JP2022046521A - Ferritic alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic alloy further improved in the corrosion resistance.

SOLUTION: The ferritic alloy comprises the following elements in following weight percentage (wt.%): C of 0.01 to 0.1; N of 0.001 to 0.1; O of 0.2 or less; Cr of 4 to 15; Al of 2 to 6; Si of 0.5 to 3; Mn of 0.4 or less; Mo+W of 4 or less; Y of 1.0 or less; Sc, Ce and/or La of 0.2 or less; Zr of 0.40 or less; RE of 1.0 or less; and the balance consisting of Fe and normally occurring impurities, where the following inequality has to be fulfilled: 0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022.

SELECTED DRAWING: None

COPYRIGHT: (C)2022,JPO&INPIT

Description

The present disclosure relates to the ferrite alloy according to the premise of claim 1. The present disclosure further relates to the use of said ferrite alloys and to objects or coatings made from the alloys.

FeCrAl alloys containing ferrite alloys, such as chromium (Cr) at a level of 15-25% by weight and aluminum (Al) at a level of 3-6% by weight, are protected when exposed to temperatures of 900-1300 ° C. It is well known that it can form a sex α-alumina (Al ₂ O ₃ : aluminum oxide) scale. The lower limit of the Al content for forming and maintaining the alumina scale depends on the conditions exposed. However, if the level of Al is too low at high temperatures, selective oxidation of Al will fail, forming less stable and less protective scales based on chromium and iron.

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

Another problem that arises at low temperatures, i.e. below 900 ° C., is the long-term embrittlement phenomenon that results from the low temperature miscibility gap for Cr in FeCrAl alloy systems. This miscibility gap is present for Cr levels above about 12% by weight at 550 ° C. In recent years, in order to avoid this phenomenon, alloys having a relatively low Cr level of about 10 to 12% by weight have been developed. This group of alloys has been found to work very well with molten lead at controlled low pressure _O2 .

European Patent Application No. 0475420 is a quench solidification consisting substantially of about 1.5-3 wt% Cr, Al, Si, and REM (Y, Ce, La, Pr, Nd, the balance is Fe and impurities). With respect to a patently solidified ferrite alloy. The sheet can further contain from about 0.001 to 0.5% by weight of at least one element selected from the group consisting of Ti, Nb, Zr, and V. This sheet has a particle size of about 10 μm or less. European Patent Application No. 0754420 discusses the addition of Si to improve the fluidity of molten alloys, but its success is limited due to the reduced ductility.

European Patent Application No. 0091526 relates to alloys that are resistant to periodic oxidation by heat and can be hot-worked, and more specifically to iron-chromium-aluminum alloys with rare earth additives. Upon oxidation, the alloy forms an oxide with a desirable whisker-like structure on the surface of the catalytic converter. However, the alloy thus obtained does not have high temperature resistance.

Therefore, there is still a need to further improve the corrosion resistance of ferrite alloys, which allows 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-mentioned problems.

Thus, the present disclosure relates to a ferrite alloy that provides a combination of good oxidation resistance and excellent ductility, which comprises the following composition by weight% (wt%):
C: 0.01-0.1
N: 0.001 to 0.1
O: ≤0.2
Cr: 4 to 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 is Fe, and normally present impurities, and the following equation must be satisfied:
0.014 ≦ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≦ 0.022.

Therefore, there is a certain relationship between the contents of Cr, Si and Al in the alloy according to the present disclosure, and when this is satisfied, it has excellent oxidation resistance and ductility, and in combination with increased high temperature corrosion resistance. An alloy with reduced brittleness can be obtained.

The present disclosure also relates to articles and / or coatings containing ferrite alloys according to the present disclosure. The present disclosure further relates to the use of ferrite alloys as previously or as defined below for making articles and / or coatings.

a shows the Si level on the horizontal axis for the phase at Fe-10% Cr-5% Al, and b shows the Si level on the horizontal axis for the phase at Fe-20% Cr-5% Al. I am disclosing what I took. These graphs were created using the database TCFE7 and Thermocalc software. a to e are exposed to a large amount of potassium-containing biomass (wood pellet) ash, exposed 50 times in a 1-hour cycle at 850 ° C., and then the two alloy-polished portions according to the present disclosure are referred to as three types. It is disclosed in comparison with the alloy for use.

