CN110709529A

CN110709529A - Ferritic alloy

Info

Publication number: CN110709529A
Application number: CN201780091046.0A
Authority: CN
Inventors: 保·荣松
Original assignee: Sandvik Intellectual Property AB
Current assignee: Cantel Ltd
Priority date: 2017-05-24
Filing date: 2017-05-24
Publication date: 2020-01-17
Also published as: JP2020521051A; CA3062819A1; BR112019024471A2; US20200181745A1; JP7267936B2; EP3631034A1; WO2018215065A1

Abstract

A ferritic alloy comprising the following elements in weight% [ wt% ]: c0.01 to 0.1; n: 0.001 to 0.1; o: less than or equal to 0.2; cr 4 to 15; al 2 to 6; si 0.5 to 3; mn: less than or equal to 0.4; mo + W is less than or equal to 4; y is less than or equal to 1.0; sc, Ce, La and/or Yb is less than or equal to 0.2; zr is less than or equal to 0.40; RE is less than or equal to 3.0; the balance being Fe and impurities that normally occur, 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 containing chromium (Cr) contents of 15-25 wt.% and aluminium (Al) contents of 3-6 wt.%, are well known for their ability to: that is, protective alpha-alumina (Al) forms when subjected to temperatures between 900 ℃ and 1300 ℃₂O₃) Alumina, scale (scale). The lower limit of the Al content to form and maintain the alumina scale varies with the contacting conditions. However, at higher temperatures, the effect of too low an Al content level is that the selective oxidation of Al will fail and a less stable and less protective chromium and iron based scale will form.

It is generally believed that FeCrAl alloys generally 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, since the diffusion of oxygen and aluminum to the oxide-metal interface is relatively slow at lower temperatures, and thus the rate of formation of alumina scale is low, which means that there will be a risk of severe corrosion attack and the formation of less stable oxides.

Another problem that arises at lower temperatures, i.e. temperatures below 900 c, is the long-term embrittlement phenomena caused by the low temperature miscibility gap of Cr in FeCrAl alloy systems. Miscibility gaps exist at 550 ℃ with Cr contents above about 12 wt.%. Recently, to avoid this phenomenon, alloys have been developed with lower Cr contents of about 10-12 wt% Cr. Such alloys have been found to be under controlled and low pressure O₂The performance in molten lead of (2) is very good.

EP 0475420 relates to a fast solidifying ferritic alloy foil consisting essentially of: cr, Al, about 1.5-3 wt% Si, and REM (Y, Ce, La, Pr, Nd), the balance being Fe and impurities. The foil may also 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 075420 discusses the addition of Si to improve the flow characteristics of the alloy melt, but with limited success due to reduced ductility.

EP 0091526 relates to alloys resistant to thermal cyclic oxidation and hot-workable, more particularly to iron-chromium-aluminum alloys containing rare earth additions. In oxidation, the alloy will produce the desired whisker-textured oxide on the catalytic converter surface. However, the resulting alloy does not provide high temperature resistance.

Thus, there remains a need to further improve the corrosion resistance of ferritic alloys so that they can be used in corrosive environments during high temperature conditions. An aspect of the present disclosure is to solve or at least reduce the above-mentioned 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 impurities that normally occur, 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 contents of Cr and Si and Al 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 articles and/or coatings.

Drawings

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

Fig. 2a to 2e disclose a comparison of polished cross sections of two alloys according to the present disclosure and three reference alloys after contacting biomass (wood chip) ash containing a large amount of potassium at 850 ℃ and subjected to 50 1 hour cycles.

Detailed Description

As already mentioned 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 alloys as defined above or below, i.e. alloys containing the alloying elements and in the ranges mentioned herein, unexpectedly form a protective surface layer containing aluminium-rich oxides 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 the undesired brittle sigma phase will be reduced or even avoided after prolonged exposure to the temperature ranges mentioned herein. 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 be produced and formed into 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. According to one embodiment, 0.015. ltoreq. Al +0.5Si (Cr +10Si + 0.1). ltoreq.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 form on articles and/or coatings made from the alloys, which will prevent corrosion, oxidation, and embrittlement of the articles and/or coatings. In addition, the ferritic alloys of the present disclosure can provide protection against corrosion, oxidation, and embrittlement at temperatures as low as 400 ℃, as a protective surface layer containing aluminum-rich oxides will form on the surface of articles and/or coatings made therefrom. In addition, the alloy according to the present disclosure will also perform well at temperatures up to about 1100 ℃, and it shows a reduced tendency to long-term embrittlement in the temperature range of 400 to 600 ℃.

