Alumina-based fused grain

Abstract

The invention relates to a fused grain having: - a chemical composition such that, in mass percentages based on oxides: 2.30% < MgO < 4.80%, Cr₂O₃ < 0.2%, Na₂O < 0.1%, Oxides other than MgO, Cr₂O₃, Na₂O, and Al₂O₃ < 1.5%, Al₂O₃: the remainder to 100%; - a crystallographic composition characterized by an X-ray diffraction pattern exhibiting a "reference peak" in the angular range 2θ between 18.1° and 20.1°, a first peak in the angular range 2θ between 12.3° and 13.8°, and optionally, a second peak in the angular range 2θ between 25.2° and 25.8°, or "corundum phase peak," each peak having a respective height, in number of counts, the ratio R² of the height of the first peak to The height of the reference peak is greater than 0.2, and the ratio R3 of the height of the corundum phase peak to the height of the reference peak is less than 2. No abbreviated figure.

Description

Title of the invention: Alumina-based fused grain technical field

[0001] The present invention relates to a fused alumina-based grain, a mixture of said grains, a method for manufacturing said mixture of grains, and an abrasive tool comprising said mixture of grains. The invention further relates to uses of the grains according to the invention for abrading a surface made of hardened steel. Prior art

[0002] Abrasive tools are generally classified according to the conditioning of their abrasive grains: loose abrasives (powders of grains not fixed to a support, used by projection or suspension), coated abrasives (support such as cloths or papers, on which the grains are arranged in a few layers) and agglomerated abrasives (circular grinding wheels, sticks, etc.).

[0003] In bonded abrasives, the abrasive grains are pressed together with an organic or vitreous binder, typically a binder composed of oxides, essentially silicate. The abrasive grains must themselves exhibit good mechanical properties against abrasion and good mechanical cohesion with the binder; that is, the interface with the binder must be solid.

[0004] Among abrasive grains, a distinction is made between fused grains and sintered grains, which have different microstructures. The problems posed by sintered and fused grains, and the technical solutions adopted to solve them, are therefore generally different. A composition developed to manufacture a fused grain is therefore not necessarily usable a priori for manufacturing a sintered grain with the same properties, and vice versa.

[0005] In the range of fused grains, alumina and zirconia-based materials have been known since US-A-3,181,939. These grains are generally composed of 10 to 60% zirconia, 0 to 10% of an additive, the remainder being alumina. In practice, the zirconia content of commercial products is either around 25% or around the alumina-zirconia eutectic value, which is approximately 42% zirconia, generally between 35 and 50%, as described in US-A-3,891,408.

[0006] US-A-4,457,767 also describes molten grains. For example, one might cite The grains marketed by Saint-Gobain (France) under the name NZPlus# typically contain 39% zirconia by mass and 0.8% Y2O3, less than 0.5% impurities, the remainder being alumina. Mixtures of these grains are widely used in coated abrasives and grinding wheels. abrasives with organic binder, particularly in operations with high material removal rates (roughing, cutting...), especially on stainless steel.

[0007] The Applicant has developed many types of fused abrasive grains based on alumina containing magnesia MgO.

[0008] In particular, he has developed slowly cooled, molten aluminous grains, called MA88K, marketed by Saint-Gobain Speciality Grains and Powders, or corundum-based molten grains described in FR2853898A1, which contain between 1.5% and 6.5% MgO, or in WO2004 / 094554, which contain between 2.2% and 6.5% MgO, as a mass percentage based on the oxides. These latter grains are manufactured by melting raw materials. The molten material is then rapidly cooled to promote the formation of fine, oriented structures, for example, by means of a casting device between thin metal plates such as that described in US patent 3,993,119. The cooled material is finally ground, for example by means of roller mills, then sieved and classified into series of particle size distributions, or "grits", meeting precise standards, for example FEPA.

[0009] The Applicant has also developed grains, described in application WO2023111156, which have a MgO content of 2.5% to 5.8%, but also 0.2% to 4.5% of Cr2O3, which increases the manufacturing cost.

[0010] The Applicant has further developed grains containing an MgO content of 4.8% to 7.2%, as described in the co-pending application FR2306000. Advantageously, unlike the grains described in WO2023111156, these grains contain substantially no Cr2O3. These grains require additional calcination heat treatment for abrasion of hardened steel. Finally, their high MgO content makes the manufacture of the grains more complex and increases their cost.

[0011] The Applicant has further developed fused alumina grains coated at least partially with a silica-containing coating. The addition of a silica-containing coating also increases the manufacturing cost.

[0012] There is a permanent need to improve abrasive grains that do not contain substantially Cr2O3 and that, for the abrasion of a hard steel surface, have a higher efficiency than prior art grains and a specific energy substantially equal to or lower than that of prior art grains.

[0013] One object of the invention is to meet, at least partially, this need. Summary of the invention

[0014] According to a first principal aspect of the invention, this goal is achieved by means of a molten grain having: - a chemical composition such that, in mass percentages based on oxides: 0.30% < MgO < 2.30%, Cr2O3 < 0.2%, Na2O < 0.1%, Oxides other than MgO, Cr2O3, Na2O and Al2O3 < 1.5%, Al2O3: complement to 100%; - a crystallographic composition characterized by an X-ray diffraction diagram showing a first peak in the angular range 20 between 17° and 18° and, optionally, a second peak in the angular range 20 between 43° and 44°, or "corundum phase peak", each peak having a respective area, the area being in counts x degrees (counts times degrees), the ratio RI of the area of ​​the first peak to the sum of the area of ​​the first peak and the area of ​​the corundum phase peak being greater than 0.02.

[0015] This RI report is designated by "signature DRX1".

