ES2842424T3 - Ferritic alloy - Google Patents

Abstract

A ferritic alloy comprising the following elements in% by weight [% by weight] C 0.01 to 0.1; N: 0.001 to 0.1; O: <= 0.2; Cr 4 to 15; Al 2 to 6; If 0.5 to 3; Mn: <= 0.4; Mo + W <= 4; Y <= 1.0; Sc, Ce and / or La <= 0.2; Zr 0.05 to 0.40; RE <= 1.0; the remainder being Fe and the impurities that normally occur and the following equation must also be met (elements in weight fraction): ** (See formula) **

Description

DESCRIPTION

Ferritic alloy

Technical field

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

Background and introduction

Ferritic alloys, such as FeCrAl alloys comprising chromium (Cr) levels from 15 to 25% by weight and aluminum (Al) levels from 3 to 6% by weight, are well known for their ability to form scale of a-protective alumina (ALO ³ ), aluminum oxide, when exposed to temperatures between 900 and 1300 ° C. The lower limit of Al content to form and maintain the alumina scale varies with exposure conditions. However, the effect of too low an Al level at higher temperatures is that selective oxidation of Al will fail and less stable and less protective chromium and iron based scale will form.

It is commonly accepted that FeCrAl alloys will not normally form the protective α-alumina layer if exposed to temperatures below about 900 ° C. There have been attempts to optimize the FeCrAl alloy compositions so that they form the protective α-alumina at a temperature below about 900 ° C. However, in general, these attempts have not been very successful because the diffusion of oxygen and aluminum to the oxide-metal interface will be relatively slow at lower temperatures and therefore the rate of formation of the alumina scale will be low. , which means that there will be a risk of serious corrosion attacks and the formation of less stable oxides.

Another problem that arises at lower temperatures, ie temperatures below 900 ° C, is the long-term embrittlement phenomenon that arises from a low-temperature miscibility gap for Cr in the FeCrAl alloy system. The miscibility gap exists for Cr levels above about 12% by weight at 550 ° C. Recently, alloys with lower Cr levels of about 10 to 12% by weight Cr have been developed to avoid this phenomenon. This group of alloys has been found to work very well in molten lead with O ² at controlled low pressure.

EP 0475 420 relates to a rapidly solidified ferritic alloy sheet consisting essentially of Cr, Al, Si, about 1.5 to 3% by weight and REM (Y, Ce, La, Pr, Nd, the remainder Fe and impurities. The sheet may further contain about 0.001 to 0.5% by weight of at least one element selected from the group consisting of Ti, Nb, Zr, and V. The sheet has a grain size of no more than about 10 gm EP 075420 discusses Si additions to improve flow properties of molten alloy, but success was limited due to reduced ductility.

EP 0091 526 relates to oxidation resistant and hot workable thermocyclic alloys, more particularly to iron-chromium-aluminum alloys with rare earth additions. On oxidation, the alloys will produce a desirable whisker textured oxide on catalytic converter surfaces. However, the alloys obtained did not provide high temperature resistance.

Therefore, 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. Aspects of the present disclosure are to solve or at least reduce the problems mentioned above.

Disclosure summary

Therefore, the present disclosure refers to a ferritic alloy as described in claim 1.

Therefore, there is a relationship between the content of Cr and Si and Al in the alloy according to the present disclosure, which if met will provide an alloy with excellent oxidation resistance and ductility and also reduced brittleness in combination with increased resistance to high temperature corrosion.

The present disclosure also refers to an object and / or a coating comprising the ferritic alloy according to the present disclosure. Furthermore, the present disclosure also relates to the use of the ferritic alloy as defined hereinabove or hereinafter to make an object and / or a coating.

Brief description of the figures

Figure 1 a and Figure 1 b disclose the phases at the level of Fe - 10% Cr - 5% Al versus Si (figure 1a) and the level of Fe - 20% Cr - 5% Al versus Si (figure 1 b). The diagram has been made using the TCFE7 database and Thermocalc software.

Figures 2a ae disclose polished sections of two alloys in accordance with the present disclosure compared to three reference alloys after exposure to 50 cycles of 1 hour at 850 ° C exposed to biomass ash (wood granules) containing large amounts potassium.