As mentioned above, the present disclosure yields ferrite alloys containing the following elements by weight% (wt%):
C: 0.01-0.1
N: 0.001 to 0.1
O: ≤0.2
Cr: 4 to 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 is Fe, and normally present impurities, and the following equation must be satisfied:
0.014 ≦ (Al + 0.5Si) (Cr + 10Si + 0.1) ≦ 0.022.

Surprisingly, as specified above or below, alloys containing alloying elements in the range described herein are surprisingly aluminum-rich oxidations, even at as low as 4% by weight of chromium. It was found to form a protective surface layer containing the material. This is very important for both the processability of the alloy and the long-term phase stability. This is because the undesired brittle sigma phase is reduced or even avoided after prolonged exposure in the temperature range described herein. Thus, the interaction of Si with Al and Cr enhances the formation of a stable and continuous protective surface layer containing aluminum-rich oxides, and Si can be added by using the above equation. It is possible to obtain a ferrite alloy that can be manufactured and formed into various articles. Surprisingly, the inventors have adjusted the amounts of Si and Al and Cr to meet the following conditions (all numbers for elements are fractions by weight):
0.014 ≤ (Al + 0.5Si) x (Cr + 10Si + 0.1) ≤ 0.022
It has been found that the obtained alloy has a combination of excellent oxidation resistance, processability and shape stability within the Cr range 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.

The ferrite alloys of the present disclosure are particularly useful at low temperatures below about 900 ° C. That is, 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 is corroded on the article and / or coating. This is because it prevents oxidation and embrittlement. In addition, the ferrite alloys according to the present disclosure can provide protection against corrosion, oxidation and embrittlement at temperatures as low as 400 ° C. This is because the protective surface layer containing aluminum-rich oxides is formed on the surface of the ferrite alloy articles and / or coatings according to the present disclosure. Moreover, the alloys according to the present disclosure are also excellent at temperatures up to about 1100 ° C, reducing the long-term embrittlement tendency over the temperature range of 400-600 ° C.

The alloys according to the present disclosure can be used in the form of coatings. In addition, the article may also include alloys according to the present disclosure. According to the present disclosure, the term "coating" refers to an embodiment in which the ferrite alloy according to the present disclosure exists in the form of a layer exposed to a corrosive environment (ie, in contact with a base material), wherein the means for achieving corrosion. And the method does not matter, and the relative thickness relationship between the layer and the base material does not matter. Thus, examples are, but are not limited to, PVD coatings, claddings, or compounds or composites. The purpose of this alloy is that the underlying material should be protected from both corrosion and oxidation. Examples of suitable articles are, but are not limited to, compound tubes, tubes, boilers, gas turbine members, and steam turbine members. Other examples include superheaters, water walls in power plants, containers or components in heat exchangers (eg, for modifying hydrocarbons or gases containing CO / CO ₂ or for other treatments). Stuff), components used in connection with industrial heat treatment of steel and aluminum, powder metallurgy, gas and electric heating elements.

In addition, the alloys according to the present disclosure are suitable for use in environments with corrosive conditions. Examples of such environments include exposure to salts, liquid lead, and other metals, exposure to ash or high carbon content deposits, low _O2 partial pressure and / or high in the combustion atmosphere. Environments such as N ₂ and / or an atmosphere with high carbon activity are included, but not limited to these.

In addition, the ferrite alloys according to the present disclosure can be made using commonly performed solidification rates from conventional metallurgy to quench solidification. Alloys according to the present disclosure are also suitable for making all kinds of articles (eg, wires, strips, bars and plates) that have been stamped and extruded. As will be appreciated by those skilled in the art, the degree of hot and cold plastic deformation, as well as the grain structure and grain size, will vary depending on the shape of the article and the manufacturing route.

