Alumina-based fused grain

Abstract

The invention relates to an abrasive tool that comprises fused grains having: • - a chemical composition such that, in percentages by weight on the basis of the oxides: MgO: 4.8% to 7.2%, Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and Al2O3: < 1.5%; Al2O3: the remainder to make up 100%; • - a crystallographic composition characterised by an X-ray diffraction pattern having a "reference peak" in the 2θ angular range of between 18.1° and 20.1°, a first peak in the 2θ angular range of between 12.3° and 13.8° and, optionally, a second peak in the 2θ angular range of between 25.2° and 25.8°, or the "corundum phase peak", each peak having a respective height, in terms of number of counts, the ratio R1 of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1.

Description

[0001] Description

[0002] Title: Alumina-based fused grain

[0003] Technical field

[0004] The present invention relates to a fused alumina-based grain, a mixture of said grains, as well as a method for manufacturing said mixture of grains and an abrasive tool comprising said mixture of grains. The invention also relates to uses of grains according to the invention for abrading a surface made of a hard steel.

[0005] Prior art

[0006] It is well known that the choice of grains for abrading a surface depends closely on the nature of the surface. To abrade a surface made of hard steel, we know how to use MA88K grains marketed by the company Saint-Gobain Speciality Grains and Powders.

[0007] Abrasive tools are generally classified according to the packaging of their abrasive grains: loose abrasives (powders of grains not fixed to a support, used in projection or in suspension), applied abrasives (support such as cloth or paper, on which the grains are arranged in several layers) and agglomerated abrasives (circular wheels, stick wheels, etc.).

[0008] In bonded abrasives, the abrasive grains are pressed with an organic or vitreous binder, typically a binder consisting of oxides, essentially silicate. The abrasive grains must themselves have good mechanical abrasion properties, and have good mechanical cohesion with the binder, i.e. the interface with the binder must be solid.

[0009] Among abrasive grains, a distinction is made between fused grains and sintered grains, which have different microstructures. The problems posed by sintered grains and by fused grains, and the technical solutions adopted to resolve them, are therefore generally different. A composition developed to produce a fused grain is therefore not a priori usable to produce a sintered grain with the same properties, and vice versa.

[0010] FR 2 928 916 discloses in particular fused grains developed by the Applicant. These grains require a silica coating before being incorporated into an abrasive tool. WO2023111157 also discloses fused grains developed by the Applicant. These grains require the presence of more than 0.2% Cr2O3, preferably at least 1.0% Cr2O3.

[0011] There is a continuing need to improve the performance of fused alumina-based grains used to abrade a surface of hard steel, particularly to increase efficiency while maintaining energy efficiency greater than or substantially equal to that of prior art grains.

[0012] One aim of the invention is to meet, at least partially, this need.

[0013] Summary of the invention

[0014] According to the invention, this aim is achieved by means of a fused grain, having:

[0015] - a chemical composition such that, in mass percentages based on oxides:

[0016] MgO: 4.8% to 7.2%;

[0017] Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and A12O3: < 1.5%;

[0018] A12O3: 100% complement;

[0019] - a crystallographic composition characterized by a diffraction pattern

[0020] X having a “reference peak” in the angular range 29 between 18.1° and 20.1°, a first peak in the angular range 29 between 12.3° and 13.8° and, optionally, a second peak in the angular range 29 between 25.2° and 25.8°, or “corundum phase peak”, each peak having a respective height, in number of shots, the ratio RI of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1.

[0021] As will be seen in more detail in the remainder of the description, the inventors have discovered that, for the abrasion of a surface made of hard steel, the above chemical and crystallographic compositions provide superior performance and energy efficiency at least equal to those of known alumina-based fused grains.