[0016] The performance obtained with molten grains according to the first aspect of the invention proved to be exceptional.

[0017] Without being bound by this theory, the inventors discovered that the above DRX1 signature, which appears to correspond to the presence of a crystallized phase comprising the elements aluminium, magnesium and oxygen and exhibiting, on an X diffraction diagram, a diffraction peak whose maximum intensity is located in an angular range 20 between 17° and 18°, and a diffraction peak whose maximum intensity is located in an angular range 20 between 21.3° and 22.2°, combined with a reduced Na2O content, leads to remarkable performance, despite a low Cr2O3 content, and without it being necessary to at least partially cover these grains with a coating comprising silica.

[0018] This discovery was made unexpectedly. The inventors were initially surprised to find that some grains exhibited significantly superior performance compared to other grains with the same chemical composition. Subsequently, by comparing X-ray diffraction patterns, they observed that the highest-performing grains all displayed the DRX1 signature according to the invention, unlike the other grains.

[0019] In addition, the abrasive grains are advantageously usable for both the abrasion of hard steel and stainless steel.

[0020] In continuing their research, the inventors examined the possible correlation between the X-ray diffraction pattern of a grain and its performance. They thus discovered that the DRX1 signature of the grains according to the invention could be obtained by "Stabilization" of the molten material bath before solidification by cooling. By "stabilization," it is understood that the molten material must have received a minimum amount of energy before being cooled. While not bound by this theory, the inventors consider that this minimum quantity of specific electrical energy, in kWh per kilogram of material loaded into the furnace, leads to a particularly intimate mixing of the constituents, and in particular of the magnesia. According to the inventors, such an intimate mixing cannot be obtained during the usual preparation of the initial charge that forms the bath.

[0021] Determining the minimum specific electrical energy quantity depends on the composition of the melt and the furnace used. For a given composition, however, simple tests allow it to be determined, in particular by carrying out a series of pours, with a new starting charge and additional electrical energy being supplied after each pour, until the appearance of the DRX1 signature is observed. To increase the specific electrical energy quantity, the duration of electrical energy supply, measured between feeding the furnace with a new starting charge and the partial pouring of the molten material resulting from the melting of said starting charge, can also be increased.

[0022] In particular, and surprisingly, the inventors found that stabilizing the molten material bath leads to superior performance compared to Example 1 of WO2004 / 094554 carried out without stabilization for substantially identical MgO contents, as illustrated by Examples 4 and 5 below.

[0023] A molten grain according to the first principal aspect of the invention may further exhibit one or more of the following optional characteristics: - MgO > 0.40%, preferably MgO > 0.60%, preferably MgO > 0.70%; - MgO < 1.60%, preferably MgO < 1.30%, preferably MgO < 0.85%; - Cr2O3 < 0.15%, preferably Cr2O3 < 0.10%, preferably Cr2O3 < 0.05%; - Na2O < 0.08%, preferably Na2O < 0.05%; - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.4%, preferably less than 1.0%, preferably less than 0.4%; - SiO2 < 0.3%, preferably SiO2 < 0.1%, in mass percentages on the basis of oxides; - CaO <0.1%, preferably CaO < 0.08%, in mass percentages on the basis of oxides; - the oxide content is greater than 96%, in mass percentages based on the mass of the melted grain; - the RI ratio is greater than or equal to 0.05, preferably greater than or equal to 0.08; - the RI ratio is less than or equal to 0.95; - the molten grain is not coated, even partially, with a coating containing silica.

[0024] Beyond the 2.30% threshold, the inventors were no longer able to obtain the advantageous properties of the grains described above. However, by continuing their investigations, they discovered that a second DRX2 signature could be associated with advantageous properties.

[0025] According to a second principal aspect of the invention, the invention thus relates to a molten grain having: - a chemical composition such that, in mass percentages based on oxides: 2.30% < MgO < 4.80%, Cr2O3 < 0.2%, Na2O < 0.1%, Oxides other than MgO, Cr2O3, Na2O and A12O3 < 1.5%, A12O3: complement to 100%; - a crystallographic composition characterized by an X diffraction pattern showing a "reference peak" in the angular range 20 between 18.1° and 20.1°, a first peak in the angular range 20 between 12.3° and 13.8° and, optionally, a second peak in the angular range 20 between 25.2° and 25.8°, or "corundum phase peak", each peak having a respective height, in number of counts, the ratio R2 of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R3 of the height of the corundum phase peak to the height of the reference peak being less than 2.

[0026] The R2 and R3 ratios are designated together by "signature DRX2".

[0027] This second signature can be obtained by calcining the grains at a temperature between 750°C and 1225°C.

[0028] The inventors have discovered that calcination does not need to be at high temperature, typically above 1250°C, to achieve maximum performance improvement.

[0029] Combined with a reduced Na2O content, this second signature is associated with remarkable performance, despite a low Cr2O3 content, and without the need to at least partially cover these grains with a coating containing silica.

[0030] A molten grain according to the second main aspect of the invention may further exhibit one or more of the following optional characteristics: - MgO > 2.50%, preferably MgO > 2.70%, preferably MgO > 2.80%; - MgO < 4.50%, preferably MgO < 4.20%, preferably MgO < 4.10%; - Cr2O3 < 0.15%, preferably Cr2O3 < 0.10%, preferably Cr2O3 < 0.05%; - Na2O < 0.08%, preferably Na2O < 0.05%; - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.4%, preferably less than 1.0%, preferably less than 0.4%; - SiO2 < 0.3%, preferably SiO2 < 0.1%, in mass percentages on the basis of oxides; - CaO <0.1%, preferably CaO < 0.08%, in mass percentages on the basis of oxides; - the oxide content is greater than 96%, in mass percentages based on the mass of the melted grain; - the R2 ratio is greater than 0.25, preferably greater than 0.3; - the R2 ratio is less than 1.0, preferably less than 0.8; - the R3 ratio is less than 1.8, preferably less than 0.5; - the molten grain is not coated, even partially, with a coating containing silica.

