EP1422306A2 - Ti(C,N)-(Ti,Nb,W)(C,N)-Co alloy for super-finishing turning cutting tool applications - Google Patents

Abstract

The present invention relates to a titanium based carbonitride alloy containing Ti, Nb, W, C, N and Co for metal cutting applications, in particularly superfinishing turning operations. The alloy contains in addition to Ti 3-<9 at% Co with only impurity levels of Ni and Fe, 2-<4 at% Nb, 3-8 at% W and has a C/(C+N) ratio of 0.50-0.75. The amount of undissolved Ti(C,N) cores (A) should be kept between 26 and 37 vol% of the hard constituents, the balance being one or more complex carbonitrides (B) containing Ti, Nb and W. The invented alloy is particularly useful for superfinishing of steel and cast iron.

Description

The present invention relates to a sintered carbonitride alloy with Ti as the main component and a Ni-free binder phase which has improved properties particularly when used as cutting tool material in superfinishing turning operations particularly for superfinishing of steel and cast iron. More particularly, the present invention relates to a carbonitride-based alloy of specific composition, for which the amount of undissolved Ti(C,N) cores is optimized for maximal abrasive wear resistance, while the Co and Nb contents are simultaneously optimized to give the desired toughness and resistance to plastic deformation.

Titanium-based carbonitride alloys, so called cermets, are produced by powder metallurgical methods and comprise carbonitride hard constituents embedded in a metallic binder phase. The hard constituent grains generally have a complex structure with a core, most often surrounded by a rim of other composition. In addition to Ti, group VIa elements, normally both Mo and W are added to facilitate wetting between binder and hard constituents and to strengthen the binder by means of solution hardening. Group IVa and/or Va elements, e.g. Zr, Hf, V, Nb, and Ta, are also added in all commercial alloys available today. The carbonitride forming elements are usually added as carbides, nitrides and/or carbonitrides. The metallic binder may contain one or more of Ni, Co and Fe.

Compared to WC-Co based materials, cermets have excellent chemical stability when in contact with hot steel, even if the cermet is uncoated, but have substantially lower strength. This makes them most suited for finishing operations, which generally are characterized by limited mechanical loads on the cutting edge and a high surface finish requirement on the finished component.

Cermets comprise carbonitride hard constituents embedded in a metallic binder phase generally of Co and Ni. Cermets are produced using powder metallurgical methods. Powders forming binder phase and powders forming hard constituents are mixed, pressed and sintered. The carbonitride forming elements are added as simple or complex carbides, nitrides and/or carbonitrides. During sintering the hard constituents dissolve partly or completely in the liquid binder phase. Some, such as WC, dissolve easily whereas others, such as Ti(C,N), are more stable and may remain partly undissolved at the end of the sintering time. During cooling the dissolved components precipitate as a complex phase on undissolved hard phase particles or via nucleation in the binder phase forming the abovementioned core-rim structure.

During recent years many attempts have been made to control the main properties of cermets in cutting tool applications, namely toughness, wear resistance and plastic deformation resistance. Much work has been done especially regarding the chemistry of the binder phase and/or the hard phase and the formation of the core-rim structures in the hard phase. Most often only one, or at the most two, of the three properties have been possible to optimize at the same time at the expense of the third property.

US 5,308,376 discloses a cermet in which at least 80 vol% of the hard phase constituents comprises core-rim structured particles having several, preferably at least two, different hard constituent types with respect to the composition of core and/or rim(s). These individual hard constituent types each consist of 10-80%, preferably 20-70% by volume of the total content of hard constituents.

JP-A-6-248385 discloses a Ti-Nb-W-C-N-cermet in which more than 1 vol% of the hard phase comprises coreless particles, regardless of the composition of those particles.

EP-A-872 566 discloses a cermet in which particles of different core-rim ratios coexist. When the structure of the titanium-based alloy is observed with a scanning electron microscope, particles forming the hard phase in the alloy have black core parts and peripheral parts which are located around the black core parts to appear grey. Some particles have black core parts occupying areas of at least 30 % of the overall particles referred to as big cores and some have the black core parts occupying areas of less than 30 % of the overall particle area are referred to as small cores. The amount of particles having big cores is 30-80 % of total number of particles with cores.

