Technical Field The invention relates to a powder boronizing agent developed to coat metal surfaces with a metal boride layer at high temperatures. The abrasion loss for these metals coated with a metal boride layer is reduced. Thus, the economic life of the boronized metal against abrasions increases. The corrosion resistance of these metals coated with a metal boride layer also increases. Thus, the economic life of boronized metal in corrosion environments increases.

Prior Art

In order to create a very hard layer on metal surfaces, the metal surface can be coated with thermochemical diffusion methods. Thus, core of the metal can remain soft even though a very hard coating on the metal surface is obtained as a result of coating the metal surface. Methods of coating metal surfaces with thermochemical diffusion include three main methods as carburizing, nitriding and boronizing.

The carburizing process is the process of hardening the steel surface by diffusing carbon over the steel surface at a sufficient temperature to transform steel into an austenitic structure, on a steel piece with low carbon. The carburizing process is performed at a temperature between 850 and 1015 °C.

Nitriding is the process of coating the steel surface with the metal-nitrogen layer by diffusing nitrogen gas thereon. Ammonia gas (NH ₃ ) is decomposed into nitrogen and hydrogen at temperatures between 500 and 600 °C. From these decomposing gasses, nitrogen penetrates into steel by means of diffusion and forms a metal-nitride layer (Fe4N) at a very hard structure on the surface.

Boronizing is the process of coating with a metal-boride layer by diffusing boron atom on the metal surface. Boron atoms in the boronizing agent are diffused at a temperature between 800 and 1050 °C and at a time between 1 and 10 hours and form a very hard metal-boride layer (Fe2B) on the metal surface. Boronizing can be made in the forms of solid (packet), liquid and gas. In the solid boronizing process, boronizing material comprising B4C, KBF4 and SiC are used. In a liquid boronizing process, borax, boric acid, ferrosilicon, NH4CI, NaCI and SiC are used. In the gas boronizing process, trimethyl borate, triethyl borane, BF3, BCI3, H2 and Ar gasses are used. The method used is referred to as a solid boronizing process as long as the boronizing agent in the medium remains solid at the boronizing temperature (1050 °C). Parameters such as composition and concentration of the boronizing medium, substrate metal composition, process temperature affects the characteristics of the metal-boride layer. While the hardness of the substrate metal varies between 100 to 500 HV, the hardness of the metal-boride layer can be between 1000 to 2500 HV. The metal-boride layer can have a depth of 10-300 pm as from the metal surface. Hardness starts to decrease below the metal-boride layer on the metal surface. Generally, the metal hardness of the metal surface at the depth of 600 pm is the same as the metal hardness of the sub-layer. In other words, even though the metal surface is very hard, the core of the metal is soft and this allows the toughness of the metal to be high. In the boronizing process, boron is diffused into the sub layer metal. When the boride atoms move towards the metal at a high temperature (800 °C) and they settle in the lattices formed by the crystal structure of the metal, diffusion occurs. As the metal-boride layer displays a columnar microstructure, the adhesion of the metal- boride layer to the sub-layer is very strong. Therefore, in practice, a columnar microstructure is preferred as the metal-boride layer. In the solid boronizing process, metal-boride grains grow faster perpendicular to the surface. In other words, metal-boride layers form inwardly from the steel surface. The growth of metal-boride grains in other directions is quite slow. Therefore, the microstructure of the metal-boride layer is columnar.

