Patent Application
20150354031
The subject matter of the invention relates to a nickel-cobalt alloy.
An important metallic material for rotating disks in gas turbines is the nickel-base Alloy 718. The chemical composition of Alloy 718 is listed in Table 1 of the AMS 5662 standard.
The requirements applicable to the mechanical properties that Alloy 718 must have in accordance with the AMS 5662 standard are listed in Table 2. Furthermore, for use as a rotating disk in an aircraft turbine, an elongation of <0.2% is required after a creep test at a temperature of 650° C. and a load of 550 MPa after a loading time of 35 h (or after 100 h in the case of even more stringent requirements), while high cycle numbers to failure are expected in the low cycle fatigue/LCF test. Depending on test condition, cycle numbers of several 10,000 cycles up to cycles of more than 100,000 are required, as specified on the basis of different disk designs. In accordance with the AMS 5662 standard, the mechanical requirements must be satisfied after a three-stage annealing process—one hour of solution annealing at an annealing temperature between 940 and 1000° C.+precipitation hardening at 720° C. for 8 h+620° C. for 8 h.
Essentially two precipitation phases are responsible for the high strength properties of nickel-base Alloy 718. They are on the one hand the γ″-phase Ni3Nb and on the other hand the γ′-phase Ni3(Al, Ti). A third important precipitation phase is the δ-phase, which limits Alloy 718 to a maximum temperature of 650° C., since above that temperature the metastable γ″-phase is transformed to the stable δ-phase. As a consequence of this transformation, the material loses its creep-strength properties. In the course of the process of manufacture of Alloy 718 material from the remelted ingot to the semifinished form of a forged billet, however, the δ-phase plays an important role in achieving a very fine-grained homogeneous grain structure during the forging process. During forging heats in the range of the precipitation temperature of the δ-phase, small proportions at precipitates of δ-phase result in grain refinement. This fine grain of the billet microstructure is preserved or becomes even more fine-grained due to hot forming during the manufacture in particular of turbine disks, even though forging in this case takes place at a temperature below the δ-phase solution temperature. The very fine-grained microstructure is a prerequisite for very high cycle numbers to failure in the LCF test. Since the precipitation temperature of the γ′-phase of Alloy 718 is very much lower than the δ-phase solution temperature of approximately 1020° C., Alloy 718 has a broad window of forming temperature, and so forging from ingot to billet or from billet to turbine disk is unproblematic as regards possible surface disruptions due to γ′-phase precipitates, which may occur during forging at very low temperatures. Thus Alloy 718 is very amenable to the hot-forming process. Nevertheless, one disadvantage is the relatively low application temperature of Alloy 718, up to 650° C.
Another nickel alloy known as “Waspaloy” is characterized by good microstructural stability at higher temperatures, up to approximately 750° C., and so its application temperature is approximately 100 K higher than that of Alloy 718. Waspaloy achieves its microstructural stability up to higher temperatures by higher alloying proportions of the elements Al and Ti. Herewith Waspaloy exhibits a high solution temperature of the γ′-phase, which in turn permits a higher application temperature. The chemical composition of Waspaloy is listed in Table 3 in accordance with the AMS 5704 standard.
The requirements imposed on the mechanical properties that Waspaloy must achieve in accordance with the AMS 5704 standard are listed in Table 4. Furthermore, for use as a rotating disk in an aircraft turbine, an elongation of <0.2% is required after a creep test at a test temperature and a test load after a loading time of 35 h (or after 100 h in the case of even more stringent requirements), while high cycle numbers to failure are expected in the low cycle fatigue/LCF test. In this connection, depending on test condition, cycle numbers of several 10,000 cycles up to cycles of more than 100,000 are required, as specified on the basis of different disk designs. In accordance with the AMS 5704 standard, the mechanical requirements must be satisfied after a three-stage annealing process—four hours of solution annealing at an annealing temperature between 996 and 1038° C.+stabilization annealing at 845° C. for 4 h+precipitation hardening at 760° C. for 16 hours.