[0022] A melted grain according to the invention may also have one or more of the following optional characteristics:

[0023] - MgO > 5.9%, preferably MgO > 5.8%; - MgO < 7.0%, preferably MgO < 6.5%;

[0024] - Cr2O3: < 0.10%, preferably Cr2O3: < 0.03%;

[0025] - the content of oxides other than MgO, C^Ch and AI2O3 is less than 1.0%, preferably less than 0.4%;

[0026] - Na2O < 0.3%, preferably Na2O < 0.1%, in mass percentages based on the oxides;

[0027] - SiO2 < 0.3%, preferably SiO2 < 0.1%, in mass percentages based on the oxides;

[0028] - the oxide content is greater than 96%, in percentages by mass based on the mass of the molten grain;

[0029] - the ratio R2 is less than 0.7, preferably less than 0.3, preferably less than 0.05;

[0030] - the RI ratio is greater than 0.25, preferably greater than 0.3;

[0031] - the RI ratio is less than 0.5;

[0032] - the release of gaseous hydrogen by hot acid attack for a mixture made up of said grains is less than 200 cm 3 / 100 g.

[0033] The invention also relates to a mixture of grains comprising, in mass percentage, more than 80% of melted grains according to the invention.

[0034] 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, 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, f) calcination of the solid mass, before or after an optional particle size reduction, at a temperature greater than or equal to 750°C and less than or equal to 1225°C.

[0035] The inventors consider that the RI and R2 peaks of a grain according to the invention are, with the chemical composition, a signature of this process. Conventionally, specialists in the abrasion of a surface made of hard steel consider as a general rule that it is necessary, to machine such a surface, to use a grain with high hardness, such as a fused corundum-based grain described in FR2853898A1.

[0036] Starting from FR2853898A1, the inventors tested calcination temperatures lower than those recommended in this document. They observed, on the X-ray diffraction patterns, a sudden decrease in the height of the peaks of the corundum phase. However, and contrary to the general rule above, the inventors tested these grains to abrade a surface made of hard steel. Surprisingly, they found that certain fused grains containing little or no corundum could lead to superior performances than those of corundum-based fused grains, provided that they have a magnesia content in a specific range and that their manufacture includes calcination in a particular temperature range.

[0037] This discovery is particularly illustrated by the comparison of examples 2 and 8, according to the invention, on the one hand, and 3, outside the invention and in accordance with the teaching of FR2853898A1, on the other hand. The grains according to the invention prove to be much more effective for treating a surface made of hard steel.

[0038] A manufacturing method according to the invention may also have one or more of the following optional characteristics:

[0039] - the solid mass resulting from step c) is ground during 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;

[0040] - the method comprises a said step e), step d) and step e) being carried out before step f);

[0041] - 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

[0042] 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.

[0043] The invention also 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 strip or a disc, this tool being remarkable in that at least a part, preferably more than 50%, preferably more than 70%, preferably more than 80%, preferably more than 90%, preferably more than 95%, preferably more than 99%, in mass percentage, preferably all of said grains are in accordance with the invention and / or manufactured according to a method according to the invention.

[0044] The abrasive tool may be a grinding wheel, and in particular a grinding wheel, a precision grinding wheel, a sharpening wheel, a cutting wheel, a grinding wheel, a roughing or deburring wheel, a driving wheel, a portable grinding wheel, a grinding wheel for foundries, a drill grinding wheel, a grinding wheel on a rod, a cylindrical grinding wheel, a cone grinding wheel, a disc grinding wheel or a segment grinding wheel.

[0045] The invention finally relates to a method for treating a surface made of hard steel, said method comprising an operation of abrasion of said surface with a mixture of grains according to the invention or manufactured according to a manufacturing method according to the invention.

[0046] Preferably, a treatment method according to the invention comprises

[0047] - a production of a mixture of grains according to the invention, preferably according to a production method according to the invention, then

[0048] - an operation of abrasion of said surface with said mixture of grains, preferably after putting the mixture of grains in the form of an abrasive tool according to the invention.

[0049] Definitions

[0050] In this description, unless otherwise stated, all compositions of a grain or mixture of grains are given in mass percentages, based on the total mass of oxides in the grain or mixture of grains.

