Ferritic alloy

Abstract

To provide a ferritic alloy with further improved corrosion resistance. A ferritic alloy containing the following elements in the following weight percent (wt%): C: 0.01-0.1, N: 0.001-0.1, O: ≦0.2, Cr: 4-15, Al: 2-6, Si: 0.5-3, Mn: ≦0.4, Mo+W: ≦4, Y: ≦1.0, Sc, Ce and / or La: ≦0.2, Zr: ≦0.40, RE: ≦1.0, the balance being Fe and normally present impurities, and the following equation must be satisfied: 0.014≦(Al+0.5Si)(Cr+10Si+0.1)≦0.022.

Description

Technical Field

[0001] The present disclosure relates to a ferrite alloy described in the preamble of claim 1. The present disclosure further relates to the use of the ferrite alloy and to an object or coating made from this alloy.

Background Art

[0002] Ferrite alloys, such as FeCrAl alloys containing chromium (Cr) at a level of 15 to 25 wt% and aluminum (Al) at a level of 3 to 6 wt%, are well known to be capable of forming a protective α-alumina (Al 2 O 3 : aluminum oxide) scale when exposed to temperatures in the range of 900 to 1300 °C. The lower limit of the Al content for forming and maintaining the alumina scale varies depending on the conditions to which it is exposed. However, if the level of Al is too low at high temperatures, selective oxidation of Al will fail and a scale based on chromium and iron, which is less stable and has lower protective power, will be formed.

[0003] There is a common understanding that FeCrAl alloys generally do not form a protective α-alumina layer even when exposed to temperatures below about 900 °C. Attempts have been made to optimize the composition of FeCrAl alloys so that a protective α-alumina is formed at temperatures below about 900 °C. However, generally these attempts have not been successful. This is because the diffusion of oxygen and aluminum to the oxide / metal interface is relatively slow at low temperatures, which results in a slow formation rate of the alumina scale, meaning there is a risk of severe corrosion attack and the formation of less stable oxides.

[0004] Another problem that occurs at low temperatures, i.e., temperatures below 900 °C, is a long-term embrittlement phenomenon resulting from the low-temperature miscibility gap for Cr in the FeCrAl alloy system. This miscibility gap exists for Cr levels above approximately 12 wt% at 550 °C. In recent years, alloys with relatively low Cr levels of about 10 - 12 wt% of Cr have been developed to avoid this phenomenon. This group of alloys has been found to work very well in molten lead under controlled low pressure O 2 and has been found to work very well in molten lead under controlled low pressure O

[0005] European Patent Application No. 0475420 relates to a rapidly solidified ferrite alloy consisting essentially of about 1.5 - 3 wt% Cr, Al, Si, and REM (Y, Ce, La, Pr, Nd, the balance being Fe and impurities). This sheet may further contain at least one element selected from the group consisting of Ti, Nb, Zr, and V in an amount of about 0.001 - 0.5 wt%. This sheet has a particle size of about 10 μm or less. European Patent Application No. 075420 discusses the addition of Si to improve the fluidity of the molten alloy, but its success is limited because the ductility decreases.

[0006] European Patent Application No. 0091526 relates to an alloy that is resistant to periodic oxidation by heat and is hot workable, and more specifically, to an iron-chromium-aluminum alloy having a rare earth additive. Upon oxidation, the alloy forms an oxide having a desirable whisker-like structure on the surface of the catalytic converter. However, the alloys thus obtained do not have high-temperature resistance.

