpiece, where they react with metallic elements such as iron, as well as nickel, cobalt, molybdenum, tungsten, and titanium, forming metal borides.

[0010] The thermochemical boriding of ferrous materials yields a surface compound layer, which can be either single-phase or dual-phase, consisting of FeB and / or Fe2B. Boron atoms, characterized by their relatively small size and high mobility, diffuse into the substrate material to form hard borides. For ferrous materials, the thermochemical bonding treatment leads to the formation of a single-layer (Fe2B) or a dual-layer (FeB + Fe2B), with a particular stoichiometry. The thickness of the boride layer is governed by temperature and processing time. Furthermore, the properties of this boride layer are dictated by the physical form of the boriding agent, the bonding temperature, the processing duration, and the chemical constitution of the material being bonded. Due to very limited solubility of boron in iron, a diffusion zone is nearly absent below the compound layer. The FeB phase is notably more brittle and harder than Fe2B, exhibiting a higher coefficient of thermal expansion. As a result, cracking is commonly observed in dual-phase boride layers. Hence, the sole formation of Fe2B is the desired outcome of thermochemical boriding. The Fe2B phase possesses both high hardness (1500-1800 HV) and a low coefficient of friction, making it well-suited for applications requiring wear and friction resistance.

[0011] Thermochemical boriding can be performed in solid, liquid, gaseous, or plasma media. The following provides a general overview of these methods. It’s essential to emphasize that the objective in all these methods is to achieve the formation of the Fe2B iron boride compound on the surface. In pack boriding, a type of solid boriding, parts are packed within a container and surrounded by a powder mixture typically containing boron carbide (B4C), silicon carbide (SiC), and KBF4 as an activator. The sealed container is then heated to the boriding temperature in a furnace. In paste boriding, a powder (B2O3 + SiC) mixed with a flux additive is used. The paste can be applied to the surface of the parts by brushing or spraying. Paste boriding is easy and allows for partial treatment. The pasted parts are processed in a conventional furnace under a protective atmosphere (argon, hydrogen, nitrogen). In recent years, a plasma paste boriding method has been developed. Liquid boriding involves immersing the components in a molten salt consisting of activators and reducing agents. A significant disadvantage of this method is that boriding cannot be performed if the bath temperature falls below 850°C due to the reduced fluidity of the molten borax. In fluidized bed boriding, boriding granules are used, which are typically fluidized by a flow of hydrogen or a hydrogen-nitrogen gas mixture. In electrolytic liquid boriding, the parts are placed in a salt bath. This process is carried out in an electrolytic salt bath where the parts act as the cathode and graphite electrodes in the bath act as the anode. Electrolytic liquid boriding allows for shorter processing times when a high current density is applied, but it often produces a dualphase layer containing a large, and often undesired, amount of FeB.

[0012] The gas mixtures used in gas boriding are diborane (B2H6) and hydrogen (H2), or BF3, BCl3, BBr3, and hydrogen. All gaseous boriding environments are toxic or produce toxic, flammable, and corrosive components that are not easy to handle.

[0013] In conclusion, the thermochemical boriding process performed using known techniques is generally carried out at temperatures between approximately 700 and 1000 degrees Celsius, typically involving holding times of 1 to 12 hours. The thickness of the boride layer achieved on the surface varies between 20 microns and 200 microns, depending on the processing parameters and the chemical composition of the material being coated.

[0014] BRIEF DESCRIPTION OF THE INVENTION

[0015] This invention relates to an alloy powder for coating workpieces, specifically a powder that creates a hard alloy coating and, more particularly, one that forms the Fe2B compound on the surface. The iron (Fe) and boron (B) based powder of this invention creates a hard surface coating, particularly useful for wear-resistant steel applications, providing a significant alternative to thermochemical boronizing

[0016] The alloy coating produced by the claimed alloy powder can comprise, in addition to iron (Fe) and boron (B), chromium (Cr), nickel (Ni), carbon (C), manganese (Mn), silicon (Si), niobium (Nb), molybdenum (Mo), vanadium (V), and titanium (Ti). Generally, the chemical composition of the coating powder according to the present invention is formulated as follows by weight percentage: 7.1-10% boron (B), 0-5% chromium (Cr), 0-5% nickel (Ni), 0-0.5% carbon (C), 0-2% manganese (Mn), 0-2% silicon (Si), 0-5% niobium (Nb), 0-5% molybdenum (Mo), 0-5% vanadium (V), 0-5% titanium (Ti), and the balance iron (Fe).

[0017] The surface coating powder that is the subject of this invention can be manufactured to the desired particle size using established powder production techniques such as gas atomization, water atomization, and mechanical milling. The inventive iron (Fe) and boron (B) based powder can be applied to the surface with the desired thickness utilizing diverse coating methods including electric arc spraying, flame spraying, plasma spraying, high-velocity oxy-fuel (HVOF) spraying, cold spraying, plasma transferred arc (PTA) welding, tungsten inert gas (TIG) welding, and laser cladding.

[0018] One of the aims of the invention is to obtain an iron (Fe) - boron (B) alloy powder, primarily consisting of the single-phase Fe2B phase, for use in coating a workpiece, and to provide an alternative to coating methods performed with thermochemical boriding. Another aim of the invention is to provide a hard coating alloy powder to increase the material life against wear in steels that are subjected to wear. The way in which the intended aims of our invention are set forth and their details are explained in detail below.