[0031] The invention further relates to a mixture of grains comprising, by mass percentage, more than 80% of molten grains according to the first aspect of the invention or according to the second aspect of the invention or of molten grains selected from molten grains according to the first main aspect of the invention and molten grains according to the second main aspect of the invention.

[0032] The invention also relates to a method for manufacturing a mixture of molten grains according to the invention, said method comprising the following successive steps: a) mixing of raw materials so as to form a starting charge suitable for the manufacture of said mixture of grains, b) melting, preferably in a reducing medium, of said starting charge until a molten material is obtained, c) cooling of said molten material so as to solidify it entirely in less than 3 minutes, and obtain a solid mass, said solid mass from step c) being ground in a step d) so as to obtain a particulate mixture if it is not in the form of a particulate mixture.

[0033] A manufacturing process according to the invention may further have one or more of the following optional features: - in step b), sufficient energy is supplied so that the solid mass exhibits the DRX1 signature; - the process includes, subsequent to step c), in particular to obtain the DRX2 signature, a following step: f) calcination of the solid mass and / or particulate mixture, before or after optional particle size reduction, at a temperature greater than or equal to 750°C and less than or equal to 1225°C; - the solid mass from step c) is ground in a step d), before or after step f), so as to be in the form of a particulate mixture, the process preferably comprising a step e) of particle size selection applied to the particulate mixture; - the process includes a said step e), step d) and step e) being preferably carried out before step f); - in step f), the calcination temperature is greater than or equal to 800°C and less than or equal to 1200°C, preferably greater than or equal to 950°C and less than or equal to 1150°C, and / or the calcination temperature is maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 2 hours.

[0034] Step b) is advantageously not indispensable for manufacturing grains according to the second main aspect of the invention.

[0035] Step f) is advantageously not indispensable for manufacturing grains according to the first main aspect of the invention.

[0036] In one embodiment, a step f) is however provided in a process adapted to manufacture grains according to the first main aspect of the invention, in particular after having supplied, in step b), sufficient energy so that the solid mass exhibits the signature DRX1.

[0037] The invention further relates to an abrasive tool comprising grains bound by a binder and agglomerated, for example in the form of a grinding wheel, or deposited on a support, for example a belt or a disc, this tool being remarkable in that at least a part, preferably more than 20%, preferably more than 30%, preferably more than 40%, preferably more than 50%, preferably more than 70%, preferably more than 80%, preferably more than 90%, by mass percentage, preferably all of said grains conform to the invention and / or are manufactured according to a process according to the invention.

[0038] The abrasive tool can be a grinding wheel, and in particular a grinding wheel, a precision grinding wheel, a sharpening wheel, a cutting wheel, a solid cutting wheel, a deburring or roughing wheel, a drive wheel, a portable grinding wheel, a foundry grinding wheel, a drill wheel, a mounted wheel, a cylindrical, conical, disc or segmented grinding wheel.

[0039] The invention further relates to a method for treating a surface in hardened steel, said method comprising an operation of abrading said surface with a mixture of grains according to the invention or manufactured according to a manufacturing process according to the invention, said process being adapted to obtain the DRX1 signature and / or the DRX2 signature.

[0040] Preferably, a processing method according to the invention comprises - manufacturing a mixture of grains according to the invention, preferably following a manufacturing process according to the invention adapted to obtain the DRX1 signature and / or the DRX2 signature, then - an abrasion operation of said surface with said mixture of grains, preferably after putting the mixture of grains into the form of an abrasive tool according to the invention.

[0041] The inventors have also discovered that the grains according to the invention are particularly effective for the abrasion of hard steel.

[0042] This result is all the more surprising since the inventors have found that the presence of MgO leads to a very clear decrease in the corundum content in the grains according to the invention, whereas as a general rule, it is considered necessary, in order to machine a hard steel surface, to use a fused grain based on corundum.

[0043] The invention thus also relates to the use of a mixture of grains according to the invention or manufactured according to a manufacturing process according to the invention, said process being adapted to obtain the DRX1 signature and / or the DRX2 signature, for abrading a hard steel surface.

[0044] The invention relates in particular to a kit comprising: - a mixture of grains according to the invention or manufactured according to a manufacturing process according to the invention, said process being adapted to obtain the DRX1 signature and / or the DRX2 signature, and - a document specifying that the mixture of grains is usable for abrading a hard steel surface. Definitions

[0045] In this description, unless otherwise stated, all compositions of a grain or mixture of grains are given as mass percentages, based on the total mass of oxides in the grain or mixture of grains.

[0046] The oxide contents of a grain according to the invention, including for "oxides other than MgO, Cr2O3, CaO and Al2O3", refer to the overall contents for each of the corresponding chemical elements, expressed in the form of the most stable oxide, according to the usual convention of the industry; therefore, sub-oxides and possibly nitrides, oxynitrides, carbides, oxycarbides, carbonitrides, or even the metallic species of the aforementioned elements are included.

[0047] By "impurities" is meant the unavoidable constituents necessarily introduced with the raw materials. In particular, compounds belonging to the group Oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides, and metallic species of silicon, sodium and other alkali metals, iron, and vanadium are impurities. Examples of sources of Al and Mg include SiO2, Fe2O3, Na2O, and CaO.

[0048] By “precursor” of an oxide, we mean a constituent capable of providing said oxide during the manufacture of a grain or a mixture of grains according to the invention.

