JP2024079699A - Ferrite Alloy - Google Patents

Abstract

[Problem] To provide a ferritic alloy with improved corrosion resistance. [Solution] 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, with the balance being Fe and normally occurring impurities, and the following equation must be satisfied: 0.014≦(Al+0.5Si)(Cr+10Si+0.1)≦0.022. [Selected Figure] None

Description

The present disclosure relates to a ferritic alloy according to the preamble of claim 1. The present disclosure further relates to the use of said ferritic alloy and to an object or coating made from said alloy.

Ferritic alloys, such as FeCrAl alloys containing chromium (Cr) at levels of 15-25 wt. % and aluminum (Al) at levels of 3-6 wt. %, are well known to be capable of forming protective alpha-alumina (Al ₂ O ₃ : aluminum oxide) scales when exposed to temperatures of 900-1300° C. The lower limit of the Al content for forming and maintaining the alumina scale varies depending on the conditions of exposure. However, if the Al level is too low at high temperatures, the selective oxidation of Al will fail and less stable and protective chromium and iron based scales will form.

It is commonly understood 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 FeCrAl alloys so that protective α-alumina forms at temperatures below about 900°C. In general, however, these attempts have been unsuccessful because the diffusion of oxygen to the oxide/metal interface and into aluminum is relatively slow at low temperatures, slowing the rate of alumina scale formation, which means there is a risk of severe corrosive attack and the formation of less stable oxides.

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

EP 0475420 relates to a rapidly solidified ferritic alloy consisting essentially of about 1.5-3 wt. % Cr, Al, Si, and REM (Y, Ce, La, Pr, Nd, balance Fe and impurities). The sheet may further include about 0.001-0.5 wt. % of at least one element selected from the group consisting of Ti, Nb, Zr, and V. The sheet has a grain size of about 10 μm or less. EP 075420 discusses the addition of Si to improve the fluidity of the molten alloy, but with limited success due to reduced ductility.

European Patent Application No. 0091526 relates to alloys that are resistant to thermal cyclic oxidation and are hot workable, more specifically to iron-chromium-aluminum alloys with rare earth additives. Upon oxidation, the alloys form oxides with a whisker-like structure that is desirable on the catalytic converter surface. However, the alloys thus obtained do not have high temperature resistance.

Therefore, there remains a need to further improve the corrosion resistance of ferritic alloys, so that such ferritic alloys can be used in corrosive environments during high temperature conditions. Aspects of the present disclosure should solve or at least reduce the above-mentioned problems.

The present disclosure thus relates to a ferritic alloy that provides a combination of good oxidation resistance and excellent ductility, the alloy comprising the following composition 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: ≦0.40
RE:≦1.0
The balance is Fe, and any impurities normally present, and the following equation must be satisfied:
0.014≦(Al+0.5Si)×(Cr+10Si+0.1)≦0.022.

Therefore, a relationship exists between the Cr, Si and Al contents in the alloys of the present disclosure which, when satisfied, results in an alloy with excellent oxidation resistance and ductility, as well as reduced brittleness in combination with increased hot corrosion resistance.

The present disclosure also relates to articles and/or coatings containing ferritic alloys according to the present disclosure. The present disclosure further relates to the use of ferritic alloys as defined above or below to make articles and/or coatings.

Graph a shows the Si level on the horizontal axis for the phase in Fe-10%Cr-5%Al, and graph b shows the Si level on the horizontal axis for the phase in Fe-20%Cr-5%Al. These graphs were created using the database TCFE7 and Thermocalc software. a-e disclose polished parts of two alloys according to the present disclosure compared to three reference alloys after 50 cycles of 1 hour at 850° C. exposed to biomass (wood pellet) ash containing large amounts of potassium.

As previously mentioned, the present disclosure provides a ferritic alloy comprising 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: ≦0.40
RE:≦1.0
The balance is Fe, and any impurities normally present, and the following equation must be satisfied:
0.014≦(Al+0.5Si)(Cr+10Si+0.1)≦0.022.