US 6,004,371 discloses a cermet comprising different microstructural components, namely cores which are remnants of and have a metal composition determined by the raw material powder, tungsten-rich cores formed during the sintering, outer rims with intermediate tungsten content formed during the sintering and a binder phase of a solid solution of at least titanium and tungsten in cobalt. Toughness and wear resistance are varied by adding WC, (Ti,W)C, and/or (Ti,W)(C,N) in varying amounts as raw materials.

US 3,994,692 discloses cermet compositions with hard constituents consisting of Ti, W and Nb in a Co binder phase. The technological properties of these alloys as disclosed in the patent are not impressive.

A significant improvement compared to the above disclosures was presented in US 6,344,170. By optimizing composition and sintering process using the Ti-Ta-W-C-N-Co system improved toughness and resistance to plastic deformation was accomplished. The two parameters that were used to optimize toughness and resistance to plastic deformation were Ta and Co content. The use of pure Co-based binder implied a major advantage over mixed Co-Ni-based binders with respect to the toughness behaviour due to the differences in solution hardening behaviour between Co and Ni. There is, however, no teaching how to optimize abrasive wear resistance simultaneously to the other two performance parameters. Hence, the abrasive wear resistance is still not optimal, which is crucial for most finishing operations.

It is an object of the present invention to solve the problem described above and others.

It is a further object to provide a cermet material with substantially improved wear resistance while maintaining toughness and resistance to plastic deformation on the same level as state-of-the-art cermets.

It has been found possible to design and produce a material with substantially improved wear resistance while maintaining toughness and resistance to plastic deformation on the same level as state-of-the-art cermets. This has been achieved by working with the alloy system Ti-Nb-W-C-N-Co.

Within the system Ti-Nb-W-C-N-Co a set of constraints has been found rendering optimum properties for the intended application areas. More specifically, the abrasive wear resistance was maximized for a given level of toughness and resistance to plastic deformation by optimizing the amount of undissolved Ti(C,N) cores. The amount of undissolved Ti(C,N) cores can be varied independently from other parameters, such as Nb and binder content. Hence, it has been possible to simultaneously optimize all three main cutting performance criteria, i.e. toughness, abrasive wear resistance and resistance to plastic deformation.

Fig. 1 shows the microstructure of an alloy according to the invention in which

A depicts undissolved Ti(C,N)-cores

B depicts a complex carbonitride phase sometimes surrounding the A-cores and

C depicts the Co binder phase.

In one aspect, the present invention provides a titanium based carbonitride alloy containing Ti, Nb, W, C, N and Co, which is particularly useful for superfinishing operations. When observed in back scattering mode in a scanning electron microscope the structure has black cores of Ti(C,N), A, a grey complex carbonitride phase, B, sometimes surrounding the A-cores and an almost white Co binder phase, C, as depicted in Fig. 1.

According to the present invention it has unexpectedly been found that the abrasive wear resistance could be maximized for a given level of toughness and resistance to plastic deformation by optimizing the amount of undissolved Ti(C,N)-cores (A). A large amount of undissolved cores is favourable for the abrasive wear resistance. However, the maximum amount of these cores is limited by the demand for sufficient toughness for a specific application since toughness decreases at high levels of undissolved cores. This amount must therefore be kept between 26 and 37 vol% of the hard constituents, preferably 27 and 35 vol%, most preferably 28 and 32 vol%, the balance being one or more complex carbonitride phases containing Ti, Nb and W.

The composition of the Ti(C,N)-cores can be more closely defined as TiC_xN_1-x. The C/(C+N) atomic ratio, x, in these cores should be in the range 0.46-0.70, preferably 0.52-0.64, most preferably 0.55-0.61.

The overall C/(C+N) ratio in the sintered alloy should be in the range 0.50-0.75.

