Molten alumina-zirconia grains

Abstract

A fused grain having the following chemical composition in percent by weight in relation to the oxides: ZrO.sub.2: 16% to 30%, provided that HfO.sub.2<2%, Al.sub.2O.sub.3: percentage needed to bring the total to 100%, Cr.sub.2O.sub.3: ≥0.2%, TiO.sub.2: ≥0.5%, Cr.sub.2O.sub.3+TiO.sub.2: <7%, other elements: <3%, provided that SiO.sub.2+CaO+MgO<1.5%.

Claims

1. A fused grain exhibiting the following chemical analysis, as percentages by weight based on the oxides: ZrO.sub.2: 16% to 30%, provided that HfO.sub.2<2%, Al.sub.2O.sub.3: remainder to 100%, Cr.sub.2O.sub.3: ≥0.2% and ≤4%, TiO.sub.2: ≥0.5%, Cr.sub.2O.sub.3+TiO.sub.2: <7%, Other elements: <3%, provided that SiO.sub.2+CaO+MgO<1.5%.

2. The grain as claimed in claim 1, in which Cr.sub.2O.sub.3>0.4%.

3. The grain as claimed in claim 1, in which the TiO.sub.2 content is less than or equal to 6%, as percentages by weight based on the oxides.

4. The grain as claimed in claim 1, in which the ZrO.sub.2 content is greater than 18%, and/or in which the Cr.sub.2O.sub.3 content is greater than 0.5%, and/or in which the TiO.sub.2 content is greater than 0.8%, as percentages by weight based on the oxides.

5. The grain as claimed in claim 4, in which the ZrO.sub.2 content is greater than 20%, and/or in which the TiO.sub.2 content is greater than 1%, as percentages by weight based on the oxides.

6. The grain as claimed in claim 1, in which the ZrO.sub.2 content is less than 29%, and/or in which the Cr.sub.2O.sub.3 content is less than 3.2%, and/or in which the TiO.sub.2 content is less than 4.4%, as percentages by weight based on the oxides.

7. The grain as claimed claim 6, in which the ZrO.sub.2 content is less than 27%, and/or in which the Cr.sub.2O.sub.3 content is less than 2.2%, and/or in which the TiO.sub.2 content is less than 2.8%, as percentages by weight based on the oxides.

8. The grain as claimed in claim 1, in which the Cr.sub.2O.sub.3+TiO.sub.2 summed content is greater than 1.5% and less than 3.3%, as percentages by weight based on the oxides.

9. The grain as claimed in claim 1, in which the TiO.sub.2 content is greater than 4% and less than 6.5%, and in which the Cr.sub.2O.sub.3+TiO.sub.2 summed content is greater than 4.4% and preferably less than 6.9%, as percentages by weight based on the oxides.

10. The grain as claimed in claim 1, in which the content of other elements is less than 2%, as percentages by weight based on the oxides.

11. The grain as claimed in claim 1, in which the SiO.sub.2+CaO+MgO content is less than 1%.

12. The grain as claimed in claim 1, in which the SiO.sub.2 content is less than 1%, and/or the MgO content is less than 0.5%, and/or the CaO content is less than 0.5%, and/or the Na.sub.2O content is less than 0.1%, as percentages by weight based on the oxides.

13. The grain as claimed in claim 12, in which the SiO.sub.2 content is less than 0.8%, and/or the MgO content is less than 0.3%, and/or the CaO content is less than 0.3%, and/or the Na.sub.2O content is less than 0.05%, as percentages by weight based on the oxides.

14. A mixture of grains comprising, as percentages by weight, more than 80% of abrasive grains as claimed in claim 1.

15. An abrasive tool comprising grains bound by a binder, bonded or deposited on a support, at least a portion of said grains being in accordance with claim 1.

16. The abrasive tool as claimed in claim 15, wherein said at least a portion of said grains comprising more than 80% of said grains.

17. The abrasive tool as claimed in claim 15, which is provided in the form of a grinding wheel.

18. The grain as claimed in claim 10, in which the content of other elements is less than 1%, as percentages by weight based on the oxides.

