FR2458518A1 - Sintered aluminium oxide cutting plate - contains zirconium oxide and magnesium oxide for machining case-hardened steel - Google Patents

Abstract

The cutting plate, for machining case hardened and heat treatable steel at cutting speeds greater than 500 m/min., is composed of sintered Al oxide contg. Zr oxide and has the compsn. 70-90wt.% Al oxide, 10-30wt.% Zr oxide, 0.1-0.5wt.% Mg oxide, and less than 0.6wt.% oxide impurities. The material has a porosity less than 2%, mean grain size greater than 1.7 micron (90% of particles 1.4-1.6 micron), and a fracture toughness at room temp. at least 190 N/cubic mm and at 1000C at least 140 (130) N/cubic mm, measured on cut prism-shaped bars 2.5 mm wide, 3.5 mm high and 12 mm long, provided with a saw cut 120 micron wide, 0.8+-0.1 mm deep and notch radius 50-60 micron. The cutting plate has a longer life by being less brittle. Used for cutting shafts, spindles and similar workpieces of case-hardened and heat treatable steel.

Description

"Cutting insert for work by chip removal
and its manufacturing process. "
The present invention relates to a cutting insert for working, by removing chips, metal parts such as shafts, axes, etc., in particular of hardened steel and tempered steel, preferably for use at cutting speeds greater than 500 m / min, constituted by aluminum oxide sintered at high temperature with additions of zirconium oxide.

 For cutting work, sintered aluminum oxide cutting inserts have given exceptional results over the past decades and have secured an ever-increasing share in the range of cutting materials, in particular for cases where machining operations are carried out at high cutting speed and therefore at high temperatures. However, despite the great hardness and resistance to wear of the aluminum oxide cutting inserts, efforts have been made to further improve the properties of this material, since it has a certain brittleness which fact that the cutting time of the cutting edges of the insert is not entirely satisfactory.

 For this reason, a whole series of proposals have already been made with a view to imparting, through additions, greater ductility to cutting inserts based on aluminum oxide which are relatively fragile, i.e. increase their breaking strength. Additives of this kind can, for example, be metals, and one then obtains what are called "cermets" (ceramic to metal).

Additions of metal carbides, nitrides and borides, such as, for example, titanium carbide, which increase wear resistance, have also been very satisfactory.

A previous proposal, made by the Company request resse in the German patent application published n 23 07 654, envisages to use as material for cutting inserts, the zirconium oxide stabilizes beforehand, at
namely a partially stabilized zirconium oxide with a proportion of cubic phase of between 75 and .95 X. In German patent application No. 27 41 295, this suggestion is taken up and modified in such a way that 0.5 to 35% ( by weight) of such partially stabilized zirconium oxide are incorporated into an aluminum oxide matrix a. This has already made it possible to improve the shape stability of these cutting inserts. However, the relatively high content of stabilizer constituted by the 4 X (by weight) of calcium oxide results in such a reduction in the hot resistance of these inserts. their capacity at high cutting speeds is even less than that of a ceramic based on pure aluminum oxide.

 The problems posed by this cutting ceramic based on aluminum oxide comprising additives generally relate to the fact that with too small additions the desired effect, for example to avoid brittleness, is not sufficiently reached, and with greater additions the resistance to high temperatures, as they occur with high cutting speeds, decreases again.

 The object of the present invention is to break this vicious circle and to develop a cutting insert whose capacity is markedly improved, that is to say which allows a greater production to be obtained, therefore a greater number of pieces per cutting insert and per time unit. This is absolutely and particularly necessary for the work of steel parts, especially hardened steel and tempered steel parts such as shafts, axes, etc.

