CA1056605A - Zirconia chromia alumina abrasive - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Abrasive materials comprising a fused and solidified crystalline mixture of zirconia, chromia, eutectic and in other examples zirconia in a chromia alumina solid solution phase, are described. The fused and solidified mass is finely crystalline, has a low porosity and the abrasive grains are essentially free of such an amount of chromium metal as might detrimentally affect the grinding action when the grains are bonded in a grinding wheel and used for a normal grinding activity.

Description

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This invention relates to zirconia alumina abrasives.
In the last few years improved abrasive systems have been developed based on the alumina zirconia alloy system described in the Marshall & Roschuk United States Patent 3,181,939. The present invention is the result of efforts to produce better and/or e~uivalent pPrformance with cornpositions other than alumina zirconia.
Many materials have been recommended in the past for the production of abrasive grits. For example~ United States Patent 2,279,260 to Benner et al discusses abrasives formed of alumina chromia alloys which require the presence of a signi-ficant percentage of magnesia to make a useful product. ; ;
The Polch United States Patent 2,769,699 describes `
the use of a crushed, fused zirconia grain (which may or may ' 15 not be stabilized with lime or magnesia) as an abrasive .. , ~
material.
Rosenberg et al United States Patent 2,768,887 is directed to a furnacing procedure for making an abrasive .
;~ material consisting predominantly of alumina and a relatively very small amount of chromium oxide with perhaps very small portions of other metallic oxides including the oxide of zirconium. In addition the fusion is described as being per-; formed under reducing conditions, or at least under conditions where graphite is present and car~on infiltration of the fused :
mass takes place from the electrodes whereupon any chromia

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or other metal oxide addition present is reduced to produce significant amounts of free chromium or other metal in the molten bath, which reduced material remains in the ~olidified mass in an intercrystalline position as a metal or metal carbide. Rosenberg specifies that the oxide~ other than alumina are preferably present in amounts less than 5~ and are usually considerably less than 5~ of the total weight of material.
Baumann et al United States Patent 2,418,496 shows an abrasive grit wherein the basic alumina abrasive is toughened by the addition of either chromia, vanadia, iron oxide, and/or manganese oxide in solid solution with the , alumina. This patent describes a used alumina chromia ; product that has been quick chilled and crushed to make abrasive grains.
Robie United States Patent 2,877,104, describes an ! abrasive made predominantly from zirconia and titania with a ' small amount of other oxides being present. Among these .~, .
other oxides are listed oxides of the trivalent elements chromium and aluminum. There are no examples of materials containing oxides o~ or giving proportions for fusions of the oxides of chromium, aluminum and/or zirconium. The only trivalent material shown in the examples are fusions of ~` Zr2 and TiO2 with FeO in the presence of a significant ; 25 amount of silica. The patent only states generally and in an incidental way that an abrasive can be made with a composi-tion of chromia alumina zirconia and silica but, as already stated, no proportions for these ingredients are given.
German Patent Application 2,160,705 published on June 14, 1973 is another disclosure like Robie. This appli-

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, ~35 ca~ion i~ enti~led "An Abra~iva With H:Lgh ~rain ~oughn~
and a Proce~s ~or it~ Manu~acture". It ~imply suggests ~
: that an abra~ive may be mad~ uqlng alumina zirconia and ~:.
chromia in a ~usion, but no Qxamples or o~her d~talls are included in the de~cription which i~ concern2~ with a cooling proc~dur~ ~or the ~usion.
According to the invention ~here i~ provided abr~slv~ ~
grit3 of a ~u~ed, solidif~ed, crus~ed composltion which i3 ~ ~ ' e~s~ntially ~r~e o~ pora~ and which comprises by weight ~rom 10~ to 79~ ZrO2, 5% to 15~ Cr2O3 and 15~ to 85~ A12O3, the :
composition includlng less than 0.15~ soda and le88 than 2.25%
free chromium mstal w~th the total o~ impurities being les~ ;
than 5 I ~he i~vention is ~llustrated in the aacompanying :~
; 15 drawmg~ in whlch~
; Fig. 1 is a ternary diagram oP the ~u~ed zirconia, alumina, chromia ~y~tem of this invention; :~
Fig. 2 i9 a picture mad~ in an electron microscope .
at 10~0 time~ magn~ication, o~ a typical alumina chromla zirconia crystalline mass ~howing one ~orm o~ thi~'inventio~s.and Fig. 3 is a pictur~ mad~ in an electron microscope at 2400 times magni~ica~ion o~ an eutectlc chromia zirconia compo~ition showing anoth~r ~orm o~ this invention.
Specl~lcally ther~ are provlded abr~lve mater-' ~:

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ials which result from a fusion of zirconia and chromia in one form and zirconia with solid solution chromia and alumina in another form. Serviceable and novel abrasives result from making fusions in the binary zirco~ia chromia syqtem for precision grinding, and other u~eful abrasives of this dis-cloaure result from producing a fused ternary, zirconia chromia alumina product for snagying grinding in a given composition range. The abrasive material of this invention ~; is formed under fusion conditions which tend to prevent insofar as possible the reduction of the chromia whereby to minimize the presence of free chromium metal in the fused system for the eutectic and binary precision grinding abra-sives although the presence of some metal inclu~ions can be tolerated in abrasive grains having particular use for snagging grinding. It is equally important that the system be free of gas generating compounds, so that the solidified product has a density near theoretical. The product is pre-; ferably rapidly cooled to encourage the production of finer crystals in the solidified mass.
As is well known in producing a metal oxide fusion in a large arc furnace when Cr203 is present, due to the relatively high conductivity of the fused chromia, about a 25% chromia content is the practical limit that may be -~ tolerated except in a range close to the eutectic propor-tions where fusions can be more easily controlled. While fusions have been made with higher percentages of chromia above 25-26% present the con~rol of the arc becomes very difficult except at about the eutectic range. The spacing of the electrodes adjacent the surface of the molten bath is critical and a preferred source of energy is usually of 1'1;','~ .
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relatively low voltage and high amperage, but the furnace is preferably operated with as much space as possible between the electrode and the bath to prevent reduction of the chromia. In runs made during the development of this product, fusions have been made showing as little as 0.27%
chromium metal present and other fusions showing as much as 2.25% o chromium me~al content by weight. In precision abrasives as stated above a minimum of chromium metal is desired while with a snagging grain the higher limit o metal inclusion that can be tolerated is about 2.25~ in the final product.
The product resulting from chill cooling the ;
molten ma~s produced includes essentially two phases, one is zirconia crystal~ and the other is crystals o solld solution chromia and alumina. In the ternary non eutectic compositions, the solidified mass includes primary crystals o either 2rO2 or solid solution Cr203 A1203 supported in a ; second eutectic phase which in turn is constituted of much finer zirconia crystals apparently present in the form o rods or platelets disposed in parallel xelation solidified in a chromia aLumina solid solution mass. In solidified compositions falling along the ternary eutectic line there are virtually no primary ZrO2 or Cr203 Al203 solid solution .. . .
crystals, it is all the eutectic phase just described. Above a 5% addition of chromLa by weight is needed to provide sufficient chromia in solid solution with the alumina to show a noticeable diference in ~nagging grinding as com-, pared with the known alumina zirconia snagging abrasives.
A typical cxystalline structure that results rom ;
following this teaching is shown in Fig. 2. The larger ..
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primary crystal 10 there shown is a chromia alumina solid solution that first precipitated out of the molten bath when it was chilled. It has been noted that the chromia tends to be unevenly distributed throughout ~uch solid solution crystals and tends to partially concentrate to a small extent in an internal phase of each of the various primary A1203 Cr203 crystals. The zirconia in this partic- ;
ular composition crystallized out of the fusion in the form of rods or pla~elets 11 which appear as the lighter colored elements in Fig. 2, these rods or platelets are gathered together in cells or colonies that are supported in a olidified mass of extremely finely crystalline chromia alumina solid solution 12 that appears as the darker pha~e i between the zirconia rods or platelets.
Referring to the ternary diagram of Fig. 1 the point X on the zirconia alumina line, at about 56% A1203 an~
44% ZrO2, is the eutectic composition that results from fusing these two compounds togèther. Similarly the point ~
on the zirconia chromia line, at about 54% ZrO2 and 46% Cr203, indicates the eutectic composition for such a binary fusion.
~he line X-Y indicates the full range of the apparent eutectic compositions that can be formed by fusing these three com-pounds, A1203 Cr203 ZrO2 together.
The preferred ternary system of this in~ention for producing abrasi~e grain suggested for use as a snagging abrasive like that shown in Fig. 2 is included within the :., parallelogram shape ABCF of Fig. 1, wherein a range of com-'~ positions of zirconia from L0% tv about 70% by weight is shown combined with the chromia alumina addition that may be varied in the final composition in from an amount of 5%