[0051] The oxide contents of a grain according to the invention, including for “oxides other than MgO, Cr2O3 and AI2O3”, relate to the overall contents for each of the corresponding chemical elements, expressed in the form of the most stable oxide, according to the usual industry convention; therefore included are sub-oxides and possibly nitrides, oxynitrides, carbides, oxycarbides, carbonitrides, or even the metallic species of the aforementioned elements.

[0052] By "impurities" is meant the unavoidable constituents, necessarily introduced with the raw materials. In particular, the compounds belonging to the group of oxides, nitrides, oxynitrides, carbides, oxycarbides, carbonitrides and metallic species of silicon, sodium and other alkalis, iron, and vanadium are impurities. By way of examples, in the sources of Al and Mg, mention may be made of SiCl, Fe2O3 or Na2O. By "precursor" of an oxide is meant a constituent capable of providing said oxide during the manufacture of a grain or a mixture of grains according to the invention.

[0053] A “grain” is a particle with all dimensions less than 20 mm.

[0054] An "alumina-based grain" is a grain containing more than 85% alumina by mass, as a percentage based on oxides.

[0055] By "molten grain", or more broadly "molten product", we mean a solid grain (or product) obtained by solidification, by cooling, of a molten material.

[0056] A "molten material" is a mass made liquid by heating a starting charge, which may contain some solid particles, but in insufficient quantity for them to structure said mass. To retain its shape, a molten material must be contained in a container. The molten grains according to the invention are conventionally obtained by melting at more than 1900°C.

[0057] The median size of a powder is the size dividing the particles into first and second populations of equal mass, these first and second populations containing only particles having a size greater than, equal to, or less than, respectively, the median size. The median size of a powder can be determined using a particle size distribution carried out using a laser particle size analyzer.

[0058] In this description, "hard steel" means a steel having a Rockwell hardness greater than or equal to 55 HRC. Detailed description

[0059] The following description is provided for illustrative purposes and does not limit the invention.

[0060] The chemical composition of a fused grain according to the invention, and preferably of a mixture of grains according to the invention, preferably has one or more of the following optional and preferred characteristics:

[0061] - MgO > 4.9%, preferably MgO > 5.0%, preferably MgO > 5.2%, preferably MgO > 5.5%, preferably MgO > 5.8%, and / or preferably MgO < 7.1%, preferably MgO < 7.0%, preferably MgO < 6.8%, preferably MgO < 6.5%, in mass percentages based on the oxides;

[0062] - G2O3 < 0.15%, preferably G2O3 < 0.10%, preferably G2O3 < 0.05%, preferably Cr2O3 < 0.03%, preferably Cr2O3 < 0.01%, in mass percentages based on the oxides;

[0063] - Cr2O3 is not voluntarily added to the starting charge, i.e. is preferably an impurity;

[0064] - the content of oxides other than MgO, C^Ch and AI2O3 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 based on the oxides;

[0065] - oxides other than MgO and AI2O3 are preferably impurities;

[0066] - the Na2O content is less than 0.3%, preferably less than 0.25%, preferably less than 0.2%, preferably less than 0.15%, preferably less than 0.1%, in mass percentages based on the oxides;

[0067] - the SiCh 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 based on the oxides;

[0068] - 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 percentages by mass based on the mass of the molten grain;

[0069] - the release of hydrogen gas by hot acid attack, expressed in volume of gas per 100 grams of grains, is less than 200 cm 3 / 100 g, preferably less than 100 cm 3 / 100 g, and preferably greater than 2 cm 3 / 100 g. It is measured as described in detail in the examples.

[0070] The X-ray diffraction pattern of a fused grain according to the invention, and preferably of a mixture of grains according to the invention, preferably has one or more of the following optional characteristics: the ratio R2 is less than 0.9, preferably less than 0.7, preferably less than 0.5, preferably less than 0.3, preferably less than 0.1, preferably less than 0.05; the ratio RI is greater than 0.25, preferably greater than 0.3, and preferably less than 0.5, preferably less than 0.4.