[0007] Therefore, there is still a need to further improve the corrosion resistance of ferrite alloys, thereby enabling such ferrite alloys to be used in corrosive environments during high-temperature conditions. Aspects of the present disclosure should solve or at least reduce the above-described problems. SUMMARY OF THE INVENTION

[0008] Therefore, the present disclosure relates to a ferritic alloy that provides a combination of good oxidation resistance and excellent ductility, the alloy comprising the following composition in weight % (wt%): C: 0.01 to 0.1 N: 0.001 to 0.1 O: ≤ 0.2 Cr: 4 to 15 Al: 2 to 6 Si: 0.5 to 3 Mn: ≤ 0.4 Mo + W ≤ 4 Y: ≤ 1.0 Sc, Ce, and / or La ≤ 0.2 Zr: ≤ 0.40 RE: ≤ 1.0 The balance is Fe and normally present impurities, and the following equation must be satisfied: 0.014 ≤ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≤ 0.022.

[0009] Thus, in the alloy according to the present disclosure, there is a relationship among the contents of Cr, Si, and Al, and when this is satisfied, an alloy is obtained that has excellent oxidation resistance and ductility, and a reduced brittleness in combination with increased high-temperature corrosion resistance.

[0010] The present disclosure also relates to articles and / or coatings containing the ferritic alloy according to the present disclosure. The present disclosure further also relates to the use of the ferritic alloy as defined above, previously, or hereinafter for making articles and / or coatings.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0012] As described above, the present disclosure provides a ferrite alloy containing the following elements by weight% (wt%): C: 0.01 to 0.1 N: 0.001 to 0.1 O: ≤0.2 Cr: 4 to 15 Al: 2 to 6 Si: 0.5 to 3 Mn: ≤0.4 Mo + W ≤ 4 Y: ≤1.0 Sc, Ce, and / or La ≤ 0.2 Zr: ≤0.40 RE: ≤1.0 The balance is Fe and normally present impurities, and the following equation must be satisfied: 0.014 ≤ (Al + 0.5Si)(Cr + 10Si + 0.1) ≤ 0.022.

[0013] Surprisingly, as defined hereinbefore or hereinafter, i.e. alloys containing alloying elements within the ranges stated herein, have unexpectedly been found to form a protective surface layer containing an aluminum-rich oxide even when the chromium level is as low as 4 wt%. This is highly important for both the workability of the alloy and its long-term phase stability, since after exposure for long periods in the temperature ranges described herein, the undesired brittle σ-phase is reduced or even avoided. Thus, the interaction between Si, Al and Cr enhances the formation of a stable and continuous protective surface layer containing an aluminum-rich oxide, enabling the addition of Si by using the above equation and obtaining a ferritic alloy that can be both manufactured and formed into various articles. The inventors have surprisingly found that when the amounts of Si, Al and Cr are adjusted to satisfy the following conditions (all figures for the elements are in weight fractions): 0.014 ≦ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≦ 0.022 It has been found that the resulting alloys have a combination of excellent oxidation resistance, workability and shape stability within the Cr ranges of the present disclosure. According to one embodiment, 0.015 ≦ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≦ 0.021, for example 0.016 ≦ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≦ 0.020, for example 0.017 ≦ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≦ 0.019.

[0014] The ferrite alloys of the present disclosure are particularly useful at low temperatures of less than about 900 °C. This is because a protective surface layer containing an aluminum-rich oxide is formed on an article and / or coating made of a ferrite alloy according to the present disclosure, and this protective surface layer prevents corrosion, oxidation, and embrittlement of the article and / or coating. Further, the ferrite alloys according to the present disclosure can provide protection against corrosion, oxidation, and embrittlement at a low temperature of 400 °C. This is because a protective surface layer containing an aluminum-rich oxide is formed on the surface of an article and / or coating made of a ferrite alloy according to the present disclosure. Further, the alloys according to the present disclosure are also excellent at temperatures up to about 1100 °C, and the tendency for embrittlement over a long period of time in the temperature range of 400 - 600 °C is reduced.