19. The grain as claimed in claim 11, in which the SiO.sub.2+CaO+MgO content is less than 0.8%.

20. A fused grain exhibiting the following chemical analysis, as percentages by weight based on the oxides: ZrO.sub.2: 16% to 30%, provided that HfO.sub.2<2%, Al.sub.2O.sub.3: remainder to 100%, Cr.sub.2O.sub.3: ≥0.2%, TiO.sub.2: ≥0.5%, Cr.sub.2O.sub.3+TiO.sub.2: <5.2%, Other elements: <3%, provided that SiO.sub.2+CaO+MgO<1.5%.

21. The grain as claimed in claim 20, in which Cr.sub.2O.sub.3+TiO.sub.2<4%.

22. The grain as claimed in claim 20, in which Cr.sub.2O.sub.3+TiO.sub.2<3.3%.

23. A fused grain exhibiting the following chemical analysis, as percentages by weight based on the oxides: ZrO.sub.2: 16% to 30%, provided that HfO.sub.2<2%, Al.sub.2O.sub.3: remainder to 100%, Cr.sub.2O.sub.3: ≥0.2%. TiO.sub.2: ≥4%, Cr.sub.2O.sub.3+TiO.sub.2: <7%, Other elements: <3%, provided that SiO.sub.2+CaO+MgO <1.5%.

24. The grain as claimed in claim 23, in which TiO.sub.2<6.5%.

Description

DETAILED DESCRIPTION

(1) The description which follows is provided for illustrative purposes and does not limit the invention.

(2) Fused grains according to the invention may be manufactured according to the abovementioned stages a) to e), which are conventional for the manufacture of alumina-zirconia grains. The parameters may, for example, take the values of the process used for the examples below.

(3) In stage a), starting materials are conventionally metered out, so as to obtain the desired composition, and are then mixed in order to form the feedstock.

(4) The metals Zr, Hf, Al and Ti in the feedstock are found substantially in full in the fused grains.

(5) However, the element chromium may be partially volatilized, in particular in an oxide form, during the melting. A person skilled in the art knows how to consequently adjust the composition of the feedstock.

(6) The metals Zr, Hf, Al, Cr and Ti are preferably introduced into the feedstock in the form of oxides ZrO.sub.2, HfO.sub.2, Al.sub.2O.sub.1, Cr.sub.2O.sub.3 and TiO.sub.2. They may also be conventionally introduced in the form of precursors of these oxides.

(7) In one embodiment, the feedstock consists of oxides ZrO.sub.2, HfO.sub.2, Al.sub.2O.sub.3, Cr.sub.2O.sub.3 and TiO.sub.2 and/or of precursors of these oxides, and of a source of carbon.

(8) Preferably, the feedstock comprises an amount of carbon, preferably in the form of coke, of between 1% and 4%, based on the weight of the feedstock.

(9) It is considered that a content of “other elements” of less than 3% in the grains does not suppress the technical effect provided by the invention, provided that SiO.sub.2+CaO+MgO<1.5%.

(10) If SiO.sub.2+CaO+MgO≥1.5%, the abrasive performance qualities are inadequate.

(11) The “other elements” are preferably impurities. Preferably, the content of impurities is less than 2%, less than 1%, indeed even less than 0.5%.

(12) In stage b), use is preferably made of an electric arc furnace, preferably of Héroult type with graphite electrodes, but any furnace known may be envisaged, such as an induction furnace or a plasma furnace, provided that they make it possible to melt the feedstock. The starting materials are preferably melted in a reducing environment (with in particular an addition of a source of carbon, for example petroleum coke, pitch or coal, to the furnace), preferably at atmospheric pressure.

(13) Preferably, use is made of an electric arc furnace, comprising a vessel with a capacity of 80 liters, with a melting energy before pouring of at least 1.5 kWh per kg of starting materials for a power of at least 150 kW, or an electric arc furnace with a different capacity employed under equivalent conditions. A person skilled in the art knows how to determine such equivalent conditions.

(14) In stage c), the cooling has to be rapid, that is to say so that the molten material has completely solidified in less than 3 minutes. For example, it may result from a pouring into molds, as described in U.S. Pat. No. 3,993,119, or from a quenching.

(15) If stage c) does not make it possible to obtain a powder of grains, or if these grains do not exhibit a particle size distribution suited to the application targeted, a grinding (stage d)) may be carried out, according to conventional techniques.

(16) In stage e), if the preceding stages do not make it possible to obtain a powder of grains exhibiting a particle size distribution suited to the application targeted, a particle size selection, for example by sieving or cycloning, may be carried out.

(17) The processes for the manufacture of the abrasive tools according to the invention are well known.