[0071] Grain mixture A grain mixture according to the invention comprises, in mass percentages, preferably more than 85%, preferably more than 90%, preferably more than 95%, preferably more than 99%, preferably substantially 100% of melted grains according to the invention.

[0072] Preferably, a mixture of grains according to the invention respects a particle size distribution in accordance with those of the mixtures or “grits” provided by the FEPA Standard 43-GB-1984, R1993 and the FEPA Standard 42-GB-1984, R1993.

[0073] Preferably, a grain mixture according to the invention has a mass rejection at the 16 mm sieve, preferably at the 9.51 mm sieve, measured using a Ro-Tap® sieve, of less than 1%, as a percentage by mass.

[0074] Method for manufacturing a mixture of molten grains according to the invention

[0075] Fused grains according to the invention can be manufactured according to steps a) to f) mentioned above. Unlike the grains described in FR 2 928 916, they are not coated with a silica coating. The parameters can, for example, take the values ​​of the process used for the examples below.

[0076] In step a), raw materials are conventionally dosed to obtain the desired composition, then mixed to form the starting charge.

[0077] The elements Al and Mg in the starting charge are found substantially entirely in the melted grains. The element Mg, particularly in the form of oxides, can, however, be subject to fly-off phenomena during melting. A person skilled in the art knows how to adapt the composition of the starting charge accordingly.

[0078] 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 to those skilled in the art.

[0079] The element Mg is preferably introduced into the starting charge in the form of the oxide MgO. It can also be conventionally introduced in the form of precursors of this oxide, for example in the form of MgCCL.

[0080] The element Al is preferably at least partly introduced into the feedstock in the form AI2O3 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 feedstock partly in the form AI2O3 and partly in a metallic form.

[0081] In a preferred embodiment, the starting charge comprises at least one compound creating a reducing medium upon melting.

[0082] 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.

[0083] More preferably, said at least one compound creating a reducing medium during melting and used in the starting charge is chosen from petroleum coke, aluminum and their mixtures.

[0084] A person skilled in the art knows how to determine the quantity of compound creating a reducing medium during fusion, in the starting charge, to obtain, in step b), fusion in a reducing medium.

[0085] Preferably, the starting charge contains an amount of compound creating a reducing medium upon melting greater than 1%, preferably greater than 1.5% and preferably less than 5%, preferably less than 4%, as a percentage by mass based on the starting charge.

[0086] In step b), an electric arc furnace is preferably used, preferably of the Héroult type with graphite electrodes, but all known furnaces are possible, such as an induction furnace or a plasma furnace, provided that they allow the starting charge to be melted in a reducing medium.

[0087] 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.

[0088] Preferably, the starting charge contains elements that create a reducing medium during melting.

[0089] Preferably, the raw materials are melted at atmospheric pressure.

[0090] Preferably, an electric arc furnace is used, comprising a 70-litre tank, with a pre-casting melting energy of more than 2 kWh per kg of raw materials for a power of more than 220 kW, or an electric arc furnace of different capacity operated under equivalent conditions. A person skilled in the art knows how to determine such equivalent conditions.

[0091] In step c), the cooling must be rapid, i.e. so that the molten material is completely solidified in less than 3 minutes. For example, it may result from casting into molds such as described in US 3,993,119 or from quenching.

[0092] 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.

[0093] In one embodiment, step c) results in a solid mass in the form of a particulate mixture.

[0094] 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, using conventional techniques.

[0095] A particle size selection (step e)), for example by sieving or cycloning, can be implemented to obtain a mixture of grains having a particle size suitable for the intended application.

[0096] Step d) and / or step e) may be carried out before or after the calcination step f). It / they are preferably carried out before step f).

[0097] 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 1000 ppm, said content being measured using a carbon-sulfur analyzer model CS744, marketed by the company LECO.

[0098] In step f), 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.

[0099] If the 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).

[0100] 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 preferably being maintained for a time 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.

[0101] Preferably, step f) is carried out at atmospheric pressure.

