CN110709529B

CN110709529B - Ferritic alloys

Info

Publication number: CN110709529B
Application number: CN201780091046.0A
Authority: CN
Inventors: 保·荣松
Original assignee: Cantel Ltd
Current assignee: Cantel Ltd
Priority date: 2017-05-24
Filing date: 2017-05-24
Publication date: 2025-02-28
Anticipated expiration: 2037-05-24
Also published as: CA3062819A1; JP2020521051A; EP3631034A1; WO2018215065A1; JP7267936B2; CN110709529A; US20200181745A1; BR112019024471A2

Abstract

一种铁素体合金，所述铁素体合金包含以重量％[wt％]计的以下元素：C 0.01至0.1；N：0.001至0.1；O：≤0.2；Cr 4至15；Al 2至6；Si 0.5至3；Mn：≤0.4；Mo+W≤4；Y≤1.0；Sc、Ce、La和/或Yb≤0.2；Zr≤0.40；RE≤3.0；余量为Fe和常规出现的杂质，并且还必须满足以下等式：0.014≤(Al+0.5SQ(Cr+10Si+0.1)≤0.022。A ferritic alloy comprising the following elements 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, La and/or Yb≤0.2; Zr≤0.40; RE≤3.0; the balance being Fe and conventional impurities, and must also satisfy the following equation: 0.014≤(Al+0.5SQ(Cr+10Si+0.1)≤0.022.

Description

Ferritic alloy

Technical Field

The present disclosure relates to a ferritic alloy according to the preamble of claim 1. The disclosure also relates to the use of the ferritic alloy and to articles or coatings made therefrom.

Background

Ferritic alloys, such as FeCrAl alloys comprising a chromium (Cr) content of 15-25 wt% and an aluminum (Al) content of 3-6 wt%, are well known for their ability to form protective alpha-alumina (Al ₂O₃), alumina, scale when at temperatures between 900 ℃ and 1300 ℃. The lower limit of the Al content of the alumina scale formed and maintained varies with the contact conditions. However, at higher temperatures, the effect of too low an Al content level is that the selective oxidation of Al will fail and will form less stable and less protective chromium and iron based scale.

It is generally believed that FeCrAl alloys typically do not form a protective alpha-alumina layer if subjected to temperatures below about 900 ℃. There have been many attempts to optimize the composition of FeCrAl alloys such that they will form protective alpha-alumina at temperatures below about 900 ℃. However, in general, these attempts have not been very successful because the diffusion of oxygen and aluminum into the oxide-metal interface will be relatively slow at lower temperatures, and thus the rate of formation of alumina scale will be low, meaning that there will be a risk of severe corrosive attack and formation of less stable oxides.

Another problem that arises at lower temperatures, i.e. below 900 ℃, is the long-term embrittlement phenomenon caused by the low-temperature miscibility gap of Cr in FeCrAl alloy systems. At 550 ℃, miscibility gaps exist at Cr contents above about 12 wt.%. Recently, to avoid this phenomenon, alloys having a lower Cr content of about 10-12 wt% Cr have been developed. Such alloys have been found to perform very well in the molten lead of controlled and low pressure O ₂.

EP 0 475 420 relates to a rapidly solidifying ferritic alloy foil consisting essentially of Cr, al, about 1.5-3% by weight Si, and REM (Y, ce, la, pr, nd), the balance being Fe and impurities. The foil may further contain about 0.001 to 0.5 wt% of at least one element selected from the group consisting of Ti, nb, zr and V. The foil has a grain size of no greater than about 10 μm. EP 075 420 discusses the addition of Si to improve the flow characteristics of the alloy melt, but with limited success due to the reduced ductility.

EP 0091 526 relates to heat cycle oxidation resistant and hot workable alloys and more particularly to iron-chromium-aluminum alloys containing rare earth additives. In oxidation, the alloy will produce the desired whisker texture oxide on the catalytic converter surface. However, the resulting alloy does not provide high temperature resistance.

Thus, there remains a need for further improvements in the corrosion resistance of ferritic alloys so that they can be used in corrosive environments during high temperature conditions. Aspects of the present disclosure are directed to solving, or at least reducing, the above-described problems.

