Related Technical Field of the Invention

This invention relates to a boriding agent to be used in material surface hardening method and in solid boriding method developed for overcoming the problem of lifetime shortening of metals that arise due to wear thereof.

The invention, as a boriding agent, relates particularly to material surface hardening method which is advantageous thanks to being economically cost efficient, and in which ground colemanite that enhances hardness and wear values of boronized metal material is used.

State of the Art Regarding the Invention (Prior Art)

Boriding is defined as the coating process of metal surface by a particularly hard metal boride layer in order to prevent wear of metal surface. In boriding process, boron atoms are diffused into the surface of a metal at high temperatures. Thus, surface hardness, wear resistance and corrosion resistance of boronized material are increased. Boronized material preserves these features also at high temperatures. When boriding duration and temperature increase, thickness of metal boride layer is increased. Boriding method attracts the interests of researchers particularly in recent years and very intensive researches are carried out concerning this subject. Boriding process can be applied to iron-based materials, non-iron materials and to materials produced by powder metallurgy in a wide range of areas. FeB and Fe ₂ B layers occur on surface by the boriding process. These layers increase the corrosion resistance and wear resistance of material.

Boriding process is realized by three different method which are solid, liquid and gas. The gases used in gas boriding method are toxic and explosive, whereas gas boriding agents are very expensive materials and are difficult to obtain. Also, in the liquid boriding method, the installation is expensive, wherein salt layer is accumulated at the end of process and the layer thickness is not equal on entire surface since the temperature distribution in the boriding bath is not homogeneous. Solid boriding method is the most common boriding technique since its application is easy, it requires simple equipment, it is economic and safe and since modification can be made to chemical composition of powder mixture used. In solid boriding process; boriding temperature is between 800-1100°C and boriding duration changes between 1 to 10 hours. In the boriding process; metal boride compounds such as iron diboride (Fe ₂ B), iron boride (FeB), chrome boride (CrB), manganese boride (MnB), Nickel boride (NiB) occur as result of reaction of metal substrate and boron atoms. For example; on metal surface, Fe ₂ B phase occurs when the metal substrate is iron, NiB phase occurs when the metal sublayer is nickel. In the boriding process of steel; Fe ₂ B phase occurs at the beginning of boriding duration, and FeB phase starts to occur as a second phase as the boriding duration proceeds.

The component to be subjected to process in solid boriding should usually be left inside the powder boriding environment for 1 to 10 hours at 800-1100 °C. This method which is similar to pack cementing can be conducted in inert atmosphere as well as in normal atmosphere on condition of being in tightly closed boxes. Main compositions of boron environment are boron carbide (B ₄ C), amorphous boron and ferro-boron. Boron carbide is preferred while it is cheaper in comparison to others. In addition to main boriding source, activators such as ammonium chloride (NH ₄ CI), barium fluoride (BaF), sodium tetrafluoroborate (NaBF ₄ ), ammonium fluoride (NH ₄ F), sodium carbonate (Na ₂ CC>3), potassium tetrafluoroborate (KBF ₄ ) and sodium hexafluoroaluminate (Na ₂ AIFe). Due to thermal expansion differences between the phases Fe ₂ B and FeB formed as result of boriding process, cracking can be occurred on the surface. Therefore, the phase Fe ₂ B is preferred in industry. When B ₄ C as boron source, KBF ₄ as activator, SiC as deoxidant and filling material is used, single phase (Fe ₂ B) boride layers or layers FeB proportion of which is significantly decreased are obtained.

Many studies were carried out in literature in order to enhance boriding methods and to overcome current problems. One of these studies is the invention being subjected to patent numbered TR 2016 03349 B; wherein it discloses boriding composition used in solid boriding process applied for increasing surface hardening and wear resistances of machine components/parts in machine manufacturing industry. This composition comprises disodium octaborate tetrahydrate, activator and deoxidant-diluter. Another study in the literature is the invention subjected to patent No DE19830654.7. The invention relates to a boriding substance mainly comprised of boron yielding substances, activators and amount enhancer substance resistant to fire in the remaining, not reacting with the other compounds, wherein it comprises potassium tetrafluoroborate of l%-5% by weight and calcium fluoride of 5%-40% by weight composition as activator substance in comparison to total amount of boriding substance. By this boriding substance, on the work pieces made of materials containing iron, forming of single phase boride layers comprising Fe ₂ B is possible. The substance causes that fluorine and fluoride have lower emissions.

Another state of the art study is the patent No TR 2013 07208 B, the subject matter of this patent relates to a boriding method comprising the process steps of powdering ferroboron alloy, mixing the ferroboron powder with silicate, activating the mixture by grinding (mechanical alloying), applying (boriding) the gelled mixture on the metal surface, wherein thermal and mechanical resistances and lifetime of iron based metal surfaces are enhanced by being coated by boron atoms.

US patent application No US 4,637,837 A of the state of the art relates to a method concerning a boriding process of metals and metal alloys in a fluidized bed between 580-1300 °C. The solid boriding agent used in that method is boron carbide (B ₄ C). Another patent application of the state of the art is the patent application No CA 930652 A, wherein it relates to a method for solid boriding process of metals particularly of steel. In the method subjected to this application, ferrous boron alloy, amorphous boron, boron carbide, borax and mixtures thereof are used as boriding agent. Solid boriding method has two basic problems:

1) Lifetime of metals is decreased due to wear of metals.

2) Corrosion rate of metals is high in corrosion environments. This decreases the lifetime of metal.

Said invention relates to a boriding agent to be used in solid boriding method, wherein it enhances lifetime of metals. In conclusion, the need for material surface hardening method which eliminates the drawbacks existing in the state of the art, and the insufficiency of the existing solutions have made it essential to make an improvement in the related technical field.