[0102] Step f) can be carried out in a reducing, neutral or oxidizing atmosphere.

[0103] Preferably, step f) is carried out in an oxidizing atmosphere, preferably in air.

[0104] Preferably, step f) is carried out in 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 preferably being 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.

[0105] Method of manufacturing an abrasive tool according to the invention

[0106] Methods of manufacturing abrasive tools are well known and can be used to manufacture an abrasive tool according to the invention.

[0107] The abrasive tools may in particular be formed by agglomeration of 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 belt or a disc, by means of a binder.

[0108] The binder may be inorganic, in particular a glass (for example, a binder consisting of oxides, substantially consisting of silicate(s) may be used) or organic.

[0109] An organic binder is well suited. The binder may be, in particular, a thermosetting resin. It is preferably chosen from the group consisting of phenolic, epoxy, acrylate, polyester, polyamide, polybenzimidazole, polyurethane, phenoxy, phenol-furfural, analine-formaldehyde, urea-formaldehyde, cresol-aldehyde, resorcinol-aldehyde, urea-aldehyde, melamine-formaldehyde resins, and mixtures thereof.

[0110] 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, 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, vinylidene chloride and vinyl chloride copolymer, polyvinylidene chloride, polyvinyl chloride, and mixtures thereof. The binder may also contain reinforcing fibers such as glass fibers.

[0111] Conventionally, a mixture of grains according to the invention is mixed with a said binder optionally containing organic or inorganic fillers.

[0112] Unlike the grains described in FR 2 928 916, the grains are not coated with a silica coating.

[0113] The mixture obtained, 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. Then the binder is activated, for example by heating, to bind the grains together and / or with the optional support. After hardening of the binder and optionally demolding, an abrasive tool according to the invention is obtained.

[0114] Examples

[0115] The following non-limiting examples are given for the purpose of illustrating the invention.

[0116] Measurement protocols

[0117] The following measurement protocols were used to determine some properties of fused grain mixtures. They provide an excellent simulation of the actual behavior of grains when used for abrasion.

[0118] In order to evaluate the abrasive performance of a mixture of grains from the examples, a 1.02 gram monolayer of this mixture is applied to a 12.7 cm diameter metal grinding wheel, said grains being bonded using a phenolic resin. The surface of a 52100 hard steel plate with a Rockwell hardness between 60 and 63 HRC, with dimensions of 20.5 cm x 7.6 cm x 6.0 cm, is then machined with the resulting grinding wheel, under water spray, with 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, E to t, is recorded.

[0119] After complete wear of the grinding wheel, the mass of machined steel (i.e. the mass of steel removed by the grinding operation) is measured "M a » and the volume of steel removed by the grinding operation « V a ".

[0120] To evaluate the yield, we conventionally calculate 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.

[0121] To assess energy efficiency, the specific machining energy, Es, is conventionally calculated, equal to the energy required to remove a unit volume of steel (Es = Etot / V a ).

[0122] To determine the composition of the fused grains, a bead of a mixture of these grains is made by melting the mixture, and then chemical analysis is carried out by X-ray fluorescence, except for the measurement of the carbon content.

[0123] The median size of a powder is conventionally measured using a laser granulometer model LA950V2 marketed by the Horiba company.

[0124] The quantity of hydrogen gas released by hot acid attack is measured after hot attack of the grains with a mixture of hydrochloric and hydrofluoric acid. This evaluates the reoxidation of under-oxidized species (sub-oxides, up to the metal).

[0125] For this, after magnetic separation, the grains are ground in a grinding bowl made of an oxidized material (for example, a fused alumina-zirconia-silica material) until a powder is obtained that passes through a square mesh sieve with an opening equal to 160 μm. 5 g of said powder is taken and placed in a polypropylene reactor with a volume of 100 cm 3. Then add 25 ml of the following acid mixture: (for one liter) 250 ml of 40% HF, 375 ml of 37% HCl, and 375 ml of water. After closing the reactor, the attack is carried out at 85°C, in a water bath, for 25 minutes, stirring regularly. After cooling the reactor, preferably to a temperature of 20°C, approximately 0.5 ml is taken using a syringe, through a septum, and injected into a gas chromatograph with catharometer detection (with, for the separation column, a 5 angstrom molecular sieve and argon as the carrier gas). The result is expressed as the volume of gas under standard conditions per 100 g of ground grains.