However, the high γ′ solution temperature of approximately 1035° C. is also the cause of the poor hot formability of Waspaloy. At a surface temperature of approximately 980° C., deep discontinuities caused by γ′-phase precipitates may develop at the surface of the forged pieces during processes of forging from the remelted ingot to billets or from the billet to turbine disks. Thus the window of forming temperature for Waspaloy is relatively small, necessitating several forming heats due to multiple exposures in heating furnaces, in turn resulting in a longer process duration and therefore higher manufacturing costs. Because of the necessarily higher forging temperatures and the absence of a grain-refining δ-phase, a very fine grain microstructure in the billet forged from Waspaloy is not achievable, in contrast to what can be illustrated for Alloy 718.
For aircraft applications, Alloy 718 and Waspaloy are smelted as the primary heat in a VIM furnace then cast as round electrodes in chill molds. After further processing steps, either the electrodes are remelted in the ESR or VAR double-melt smelting process or VAR resmelted ingots are produced in the VIM/ESR/VAR triple-melt process. Before the resmelted ingots can be hot-formed, they are subjected to homogenization annealing. Thereafter the resmelted ingots are forged in several forging heats to billets, which in turn are used as forging stock for the manufacture, for example, of turbine disks.
U.S. Pat. No. 6,730,264 discloses a nickel-chromium-cobalt alloy of the following composition: 12 to 20% Cr, up to 4% Mo, up to 6% W, 0.4 to 1.4% Ti, 0.6 to 2.6% Al, 4 to 8% Nb (Ta), 5 to 12% Co, up to 14% Fe, up to 0.1% C, 0.003 to 0.03% P, 0.003 to 0.015% B, the rest nickel.
DE 69934258 T2 discloses a process for manufacturing an object formed from Waspaloy, which process includes the following steps:
The invention is based on the object of providing an alloy in which the previously described advantages of the two known alloys, Alloy 718 and Waspaloy, i.e., the good hot formability of Alloy 718 and the microstructural stability of Waspaloy up to higher temperatures of approximately 750° C., can be combined.
This task is accomplished by an Ni—Co alloy with 30 to 65 wt % Ni, >0 to max. 10 wt % Fe, >12 to <35 wt % Co, 13 to 23 wt % Cr, 1 to 6 wt % Mo, 4 to 6 wt % Nb+Ta, >0 to <3 wt % Al, >0 to <2 wt % Ti, >0 to max. 0.1 wt % C, >0 to max. 0.03 wt % P, >0 to max. 0.01% wt Mg, >0 to max. 0.02% wt B, >0 to max. 0.1% wt Zr, which alloy satisfies the requirements and criteria listed below:
Advantageous improvements of the inventive alloy are specified in the associated dependent claims.
On the basis of the parameters mentioned in claim 1, the inventive alloy no longer exhibits the disadvantages of Alloy 718, namely the relatively low application temperature, and of Waspaloy, namely the poor hot formability.
The inventive alloy preferably satisfies the requirement “945° C.≦γ′-solvus temperature 1000° C.”.
It is of particular advantage when Co contents between 11.5 and 35 at % can be adjusted at a ΔT (δ−γ′)≧80 K and Al+Ti≦4.7 atomic %.
The inventive alloy advantageously has a temperature interval between δ-solvus and γ′-solvus temperatures equal to or greater than 140 K and at the same time a Co content between 15 and 35 at %.
According to a further improvement of the invention, the Ti content in the alloy is adjusted to ≦0.8 atomic % and more preferably to a content of ≦0.65 atomic %.
Restricting the (Nb+Ta) contents to values between 4.7 and 5.7 wt % may also contribute to improving the good hot deformability of Alloy 718 and the microstructural stability of Waspaloy up to higher temperatures of approximately 750° C.