Disclosure of Invention

Accordingly, the present disclosure relates to a ferritic alloy that will provide a combination of good oxidation resistance and excellent ductility, comprising the following composition in weight percent (wt%):

C0.01 to 0.1;

N:0.001-0.1;

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, la and/or Yb is less than or equal to 0.2;

Zr≤0.40;

RE≤3.0;

the balance being Fe and conventionally occurring impurities, and must also satisfy the following equation:

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022。

Thus, there is a relationship between the Cr and Si and Al content in the alloy according to the present disclosure, which if satisfied, would provide an alloy having excellent oxidation resistance and ductility, as well as reduced brittleness and increased high temperature corrosion resistance.

The present disclosure also relates to an article and/or coating comprising a ferritic alloy according to the present disclosure. In addition, the present disclosure also relates to the use of a ferritic alloy as defined above or below for the manufacture of an article and/or a coating.

Drawings

FIGS. 1a and 1b disclose a phase diagram of Fe-10% Cr-5% Al with respect to Si content (FIG. 1 a) and a phase diagram of Fe-20% Cr-5% Al with respect to Si content (FIG. 1 b). The graph is made using the databases TCFE7 and Thermocalc software.

Fig. 2 a-2 e disclose a comparison of polished sections of two alloys according to the present disclosure with polished sections of three reference alloys after contacting biomass (wood chip) ash containing a significant amount of potassium at 850 ℃ and being subjected to 501 hour cycles.

Detailed Description

As has been described above, the present disclosure provides a ferritic alloy comprising, in weight percent (wt%):

C0.01 to 0.1;

N:0.001-0.1;

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, la and/or Yb is less than or equal to 0.2;

Zr≤0.40;

RE≤3.0;

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022。

It has surprisingly been found that an alloy as defined above or below, i.e. an alloy containing said alloying elements and within the ranges mentioned herein, unexpectedly forms a protective surface layer containing aluminium rich oxide even at chromium contents as low as 4 wt.%. This is very important for both the workability and the long-term phase stability of the alloy, since after prolonged exposure to the temperature ranges mentioned herein, the undesirable brittle sigma phase will be reduced or even avoided. Thus, the interaction between Si and Al and Cr will promote the formation of a stable and continuous protective surface layer containing aluminum rich oxide, and by using the above equation Si will be added and still obtain a ferritic alloy that can produce and form different articles. The inventors have surprisingly found that if the amounts of Si and Al and Cr are balanced such that the following conditions are met (all numbers of elements are weight fractions):

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022,

The resulting alloy will have excellent oxidation resistance and a combination of workability and formability within the Cr range of the present disclosure. In accordance with one embodiment of the present invention, less than or equal to 0.015 percent (Al+0.5Si) (Cr+10Si+0.1) less than or equal to 0.021, for example, 0.016. Ltoreq.Al+0.5Si) (Cr+10Si+0.1. Ltoreq.0.020, for example, 0.017. Ltoreq.Al+0.5Si (Cr+10Si+0.1. Ltoreq.0.019).

The ferritic alloys of the present disclosure are particularly useful at temperatures below about 900 ℃ because a protective surface layer containing aluminum-rich oxides will be formed on articles and/or coatings made from the alloys, which will prevent corrosion, oxidation, and embrittlement of the articles and/or coatings. Furthermore, the ferritic alloys of the present disclosure may provide protection against corrosion, oxidation, and embrittlement at temperatures as low as 400 ℃ because a protective surface layer containing aluminum-rich oxides will be formed on the surface of the articles and/or coatings made therefrom. In addition, the alloy according to the present disclosure will also perform excellent at temperatures up to about 1100 ℃ and it will exhibit reduced long-term embrittlement tendency over a temperature range of 400 to 600 ℃.

The alloys of the present disclosure may be used in the form of a coating. Additionally, the article may also comprise an alloy of the present disclosure. According to the present disclosure, the term "coating" is intended to refer to an embodiment wherein the ferritic alloy according to the present disclosure is present in the form of a layer that is placed in a corrosive environment in contact with the substrate, regardless of the means and method by which it is achieved, and regardless of the relative thickness relationship between the layer and the substrate. Thus, examples thereof are, but are not limited to, PVD coatings, overlays or compounds or composites. The purpose of the alloy should be to protect the underlying material from corrosion and oxidation. Examples of suitable articles are, but are not limited to, composite pipes, 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 treatment of hydrocarbons or CO/CO ₂ -containing gases), components used in connection with industrial heat treatment of steel and aluminum, powder metallurgy processes, gas and electric heating elements.

