EP0433856A1 - Mixed hard metal materials based on borides, nitrides and iron group matrix metals - Google Patents

Abstract

The invention relates to mixed sintered metal materials based on high-melting borides and nitrides and low-melting iron binder metals having the composition: (1) 40-97% by volume of borides, such as titanium diboride and zirconium diboride; (2) 1-48% by volume of nitrides, such as titanium nitride and zirconium nitride; (3) 0-10% by volume of oxides, such as titanium oxide and zirconium oxide, with the proviso that components (2) and (3) may also be present as oxynitrides such as titanium and zirconium oxynitride; and (4) 2-59% by volume of low-carbon binder metals, such as iron and iron alloys and to processes for preparing the same.

Description

Hard metals, which are understood to mean sintered materials made of metallic hard materials based on high-melting carbides of the metals from groups 4b to 6b of the periodic table and low-melting binder metals from the iron group, in particular cobalt, have long been known. They are mainly used for machining technology and to combat wear. For the production of these hard metals from the usually powdery hard materials, the metal binders are required, which must wet the hard material during the sintering process with the formation of an alloy (solution). This is the only way to create the tough, hard microstructure of the hard metals suitable for use, among which the WC-Co and TiC-WC-Co systems are best known. It is also known that binders from the iron group are also suitable for other high-melting metallic hard materials, such as borides and nitrides (cf. "Ullmanns Enzyklopadie der techn. Chemie", Vol. 12, 4th Edition 1976, Chapter "Hard Metals" , Pp. 515-521).

The systems TiB₂-Fe, Co or Ni and ZrB₂ and Fe, Co or Ni were already investigated in the 1960s. It was found that such alloys based on TiB₂ with up to 20% Fe as binders are considerably harder than such based on WC-Co and TiC-WC-Co. Alloys based on ZrB₂ with Co and Ni are brittle and not resistant to oxidation, while Fe reacts with ZrB₂ to form tetragonal Fe₂B and is therefore not considered as a binder (cf. works by VF Funke et al. And ME Tyrrell et al., Ref in the book "Boron and Refractory Borides", Ed. by VJ Matkovich, Springer-Verlag, Berlin-Heidelberg-New York, 1977 in Chapter XIV, p. 484 in connection with plate 7 and p. 488 in connection with plate 8th).

From these results it was concluded that the suitable binder for these borides had apparently not yet been found, which would compensate for the disadvantages of excessive brittleness and thus the industrial use of such alloys in the field of cutting materials and other applications which have high demands on corrosion and heat - and / or resistance to oxidation, could enable (see loc. cit., p. 489).

Alloys based on nitrides and carbonitrides of titanium and zirconium with a very high proportion of the binder, in particular iron, (at least 50% and more) are particularly tough, but no longer very hard (HV 1050 - 1175) (cf. US-A -4,145,213 by Oskarsson et al.). Such materials are believed to be less brittle than the boride-based systems mentioned above. However, due to their low hardness, they are not suitable for processing hard and high-temperature materials such as SiC-reinforced aluminum alloys.

Combinations based on diborides, in particular titanium and zircon, with carbides and / or nitrides, in particular titanium nitride and titanium carbide, and with boride-based binders, such as in particular Co, Ni or Fe boride, do not solve the problem, because Because of the boride binder, which is to be understood in particular as CoB, such substances are very hard and strong, but are particularly brittle (cf. US Pat. No. 4,379,852 by Watanabe et al.).

Finally, attempts have also already been made to add graphite to the known system based on titanium boride and optionally titanium carbide with binders made of iron, cobalt and nickel or alloys thereof, which is intended to react with existing oxygen during the sintering process. This is intended to achieve cutting materials that are both sufficiently hard and tough, so that they can be used in particular for the machining of aluminum and aluminum alloys (cf. EP-B-148 821 by Moskowitz et al., Which was published on PCT - Registration based on WO 84/04713). However, the reaction of graphite with titanium boride in the presence of iron favors the formation of the undesired Fe₂B phase, which is not only less hard than titanium diboride, but also reduces the proportion of the ductile iron binding phase, so that the resulting materials are not only less hard, but are also less tough.

It is therefore the task to provide hard metal mixtures based on high-melting borides and nitrides of metals from group 4b of the periodic table and low-melting binding metals made of iron or iron alloys, which are high-density, very hard, tough and strong are, so that they can be used in particular as cutting materials for hard and heat-resistant materials.