Surprisingly, it has been found that alloys containing alloying elements as defined above or below, i.e. in the ranges described herein, unexpectedly form a protective surface layer containing aluminum-rich oxides, even at chromium levels as low as 4% by weight. This is very important for both the workability and the long-term phase stability of the alloy, since the undesirable brittle σ-phase is reduced or even avoided after long exposure to the temperature ranges described herein. Thus, the interaction of Si, Al and Cr enhances the formation of a stable and continuous protective surface layer containing aluminum-rich oxides, and by using the above equation, it is possible to add Si and obtain ferritic alloys that can both be manufactured and formed into various articles. The inventors have been surprised to find that when the amounts of Si, Al and Cr are adjusted to satisfy the following conditions (all numbers for the elements are weight fractions):
0.014≦(Al+0.5Si)×(Cr+10Si+0.1)≦0.022
The resulting alloy has been found to have a combination of excellent oxidation resistance, workability, and form stability within the Cr ranges disclosed herein. According to one embodiment, 0.015≦(Al+0.5Si)×(Cr+10Si+0.1)≦0.021, such as 0.016≦(Al+0.5Si)×(Cr+10Si+0.1)≦0.020, such as 0.017≦(Al+0.5Si)×(Cr+10Si+0.1)≦0.019.

The ferritic alloys of the present disclosure are particularly useful at low temperatures below about 900°C because a protective surface layer containing aluminum-rich oxides is formed on the ferritic alloy articles and/or coatings according to the present disclosure, which protects the articles and/or coatings from corrosion, oxidation, and embrittlement. Furthermore, the ferritic alloys of the present disclosure can provide protection against corrosion, oxidation, and embrittlement at temperatures as low as 400°C because a protective surface layer containing aluminum-rich oxides is formed on the surfaces of the ferritic alloy articles and/or coatings according to the present disclosure. Furthermore, the alloys of the present disclosure also excel at temperatures up to about 1100°C, and have a reduced tendency to embrittle over long periods of time in the temperature range of 400-600°C.

The alloy according to the present disclosure can be used in the form of a coating. Furthermore, an article can also include the alloy according to the present disclosure. According to the present disclosure, the term "coating" refers to an embodiment in which the ferritic alloy according to the present disclosure is present in the form of a layer exposed to a corrosive environment (i.e. in contact with a base material), regardless of the means and method by which the corrosion is accomplished and regardless of the relative thickness relationship between the layer and the base material. Examples are therefore, but are not limited to, PVD coatings, claddings, or compound or composite materials. The purpose of the alloy is that it should protect the underlying material from both corrosion and oxidation. Examples of suitable articles are, 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, components in vessels or heat exchangers (e.g. for reforming or other processing of gases containing hydrocarbons or CO/ _CO2 ), components used in connection with industrial heat treatment of steel and aluminum, powder metallurgy, gas and electric heating elements.

Additionally, alloys according to the present disclosure are suitable for use in environments having corrosive conditions, examples of which include, but are not limited to, exposure to salt, liquid lead, and other metals, exposure to deposits with high ash or carbon content, combustion atmospheres with low _O2 partial pressure and/or high _N2 and/or high carbon activity.

Furthermore, the ferritic alloys according to the present disclosure can be produced using any of the commonly occurring solidification rates from conventional metallurgy to rapid solidification. The alloys according to the present disclosure are also suitable for producing all kinds of pressed and extruded articles (e.g., wire, strip, bar and plate). 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 and manufacturing route of the article.

The functions and actions of the essential alloying elements for the alloys defined above and below are given in the following paragraphs. The enumeration of the functions and actions of each alloying element should not be considered exhaustive, and there may be additional functions and actions for these alloying elements.

Carbon (C)
Carbon may be present as an unavoidable impurity resulting from the manufacturing process. Carbon may be included in the ferritic alloy as defined above or below to increase strength by precipitation hardening. To have a significant effect on strength in the alloy, carbon is desirably present in an amount of at least 0.01 wt.%. At levels that are too high, carbon may make the material difficult to form and may have a negative effect on corrosion resistance. Thus, the maximum amount of carbon is 0.1 wt.%. The carbon content may be, for example, 0.02-0.09 wt.%, such as 0.02-0.08 wt.%, for example 0.02-0.07 wt.%, such as 0.02-0.06 wt.%, for example 0.02-0.05 wt.%, for example 0.01-0.04 wt.%.

Nitrogen (N)
Nitrogen may be present as an unavoidable impurity resulting from the manufacturing process. Nitrogen may be included in the ferritic alloy as defined above or below, in order to increase the strength by precipitation hardening, especially when applying the powder metallurgical route. At too high a level, nitrogen may make the alloy more difficult to form and may also have a negative effect on corrosion resistance. The maximum amount of nitrogen is therefore 0.1 wt.%. Suitable ranges for nitrogen are, for example, 0.001-0.08 wt.%, such as 0.001-0.05 wt.%, such as 0.001-0.04 wt.%, such as 0.001-0.03 wt.%, such as 0.001-0.02 wt.%.

