JP4170402B2 - Titanium-based carbonitride alloy with nitrided surface region - Google Patents

Abstract

According to the present invention an uncoated titanium based carbonitride cutting tool insert with superior plastic deformation resistance and wear resistance is provided. This is accomplished by heat treating the material in nitrogen atmosphere under conditions to obtain a nitrogen rich surface zone, also containing substantial amounts of binder phase.

Description

結果を比較すると、窒化処理工程は耐摩耗性と耐塑性変形性の双方を劇的に改良した。窒化工程を除いた本発明にしたがう未被膜インサートは、この試験に含まれているために意味のないことであることに注目すべきである。冷却剤のない耐塑性変形性は１〜３分以上持ちこたえるためには不十分である。The present invention relates to a liquid phase sintered body of a carbonitride alloy having titanium as a main component, the sintered body having improved properties and more specifically combining high wear resistance and plastic deformation resistance. Used as a cutting tool material in cutting operations that require a sharp cutting edge. This could be achieved by heat treating this material in a nitrogen atmosphere.
Titanium-based carbonitride alloys, called cermets, are well established today as insert materials in the metal cutting industry and are particularly used for finishing. These consist of a carbonitride hard structure surrounded by a metal binder phase. Hard constituent grains are generally complex structures having a core surrounded by a rim of another composition.
In addition to titanium, group VIa elements, both molybdenum and tungsten, and sometimes chromium, are added to promote the wettability of the binder and hard component and to strengthen the binder phase by the solid solution effect. Also, IVa group elements and / or Va group elements, Zr, Hf, V, Nb and Ta, are added to all commercially available alloys available today. All these additive elements are generally added as carbides, nitrides and / or carbonitrides. The particle size of the hard component is generally <2 μm. In general, the binder phase is primarily a solid solution of both cobalt and nickel. The amount of binder phase is generally 3-25 wt%. Sometimes other elements, such as aluminum, are added on top of it, this aluminum being said to cure the binder phase and to improve the wettability between the hard component and the binder phase.
The main advantage that cermets have over WC-Co based materials is that relatively high wear resistance and chemical inertness can be achieved without imparting a surface coating. This porosity is mainly used for final finishing operations requiring a sharp cutting edge, and chemical inertness is mainly used for cutting with low feed and high speed. However, these desired properties are achieved at the expense of toughness, blade safety and manufacturability. The most convenient materials contain large amounts of nitrogen (mostly over 50% N (N + C)), which can be sintered by conventional methods that have been difficult due to the porosity due to decarburization. The high nitrogen content also makes the material difficult to polish, which is necessary to achieve a sharp, defect-free blade and severe dimensional tolerances, ideally for final finishing operations One skilled in the art would like to have a cermet with a wear-resistant, but uncoated, like PVD or CVD coating material, with the nitrogen content adjusted to be low for ease of manufacture. It is out.
U.S. Pat. No. 4,447,263 discloses a titanium-based carbonitride alloy with a wear-resistant surface phase of carbonitride or oxycarbonitride alone or in combination where there is no binder phase in the surface phase. Is disclosed. This phase is achieved by heat treatment at 1100-1350 ° C. in a N ₂ , CO and / or CO ₂ atmosphere under pre-pressure.
Another example is shown in US Pat. No. 5,336,292, in which the surface layer contains a small amount of binder phase, but from the inside of the material by a sharp interface to the area rich in binder phase. Are separated. This layer may optionally N ₂ and / or NH ₃ in CH _4, at least one of CO and CO ₂ and in the combination atmosphere, in an atmosphere of 1 to 25 hours atmospheric or higher pressure at 1100 to 1350 ° C. Achieved by heat treatment.
It is an object of the present invention to provide a sintered titanium-based carbonitride alloy, which is heat treated to obtain a high surface area of 5-60 μm with a high nitrogen content. This heat treatment is performed as a processing step included in the cooling process of the sintering cycle, or as another processing step, for example, a final manufacturing step after any grinding operations are performed.
FIG. 1 is a 2000 × photomicrograph showing a portion of the insert of the present invention.
FIG. 2 is an EMPA (Electron Microprobe Analysis) line scan of Co, N, W, Ti and C of some of the inserts of the present invention.
FIG. 3 is an X-ray diffraction diagram of the heat-treated surface of the insert of the present invention.
The sintered titanium-based carbonitride alloy of the present invention contains 2 to 15 atomic percent, preferably 2 to 6 atomic percent tungsten and / or molybdenum. This alloy contains 0 to 15 atomic percent of elements IVa and / or Va, apart from titanium, preferably 0 to 5 atomic percent of tantalum and / or niobium. As the binder phase forming element, 5 to 25 atomic%, preferably 9 to 16 atomic% of cobalt is added. This alloy has N / (C + N) in the range of 10-60 atomic%, preferably 10-40 atomic%. Apart from C, N, Ti, W, Ta and Co, the most preferred elements are not arbitrarily added. In a surface region thicker from 5 to 60 μm, preferably from 15 to 50 μm, most preferably from 20 to 40 μm, the nitrogen content decreases towards the surface. This abundance is mainly due to the presence of TiN grains formed during heat treatment and can be distinguished by X-ray diffraction. These TiN grains can be grown separately, but can also grow epitaxially, surrounding the carbonitride grains and at least partially forming an outer shell. Furthermore, the nitrogen-rich region is nearly identical to the body and has a binder phase content that is distributed throughout the surface. The cobalt content of the surface is 50-150% of the volume of the body, preferably 75-130%, most preferably 90-125%, depending on whether a Co gradient towards the surface exists before heat treatment is there. That is, this abundant region is not a coating and is substantially free of a binder phase without a hard phase layer. In another embodiment, the Co content in this surface region is substantially the same as the interior of the object. The surface X-ray diffractogram shows the Ti-containing hard phase as a prominent peak, one with a peak originating from TiN and the other peak originating from a mixed cubic carbonitride phase. The strength ratio of TiN (200) / TiCN (200) is> 0.5, preferably> 1 and most preferably> 1.5. A prominent peak generated from the Co-based binder phase is also observed in the same diffraction diagram.