• (1) 40 bis 97 Vol.-% Boriden, ausgewählt aus der Gruppe von Titandiborid, Zirkondiborid und Mischkristallen hiervon,
• (2) 1 bis 48 Vol.-% Nitriden, ausgewählt aus der Gruppe von Titannitrid und Zirkonnitrid,
• (3) 0 bis 10 Vol.-% Oxiden, ausgewählt aus der Gruppe von Titanoxid und Zirkonoxid, wobei die Komponenten (2) und (3) auch ganz oder teilweise in Form von Oxynitriden, ausgewählt aus der Gruppe von Titanoxynitrid und Zirkonoxynitrid, vorhanden sein können, und
• (4) 2 bis 59 Vol.-% kohlenstoffarmen Eisen und Eisenlegierungen

und haben folgende Eigenschaften:The mixing materials according to the invention consist of

• (1) 40 to 97% by volume of borides selected from the group of titanium diboride, zirconium diboride and mixed crystals thereof,
• (2) 1 to 48% by volume of nitrides selected from the group of titanium nitride and zirconium nitride,
• (3) 0 to 10% by volume of oxides, selected from the group of titanium oxide and zirconium oxide, components (2) and (3) also being present in whole or in part in the form of oxynitrides, selected from the group of titanium oxynitride and zirconium oxynitride can be, and
• (4) 2 to 59% by volume of low carbon iron and iron alloys

and have the following properties:

Density at least 97% TD, based on the theoretically possible density of the entire mixed material, grain size of the hard material phase maximum 5.5 µm, hardness (HV 30) at least 1200, flexural strength (measured according to the 4-point method at room temperature) at least 1,000 MPa and Breaking resistance K _IC at least 8.0 MPa m ^1/2 .

Tungsten carbide mixed materials, in which the hard material components consist of titanium boride and titanium nitride, together, preferably 50-97% by volume, have proven particularly useful Make up 50 - 90 vol .-%, and in particular about 80 vol .-%, of the entire mixed material. 2.5-40% by volume of the hard material components preferably consist of titanium nitride. The missing proportion of up to 100% by volume in the entire mixed material is distributed among the oxides, which may be present, preferably titanium oxide, with a proportion between 0 to 10% by volume and the metallic binding phase from the low-carbon iron or Iron alloy. Alloy components for low-carbon iron types are preferably chromium or chromium-nickel mixtures.

The hard metal mixing materials according to the invention can be produced by processes known per se, for example by pressure-free sintering of fine starting powder mixtures or by infiltration of porous moldings from the hard material components with the low-carbon binder.

Very fine and very pure starting powders are advantageously used as the starting material for carrying out these processes. The borides and nitrides selected as hard material components should be as free as possible from carbon-containing impurities which have a disadvantageous effect on the formation of the microstructure in the finished sintered body. For example, titanium diboride, which may contain boron carbide in its manufacture, can react not only with graphite, as already mentioned above, but also with boron carbide in the presence of iron to form the undesired Fe₂B phase during the sintering process, as the following equations illustrate:

Oxygen, which is predominantly in the form of adhering oxides of titanium and zirconium, which includes, for example, TiO₂, Ti₂O₃ and / or TiO and the corresponding oxides of zirconium, does not, however, interfere and can contain up to about 2% by weight in the Starting powders are tolerated. In addition, it was found that the separate addition of such oxides, in particular titanium oxide, does not interfere with the sintering process and that, for example, up to 10% by volume of titanium oxide in the finished mixed material, its properties remain practically unchanged.

In addition to the form of the oxides, the oxygen can also be present, in whole or in part, in the form of so-called oxynitrides of titanium and zircon. This includes titanium and zirconium nitrides, in which some of the nitrogen atoms have been replaced by oxygen atoms according to the formulas Ti (O, N) and Zr (O, N), since nitrogen and oxygen in the titanium nitride or zirconium nitride lattice can be exchanged indefinitely below Formation of mixed crystals without mixing gap.

Iron types with a C content of less than 0.1, preferably less than 0.05% by weight are advantageously used as the low-carbon binder metals. Carbonyl iron powders with an Fe content of 99.95 to 99.98% by weight have proven particularly useful. These low-carbon types of iron can contain, for example, chromium in amounts of approximately 12% by weight or nickel-chromium mixtures of, for example, 8% by weight of nickel and 18% by weight of chromium as alloy components.

In order to avoid contamination with carbon in particular, it is advantageous to autogenously grind these starting powders, which must already be sufficiently pure from the production stage. Known grinding units can be used for this purpose, such as ball mills, planetary ball mills and attritors, in which grinding media and grinding vessels are made of material of their own, which in the present case means, for example, titanium diboride and low-carbon iron types.

When grinding with grinding media made of titanium diboride, in particular coarse starting powders can be comminuted to the desired fineness, while grinding media made from low-carbon iron types are suitable for sufficient mixing of the starting powders, since the crushing effect of the hard material components is only slight in this case. In this case, the desired particle size distribution of the starting powders must therefore be present before grinding.

