# Patent

## Title
High-temperature alloy

## Abstract

An iron-based high-temperature alloy is disclosed which contains the following chemical composition: 20% by weight Cr; 5 to 6% by weight Al; 4% by weight Ta; 4% by weight Mo; 3 to 4% by weight Re; 0.2% by weight Zr; 0.05% by weight B; 0.1% by weight Y; 0.1% by weight Hf; 0 to 0.05% by weight C; and remainder Fe and unavoidable impurities. The alloy can be produced at low cost and can possess outstanding oxidation resistance and good mechanical properties at temperatures up to 1200° C.

## Background
RELATED APPLICATION
This application claims priority under 35 U.S.C. §119 to Swiss Patent No. 01174/08 filed in Switzerland on Jul. 25, 2008, the entire content of which is hereby incorporated by reference in its entirety.
FIELD
The disclosure concerns the field of materials science. It relates to an iron-based high-temperature alloy which, for example, contains approximately 20% by weight Cr and several % by weight Al, as well as small amounts of other constituents, and which can possess good mechanical properties and oxidation resistance at operating temperatures up to 1200° C.
BACKGROUND INFORMATION
Iron-based ODS (oxide-dispersion-strengthened) materials, for example ferritic ODS FeCrAl alloys, have been known for some time. On account of their outstanding mechanical properties at high temperatures, they are, for example, used for components that are subjected to extreme thermal and mechanical stress, such as gas turbine blades or vanes.
These materials can also be used for tubes to protect thermocouples which are used, for example, in gas turbines with sequential combustion for temperature control and are exposed to extremely high temperatures and oxidizing atmospheres.
Table 1 specifies nominal chemical compositions (in % by weight) of known ferritic iron-based ODS alloys:







TABLE 1







Nominal composition of known ODS-FeCrAlTi alloys











Addition of reactive




elements (in the


Alloy
Constituent
form of an oxide













designation
Fe
Cr
Al
Ti
Si
dispersion)





Kanthal
Rem.
20.0
5.5
0.03
0.23
ZrO2—Al2O3


APM


MA 956
Rem.
20.0
4.5
0.5
—
Y2O3—Al2O3 (0.5 Y2O3)


PM 2000
Rem.
19.0
5.5
0.5
—
Y2O3—Al2O3 (0.5 Y2O3)









The operating temperatures of these metallic materials reach up to, for example, approximately 1350° C. They have potential properties that are more typical of ceramic materials.
The materials mentioned can have very high creep rupture strengths at very high temperatures and can also provide outstanding high-temperature oxidation resistance by forming a protective Al2O3 film, as well as a high resistance to sulfidizing and vapor oxidation. They can have highly pronounced directional-dependent properties. For example, in tubes, the creep strength in the transverse direction is approximately 50% of the creep strength in the longitudinal direction.
ODS alloys of this type are produced by powder metallurgical processes, using mechanically alloyed powder mixtures that are compacted in a known way, for example by extrusion or by hot isostatic pressing. The compact is subsequently highly plastically deformed, usually by hot rolling, and subjected to a recrystallization annealing treatment. This type of production, but also the material compositions described, results in, inter alia, these alloys being very expensive and having anisotropic properties.
Furthermore, various Ni-based wrought alloys such as, for example, Hastelloy X and Haynes 214 are known, and can be produced at a lower cost than the materials mentioned above and do not have anisotropic properties. These alloys have the following chemical compositions:







TABLE 2







Nominal composition of known Ni-based wrought alloys








Alloy



desig-
Constituent


















nation
Ni
Cr
Co
Mo
W
Fe
Mn
Si
C
Al
Y





















Hastelloy
Rem.
22
1.5
9
0.6
18.5
0.5
0.5
0.1
0.3
—


X


Haynes
Rem.
16
—
—
—
3
—
—
0.04
4.5
0.01


214









According to the company brochure, the material Haynes 214 should be the most oxidation-, carburization- and chlorination-resistant alloy commercially available as a wrought alloy, with effective use being possible at 2200° F. (approximately 1205° C.) for long-term stress and at 2400° F. (approximately 1316° C.) for short-term stress. However, properties of this alloy at very high temperatures are not as good as the outstanding properties of the ODS alloys mentioned above.
SUMMARY
An iron-based high-temperature alloy chemical composition is disclosed, comprising (e.g. consisting of):
20% by weight Cr;
5 to 6% by weight Al;
4% by weight Ta;
4% by weight Mo;
3 to 4% by weight Re;
0.2% by weight Zr;
0.05% by weight B;
0.1% by weight Y;
0.1% by weight Hf;
0 to 0.05% by weight C;
and remainder Fe and impurities.
A method is disclosed for producing a high-temperature alloy containing:
20% by weight Cr;
5 to 6% by weight Al;
4% by weight Ta;
4% by weight Mo;
3 to 4% by weight Re;
0.2% by weight Zr;
0.05% by weight B;
0.1% by weight Y; 0.1% by weight Hf;
0 to 0.05% by weight C;
and remainder Fe and impurities, the method comprising: melting elements corresponding to the alloy chemical composition by an arc; and rolling the alloy chemical at approximately 900-800° C.



BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the disclosure are discussed with respect to the drawing.
The single FIGURE shows oxidation behavior at 1200° C./12 h for two high-temperature alloys according to the disclosure as compared with the known alloys PM 2000, Hastelloy X and Haynes 214.



The disclosure is explained in more detail below on the basis of exemplary embodiments and the drawing.
DETAILED DESCRIPTION
Exemplary embodiments as disclosed herein are directed to developing an iron-based material that is suitable for various applications (such as protective tubes for thermocouples which can be used at extremely high temperatures in gas turbines), and costs less than the known PM 2000 material, but has at least equally good oxidation resistance. Exemplary material according to the disclosure can be well-suited for hot working and have very good mechanical properties.
An exemplary high-temperature alloy of the FeCrAl type disclosed herein can have a chemical composition which contains (e.g., consists of):
20% by weight Cr;
5 to 6% by weight Al;
4% by weight Ta;
4% by weight Mo;
3 to 4% by weight Re;
0.2% by weight Zr;
0.05% by weight B;
0.1% by weight Y;
0.1% by weight Hf;
0 to 0.05% by weight C; and
remainder Fe and impurities (e.g., unavoidable impurities). Exemplary compositions as disclosed herein can consist of any one or more of the above elements in the percentages by weight listed, including any specific percentage by weight which falls within a range specified for any given element. All percentages by weight specified herein are approximate (e.g., ±10%).
The high Cr content (e.g., 20% by weight) can ensure that the material has a good oxidation and corrosion behavior. Cr can also have a positive effect on the ductility.
The alloy contains about 5-6 (e.g., preferably 5.5%) by weight Al. This forms a protective Al2O3 film on the surface of the material, which can increase the high-temperature oxidation resistance.
If the Ta and Mo contents are lower than the values of 4% by weight specified for each, the high-temperature strength can be reduced too much; if they are higher, the oxidation resistance can be reduced in an undesirable manner and the material also becomes too expensive.
It has surprisingly been found that it is not necessary, as is the case with the known ODS alloys and described above, to add titanium. Ti and Cr act as solid-solution strengtheners. In the range of about 4% by weight, Mo has a similar effect but is much less expensive than Ti. In addition, if it is added together with Zr, as is the case in the present disclosure, Mo leads to improved tensile strengths and creep rupture strengths.
Ta, Zr and B are elements that act as dispersion strengtheners. The interaction of these constituents with the other constituents (e.g., the Cr, the Mo and the Ta) can lead to good strength values, while Al, Y and also Zr and Hf increase the oxidation resistance. Cr can have a positive effect on the ductility.
Rhenium can be particularly important. The addition of about 3-4% by weight Re can, for example, improve the creep rupture strength of the material at very high temperatures but, at the same time, also increases the oxidation resistance. Re is a solid-solution strengthener and can have a very strong effect in improving the creep properties at high temperatures. It can increase the activity of Al to form Al2O3.
Re has a hexagonally tightly packed crystal structure that differs greatly from the cubic lattice structure of Fe, Mo, Al, Ta, Cr. This difference in the crystal structure of Re means that it acts as a solid-solution strengthener.
On account of its chemical composition (e.g., combination of the specified elements in the specified ranges), the material according to the disclosure can have outstanding properties at temperatures of 1200° C. (e.g., a good creep rupture strength and extremely high oxidation resistance).
Known alloys (ODS FeCrAl comparative alloy PM 2000 produced by powder metallurgical means, as well as the wrought alloys Hastelloy X and Haynes 214—see table 2 for the composition) and the alloys according to the disclosure listed in table 3 were investigated with regard to the oxidation behavior at very high temperatures, in this case 1200° C. The alloying constituents of the alloys 2025 and 2022 according to the disclosure are specified in % by weight:







TABLE 3







Compositions of the investigated alloys according to the disclosure








Alloy
Constituent


















designation
Fe
Cr
Al
Ta
Mo
Re
Zr
B
Y
Hf
C





2022
Rem.
20
5.5
4
4
4
0.2
0.05
0.1
0.1
—


2025
Rem.
20
5.5
4
4
3
0.2
0.05
0.1
0.1
0.05









Exemplary alloys according to the disclosure were produced by arc melting of the elements specified and then rolled at temperatures of 900-800° C. Specimens for determining the oxidation resistance and the mechanical properties were produced therefrom.
In the single FIGURE, the change in weight at 1200° C. is represented as a function of time over a time period of 12 hours for the alloys specified. As expected, the very costly known comparative alloy PM 2000, produced by a powder metallurgical process, shows the smallest changes in weight, and therefore the best oxidation resistance, under these test conditions. A virtually equally good progression of this property is also shown by the alloy 2022 according to the disclosure, this alloy differing from the other alloy 2025 according to the disclosure merely in that it contains no carbon and has a 1% by weight higher Re content. Under the test conditions mentioned above, the oxidation behavior of the other known investigated wrought alloys (Hastelloy X and Haynes 214) is much worse than that of the alloys according to the disclosure. By way of example, the change in weight of the Hastelloy specimens can be approximately 2-2.5 times greater than that of the alloys according to the disclosure after age-hardening for 12 hours at 1200° C.
For exemplary alloys according to the disclosure, the yield strength at 1000° C. is approximately 60 MPa, whereas the comparative alloy PM 2000 has a yield strength at 1000° C. of approximately 90 MPa. However, if this is considered in conjunction with the outstanding oxidation behavior of these alloys at 1200° C. (see FIGURE), this represents a very good combination of properties. The lower strength of the alloys according to the disclosure as compared with PM 2000 is additionally entirely sufficient for the intended purpose (protective tube for a sheathed thermocouple).
The materials according to the disclosure are, for example, also well-suited for hot rolling and have good plastic deformability.
It is clear that a combination of Mo and Ta in equal amounts can have, for example, good effect on the oxidation behavior at 1200° C. In the range specified, Ta, for example, can increase the activity of Al and improve the oxidation resistance.
Protective tubes for sheathed thermocouples can be advantageously produced from exemplary materials according to the disclosure. Thermocouples of this type are used, for example, in gas turbines with sequential combustion for temperature control and are exposed there to oxidizing atmospheres.
Exemplary alloys according to the disclosure can have very high oxidation resistance at 1200° C. Although the strength values of the alloys according to the disclosure can be somewhat lower than those of the alloy PM 2000 at high temperatures, they are still sufficiently high. Since exemplary alloys according to the disclosure can be less expensive than PM 2000 (less expensive constituents, simpler production), they are outstandingly suitable as a substitute for PM 2000 for the areas of use described above.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

## Claims

1. An iron-based high-temperature alloy chemical composition, consisting of:
20% by weight Cr;
5 to 6% by weight Al;
4% by weight Ta;
4% by weight Mo;
3 to 4% by weight Re;
0.2% by weight Zr;
0.05% by weight B;
0.1% by weight Y;
0.1% by weight Hf;
0 to 0.05% by weight C; and
remainder Fe and impurities.

2. The high-temperature alloy as claimed in claim 1, wherein the Al content is 5.5% by weight.

3. The high-temperature alloy as claimed in claim 2, wherein the C content is 0.05% by weight.

4. The high-temperature alloy as claimed in claim 3, wherein the Re content is 3% by weight.

5. The high-temperature alloy as claimed in claim 2, wherein the Re content is 4% by weight.