The value ranges for a ratio of two element contents are different when expressed in atomic and weight percent. At the structural level, atomic proportions are essential. The contents of the elements essential for the inventive alloy, namely Al, Ti and Co, are presented in atomic % especially in Table 6a.
The inventive alloy may also contain the following elements as residual elements:
If appropriate for the respective application, the inventive alloy may if necessary also contain the following elements
In the inventive alloy, the elements listed below may be adjusted as follows:
Depending on area of application of the inventive alloy, it may be appropriate from cost viewpoints to substitute part of the elements Ni and/or Co with the less expensive element Fe.
The inventive alloy is preferably usable as a component in an aircraft turbine, especially a rotating turbine disk, as well as a component of a stationary turbine.
The alloy may be produced in the following semifinished forms: strip, sheet, wire, bar.
The material is creep-resistant at high temperature and, besides the already mentioned applications, can also be used for the following service areas: in engine construction, in exhaust-gas systems, as heat shields, in furnace construction, in boiler construction, in power-plant construction, especially as superheater pipes, as structural parts in gas and oil extraction engineering, in stationary gas and steam turbines and also as a weld filler for all of the said applications.
The present invention describes a nickel alloy, especially for critical rotating components of an aircraft turbine. The inventive alloy has a high microstructural stability at high temperatures and therefore offers the possibility of application at thermal loads up to 100 K hotter than for the known nickel-base Alloy 718. Furthermore, the inventive alloy is characterized by better formability than the nickel alloy known as Waspaloy. The alloy of the present invention offers technological properties that permit applications in gas turbines in the form of disks, blades, holders, housings or shafts.
The present alloy describes the chemical composition, the technological properties and the processes for the manufacture of semifinished products made from the material of the inventive nickel-cobalt alloy.
The properties of the inventive alloy are discussed hereinafter:
Numerous laboratory heats with different chemical compositions were produced by means of a laboratory vacuum arc furnace.
Each heat was cast into a heavy-duty cylindrical copper chill mold with a diameter of 13 mm. During smelting, three bars with a length of approximately 80 mm were produced. All alloys were homogenized after smelting. The entire process took place in the vacuum furnace and consisted of 2 stages: 1140° C./6 h+1175° C./20 h. This was followed by quenching in an argon atmosphere. Hot forming for the smelted alloys was carried out using a rotary swaging machine. The bars had a diameter of 13 mm at the beginning and were reduced in diameter by four rotary swaging operations of one millimeter each to obtain the final diameter of 9 mm.
Table 1 discloses the chemical composition of Alloy 718 corresponding to the prior art as specified by the valid AMS 5662 standard, while Table 2 presents the mechanical properties of that alloy.
Table 3 discloses the chemical composition of Waspaloy corresponding to the prior art as specified by the valid AMS 5662 standard, while Table 4 presents the mechanical properties of that alloy.
The inventive chemical compositions of the laboratory heats are listed in Table 5. At the bottom, the known alloys A718, A718 Plus and Waspaloy are also included as reference materials. In addition to the reference materials, the test alloys are identified with the letters V and L plus 2 numerals each. The chemical compositions of these test alloys include variations in the contents of the elements Ti, Al, Co and Nb.
When the contents of the elements Ti, Al and Co as well as the sum of Al+Ti and the Al/Ti ratio of the contents of the elements are expressed in atomic percent, very good technological properties are obtained in selected ranges for the γ′-solvus temperature, the difference between δ-solvus and γ′-solvus temperatures, the absence of primary delta phase and absence of the η-phase, the microstructural stability at 800° C. after aging annealing tests for 500 h and the mechanical hardness HV after a standard heat treatment comprising solution annealing and two-stage precipitation-hardening annealing for A718 (980° C./1 h+720° C./8 h+620° C./8 h, see the AMS 5662 standard).
Table 6a lists the contents in atomic percent of the elements Al, Ti and Co as well as the sum of the Al+Ti contents (in atomic percent) and the Al/Ti ratios for the test alloys and the 3 reference materials of Table 5.
