Method of making an extremely fine-grained titanium-based carbonitride alloy

Abstract: According to the present invention there is now provided a method of making a sintered tianium-based carbonitride alloy. According to the method melt-metallurgical raw materials containing the metallic alloying elements for hard constituent forming as well as binder phase forming elements are melted and cast, using no intentional additions of the elements C, N, B and O, to a pre-alloy which in solidified condition essentially consists of brittle intermetallic phases with hard constituent forming and binder phase forming elements mixed in atomic scale. The pre-alloy is crushed and/or milled to powder with grain size < 50 µm. The powder is carbonitrided for simultaneous formation in situ of extremely fine-grained, < 0.1 µm, hard constituent particles enclosed in their binder phase. The obtained powder is milled together with lubricant and possible additions of powders of metals, carbides and/or nitrides from the groups IV, V or VI in the periodic table in order to obtain desired final analysis after which the powder mixture is compacted and sintered.

Description: The present invention relates to a method of making an extremely fine-grained titanium-based carbonitride alloy. Titanium-based carbonitrides, often named cermets, are known for having considerably better wear resistance but at the same time inferior toughness behaviour than conventional, i.e. WC-Co based, cemented carbide at the same content of hard constituents. Such carbonitride alloys are therefore used most often at extreme finishing at high speed and during stable conditions at which they generate very fine surfaces on the work piece and at the same time maintain the tolerances for long time because of the superior wear resistance. One reason for the better wear resistance of titanium-based hardmaterials compared to tungsten-based ones is that the titanium hard constituents have much better chemical stability than tungsten hard constituents. The very much active diffusional wear mechanism at high temperature has thus essentially lower effect for titanium-based hardmaterials. Another effect of the good chemical stability is a decreased tendency to clad of the work-piece material onto the tool. Methods used to improve the toughness behaviour are to increase the content of binder phase which leads to impaired high temperature properties and decreased wear resistance. Alternatively, an improved toughness behaviour at maintained binder phase content can be obtained by increasing the grain size. The established experience within the powder metallurgy and particularly within the cemented carbide technique and industry is that a reduction of the grain size at maintained binder phase content leads to increased hardness and decreased toughness. The increasing hardness and the decreasing toughness have been related to the decrease of the free mean path length in the binder phase. This is well known to those skilled in the art and it is therefore logical to increase the grain size in order to increase the toughness. Fig. 1 shows in 5300 X the structure of a conventional titanium-based carbonitride alloy. Fig. 2 shows in 5300 X the structure of titanium-based carbonitride alloy according to the invention. According to the present invention it has now been surprisingly found that an opposite effect to the expected will be obtained at a sufficient decrease of the free mean path length. Contrary to all established knowledge a considerably improved toughness behaviour is obtained. The structure of a "normal" titanium-based carbonitride alloy is shown in Fig. 1. Such material is well known and gives as earlier been mentioned very good wear resistance but in many cases insufficient toughness behaviour. Intermittent cutting gives often great failures in such material. The hardness of the material according to Fig. 1 is 1650 HV3. It has now been found that a material with considerably improved toughness behaviour can be obtained by maintaining the same binderphase content as in the material according to Fig. 1, even the same total chemical composition, but changing the grain size of the hard constituents down to a mean grain size of 0.5-1.0 µm. The hardness of said material is 1700 HV3. The structure of material according to the present invention is shown in Fig. 2. It has also been found that the unexpected effect of increased toughness behaviour at decreased grain size and unchanged binderphase content is strengthened at a binderphase content < 20 % by volume, preferably < 18 % by volume and mostly < 16 % by volume. At the same time it is difficult to obtain so fine-grained structures with a homogenous composition in the microstructure at binderphase contents > 5 % by volume, preferably > 7 % by volume. A method of producing a sufficiently fine grain size is to start from melt-metallurgically produced intermetallic prealloys, i.e. without interstitial alloying elements such as carbon, oxygen and nitrogen, which then are