This problem is solved, according to the present invention, by a cutting insert for the work by removal of shavings, of the type described in the preamble, which is produced with a sintered aluminum oxide, at high temperature with additions of zirconium, and which, according to the present invention, has a combination of the following characteristics: a) Chemical composition:
70 to 90 X (by weight) aluminum oxide
10 to 30 X (by weight) zirconium oxide 0.1 to 0.5% (by weight) magnesium oxide
various impurities in the form of oxides constituting less than 0.6% (by weight) and all the contents complementing each other at 100%.

b) Porosity less than 2% c) Average grain size less than 1.7 lum d) Resistance (tenacity) to breaking KI of at least
190 N / mm ss at room temperature and 140 N / mm3 / 2
at lOOO'C, measured on 2.5 mm prismatic bars
wide, 3.5 mm high and 12 mm long, thimble
cut into cutting inserts, and having a
notch 120 pm wide, 0.8 + 0.1 mm deep,
the radius of the notch being between 50 and 60
Sintered bodies comprising a matrix of very varied materials, including aluminum oxide and incorporated particles of unstabilized zirconium oxide, are known, for example from German patent application No. 25 49 652, in public inspection in which they have been proposed for gas turbine components which must have a high resistance to temperature variations. However, the fact that the combined or combined action of all the characteristics claimed makes it possible to obtain a cutting insert whose properties guarantee a capacity clearly greater than that of all other cutting inserts known to date, and in particular for steel machining, this in particular at high cutting speeds, i.e. at speeds equal to or greater than 500 m / min, is quite surprising. This fact was particularly surprising because that, in the case of cutting inserts, the existing stresses are completely different from those which occur, for example, for gas turbines, and because the cutting inserts undergo in particular heavy wear as well as stresses on the cutting edges, and that interactions occur with the material being machined, on which large additions of materials increasing ductility generally have a negative influence.

 It is not yet possible at present to establish with precision which of the characteristics mentioned above has the greatest importance. A very high purity of the various constituent materials and the maintenance of the content of mangesium oxide at a well-defined maximum value are factors of considerable influence.

Small amounts of magnesium oxide ranging from 0.1 to 0.5% by weight are sufficient to prevent, in known manner, an uncontrolled magnification of the grains. On the other hand, due to the small amount of magnesium oxide and very small amounts of the various impurities present in the form of oxides, the zirconium oxide added to the aluminum oxide occurs in its monoclinic form. in the starting material. During the sintering process, this oxide changes to the tetragonal phase which, upon cooling, reversibly converts back to the monoclinic phase. preferably even less than 0.5 #m, the transformation into monoclinic phase is delayed, which creates latent tensions in the crystal structure, tensions which are able to take up a large part of the forces caused during work by removal of shavings , probably has a considerable influence.

The low porosity characteristic, that is to say a porosity of less than 2%, preferably even less than 1%, is of quite considerable importance for increasing the cutting capacity and in particular for the resistance of the cutting edges of the cutting insert according to the present invention. We therefore ensure, by appropriate provisions, that the pores proper are as small as possible, preferably less than 2 # m (median value). 32
The breaking strength KIc, which is at least m of 190 N lm3 / 2 at ordinary temperature and above all it causes that at 1000 C, therefore has temperatures which are easily reached during work by removal of chips at speeds in addition, the breaking strength KIc ', which still reaches at least 140 N / mm /, has a decisive influence. The tensile strength KIc is preferably even greater than 200 N / mm / at the ordinal temperature and at least 150 Vmm at looooc.

 The need to impart low porosity to the cutting insert according to the present invention goes hand in hand with a high density of the material, which must reach at least 98% of the theoretical density. With a preferential content of zirconium oxide, between 13 and 17 by weight, the density is preferably at least 4.1, in the case of a sintered cutting insert in the usual manner.

The high KIC value guarantees that during the cutting process and in the presence of the strong stresses suffered by the cutting edges of the cutting insert, the grains do not escape from the crystal lattice, which results in an increase in the duration cutting tool or allows to machine with it, and at high speeds, materials for which we could not previously use economically conventional cutting ceramics.

The fact that relatively high proportions of zirconium oxide, which in itself leads to a reduction in resistance to high temperatures, makes it possible to obtain an increase in production when all the characteristics are combined in the claimed manner, which gives a crystal structure with combinations of optimal properties compared to all cutting inserts known to date, is quite surprising, It is the density and the solidity of the crystal lattice, which are such that, even in presence of overloads, there is no longer any breakage of entire angles of the cutting insert making it unusable, which are of particular importance.

 To obtain this very particular combination of properties, which hitherto was considered impractical, and to which is due the appreciable increase in the specific load capacity of the cutting edges, the method of manufacturing the cutting insert according to the present invention has special significance.