Furthermore, Table 6b contains the calculated solvus temperatures of the δ-phase and of the γ′-phase as well as the temperature difference ΔT (δ−γ′) calculated therefrom between the δ-solvus and γ′-solvus temperatures. Table 6b also indicates the mechanical hardness values 10 HV determined for the test alloys (after three-stage precipitation-hardening heat treatment of 980° C./1 h+720° C./8 h+620° C./8 h in accordance with the AMS 5662 standard for A718). Moreover, Table 6b indicates remarks on the occurrence of the η-phase (calculated or observed).
The criteria for selection of the inventive alloy are explained and exemplary test alloys are indicated in the following descriptions.
For reasons of strength and microstructural stability, the γ′-solvus temperature of the inventive alloy should be 50 K higher than that of alloy A718, which has a γ′-solvus temperature of approximately 850° C. On the other hand, the γ′-solvus temperature of the inventive alloy should be lower than or equal to 1030° C. This 1030° C. corresponds approximately to the γ′-solvus temperature of Waspaloy. A higher γ′-solvus temperature would influence the hot formability very negatively since, in the forging process, for example, γ′-precipitates already lead to extensive precipitation hardening of the surface of the forged piece if the surface temperatures of the forged piece are slightly below the γ′-solvus temperature, and this in turn may lead to considerable disruptions of the surface of the forged piece during further forming by forging.
Thus the requirement 900° C.≦γ′-solvus T≦1030° C. should be satisfied.
In
From
For even better hot formability, the γ′-solvus temperature of the inventive alloy should be <1000° C., and for microstructural stability at even higher temperature it should be >945° C. The test alloys V14, V16, V17, V20, V21, V22 L04, L15, L16, L17 and L18 are exemplary alloys for this range. The temperature range bounded between 945° C. and 1000° C. is evident from
The Co content of the test alloys influences the δ-solvus and γ′-solvus temperatures and thus ΔT (δ−γ′). The Co content of the inventive alloy is not permitted to be too high, to ensure that no primary δ-phase develops. This restricts the Co content to <35 at %. Exemplary alloys in which primary δ-phase develops are the test alloys L12 and L13, both of which have a Co content of approximately 50 at %.
During the forging process, minor proportions of δ-phase are consumed for grain refining of the microstructure. In other words, forging in the last forging heats is carried out starting from a temperature slightly below the δ-solvus temperature, in order to produce a very fine-grained microstructure of the respective forged piece. On the other hand, in order to make it possible to work with a sufficiently broad window of forging temperatures, the γ′-solvus temperature cannot be permitted to be too high, and it must lie well below the δ-solvus temperature of the inventive alloys. For the window of forging temperature to be sufficiently broad, it must be ≧80 K. Therefore the difference ΔT (δ−γ′) between δ-solvus temperature and γ′-solvus temperature must be ≧80 K.
From
A further criterion results from the requirement that states that the microstructure of the inventive alloy should be stable at an aging temperature of 800° C. (after 500 h). This criterion is satisfied by the inventive alloys that have an Al/Ti ratio of ≧5.0. Exemplary alloys for this condition are the test alloys V13, V15, V16, V17, V21 and V22.
Table 7 lists exemplary test alloys for the requirement of the Al/Ti ratio of the inventive alloy.
a to 5e show exemplary SEM photographs for the test alloys L4, V10, V15, V16 and V17 after aging annealing for 500 h at 800° C.
By way of further description of the subject matter of the invention,
Furthermore, it was observed that, in the creep and stress rupture test at 700° C., A 780 also achieves the desired mechanical properties of creep elongation much smaller than 0.2% as well as much longer times to failure of >23 h in the stress rupture test—under otherwise identical test conditions where these properties are achieved by A 718 only at test temperatures up to 650° C.
Table 8 shows the batches 25 to 27 indicated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2013 002 483.8 | Feb 2013 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DE2014/000053 | 2/13/2014 | WO | 00 |