Furthermore, the alloys according to the present disclosure are suitable for use in environments having corrosive conditions. Examples of such environments include, but are not limited to, contact salts, liquid lead and other metals, contact ash or high carbon content deposits, combustion atmospheres, atmospheres with low pO ₂ and/or high N ₂, and/or high carbon activity environments.

In addition, the ferritic alloys of the present disclosure may be manufactured by using solidification rates that range from conventional metallurgy to normal occurrence of rapid solidification. The alloys of the present disclosure are also suitable for use in the manufacture of all types of wrought and extruded articles, such as filaments, belts, rods and plates. As is well known to those skilled in the art, the amount of thermoplastic deformation and cold plastic deformation, as well as the grain structure and grain size, vary between article forms and production routes.

The function and effect of the basic alloying elements of the alloys defined above and below will appear in the following paragraphs. The list of functions and roles of the individual alloying elements should not be considered to be all, as other functions and roles may also exist for the alloying elements.

Carbon (C)

Carbon may be present as an unavoidable impurity generated in the production process. Carbon may also be included in the ferritic alloy as defined above or below to increase strength by precipitation hardening. In order to have a significant effect on the strength of the alloy, carbon should be present in an amount of at least 0.01 wt.%. At too high a level, carbon can lead to difficulties in forming materials and also negatively impact corrosion resistance. Thus, the maximum amount of carbon is 0.1 wt%. For example, the carbon content is 0.02 to 0.09 wt.%, e.g., 0.02 to 0.08 wt.%, e.g., 0.02 to 0.07 wt.%, e.g., 0.02 to 0.06 wt.%, e.g., 0.02 to 0.05 wt.%, e.g., 0.01 to 0.04 wt.%.

Nitrogen (N)

Nitrogen may be present as an unavoidable impurity resulting from the production process. Nitrogen may also be included in the ferritic alloy as defined above or below to improve strength by precipitation hardening, especially when a powder metallurgy process route is applied. At too high a level, nitrogen can lead to difficulties in alloying and 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 to 0.08 wt.%, for example, 0.001 to 0.05 wt.%, for example, 0.001 to 0.04 wt.%, for example, 0.001 to 0.03 wt.%, for example, 0.001 to 0.02 wt.%.

Oxygen (O)

Oxygen may be present in the alloy as defined above or below as an impurity produced by the production process. In that case, the amount of oxygen may be up to 0.02 wt%, for example up to 0.005 wt%. If oxygen is intentionally added to provide strength by dispersion strengthening, the alloy as defined above or below contains at most or equal to 0.2 wt.% oxygen when the alloy is manufactured by a powder metallurgy process route.

Chromium (Cr)

Chromium exists primarily as a matrix solid solution element in the alloys of the present disclosure. Chromium promotes the formation of an alumina layer on the alloy by the so-called third elemental effect, i.e. by forming chromium oxide during the transient oxidation phase. To achieve this, chromium should be present in the alloy as defined above or below in an amount of at least 4 wt%. In the alloys of the present disclosure, cr also enhances the susceptibility to forming brittle sigma phases and Cr ₃ Si. This effect occurs at about 12 wt% and is enhanced at levels above 15 wt%, so the limit for Cr is 15 wt%. Furthermore, from an oxidizing point of view, a content higher than 15% by weight will lead to an undesired contribution of Cr 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 or below. When exposed to oxygen at high temperatures, aluminum forms a dense and thin oxide Al ₂O₃ by selective oxidation, which protects the underlying alloy surface from further oxidation. The amount of aluminum should be at least 2 wt.% to ensure that a protective surface layer containing aluminum-rich oxide is formed and also to ensure that sufficient aluminum is present to repair the protective surface layer when damaged. However, aluminum has a negative effect on formability, and a large amount of aluminum may cause cracks to form in the alloy during its machining. Therefore, the amount of aluminum should not exceed 6 wt.%. For example, the aluminum may be 3-5 wt.%, such as 2.5-4.5 wt.%, such as 3-4 wt.%.