[0049] A “grain” is a particle whose dimensions are all less than 20 mm.

[0050] A grain containing more than 85% by mass is called an "alumina-based grain". of alumina, as a percentage based on oxides.

[0051] By "molten grain", or more broadly "molten product", we mean a solid grain (or product) obtained by solidification, by cooling, of a molten material.

[0052] A "molten material" is a mass rendered liquid by heating a starting charge, which may contain some solid particles, but in insufficient quantity for them to structure said mass. To maintain its shape, a molten material must be contained within a vessel. The molten grains according to the invention are conventionally obtained by melting at over 1900°C.

[0053] The "median size" of a powder is defined as the size that divides the particles into first and second populations equal in mass, these first and second populations consisting only of particles with a size greater than or equal to, or less than, respectively, the median size. The median size of a powder can be determined using a particle size distribution obtained with a laser particle size analyzer.

[0054] In this description, "hard steel" means steel having a Rockwell hardness greater than or equal to 55 HRC.

[0055] Unless otherwise specified or in the event of technical incompatibility, a feature of the "invention" is applicable to the first and second principal aspects of the invention. Detailed description

[0056] The following description is provided for illustrative purposes only and does not limit the invention. Melted grain

[0057] A molten grain according to the first or second principal aspect of the invention, and preferably a mixture of such grains according to the invention, preferably has one or more of the following optional and preferred characteristics: - Cr2O3 < 0.15%, preferably Cr2O3 < 0.10%, preferably Cr2O3 < 0.05%, in mass percentages on the basis of oxides; - Cr2O3 is not intentionally introduced into the starting charge, that is to say, it is preferably an impurity; - the Na2O content is less than 0.08%, preferably less than 0.05%, in mass percentages on the basis of oxides; - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.4%, preferably less than 1.3%, preferably less than 1.0%, preferably less than 0.9%, preferably less than 0.8%, preferably less than 0.7%, preferably less than 0.6%, preferably less than 0.5%, preferably less than 0.4%, in mass percentages on the basis of oxides; - oxides other than MgO and Al2O3 are preferably impurities; - the SiO2 content is less than 0.3%, preferably less than 0.2%, preferably less than 0.15%, preferably less than 0.1%, preferably less than 0.08%, preferably less than 0.05%, in mass percentages on the basis of oxides; - the CaO content is less than 0.1%, preferably less than 0.08%, preferably less than 0.05%, in mass percentages on the basis of oxides; - the oxide content is greater than 96%, preferably greater than 97%, or even greater than 98%, or even greater than 99%, or even greater than 99.4%, or even greater than 99.5%, or even greater than 99.6%, or even greater than 99.7%, in mass percentages on the basis of the mass of the molten grain;

[0058] - in one embodiment, the grains are not coated, even partially of a coating containing silica, in particular a coating containing more than 50% silica by mass;

[0059] - in one embodiment, the grains are not coated. A molten grain according to the first principal aspect of the invention, and preferably a mixture of such grains according to the invention, preferably has one or more of the following optional and preferred characteristics: - MgO > 0.40%, preferably MgO > 0.50%, preferably MgO > 0.60%, preferably MgO > 0.70%, preferably MgO > 0.80%, and / or preferably MgO < 1.60%, preferably MgO < 1.40%, preferably MgO < 1.30%, preferably MgO < 1.20%, in mass percentages on the basis of oxides; - in one embodiment, MgO < 0.87%, preferably MgO < 0.85%, or even MgO < 0.80%, in mass percentages on the basis of oxides; - the RI ratio is greater than or equal to 0.05, preferably greater than or equal to 0.07, preferably greater than or equal to 0.08, preferably greater than or equal to 0.10; - in one embodiment, the RI ratio is greater than or equal to 0.05, preferably greater than or equal to 0.07, preferably greater than or equal to 0.08, preferably greater than or equal to 0.10, and less than or equal to 0.95, preferably less than or equal to 0.90, preferably less than or equal to 0.85, preferably less than or equal to 0.80, preferably less than or equal to 0.70, preferably less than or equal to 0.60.

[0060] A molten grain according to the second main aspect of the invention, and preferably a mixture of such grains according to the invention, preferably has one or more of the following optional and preferred characteristics: - MgO > 2.50%, preferably MgO > 2.70%, preferably MgO > 2.80%, and / or preferably MgO < 4.50%, preferably MgO < 4.20%, preferably MgO < 4.10%, in mass percentages on the basis of oxides; - the R2 ratio is greater than 0.25, preferably greater than 0.3, and preferably less than 1.0, preferably less than 0.9, preferably less than 0.8; - the R3 ratio is less than 1.8, preferably less than 1.5, preferably less than 1.3, preferably less than 1.0, preferably less than 0.9, preferably less than 0.7, preferably less than 0.5, preferably less than 0.3. Grain mixture

[0061] A mixture of grains according to the invention comprises, by mass percentages, preferably more than 85%, preferably more than 90%, preferably more than 95%, preferably more than 99%, preferably substantially 100% of molten grains - according to the first principal aspect of the invention or - according to the second principal aspect of the invention or - chosen from molten grains according to the first main aspect of the invention and molten grains according to the second main aspect of the invention.

[0062] Preferably, a mixture of grains according to the invention complies with a particle size distribution conforming to those of the mixtures or "grits" provided by FEPA Standard 43-GB-1984, R1993 and FEPA Standard 42-GB-1984, R1993.

[0063] Preferably, a mixture of grains according to the invention has a mass retention on the 16 mm sieve, preferably on the 9.51 mm sieve, measured using a Ro-Tap® sieve-maker, of less than 1%, by mass percentage.

