EP0330913A2 - Process for preparing a sintered hard metal, and sintered hard metal obtained thereby - Google Patents

Abstract

To improve the heat resistant properties of sintered hard metals, in particular with a view to achieving greater cutting powers during use as the cutting tool, it is proposed to alloy aluminium-containing complex nitrides and/or aluminium-containing complex carbides, in particular from the family comprising the H, chi or kappa phases, with the binder metal to which at least one hard material phase has been added.

Description

The invention relates to a method for producing a sintered hard metal body, which consists of at least one hard material from the field of carbides, nitrides and / or carbonitrides of the transition metals of groups 4, 5 and / or 6 of the periodic table and at least one of the binder metals iron, nickel and Contains cobalt, the hard material being present as carbide and / or mixed carbide and / or nitride and / or mixed nitride in the form of cubic crystals or mixed crystals, and is produced by mixing and grinding powdered starting materials and by pressing and subsequent sintering of the starting powder mixture. The invention also relates to a sintered hard metal body which can be produced by means of the method according to the invention.

Processes for the production of sintered hard metal bodies are basically from e.g. Kieffer-Benesovsky, "Hartmetall", 1965, Springer-Verlag, as well as "Hartmetall für die Praktiker, build, manufacture, properties and industrial application of a modern material group", VDI-Verlag GmbH, 1988, as well as the possible compositions of the carbide body . In particular, it is known that the proportion of binder is between 3 and 30% by mass.

Sintered hard metals based on titanium carbide (US Pat. No. 2,967,349) or titanium carbonitride as hard material phase (AT-PS 2 99 561, US Pat. No. 3,994,692) - which are each bound by a nickel-molybdenum binder - are known to be distinguished compared to conventional hard metal with tungsten carbide as the one hard material phase and cubic titanium mixed carbides - in which some of the titanium atoms are replaced by tantalum, niobium, tungsten - as the second hard material phase and cobalt as binder metal due to increased wear resistance. However, titanium carbide and titanium carbonitride hard metal can only be used to a limited extent as cutting tools, especially at high cutting speeds and with cyclical thermal loads (such as during milling); under the effect of the high temperatures occurring at the cutting edge, the binder metal loses its strength and tends to undergo plastic deformation under the influence of the cutting forces. The significantly lower thermal conductivity of TiC-Mo, Ni and Ti (C, N) -Mo, Ni hard metals compared to tungsten carbide leads to heat build-up, especially at the most stressed point.

In order to eliminate this disadvantage of the TiC-Mo, Ni and Ti (C, N) -Mo, Ni hard metals, which are superior in terms of their wear resistance, the proposal has already been made to sinter carbonitride hard metals with the addition of tungsten carbide and an alloyed nickel or cobalt binder (US-PS 38 40 367, DE-OS 25 46 623). Because of the willingness of Ti (C, N) to react with tungsten carbide, the hard metal body has to be sintered under a nitrogen partial pressure depending on the composition and the sintering temperature, which creates microporosity in the structure and thus causes a reduction in the quality of the hard metal.

US Pat. No. 3,971,656 describes a hard metal in which the hard material particles consist of two phases, namely a titanium and nitrogen-rich carbonitride mixed phase inside the hard material particle and another phase which is rich in metals of the 6th group in the periodic table and is low in nitrogen and which envelops the carbonitride mixed phase. It is known that titanium nitride increases the crust resistance of hard metals during cutting operations compared to titanium carbide. According to the teaching of US Pat. No. 3,971,656, it is assumed that the equilibrium is established within the hard material particle consisting of two mixed phases. The core of the hard material particle therefore consists of relatively carbon-rich carbonitride, since unalloyed titanium nitride cannot be in equilibrium with the required second (Mo, W) - rich phase. According to US Pat. No. 3,971,656, a hard metal is thus created whose wear resistance is not yet optimal.