The alloys of the present disclosure may be used in the form of coatings. Additionally, articles can also comprise the alloys of the present disclosure. According to the present disclosure, the term "coating" is intended to refer to the following embodiments: wherein the ferritic alloy according to the present disclosure is present in the form of a layer which is placed in a corrosive environment in contact with the substrate, regardless of the means and method for achieving it, and regardless of the layer and the substrateHow the relative thickness relationship between the materials is. Examples thereof are thus, but not limited to, PVD coatings, cladding 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, waterwalls in power plants, vessels or heat exchangers (e.g. for hydrocarbons or CO/CO-containing₂Gas reforming or other processing) components used in connection with industrial heat treatment of steel and aluminum, powder metallurgy processes, gas and electric heating elements.

Further, the alloy according to the present disclosure is suitable for use in environments with corrosive conditions. Examples of such environments include, but are not limited to, exposure to salts, liquid lead and other metals, exposure to ash or high carbon content deposits, combustion atmospheres, having low pO₂And/or high N₂And/or a high carbon activity environment.

In addition, the ferritic alloys of the present disclosure can be manufactured by using normally occurring solidification rates ranging from conventional metallurgy to rapid solidification. The alloys of the present disclosure are also suitable for use in the manufacture of all types of forged and extruded articles, such as wires, ribbons, rods and plates. As is well known to those skilled in the art, the amount of thermoplastic 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 be presented in the following paragraphs. The list of functions and effects of the individual alloying elements should not be regarded as complete, since further functions and effects may also be present for the alloying elements.

Carbon (C)

Carbon may be present as an inevitable impurity generated during the production process. Carbon may also be included in the ferritic alloy as defined above or below to improve 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 may lead to difficulties in forming the material and also negatively affect the corrosion resistance. Thus, the maximum amount of carbon is 0.1 wt%. For example, the carbon content is 0.02 to 0.09 wt.%, such as 0.02 to 0.08 wt.%, such as 0.02 to 0.07 wt.%, such as 0.02 to 0.06 wt.%, such as 0.02 to 0.05 wt.%, such as 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 applying powder metallurgical process routes. At too high a level, nitrogen can lead to difficulties in forming the alloy and also have a negative effect on corrosion resistance. Thus, the maximum amount of nitrogen is 0.1 wt.%. Suitable ranges for nitrogen are, for example, from 0.001 to 0.08 wt%, such as from 0.001 to 0.05 wt%, such as from 0.001 to 0.04 wt%, such as from 0.001 to 0.03 wt%, such as from 0.001 to 0.02 wt%.

Oxygen (O)

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

Chromium (Cr)

Chromium is present in the disclosed alloys primarily as a matrix solid solution element. Chromium promotes the formation of an alumina layer on the alloy by the so-called tertiary elemental effect, i.e. by forming chromium oxide in the transient oxidation stage. 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 formation of brittle sigma phases and Cr₃Sensitivity of Si. This effect occurs at about 12 wt% and is enhanced at levels above 15 wt%, so the limit for Cr is 15 wt%. Also from an oxidation point of view, a content higher than 15 wt% would lead to an undesired contribution of Cr to the protective oxide scale. According to one implementationIn this way, the content of Cr 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)

Aluminium is an important element in the alloys as defined above or below. When exposed to oxygen at high temperatures, aluminum forms a dense and thin oxide Al by selective oxidation₂O₃This will protect 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 large amounts of aluminum can lead to the formation of cracks in the alloy during its machining. Therefore, the amount of aluminum should not exceed 6 wt%. For example, the aluminum may be 3 to 5 weight percent, such as 2.5 to 4.5 weight percent, such as 3 to 4 weight percent.

Silicon (Si)

In commercial FeCrAl alloys, silicon is typically present at levels up to 0.4 wt%. In ferritic alloys as defined above or below, Si will play a very important role, since it has been found that Si has a great effect on improving oxidation resistance and corrosion resistance. The upper limit of Si is due to loss of processability under hot and cold conditions and formation of brittle Cr during long term exposure₃The sensitivity of the Si and sigma phases is set to increase. Therefore, the addition of Si must be performed in relation to the contents of Al and Cr. 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 amounts 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 oxides and/or nitrides. The maximum amount of yttrium present in the dispersion-hardened alloy in the form of an oxygen-containing yttrium compound may be up to 1.0% by weight.

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

Scandium, cerium, lanthanum, and ytterbium are interchangeable elements and may be added individually or in combination in a total amount of up to 0.2 wt.% to improve oxidation properties, alumina (Al)₂O₃) Self-repairing of layers or alloys with Al₂O₃Adhesion between layers.

Molybdenum (Mo) and tungsten (W)

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

Reactive Element (RE)

By definition, reactive elements have a 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 therefore they are strong carbide formers. These elements are added to improve the oxidation properties of the alloy. The total amount of said elements is at most 3.0 wt.%, such as more than 1.0 wt.%, for example 1.5 to 2.5 wt.%.

The maximum amount of each reactive element will depend primarily on the tendency of the element to form undesirable intermetallic phases.