Brief Description and Objects of the Invention

The present invention relates to a boriding agent to be used in material surface hardening method and in solid boriding method meeting the above-mentioned requirements, eliminating the drawbacks and introducing several additional advantages and developed for overcoming the problem of lifetime shortening of metals that arise due to wear thereof. Object of the present invention on the basis of the state of the art is to ensure that the total cost of boriding process is lowered by means of using ground colemanite as boriding agent in material surface hardening method instead of using boron carbide which is relatively more expensive. Another object of the invention is to ensure that hardness of boronized sample is enhanced by means of structure of powder boriding agent used in material surface hardening method.

In solid boriding method regarding the invention, ground powder colemanite is used as boriding agent.

Cost of subject matter boriding agent is comparatively lower than the present art solid boriding agents.

Hardness and wear values of the material boronized by the subject matter boriding agent is higher than hardness and wear values of the material boronized by the present art boriding agent.

Furthermore, although the solid boriding method performed using subject matter boriding agent takes a shorter time with respect to the current method, it has higher hardness and wear values than the current method.

Definitions of the Figures Describing the Invention

Figure 1: XRD analysis of AISI 1010 steel sample boronized for 4 hours at 950°C.

Figure 2: Hardness analysis of AISI 1010 steel sample boronized for 4 hours at 950 °C. Figure 3: Hardness analysis of non-boronized original AISI 1010 steel sample.

Figure 4: SEM view of iron boride layer thickness.

Figure 5: SEM view of microstructure of iron boride layer.

Detailed Description of the Invention

This detailed description has been given solely for better understanding of the material surface hardening method developed for overcoming the problem of lifetime shortening of the subject matter metals that arise due to wear thereof as an example and does not have any limiting effect. The subject matter solid boriding agent comprises; ground calcined colemanite (CaO.3B2O3.5H2O), potassium tetrafluoroborate (KBF ₄ ), silicium carbide (SiC) and starch.

Said boriding agent is in powder form, wherein it comprises 1-40% by weight calcined colemanite, 1-30% by weight KBF _4, 40-95% by weight SiC and 1-30% by weight starch. Calcined colemanite containing boron oxide in proportion of 40-50% B2O3 by heating the ground colemanite, which contains 38-45% B ₂ 0 ₃ and which is used in the invention, at 590- 650 °C for 150-180 minutes in a shuttle kiln. Particle size of said calcined colemanite is approximately 45pm.

Preparation method of the subject matter boriding agent comprises the following process steps;

i) Calcining of ground colemanite by heating,

ii) Mixing of calcined colemanite, potassium tetrafluoroborate, silicon silicon carbide and starch, which have been obtained, at 20-50 rpm for 120-240 minutes.

Solid boriding method in which the subject matter solid boriding agent in powder state is used comprises the following process steps;

- Addition of powder boriding agent into ladle starting from bottom of the ladle,

Addition of the sample into the powder boriding agent in the ladle and addition of deoxidant material on that,

Placing the ladle vertically in a tightly closed manner into furnace and heating at 950 °C for 4 hours,

- Leaving the heated ladle for cooling in the furnace,

Acquiring of boronized sample by opening the cover of the ladle at the end of cooling. The hardness value of steel boronized at 950 °C for 4 hours by the subject matter powder boriding agent were measured as 2277 HV in average. The hardness value of steel boronized at 900 °C for 4 hours were measured between 1881-2465 HV and the hardness value of 2138 HV in average was obtained.

Cylindrical AISI 1010 steel sample having 15 mm length and diameter was chosen as metal substrate for boriding process in the work in which the subject matter boriding agent is used. All the materials herein were weighted by 0.01 g precision scale. A furnace that can be maximum 1200 °C was used for heating the ladle. Boronized samples were ground by 1200 mesh sandpaper, wherein each test sample were subjected to XRD analysis at 2-90° and 2 degrees/min. scan rate by Rigaku Ultima-IV device. Hardness values of the samples were measured by Galileo Durometria device and their Vickers microhardness measurements were made with 200 g load. Their microscopic views were taken using Nikon microscope. They were polished 45 min. by 60 mesh, 5 min. by 240 mesh, 5 min. by 400 mesh, 5 min. by 800 mesh, 5 min. by 1200 mesh, 10 min. by alumina paste. The sample was kept by tongs inside a solution containing 5% nitric acid and %95 methanol for 20 seconds by way of immersing. Then, the sample was immersed into methanol and water respectively and dried.

XRD analysis of AISI 1010 steel boronized for 4 hours at 950°C is shown in Fig. 1. Examining the diagram; it can be seen that greater part of major peaks is Fe ₂ B phase as result of boriding at 950 °C for 4 hours. In the boriding for 4 hours, while the main phase is Fe ₂ B, FeB phase is so small that it can be disregarded.

It can be seen in Fig. 4 that boride layer thickness is between 160 - 188 pm in SEM view of the sample boronized for 4 hours while the boriding temperature is fixed at 950°C. SEM analysis view of microstructure of iron boride layer formed as result of boriding of AISI 1010 steel at 950 °C for 4 hours is shown in Fig. 5. In this SEM analysis view, it was determined that iron boride layer appeared to be as sawtooth. Hardness of non-boronized AISI 1010 steel sample was measured as 262 HV. Hardness of AISI 1010 steel sample boronized for 4 hours at 950°C was obtained as 2277 HV in average. As can be seen in Figures 2 and 3, hardness of AISI 1010 steel is increased 8.5 times as result of boriding by powder boriding agent. The subject matter of the invention yields similar results as existing applications used as boriding agent in material surface hardening method.

Hence, boriding process performed by the method developed ensures obtaining required surface hardness and strength for enhancing lifetime of metal material beside providing financial advantages.