A preferred manufacturing process is characterized by the fact that intimately mixed, with the addition of compression aids - 70 to 90% by weight of aluminum oxide having a content
minimum in Al203 of 99% and whose grain size
troyenne corresponds to a median value lower than l, Omi, .10 to 30%
by weight of zirconium oxide. having a ZnO2 content of 99%
and whose average grain size corresponds to a median value
less than l, O, mstet
- 0.1 to 0.5% by weight of pure magnesium oxide, the content of impurities contained in the form of oxides in this initial pulverulent mixture being less than 0.6 X by weight, than cutting inserts having the desired geometric shape are produced by compression, from this mixture, and these plates are sintered for at least three hours at temperatures between 1500 and 160C. The shaped body is preferably sintered at a temperature between 1520 and 1560-C for 6 to 10 hours.

There is a general relationship between duration and sintering temperature, which is such that the chosen sintering times will be longer at lower sintering temperatures, and will be shorter at more sintering temperatures. high. But below a sintering temperature of 1500C, the shaped body generally no longer has the necessary high density and low porosity. Above 1600-C, the grain magnification increases, on the other hand, markedly, the grain size of the finished sintered product increases, above 1.7 μm (average value), the uniformity of the cris talline structure decreases and the properties of the cutting insert therefore deteriorate considerably, in particular at the high cutting speeds that we strive to achieve.

The need to use very pure starting materials has already been mentioned previously. Particularly favorable results are obtained, in particular with regard to the service life of the cutting inserts, when the degree of purity is even higher, both for the aluminum oxide used and for the zirconium oxide, and the content in Al203 or Zr02 exceeds me erit * e your proportion of 99.9 X.

As regards the content of ZrO2, it should be noted that it may also contain a proportion of hafnium oxide. We know that these two elements and their compounds are so close in their properties that they occur together in nature and are very difficult to separate from each other. This is why we do not usually eliminate the hafnium oxide fraction, which can reach 2%. The indicated proportions of 99% by weight and 99.9% by weight of ZrO2 therefore relate to the whole Zr02 + Hf02.

It is therefore essential, and it is in this that the subject of the present application is distinguished from the cutting inserts known up to now which are based on aluminum oxide and zirconium oxide, that the various impurities contained in the form of oxides, such as SiO2, calcium oxide, etc., in the initial pulverulent mixture, represent a proportion less than 0.6 X by weight, preferably even less than this value of a power of ten. To guarantee this low content, it is obviously necessary that the magnesium oxide, added as a grain growth inhibitor, is also very pure. Advantageously, a powdered aluminum oxide is used, to which one has already added during its manufacturing, the necessary amount of pure magnesium oxide.

 The initial grain size of the starting materials in the pulverulent state also obviously has a considerable influence. To allow the desired very fine and homogeneous structure to be obtained, the median value of aluminum oxide and zirconium oxide is less than 1.0 μm and, in the case of zirconium oxide, is even preferably less than 0.5 lum. The "median value" is defined as follows.

The particle size distribution is usually represented in the form of a cumulative distribution, so that the size parameters are represented as a function of the frequency of the percentages up to 100%. The cumulative distribution curves are usually found in simplified form, with an indication of the median value. This is defined as the point of intersection of the sum curve with the line of 50% of the frequency function, and it thus designates the average size (particle size or pore size), at which 50% are higher and 50 X are lower.

The pressing of the initial powder mixture is advantageously carried out under a pressure of at least 6000 N / cm 2, preferably under a pressure of between 12,000 and 16,000 2
N / cm. This is also done to a certain extent in accordance with a law, namely that at a low pressure the chosen sintering temperature is a little higher, and at a higher pressure the chosen sintering temperature is a little lower .

The most diverse materials are suitable as compression aids. Polyvinyl alcohols, which it is recommended to use in a 1 × solution, have been particularly satisfactory. The compression humidity of the initial powder is approximately 4 s by weight.

The quality of the cutting insert, and in particular its high density and low porosity, are also strongly influenced by the speed of heating and cooling, that is to say by the time necessary to heat the preform pressed to the specified sintering temperature and the time required to cool it back to room temperature. This heating rate and this cooling rate are preferably around 200C per hour.