The average grain size of the undissolved cores, A, should be 0.1-2 µm and the average grain size of the hard phase including the undissolved cores 0.5-3 µm.

The Nb and Co contents should be chosen properly to give the desired properties for an envisioned application area.

Superfinishing applications place extremely high demands on resistance to plastic deformation and high demands on abrasive wear resistance while requiring moderately high toughness. This combination is best achieved by Nb contents of 2 to <4 at%, preferably 3 to <4 at% and Co contents of 3 to <9 at%, preferably 5 to <9 at%. The W content should be 3 to 8 at%, preferably less than 4 at%, to avoid an unacceptably high porosity level.

For cutting operations requiring extremely high wear resistance it is advantageous to coat the body of the present invention with a thin wear resistant coating using PVD, CVD, MTCVD or similar techniques. It should be noted that the composition of the insert is such that any of the coatings and coating techniques used today for WC-Co based materials or cermets may be directly applied, though of course the choice of coating will also influence the deformation resistance and toughness of the material.

In another aspect of the invention, there is provided a method of manufacturing a sintered titanium-based carbonitride alloy in which hard constituent powders of TiC_xN_1-x, wherein x is 0.46-0.70, preferably 0.52-0.64, most preferably 0.55-0.61, NbC and WC, are mixed with powder of Co to a composition as defined above and pressed into bodies of desired shape. Sintering is performed in a N₂-CO-Ar atmosphere at a temperature of 1370-1500 C for 1.5-2h, preferably using the technique described in EP-A-1052297. In order to obtain the desired amount of undissolved Ti(C,N) cores the amount of Ti(C,N) powder should be 50-70 wt-%, its grain size 1-3 µm and the sintering temperature and sintering time have to be chosen adequately. It is within the purview of the skilled artisan to determine by experiments the conditions necessary to obtain the desired microstructure according to this specification.

Example 1

A powder mixture of nominal composition (at%) Ti 39.4, W 3.9, Nb 3.7, Co 6.2 and a C/(N+C) ratio of 0.82 (Alloy A) was prepared by wet milling of

61.2 wt-% TiC_0.58N_0.42 with a grain size of 1.43 µm,

10.0 wt-% NbC grain size 1.75 µm,

19.3 wt-% WC grain size 1.25 µm, and

9.5 wt-% Co.

The powder was spray dried and pressed into TNMG160408-PF inserts. The green bodies were dewaxed in H₂ and subsequently sintered in a N₂-CO-Ar atmosphere for 1.5 h at 1480 °C according to EP-A-1052297, which was followed by suitable edge treatment. Polished cross sections of inserts were prepared by standard metallographic techniques and characterized using scanning electron microscopy. Fig. 1 shows a scanning electron micrograph of such a cross section, taken in back scattering mode. As indicated in Fig. 1, the black particles (A) are the undissolved Ti(C,N) cores and the light grey areas (C) are the binder phase. The remaining grey particles (B) are the part of the hard constituents consisting of carbonitrides containing Ti, Nb and W. Using image analysis, the amount of undissolved Ti(C,N) cores was determined to be 29.4 vol% of the hard constituents.

Example 2 (comparative)

Inserts of a prior art composition (Alloy B) were manufactured and characterized in the same manner as described in Example 1. The composition of Alloy B is (at %) Ti 39.4, W 3.9, Ta 3.7, Co 6.2 with a N/(N+C) ratio of 0.38. Characterization was carried out in the same manner as described in Example 1. Using image analysis, the amount of undissolved Ti(C,N) cores was determined to be 36.2% of the hard constituents.

Example 3

Cutting tests in a workpiece requiring a cutting tool with high toughness were done with the following cutting data:

Workpiece material: SS2234,

V=240 m/min, f=0.25 mm/r, d.o.c.=0.5 mm, with coolant.