Oxygen (O)
Oxygen may be present in the alloy as defined above or below as an impurity resulting from the manufacturing process, in which case the amount of oxygen is at most 0.02 wt.%, such as at most 0.005 wt.%. When oxygen is intentionally added to provide strength through dispersion strengthening, as when the alloy is made via the powder metallurgical route, the alloy contains at most 0.2 wt.%, or 0.2 wt.%, of oxygen, as defined above or below.

Chromium (Cr)
Chromium is present in the alloy according to the present disclosure primarily as a solid solution element of the matrix. Chromium promotes the formation of an aluminum oxide layer in the alloy by the so-called "third element effect", i.e. by forming chromium oxide in a transitional oxidation state. To this end, chromium is desirably present in the alloy as defined above or below in an amount of at least 4 wt. %. In the inventive alloy according to the present disclosure, Cr also enhances the susceptibility to the formation of brittle σ-phase and Cr ₃ Si. This effect is present at about 12 wt. % and is enhanced at levels above 15 wt. %, so that the upper limit for Cr is 15 wt. %. From an oxidation standpoint, at levels higher than 15 wt. %, Cr also makes an undesirable contribution to the protective oxide scale. According to one embodiment, the Cr content is 5-13 wt. %, such as 5-12 wt. %, such as 6-12 wt. %, such as 7-11 wt. %, such as 8-10 wt. %.

Aluminum (Al)
Aluminum is an important element in the alloy as defined above and below. When exposed to oxygen at high temperatures, aluminum selectively oxidizes to form a dense, thin oxide (Al ₂ O ₃ ) which protects the underlying alloy surface from further oxidation. The amount of aluminum is desirably at least 2 wt. % to ensure that a protective surface layer containing an aluminum-rich oxide is formed and that sufficient aluminum is present to repair the protective surface layer if damaged. However, aluminum has a negative effect on formability and high amounts of aluminum can cause cracks to form in the alloy during mechanical working of the alloy. Therefore, the amount of aluminum is desirably not more than 6 wt. %. Aluminum may, for example, be present in an amount of 3-5 wt. %, such as 2.5-4.5 wt. %, for example 3-4 wt. %.

Silicon (Si)
In commercial FeCrAl alloys, silicon is often present at a level of up to 0.4 wt. %. As defined above and below, Si plays an important role in ferritic alloys, since it has been found to have a positive effect in improving oxidation and corrosion resistance. The upper limit of Si is determined by the loss of workability in hot and cold conditions, and by the increased susceptibility to the formation of brittle Cr ₃ Si and σ phases during long-term exposure. The addition of Si must therefore be made in relation to the Al and Cr contents. The amount of Si is therefore 0.5-3 wt. %, such as 1-3 wt. %, such as 1-2.5 wt. %, such as 1.5-2.5 wt. %.

Manganese (Mn)
Manganese may be present as an impurity in the alloy as defined above or below, up to 0.4% by weight, for example 0-0.3% by weight.

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

Scandium (Sc), Cerium (Ce), and Lanthanum (La)
Scandium, cerium, and lanthanum are interchangeable elements and may be added individually or in combination up to a total of 0.2 _wt . % to improve the oxidation properties, the self-healing properties of the aluminum oxide ( _Al2O3 ) layer, or the adhesion of the alloy to _{the Al2O3} layer.

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

Reactive Elements (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 this sense, as they have a high affinity for carbon and are therefore strong carbide formers. These elements are added to improve the oxidation properties of the alloy. The total amount of the elements is up to 1.0 wt%, such as up to 0.4 wt%, such as up to 0.15 wt%.

The maximum amount of each reactive element is primarily determined by the element's tendency to form unfavorable intermetallic phases.

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

However, Zr levels above 0.40 wt.% have an effect on oxidation due to the formation of Zr-rich intermetallic inclusions, and levels below 0.05 wt.% are too low to achieve both objectives, regardless of the C and N contents. Thus, when Zr is present, its range is 0.05-0.40 wt.%, e.g., 0.10-0.35 wt.%.

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 may be important to achieve better oxidation resistance for the protective surface layer (i.e. alumina scale). Thus, the inventors have surprisingly found that when Zr is added to the alloy, the alloy contains N and C in the following conditions (element contents in weight %):

If the above formula is satisfied,
The resulting alloy has been found to have good oxidation resistance.

The balance in ferritic alloys is Fe, as defined above and below, and unavoidable impurities. Examples of unavoidable impurities are elements or compounds that are not added for any purpose, but cannot be completely avoided, for example because they are normally present as impurities in the materials used to make the ferritic alloy.