Another way to create cemented carbide with improved high temperature strength is to increase the heat resistance of the binder metal. For example, in addition to molybdenum, which nickel can harden through solid solution hardening, the binder metal was also alloyed with aluminum in order to emulate the effect of γ′-hardening (hardening through the precipitation of coherent particles with an automotive structure) known from the superalloys in the binder phase. The occurrence of γ′-phases was demonstrated by electron microscopic investigations of aluminum alloy binder phases in Ti (C, N) -Mo, Ni hard metals. The addition of aluminum resulted in an increase in the hardness measured at room temperature, which, however, is associated with a decrease in the bending strength (H. Doi and K. Nishigaki: in HH Hausner (ed.) Modern Development in P / M 10, 525-542; (D. Moskowitz and M. Humenik; in HH Hausner (ed.) Modern Development in P / M 14, 307, 1980). In the process in question, the aluminum portion of the carbide approach was powdered in the form of very fine-grained Ni-Al Alloys with grain sizes in the µm range are added, the production of which is extremely difficult and complex because of the very high plasticity of the intermetallic alloys in the Ni-Al system. In order to achieve optimum properties of the binder metal, the The prescribed carbon content of the sintered alloy is strictly adhered to so that the amount of titanium required for a coherent elimination of the γ′-phase from the hard material dissolves. Only when the ratio of the proportion of aluminum and titanium dissolved in the binder metal is approximately the same is a noticeable influence on the properties of the binder metal to be expected. If the titanium content is too high, the γ′-excretion becomes metastable; in the absence of titanium, the coherence voltage becomes too low, which reduces the hardening effect at medium temperatures.

AlN is added to the binder described in DE-PS 28 30 010 to improve the heat resistance; this remains in the structure as a "dispersed phase" and improves hardness. However, under sintering conditions, AlN forms neither with TiC nor with TiN mixed crystals, is a non-metallic hard material that does not have good wetting properties, is also non-resistant to atmospheric moisture in finely divided form and decomposes under the action of it to Al (OH) ₃ and NH₃. This has a particularly disadvantageous effect when grinding with grinding liquids that are not entirely free of water.

The object of the invention is to enable the production of a sintered hard metal which, while avoiding the disadvantages described above, has increased wear resistance even at higher temperatures. The sintered hard metal should in particular also be usable as a cutting tool or cutting plate and, particularly in the machining of short and long chipping workpiece materials, should have significantly improved cutting performance.

The task related to the method is achieved by the measures listed in claim 1. Subclaims 2 to 14 describe further developments of this method. Aluminum-containing complex carbides or complex nitrides should preferably be used, furthermore those complex carbides or complex nitrides which contain substances which have the same or similar effects to aluminum. In particular, the substances NbCrN, TaCrN, V₅Si₃N _1-x , Mo₅Si₃C _0.6 offer.

In an advantageous development of the method according to the invention, aluminum-containing complex carbides and / or nitrides from the family of the H phases and / or Chi phases and / or Kappa phases are used.

The term complex carbides is used inter alia. in "Angew. Chem.", 84th year, 1972, No. 20, pp. 973 ff. Further information about crystal chemistry is e.g. in Peter S. Rudman, John Stringer, Robert I. Jaffee: "Phase Stability in Metals and Alloys", Mc-Graw-Hill Book Company, New York, 1967, pp. 319 to 336, and "Journal of the Institute of Metals ", 1969, Vol. 97, pp. 180 to 186.

When sintering a starting powder mixture compressed by pressing from the hard and wear-resistant carbides and / or nitrides of the transition metals with addition of at least one complex carbide and / or nitride (in particular from the family of the H, Chi or Kappa phases) and nickel and / or Cobalt and / or iron surprisingly form particularly hard and wear-resistant alloys, which are superior to conventional hard metals, especially when machining short and long-chipping materials in a continuous and interrupted cut and when milling.

The following compounds are suitable as aluminum-containing complex carbides or complex nitrides from the family of the H, Chi and Kappa phases:

Ti₂AlN, Ti₂AlC, V₂AlC, V₂AlN, Nb₂AlC, Ta₂AlC, Cr₂AlC, Nb₃Al₂C, Ta₃Al₂C, Nb₃AlN, Mo₃Al₂C, MoCr₂Al₂C, Mo-Ni-Al-C, Mo-Co-Al-C, Mo-Mn-Al-C, W- Mn-Al-C, W-Fe-Al-C.