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chromia r~present~d by line A~B and up to 25% Cr2O3 indic~t~d by line C-F of ~he ~usion, ~ith alumlna belng pr~nt and variabla withln a rang~ o~ ~rom 5% rapr~0nt~d by point C to pre~erably a~ much aa abou~ 85~ indica~ed ~y polnt A. On tho other hand, an abra~lve grain ~avlng primary utility ~or pre cision grlndlng result~ ~rom ~u3ing a mixtuxe o~ th0 ingred-iant~ to produce the zirconia chromia eutectic a~ shown in ~ig. 3 or a ternary mixture ~alllng clo~e to the appar0nt eutectlc llne X-Y o~ ~ig. 1. Such preoi~ion abra~ives will ~all within ~ho txapez~idal area de~ined by DEHG in Fig. 1.
Compo~ition lying within the area DEHG ar~ claim~ in Canadian Patent Application 215,124 and do not ~orm a part o~ the lnv~n-tion o~ the present applicatlon.
E~AMPLE I ,"
;l 15 A lS0 lb. mixture o~ 56 parts by weight o a?umina,~
14 parts by weigh~ o chromià and 30 parts by weight o zir-conia was made. Thls mixture wa~ ~ed to a sm~ll electric arc ~urnace having a car~on rod therein to a~ in starting the ~usion. The ~urnace was operated at a relatively l~w voltage . .
~103-135 volts) and low power (130-150 kilowatts) in order ~o ;~
make it po~3ible to keep the car~on electrodes in ~ust the -~ ?
proper epaced rolation ~o the bath to prevent the in~iltration i o~ ~urther carbon in the ~urnace to minimize the reduction o~
chromia~ Wh~n a molten bath had boen establi~hed an addition-al charge o~ 200 lbs. o~ tha alumina chromia zirconia mixture J wa~ ~ed into th~ ~urnace undor non-reducing condi~ion~ inso~
far as conveniently possible. A~ter the bath had been re~der-' ed molten and h~l~ for about 20 mlnutes, ~he fu~d ma~s wa3 f 'l ca8t into a quick chilling mold to provide Por the very rapid 9 ~ 30 coollng o~ tho m~lt. The cooled product wa~ crushed and -~
~ ~ varioua grit ~iZ~5 8 to 60 m0~h wer~ prepared by normal ;~
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The material produced in Example I had the following composition by weight percent.
TABLE I
A1203 ~ 59.64~ SiO2 - 0.35~ CaO - 0.14% C - 0.02%
Cr~03 9.81~ Fe203 - 0.46~ MgO - 0.02%
Zr2 ~ 27.14% TiO2 - 0.13~ Na20 - 0.04%
Cr metal 2.25%
This material was tested in a lab~ratory snagging grinding test against a standard commercial alumina zirconia abrasive of 25% zirconia with the results set forth in Table II. Billets of alloy steel were ground with wheels operating at a pressure o 400 lbs.
In laboratory comparative tests for new snagging abrasives, two wheels are made, one with the new abrasive and one with a standard abrasive for compaxison. In the case of this invention there were made 2 - 16" wheels with snagging abrasive grains, one with a commercial cofused alumina zirconia grain with about 25% zirconia present, and the other with the grain to be tested, the wheels being made with a standard resin bond in the form of laminar wheels. The center sections of the5e wheels were 1" thick with two side faces 17/32nds of an inch thick. A typical center section for each . .
wheel include~ grits of commercial alumina 2irconia or the grain to be tested, in the 10 to 12 grit range for example with the side faces having 16 grit fused crushed alumina grits in the same bond.
In running the test ~hese laminar wheels are com-pared in a grinder having a hydraulic pressure means to maintain the wheel being tested under a prassure of about 400 lbs. against the billet. The wheels are run under simulated _ 9- , ;.' ' :