The functions and actions of the alloying elements essential for the alloys specified above and below are shown in the following paragraphs. The enumeration of the functions and actions of each alloying element should not be considered complete and additional functions and actions may exist for these alloying elements.

Carbon (C)
Carbon can exist as an unavoidable impurity resulting from the manufacturing process. Carbon may be included earlier or in the ferrite alloy as specified below in order to increase the strength by precipitation hardening. Carbon is preferably present in an amount of at least 0.01% by weight in order to have a significant effect on strength in the alloy. If that level is too high, carbon can make the formation of the material difficult and can have a negative effect on corrosion resistance. Therefore, the maximum amount of carbon is 0.1% by weight. The carbon content is, for example, 0.02 to 0.09% by weight, for example 0.02 to 0.08% by weight, for example 0.02 to 0.07% by weight, for example 0.02 to 0.06% by weight, for example. It is 0.02 to 0.05% by weight, for example, 0.01 to 0.04% by weight.

Nitrogen (N)
Nitrogen can be present as an unavoidable impurity resulting from the manufacturing process. Nitrogen may be included earlier or in the ferrite alloy as specified below, especially when the powder metallurgy route is applied, in order to increase the strength by precipitation hardening. If that level is too high, nitrogen can make alloy formation difficult and can have a negative effect on corrosion resistance. Therefore, the maximum amount of nitrogen is 0.1% by weight. Suitable ranges for nitrogen are, for example, 0.001 to 0.08% by weight, for example 0.001 to 0.05% by weight, for example 0.001 to 0.04% by weight, for example 0.001 to 0.03% by weight. %, For example 0.001 to 0.02% by weight.

Oxygen (O)
Oxygen can be present as an impurity resulting from the manufacturing process, either earlier or in the alloy as specified below. In this case, the amount of oxygen is up to 0.02% by weight, for example up to 0.005% by weight. If oxygen is intentionally added to provide strength through dispersion strengthening, the alloy will be oxygenated at maximum, as when making the alloy through the powder metallurgy pathway, either earlier or as specified below. Contains 0.2% by weight or 0.2% by weight.

Chromium (Cr)
Chromium is mainly present in the alloys according to the present disclosure as a solid solution element of the matrix. Chromium promotes the formation of aluminum oxide layers in alloys by the so-called "third element effect", i.e., by forming chromium oxide in a transitional oxidation state. To this end, chromium is preferably present in an amount of at least 4% by weight, either earlier or in the alloy as specified below. In the original alloy according to the present disclosure, Cr also enhances the easiness of forming a brittle σ phase and Cr _3Si . This effect appears at about 12% by weight and is enhanced at levels above 15% by weight, so the upper limit for Cr is 15% by weight. From the point of view of oxidation, at a level higher than 15% by weight, Cr makes an undesired contribution to the protective oxide scale. According to one embodiment, the Cr content is 5-13% by weight, such as 5-12% by weight, such as 6-12% by weight, such as 7-11% by weight, such as 8-10% by weight.

Aluminum (Al)
Aluminum is an important element in alloys, as defined above or below. When exposed to oxygen at high temperatures, aluminum forms a dense, thin oxide (Al ₂ O ₃ ) by selective oxidation, which protects the underlying alloy surface from further oxidation. The amount of aluminum ensures that a protective surface layer containing an aluminum-rich oxide is formed, and that there is sufficient aluminum to repair the protective surface layer in case of damage. It is desirable that it is at least 2% by weight in order to guarantee. However, aluminum has a negative effect on formability, and high amounts of aluminum can cause cracks to form in the alloy during mechanical machining of the alloy. Therefore, it is desirable that the amount of aluminum does not exceed 6% by weight. Aluminum may be present, for example, in 3-5% by weight, for example 2.5-4.5% by weight, for example 3-4% by weight.