[0015] The alloys according to the present disclosure can be used in the form of a coating. Further, an article can also contain an alloy according to the present disclosure. According to the present disclosure, the term "coating" refers to an embodiment in which a ferrite alloy according to the present disclosure exists in the form of a layer that is exposed to a corrosive environment (i.e., in contact with a base material), where the means and methods of achieving corrosion are not questioned, nor is the relative thickness relationship between the layer and the base material. Thus, examples thereof include, but are not limited to, PVD coatings, claddings, or compounds or composite materials. The purpose of this alloy is to protect the underlying material from both corrosion and oxidation. Examples of suitable articles include, but are not limited to, compound tubes, tubes, boilers, gas turbine components, and steam turbine components. Other examples include superheaters, water walls in power plants, members in containers or heat exchangers (e.g., for reforming a gas containing hydrocarbons or CO / CO 2 for other treatments), members used in connection with industrial heat treatments of steel and aluminum, powder metallurgy, gas and electric heating elements.

[0016] Furthermore, the alloys according to the present disclosure are suitable for use in environments having corrosive conditions. Examples of such environments include exposure to salts, liquid lead, and other metals, exposure to ash or sediments with high carbon content, exposure to combustion atmospheres with low O 2 partial pressure and / or high N 2 and / or atmospheres with high carbon activity, among others, but are not limited thereto.

[0017] Furthermore, the ferritic alloys according to the present disclosure can be produced using generally performed solidification rates ranging from conventional metallurgy to rapid solidification. The alloys according to the present disclosure are also suitable for producing any kind of articles (such as wires, strips, bars, and plates) that are press-worked and extruded. As will be understood by those skilled in the art, the degree of hot and cold plastic deformation, as well as the grain structure and grain size, vary depending on the shape of the article and the manufacturing route.

[0018] Previously, and as defined below, the functions and actions of the essential alloying elements for the alloys are shown in the following paragraphs. The listing of the functions and actions of each alloying element should not be considered complete, and there may be additional functions and actions for these alloying elements.

[0019] Carbon (C) Carbon may exist as an inevitable impurity resulting from the manufacturing process. Carbon may be included in the ferritic alloy as previously or as defined below in order to increase strength by precipitation hardening. In order to have a significant effect on the strength in the alloy, it is desirable for carbon to be present in an amount of at least 0.01 wt%. If the level is too high, carbon may make it difficult to form the material and may also have a negative effect on corrosion resistance. Therefore, the maximum amount of carbon is 0.1 wt%. The carbon content is, for example, 0.02 - 0.09 wt%, for example 0.02 - 0.08 wt%, for example 0.02 - 0.07 wt%, for example 0.02 - 0.06 wt%, for example 0.02 - 0.05 wt%, for example 0.01 - 0.04 wt%.

[0020] Nitrogen (N) Nitrogen may be present as an inevitable impurity resulting from the manufacturing process. Nitrogen may be contained in the ferrite alloy either previously or as defined hereinafter, especially when applying the powder metallurgy route, in order to increase strength by precipitation hardening. If its level is too high, nitrogen may make alloy formation difficult and may also have a negative effect on corrosion resistance. Therefore, the maximum amount of nitrogen is 0.1 wt%. Suitable ranges for nitrogen are, for example, 0.001 - 0.08 wt%, for example 0.001 - 0.05 wt%, for example 0.001 - 0.04 wt%, for example 0.001 - 0.03 wt%, for example 0.001 - 0.02 wt%.

[0021] Oxygen (O) Oxygen may be present in the alloy as an impurity resulting from the manufacturing process, either previously or as defined hereinafter. In this case, the amount of oxygen is at most 0.02 wt%, for example at most 0.005 wt%. When oxygen is intentionally added to provide strength by dispersion strengthening, the alloy contains oxygen at most 0.2 wt% or 0.2 wt%, in the same way as when producing the alloy through the powder metallurgy route, either previously or as defined hereinafter.