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snagging grinding conditions at 9500 s.f.p.m. (surface feet `~
per minute) for an initial break-in period of 20 minutes and ~ ~ .
then under the same pressure and s.f.p.m. the same wheel is run for its test in three consecutive 20 minute runs. After 5 each 20 minute period the billet is removed from under the wheel and weighed while another billet of the identical composition is put under the wheel for each of the succeeding runs. Each of the three billets is weighed in turn and the cumulative weight of metal removed is determined. The wheel ~`
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is weighed o~ce at the end of the three runs to determine the amount of wheel wear. During the running of the test, the side faces of the laminar wheels, being made of a finer alumina abrasive, act aQ soft side faces and erode away so that the es~ential grinding action against the billet is performed by the center section of the respective wheels. ~ ~;
While commercially operated snagging wheels are : -:
subjected to much more severe pressure and speed conditions ~ ;~
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in actual field operations and the above testing procedure does not duplicate such commercial grinding conditions it has , 20 been found that the above described testing procedure does provida a very accurate comparative test procedure ~or the ... . . .
evaluation of newly developed grains.
TABLE II
Relativ~ Relative Relative Wheel Material Grinding Rate of Rate of Wear Removed MR Ratio Cut Wheel WW MR "a"* WW RGR RRC ~ear RRWW
~m~rcial Al Zr abrasi~e 78.9 164 12 2.08 1.00 100 100 .
~, A1203-Cr203-ZrO2 72.5 180 5 2.48 1.19 110 92 *"a" value determination is explained in "Abrasives"
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authored by Loring Coes, Jr., copyrighted 1971 by Springer-Verlag/Wien. See Chapter 12 beginning on page 122.
The value "a" is a constan~ which is a measure o~
the destructibility factor of the abrasive in grinding a particular metal. The lower the "a" value the better the grain. This "a" value, as shown on page 126 of the Coes publication, is inversely proportional to the melting point of the abrasive grain composition in a relation to the melting point of the metal being ground. Its units are inches3/hr.
and may be calculated for a given constant force grinding, to the wheel performance as follows:
M ~ KPVW/(W+a), where M is pounds of metal removed per hour, K is the con~tant ; 15which measures the grindability of the metal, P ~s the ~orce in lbs. with which the wheel is applied to the work, V LS the ;~
surface speed of the wheel in feet per minute, W is the wheel wear cubic inches per hour.
Other examples of this invention are set forth in Table III below showing data on compositions that were fused in a large commercial arc furnace and chill cast in a mold.
TABLE III

- Mean Free Path,*
Alpha Alumina-Impact Chromia primary ~- Composition~%/wt. or K at Crystals, in A120~ Cr203 ZrO~ Impurities 210'~sec. Microns ;~ ;
62.25.3 30.3 2.2% Q.08 11.3 ~ -56.313.1 27.6 3.0% 0.08 13.6 3~ 51.215.9 2~.4 3.5~ 0.08 15.0 48.623.7 25.7 2.0% 0.08 16.2 42.~11.1 43.8 3.1% 0.12 47.87.6 41.7 ~.08 . -11- ;