Detailed description of the disclosure

As already indicated above, the present disclosure provides a ferritic alloy as described in claim 1.

Surprisingly, it has been discovered that an alloy as defined hereinbefore or hereinafter, i.e., containing the alloying elements and in the ranges mentioned herein, will unexpectedly form a protective surface layer containing Aluminum rich oxide 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 phase, after prolonged exposure in the temperature range mentioned herein, will be reduced or even reduced. will avoid. Therefore, the interaction between Si and Al and Cr will improve the formation of a stable and continuous protective surface layer containing oxide rich in aluminum, and by using the above equation, it will be possible to add Si and still obtain a ferritic alloy that will be possible both to produce and to transform into different objects. The inventor has surprisingly discovered that if the amounts of Si and Al and Cr are balanced so that the following condition is met (all the numbers of the elements are in fractions by weight):

The alloy obtained will have a combination of excellent oxidation resistance and workability and formability within the Cr range of the present disclosure. According to one embodiment, 0.015 <( Al + 0.5 Si) ( Cr + 10 Si + 0.1) <0.021, such as 0.016 <( Al + 0.5 Si) ( Cr + 10 Si + 0 , 1) <0.020, such as 0.017 <( Al + 0.5 Si) ( Cr + 10 Si + 0.1) <0.019.

The ferritic alloy of the present disclosure is especially useful at temperatures below about 900 ° C, as a protective surface layer containing aluminum-rich oxide will form on an object and / or a coating made from said alloy, which will prevent corrosion, oxidation and embrittlement of the object and / or the coating. In addition, the present ferritic alloy can provide protection against corrosion, oxidation and embrittlement at temperatures as low as 400 ° C as a protective surface layer containing aluminum rich oxide will form on the surface of the object and / or coating manufactured with the herself. Furthermore, the alloy according to the present disclosure will also perform excellently at temperatures up to about 1100 ° C and will show a reduced tendency for long-term embrittlement in the temperature range of 400 to 600 ° C.

The present alloy can be used in the form of a coating. Furthermore, an object may also comprise the present alloy. In accordance with the present disclosure, the term "coating" is intended to refer to embodiments in which the ferritic alloy according to the present disclosure is present in the form of a layer exposed to a corrosive environment that is in contact with a base material. , regardless of the means and procedures to achieve it, and regardless of the relative thickness relationship between the layer and the base material. Therefore, examples of this, but not limited to, are a PVD coating, a coating, or a composite or composite material. The purpose of the alloy is to protect the material underneath from corrosion and oxidation. Examples of suitable objects, but not limited to, are a composite tube, a tube, a boiler, a gas turbine component, and a steam turbine component. Other examples include a superheater, a wall of water in a power plant, a component in a vessel or a heat exchanger (for example, for reforming or other processing of hydrocarbons or gases containing CO / CO ² ), a component used in related to the industrial heat treatment of steel and aluminum, powder metallurgy processes, electric and gas heating elements.

Furthermore, the alloy according to the disclosure is suitable for use in environments that have corrosive conditions. Examples of such environments include, but are not limited to, exposure to salts, liquid lead and other metals, exposures to ash or high carbon deposits, combustion atmospheres, atmospheres with low pO ² activity environments and / or high activity in N ² and / or high activity in carbon.

Furthermore, the present ferritic alloy can be manufactured using normally occurring solidification rates ranging from conventional metallurgy to rapid solidification. The present alloy will also be suitable for making all kinds of objects both forged and extruded, such as a wire, a strip, a bar and a plate. The amount of hot and cold plastic deformation, as well as grain structure and grain size, as those skilled in the art know, will vary between object shapes and production pathway.

The functions and effects of the essential alloying elements of the alloy defined earlier herein and hereinafter will be presented in the following paragraphs. The list of functions and effects of the respective alloying elements should not be considered complete as there may be other functions and effects of such alloying elements.