 The olon cutting inserts of the present invention are advantageously cooked under an oxidizing atmosphere, which constitutes a considerable advantage compared to already known cutting inserts comprising additions of carbides or nitrides. Due to the presence of these additives, these cutting inserts can only be sintered under a reducing or inert atmosphere, or else under vacuum, which obviously requires the use of much larger equipment and makes these cutting inserts very expensive. However, the cutting inserts according to the present invention can obviously also be sintered according to another method, according to the subject of the present invention, for example by hot compression. In this case, the sintering times are much shorter and These temperatures are on average about lOOC higher.

 As has already been mentioned several times previously, the high KIC value, in particular that which is obtained at 1 0000C, that is to say at the high temperatures which prevail for high cutting speeds, constitutes a particularly characteristic. important cutting inserts according to the present invention. On the attached graph, the KIC values of a sintered ceramic based on Al 2 O 3 containing ZrO 2 and TiC are compared, according to German patent application No. 27 41 295 (curve 1), with those obtained with a ceramic. according to the present invention (curve 2). There is clearly a great agreement of the values K Ic at ordinary temperature, but a much more rapid drop in KIC is observed with the increase in temperature in the case of ceramic corresponding to l 'Prior state of the art, that in the case of that of the invention.

 The determination of the KIC (Critical Stress Intensity Factor) value, which is a measure of the ultimate strength, is explained below.

Dimension of test pieces
Prismatic bars with a width B = 2.5 mm, a height W = 3.5 mm and a length L = 12 mm are cut from cutting inserts to the desired dimensions. Then, using a copper disc garnished with diamonds, a saw cut simulating a crack, with a width d = 120 pm and a depth a = 0.8 + 0.1 mm over width B for L / 2. The radius of 11 notches is between 50 and 60 tlm.

Procedure applied for the test
The notched test piece is loaded until failure during a three-point bend test. During the test, it is then placed upright across its width over a distance
S = 11 mm and the test load is applied to the side opposite the saw cut. At the point where the bending moment reaches its maximum, the folding speed is 0.25 mm / min.

dans laquelle

qui est la résistance à la rupture rapportée à la section transversale ( W.B) et Y est une constante consignée sur des tables, qui n'est fonction que du rapport a/W, F B étant la charge de rupture.The KIC value of the breaking strength is determined according to the expresion:

in which

which is the breaking strength reported in cross section (WB) and Y is a constant recorded on tables, which is a function only of the ratio a / W, FB being the breaking load.

The dimensions of the test pieces and the procedure applied for the tests were established on the basis of ASTM E 399-72 roaring the standard test methods for the determination of the breaking strength of metallic materials. Ceramic materials, with regard to the adjustment of the assumed plane elongation state and the appearance of plastic zones, behave in a non-critical manner. The simulation of sharp cracks by fine saw cuts is a usual facility for measuring the KIC value of ceramic materials [See
TR WISHAW et ai, ENG.FRAe0MECH.1 (1968) 191, RF PABST,
Dissertation, STUTTGART 1972, RL BERTOLOTTI, 2.AMER. CERDAN.

SOC. 56 (1937> 107].

 The present invention is illustrated by the following descriptive and nonlimiting example relating to the manufacture of a cutting insert.

 42 kg of A1203 having an MgO content of 0.2 X by weight and 7.5 kg of unstabilized ZrO2 are dispersed in distilled water and the whole is ground for 30 minutes in an oscillating ball mill to obtain a homogeneous mixture.

 After grinding, 1% of polyvinyl alcohol is added, the suspension is vigorously stirred for 10 minutes at 500 rpm, and it is passed through a 40 μm sieve. The sieved suspension is dried and granulated in a spray dryer. The granulated powder and therefore fluid is compressed in automatic powder presses, under a pressure of 12000 N / cm2, to form cutting inserts having the desired geometric shape and a green density of 2.52 g / cm.

After this shaping process, the shaped body is sintered at 15500C for 8 hours. The heating and cooling rate is 200 / h.

 After sintering, the cutting inserts are ground and bevelled using diamond wheels until their final geometric shape is obtained, in accordance with the stipulations of the SNG standards.