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(The % impurities above are residual amounts remaining after fusion, which are impurities normally present in ores from which the A1203-ZrO2-Cr203 were obtained.) *The mean free path reported here was determined based on the discussion given in "Quantitative Microscopy" by Robert T. DeHoff and Frederick N. Rhines, copyrighted 1968 by McGraw Hill Book Company, New York, Chapter 9, p. 283. The original reference given there is the article of R. L. Fullman entitled "Measurement of Particle Sizes in Opaque BodieSI' in Trans. Met. Soc. AIME, 197, 447 (1953). "Mean Free Path" is the average separation of the alpha alumina chromia solid solution crystal-matrix interfaces measured through the crystals in all directions. Thus, the mean free path listed above represents an average crystal size o the primary alumina chromia solid solution. The matrix in this case is the eutectic phase formed between the said solid solution and zirconia.
All of the fusions set forth in Table III were made in a casting furnace and during each run a number of mo~ds like those disclosed in the Sco~t application were filled.
The resulting product after being dumped from the molds was crushed and the portion passing through a 6 mesh Tyler screen and retained on 24 mesh,- was collected. In the different runs from 84~ to 87% of the initially crushed material was recovered. This grain was then subjected to urther treatment to ~hape the grains and eliminate weak grains. Before shaping, ~o :
a sample of the 6 to 24 mesh grain retained on a 14 mesh Tyler screen taken rom the 62.2~ A1203-5.3% Cr203-30.3% 2rO2 product was tested and ound to have an impact K value of 0.13~
Ater shaping, another portion o the sample o the 14 mesh grain of that composition was tested and had a K value of 0.08. The K value mentioned was determined by impacting the grains with a paddle moving at a rate of 210' per second in a device as described in an article "Single Impact Testing of Brittle ~aterials" by Karpinski and Tervo pu~lished in the June 1964 Transactions, of the Society of Mining Engineers, -pages 126 to 130. (In the article "r" is used instead of the ,~ . . .
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term now preferred, namely K value). K value is an indication of the resistance o:E a sample of grain to impact breakdown and the lower the K value the tougher the grain, that is, fewer grains of the given test sample are broken down in the impact test machine. K value is merely an abstract com-parative value. Thus it is seen that the above mentioned crushed grain having a K value of 0.130 was improved as to its resistance to impact breakage to have a K value of 0.08 after the grain shaping step.
All of the fusions listed in Table III we:re sub~
jected to a shaping procedure and recoveries of from 42% to as much as 5896 of the several Eusions were made.
The last two fu8ions in Table III were de~igned to all substantially along line X-Y of Fig. 1. A typical eutectic fusion is shown in Fig. 3. In these, substantially all eutectic compositions, no primary crystals solidified out of the fusion as would happen if there were an excess of alumina or zirconia present for example. The all eutectic composition solidifies to produce a product made up of a eutectic of chromia 12 or of solid solution alumina chromia (the darker area in Fig. 3) wit~ zirconia crystals 11 ~the lighter area in Fig. 3) dispersed throughout in the form of parallel rods or platelets. The eutectic solidified in~
so-called colonies having an estimated avexage diameter o ::;
35.1 microns in the 42% A1203-11.1% Cr203-43.8% ZrO2 fusion and 31.9microns in the 47.8% A1203-7.6% Cr203 41.7% ZrO2 composition.
Grains were made from a zirconia chromla fusion in eutectic proportions as represented by point Y in Fig. l of about 48% chromia and about 52~ zirconia with clS much as 396 .;: .
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impurities presen~ and upon solidification 60 me~h grains were used in making a 5" resin bonded wheel for comparison with a premium grade of conventional commercial fused 60 mesh alumina abrasive used for precision grinding. Tests were run on a surface grinder at 5500 s.f.p.m. wheel speed, at 50' per minute traverse and a 1 mil feed, grinding two different steels under wet grinding conditions with a rim of resin bonded abrasive on the surface of a 5" x 3/16" x 1-1/4" wheel. The impurities found in the chromia zirconia eutectic composition consisted of SiO2 0.28%, Na20 0.14%, C 0.06% and Fe203 2.8~.
The test results were recorded as follows:
STEEL I

W.W. M.R. Average PeakPower mills mills G-Ratio Watts Commercial abrasives 24.2 8 3.60 1350 ;`~
Cr23 Zr2 30.5 5.S 2.00 1850 Cr203 Zr2 28.0 5.4 2.05 2000 STEEL II
Commercial abrasives 9.8 14.0 15.3 1000 Cr23 Zr2 11.3 14.4 1~.7 1150 Cr203 Zr2 12.0 13.8 11.9 1350 While the grinding ratio of the eutectic composition is less than that of the standard in each case it should be ; 2S noted that there were substantial percentages of both soda and silica impurities present therein which if eliminated as known in the art, would upgrade the abrasive grindability of the product. The product in the eutectic range thus appears . , .
to have good utility for precision grinding uses.

The commercial abrasive with which the zirconia . . .

~LO~ 6~5 chromia eutectic grain was compared, is a premium type abrasive monocrystalline grain crystallized with a decompos-; able sulfide matrix.
In some more recent tests of the eutectic grain, a comparison was made between a purer ZrO2.Cr2O~ eutecticand commercial abrasive as well as another commercial precision abrasive. The latter is a fused crushed alumina grain recommended for use in precision ~rinding and other uses the grain comprising about 50% monocrystalline grains as compared with 100~ monocrystalline grain of the irst commercial abrasive. This eutectic grain was analyzed to ~6~ ZrO2, 43.80~ Cr2O3, 2.S% A12O3, 0-14% SiO2, 0.36~ Fe2O3, 0.08% ~io2, 0.06% CaO, 0.02~ MgO, 0.02~ Na2O
and 0.56% chromium metal. It will be noted that the soda con~ent of this grain is much less than that used in the test reported above. This eutectic composition was tested in grinding against the same two steels as previously, with the following results~
Steel I
W.W. M.R. Average Peak inch~8i inches G-Ratio Pcwer Watts First Commercial abrasive .0232 .0077 3.88 1250 Cr2O3 Zr2 .027 .0080 3.67 1650 Steel II
W.W. M.R. Average Peak inches inches G-Ratio Power Watts ~-First commercial ~ abrasive .0076 .0155 24.3* 500 ., , Second Commercial abrasive .0110 .0130 14.3 800 -.. . ..
cr23 zro2 .0080 .0160 25.2 950 * This g ratio is not typical of the first commercial abrasive and appears to be a freak for which there is no quick -15~