In the boronizing process, the first layer formed on the steel surface is the Fe ₂ B phase. If there is enough boron in the boronizing agent, a boride-rich FeB phase can form on the Fe ₂ B. Additionally, although the Fe ₂ B phase forms in a short-term boronizing process, FeB phase forms in a long-term boronizing process. As the brittleness of the FeB phase is higher than the brittleness of the Fe ₂ B phase, it is undesirable for the FeB phase to form during the boronizing process. At the last stage of boronizing process, only the FeB phase forms at the outer surface of the sample. As the FeB phase does not take as long as the Fe ₂ B phase, the texture of the FeB phase is not as strong as that of Fe ₂ B. As a result of the steels being boronized, there are three different layers on the steel surface; such as a metal-boride layer, transition layer, and matrix layer. The thickness of the transition layer is larger than the metal-boride layer. However, the hardness of the transition layer is lower than that of the metal-boride layer. The steel itself is available under the transition layer, and this is called the matrix layer. While the metal-boride layer in low- carbon steel has a columnar structure, the metal-boride layer in high-carbon steel has a flat structure. In high-carbon steels, the Fe B phase is stable. As the carbon element is not dissolved in the coating (metal-boride) layer, the carbon element diffuses towards the matrix. The carbon element forms carbides such as Fe C, Fe C on the transition layer. The chrome element dissolves in the metal-boride layer. It forms chrome-boride phases such as CrB and Cr B. While the metal-boride layer is columnar in steels with low-chrome content, it forms a flat structure in steels with high chrome content. An increase in the chrome rate in steels results in a decrease in the thickness of the metal-boride layer. While the metal-boride layer in low-nickel steel has a columnar structure, the metal-boride layer in high-nickel steels has a flat structure and it has high porosity.

In the process of boronizing steels, the thickness of the metal-boride layer varies depending on the steel standard type. While the metal-boride layer thickness resulting from boronizing at AISI 1010 steel at a temperature of 950 °C for 4 hours is 150 pm, metal-boride layer thickness resulting from boronizing of AISI 304 steel is 35 pm. This is because there are high amounts of alloy elements such as chrome and nickel in AISI 304 steel. In addition, although the micro-structure of the metal-boride layer formed in AISI 1010 is columnar, the microstructure of the metal-boride layer formed in AISI 304 has a flat structure.

Boron resources

Boron carbide, which is widely used as a boron resource in the solid boronizing agent, is an amorphous boron or crystal boron. As a result of boronizing steels with solid boronizing agent, Fe B phase is formed. This Fe B phase increases the hardness and abrasion resistance of the steel surface. In order to diffuse boron into the steel surface at high temperatures, there should be a boron resource in the boronizing agent. The rate of this boron resource in the boronizing agent affects the thickness of the metal-boride layer. When the rate of boron resource in the boronizing agent is low, the metal-boride layer has a smaller thickness. Activators

Alkali and alkaline-earth metals are used as activators. Examples of these include potassium fluoroborate, sodium bromide, sodium chloride, sodium fluoride, potassium chloride, potassium fluoride, calcium fluoride or barium fluoride. In order to coat steel surfaces with Fe2B phase by a solid boronizing agent, the boron in the boron resource must be diffused into the steel surface. The activator enables the boron atoms to diffuse into the sub-layer metal when heated in an anaerobic medium at high temperature. In solid boronizing agents, materials such as KBF4, NaBF4 or NaF are used as an activator.

In an exemplary embodiment for boronizing steel, when potassium tetrafluoroborate which acts as an activator is heated to 700 °C, boron trifluoride (BF3) is formed as the first product.

As a result of the reaction of boron trifluoride gas molecules, boron carbide molecules and iron, Fe2B is formed on the metal surface. After the formation of Fe2B on the metal surface with the effect of the activator, boron carbide decomposes by the interaction between Fe2B and boron carbide and releases boron and carbon. These boron atoms diffuse into the metal surface and form a layer of metal-boride compounds (Fe2B) deeper in the metal surface.

The boron mass transfer mechanism during the solid boronizing process of steels is explained by the following reactions, by Spence and Makhlouf (2005).

Potassium tetrafluoroborate, which acts as an actuator, begins to decompose at 530 °C (equation 1).

BF3 gas resulting from this reaction in turn reacts with the boron carbide acting as the iron and boron resource on the steel surface (equation 2).

2Fe(solid) + ^/j^BF3(gasj + ³ /i3B4C(solid ) After Fe2B is formed on the steel surface, it decomposes by interacting with the excessive B4C, Fe2B in the medium (equation 3).