The aluminum-containing complex carbides and nitrides are produced by reaction of the nitride or carbide of aluminum with the powdery transition metals or by reaction of the nitrides or carbides of the transition metals with aluminum. They become common in the carbide industry Chen crushing methods pulverized and processed with the other alloy components of the hard metal in a conventional manner to a sintered hard metal body - in particular to cutting tools or cutting plates.

The relative proportions between the aluminum-containing complex carbide or nitride and binder metal are selected so as to achieve optimal properties so that - assuming that the entire aluminum content of the complex carbide or nitride remains in the sintered (i.e. finished) hard metal body - the aluminum content of the binder metal 20 Mass%, preferably 10 mass%, does not exceed; in the sintered hard metal body, the minimum aluminum content in the binder metal should be in the order of 1% by mass.

Particularly favorable results can be achieved if the aluminum content of the binder metal is between 2 and 8% by mass.

The complex carbides and nitrides are largely resistant to the commonly used grinding aids. A chemical attack on the complex carbides and nitrides or hydrolysis of these compounds is not to be feared.

The complex carbides and nitrides in question decompose in the presence of nickel and / or cobalt at the sintering temperatures usually used (about 1350 to 1550 ° C.), the monocarbides or mononitrides of the transition metals of the 4th to 4th 6. Eliminate group of the periodic table, while aluminum is dissolved in excess of the nickel cobalt, solidifies the binder by solid solution hardening and, if a minimum content of aluminum in the binder metal is exceeded, is excreted as a γ'-phase when cooling (e.g. H. Nowotny et al : Montash. Chem. 114 (1985), 127-135). In the case of complex carbides with chromium, molybdenum and tungsten as transition metal components, part of the transition metal diffuses into the hard material particles; another part remains dissolved in the binder metal and strengthens the binder metal by means of mixed crystal hardening.

The monocarbides and nitrides of the transition metals which form during the reaction of the complex carbides and nitrides with the liquid binder metal are epitaxially deposited on the surface of the hard material particles and completely envelop the hard material particle. At sintering temperatures between 1350 ° C and 1550 ° C and sintering times of up to 2 hours, the diffusion rates in the hard material particles are not sufficient to bring about a metallurgical equilibrium between the hard material particle in question and its shell made of monocarbides or nitrides of the transition metals. Rather, the shell made of monocarbides or nitrides of the transition metals forms a diffusion-inhibiting barrier layer, which also prevents further material exchange between the hard material particle in question and the binder metal. The chemical composition of the core of the coated hard material particle in the sintered hard metal is thus essentially identical to the chemical composition of the corresponding hard material particle in the starting powder mixture from which the hard metal body was produced by pressing and sintering. The cubic mixed crystal forming the coated hard material particle also remains in an imbalance state in the sintered hard metal body. This phenomenon is noticeable in metallographic grinding in that even fine-grained hard material particles have a clearly recognizable edge zone. From the core zone of the hard metal particle, this edge zone made of monocarbides and nitrides of the transition metals can be clearly distinguished both with regard to their metal components (generally: transition metals of the 4th and 6th group of the periodic table) and their non-metal components (carbon and nitrogen).

The sintered hard metal according to the invention combines the favorable properties of the carbides of the transition metals in the peripheral zone, which are readily wettable by the conventional binder metals, with the high wear resistance of the nitrides in the core and, due to the content of titanium and aluminum in the binder metal, has such a high wear resistance that the cutting tools produced therefrom or cutting inserts have significantly improved cutting performance. Another advantage of the hard metal according to the invention is that during the implementation of the complex carbides and -ni tride with the liquid binder metal forming monocarbides and nitrides of the transition metals are epitaxially deposited on the surface of the hard material particles and thus prevent a further change in the hard material core under the effect of the liquid binder metal. In this way it is possible, for example, to largely maintain the nitrogen content of a fine-grained titanium nitride in the core of the hard material particles even when sintered in a vacuum, for example if titanium nitride with Ti₂AlC or V₂AlC and nickel are used.