Silicon (Si)

In commercial FeCrAl alloys, silicon is typically present at a level of up to 0.4 wt.%. In ferritic alloys as defined above or below, si will play a very important role, as Si has been found to have a great effect on improving oxidation resistance and corrosion resistance. The upper limit of Si is set by the loss of workability under hot and cold conditions and the increased susceptibility to formation of brittle Cr ₃ Si and sigma phases during long-term exposure. Therefore, si must be added in relation to the Al and Cr contents. Thus, the amount of Si is 0.5-3 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 in an amount of up to 0.4 wt%, for example 0-0.3 wt%.

Yttrium (Y)

In melt metallurgy, yttrium may be added in an amount of up to 0.3 wt.% to improve the adhesion of the protective surface layer. Furthermore, in powder metallurgy, if yttrium is added to produce a dispersion with oxygen and/or nitrogen, the yttrium content is in an amount of at least 0.04 wt.% to achieve the desired dispersion hardening effect by the oxide and/or nitride. The maximum amount of yttrium present in the form of an yttria-containing compound in the dispersion-hardened alloy may be up to 1.0 wt.%.

Scandium (Sc), cerium (Ce), lanthanum (La), ytterbium (Yb)

Scandium, cerium, lanthanum, and ytterbium are interchangeable elements, and may be added alone or in combination in a total amount of up to 0.2 wt% to improve the oxidation properties, self-repair of the aluminum oxide (Al ₂O₃) layer, or adhesion between the alloy and the Al ₂O₃ layer.

Molybdenum (Mo) and tungsten (W)

Both molybdenum and tungsten have a positive effect on the heat strength of the alloy as defined above or below. Mo also has a positive effect on the wet corrosion properties. They may be added individually or in combination in amounts of up to 4.0 wt.%, for example 0-2.0 wt.%.

Reactive Element (RE)

By definition, reactive elements have a relatively high reactivity with carbon, nitrogen and oxygen. Titanium (Ti), niobium (Nb), vanadium (V), hafnium (Hf), tantalum (Ta) and thorium (Th) are reactive elements in the sense of having a high affinity for carbon, and thus they are strong carbide formers. These elements are added to improve the oxidation properties of the alloy. The total amount of the elements is at most 3.0 wt%, for example over 1.0 wt%, for example 1.5 to 2.5 wt%.

The maximum amount of the various reactive elements will depend primarily on the propensity of the elements to form unfavorable intermetallic phases.

Zirconium (Zr)

Zirconium is commonly referred to as a reactive element because it is very reactive with oxygen, nitrogen and carbon. In the alloys of the present disclosure Zr has been found to have a dual role as it will be present in the protective surface layer containing the aluminum-rich oxide thereby improving oxidation resistance and also forming carbides and nitrides. Therefore, to achieve optimal properties of the protective surface layer containing the aluminum-rich oxide, it is advantageous to include Zr in the alloy.

However, zr content higher than 0.40 wt% will affect oxidation due to formation of Zr-rich intermetallic inclusions, and Zr content lower than 0.05 wt% will be too small to satisfy dual purposes, irrespective of the contents of C and N. Thus, if Zr is present, the range is between 0.05-0.40 wt%, e.g., 0.10 to 0.35 wt%.

Furthermore, it has been found that the relationship between Zr and N and C may be important in order to achieve even better oxidation resistance of the protective surface layer, i.e. the alumina scale. Thus, the inventors have surprisingly found that if Zr is added to the alloy and the alloy also comprises N and C, and if the following conditions are met (the element content is given in weight%), the resulting alloy will achieve good oxidation resistance:

Such as Such as

The balance in the ferritic alloy as defined above or below is Fe and unavoidable impurities. Examples of unavoidable impurities are elements and compounds which are not intentionally added but cannot be completely avoided, since they are usually present as impurities in, for example, materials for the production of ferritic alloys.

FIGS. 1a and 1b show that in Si-containing ferritic alloys, higher Cr tends to form Si ₃ Cr inclusions, while 20% Cr also tends to promote the formation of undesirable brittle sigma phases after prolonged exposure in the focus temperature region. Although only two Cr levels, 10% and 20%, are shown in the figure, the tendency of the embrittling phase to increase with increasing Cr levels is clearly demonstrated. It should be noted that at 10% Cr no sigma phase is present, whereas at higher Si content at both Cr levels the amount of Cr ₃ Si phase increases. Thus, these figures show that there is a problem when using Cr levels of about 20%.