The powder mixtures obtained after the mixed grinding are optionally mixed with temporary binders or pressing aids and made free-flowing by spray drying. They are then pressed by customary measures, such as cold isostatic pressing or die pressing, to form green bodies of the desired shape with a density of at least 60% TD. An annealing treatment at about 400 ° C removes binders or pressing aids without leaving any residue. The green bodies are then heated in the absence of oxygen to temperatures in the range from 1350 ° C. to 1900 ° C., preferably from 1550 ° C. to 1800 ° C., and 10 to 150 minutes, preferably 15, until a liquid iron-rich phase is formed to 45 Minutes, held and then slowly cooled to room temperature. This sintering process is advantageously carried out in furnace units which are equipped with metallic heating elements, for example made of tungsten, tantalum or molybdenum, in order to avoid unwanted carburization of the sintered bodies.

Subsequently, the sintered bodies, expediently before cooling to room temperature, by applying pressure by means of a gaseous pressure transmission medium such as argon, at temperatures from 1200 ° C to 1400 ° C under a pressure of 150 to 250 MPa, preferably about 200 MPa, 10 continue to heat up to 15 minutes. This shell-free, hot isostatic post-compression practically eliminates all remaining pores, so that the finished hard metal mixture has a density of 100% TD.

As an alternative to this sintering process, the hard material components, for example titanium boride, titanium nitride and optionally titanium oxide, can be ground autogenously per se and these powder mixtures can be molded into green bodies with a density of 50 to 60% TD. These porous green bodies are then surrounded in a refractory crucible, for example made of boron nitride or aluminum oxide, with a powder bed of the desired binding metal, which only partially covers the surface of the porous body. The crucibles are then heated in furnace units with metallic heating elements (W, Ta, Mo) in a vacuum free of carbon impurities to temperatures above the melting point of the metallic binding phase, whereby the liquid binding metal penetrates the porous green body by infiltration until its pores are practically completely closed are. In this case, too, will be practical pore-free mixing materials obtained, which also have a density of almost 100% TD. The time required for this is essentially determined by the time required for the binder metal to melt. The process is generally completed in a period of 30 seconds to 30 minutes depending on the size of the workpiece.

The hard metal mixing materials according to the invention produced in this way are not only very dense, but also very hard, tough and strong. The desired combination of toughness and hardness can be varied over a wide range via the mixing ratio of the hard materials, since titanium nitride, for example, is somewhat tougher than titanium diboride with a somewhat lower hardness. For example, even small amounts of titanium nitride can considerably reduce the crater wear that usually occurs with indexable inserts, although such an influence was not to be expected from a hard material component that was softer than titanium diboride. Due to the combination of properties, which can be precisely adapted to the intended use, the mixed materials according to the invention are also suitable as cutting tools for machining very hard materials, for example with SiC-reinforced aluminum alloys and nickel-based superalloys, as well as for impact-free machining such as core drilling or sawing of building materials containing silicon dioxide, for example concrete.

The production of hard metal mixed materials according to the invention is described in more detail in the following examples.

In Examples 1 to 7, hard materials and binding metals were used with the following powder analyzes:

example 1

1350 g of titanium diboride with an average particle size of 5 microns, 50 g of titanium nitride with an average particle size of 2 microns and 600 g of carbonyl iron powder with an average particle size of 20 microns were together with 2 g of paraffin and 10 dm³ of heptane in a grinding container made of hot-pressed titanium diboride with grinding balls ground from titanium diboride at 120 rpm for 2 hours. A free-flowing powder was produced from the comminuted powder mixture with an average particle size of 0.7 μm (FSSS) and this was pressed under a pressure of 320 MPa in a die press into green bodies in the form of rectangular plates with the dimensions 53 × 23 mm. The green bodies were then densely sintered in a furnace with tungsten heating elements under vacuum in the presence of a carbon-free residual gas at 1700 ° C. for 30 minutes and then slowly cooled to room temperature.

Example 2

1570 g of titanium diboride with an average particle size of 5 microns, 110 g of titanium nitride of the same particle size and 300 g of carbonyl iron powder with an average particle size of 20 microns were together with 1 wt .-% paraffin and 10 dm³ heptane in a grinding container made of V2A steel with carbonyl iron balls for 2 hours ground at 120 rpm. The powder mixture obtained in this way was prepared and sintered as described in Example 1.

Example 3

From the same amounts of titanium diboride, titanium nitride and carbonyl iron and under the same conditions as described in Example 1, green bodies were produced in the form of plates which were stored in a carbon-free vacuum at 1650 ° C. for 15 minutes were sintered. After lowering the temperature to 1200 ° C., these presintered plates were post-hot isostatically compressed in the same furnace chamber under an argon gas pressure of 200 MPa for 15 minutes and then slowly cooled to room temperature.