Carbon (C)

Carbon can be present as an unavoidable impurity resulting from the production process. Carbon can also be included in the ferritic alloy as defined hereinabove or hereinafter to increase strength by precipitation hardening. To have a noticeable effect on the strength of the alloy, carbon must be present in an amount of at least 0.01% by weight. At too high levels, carbon can result in difficulties in forming the material and also a negative effect on corrosion resistance. Therefore, the maximum amount of carbon is 0.1% by weight. For example, the carbon content is 0.02-0.09% by weight, such as 0.02-0.08% by weight, such as 0.02-0.07% by weight, such as 0.02. - 0.06% by weight, such as 0.02-0.05% by weight, such as 0.01-0.04% by weight.

Nitrogen (N)

Nitrogen can be present as an unavoidable impurity resulting from the production process. Nitrogen may also be included in the ferritic alloy as defined hereinbefore or hereinafter to increase strength by precipitation hardening, particularly when applying a powder metallurgical process route. At too high levels, nitrogen can result in difficulties in alloying and also have a negative effect on corrosion resistance. Therefore, the maximum amount of nitrogen is 0.1% by weight. Suitable ranges of nitrogen are, for example, 0.001-0.08% by weight, such as 0.001-0.05% by weight, such as 0.001-0.04% by weight, such as 0.001-0.03% by weight. weight, such as 0.001-0.02% by weight.

Oxygen (O)

Oxygen may exist in the alloy as defined hereinbefore or hereinafter as an impurity resulting from the production process. In that case, the amount of oxygen can be up to 0.02% by weight, such as up to 0.005% by weight. If oxygen is deliberately added to provide strength by dispersion reinforcement, such as when the alloy is manufactured by a powder metallurgical process route, the alloy as defined hereinbefore or hereinafter comprises up to or equals 0.2% by weight of oxygen.

Chromium (Cr)

Chromium is present in the present alloy primarily as a solid matrix solution element. Chromium promotes the formation of the aluminum oxide layer in the alloy through the so-called third element effect, that is, through the formation of chromium oxide in the transient oxidation stage. Chromium should be present in the alloy as defined hereinbefore or hereinafter in an amount of at least 4% by weight to serve this purpose. In the present alloy according to the invention, Cr also improves the susceptibility to form a brittle o and Cr3Si phase. This effect arises at about 12% by weight and is enhanced at levels above 15% by weight, whereby the Cr limit is 15% by weight. Also from an oxidation point of view, levels greater than 15% by weight will result in an undesirable contribution of Cr in the protective oxide scale. According to one embodiment, the Cr content is 5 to 13% by weight, such as 5 to 12% by weight, such as 6 to 12% by weight, such as 7 to 11% by weight. , such as 8 to 10% by weight.

Aluminum (Al)

Aluminum is an important element in the alloy as defined hereinabove or hereinafter. Aluminum, when exposed to oxygen at high temperature, will form the thin, dense oxide, AbO3, through selective oxidation, which will protect the underlying alloy surface from further oxidation. The amount of aluminum must be at least 2% by weight to ensure that a protective surface layer is formed containing aluminum rich oxide and also to ensure that there is enough aluminum present to cure the protective surface layer when it is damaged. However, aluminum has a negative impact on formability and large amounts of aluminum can result in the formation of cracks in the alloy during mechanical working of the alloy. Consequently, the amount of aluminum should not exceed 6% by weight. For example, aluminum can be 3 to 5% by weight, such as 2.5 to 4.5% by weight, such as 3 to 4% by weight.

Silicon (Si)

In commercial FeCrAl alloys, silicon is usually present in levels of up to 0.4% by weight. In ferritic alloys as defined hereinabove or hereinafter document, Si will play a very important role as it has been found to have a great effect in improving resistance to oxidation and corrosion. The upper limit of Si is established by the loss of workability in hot and cold conditions and an increased susceptibility to Cr3Si and phase or brittle formation during long-term exposure. Therefore, the Si additions must be made in relation to the Al and Cr content. Therefore, the amount of Si is between 0.5 and 3% by weight, such as 1 to 3% by weight. weight, such as 1 to 2.5% by weight, such as 1.5 to 2.5% by weight.

Manganese (Mn)

Manganese may be present as an impurity in the alloy as defined hereinbefore or hereinafter up to 0.4% by weight, such as 0 to 0.3% by weight.