carburized, nitrided and/or carbonitrided in solid phase. A material according to said constituent is known by the Swedish patent No. 7505630-9, but it relates to hard materials with 30-70 % by volume of hard constituents and with properties in the gap between conventional cemented carbide, i.e. WC-Co based, and high speed steel. The present invention relates to a material with more than 70 % by volume of hard constituents and lies regarding its properties on the other side of cemented carbide, i.e. the more wear resistant but at the same time less tough side. The material according to the Swedish patent No. 7505630-9 is based upon the established knowledge that a decreased grain size of the hard constituents gives an increased hardness and consequently the binderphase content could be strongly increased but the material as such remained a hard material. Also EP-A-214 944 relates to hard materials having, in particular, less than 70 % by volume of hard constituents. EP-A-214 944 discloses in more detail and as variants the material and the procedure known by Swedish patent No. 7505630-9. The present invention relates to a method as disclosed in present claim 1. The constituents other than Ti are Zr, Hf, V, Nb, Ta, Cr, Mo and/or W. Small additions of Al can also occur, but they are mainly in the binder phase, which is based on Fe, Ni and/or Co, preferably Ni and Co. The material manufactured according to the present invention is suitably produced by melting of melt-metallurgical raw materials containing the metallic alloying elements for the hard constituent forming as well as the binder phase forming elements but without intentional additions of the elements C, N, B and O. The melt is then cast to an intermetallic pre-alloy which in solidified condition essentially consists of brittle intermetallic phases with hard constituent forming and binder phase forming elements mixed in atomic scale. Said alloy can have a composition which completely or almost completely corresponds to the finally intended one. But it can also be a so called base alloy meaning that it can be used for many different grades by adjusting the composition in connection with the final milling. It has been found that e.g. the tungsten or molybdenum content influences how much nitrides can be present in the final alloy. Thus, a high content of nitrides demands low amounts of particularly tungsten but also limited contents of molybdenum and it can be suitable to have only a small amount Mo+W, < 10 %, preferably < 7 %, in the base alloy. Said metals are also difficult to melt and get uniformly distributed in the pre-alloy when applied in great amounts. The base alloy is produced melt-metallurgically under inert gas atmosphere or in vacuum. Also the casting is protected in the same way. The alloy is then disintegrated into powder form. This can be done e.g. directly from the melt by inert gas granulation in an explosion-proof equipment or by mechanical dividing of the solidified ingot. The final disintegration of the pre-alloy should be performed in a protected environment, suitably wet milling in an oxygen-free environment, i.e. in an oxygen-free milling liquid and where also the air in the gas space of the mill has been replaced by e.g. argon or nitrogen. It has been found that some nitriding here means no drawback. In connection with the final milling the carbon intended for the later carburizing can be added in solid state. Hereby a fine distribution of the carbon is obtained so that the reaction in a later step starts at about the same time in the whole charge. After milling of the pre-alloy to desired grain size, < 50 µm, preferably < 30 µm, the milling liquid is removed and carbonitriding of the base alloy is performed at so low temperature that no melt will ever be present. In order to obtain fine-grained hard constituents the temperature is < 1200 °C, preferably < 1100 °C. It is important that removal and carbonitriding are performed in a closed system, which is protected from contact with the air atmosphere. Otherwise, an uncontrolled reaction can take place. When all the reactive metals in the base alloy, i.e. the hard constituent formers, have reacted with carbon and/or nitrogen the furnace charge can cool to room temperature. Not until now the furnace charge can be exposed to the air atmosphere because now only stable compounds are present. The powder consisting of extremely fine-grained hard constituent particles, < 0.2 µm, preferably ≦ 0.1 µm, enclosed in their binder phase are milled together with lubricant and possible other additions of powders of metals, carbides and/or nitrides from the groups IV, V or VI in the periodic table e.g. WC, W, TiC, TiN, TaC etc in order to give the desired final composition after which the obtained powder mixture is pressed and sintered. To the same base alloy additions of various amounts of carbon and nitrogen can give powders with completely different properties in the final product because of changes in the carbon/nitrogen balance. Thus, e.g. a