(18) The bonded abrasive tools, in particular a grinding wheel, may be formed by pressing into shape a mixture of abrasive grains and of a binder. In an abrasive tool according to the invention, the binder may be vitrified (for example, a binder consisting of oxides, essentially silicate) or organic. An organic binder is highly suitable.

(19) The binder may in particular be a thermosetting resin. It may be chosen from the group consisting of phenolic, epoxy, acrylate, polyester, polyamide, polybenzimidazole, polyurethane, phenoxy, phenol-furfural, aniline-formaldehyde, urea-formaldehyde, cresol-aldehyde, resorcinol-aldehyde, urea-aldehyde or melamine-formaldehyde resins, and mixtures of these.

(20) Usually, the binder represents between 2% and 60%, preferably between 20% and 40%, by volume of the mixture. The binder may also incorporate organic or inorganic fillers, such as hydrated inorganic fillers (for example alumina trihydrate or boehmite) or nonhydrated inorganic fillers (for example molybdenum oxide), 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, fibers, sulfides, chlorides, sulfates, fluorides, and mixtures of these. The binder may also contain reinforcing fibers, such as glass fibers.

Examples

(21) The following nonlimiting examples are given for the purpose of illustrating the invention.

(22) The products given as examples were prepared from the following starting materials: Alumina powder sold under the name AR75 by Alteo, exhibiting an alumina content of greater than 99.4% and a sodium oxide content of less than 2500 ppm; Zirconia powder having a mean zirconia content of greater than 85%, containing on average 5% of silica, an alumina content of less than 10%, a hafnium oxide content of less than 2%, a content of other oxides of less than 1% and a maximum size equal to 13 mm; Titanium oxide powder, “Rutile Sand Premium Grade”, sold by Traxys FrancePra, exhibiting a TiO.sub.2 content >95%, and 80% by weight of the particles of which exhibit a size of less than 106 μm; Pigmentary chromium oxide Cr.sub.2O.sub.3 powder sold under the name Bayoxide® C GN-R by Lanxess, exhibiting a Cr.sub.2O.sub.3 content of greater than 98.5% by weight; Pitch coke sold by Altichem, with a size of between 1 and 4 mm.

(23) The grains were prepared according to the following conventional process, well known to a person skilled in the art: a) mixing the starting materials so as to form a feedstock, b) melting in a single-phase electric arc furnace of Héroult type comprising graphite electrodes, with a furnace vessel having a capacity of 80 liters and a diameter of 0.8 m, a voltage of 145-150V, a current of 1700 A and a specific electrical energy supplied equal to 1.7 kWh/kg charged, c) sudden cooling of the molten material by means of a device for casting between thin metal plates, such as that presented in the patent U.S. Pat. No. 3,993,119, so as to obtain a completely solid sheet, constituting a solid mass. d) grinding said solid mass cooled in stage c), so as to obtain a mixture of grains, e) selection by sieving of the grains between 500 and 600 μm.
The compositions of the feedstocks, as percentages by weight, used in stage a) to manufacture the grains of the different examples are provided in the following table 1.

(24) TABLE-US-00001 TABLE 1 Alumina Zirconia Titanium Chromium Pitch Examples powder powder oxide powder oxide powder coke Comp1 66 25.5 0 6.6 1.9 Comp2 68.3 26.7 3 0 2 Comp3 67.5 25.5 0 5.1 1.9 1 67.7 27 2.5 0.8 2 2 68.5 27 1.9 0.6 2 Comp4 69 27 2 0 2 Comp5 59.2 23.8 0 15 2 Comp6 64.8 25.2 8 0 2 3 64.7 25.3 7 1 2

(25) In order to evaluate the performance qualities and the lifetimes of the mixtures of grains, grinding wheels with a diameter of 12.6 cm, containing 1.02 grams of grains of each example, were produced according to the following method: a disk made of steel of 4140 grade, with a diameter of 12.6 cm and with a thickness equal to 6 mm, is cleaned. The edge face of the disk (defining its thickness) is then covered with a phenolic resin. A single layer of test grains is subsequently deposited uniformly over said resin, which is still sufficiently warm to remain tacky. After drying in a cycle exhibiting a total duration equal to 17 hours and a maximum temperature achieved equal to 175° C., a layer of phenolic resin is applied over the test grains and then the assembly is placed in an oven in a cycle exhibiting a total duration equal to 17 hours and a maximum temperature achieved equal to 175° C., so as to obtain the test grinding wheel.