Figures 1a and 1b show that higher Cr in Si-containing ferritic alloys favors the formation of _Si3Cr inclusions, and that 20% Cr promotes the undesirable brittle sigma phase after extended exposure in the temperature range of interest. Although these graphs are only for two Cr levels (10% and 20%), the trend for more brittle phases with more Cr is clearly shown. Note the absence of sigma phase at 10% Cr, and that higher Si contents increase the amount of _Si3Cr phase at both Cr levels. Thus, these figures show that using Cr at levels around 20% will cause problems.

When the wording "≦" or "less than" is used in the following context: "element≦number", a person skilled in the art would recognize that the lower limit of the range is 0% by weight, unless another number is specifically mentioned. Furthermore, the indefinite article "a" does not exclude a plurality.

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

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

The resulting samples were hot rolled and machined into flat rods with a cross section of 2 x 10 mm. These rods were then cut into 20 mm long sections and polished to 800 mesh with SiC paper for exposure to air and combustion conditions. Some rods were cut into rods 200 mm long x 3 x 12 mm long for tensile testing with a Zwick/Roell Z100 tensile tester at room temperature.

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

These samples were tested for yield and break stress, as well as elongation at break in a standard tensile tester, and results showing an elongation >3% are indicated in the table with an "x" in the "Workability" column. Thus, an "x" indicates an alloy that is easily hot rolled, which indicates ductility at room temperature. In the "Oxidability" column, an "x" indicates that the alloy forms a protective alumina-rich oxide scale at 950°C in air and at 850°C on biomass ash deposits.

As can be seen from the table above, the alloy according to the present disclosure exhibits good workability and good oxidation resistance.

Figures 2a)-e) show polished specimens of the present disclosure (Figures 2a) 4783 and 2b) 4779) compared to three reference alloys after 50 cycles of 1 hour at 850°C exposed to biomass (wood pellet) ash containing large amounts of potassium. These photomicrographs were taken on a JEOL FEG SEM at 100x magnification and show clear advantages between the properties of the alloys according to the present disclosure and the reference materials. As can be seen, the alloys according to the present disclosure form a thin, 3-4 μm protective alumina scale (aluminum oxide layer), whereas the stainless steels (2c: 11Ni, 21Cr, N, Ce, balance Fe) and Ni-based alloys (2e: Inconel 625: 58Ni, 21Cr, 0.4Al, 0.5Si, Mo, Nb, Fe) form a relatively thick, less protective, chromia-rich scale, and the comparative FeCrAl alloy (Alloy 4776) (Figure 2d: 20Cr, 5Al, 0.04Si, balance Fe) forms a relatively porous, non-protective alumina scale.

As can be seen from Figures 2a-e, the addition of Si, Al, and Cr in the ranges according to the present disclosure promotes alumina scale formation even at Al levels as low as about 2 wt.% and chromium levels as low as 5 wt.%.

Claims (17)

A ferritic alloy containing the following elements in the following 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: ≦0.40
RE:≦1.0
The balance is Fe, and impurities 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 by weight percentage)
0.015≦(Al+0.5Si)×(Cr+10Si+0.1)≦0.021
2. The ferritic alloy of claim 1 , The ferritic alloy according to claim 1 or 2, in which Zr is 0.05 to 0.40 wt%. A ferritic alloy according to any one of claims 1 to 3, in which Cr is 5 to 13 wt%. A ferritic alloy according to any one of claims 1 to 4, in which Cr is 6 to 12 wt%. A ferritic alloy according to any one of claims 1 to 5, in which Al is 2.5 to 4.5 wt.% or 3 to 5 wt.%. A ferritic alloy according to any one of claims 1 to 6, in which Al is 3 to 4 wt%. A ferritic alloy according to any one of claims 1 to 7, in which the Si content is 1.0 to 3 wt%. A ferritic alloy according to any one of claims 1 to 8, in which the Si content is 1.5 to 2.5 wt%. A ferritic alloy according to any one of claims 1 to 9, in which Zr is 0.10 to 0.35 wt%. C, N and Zr are represented by the following equation:

The ferritic alloy according to any one of claims 1 to 10, wherein A coating comprising the ferritic alloy according to any one of claims 1 to 11. An article comprising the ferritic alloy according to any one of claims 1 to 11. Use of the ferritic alloy according to any one of claims 1 to 11 for making coatings and/or claddings and/or articles. Use of the ferritic 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 a ferritic 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. 12. Use of the ferritic alloy according to any one of claims 1 to 11 in environments where the ferritic alloy is exposed to salts, liquid lead and other metals, to deposits with high ash or carbon content, to combustion atmospheres, to atmospheres with low _O2 partial pressure and/or high _N2 and/or high carbon activity.

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