higher content of carbon and corresponding lower content of nitrogen means a harder and more wear resistant but also less tough alloy. In the same way a higher content of nitrogen and a lower content of carbon gives a tougher but less wear resistant alloy concerning abrasive wear. Because the nitrides are more stable than the corresponding carbides the resistance to diffusional wear can be improved, however, at the same time. Diffusional wear is in most cases observed as cratering while abrasive wear usually is found as flank wear. Furthermore, additions of other hard material powders and similar can in the same way give final products having completely different properties. Because the carbonitrided base alloy is very fine-grained it can be suitable to pre-mill the "additions" before the main raw material is added. Example 1 A pre-alloy of the metals Ti, Ta, V, Co, Ni was made in a vacuum induction furnace at 1450 °C in Ar protecting gas (400 mbar). The composition of the ingot after casting in the ladle was in % by weight: Ti 66, Ta 8, V 6, Ni 8 and Co 12. After cooling the ingot was crushed to a grain size ≦ 1 mm. The crushed powder was milled together with necessary carbon addition in a ball mill with paraffin as milling liquid to a grain size < 50 µm. The pulp was poured on a stainless plate and placed in a furnace with a tight muffle. The removal of the milling liquid was done in flowing hydrogen gas at the temperature 100-300 °C. After that the powder was carbonitrided in solid phase by addition of nitrogen gas. The total cycle time was 7 h including three evacuations in order to retard the procedure. The carburizing occurs essentially at the temperature 550-900 °C. Then the final carbonitride charge cooled in nitrogen gas. The finishing powder manufacture was done in conventional ways, i.e. additional raw materials (WC and Mo₂C) were added and milled together with the carbonitride charge to final powder which was spray-dried in usual ways. Example 2 Cutting inserts of type: TNMG 160408-QF were manufactured of the alloy according to the Example 1 with the following analysis in mole-%: Ti 62.4, Ta 2.3, V 4.7, W 6.2, Mo 7.0, Co 10.0, Ni 7.4 and of a similar powder made in conventional way. The difference in composition was less than 1 %. The cutting inserts of the latter material were used as references in a toughness test. The two variants had the same edge radius and edge rounding. The cutting inserts were tested by cutting of a plank package up to failure. Cutting data at the initial engagement was: v= 110 m/min f ° = 0.11 mm/rev. a= 1.5 mm Work piece: SS 2244 The feed was incresed linearly until all the cutting inserts had failed. After that the accumulated failure frequency was determined as a function of time to failure. The value of 50 % failure frequency for a certain feed was given as comparison figure for the toughness behaviour. 30 edges per variant were tested with the following result: Feed where 50 % of the edges have failed, mm/rev. The reference 0.120 Acc. to the invention 0.145 Student's t-test shows that the confidence level for differences between the materials is > 99.99%. If the number of victories per variant is considered the material according to the invention wins in 95 % of the tests. The result can also be formulated so that cutting inserts made according to the invention will last 2.5 times longer than the reference until 50 % of the cutting inserts have failed.

Claims: Method of making a fine-grained hard material alloy, at which meltmetallurgical raw materials containing the metallic alloying elements for the hard constituent forming as well as the binder phase forming elements, but without intentional additions of the elements C, N, B and O, are melted and cast to a pre-alloy which in solidified condition essentially consists of brittle intermetallic phases with hard constituent forming and binder phase forming elements mixed in atomic scale, after which the pre-alloy is crushed and/or milled to powder with grain size < 50 µm, said powder being carbonitrided at a temperature of less than 1200°C for simultaneous formation in situ of extremely fine-grained, ≦ 0.1 µm, hard constituent particles enclosed in their binder phase, said powder being milled together with lubricant and possible additions of powders of metals, carbides and/or nitrides from the groups IVa, Va or VIa in the periodic table in order to obtain desired final analysis after which the powder mixture is compacted, characterized in that the hard material alloy is a titanium-based carbonitride alloy containing more than 70 % by volume of hard constituents, of which more than 50 mole-% of the metallic elements consists of titanium, said alloy being sintered so that the mean grain size of the hard constituents is 0.5 - 1.0 µm.

Inventors: OSKARSSON ROLF, THELIN ANDERS, WEINL GEROLD, OSKARSSON, ROLF, THELIN, ANDERS, WEINL, GEROLD
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1991
Main classification: C22C1
IPC codes: C22C32, C22C29, C22C1, B22F1, C23C8