The sintered hard metal body, which can be produced by means of the method according to the invention, is essentially characterized in that hard materials forming the starting powder mixture are essentially present in their original composition in the sintered hard metal body (i.e. after completion of the manufacturing process):

The existing carbides and / or mixed carbides and / or nitrides and / or mixed nitrides encased with a diffusion-inhibiting layer thus indicate from their structure that an equilibrium in the metallurgical sense has been avoided between the different hard materials within the hard material particle. This deliberately created imbalance condition results in the abovementioned improved wear resistance - even under extreme working conditions.

Further essential features of the sintered hard metal body are described in claims 16 to 19.

• Fig. 1 im Vergleich der Werte der Kolktiefe und des Freiflächen­verschleißes für eine Schneidplatte aus einem herkömmlichen Hartmetall bzw. aus zwei Hartmetallen, denen unterschied­liche Gehalte an Komplexnitrid aus der Familie der H-Phasen - nämlich Ti₂AlN - zugesetzt worden sind, und zwar beim Drehen von Stahl Cm45N im kontinuierlichen Schnitt,
• Fig. 2 im Vergleich die Werte für die Schlagzahlen, welche die im Zusammenhang mit Fig. 1 beschriebenen Hartmetalle beim Drehen von Stahl CK45N im unterbrochenen Schnitt erreichen.
• Fig. 3 im Vergleich die Werte der Fräslänge der im Zusammenhang mit Fig. 1 beschriebenen Hartmetall und
• Fig. 4 eine Tabelle mit acht Ausführungsbeispielen für die Zusam­mensetzung der Ausgangspulvermischung und des erfindungsge­mäßen Hartmetallkörpers.

The invention is explained in detail below with reference to the drawings using exemplary embodiments. It shows.

• Fig. 1 in comparison of the values of the crater depth and the flank wear for a cutting insert made of a conventional hard metal or of two hard metals, to which different contents of complex nitride from the family of H phases - namely Ti₂AlN - have been added, namely when turning Cm45N steel in continuous cut,
• Fig. 2 in comparison, the values for the number of blows that the hard metals described in connection with Fig. 1 achieve when turning steel CK45N in interrupted section.
• Fig. 3 in comparison the values of the milling length of the hard metal described in connection with Fig. 1 and
• Fig. 4 is a table with eight embodiments for the composition of the starting powder mixture and the hard metal body according to the invention.

The conventional hard metal used for comparison (see FIG. 1, left blocks) consists of 57% Tic, 10% TiN, 10% WC, 2% VC, 10% Mo as well as 5.5% Ni and 5.5% Co. The hard metals according to the invention with complex nitride-modified binder metal (cf. the blocks in the middle and on the right-hand side of FIG. 1) were made from the same base material with the addition of 0.6% or 2.2% Ti₂AlN while simultaneously reducing the nickel and Cobalt content to 5.2% and 4.4% in a manner known per se; in the sintered hard metal, the associated aluminum content in the binder is about 2 or slightly more than 7%.

As the illustration in question shows, the scour depth KT for cutting tests on the workpiece material Cm45N is at a cutting speed of 355 m / min, a cutting time of 12.5 min and a product of cutting depth and feed in the order of magnitude of 1.0 x 0, 1 mm² / rev for the hard metals to be compared with one another in the range between approximately 30 to 35 μm.

The open area wear VB for the conventional hard metal (left) is 450 µm and becomes smaller with increasing content of Ti₂AlN (middle and right side of the illustration). While the crater depth KT could not be improved by the addition of Ti₂AlN, the detected open area wear VB decreases with increasing Ti₂AlN content from about 450 to 280 µm.