Example 4

1300 g of titanium diboride and 175 g of titanium nitride with an average particle size <10 μm were ground together with 10 dm³ of heptane in a grinding container made of titanium diboride and titanium diboride grinding balls for 2 hours at 120 rpm. The comminuted hard material powder mixture was then cold isostatically pressed into green bodies with a density of 60% TD in a rubber sleeve. These green bodies were placed in a crucible made of aluminum oxide and surrounded with a powder mixture of carbonyl iron, which extends up to about 2 cm below the upper edge of the green bodies. The crucibles were then heated in an oven with tungsten heating elements in a carbon-free vacuum to 1700 ° C. and held at this temperature for 30 minutes. The porous green body sucks in liquid iron until the pores are practically completely closed.

Example 5

The same amounts of titanium diboride and titanium nitride as in Example 1 were mixed with 600 g of a powder made of stainless steel, which contained 18% by weight of nickel, 8% by weight of chromium and <0.05% by weight of carbon and an average starting particle size of 20 μm had ground and processed under the same conditions as described in Example 1. The sintering was carried out at a temperature of 1650 ° C.

Example 6

1030 g of titanium diboride (60% by volume), 206 g of titanium nitride (10% by volume), 164 g of titanium dioxide (10% by volume) and 600 g of carbonyl iron powder with an average particle size of the starting powder of in each case <30 μm were as in Example 1 described ground and processed.

Example 7

687 g of titanium diboride (40% by volume), 824 g of titanium nitride (40% by volume) and 600 g of carbonyl iron powder (20% by volume of Fe), each with an average particle size of the starting powder of <30 μm, were placed in a V2A grinding container Grind steel and carbonyl iron balls at 120 rpm for 2 hours. The further processing was carried out as described in Example 1.

The hard metal mixed materials produced in Examples 1 to 7 were analyzed and tested with regard to their mechanical properties. The results are shown in Tables 3 and 4.

Claims (6)

Tungsten carbide mixed materials based on high-melting borides and nitrides of metals from group 4b of the periodic table and low-melting metals made of iron and iron alloys, characterized in that the mixed materials consist of (1) 40 to 97% by volume of borides selected from the group of titanium diboride, zirconium diboride and mixed crystals thereof, (2) 1 to 48% by volume of nitrides selected from the group of titanium nitride and zirconium nitride, (3) 0 to 10% by volume of oxides, selected from the group of titanium oxide and zirconium oxide, components (2) and (3) also being present in whole or in part in the form of oxynitrides, selected from the group of titanium oxynitride and zirconium oxynitride can be, and (4) 2 to 59% by volume of low carbon iron and iron alloys exist and have the following properties:
Density at least 97% TD, based on the theoretically possible density of the entire mixed material, grain size of the hard material phase maximum 5.5 µm, hardness (HV 30) at least 1200, flexural strength (measured according to the 4-point method at room temperature) at least 1,000 MPa and breaking resistance K _IC at least 8.0 MPa m ^1/2 . Mixed materials according to Claim 1, characterized in that the hard material components (1) and (2) consist of titanium diboride and titanium nitride, which together make up 50 to 97% by volume of the total mixed material, and the hard material component (3) consists of titanium oxide with a proportion from 0.1 to 10 vol% Mixed materials according to claim 1 and 2, characterized in that the binder metal component (4) consists of a low-carbon iron alloy which contains chromium or chromium-nickel mixtures as alloy components. A process for producing the mixed materials according to claim 1, characterized in that very pure starting powders from the hard material components (1), (2) and optionally (3) and the binder metal (4) are ground autogenously and the fine starting powder mixtures obtained in this way are cold formed into green bodies pressed and then sintered without pressure in a carbon-free atmosphere and in the absence of oxygen at temperatures in the range from 1350 ° C to 1900 ° C. A method according to claim 4, characterized in that the pressurelessly sintered mixed materials are hot isostatically post-compressed using a gaseous pressure transmission medium at temperatures of 1200 ° C to 1400 ° C under a pressure of 150 to 250 MPa. A process for producing the mixed materials according to claim 1, characterized in that very pure starting powders from the hard material components (1), (2) and optionally (3) are autogenously ground, the fine starting powder mixtures obtained in this way are cold-pressed to form green bodies and these are poured into a powder bed are heated from the binder metal component (4) in a carbon-free atmosphere above the melting point of the metallic binder phase until the liquid binder metal penetrates into the porous green body by infiltration and completely closes its pores.