The present invention relates to the use of a pre-alloyed powder containing iron as binder in the manufacture of diamond tools by hot sintering.
In the manufacture of diamond tools by hot sintering, with or without pressure, of an intimate mixture of diamond and of binder, use is made, for the binder, that is to say the material forming the matrix of the tool at the end of the sintering operation, either of fine cobalt powders (1-6 µm) or of mixtures of fine powders, such as a mixture of fine cobalt, nickel and iron powders, or coarse pre-alloyed powders (less than 44 µm) such as a steel powder obtained by atomization.
The use of a fine cobalt powder has very good results from a technical standpoint; its only drawback stems from the high price of the powder.
Using mixtures of fine powders, matrices are obtained whose hardness and, consequently, the wear resistance, are relatively low.
The use of coarse pre-alloyed powders requires a sintering temperature of about 1100-1300°C, at which temperature degradation of the diamond, called graphitization, becomes appreciable.
The object of the present invention is to provide a pre-alloyed powder containing iron, whose use as binder in the manufacture of diamond tools by hot sintering avoids the aforementioned drawbacks.
For this purpose, the powder used according to the invention has an average particle size of less than 8 µm as measured with the Fisher Sub Sieve Sizer and a loss of mass by reduction in hydrogen of less than 3% as measured according to the standard ISO 4491-2:1989; this powder contains, in % by weight, 10 - 80% of iron, up to 40% of cobalt, up to 60% of nickel and up to 15% of M, M being present, at least partially, in the oxidized state and representing one or more of the elements Mn, Cr, V, Al, Mo and Ti, the other components in the powder consisting of unavoidable impurities. In fact, it has been found that such a powder, which therefore contains at most only 40% of cobalt, may be sintered at moderate temperatures (650 - 1000°C) to give a matrix having a high hardness and that, furthermore, this hardness may be easily adapted to the particular requirements of the users of diamond tools, by varying the composition of the powder.
It is necessary for the particle size to be less than 8 µm in order that the powder be sinterable at moderate temperatures; advantageously, it is less than 5 µm.
The loss of mass by reduction in hydrogen must be less than 3%; otherwise, there is a risk of producing, when the powder mixed with diamonds is sintered in a reducing atmosphere, such a great evolution of gas that porosity appears in the sintered product and/or that the graphitization of the diamond becomes too great; the said loss of mass is preferably less than 2%.
The abovementioned Fe, Co, Ni and M contents are necessary in order that the matrix have a suitable hardness and in order that this hardness be able to be adapted to the requirements of the users of diamond tools. Preference is given to an Fe content of at least 30%, a Co content ranging up to 30%, an Ni content of 10 - 30% and an M content ranging up to 10%, these contents leading to very high hardnesses. The most preferred Fe content is at least 50% and that of M equal to or less than 5%.
The present invention also relates to the above-defined pre-alloyed powder containing iron, this powder therefore being characterized in that it has an average particle size of less than 8 µm as measured with the Fisher Sub Sieve Sizer and a loss of mass by reduction in hydrogen of less than 3% as measured according to the standard ISO 4491-2:1989 and in that it contains, in % by weight, 10 - 80% of iron up to 30% of cobalt, up to 30% of nickel and up to 15% of M, M being present, at least partially, in the oxidized state and representing one or more of the elements Mn, Cr, V, At Mo and Ti, the other components in the powder consisting of unavoidable impurities.
The powder of the invention may be prepared by heating, in a reducing atmosphere, a hydroxide, oxide, carbonate, basic carbonate (mixture of hydroxide and carbonate) or mixed organic salt of the constituents of the alloy so as to obtain a pulverulent product, whose loss of mass by reduction in hydrogen is less than 3%, and by comminuting this product (the expression "constituents of the alloy" is used here to denote all the elements present in the composition of the alloy, apart from oxygen: thus, for example, Fe, Ni, Co and Mn must be regarded as constituents of the Fe-Ni-Co-Mn-O alloy).
The hydroxide, carbonate, basic carbonate and the organic salt may be prepared by adding an aqueous solution of the constituents of the alloy to an aqueous solution of, respectively, a base, a carbonate, a base and a carbonate, and a carboxylic acid, separating the precipitate thus obtained from the aqueous phase and by drying the precipitate.
The solution of the constituents of the alloy may be a chloride solution, a sulphate solution, a nitrate solution or a mixed solution of these salts.
It may be useful to add a small quantity of carbon, for example 0.05 - 3%, in the form of an organic compound, to the pre-alloyed powder in order to reduce the risk of graphitization, this risk albeit low at the moderate temperatures used for the sintering.
This example relates to the preparation of a powder according to the invention by the precipitation of a mixed oxalate and the subsequent decomposition of this oxalate.