[0022] Chromium (Cr) Chromium is mainly present in the alloy according to the present disclosure as a solid solution element of the matrix. Chromium promotes the formation of an aluminum oxide layer in the alloy by means of the so-called "third element effect", i.e., by forming chromium oxide in a transitional oxidation state. To achieve this purpose, chromium is desirably present in the alloy in an amount of at least 4 wt%, either previously or as defined hereinafter. In the inventive alloy according to the present disclosure, Cr is a brittle σ phase and Cr 3The tendency to form Si is also enhanced. This effect appears at about 12 wt%, and is enhanced at levels exceeding 15 wt%, so the upper limit of Cr is 15 wt%. From the perspective of oxidation, at levels higher than 15 wt%, Cr makes an undesirable contribution to the protective oxide scale. According to one embodiment, the Cr content is 5-13 wt%, for example 5-12 wt%, for example 6-12 wt%, for example 7-11 wt%, for example 8-10 wt%.

[0023] Aluminum (Al) Aluminum is an important element in the alloy, either previously or as defined below. When exposed to oxygen at high temperatures, aluminum forms a dense and thin oxide (Al 2 O 3 ) by selective oxidation, thereby protecting the underlying alloy surface from further oxidation. The amount of aluminum is desirably at least 2 wt% to ensure the formation of a protective surface layer containing an aluminum-rich oxide and to ensure that sufficient aluminum is present to repair the protective surface layer in case of damage. However, aluminum has a negative impact on formability, and when the amount of aluminum is large, cracks may form in the alloy during mechanical processing of the alloy. Therefore, the amount of aluminum desirably does not exceed 6 wt%. Aluminum can be present, for example, in an amount of 3-5 wt%, for example 2.5-4.5 wt%, for example 3-4 wt%.

[0024] Silicon (Si) In commercially available FeCrAl alloys, silicon is often present at levels up to 0.4 wt%. As previously or as defined below, Si plays an important role in ferritic alloys. This is because silicon has been found to have an excellent effect of improving oxidation resistance and corrosion resistance. The upper limit of Si is determined by the loss of workability under hot and cold conditions and by the brittle Cr formed during long-term exposure 3It is determined by the increased tendency of the formation of Si and the σ phase. Therefore, the addition of Si must be carried out in relation to the contents of Al and Cr. Thus, the amount of Si is 0.5 to 3% by weight, for example 1 to 3% by weight, for example 1 to 2.5% by weight, for example 1.5 to 2.5% by weight.

[0025] Manganese (Mn) Manganese can be present as an impurity in the alloy, at most 0.4% by weight, for example 0 to 0.3% by weight, either previously or as defined below.

[0026] Yttrium (Y) In solution metallurgy, yttrium can be added in an amount of at most 0.3% by weight to improve the adhesion of the protective surface layer. Further, when yttrium is added in powder metallurgy to form a dispersion with oxygen and / or nitrogen, the yttrium content is at least 0.04% by weight in order to achieve the desired dispersion hardening effect by oxides and / or nitrides. The maximum amount of yttrium in the dispersion-hardened alloy can be at most 1.0% by weight in the form of yttrium-containing oxygen compounds.

[0027] Scandium (Sc), Cerium (Ce), and Lanthanum (La) Scandium, cerium, and lanthanum are mutually exchangeable elements, and can be added individually or in combination, for a total of at most 0.2% by weight, to improve the self-healing property of the oxide layer (Al 2 O 3 ), or the adhesion between the alloy and the Al 2 O 3 layer.

[0028] Molybdenum (Mo) and Tungsten (W) Both molybdenum and tungsten have a positive effect on the hot strength of the alloy, either previously or as defined below. Mo also has a positive effect on the wet corrosion properties. These elements can be added individually or in combination, in an amount of at most 4.0% by weight, for example 0 to 2.0% by weight.

[0029] Reactive element (RE) A reactive element is defined as one that is highly reactive with carbon, nitrogen, and oxygen. Titanium (Ti), niobium (Nb), vanadium (V), hafnium (Hf), tantalum (Ta), and thorium (Th) are reactive elements in this sense. These elements have a high affinity for carbon and are thus strong carbide formers. These elements are added to improve the oxidation characteristics of the alloy. The total amount of the elements is up to 1.0 wt%, for example 0.4 wt%, for example up to 0.15 wt%.