Silicon (Si)
In commercially available FeCrAl alloys, silicon is often present at levels up to 0.4% by weight. Si plays an important role in ferrite alloys, as specified above or below. 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 in hot and cold conditions and by the increased likelihood of forming brittle Cr ₃ Si and σ phases during long-term exposure. Therefore, the addition of Si must be performed in relation to the contents of Al and Cr. Therefore, 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.

Manganese (Mn)
Manganese can be present in up to 0.4% by weight, for example 0-0.3% by weight, as an impurity in the alloy, either earlier or as specified below.

Yttrium (Y)
In dissolution metallurgy, yttrium can be added in an amount of up to 0.3% by weight to improve the adhesion of the protective surface layer. In addition, when yttrium is added to make a dispersion with oxygen and / or nitrogen in powder metallurgy, the yttrium content is to achieve the desired dispersion curing effect of the oxide and / or nitride. The amount is at least 0.04% by weight. The maximum amount of yttrium in the dispersion cured alloy can be up to 1.0% by weight in the form of the oxygen-containing yttrium compound.

Scandium (Sc), cerium (Ce), and lanthanum (La)
Scandium, cerium, and lanthanum are interchangeable elements to improve oxidation properties, self-healing of aluminum oxide (Al ₂ O ₃ ) layers, or adhesion between alloys and Al ₂ O ₃ layers. In total, up to 0.2% by weight, can be added individually or in combination.

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

Reactive element (RE)
Reactive elements are defined as those that are highly reactive with carbon, nitrogen, and oxygen. Titanium (Ti), niobium (Nb), vanadium (V), hafnium (Hf), tantalum (Ta), and thorium (Th) are reactive elements in that sense, and these elements have an affinity for carbon. It is a highly potent and therefore strong carbide-forming body. These elements are added to improve the oxidizing properties of the alloy. The total amount of elements is up to 1.0% by weight, for example 0.4% by weight, for example up to 0.15% by weight.

The maximum amount of each reactive element is mainly due to the tendency of the elements to form an inconvenient intermetallic compound phase.

Zirconium (Zr)
Zirconium is often referred to as a reactive element. That's because zirconium is highly reactive with oxygen, nitrogen, and carbon. Zr has been found to have two roles in the alloys according to the present disclosure. This is because zirconium is present in the protective surface layer containing aluminum-rich oxides, which improves oxidation resistance and also forms carbides and nitrides. Therefore, it is advantageous to include Zr in the alloy in order to achieve the best properties for the aluminum-rich oxide-containing protective surface layer.

However, at Zr levels above 0.40% by weight, Zr-rich intermetallic compound inclusions are formed, which has an effect on oxidation, and at levels less than 0.05% by weight, the C content and Regardless of the N content, it is too low to achieve the two objectives. Therefore, when Zr is present, its range is 0.05 to 0.40% by weight, for example 0.10 to 0.35% by weight.

が満たされるように含有すると、
得られる合金は、良好な耐酸化性を獲得することを見出した。 Furthermore, it has been found that the relationship between Zr, N and C can be important for achieving even better oxidation resistance for the protective surface layer (ie, alumina scale). Thus, the inventors are surprisingly concerned that when Zr is added to an alloy, the alloy provides N and C under the following conditions (element content is expressed in% by weight):

When it is contained so as to be satisfied,
It has been found that the resulting alloys acquire good oxidation resistance.

The residue in the ferrite alloy, as specified earlier or below, is Fe and unavoidable impurities. Examples of unavoidable impurities are not elements or compounds added for any purpose, but are elements or compounds that cannot be completely avoided, for example because they are usually present as impurities in materials used to make ferrite alloys. be.

FIGS. 1a and 1b show that the more Cr in the Si-containing ferrite alloy, the more likely it is that Si ₃ Cr inclusions are formed, and that 20% Cr is undesired after long exposure in the focal temperature region. It is shown that the brittle σ phase is promoted. Although these graphs only show for the two Cr levels (10% and 20%), the tendency for more Cr to increase the brittle phase is clearly shown. It should be noted that the absence of the σ phase at 10% Cr and the high Si content increase the amount of Si ₃ Cr phase at both Cr levels. Therefore, these figures show that using Cr at a level of about 20% would cause problems.