[0019] MEANING OF FIGURES

[0020] Figure 1. A scanning electron microscope (SEM) image showing the powder produced through gas atomization

[0021] Figure 2. An optical microscope image of the microstructure observed in the coating, with the chemical composition provided in Table 2

[0022] DETAILED DESCRIPTION OF THE INVENTION

[0023] The coating powder that is the subject of this invention is an alloy composition designed for the creation of hard coatings on metallic surfaces, with a particular focus on steels. The hard coating powder is particularly suited for use as a surface coating on steels exposed to wear, due to its microstructure consisting entirely of Fe2B~type iron boride. The application of coatings utilizing this developed powder offers a viable alternative to thermochemical coating processes employed for boronizing steels.

[0024] In thermochemical boriding, the steel material to be bonded is heated to high temperatures of 700 - 1000 degrees Celsius and kept at these temperatures for long periods (such as 1-12 hours). As a result, a thin (20 - 200 micron) Fe2B phase forms on the steel surface. The need for thermochemical boriding to be carried out at high temperatures and for long durations increases the cost, both economically and by potentially degrading the properties of the substrate material. The iron (Fe) and boron (B) containing powder that is the subject of this invention provides a surface that is entirely Fe2B phase, just as in the thermochemical boriding process. In the iron (Fe) -boron (B) binary phase diagram, an alloy containing 8.8% boron (B) and 91.2% iron (Fe) by weight gives a 100% Fe2B phase. Based on this, the powder of the present invention primarily provides this content. Therefore, this powder can be used for coating on desired metallic surfaces, especially steels, using various coating methods based on thermal spraying and / or melting. Therefore, coating with the powder of this patent can be a significant alternative to thermochemical boriding. Instead of creating Fe2B on the surface by waiting for a long time at high temperatures, as in the thermochemical boriding method, the direct coating of a powder with the Fe2B structure on the surface allows for the coating to be performed without having to subject the material to high temperatures for long durations. Furthermore, the difficulties encountered in the boriding of non-ferrous metallic materials or the inability to boride them can be largely overcome by the use of the powder developed in this invention. In addition, the surface of steels used in nuclear power plants can be coated with this powder to provide neutron absorption. Also, there is no limitation on the upper limit of coating thickness with coatings made with this powder, making it possible to perform coatings at desired thicknesses. Compared to thermochemical boriding, coatings made with this powder are practical and economical, and therefore, they have the capacity to be used in applications and on parts where thermochemical boriding is used.

[0025] In summary, the advantages of our invention can be listed as follows:

[0026] 1. It will provide an alternative to the boriding process performed using thermochemical methods,

[0027] 2. It will allow for the boriding of surfaces economically and in a very short time, 3. The formation of the Fe2B compound on the surface will reduce the coefficient of friction and increase corrosion and high-temperature resistance,

[0028] 4. It will allow coatings to be applied at desired thicknesses.

[0029] The surface coating powder of the present invention can be produced in different powder sizes using known powder production methods. Gas atomization and mechanical milling can be given as examples of powder production methods. Powder sizes may vary according to the coating method to be applied. The developed powder can be used to coat surfaces using different methods. These coating methods include various techniques such as electric arc spray, flame spray, plasma spray, high-velocity oxy-fuel (HVOF), cold spray, plasma transferred arc (PTA), tungsten inert gas (TIG), and laser. The chemical composition of the hard coating powder according to the present invention, in weight percentage, is given in Table 1.

[0030]

[0031] Table 1. Chemical composition of the surface coating powder described in this invention

[0032] The chemical composition of a non-limiting hard coating alloy made with the hard coating powder of the present invention is given in Table 2.

[0033]

[0034] Table 2. Chemical composition of the coating produced using the inventive powder. The hard coating applied to AISI 1020 steel using the laser melting method, and whose analysis is presented in Table 2, has been measured for hardness using a Vickers hardness tester. The minimum and maximum hardness values are provided in Table 3. As these hardness values demonstrate, the coating hardness is considerably higher than the substrate material’s hardness, with a nearly 9,6-fold increase when considering the maximum value. This significant increase in hardness is attributed to the presence of the well-known high-hardness Fe2B phase.

[0035]

[0036] Table 3. Minimum and maximum hardness values for coatings whose chemical composition is listed in Table 2

[0037] The most important components of the hard coating alloy are the elements iron (Fe) and boron (B). In the hard coating powder, iron (Fe) and boron (B) are in the form of the Fe2B compound. The melting temperature of this powder is approximately 1390 degrees. The amount of boron included in the hard coating powder of the present invention needs to be 8.8% by weight in order for the Fe2B compound to form completely in the microstructure of the powder.

[0038] Modifications to the discussed arrangements, as well as other arrangements of the invention, can be implemented without departing from the spirit and scope of the invention. All such modifications are intended to be within the scope of the invention, and it should be understood that the foregoing description is for illustrative purposes only, and not as a limitation. The claims define the scope of the present invention.

Claims

CLAIMS1. It is a coating powder for enhancing the wear resistance of steel including a carbon (C) content of 0 to 0.5%, a chromium (Cr) content of 0 to 5%, a nickel (Ni) content of 0 to 5%, a manganese (Mn) content of 0 to 2%, a silicon (Si) content of 0 to 2%, a niobium (Nb) content of 0 to 5%, a molybdenum (Mo) content of 0 to 5%, a vanadium (V) content of 0 to 5%, and a titanium (Ti) content of 0 to 5% by weight, characterized by a boron (B) content of 8.8% by weight, with the remainder being iron (Fe).

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