[0030] The maximum amount of each reactive element is mainly due to the tendency of the element to form unfavorable intermetallic compound phases.

[0031] Zirconium (Zr) Zirconium is often referred to as a reactive element because zirconium is very reactive with oxygen, nitrogen, and carbon. In the alloys according to the present disclosure, Zr has been found to have two roles. This is because zirconium is present in a protective surface layer containing an aluminum-rich oxide, thereby improving the oxidation resistance, and zirconium also forms carbides and nitrides. Therefore, in order to achieve the best properties for the protective surface layer containing an aluminum-rich oxide, it is advantageous to include Zr in the alloy.

[0032] However, at Zr levels above 0.40 wt%, Zr-rich intermetallic compound inclusions are formed, which affects oxidation. At levels below 0.05 wt%, regardless of the C content and N content, it is too little to achieve the two purposes. Therefore, when Zr is present, the range is 0.05 - 0.40 wt%, for example 0.10 - 0.35 wt%.

[0033] Furthermore, the relationship among Zr, N, and C has been found to be important for achieving even better oxidation resistance for the protective surface layer (i.e., alumina scale). Therefore, the inventors have surprisingly found that when Zr is added to the alloy, this alloy contains N and C under the following conditions (element contents are shown in wt%): TIFF0007690388000001.tif9170 is satisfied, the resulting alloy has been found to acquire good oxidation resistance.

[0034] Previously, or as defined below, the balance in the ferritic alloy is Fe and inevitable impurities. Examples of inevitable impurities are not elements or compounds added for any purpose, but rather elements or compounds that are normally present as impurities in the materials used to make the ferritic alloy and thus cannot be completely avoided.

[0035] Figures 1a and 1b show that in Si-containing ferritic alloys, the more Cr there is, the easier it is to form Si 3 Cr inclusions, and at 20% Cr, after being exposed for a long time in the temperature range of interest, the undesired brittle σ-phase is promoted. Although these graphs only show for two Cr levels (10% and 20%), the tendency for the brittle phase to increase as Cr increases is clearly shown. Note that at 10% Cr, the σ-phase does not exist, and as the Si content increases, the amount of the Si 3 Cr phase increases at both Cr levels. Therefore, these figures show that problems will occur when using Cr at a level of about 20%.

[0036] When the expression "≦" or "below" is used in the following context: "element ≦ number", those skilled in the art will understand that the lower limit of the range is 0 wt% unless otherwise specifically mentioned for other numbers. Furthermore, the indefinite article "a (one)" does not exclude the possibility of being plural.

[0037] The present disclosure will be further described by the following non-limiting examples.

Example

[0038] The test melt was produced in a vacuum melting furnace. The composition of the test melt is shown in Table 1.

[0039] The obtained samples were hot-rolled and machined into flat rods with a cross-section of 2×10 mm. Then, to expose them to air and combustion conditions, these rods were cut into 20 mm long fragments and ground to 800 mesh with SiC paper. For tensile testing at room temperature using a Zwick / Roell Z100 tensile testing machine, several rods were cut into rods with a length of 200 mm×3×12 mm.

[0040] The results of the exposure test and tensile test are shown in Table 1.

[0041] For these samples, the yield stress, fracture stress, and elongation at the fracture point in a standard tensile testing machine were tested. Results showing an elongation >3% are indicated by "x" in the "Workability" column in the table. Thus, "x" indicates an alloy that is easy to hot-roll, which represents ductility at room temperature. In the column "Oxidation resistance", "x" indicates that the alloy forms a protective alumina-rich oxide scale at 950 °C in air and at 850 °C in biomass ash deposits.

[0042] TIFF0007690388000002.tif237170TIFF0007690388000003.tif237170

[0043] Therefore, as can be seen from the above table, the alloys according to the present disclosure exhibit good workability and good oxidation resistance.