The cutting insert produced according to this example has the following properties - Specific weight 4.17N g / cm3 - Porosity 0.5% - Average grain size having
a median value of: 1.45 #m - VICKERS hardness: 19700N / mm - Resistance to breaking by bending: 580 N / mm2 - Resistance to rupture 201 N / rn.m3 / 2 at temperature
ordinary, and
150 N / mm / at 1000 C
All the indications relating to grain sizes, which are contained in the present description, are determined according to the SYNDER-GRA method.

 Such a cutting insert assumes a significantly higher load at the cutting angles. The admissible load at the cutting angles is checked during the so-called "advance" test.

 This consists in increasing the feed value, with a preset cutting depth, in this case, 3 mm and at a well defined angle of attack, in this case 850, and with a geometry of the cutting insert. determined, in this case of the SNGN 120816 T type (0.2 x 20-), until the cutting insert barely withstands an action time of 10 minutes without breaking for a cutting speed of 500 m / min.

 Under the test conditions indicated above, an advance value of 0.70 mm / revolution was obtained for the cutting insert according to the example cited.

Such a high feed value means in practice, compared to already known cutting inserts, based on Al 03 alone or based on a cutting ceramic containing
Zr02 ei Tic, according to German patent application nv 27 41 295, a considerable increase in the number of parts that can be machined with a cutting insert. It was thus possible to multiply by 6 the number of parts machined compared to a ceramic based on Al203 and compared to a ceramic based on A1203, Zr02 and TIC, this increase having been determined on the example d '' a rear drive wheel shaft in 41 Cr4V90.

Claims (3)

CLAIMS * ========================== 1. Cutting insert for work, by removing shavings, metal parts, such as trees, axes, and the like, in particular of hardened steel and tempered steel parts, preferably for use at cutting speeds greater than 500 m / min, constituted by aluminum oxide sintered at high temperature with additions of zirconium oxide, characterized in that it has a combination of the following characteristics a) Chemical composition  70 to 90% by weight of aluminum oxide  10 to 30% by weight of zirconium oxide 0.1 to 0.5% by weight of magnesium oxide various impurities in the form of oxides constituting less 0.6% by weight and all contents complementing each other  100%. b) Porosity less than 2% c) Average grain size less than 1.7 #md) Breaking strength KIc of at least 190 N / mm / at room temperature and at least 140 N / mm3 / 2 at 1000 measured on prismatic bars 2.5 mm wide, 3.5 mm high and 12 mm long, cut from the cutting inserts and having a notch 120 vm wide, 0.8 + 0.1 mm deep, the notch radius being between 50 and 60 lum. 2.- cutting insert according to claim 1, characterized in that the average grain size is between 1.4 and 1.6 Zm and that 90 s of all the grains have a size within this range.  3.- cutting insert according to one of claims 1 and 2, characterized in that the breaking strength KIc is at least 200 N / mm / at temperature  3/2 ordinary, and at least 150 N / mm at 1000C.  4.- cutting insert according to any one of claims 1 to 3, characterized in that with a preferential content of zirconium oxide between 13 and 17% by weight, the specific weight is at least 4, lg / cm3. 5.- A method for manufacturing a cutting insert according to any one of claims 1 to 4, characterized in that one intimately mixes, with the addition of compression aids - 70 to 90% by weight d aluminum oxide having a minimum content of A1203 of 99% and the average grain size of which corresponds to a median value of less than 1.0 μm, - 10 to 30% by weight of zirconium oxide having a minimum content of ZrO2 of 99 X and whose average grain size corresponds to a median value less than 1.0> im and - 0.1 to 0.5% by weight of pure magnesium oxide, the content of impurities contained in the form d oxides in this initial pulverulent mixture being less than 0.6% by weight, cutting inserts having the desired geometric shape are produced from this mixture, and these inserts are sintered for at least three hours at temperatures between 1500 * and 1600-C. 6.- Method according to claim 5, characterized in that the shaped body is sintered at a temperature between 1520 and 15600C for 6 to 10 hours.  7.- Method according to one of claims 5 and 6, characterized in that the heating rate to the sintering temperature, and the cooling rate to room temperature, once the sintering is completed, are about 2000C per hour. 8.- Method according to any one of claims 5 to 7, characterized in that the cutting inserts are sintered under an oxidizing atmosphere.