Unless another number is explicitly indicated, when the term "no more than" or "less than or equal to" is used in the context of "element no more than" below, those skilled in the art will recognize that the lower limit of the range is 0 wt%. Furthermore, the indefinite article "a" does not exclude a plurality.

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

Examples

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

The resulting samples were hot rolled and processed into flat bars with a cross section of 2mm x 10 mm. It was then cut into 20mm long specimens and ground with SiC paper to 800 mesh to contact air and combustion conditions. Some bars were cut into 200mm long by 3mm by 12mm bars for tensile testing in a Zwick/Roell Z100 tensile test apparatus at room temperature.

The results of the exposure and tensile tests are shown in table 1.

The samples were tested for yield and elongation at break in a standard tensile tester, and the results giving >3% elongation at break are designated as "x" in the "processable" column of the table. Thus, "x" represents an alloy that is easy to hot-roll and exhibits ductility characteristics at room temperature. In the "oxidation" column, "x" means that the alloy forms a protective oxygen-enriched aluminum oxide scale with biomass ash deposits in air at 950 ℃ and at 850 ℃.

Table 1-composition of melt and results of test processability and oxidation, (x) represents values between 3% and 6% elongation.

Thus, as can be seen from the above table, the alloys of the present disclosure exhibit good workability and good oxidation resistance.

Fig. 2 a) to 2 e) disclose samples of polished cross sections after contacting biomass (wood chip) ash containing a significant amount of potassium with three comparative alloys at 850 ℃ and 501 hour cycles of the present disclosure (fig. 2 a) 4783 and fig. 2 b) 4779). Micrographs were taken with a JEOL FEG SEM at 1000 x magnification and showed significant behavioral advantages between the alloys of the present disclosure and the reference material. It can be seen that on the alloys of the present disclosure, a 3-4 μm thin and protective alumina scale (alumina layer) has been formed, while on stainless steel (2 c-11Ni,21cr, n, ce, balance Fe) and Ni-based alloys (2 e-Inconel 625:58Ni,21cr,0.4al,0.5si, mo, nb, fe) a thicker and less protective chromia-rich (chromia) scale has been formed, and on the comparative FeCrAl alloy (alloy 4776) a relatively porous scale (fig. 2d-20cr,5al,0.04si, balance Fe) was formed that was not as protective alumina.

As can be seen from fig. 2a-2e, the addition of Si, al and Cr in accordance with the scope of the present disclosure will promote the formation of alumina scale at Al levels as low as about 2 wt.% and chromium levels as low as 5 wt.%.

Claims

1. A ferrite alloy, which is used for the production of a ferrite alloy, the ferrite alloy consists of the following elements in weight% [ wt% ]:

C0.01 to 0.1;

n is 0.001 to 0.1;

O:≤0.2;

cr 4 to 15;

Al 3 to 5;

Si 1.5 to 2.5;

Mn:≤0.4;

Mo+W, alone or in combination, is at most 4.0;

Y≤1.0;

Sc, ce, la and/or Yb, alone or in combination, in a total amount of at most 0.2;

zr 0.10 to 0.35;

Re=titanium (Ti), niobium (Nb), vanadium (V), hafnium (Hf), tantalum (Ta) and thorium (Th), in total amounts of more than 1.0 to 2.5 wt%;

the balance being Fe and the impurities conventionally present, and the following equation (elements in weight fraction) must also be satisfied:

0.016≤(Al+0.5Si)(Cr+10Si+0.1)≤0.020,

wherein C, N and the amount of Zr satisfy the following equation:

2. The ferritic alloy according to claim 1, wherein

Cr is 5 to 13 wt%.

3. A coating comprising the ferritic alloy according to any preceding claim.

4. An article comprising the ferritic alloy according to any one of claims 1 to 2.

5. Use of a ferritic alloy according to any of claims 1 to 2 for the manufacture of a coating and/or a covering and/or an article.

6. Use of a ferritic alloy according to any of claims 1 to 2 for manufacturing an article or a coating to be used in a corrosive environment.

7. Use of a ferritic alloy according to any of claims 1 to 2 for manufacturing an article or coating to be used in a furnace or as a heating element.

8. Use of the ferritic alloy according to any of claims 1 to 2 in environments where the ferritic alloy contacts salts, liquid lead and other metals, contacts ash or high carbon content deposits, combustion atmospheres, atmospheres with low pO ₂ and/or high N ₂ and/or high carbon activity.