This alloy need not contain nickel and / or iron apart from unavoidable impurities. Because of these higher levels of binder forming elements, the desired microstructure cannot be produced. Instead, a hard phase surface layer substantially free of binder phase is formed. This layer has been shown by previous inventors as an alternative to expensive coating operations, but has inferior properties compared to CVD and PVD coatings.
In another aspect of the invention, a sintered carbonitride alloy is provided in which carbide, carbonitride and nitride powders are mixed with Co to a defined composition and in the desired form. Press molded into green objects. The green body is liquid phase sintered in a vacuum or in a controlled gas atmosphere, preferably in the temperature range of 1370-1500 ° C., preferably using the technique described in Swedish patent 9701858-4. The cooling step directly from the sintering temperature or, as a separate step, the insert is preferably at a temperature of 1150 to 1250 ° C. in an atmosphere of nitrogen gas of 500 to 1500 mbar, preferably 1000 to 1500 mbar, preferably for 1 to 40 hours. Either heat treatment for 10 to 25 hours.
It turns out quite surprising to the above special composition and uses decarburization to enhance the chemical inertness, wear resistance and plastic deformation resistance of the cermet without obtaining a hard phase surface layer. It is possible. The reason for this has been found to be that the diffusion of nitrogen from the surface in the Co-based binder layer is much faster than the diffusion of titanium in a relatively high nitrogen pressure furnace. For this reason, TiN nucleates inside the material rather than the surface. The formation rate of TiN at a predetermined depth from the surface is determined by the nitrogen activity at that depth. Ti is most prominently extracted from the edges of the hard phase grains. That is, this edge is at least partially dissolved, resulting in a reduction in particle size. Excess Group V and Group VI elements from this edge diffuse from the surface and re-deposit into the hard phase particles present inside the material. With the latter reprecipitation process, a nitride surface area that is only slightly rich in binder phase can appear at least during relatively long processing times. If this is not desired, it can be counteracted by forming a tailored binder phase depletion in the surface area of the insert prior to heat treatment. This is preferably done by using the techniques described in the above patent specifications. When significant amounts of iron and nickel are added to the alloy, the solubility of titanium in the binder phase increases dramatically. Similarly, this increases the diffusion rate of titanium, and instead forms a hard phase surface layer.
Since this treatment is controlled by the reaction gas in the sintering atmosphere, it is a clear advantage to place the insert on a surface that has been inerted in this atmosphere. A good example of this is a tray coated with yttria as described in WO 97 / 402-03.
Example 1
A powder mixture having a chemical composition of 40.7% Ti, 3.6% W, 30.4% C, 13.9% N and 11.4% Co is Ti (C, N), Manufactured from raw powders of WC and Co. The average particle size of the Ti (C, N) and WC powders was 1.4 μm. This powder mixture was wet mixed, dried and pressed into a green body of insert of type TNMG160408-PF. This body was liquid phase sintered at 1430 ° C. for 90 minutes in an Ar atmosphere of 10 mbar. In order to achieve a macroscopic Co gradient throughout the material in this sintering process, a technique is employed that comprises a reverse phase melt formed in the center and propagating towards the surface, with a liquid binder layer at the center, and containing Co on the surface The amount was 85% of the content in the center of the alloy. This method is described in Swedish patent 9701858-4. The cooling process of this process includes a nitriding process in which the object is heat treated at 1200 ° C. for 20 hours in nitrogen gas of 1013 mbar.
The polished cross section of the insert was prepared by standard metallurgical techniques and characterized by optical and electron microscopy analysis (EMPA). The optical microscope showed that the insert was golden in color with a thick surface layer of about 40 μm in the bronze color of FIG. FIG. 2 shows a line scan analysis of Co, N, W, Ti and C EMPA ranging from the surface of the material to 500 μm. Apparently, in a thick surface area of about 30 μm, the nitrogen content increases substantially towards the surface and the Ti content increases until the W and C contents decrease. In the same region, the Co content increases to reach about 125% of the bulk amount at the surface. FIG. 3 shows the X-ray diffraction of the heat treated surface. Apparently, a given Ti-based hard phase rose in two different peak systems, and the one generated from TiN had almost twice the strength of the one generated from the carbonitride phase. The Co peak is also shown in the diffractogram.
Example 2 (comparative example)
As a reference for performance testing, the TNMG16048-PF insert is composed of 8.3% atomic percent Co, 4.2% Ni, 34.8% Ti, 2.5% Ta, 0.8% Nb, It was produced from a powder mixture consisting of 4.2% W, 2% Mo, 26.6% C and 16.6% N, and liquid crystal sintered in a conventional manner. These inserts were coated using a physical vapor deposition technique (PVD) with a Ti (C, N) layer about 4 μm thick and a TiN layer less than 1 μm thick. This was a well-established PVD coated cermet grade within the P25 range for turning.
Example 3
In order to investigate the wear resistance and plastic deformation resistance of the inserts of Example 1 and Example 2, a lengthwise turning operation was performed. Tool life is blade failure due to plastic deformation or flank wear exceeding 0.3 mm. One test was performed with a coolant primarily to test the wear resistance. Another test was performed without coolant, primarily to test plastic deformation resistance. The time required to reach the tool life was measured for each cutting edge. In each test, 3 blades per variant were tested. The speed was 275 m / min, the feed was 0.2 mm / revolution, the cut depth was 2 mm, and the workpiece material was SS2541. The results are shown in Table 1 below.