FIG. 2 shows the number of strokes of 10 cutting edges for the three previously mentioned hard metals. The cutting test was carried out on a shaft made of workpiece material Ck45N, at a cutting speed of 200 m / min with a product of cutting depth and feed of 2.5 x 0.2 mm² / rev.

While the conventional hard metal (left) only achieves a stroke rate of around 10,000, the addition of 0.6% Ti₂AlN already doubles the number of strokes to 20,000; on the other hand, the hard metal, the starting mixture of which 2.2% Ti₂AlN has been added (right block in the illustration), even withstands 160,000 blows. When turning in an interrupted cut, the hard metals designed according to the invention are clearly superior to the conventional hard metal.

When milling (cf. Fig. 3), a tool or a cutting insert made of a hard metal designed according to the invention can achieve a considerably greater cutting performance than a tool made of conventional hard metal: by adding 0.6 or 2.2% Ti₂AlN the milling path achieved increases from approximately 800 mm to 1200 mm or 1600 mm.

The milling tests, the result of which is shown in the drawing in the form of the milling path LF (in mm), were carried out on a shaft made of tempered steel 42CrMo4 at a cutting speed of 250 m / min; the associated product of depth of cut, chip cross-section and feed per tooth is 1.0 x 120 x 0.1 mm / tooth.

Tools or cutting inserts made of hard metal, to which aluminum-containing complex nitrides have been added, are - as the test results prove - clear in terms of cutting performance, especially when turning in interrupted cuts and when milling the tools or cutting inserts that have been made from conventional hard metals think.

The improved wear resistance - which makes the hard metals according to the invention also interesting for other areas of application - is based on the fact that the starting mixture for producing the hard metal or hard metal body is composed in such a way that certain chemical reactions are initiated very quickly at the beginning of the melting of the binding phase, which result in the formation of a diffusion-inhibiting layer around the surface of the hard material particles of the starting mixture. The deliberate selection of the constituents forming the starting powder mixture therefore means that no metallurgical equilibrium can be established in the finished hard metal or hard metal body. This ensures that the optimum properties of the different hard material particles for the intended applications - such as the known wear resistance of the titanium nitride and the known excellent hardness of the titanium carbide - are retained in the finished hard metal. By setting the metallurgical equilibrium, which is usually given according to the prior art, these individual properties of the hard material particles according to the invention would at least partially be lost.

In contrast to the known prior art, the invention consists in the fact that no metallurgical equilibrium is expressly sought and is present.

4 shows a table with eight exemplary embodiments for the composition of the starting powder mixture of the hard metal body according to the invention.

With the hard metals No. 1 to 4 - with the exception of the complex carbide / nitride - only powders in the form of the pure components (eg TiC, TiN, WC etc.) are used to produce the sintered hard metal body. Powdered master alloys (eg Ti (N, C), (W, Ti, Ta, Nb) C) were used for the production of hard metals No. 5 to 8. This production variant has the advantage that, compared to the production of the sintered hard metal from the pure components, due to a lower need chemical reactions between the individual components of the starting powder mixture, a product with significantly improved quality can be created.

All percentages are percentages by mass.

Claims (19)