[0064] Method for manufacturing a mixture of molten grains according to the invention

[0065] Molten grains according to the invention can be manufactured according to steps a) to e), or even a) to f) mentioned above. The parameters can, for example, take the values ​​of the process used for the examples below.

[0066] In step a), raw materials are conventionally dosed to obtain the desired composition, then mixed to form the starting charge.

[0067] The elements Al and Mg in the initial charge are found almost entirely in the molten grains. The element Mg, particularly in the form of oxides, may, however, be subject to flyaway phenomena during melting. Those skilled in the art know how to adjust the composition of the initial charge accordingly.

[0068] Choosing the raw materials for the starting charge so that the solid mass obtained at the end of step c) has a chemical analysis consistent with that of a grain according to the invention therefore poses no difficulty for a person skilled in the art.

[0069] In particular, the Na2O content must be controlled to ensure that Na2O < 0.1%.

[0070] The Mg element is preferably introduced into the starting charge in the form of MgO oxide. It can also be conventionally introduced in the form of precursors of this oxide, for example in the form of MgCO3.

[0071] The element Al is preferably introduced at least partially into the starting charge in the form of Al₂O₃ and / or in the form of precursors of this oxide, for example in the form of aluminum hydroxide and / or boehmite. Preferably, the element Al is introduced into the starting charge partly in the form of Al₂O₃ and partly in a metallic form.

[0072] In a preferred embodiment, the starting charge comprises at least one compound creating a reducing medium during melting.

[0073] Preferably, said compound is selected from a carbon source, a metal, and mixtures thereof. Preferably, the carbon source is selected from carbon, petroleum coke, pitch, coal, and mixtures thereof, preferably petroleum coke. Preferably, the metal is aluminum.

[0074] Preferably, said at least one compound creating a reducing environment during melting and used in the starting charge is chosen from petroleum coke, aluminum and their mixtures.

[0075] A person skilled in the art can determine the quantity of compound creating a reducing medium during melting, in the starting charge, to obtain, in step b), a melting in a reducing medium.

[0076] Preferably, the starting charge contains an amount of compound creating a reducing environment during melting greater than 0.5%, preferably greater than 1%, preferably greater than 1.5% and, preferably less than 5%, preferably less than 4%, as a percentage by mass on the basis of the starting charge.

[0077] In step b), an electric arc furnace is preferably used, preferably of the Hérault type with graphite electrodes, but all known furnaces are conceivable, such as an induction furnace or a plasma furnace, provided that they allow the starting charge to be melted, preferably in a reducing medium.

[0078] Melting in a reducing medium is preferably obtained by the presence, in the starting charge, of compounds creating a reducing medium during melting and / or by the fact that the electrodes are immersed in the bath of molten material.

[0079] Preferably, the starting charge contains elements that create a reducing environment during melting.

[0080] Preferably, the raw materials are melted at atmospheric pressure.

[0081] Preferably, an electric arc furnace is used, comprising a 70-liter tank, with a pre-pouring melting energy greater than or equal to 2 kWh per kg of raw materials for a power output of more than 220 kW, or an electric arc furnace of a different capacity operated under equivalent conditions. Those skilled in the art know how to determine such equivalent conditions.

[0082] To "stabilize" the molten material bath, the bath can be kept molten before step c). In a semi-continuous or continuous manufacturing process, the first pours can be discarded.

[0083] In step c), the cooling must be rapid, i.e., such that the molten material is completely solidified in less than 3 minutes. For example, it can result from pouring into molds as described in US 3,993,119 or from quenching.

[0084] Preferably, the molten material is completely solidified in less than 2 minutes, preferably in less than 1 minute, preferably in less than 40 seconds, preferably in less than 30 seconds.

[0085] In one embodiment, step c) leads to a solid mass in the form of a particulate mixture.

[0086] If step c) does not allow a mixture of grains to be obtained directly, or if these grains do not have a particle size suitable for the intended application, grinding of the solid mass (step d)) can be carried out, according to conventional techniques.

[0087] A particle size selection (step e)), for example by sieving or cycloning, can be implemented to obtain a mixture of grains with a particle size suitable for the intended application.

[0088] Step d) and / or step e) may be carried out before or after step f) of calcination. It / they are preferably carried out before step f).

[0089] Preferably, the molten grains obtained at the end of step c) and / or d) and / or e) have a carbon content greater than 15 ppm and less than 1200 ppm on the basis of the mass of said grains, said content being measured using a CS744 model carbon-sulfur analyzer, marketed by the company LECO.

[0090] In step f), which is optional according to the first main aspect of the invention and obligatory according to the second main aspect of the invention, the solid mass obtained at the end of step c), preferably after reduction in the form of a particulate mixture during step d) if the process includes such a step, or preferably after step e) if the process includes such a step, is calcined.

[0091] If calcination is carried out on the solid mass directly obtained at the end of step c), a step d) and preferably a step e) is / are then carried out after step f).

[0092] The calcination is carried out at a temperature greater than or equal to 750°C, preferably greater than or equal to 800°C, preferably greater than or equal to 900°C, preferably greater than or equal to 950°C, and less than or equal to 1225°C, preferably less than or equal to 1200°C, preferably less than or equal to 1150°C, the calcination temperature being preferably maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 1 hour, preferably greater than or equal to 2 hours, preferably greater than or equal to 3 hours, preferably greater than or equal to 4 hours, and preferably less than 100 hours, preferably less than 50 hours, preferably less than 20 hours, preferably less than 15 hours, preferably less than 10 hours.

[0093] Preferably, step f) is carried out at atmospheric pressure.

[0094] Step f) can be carried out in a reducing, neutral or oxidizing atmosphere.

[0095] Preferably step f) is carried out in an oxidizing atmosphere, preferably under air.