[0126] This release is called “release of hydrogen gas by hot acid attack”.

[0127] The X-ray diffraction diagram of the molten grains of the examples is carried out on a powder of particles having passed through a sieve with an opening equal to 40 pm, obtained by grinding said grains.

[0128] The acquisition of the X-ray diffraction pattern is carried out using a Bruker D8 Endeavor device, over an angular range 29 between 5° and 100°, with a step of 0.01°, and a counting time of 0.34 s / step. The front optics include a 0.3° primary slit, a 2.5° Soller slit and a 0.0125 mm nickel filter. The sample is rotated on itself at a speed equal to 5 rpm, using the automatic knife. The rear optics include a 2.5° Soller slit, a 0.0125 mm nickel filter and a 1D detector with an aperture equal to 4°.

[0129] The diffraction patterns are then qualitatively analyzed using the DIFFRAC.EVA software marketed by Bruker and the PDF4 + 2023 database.

[0130] Manufacturing protocol

[0131] The mixtures in the examples were prepared from the following raw materials:

[0132] - an alumina powder with a purity greater than 99.6% by mass, comprising the impurities Na?O, CaO, Fe2O3, MgO, TiCL, SiCL, and having a median size equal to 80 pm;

[0133] - a magnesia powder, with a purity greater than 99% by mass, of which more than 85% of the grains, by mass, pass through the cloth of a 45 pm sieve.

[0134] Reference example 1 (“Ref”), outside the invention, is a mixture of fused grains marketed by the company Saint-Gobain Speciality Grains and Powders under the name MA88K.

[0135] The grain mixtures of Examples 2 to 11 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% by mass of metallic aluminum shavings and 0.5% by mass of petroleum coke, b) melting in a reducing medium of said starting charge in a single-phase electric arc furnace of the Héroult type with graphite electrodes, with a furnace tank 0.8 m in diameter, a voltage of 125 V, an intensity of 1040 A and a specific electrical energy supplied of 3 kWh / kg charged, c) sudden cooling of the molten material, so that said molten material is entirely solidified in less than 3 minutes, by means of a casting device between thin metal plates such as that presented in patent US-A-3,993,119, so as to obtain a completely solid plate, constituting a solid mass,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, f) calcination 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 80°C / h.,

[0136] 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.

[0137] 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.

[0138] A positive and high value of the percentage improvement of the S ratio is sought, without a significant increase in the specific energy, preferably with a decrease in the specific energy (positive value of the percentage reduction of the specific energy Es described below). The inventors consider a percentage improvement of the S ratio greater than 5% to be significant.

[0139] Preferably, the S ratio is improved by more than 10%, preferably more than 20%, preferably more than 30%, preferably more than 40%, preferably more than 50%, preferably more than 60%.

[0140] The percentage reduction in specific energy, Es, is calculated by the following formula:

[0141] 100x(Es with the mixture of reference example 1 - Es with the mixture of the example considered) / Es of the mixture of reference example 1. The inventors consider significant a reduction of more than 5% of the specific energy Es. Preferably, the specific energy is reduced by more than 10%, preferably by more than 15% (compared to the reference).