Comparing the results, the nitriding process dramatically improved both wear resistance and plastic deformation resistance. It should be noted that uncoated inserts according to the present invention without the nitridation step are meaningless because they are included in this test. The plastic deformation resistance without coolant is insufficient to withstand more than 1 to 3 minutes.

Claims (3)

A fire containing a hard component based on at least one selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, and a Co binder phase surrounding the hard component A cutting tool insert made of a titanium-based carbonitride alloy,
The alloy has a nitrogen-rich surface area of 5-60 μm thickness ;
The nitrogen-rich surface region has the same Co binder phase content as the interior of the alloy;
The Co binder phase is entirely distributed to the surface from the interior of the alloy,
The surface is in the range Co content is 50% to 150% of the inside of the,
Cutting tool insert, wherein a call. The X-ray diffraction pattern of the surface includes two sets of peaks generated from a Ti-based hard phase and one set of peaks generated from a Co-based binder phase, and an intensity ratio of TiN (200) / TiCN (200) The cutting tool insert according to claim 1, wherein is> 0.5. Aside from unavoidable impurities, in addition to titanium, at least one of 2-15 atomic percent tungsten and molybdenum,
Aside from titanium, tungsten and molybdenum, 0-15 atomic percent of at least one element of groups IVa and Va,
5-25 atomic percent cobalt, with an average N / (C + N) ratio in the range of 10-60 atomic percent;
The cutting tool insert according to claim 1, comprising:

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