[0096] Preferably step f) is carried out under an oxidizing atmosphere, preferably in air, preferably at atmospheric pressure, at a temperature greater than or equal to 750°C, preferably greater than or equal to 800°C, preferably greater than or equal to 900°C, preferably greater than or equal to 950°C, and less than or equal to 1225°C, preferably less than or equal to 1200°C, preferably less than or equal to 1150°C, the calcination temperature being preferably maintained for a period greater than or equal to 30 minutes, preferably greater than or equal to 1 hour, preferably greater than or equal to 2 hours, preferably greater than or equal to 3 hours, preferably greater than or equal to 4 hours, and preferably less than 100 hours, preferably less than 50 hours, preferably less than 20 hours, preferably less than 15 hours, preferably less than 10 hours.

[0097] Method for manufacturing an abrasive tool according to the invention

[0098] The manufacturing processes for abrasive tools are well known and can be used to manufacture an abrasive tool according to the invention.

[0099] Abrasive tools can in particular be formed by agglomerating grains according to the invention by means of a binder, in particular in the form of a grinding wheel, for example by pressing, or be formed by fixing grains according to the invention on a support, for example a strip or a disc, by means of a binder.

[0100] The binder can be inorganic, in particular a glass (for example, a binder made up of oxides, substantially made up of silicate(s) can be used) or organic.

[0101] An organic binder is well suited. The binder may, in particular, be a thermosetting resin. It is preferably chosen from the group consisting of phenolic, epoxy, acrylate, polyester, polyamide, polybenzimidazole, polyurethane, phenoxy, phenol-furfural, analin-formaldehyde, urea-formaldehyde, cresol-aldehyde resins, resorcinol-aldehyde, urea-aldehyde, melamine-formaldehyde, and mixtures thereof.

[0102] The binder may also incorporate organic or inorganic fillers, such as hydrated (e.g., aluminum trihydrate or boehmite) or non-hydrated (e.g., molybdenum oxide) inorganic fillers, cryolite, a halogen, fluorspar, iron sulfide, zinc sulfide, magnesia, silicon carbide, silicon chloride, potassium chloride, manganese dichloride, potassium or zinc fluoroborate, potassium fluoroaluminate, calcium oxide, potassium sulfate, a copolymer of vinylidene chloride and vinyl chloride, polyvinylidene chloride, polyvinyl chloride, and mixtures thereof. The binder may also contain reinforcing fibers such as glass fibers.

[0103] Conventionally, a mixture of grains according to the invention is mixed with a binder optionally containing organic or inorganic fillers. The resulting mixture, in which the binder typically represents between 2% and 60%, preferably between 20% and 40% by volume, is shaped, for example, placed in a mold or deposited on a support. The binder is then activated, for example, by heating, to bind the grains together and / or with the optional support. After the binder has hardened and, optionally, the material has been demolded, an abrasive tool according to the invention is obtained. Examples

[0104] The following non-limiting examples are given for the purpose of illustrating the invention. Measurement protocols

[0105] The following measurement protocols were used to determine certain properties of molten grain mixtures. They allow for excellent simulation of the actual behavior of the grains when implemented for abrasion.

[0106] In order to evaluate the abrasive performance of a mixture of grains from the examples, a monolayer of 1.02 grams of this mixture is applied to a metal grinding wheel with a diameter of 12.7 cm, said grains being bonded with a phenolic resin.

[0107] The surface of a 52100 hard steel plate with a Rockwell hardness between 60 and 63 HRC, measuring 20.5 cm x 7.6 cm x 6.0 cm, is then machined with the resulting grinding wheel, under water spray, using a reciprocating motion at constant speed, maintaining a constant cutting depth of 20 µm and a grinding wheel rotation speed of 3600 rpm. The total energy developed by the grinding wheel during machining, Etot, is recorded.

[0108] After complete wear of the grinding wheel, the mass of steel machined (i.e. the mass of steel removed by the grinding operation) “Ma” and the volume of steel removed by the grinding operation “Va” are measured.

[0109] To evaluate the yield, the ratio S of the mass of machined steel divided by the mass of grains consumed during said machining, here equal to 1.02 grams, is classically calculated.

[0110] To evaluate energy efficiency, the specific machining energy, Es, is classically calculated, equal to the energy required to remove a unit volume of steel (Es = Etot / Va).

[0111] To determine the composition of the molten grains, a bead of a mixture of these grains is made by melting the mixture, then the chemical analysis is carried out by X-ray fluorescence, except for the measurement of the carbon content.

[0112] The carbon content of the molten grains of the examples is measured using a CS744 model carbon-sulfur analyzer, marketed by the company LECO.

[0113] The median size of a powder is classically measured using a LA950V2 laser particle size analyzer marketed by the company Horiba.

[0114] The X-ray diffraction pattern of the molten grains of the examples is produced on a powder of particles having passed through a sieve with an opening of 40 pm, obtained by grinding said grains.

[0115] The X-ray diffraction pattern is acquired using a Bruker D8 Endeavor instrument equipped with a copper anode, over an angular range of 5° to 100°, with a step size of 0.01° and a counting time of 0.34 s / step. The front optics include a 0.3° primary slit and a 2.5° Soller slit. The sample is rotated at a speed of 5 rpm using the automatic cutter. The rear optics include a 2.5° Soller slit, a 0.0125 mm nickel filter, and a 1D detector with a 4° aperture.

[0116] The diffraction diagrams are then qualitatively analyzed using the DIFFRAC.EVA software marketed by Bruker and the PDF5 + 2024 database.