2.47 litres of a chloride solution containing 39 g/l of Co, 25 g/l of Ni, 85 g/l of Fe and 11 g/l of Mn are added at room temperature and with stirring, to 13.64 litres of an aqueous solution of oxalic acid containing 65 g/l of C2H204·2H2O. Thus, 94% of the Co, 85% of the Ni, 81% of the Fe and 48% of the Mn are precipitated in the form of a mixed oxalate. This precipitate is separated by filtration, washed in water and dried at 100°C. The dry precipitate contains 9.2% Co, 5.3% Ni, 17.2% Fe and 1.3% Mn.
The precipitate is heated at 520°C in a stream of hydrogen for 6 hours. A pulverulent metallic product is thus obtained. Grinding this product in a mortar gives a pre-alloyed powder having a loss of mass by reduction in hydrogen of 2% and containing 27,1 % Co, 15.7% Ni, 50.8% Fe and 3.9% Mn, and the particles of which have an average diameter of 2.1 µm measured with the Fisher Sub Sieve Sizer. Examination of the powder using X-ray diffraction shows that virtually all of the Mn is present in the oxidized state.
This example relates to the preparation of a powder according to the invention by the precipitation of a mixed hydroxide and the subsequent reduction of this hydroxide.
9.4 litres of a chloride solution containing 24.4 g/l Co, 13.5 g/l Ni, 58.6 g/l Fe and 2.3 g/l Mn are added, at 80°C and with stirring, to 36.7 litres of an aqueous solution of caustic soda containing 45 g/l of NaOH. Virtually all of these elements are thus precipitated in the form of a mixed hydroxide. This precipitate is separated by filtration, washed in water, repulped at 80°C in a 45 g/l NaOH solution, separated once again by filtration, washed in water and dried at 100°C. The dry precipitate contains 14.8% Co, 8.2% Ni, 35.6% Fe and 1.4% Mn.
The precipitate is heated at 510°C in a stream of hydrogen for 7.5 hours The pulverulent metallic product thus obtained gives, after grinding in a mortar, a pre-alloyed powder having a loss of mass by reduction in hydrogen of 1.65% and containing 24.2% Co, 13.4% Ni, 58% Fe and 2.3% Mn, and the particles of which have an average diameter of 2.1 µm Examination of the powder using X-ray diffraction shows that virtually all the Mn is present in the oxidized state.
This example relates to a series of tests comparing the sinterability of two powders according to the invention, called hereinbelow powder A and powder B, of a fine Co powder (powder C) and of a Co powder obtained by atomization (powder D).
Powder A is that obtained according to Example 1 and powder B is that obtained according to Example 2. Powder C is a commercially available Co powder (1.5 µm) obtained via the oxalate route.
Powder D consists of particles having an average diameter of 9.7 µm.
A cylindrical pill, having a diameter of 4 mm and a length of 4 mm, of each of the powders to be tested is produced by cold pressing. These cylinders are heated at a rate of 5°C per minute and the change in length as a function of temperature is measured. The variation of the change (in %) in the length of the cylinders as a function of temperature is given in the figure appended hereto.
The densities (in g/cm3) of the cylinders before and after heating and the ratio between these densities are given in the table below:
These results show that the sinterability of the powders according to the invention (A and B) is superior to that of the fine Co powder (C) and far superior to that of the coarse powder D.
In this example, the mechanical properties of sintered pieces made from cobalt powder, nickel powder, iron powder, various mixtures of Co, Fe, Ni and Mn powders and various powders according to the invention are compared.
The following powders are used:
The powders were sintered by pressing for 3 minutes at 650, 700, 750, 800, 850 or 900°C under a pressure of 35 MPa in a graphite mould.
The density and the Vickers hardness of all the sintered pieces were measured. A large number of pieces were also subjected to the transverse bending test according to DIN/ISO 3325: the 45 x 10 x 6 mm sintered bar is placed so as to bear freely on two supports separated by 25 mm and the load is applied in the middle of this separation by means of a punch until the piece fails. The results are given in Tables I, II and III below, the first table referring to the elemental powders (Co, Ni, Fe) and to the mixtures of powders, the second table to the ex-oxalate powders used according to the invention and the third table to the ex-hydroxide powders used according to the invention.
N°
mm
N°
mm
N°
These results show that, after sintering, superior mechanical properties are obtained with the pre-alloyed powders according to the invention than with mixtures of elemental powders. For comparable compositions (see, for example, test No. 14 versus test No. 57) the hardness obtained with the powders of the invention is from 2 to 3 times higher than that obtained with mixtures of powders. With regard to the failure load, higher values were measured with the pre-alloyed powders than with the mixed powders within the 25 - 35% Co, 5 - 20% Ni and 45 - 55% Fe range; outside this range, the failure loads are comparable.
This example relates to the use of a powder according to the invention in the manufacture of diamond tools.
Powder obtained in Example 1 is mixed with 1 % of synthetic diamonds. The mixture is sintered by pressing under vacuum at 800°C and 35 MPa.
Microscope examination of the sintered material shows that the manganese oxide is finely dispersed in the metallic matrix, that the diamonds remain intact and that they are firmly embedded in the metallic matrix.