Yttrium (Y)

In molten metallurgy, yttrium can be added in an amount up to 0.3% by weight to improve the adhesion of the protective surface layer. In addition, in powder metallurgy, if yttrium is added to create a dispersion together with oxygen and / or nitrogen, the yttrium content is at least 0.04% by weight, to achieve the effect of hardening the desired dispersion by oxides. and / or nitrides. The maximum amount of yttrium in dispersion hardened alloys in the form of oxygen-containing Y compounds can be up to 1.0% by weight.

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

Scandium, cerium and lanthanum are interchangeable elements and can be added individually or in combination in a total amount of up to 0.2% by weight to improve the oxidation properties, the self-healing of the aluminum oxide layer (Al2O3) or the adhesion between the alloy and the Al2O3 layer.

Molybdenum (Mo) and tungsten (W)

Both molybdenum and tungsten have positive effects on the hot strength of the alloy as defined hereinbefore or hereinafter. Mo also has a positive effect on wet corrosion properties. They can be added individually or in combination in an amount of up to 4.0% by weight, such as 0 to 2.0% by weight.

Reactive elements (RE)

By definition, reactive elements 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 the sense that they have a high affinity for carbon. , so they are strong carbide formers. These elements are added to improve the oxidation properties of the alloy. The total amount of elements is up to 1.0% by weight, such as 0.4% by weight, such as up to 0.15.

The maximum amounts of the respective reactive element will depend mainly on the tendency of the element to form adverse intermetallic phases.

Zirconium (Zr)

Zirconium is often referred to as a reactive element, as it is very reactive with oxygen, nitrogen, and carbon. In the present alloy, it has been discovered that Zr has a dual role as it will be present in the protective surface layer containing aluminum rich oxide, thus improving oxidation resistance and will also form carbides and nitrides. Therefore, to achieve the best properties of the protective surface layer containing aluminum rich oxide, it is advantageous to include Zr in the alloy.

However, levels of Zr above 0.40% by weight will have an effect on oxidation due to the formation of Zr-rich intermetallic inclusions and levels below 0.05% by weight will be too small to meet the goal. dual purpose, regardless of the C and N content. Therefore, the range of Zr is between 0.05 and 0.40% by weight, such as 0.10 to 0.35.

In addition, it has also been found that the ratio of Zr to N and C may be important in achieving even better oxidation resistance of the protective surface layer, ie, the scaling of alumina. Therefore, the inventor has surprisingly found that if Zr is added to the alloy and the alloy also comprises N and C and if the following condition is met (the content of the element given in% by weight):

^{- 0.1 5} <Zr - - ■ < ^{0.1 5,} -0.15 <Z r - ^ ^ - r - ⁴

^{'7C 4N} <0.10, Z <0.10, _{or, 62} - as''62 ° as -0.05 ^<.62 alloy obtained achieve a good oxidation resistance.

The remainder in the ferritic alloy as defined hereinbefore or hereinafter is Fe and unavoidable impurities. Examples of unavoidable impurities are elements and compounds that have not been added on purpose, but cannot be completely avoided, as they are normally produced as impurities in, for example, the material used to make the ferritic alloy.

Figure 1a and Figure 1b show that a higher Cr in a ferritic alloy containing Si is prone to forming Si3Cr inclusions and at 20% Cr is also prone to promoting an unwanted brittle phase after exposure for a prolonged time in the focus temperature area. Although the diagrams are only shown for two Cr levels, 10 and 20%, the trend of embrittlement phases increasing with higher Cr is clearly demonstrated. The absence of phase or 10% Cr and the increasing amount of Cr3Si phase with higher Si content is observed at both Cr levels. Therefore, these figures show that there will be problems when using Cr levels of about 20 %.

When the terms "<" or "less than or equal to" are used in the following context: "element <number", it is known to the person skilled in the art that the lower limit of the range is 0% by weight unless indicated specifically another number Furthermore, the indefinite article "a / an" does not exclude a plurality.

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

Examples

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

The samples obtained were hot rolled and machined into flat rods with a cross section of 2 x 10 mm. They were then cut into 20mm long coupons and ground with SiC paper to 800 mesh for exposure to air and burning conditions. Some of the rods were cut into 200mm long x 3x12mm rods for room temperature tensile testing on a Zwick / Roell Z100 tensile tester.

The results of the exposure and traction tests are shown in Table 1.