[0117] For the fused grains according to the first main aspect of the invention, each surface required for the calculation of the RI ratio is selected in the X diffraction diagram using the "select area" function, and the value of the surface is the "net area" value given by the DIFFRAC.EVA software. Manufacturing protocol

[0118] The mixtures in the examples were prepared from the following raw materials: - an alumina powder of purity greater than 99.8% by mass, containing the impurities Na2O, CaO, Fe2O3, MgO, TiO2, SiO2, and having a median size of 80 pm; - a magnesia powder, of purity greater than 99% by mass, of which more than 85% of the grains, by mass, pass through the cloth of a 45 µm sieve.

[0119] Reference example 1 (“Ref”), outside the scope of this invention, is a mixture of molten grains marketed by Saint-Gobain Speciality Grains and Powders under the name MA88K.

[0120] The grain mixtures of Examples 2 to 8 were prepared according to the following manufacturing process: a) mixing of the raw materials so as to form a starting charge, said starting charge comprising 2% metallic aluminium chips and 0.5% petroleum coke, in mass percentages on the basis of said starting charge, b) melting in a reducing medium of said starting charge in a single-phase Héroult-type electric arc furnace with graphite electrodes, with a furnace vessel of 0.8 m diameter, a voltage of 170 V, a current of 1500 A and a specific electrical energy supplied equal to 2 kWh / kg charged, (c) rapid cooling of the molten material, so that said molten material is completely solidified in less than 3 minutes, by means of a casting device between thin metal plates such as that shown in US patent 3,993,119, so as to obtain a completely solid mass, in the form of a plate, d) grinding said solid mass cooled in step c) so as to reduce it to the form of a mixture of grains, e) selection by sieving using a Ro-Tap® sieve of grains having a size between 500 and 600 pm.

[0121] When a calcination step f) was carried out, it was carried out in air, at atmospheric pressure, at temperature T, the temperature T being maintained for a time t, the rate of rise to temperature T being equal to 300°C / h.

[0122] The following Table 1 provides the chemical composition of these grain mixtures, the crystallographic characteristics, the temperature T, the time t and the results obtained with said mixtures.

[0123] The percentage improvement of the S ratio is calculated by the following formula: 100x(S ratio of the mixture of the example considered - S ratio of the mixture of the reference example 1) / S ratio of the mixture of the reference example 1.

[0124] A high, positive value for the percentage improvement in the S ratio is sought, without a significant increase in specific energy, preferably with a decrease in specific energy (positive value for the percentage reduction in specific energy Es described below). The inventors consider a percentage improvement in the S ratio greater than 5% to be significant.

[0125] Preferably, the ratio S is improved by more than 10%, preferably by more than 20%, preferably by more than 30%, preferably by more than 40%, preferably by more than 50%, preferably by more than 60%.

[0126] The percentage reduction in specific energy, Es, is calculated using the following formula: 100x(Es with the mixture of reference example 1 - Es with the mixture of the example under consideration) / Es of the mixture of reference example 1.

[0127] The inventors consider a reduction of more than 5% in the specific energy Es to be significant. Preferably, the specific energy is reduced by more than 10%, preferably by more than 15% (compared to the reference).

[0128] [Tables] ex 1 (Re 0 ex 2 (*) ex 3 ex 4 (*) ex 5 ex 6 (*) ex 7 (*) ex 8 Step f) of calcination Temperature T (° C) - - 100 0 140 0 100 0 100 0 120 0 100 0 Time t (h) - - 4 2 4 2 1 4 Chemical analysis, in mass percentages based on the oxy groups of Al2O3 Complement to 100% MgO <0.03 2.56 2.56 2.56 3.50 3.60 3.60 4.50 Cr2O3 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 <0.03 Na2O 0.05 <0.05 <0.05 <0.05 <0.05 0.25 0.25 <0.05 Oxides other than MgO, Cr2O3 and Al2O3 <0.5 <0.15 <0.15 <0.15 <0.15 <0.15 <0.5 <0.5 <0.15 of which SiO2 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 of which TiO2 0.3 <0.01 <0.01 <0.01 0.03 0.1 0.1 <0.01 of which Fe2O3 0.04 <0.01 <0.01 <0.01 <0.03 <0.05 <0.05 0.02 of which CaO 0.02 0.02 0.02 0.02 0.02 0.06 <0.05 <0.05 0.02 Other characteristics Carbon content, in ppm, based on grain mass nd 100 0 800 nd 580 900 nd 510 Ratio R2 - 0 0.71 1 0.45 0.29 0.41 0.38 Ratio R3 - 0 1.27 31 0.20 <0.05 0.25 0 % improvement of S - 45 71 17 106 30 47 70 % reduction of Es - -13 0 -116 0 -16 -27 0

[0129]

[0130]

[0131]

[0132] (*) : outside invention n / a: not determined A comparison of examples 1 (reference example), 2 outside the invention, 3 according to the invention and 4 outside the invention, shows the impact of the calcination temperature: at the same MgO content of 2.56%, the mixture of grains of example 3, calcined at a temperature T of 1000°C for a time t of 4 hours, shows a percentage improvement in the positive S ratio of 71%, without degradation of the specific energy, whereas the same mixture of grains not calcined or calcined at a temperature T of 1400°C for a time t of 2 hours, shows a percentage improvement in the positive S ratio of 45% and 17%, respectively and a specific energy increase of 13% and 116%, respectively, in other words a degradation of the specific energy of 13% and 116%, respectively. A comparison of examples 1 (reference example), 5 according to the invention, 6 outside the invention, and 7 outside the invention shows the impact of the Na2O content: with substantially identical MgO content (3.50% for example 5, and 3.60% for examples 6 and 7), the grain mixture of example 5, calcined at a temperature T of 1000°C for a time t of 4 hours, shows a 106% improvement in the positive S ratio, without any degradation of the specific energy, whereas the grain mixture of example 6 calcined at a temperature T of 1000°C for a time t of 4 hours and the grain mixture of example 7 calcined at a temperature T of 1200°C for a time t equal to 1 hour, show a percentage improvement in the positive S ratio of 30% and 47%, respectively and a specific energy increase of 16% and 27%, respectively, in other words a degradation of the specific energy of 16% and 27%, respectively.