Zirconium (Zr)

Zirconium is commonly referred to as a reactive element because it is very reactive towards 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 aluminum rich oxide to improve oxidation resistance, and also form carbides and nitrides. Therefore, in order to achieve the best properties of the protective surface layer containing an aluminum-rich oxide, it is advantageous to include Zr in the alloy.

However, Zr content higher than 0.40 wt% will affect the oxidation due to the formation of Zr rich intermetallic inclusions, and Zr content lower than 0.05 wt% will not satisfy the dual purpose because it is too small, regardless 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 was found that the relation between Zr and N and C may be important in order to achieve an 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 materials, for example for producing ferritic alloys.

FIGS. 1a and 1b show that in Si-containing ferritic alloys, higher Cr tends to form Si₃Cr inclusions, while 20% Cr also tends to promote the formation of an undesirable brittle sigma phase after prolonged exposure in the focusing temperature region. Although only two Cr levels, 10% and 20%, are shown in the figure, the tendency of the embrittling phases to increase with increasing Cr levels is clearly demonstrated. It should be noted that at 10% Cr there is no sigma phase, while at both Cr levels at higher Si content, Cr₃The amount of 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 terms "element ≦" or "less than or equal to" are used in the following context "element ≦ number," one of ordinary skill in the art will recognize that the lower limit of the range is 0 wt%. In addition, the indefinite article "a" or "an" 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 sample was hot rolled and processed into flat bars having a cross section of 2mm x 10 mm. It was then cut into 20mm long samples and ground to 800 mesh with SiC paper to contact air and fire conditions. Some of the bars were cut into 200mm long by 3mm by 12mm bars for tensile testing in a Zwick/Roell Z100 tensile testing apparatus at room temperature.

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

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

Table 1-composition of the melt and results of testing processability and oxidation, (x) represents a value 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. 2a) to 2e) disclose samples that are polished sections of the present disclosure (fig. 2a)4783 and 2b)4779) and three comparative alloys after contacting biomass (wood chip) ash containing a significant amount of potassium at 850 ℃ and 50 1 hour cycles. Micrographs were taken with a JEOL FEG SEM at 1000 x magnification and show a clear behavioral advantage between the alloys of the present disclosure and the reference materials. It can be seen that on the alloys of the present disclosure, a 3-4 μm thin protective alumina scale (alumina layer) has formed, whereas on 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) a thicker and less protective chromium oxide (chromia) -rich scale has formed, and on the comparative FeCrAl alloy (alloy 4776) a scale is formed that is relatively porous and not capable of acting as a protective alumina (fig. 2d-20Cr, 5Al, 0.04Si, balance Fe).

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 contents as low as about 2 wt% and chromium contents as low as 5 wt%.

Claims

1. A ferritic alloy comprising the following elements in weight% [ wt% ]:

c0.01 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 is less than or equal to 0.2;

Zr≤0.40；

RE≤3.0；

the balance being Fe and impurities that conventionally occur, and must also satisfy the following equation (elements in weight fraction):

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022。

2. the ferritic alloy of claim 1 wherein (in terms of weight fractions of elements) 0.015 ≦ (Al +0.5Si) (Cr +10Si +0.1 ≦ 0.021.

3. The ferritic alloy of claim 1 or claim 2, wherein Zr is 0.05 to 0.40 weight percent.

4. The ferritic alloy of any of claims 1-3, wherein Cr is from 5 wt.% to 13 wt.%.

5. The ferritic alloy of any of claims 1-4, wherein RE is greater than 1.0 to 3.0 weight percent.

6. The ferritic alloy of any preceding claim, wherein Al is 2.5 wt.% to 4.5 wt.% or 3 wt.% to 5 wt.%.

7. The ferritic alloy of any preceding claim, wherein Al is 3-4 wt%.

8. The ferritic alloy of any preceding claim, wherein Si is 1.0-3 wt.%.

9. The ferritic alloy of any preceding claim, wherein Si is 1.5-2.5 weight percent.

10. The ferritic alloy of any preceding claim, wherein Zr is from 0.10 wt.% to 0.35 wt.%.

11. The ferritic alloy of any preceding claim, wherein the amounts of C, N and Zr satisfy the following equation:

12. a coating comprising the ferritic alloy of any preceding claim.

13. An article comprising the ferritic alloy of any preceding claim.

14. Use of the ferritic alloy according to any of claims 1 to 11 for the manufacture of coatings and/or coverings and/or articles.

15. Use of the ferritic alloy according to any of claims 1 to 11 for the manufacture of articles or coatings to be used in corrosive environments.

16. Use of the ferritic alloy according to any of claims 1-11 for the manufacture of articles or coatings to be used in furnaces or as heating elements.

17. Use of the ferritic alloy according to any of claims 1 to 11 in the following environment: wherein the ferritic alloy is in contact with salts, liquid lead and other metals, with ash or high carbon content deposits, in a combustion atmosphere, having a low pO₂And/or high N₂And/or a highly carbon reactive atmosphere.