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~ ~5~5 answer. The normal g ratio for grinding Steel II with that abrasive is in the order of 15 to 17.
In general the abrasive grain of the present in-vention is more useful for snagging and has a composition such that the proportions of ~irconia, chromia and alumina ~ , ~
(when present) are encompassed by the area ABCF on the ternary diagram of Fig. 1. In other cases the compositions included in the trapeæoidal area DEHG have properties suggesting their use for precision grinding activities.
The impurity content of the alumina and zirconia materials used for this abrasive material should be controlled `~
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so that it has no more impurities than are found in normal arc urnace alumina-zirconia products produced from presenkly " . .
t commercially available alumina or zirconia such as Bayer . j ~ ,, alumina or the grades of zircon and bauxite currently used in the abrasive industry. It is important, however, that the source of the chromia addition must be carefully chosen or `
the chromia must be derived from a source that has been , . . .
;~ purified so that the amount of Na2O is minimized in the final product.
The fusion process must be carried out under controlled conditions which will not give any appreciable ; amount of free chromium metal in the final product to be . ...
used for precision grinding but we can tolerate as much as 25 from about 2% to about 2.25% metal in a usion that is to ~ J :: ~
t be crushed for snagging grinding applications.
~` The abrasive should have less than about 0.15%
~; Na2O and preferably as little soda as possible also less than .3% chromium metal for precision grinding although as -. ,, ~ ~.
shown above, in some applications higher Cr metal contents, 16 ~ !
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even as high as 2.25%, are not too detrimental to the use of the grain in snagging grinding operations. ~he general level of other impurities that might normally be present in the raw materials being fused are controlled in the prep-aration of the feed to the furnace and by controlling thefurnacing operations to maintain them at levels preferably less than those specified below:
- Impurity Maximum ~ b~ Wei~
Sio2 1 . 0' ) e203 ( e ) 2.3 ) CaO

MgO )Total of all )impurities not to C )exceed 5%
Tio ., , SrO, V203, MnO2, BaO, 3 Y203, La2O3, CeO2 and other rare earth oxides *Some iron impurities over and above that which is normally present in the raw materials may come from iron borings introduced in the start up process of the arc fusion furnace.
In addition to the composition and impurity limitations on the product it is highly desirable that the product contain extremely small crystals of zirconia. By rapidly chilling this product as described in the art for alumina zirconia abrasives, an acceptable finely crystalline product results. In the preferred embodiment of the invention this small crystal size is achieved by using the quick cooling technique wherein the molten abrasive mixture is poured into a container filled with spaced apart metal plates ; to provide an almost instantaneous freezing of the abrasive product.

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While the examples given a~ove ~how the ut~lity of this product ~or the manu~aatur~ o~ grlnding wh~ol~ ~or either preci~ion or snagging grindin~, its properti~s may `~' al80 recommend it ~or use in the manu~actura o~ coa~sd : ~ :
`- 5 abra3iveg.
.' Re~erenc~ i8 hereby made to Canad~an Patent Application 215,124 dlrected to ~ub~ect matter related to .
that o~ the pr~sent application.

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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Abrasive grits of a fused, solidified, crushed composition which is essentially free of pores and which comprises by weight from 10% to 70% ZrO2, 5% to 15% Cr2O3 and 15% to 85% Al2O3, the composition including less than 0.15% soda and less than 2.25% free chromium metal with the total of impurities being less than 5%.

2. Abrasive grits according to Claim 1, wherein the relative proportions of chromia, zirconia and alumina ex-cluding any impurities is chromia 10%, zirconia 30%, alumina 60%.

3. Abrasive grits according to claim 1 or 2, wherein primary solid solution alumina chromia crystals are supported in a eutectic of zirconium oxide crystals in a chromia alumina solid solution phase.

4. Abrasive grits according to claim 1, wherein there is less than 3% iron oxide.

5. Abrasive grits according to claim 1, wherein there is, by weight, 5% to 15% chromia, 45% to 50% zirconia and 35% to 49% alumina.