[0044] Figures 2a) - e) disclose samples of the polished portions of the present disclosure (4783 in Figure 2a and 4779 in 2b) after being exposed 50 times to a cycle of 1 hour at 850 °C to a biomass (wood pellet) ash containing a large amount of potassium, compared to three reference alloys. These micrographs were taken at 100x magnification with a JEOL FEG SEM and show a clear advantage between the properties of the alloys according to the present disclosure and those of the reference materials. As can be seen therefrom, in the alloys according to the present disclosure, a protective alumina scale (aluminum oxide layer) as thin as 3 - 4 μm is formed, while in stainless steel (2c: 11Ni, 21Cr, N, Ce, balance Fe), and Ni - based alloy (2e: Inconel 625: 58Ni, 21Cr, 0.4Al, 0.5Si, Mo, Nb, Fe), a scale rich in relatively thick and less protective chromia (chromium oxide) is formed, and in the FeCrAl alloy of the comparative example (alloy 4776) (Figure 2d: 20Cr, 5Al, 0.04Si, balance Fe), a relatively porous and non - protective alumina scale is formed.

[0045] As can be seen from Figures 2a - e, by adding Si, Al, and Cr within the ranges according to the present disclosure, even when the Al level is as low as about 2 wt% and the chromium level is as low as 5 wt%, the formation of the alumina scale is promoted.

Claims

1. A ferrite alloy containing the following elements in weight percent (wt%): C: 0.01 to 0.1 N: 0.001 to 0.1 O: ≤0.2 Cr: 4 to 15 Al: 2 to 6 Si: 0.5 to 3 Mn: ≤0.4 Mo + W: ≤4 Y: ≤1.0 Sc, Ce, and / or La: ≤0.2 Zr: 0.05 to 0.40 At least one element selected from Ti, Nb, V, Hf, Ta, and Th: ≤1.0 The balance being Fe and unavoidable impurities, and the following equation must be satisfied (the elements are weight fractions): 0.014 ≤ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≤ 0.

022.

2. (The elements are weight fractions) 0.015 ≤ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≤ 0.021 The ferrite alloy according to claim 1, wherein

3. The ferrite alloy according to claim 1 or 2, wherein Cr is 5 to 13 wt%.

4. The ferrite alloy according to any one of claims 1 to 3, wherein Cr is 6 to 12 wt%.

5. The ferrite alloy according to any one of claims 1 to 4, wherein Al is 2.5 to 4.5 wt%.

6. The ferrite alloy according to any one of claims 1 to 4, wherein Al is 3 to 5 wt%.

7. The ferrite alloy according to any one of claims 1 to 6, wherein Al is 3 to 4 wt%.

8. The ferrite alloy according to any one of claims 1 to 7, wherein Si is 1.0 to 3 wt%.

9. The ferrite alloy according to any one of claims 1 to 8, wherein Si is 1.5 to 2.5 wt%.

10. The ferrite alloy according to any one of claims 1 to 9, wherein Zr is 0.10 to 0.35 wt%.

11. The amount of C, N, and Zr satisfies the following equation: The ferrite alloy according to any one of claims 1 to 10, wherein

12. A coating comprising the ferrite alloy according to any one of claims 1 to 11.

13. An article comprising the ferrite alloy according to any one of claims 1 to 11.

14. Use of the ferrite alloy according to any one of claims 1 to 11 for producing a coating and / or an article.

15. Use of the ferrite alloy according to any one of claims 1 to 11 for producing an article or a coating to be used in a corrosive environment.

16. Use of a ferrite alloy according to any one of claims 1 to 11 for making an article or coating used in a furnace or as a heating element.

17. The ferrite alloy is resistant to salts, liquid lead and other metals, ashes or high-carbon-containing deposits, the combustion atmosphere, or Low O ２ Partial pressure or high N ２ Use of the ferrite alloy according to any one of claims 1 to 11 in an environment exposed to an atmosphere having a partial pressure or high N or high carbon activity.

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