B4C(solid) + Fe2B(solid) - » 4B[Fe _2B] C(solid) (600 °C) (3) With the diffusion of the boron released into the steel, the iron boride layer grows from the steel surface towards the inside. Depending on the concentration of the boron released at that time, a FeB layer may form with a lower resistance than the Fe2B layer. Besides boron, also the carbon released may cause cementite formation with high brittleness on the steel surface and thereabouts.

When there is no boron carbide in the boronizing agent, a reaction may take place between BF ₃ gas and steel as a result of the KBF ₄ 530 °C decomposition (equation 4). Inert Refractor Filling (Diluent)

During the boronizing process, a filling material that does not get sintered at 1100 °C can be used. For this purpose, materials such as silicon carbide, alumina, zirconia are commonly used filling materials. Free boron rate released by the use of filling material and thus the formation of FeB can be restricted. DE1796212B1 discloses a powder boronizing agent comprising 5% boron carbide, 5% potassium fluoroborate and 90% silicon carbide. US6503344B2 discloses a pasty boronizing agent comprising 1 to 15% boron carbide, 1 to 15 %potassium tetrafluoroborate and 5 to 40% calcium fluoride by weight. US4637837A discloses a solid boronizing agent comprising 20950 g SiC, 810 g B ₄ C, 1160 g KBF ₄ , 2000 g sucrose, and 13000 g water. US4536224 discloses a salt bath boronizing agent comprising 50 kg BaC , 15 kg NaF, 20 kg NaCI, 5 kg B ₂ O ₃ , 10 kg B ₄ C.

The former USSR patent SU1452182A1 discloses a solid boronizing agent comprising 20 to 50% amorphous boron, 20 to 16% potassium tetrafluoroborate, 1 to 3% boron carbide by weight. The Europeant Patent document numbered EP1026282B1 discloses a pasty boronizing agent comprising 30% water, 7.5% boron carbide, 9.2% potassium tetrafluoroborate, 52.5% silicon carbide, 0.8% bentonite by weight. The PCT document numbered WO2018169827 discloses a solid boronizing agent comprising 2 to 3% boron carbide, 4 to 6% potassium tetrafluoroborate, 18 to 22% carbon black and 69 to 75% silicon carbide by weight.

Object and Brief Description of the Invention The first object of the boronizing agent developed by this invention is to form a very hard metal-boride layer on the surface of metals. It is to increase the abrasion resistance of metals by coating the metal surface with a very hard metal-boride layer and thus to increase the economic life thereof. The second object of this boronizing agent is to obtain a new surface by metal-boride coating on the surface of metals and to increase the corrosion resistance of metals. That is because metal-boride coatings forming on metal surfaces have a higher corrosion resistance than the sub-layer metal.

In line with the objects of the invention, a boronizing agent has been developed having lower cost compared to solid boronizing agents comprising boron resources like carbide, which are hard to process due to their hardness. The boronizing agent developed, enables to limit surface characteristics of steel, especially FeB formation.

Description of the Figures Explaining the Invention

Below are provided the figures and related descriptions that are used to explain the boronizing agent developed by this invention.

Figure 1 is the flow chart of the method which enables preparing a boronizing agent according to the invention.

Figure 2 is the flow chart of a typical test method that can be used to evaluate the efficiency of the boronizing process performed by using a boronizing agent according to the invention.

Figure 3 is a schematic view of a typical test assembly on which the boronizing process is performed by using a boronizing agent according to the invention.

Figure 4 is the SEM view (scanning electron microscope) of the AISI 1010 steel boronized with the boronizing agent according to the invention at 950 °C for 4 hours. Figure 5 is the result of XRD (X-Ray Diffraction) analysis of the AISI 1010 steel boronized with the boronizing agent according to the invention at 950 °C for 4 hours.

Definition of Elements Composing the Invention

In order to explain the boronizing agent developed by this invention, the parts and pieces in the figures are numbered and the equivalent of each number is given below.

1. Boronizing agent

2. Box

3. Sample

4. De-oxidizing material

5. Cover

6. Furnace

Detailed Description of the Invention

The inventive boronizing agent (1) comprises only potassium tetrafluoroborate (KBF4) as a boron resource and does not contain any other boron compound. The boronizing agent (1) comprises potassium tetrafluoroborate (KBF4) as an activator. The tetrafluoroborate (KBF4) in the boronizing agent is used as both boron resource and activator. The boronizing agent (1) comprises silicon carbide (SiC) as diluent.