[0133] A comparison of examples 1 (reference example) and 8 according to the invention shows the positive impact of the presence of 4.50% of MgO in the mixture of grains of example 8, calcined at a temperature T equal to 1000°C for a time t equal to 4 hours: the ratio S is improved by 70% without degradation of the specific energy.

[0134] As is now clearly apparent, the invention provides a mixture of alumina-based molten grains exhibiting better yield and energy efficiency than the known alumina-based molten grains of the reference example for the abrasion of a surface in hard steel.

[0135] Of course, the present invention is not limited to the embodiments described, which are provided by way of illustrative and non-limiting examples.

[0136] In particular, the molten grains according to the invention are not limited to particular shapes or dimensions. They could be used in applications other than the abrasion of a surface in hardened steel.

Claims

Demands

1. Melted grain exhibiting - a chemical composition such that, in mass percentages on the basis of oxides: 2.30% < MgO < 4.80%, Cr2O3 < 0.2%, Na2O<0.1%, Oxides other than MgO, Cr2O3, Na2O and Al2O3 < 1.5%, Al2O3: complement to 100%; - a crystallographic composition characterized by an X diffraction pattern showing a "reference peak" in the angular range 20 between 18.1° and 20.1°, a first peak in the angular range 20 between 12.3° and 13.8° and, optionally, a second peak in the angular range 20 between 25.2° and 25.8°, or "corundum phase peak", each peak having a respective height, in number of counts, the ratio R2 of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R3 of the height of the corundum phase peak to the height of the reference peak being less than 2.

2. Melted grain according to the preceding claim, wherein - MgO > 2.50%, and / or - MgO < 4.50%, and / or - Cr2O3 < 0.15%, and / or - Na2O < 0.08%, and / or - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.4%.

3. Melted grain according to the immediately preceding claim, wherein - MgO > 2.70%, and / or - MgO < 4.20%, and / or - Cr2O3 < 0.10%, and / or - Na2O < 0.05%, and / or - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 1.0%.

4. Melted grain according to the immediately preceding claim, wherein - MgO > 2.80%, and / or - MgO < 4.10%, and / or - Cr2O3 < 0.05%, and / or - - the content of oxides other than MgO, Cr2O3, Na2O and Al2O3 is less than 0.4%.

5. Melted grain according to any one of the preceding claims, wherein - SiO2 < 0.3%, by mass percentages on the basis of oxides, and / or - CaO < 0.1%, by mass percentages on the basis of oxides.

6. Melted grain according to the immediately preceding claim, wherein - SiO2 <0.1%, by mass percentages on the basis of oxides, and / or - CaO < 0.08%, by mass percentages on the basis of oxides.

7. Melted grain according to any one of the preceding claims, wherein the oxide content is greater than 96%, in mass percentages on the basis of the mass of the melted grain.

8. Melted grain according to any one of the preceding claims, wherein - the R2 ratio is greater than 0.25, and / or - the R2 ratio is less than 1.0, and / or - the R3 ratio is less than 1 R

9. 1. Melted grain according to the immediately preceding claim, wherein - the ratio R2 is greater than 0.3, and / or - the ratio R2 is less than 0.8, and / or - the ratio R3 is less than 0.

5.

10. Melted grain according to any one of the preceding claims, said grain not being coated, even partially, with a coating comprising silica.

11. Grain mixture comprising, by mass percentage, more than 80% of melted grains according to any one of the preceding claims.

12. A method for manufacturing a mixture of molten grains according to the immediately preceding claim, said method comprising the following successive steps: a) mixing raw materials so as to form a starting charge suitable for manufacturing said mixture of grains, b) melting, preferably in a reducing medium, of said starting charge until a molten material is obtained, c) cooling of said molten material so as to solidify it entirely in less than 3 minutes, and obtain a solid mass, said solid mass from step c) being ground in a step d) so as to obtain a particulate mixture if it is not in the form of a particulate mixture, f) calcination of the solid mass and / or the particulate mixture at a temperature greater than or equal to 750°C and less than or equal to 1225°C.

13. A method according to the immediately preceding claim, comprising a step e) of particle size selection applied to the particulate mixture.

14. A method according to the immediately preceding claim, comprising a step e), step d) and step e) being carried out before step f).

15. A method according to any one of the three immediately preceding claims, wherein in step f), - the calcination temperature is greater than or equal to 800°C and less than or equal to 1200°C, and / or - the calcination temperature is maintained for a period greater than or equal to 30 minutes.

16. A method according to the preceding claim, wherein in step f), - the calcination temperature is greater than or equal to 950°C and less than or equal to 1150°C, and / or - the calcination temperature is maintained for a period greater than or equal to 2 hours.

17. Abrasive tool comprising grains - bound by a binder and agglomerated, or - deposited on a support, at least a part of said grains conforming to any one of claims 1 to 10.

18. Abrasive tool according to the immediately preceding claim, comprising more than 30%, by mass percentage, of grains according to any one of claims 1 to 10.

19. Abrasive tool according to any one of the two immediately preceding claims, in the form of a grinding wheel, a strip or a disc.

20. A method for treating a surface in hard steel, said method comprising an abrasion operation of said surface with a mixture of grains according to claim 11 or manufactured according to a method according to any one of claims 12 to 16.