(26) Plates made of 304 stainless steel, with dimensions of 20.5 cm×7.6 cm×6 cm, were subsequently machined at the surface with these grinding wheels, with a to-and-fro movement at a constant speed while maintaining a constant cutting depth of 40 μm and a rotational speed of the grinding wheel of 3600 rev/min. The maximum power developed by the grinding wheel during the machining, P.sub.max, was recorded.

(27) After the grinding wheel has been completely worn away, the weight of machined steel (that is to say, the weight of steel removed by the grinding operation), “Ma”, and the weight of grinding wheel consumed, “Mm”, were measured. The S ratio is equal to the Ma/Mm ratio.

(28) The cutting efficiency is determined by measuring the maximum power developed by the grinding wheel during the machining test, P.sub.max, and the lifetime of the grinding wheel, t.sub.max, the lifetime of a grinding wheel being regarded as complete when all the grains of the grinding wheel have been consumed.

(29) The chemical compositions of different mixtures of grains tested are provided in table 2. The results obtained with these mixtures are provided in table 3.

(30) To highlight the respective effects of the titanium oxide and of the chromium oxide, the examples to be compared should exhibit the same total content of these two oxides. Example 1 should thus be compared with comparative example 1 or comparative example 2. Example 2 should thus be compared with comparative example 3 or comparative example 4. Example 3 should be compared with comparative example 5 or comparative example 6.

(31) The percentage of improvement in the S ratio is calculated by the following formula:
100.(S ratio of the product of the example considered−S ratio of the product of the reference example)/S ratio of the product of the reference example,
the reference example being comparative example 1 or comparative example 2 for example 1, comparative example 3 or comparative example 4 for example 2, and comparative example 5 or comparative example 6 for example 3.

(32) The percentage of reduction in the maximum power developed by the grinding wheel during the test, P.sub.max, is calculated by the following formula:
100.(P.sub.max, with the product of the reference example−P.sub.max, with the product of the example considered)/P.sub.max of the product of the reference example,
the reference example being comparative example 1, comparative example 2, comparative example 3, comparative example 4, comparative example 5 or comparative example 6, as for the determination of the percentage of improvement in the S ratio. A positive and high value for the percentage of reduction in the maximum power developed by the grinding wheel during the test, P.sub.max, is desired.

(33) The percentage of improvement in the lifetime of the grinding wheel, t.sub.max is calculated by the following formula:
100.(t.sub.max of the product of the example considered−t.sub.max of the product of the reference example)/t.sub.max of the product of the reference example,
the reference example being comparative example 1, comparative example 2, comparative example 3, comparative example 4, comparative example 5 or comparative example 6, as for the determination of the percentage of improvement in the S ratio. A positive and high value for the percentage of improvement in the lifetime of the grinding wheel, t.sub.max, is desired.

(34) The results obtained are summarized in the following tables 2, 3 and 4.

(35) Comparative examples 2, 4 and 6 are mixtures of grains according to U.S. Pat. No. 5,143,522 and comparative examples 1, 3 and 5 are mixtures of grains according to U.S. Pat. No. 4,035,162.

(36) The grains of the comparative examples were sieved between 500 and 600 μm.

(37) TABLE-US-00002 TABLE 2 Other elements, expressed in the form of ZrO.sub.2 + HfO.sub.2 Cr.sub.2O.sub.3 TiO.sub.2 Cr.sub.2O.sub.3 + TiO.sub.2 oxides (%) Example (%) Al.sub.2O.sub.3 (%) (%) (%) (%) Total SiO.sub.2 Comp1 24.4 Remainder to 100% 2.54 0.10 2.64 <1.23 0.53 Comp2 26.0 Remainder to 100% 0.03 2.77 2.80 <1.30 0.60 Comp3 25.0 Remainder to 100% 1.93 0.09 2.02 <1.22 0.52 1 26.0 Remainder to 100% 0.61 2.11 2.72 <0.90 0.20 2 24.5 Remainder to 100% 0.46 1.60 2.06 <0.83 0.13 Comp4 25.9 Remainder to 100% 0.01 2.19 2.19 <0.65 0.30 Comp5 24.5 Remainder to 100% 6.95 0.10 7.05 <0.53 0.35 Comp6 24.1 Remainder to 100% 0.01 7.09 7.10 <0.67 0.43 3 24.4 Remainder to 100% 0.77 6.17 6.94 <0.47 0.27

(38) In all the examples, Na.sub.2O<0.05%, MgO<0.05%, CaO<0.05%, SiO.sub.2+CaO+MgO<0.8%, based on the oxides. Carbon C always represents less than 0.20% of the weight of the grains.