Results:

Number of passes to breakage (5 edges tested):

	Edge number
	1	2	3	4	5
Alloy A	185	172	210	176	194
Alloy B	160	84	120	145	98

Example 4

Wear resistant tests of Alloys A and B by longitudinal turning were done using the following cutting data:

Workpiece material: Ovako 825B

V = 250 m/min, f= 0.15 mm/r, d.o.c.= 1 mm, with cooling

Tool life criterion was Vb ≥ 0.3 mm.

Results:

Tool life in minutes (average of 3 edges):

Alloy A: 32

Alloy B: 31

From examples 3 and 4 it is obvious that the alloy produced according to the invention has significantly improved toughness compared to the prior art material while showing comparable wear resistance.

Example 5 (comparative)

An Alloy C of the same nominal composition as Alloy A was produced and characterized in an identical manner except for the sintering temperature which was 1510 °C. Using image analysis, the amount of undissolved Ti(C,N) cores was determined to be 25.0 vol% of the hard constituents.

Example 6

Wear resistant tests of Alloys A and C by longitudinal turning were done using the following cutting data:

Workpiece material: Ovako 825B,

V=250 m/min, f= 0.15 mm/r, d.o.c.= 1 mm, with cooling

Tool life criterion was Vb ≥ 0.3 mm.

Results:

Tool life in minutes (average of 3 edges):

Alloy A: 32

Alloy C: 26

Example 7

Plastic deformation resistance for Alloys A and C was determined in a test with facing towards centre in a tube blank, with the following cutting data:

Workpiece material: SS2541

V= varying between 500 and 700 m/min, f = 0.3 mm/r, d.o.c. = 1 mm, no coolant

The result below shows the cutting speed in m/min when the edges were plastically deformed (average of 3 edges):

A: 600

C: 550

Example 8

Cutting tests in a workpiece requiring a cutting tool with high toughness were done with the following cutting data:

Workpiece material: SS2234,

V = 240 m/min, f=0.25 mm/r, d.o.c.=0.5 mm, with coolant.

Results:

Number of passes to breakage (average of 5 edges):

	Edge number
	1	2	3	4	5
Alloy A	185	172	210	176	194
Alloy C	189	169	205	183	187

From these results it was concluded that no differences in toughness between Alloys A and C were observed.

It is obvious from examples 6 through 8 that the alloy produced according to the invention has cutting properties that are superior over the comparative material.

Claims (6)

1. A titanium based carbonitride alloy containing Ti, Nb, W, C, N and Co, for superfinishing operations comprising hard constituents with undissolved Ti(C,N)-cores
   characterized in containing in addition to Ti 3-<9 at% Co with only impurity levels of Ni and Fe, 2-<4 at% Nb, 3-8 at% W, C and N having a C/(N+C) ratio of 0.50-0.75, and wherein the amount of undissolved Ti(C,N) cores is between 26 and 37 vol% of the hard constituents the balance being one or more complex carbonitride phases.
2. The alloy according to claim 1, characterized in that the alloy contains 5-<9 at% Co.
3. The alloy according to claim 1, characterized in that the alloy contains 3-<4 at% Nb.
4. The alloy according to claim 1, characterized in that the alloy contains 3-4 at% W.
5. The alloy according to claim 1, characterized in that the amount of undissolved Ti(C,N) cores is between 27 and 35 vol% of the hard constituents, the balance being one or more complex carbonitride phases.
6. A method of manufacturing a sintered titanium-based carbonitride alloy containing Ti, Nb, W, C, N and Co, for superfinishing operations comprising hard constituents with undissolved Ti(C,N)-cores by mixing hard constituent powders of TiC_xN_1-x, x having a value of 0.46-0.70, NbC and WC with powder of Co to a desired composition, pressed into bodies of desired shape and sintered in a N₂-CO-Ar atmosphere at a temperature in the range 1370-1500 °C for 1.5-2 characterized in that in order to obtain the desired amount of undissolved Ti(C,N) cores the amount of Ti(C,N) powder is 50-70 wt-% of the powder mixture, its grain size is 1-3 µm, and the sintering temperature and sintering time are chosen to give an amount of undissolved Ti(C,N) cores between 26 and 37 vol% of the hard constituents.

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