The samples were tested for breaking stress and elongation as well as elongation at break on a standard tensile testing machine and the result giving> 3% elongation at break is designated as "x" in the column. "Workable" table. Therefore, the "x" designates an alloy that is easily hot rolled and exhibits ductile behavior at room temperature. In the "Oxidation" column, the "x" indicates that the alloy forms a protective oxide scale rich in alumina at 950 ° C in air and at 850 ° C with biomass ash deposition.

Table 1: The composition of the melts and the results of the workability and oxidation test. The ( x) indicates an elongation value between 3 and 6%.

Therefore, as can be seen from the table above, the alloys of the present disclosure show good workability and good oxidation behavior.

Figures 2a to e disclose samples that are polished sections of the present disclosure (Figures 2a) 4783 and 2b) 4779) compared to three comparative alloys after exposure to 50 cycles of 1 hour at 850 ° C exposed to biomass ash ( wood granules) that contain large amounts of potassium. The micrographs are taken on a JEOL FEG SEM at 1000-fold magnification and show a clear performance advantage between the alloys of the present disclosure and the reference materials. As can be seen, in the alloys of the present disclosure, a 3-4 gm thick protective alumina scale (aluminum oxide layer) has been formed, while a chromium-rich scale (chromium oxide) is formed. thicker and less protective on stainless steel (2c - 11 Ni, 21 Cr, N, Ce, Fe remainder) and a Ni-based alloy (2e - Inconel 625: 58Ni, 21Cr, 0.4Al, 0.5Si , Mo, Nb, Fe), and a relatively porous and not so protective alumina scale is formed in the comparative FeCrAl alloy (alloy 4776) (Figure 2d - 20Cr, 5Al, 0.04 Si, Fe residue).

As can be seen in Figures 2a-e, the addition of Si, Al and Cr in accordance with the ranges according to the present disclosure will promote the formation of alumina scale at Al levels as low as about 2% by weight and longer. chromium levels as low as 5% by weight.

Claims (15)

1. A ferritic alloy comprising the following elements in% by weight [% by weight] C 0.01 to 0.1; N: 0.001 to 0.1; O: <0.2; Cr 4 to 15; Al 2 to 6; If 0.5 to 3; Mn: <0.4; Mo + W <4; Y <1.0; Sc, Ce and / or La <0.2; Zr 0.05 to 0.40; RE <1.0; the remainder being Fe and the impurities that normally occur and the following equation must also be met (elements in weight fraction):

2. The ferritic alloy according to claim 1, wherein (elements in fractions by weight)

The ferritic alloy according to any one of claims 1 to 2, wherein the Cr is 5 to 13% by weight. 4. The ferritic alloy according to any of claims 1 to 4, wherein the Cr is 6 to 12% by weight. 5. The ferritic alloy according to any of the preceding claims, wherein the Al is 2.5 to 4.5% by weight or 3 to 5% by weight. The ferritic alloy according to any of the preceding claims, wherein the Si is 1.0 to 3% by weight. 7. The ferritic alloy according to any of the preceding claims, wherein the Si is 1.5 to 2.5% by weight. 8. The ferritic alloy according to any of the preceding claims, wherein the Zr is 0.10 to 0.35% by weight. 9. The ferritic alloy according to any of the preceding claims, wherein the amount of C, N and Zr meets the following equation: -0.15 < ^{Z r} - 0.62 <0.15 10. A coating comprising the ferritic alloy according to any of the preceding claims. 11. An object comprising the ferritic alloy according to any of the preceding claims. 12. Use of the ferritic alloy according to any one of claims 1 to 11 to manufacture a coating and / or a coating and / or an object. 13. Use of the ferritic alloy according to any one of claims 1 to 11 to manufacture an object or a coating for use in corrosive environments. 14. Use of the ferritic alloy according to any of claims 1 to 11 to manufacture an object or a coating for use in a furnace or as a heating element. 15. Use of the ferritic alloy according to any of claims 1 to 11 in environments where the ferritic alloy is exposed to salts, liquid lead and other metals, is exposed to ash or high carbon deposits, atmospheres of combustion, atmospheres with low pO2 activity and / or high N2 activity and / or high carbon activity.