(39) TABLE-US-00003 TABLE 3 P.sub.max S ratio % of % of % of t.sub.max % of % of % of reduction/ reduction/ reduction/ % of % of % of improvement/ improvement/ improvement/ example example example improvement/ improvement/ improvement/ Example example comp1 example comp2 example comp3 comp1 comp2 comp3 example comp1 example comp2 example comp3 1 40 3 — 23 9 — 14 17 — 2 — — 56 — — 33 — — 37

(40) TABLE-US-00004 TABLE 4 P.sub.max S ratio % of % of % of t.sub.max % of % of % of reduction/ reduction/ reduction/ % of % of % of improvement/ improvement/ improvement/ example example example improvement/ improvement/ improvement/ Example example comp4 example comp5 example comp6 comp4 comp5 comp6 example comp4 example comp5 example comp6 2 5 — — 25 — — 28 — — 3 — 25 1 — 14 13 — 20 22

(41) The inventors consider that there exists a good compromise between the S ratio, the maximum power developed by the grinding wheel during the machining test, P.sub.max, and the lifetime of the grinding wheel, t.sub.max, when: on the one hand, the S ratio is identical to or greater than the products of the reference examples, and on the other hand the maximum power developed, P.sub.max, is reduced by at least 5%, with respect to the products of the reference examples, and/or the lifetime of the grinding wheel, t.sub.max, is improved by at least 6%, with respect to the products of the reference examples.

(42) Preferably, the S ratio is improved by at least 5%, preferably by at least 10%, preferably by at least 15%, preferably by at least 20%, indeed even by at least 25%, and/or the maximum power developed, P.sub.max, is reduced by at least 10%, preferably by at least 15%, indeed even by at least 20%, indeed even by at least 25%, and/or the lifetime of the grinding wheel, t.sub.max, is improved by at least 10%, preferably by at least 15%, indeed even by at least 20%.

(43) A comparison of examples 1 and comp1 shows the importance of a minimum TiO.sub.2 content, for a Cr.sub.2O.sub.3+TiO.sub.2 sum of approximately 2.7%: the S ratio is improved by 40%, P.sub.max, is reduced by 23% and t.sub.max is improved by 14%.

(44) A comparison of examples 2 and comp3 also shows the importance of a minimum TiO.sub.2 content, for a Cr.sub.2O.sub.3+TiO.sub.2 sum of approximately 2.1%: the S ratio is improved by 56%, P.sub.max is reduced by 33% and t.sub.max is improved by 37%.

(45) A comparison of examples 3 and comp5 also shows the importance of a minimum TiO.sub.2 content, for a Cr.sub.2O.sub.3+TiO.sub.2 sum of approximately 7.0%: the S ratio is improved by 25%, P.sub.max is reduced by 14% and t.sub.max is improved by 20%.

(46) A comparison of examples 1 and comp2 shows the importance of a minimum Cr.sub.2O.sub.3 content: the S ratio is improved by 3%, P.sub.max is reduced by 9% and t.sub.max is improved by 17%.

(47) A comparison of examples 2 and comp4 also shows the importance of a minimum Cr.sub.2O.sub.3 content: the S ratio is improved by 5%, P.sub.max is reduced by 25% and t.sub.max is improved by 28%.

(48) A comparison of examples 3 and comp6 also shows the importance of a minimum Cr.sub.2O.sub.3 content: the S ratio is improved by 1%, P.sub.max is reduced by 13% and t.sub.max is improved by 22%.

(49) Examples 1, 2 and 3 according to the invention thus observe the desired compromise.

(50) These comparisons clearly show the advantage of the simultaneous presence of Cr.sub.2O.sub.3 and TiO.sub.2 within the claimed ranges.

(51) As is now clearly apparent, the invention provides a mixture of abrasive fused alumina-zirconia grains exhibiting an exceptional abrasive performance, an exceptional endurance and an exceptional cutting efficiency.

(52) Of course, the present invention is not, however, limited to the embodiments described and represented, which are provided by way of illustrative and nonlimiting examples.