1. A process for producing a sintered hard metal body, which consists of at least one hard material from the carbide, nitride and / or carbonitride range of transition metals of groups 4, 5 and / or 6 of the periodic table and contains at least one of the binding metals iron, nickel and cobalt , wherein the hard material is present as carbide and / or mixed carbide and / or nitride and / or mixed nitride in the form of cubic crystals or mixed crystals and is produced by mixing and grinding powdered starting materials and by pressing and then sintering the starting powder mixture, characterized in that the starting powder mixture at least one complex carbide and / or nitride is added, which disintegrates at the beginning of the melting of the binder phase to form a transition metal carbide and / or nitride and grows on the surface of the hard material particles of the starting powder mixture to form a diffusion-inhibiting layer. 2. The method according to claim 1, characterized in that up to 3 mass% - based on the total starting powder mixture - complex carbide and / or nitride is added. 3. The method according to claim 1 or 2, characterized in that an aluminum-containing complex nitride or aluminum-containing complex carbide is added to the starting powder mixture. 4. The method according to at least one of claims 1 to 3, characterized in that an aluminum-containing complex nitride or aluminum-containing complex carbide from the family of the H phases is added to the starting powder mixture. 5. The method according to claim 4, characterized in that Ti₂AlN, Ti₂AlC, V₂AlC, Nb₂AlC, Ta₂AlC or Cr₂AlC is added. 6. The method according to at least one of claims 1 to 3, characterized in that an aluminum-containing complex nitride or aluminum-containing complex carbide from the family of the chi phases is added to the starting powder mixture. 7. The method according to claim 6, characterized in that Nb₃Al₂C, Ta₃Al₂C, Nb₃AlN or Mo₃Al₂C is added. 8. The method according to at least one of claims 1 to 3, characterized in that an aluminum-containing complex nitride or aluminum-containing complex carbide from the family of kappa phases is added to the starting powder mixture. 9. The method according to claim 8, characterized in that Mo-Ni-Al-C, Mo-Co-Al-C, Mo-Mn-Al-C, W-Mn-Al-C or W-Fe-Al-C is added. 10. The method according to at least one of claims 1 to 9, characterized in that the aluminum-containing complex carbide or aluminum-containing complex nitride is added in such an amount that in the sintered hard metal body, the aluminum content in the binder metal 20 mass%, preferably 10 mass%, not exceeds. 11. The method according to at least one of claims 1 to 10, characterized in that the aluminum-containing complex carbide or aluminum-containing complex nitride is added in such an amount that the aluminum content in the sintered hard metal body does not exceed 2 to 8% by mass in the binder metal. 12. The method according to any one of claims 1 to 11, characterized in that one or more of the following complex carbides or nitrides is added to the starting powder mixture: Ti₂AlN, Ti₂AlC, V₂AlC, Nb₂AlC, Ta₂AlC, Cr₂AlC, Nb₃Al₂C, Ta₃Al₂C, Nb₃AlN, Mo₃Al₂C, MoCr₂Al₂C, Mo-Ni-Al-C, Mo-Co-Al-C, Mo-Mn-Al-C, W-Mn-Al-C, W-Fe-Al-C, NbCrN, TaCrN, V₅Si₃N _{1 -x} , Mo₅Si₃C _0.6 , Ni-Mo-N. 13. The method according to any one of claims 1 to 12, characterized in that one or more of the following complex carbides or nitrides is added: Ti₂AlC, Ti₂AlN, V₂AlC, Nb₂AlC, Ta₂AlC, NbCrN, TaCrN. 14. The method according to any one of claims 1 to 13, characterized in that one or more of the following complex carbides or nitrides is added: Ti₂AlC, Ti₂AlN, V₂AlC, Ta₂AlC. 15. Sintered hard metal body, which is produced by a method according to at least one of claims 1 to 14 and which consists of at least one hard material from the range of carbides, nitrides and / or carbonitrides of the transition metals of groups 4, 5 and / or 6 of the periodic table and at least one of the binder metals contains iron, nickel and cobalt, the hard material being present as carbide and / or mixed carbide and / or nitride and / or mixed nitride in the form of cubic crystals or mixed crystals, characterized in that the hard materials of the starting powder mixture are essentially in their original composition are included. 16. Sintered hard metal body according to claim 15, characterized in that the hard materials of the starting powder mixture are surrounded by a diffusion-inhibiting shell made of epitaxially deposited monocarbides and / or nitrides and / or mixed carbides and / or mixed nitrides. 17. Sintered hard metal body according to claim 15 or 16, characterized in that the complex carbide and / or complex nitride portion of the total starting mixture before sintering is a maximum of 3%. 18. Sintered hard metal body according to one of claims 11 to 17, characterized in that in the sintered hard metal body the aluminum content in the binder metal does not exceed 20% by weight, preferably 10% by weight. 19. Sintered hard metal body according to one of claims 15 to 18, characterized in that in the sintered hard metal body the aluminum content in the binder metal does not exceed 2 to 8 wt .-%.

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