[0142] [Table 1] (*): excluding invention nd: not determined

[0143] A comparison of examples 1 (reference example), 2 according to the invention and 3 outside the invention and in accordance with the teaching of FR2853898A1, shows the impact of the calcination temperature: at the same MgO content equal to 6.3%, the mixture of grains of example 2, calcined at a temperature T equal to 1200°C for a time t equal to 2 hours, has a percentage improvement in the positive S ratio of 30% and a reduced specific energy of 6%, whereas the same mixture of grains calcined at a temperature T equal to 1250°C for a time t equal to 2 hours, has a percentage improvement in the negative S ratio of 25% and an increase in the specific energy of 20%.A comparison of Examples 1 (reference example), 4 and 5 outside the invention, and 6 according to the invention, shows the positive impact of the presence of 4.8% of MgO in the grain mixture of Example 6, calcined at a temperature T equal to 1000°C for a time t equal to 4 hours: the S ratio is improved by 70% without degradation of the specific energy. The grain mixtures of Examples 4 and 5, outside the invention, having an MgO content equal to 3.6% and calcined at a temperature T equal to 1000°C and 1200°C, respectively, for a time t equal to 2 hours and 1 hour respectively, have an improved S ratio of 30% and 47% respectively, and an increase in the specific energy of 16% and 27% respectively.

[0144] A comparison of reference examples 1, and 7 and 8 according to the invention, shows that the grain mixtures of example 7 and example 8, both calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 5.1% and 6.3% respectively, have an improved S ratio of 68% and 80% respectively, and a reduction in specific energy of 6% and 22% respectively.

[0145] A comparison of reference examples 1 and 9 outside the invention shows that the mixture of grains of example 9 calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 8.2%, presents an improvement in the S ratio of 15% and an increase in the specific energy of 22%.

[0146] A comparison of examples 1, reference, and 10 outside the invention, shows that the mixture of grains of example 10 calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 8.2%, presents a decrease in the S ratio of 10% and an increase in the specific energy of 35%.

[0147] A comparison of examples 1, reference, and 11 outside the invention, shows that the mixture of grains of example 11 calcined at a temperature T equal to 1000°C for a time t equal to 4 hours, having an MgO content equal to 7.6%, presents an increase in the S ratio of 47% and an increase in the specific energy of 12%.

[0148] As now clearly appears, the invention provides a mixture of alumina-based fused grains having better efficiency and energy efficiency than the known alumina-based fused grains of the reference example for abrasion of a surface of a hard steel.

[0149] Of course, the present invention is not limited to the described embodiments provided as illustrative and non-limiting examples. In particular, the fused 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 made of hard steel.

Claims

CLAIMS 1. Abrasive tool comprising grains bound by a binder and agglomerated or deposited on a support, at least a portion of said grains each being a fused grain having - a chemical composition such that, in mass percentages based on oxides: MgO: 4.8% to 7.2%; Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and A12O3: < 1.5%; A12O3: 100% complement; the oxide content is greater than 96%, in percentages by mass based on the mass of the molten grain, - a crystallographic composition characterized by a diffraction pattern X having a “reference peak” in the angular range 29 between 18.1° and 20.1°, a first peak in the angular range 29 between 12.3° and 13.8° and, optionally, a second peak in the angular range 29 between 25.2° and 25.8°, or “corundum phase peak”, each peak having a respective height, in number of shots, the ratio RI of the height of the first peak to the height of the reference peak being greater than 0.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1, said molten grains being manufactured according to a method comprising the following successive steps: a) mixing raw materials so as to form a starting charge suitable for the manufacture of said molten grains, b) melting in a reducing medium of said starting charge until a molten material is obtained, c) cooling said molten material so as to completely solidify it in less than 3 minutes,and obtaining a solid mass, d) before or after step f), grinding the solid mass from step c) so as to obtain a particulate mixture, f) calcining the particulate mixture at a temperature greater than or equal to 750°C and less than or equal to 1225°C so as to obtain said fused grains.

2. Abrasive tool according to the preceding claim, in which the molten grain has a chemical composition such that - MgO > 5.0%, and / or - MgO < 7.0%, and / or - G2O3: < 0.10%, and / or - the content of oxides other than MgO, Cr2O3 and AI2O3 is less than 1.0%.

3. An abrasive tool according to the immediately preceding claim, wherein the fused grain has a chemical composition such that - MgO > 5.8%, and / or - MgO < 6.5%, and / or - Cr2O3: < 0.03%, and / or - the content of oxides other than MgO, Cr2O3 and AI2O3 is less than 0.4%.