The said boronizing agent (1) is in powder formand preferably comprises 1 to 50% KBF4 by weight and 50 to 99% SiC by weight. The particle size of the boronizing agent is preferably at least lOOpm, at least lOOpm for KBF4 and at least lOOpm for SiC.

Potassium fluoroborate, which is used as a boron resource, starts to decompose at 530 °C as shown in equation 1.

There is no boron resource other than potassium tetrafluoroborate in the medium during the use of a boronizing agent (1) according to the invention. BF3 gas, which is formed as a result of decomposition of potassium tetrafluoroborate, reduces iron on the steel surface at a temperature of 700 °C (equation 4).

2BF3(gas) + 4F6(solid) 2F62B(solid) + 3F2(gas) (4)

Thus, an iron boride layer can be formed on the steel surface according to equation 4. With the boronizing agent (1) of the invention, free boron is not exposed in the application area and FeB formation due to the free boron concentration is not observed. However, after the formation of Fe ₂ B on the steel surface, Fe ₂ B can react with excess BF ₃ gas in the environment (equation 5).

2BF3(gas) + 2Fe2B(solid) - » 4FeB(solid) + 3F2(gas) (5) After the formation of an iron boride layer on the steel surface, this layer should be enlargeable as much as desired and the iron boride formation should be limitable to Fe ₂ B. For this reason, silicon carbide is also used together with potassium tetrafluoroborate. Since the amount of boron trifluoride released is proportional to the amount of potassium tetrafluoroborate in the boronizing agent (1), boron trifluoride concentration in the medium of the boronizing process can be taken under control by increasing the rate of silicon carbide in the boronizing agent (1). In this way, the reaction rate of boron trifluoride with Fe ₂ B is ensured to remain below the rate of diffusion into the steel, and the formation of the FeB layer can be prevented while the Fe ₂ B layer is growing.

Silicon carbide also reacts with fluorine in order to prevent the damage that fluorine released according to equations 4 and 5 can cause on steel (equation 6).

4F2(gas) + SiQsolid) SiF 4(gas) + CF 4(gas) (6)

The method for preparing the boronizing agent (1) includes the following two processing steps.

(101) Weighing solid potassium tetrafluoroborate and silicon carbide from 1 to 50% by weight and 50 to 99% by weight, respectively,

(102) mixing potassium tetrafluoroborate and silicon carbide for at 5 to 60 rpm 1 to 72 hours, preferably at 10 to 30 rpm for 48 hours. In one embodiment of the invention, either starch or active carbon can also be added to the boronizing agent (1). In this case, either starch or active carbon is also weighed in the first step of the method for preparing the boronizing agent (1) and, either starch or active carbon is mixed with other materials in the second step. A typical test method that can be used to evaluate the effectiveness of the boronizing process performed with a powder boronizing agent (1) according to the invention includes the following processes.

(201) Adding the boronizing agent (1) into a steel box (2), starting from the bottom of the box,

(202) Placing the samples (3) in the boronizing agent (1) in the box (2) with a distance of at least 1 cm between each sample (3),

(203) adding a de-oxidizing material (4) on the boronizing agent (1) in the box (2),

(204) closing the cover (5) of the box (2) and placing it in the furnace (6),

(205) heating the box (2) in the furnace (6) for 4 hours at 950 °C,

(206) leaving the box (2) to cool in the furnace (6),

(207) opening the cover (5) of the box (2) and obtaining the boronized samples (3).

Samples (3) obtained by this test method were analyzed by scanning electron microscopy and X-ray crystallography. The results obtained are presented in Figure-4 and Figure-5, respectively. The formation of an iron boride layer on this boronized sample surface with a thickness of 148 pm has been measured by SEM analysis. As is seen, the boronizing process with the desired quality can be performed on the steel with the boronizing agent of the invention.

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