When the expression "≤" or "less than or equal to" is used in the following context: "elements ≤ number", those skilled in the art will have a lower limit of 0% by weight, unless otherwise noted. It turns out that there is. Furthermore, the indefinite article "a (one)" does not preclude being plural.

The present disclosure is further described by the following non-limiting examples.

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

The obtained sample was hot-rolled and machined into a flat rod with a cross section of 2 x 10 mm. These rods were then cut into 20 mm long pieces and polished to 800 mesh with SiC paper for exposure to air and combustion conditions. Several rods were cut into 200 mm × 3 × 12 mm length rods for tensile testing with the Zwick / Roell Z100 tensile tester at room temperature.

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

Yield stress and breaking stress, as well as breaking point elongation in a standard tensile tester, were tested on these samples and the results showing> 3% elongation were listed as "x" in the "Workability" column in the table. It is shown. Therefore, "x" indicates that the alloy is easy to be hot-rolled, which indicates ductility at room temperature. In the column "oxidizing", "x" indicates that the alloy forms a protective alumina-rich oxide scale at 950 ° C in air and at 850 ° C for biomass ash deposits.

Therefore, as can be seen from the above table, the alloy according to the present disclosure exhibits good processability and good oxidizing property.

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

As can be seen from FIGS. 2a-e, by adding Si, Al and Cr in the range according to the present disclosure, even if the Al level is as low as about 2% by weight and the chromium level is as low as 5% by weight. , Alumina scale formation is promoted.

Claims (17)

Ferrite alloy containing the following elements in the following weight% (wt%):
C: 0.01-0.1
N: 0.001 to 0.1
O: ≤0.2
Cr: 4 to 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 is Fe and impurities that are normally present, and the following equation must be satisfied (elements are weight fractions):
0.014 ≦ (Al + 0.5Si) × (Cr + 10Si + 0.1) ≦ 0.022. (Elements are weight fractions)
0.015 ≤ (Al + 0.5Si) x (Cr + 10Si + 0.1) ≤ 0.021
The ferrite alloy according to claim 1. The ferrite alloy according to claim 1 or 2, wherein Zr is 0.05 to 0.40% by weight. The ferrite alloy according to any one of claims 1 to 3, wherein Cr is 5 to 13% by weight. The ferrite alloy according to any one of claims 1 to 4, wherein Cr is 6 to 12% by weight. The ferrite alloy according to any one of claims 1 to 5, wherein Al is 2.5 to 4.5% by weight or 3 to 5% by weight. The ferrite alloy according to any one of claims 1 to 6, wherein Al is 3 to 4% by weight. The ferrite alloy according to any one of claims 1 to 7, wherein Si is 1.0 to 3% by weight. The ferrite alloy according to any one of claims 1 to 8, wherein Si is 1.5 to 2.5% by weight. The ferrite alloy according to any one of claims 1 to 9, wherein Zr is 0.10 to 0.35% by weight. C, N and Zr have the following equations:

The ferrite alloy according to any one of claims 1 to 10, which satisfies the above conditions. A coating comprising the ferrite alloy according to any one of claims 1 to 11. An article comprising the ferrite alloy according to any one of claims 1 to 11. Use of the ferrite alloy according to any one of claims 1 to 11 for making coatings and / or cladding and / or articles. Use of the ferrite alloy according to any one of claims 1 to 11 for making an article or coating to be used in a corrosive environment. Use of the ferrite alloy according to any one of claims 1 to 11 for making an article or coating to be used in a furnace or as a heating element. Ferrite alloys in salts, liquid lead and other metals, in ash or high carbon content deposits, in combustion atmospheres, in atmospheres with low _O2 partial pressure and / or high _N2 and / or high carbon activity. Use of the ferrite alloy according to any one of claims 1 to 11 in an exposed environment.

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