4. An abrasive tool according to any preceding claim, wherein the fused grain has a chemical composition such that - Na2O < 0.3%, in mass percentages based on oxides, and / or - SiO2 < 0.3%, in mass percentages based on oxides.

5. An abrasive tool according to the immediately preceding claim, wherein the fused grain has a chemical composition such that - Na2O < 0.1%, in mass percentages based on oxides, and / or - SiO2 < 0.1%, in mass percentages based on oxides.

6. An abrasive tool according to any preceding claim, wherein the fused grain has a crystallographic composition such that - the R2 ratio is less than 0.7, and / or - the RI ratio is greater than 0.

25.

7. An abrasive tool according to the immediately preceding claim, wherein the fused grain has a crystallographic composition such that - the R2 ratio is less than 0.3, and / or - the RI ratio is greater than 0.

3.

8. Abrasive tool according to the immediately preceding claim, in which the fused grain has a crystallographic composition such that the ratio R2 is less than 0.

05.

9. An abrasive tool according to any preceding claim, wherein the fused grain has a crystallographic composition such that the RI ratio is less than 0.

5.

10. An abrasive tool according to any preceding claim, wherein the release of hydrogen gas by hot acid attack for a mixture consisting of said molten grains is less than 200 cm 3 / 100 g, a release of hydrogen gas by hot acid attack of a mixture of grains being measured according to the following steps: - after magnetic separation, grinding of the grains in a grinding bowl made of an oxidized material until a powder is obtained passing through a square mesh sieve with an opening equal to 160 pm; - taking 5 g of said powder and placing said 5 g in a polypropylene reactor with a volume of 100 cm 3 ; - addition of 25 ml of an acid mixture comprising, for one liter, 250 ml of 40% HF, 375 ml of 37% HCl, and 375 ml of water; - closure of the reactor; - attack at 85°C, in a bain-marie, for 25 minutes, stirring regularly; - cooling of the reactor; - sampling, using a syringe, through a septum, of approximately 0.5 ml and injection of said 0.5 ml into a gas chromatograph in catharometer detection, with, for the separation column, a 5 angstrom molecular sieve and argon as carrier gas; the release of hydrogen gas by hot acid attack being the volume of gas obtained expressed under normal conditions for 100 g of ground grains.

11. An abrasive tool according to any one of the preceding claims, wherein said at least a portion of said grains comprises, in mass percentage, more than 80% of said fused grains.

12. Abrasive tool according to any one of the preceding claims, in which the method comprises a step e) of particle size selection applied to the particulate mixture.

13. Abrasive tool according to the immediately preceding claim, step d) and step e) being carried out before step f).

14. An abrasive tool according to any preceding claim, 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.

15. Abrasive tool according to the immediately 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.

16. An abrasive tool according to any preceding claim, in the form of a grinding wheel, belt or disc.

17. Method for treating a surface of a steel having a Rockwell hardness greater than or equal to 55 HRC, said method comprising an operation of abrasion of said surface with - a mixture of grains comprising, in mass percentage, more than 80% of molten grains having - a chemical composition such that, in mass percentages based on oxides: MgO: 4.8% to 7.2%; Cr2O3: < 0.2%; oxides other than MgO, Cr2O3 and A12O3: < 1.5%; A12O3: 100% complement; - a crystallographic composition characterized by a diffraction pattern X having a “reference peak” in the angular domain 29 between 18.1° and 20.1°, a first peak in the angular range 29 between 12.3° and 13.8° and, optionally, a second peak in the angular range 29 between 25.2° and 25.8°, or “corundum phase peak”, each peak having a respective height, in number of shots, the ratio RI of the height of the first peak to the height of the reference peak being greater than 9.2 and the ratio R2 of the height of the corundum phase peak to the height of the reference peak being less than 1, or - with an abrasive tool according to any one of the preceding claims.

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