A new iron chromium aluminum alloy

Abstract

The present disclosure relates in general to an iron chromium aluminum (FeCrAl) alloy comprising in weight %, wt%, C equal to or less than 0.3; N equal to or less than 0.2; Ta 0.6 – 4.5; Cr 9 – 12; Al 3 – 6; Si equal to or less than 1.5; Mn equal to or less than 0.5; Mo equal to or less than 3.5; Nb equal to or less than 2.5; Ti equal to or less than 1.0; Zr equal to or less than 1.0; Hf equal to or less than 1.0; Balance Fe and unavoidable impurities. and its use. The present disclosure further relates an object or a coating manufactured thereof.

Description

[0001] A new iron chromium aluminum alloy

[0002] TECHNICAL FIELD

[0003] The present disclosure relates in general to an iron chromium aluminum (FeCrAl) alloy and its use. The present disclosure further relates an object or a coating manufactured thereof.

[0004] BACKGROUND

[0005] Conventionally manufactured FeCrAl alloys having a chromium (Cr) content of 15 to 25 wt% and an aluminium (Al) content from 3 to 6 wt% are well known for their ability to form protective a-alumina (AI2O3), aluminium oxide, scales 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.

[0006] Products composed of these alloys are good in applications where there is a need for good oxidation resistance. However, if these products cannot retain their geometric shape when subjected to mechanical load, they will quickly fail due to excessive deflections over time.

[0007] There is a desire within the present field for having an alloy which is able to be load bearing, and which has good oxidation resistance in both air and in molten lead. Additionally, there is also a desire to have an alloy which will not become brittle at temperatures of 450 to 500°C and which will therefore be able to carry load and remain oxidation resistant at these temperatures in both air and molten lead and retain its properties at higher temperature ranges.

[0008] The present disclosure aims at solving or at least reducing the above-mentioned problems.

[0009] SUMMERY

[0010] An aspect of the present disclosure is to provide a FeCrAl alloy which will have a good creep strength and a good oxidation resistance in air and molten lead.

[0011] Furthermore, an additional aspect of the present disclosure to provide a FeCrAl alloy with very good creep strength and very good oxidation resistance at temperatures up to 1300°C. Thus, these properties are present at higher temperatures.

[0012] Yet a further aspect of the present disclosure is to provide a FeCrAl alloy, which will be suitable as construction material and which will be able to carry load in for example high temperature applications, such as boilers, furnaces and heat and energy generating plants and processes. Other suitable applications may include materials for atube, a composite tube, a sheet, and a plate, a strip, a wire and an overlay weld. Other applications may include a coating.

[0013] Furthermore, an aspect of the present inventive alloy is that it may be manufactured through a conventional steel manufacturing process.

[0014] Additionally, there is an aspect of the present invention to provide an object (product) comprising or consisting of the FeCrAl alloy as defined hereinbelow. Examples of an object are atube, a composite tube, a sheet, and aplate, a strip, a wire and an overlay weld.

[0015] Further, another aspect of the present invention is to provide a coating comprising or consisting of the FeCrAl alloy as defined hereinbelow.

[0016] A FeCrAl-alloy is an alloy which contains iron (Fe), chromium (Cr) and aluminum (Al). Such an alloy is characterized in that a layer of alumina is formed on the surface of an object during operation. Furthermore, these alloys typically contain carbon (C), which may be added deliberately, for example to increase strength, or may exist in the alloy as an impurity resulting from the production process or alloying materials.

[0017] In accordance with the present disclosure, an iron chromium aluminum alloy (FeCrAl alloy) is provided having the following composition in weight% (wt%):

[0018] C equal to or less than 0.3;

[0019] N equal to or less than 0.2 ;

[0020] Ta 0.6 - 4.5;

[0021] Cr 9 - 12;

[0022] Si equal to or less than 1.5;

[0023] Mn equal to or less than 0.5;

[0024] Mo equal to or less than 3.5;

[0025] Nb equal to or less than 2.5;

[0026] Ti equal to or less than 1.0;

[0027] Zr equal to or less than 1.0;

[0028] Hf equal to or less than 1.0;

[0029] Balance Fe and unavoidable impurities.

[0030] The inventors have surprisingly found that the present FeCrAl alloy will have a combination of high creep strength and good oxidation properties even though it has been produced through a conventional steel manufacturing process, i.e., by using melting and casting prior to hot working and cold working. High creep strength in a FeCrAl alloy has so far only been achievable in alloys produced through powder metallurgy, i.e., by atomization of powder followed by hot isostatic pressing instead of melting and casting. Avoiding embrittlement in the 450 to 500°C temperature range while retaining good oxidation resistance in lead has so far only been achieved for alloys with unsatisfactory load bearing capacity. Hence, according to embodiments, the present FeCrAl alloy is a conventional steel manufactured FeCrAl alloy.

[0031] According to the present disclosure, the manufacturing process comprises the steps of a) melting; b) casting; c) hot working; d) optionally cold working; and e) optionally heat treatment.

[0032] The present disclosure also relates to the use of the FeCrAl alloy as defined hereinabove or hereinafter in a molten lead environment or in an energy producing plant for molten lead. BRIEF DESCRIPTION OF DRAWINGS

[0033] The present disclosure is also described in the non-limiting figures:_

[0034] Figure 1 shows SEM images of alloys having different Ta content;

[0035] Figure la shows an alloy comprising 0.3 wt% Ta;

[0036] Figure lb shows an alloy with 0.6 wt% Ta;

[0037] Figure 1c shows an alloy with 1.1 wt% Ta;

[0038] Figure 2 shows a graph wherein with minimum creep rate as a function of

[0039] Ta content at 1000 °C with a 2.50 MPa load; and

[0040] Figure 3 shows the gross mass gain for several samples during oxidation resistance testing.

[0041] DETAILED DESCRIPTION

[0042] The present disclosure relates to an iron-chromium-aluminium (FeCrAl) alloy having the following composition in weight%, wt%:

[0043] C equal to or less than 0.3;

[0044] N equal to or less than 0.2 ;

[0045] Ta 0.6 - 4.5;

[0046] Cr 9 - 12;

[0047] Al 3 - 6;

[0048] Si equal to or less than 1.5;

[0049] Mn equal to or less than 0.5;

[0050] Mo equal to or less than 3.5;

[0051] Nb equal to or less than 2.5;

[0052] Ti equal to or less than 1.0;

[0053] Zr equal to or less than 1.0;

[0054] Hf equal to or less than 1.0;

[0055] Balance Fe and unavoidable impurities.

[0056] The inventors have surprisingly found that tantalum in the ranges as disclosed hereinabove or hereinafter will have a very positive impact on the creep strength without impairing the oxidation resistance. The improvement is in the magnitude that an object containing the present alloy will be able to carry load even in high temperature applications. By being able to carry a load over time with minimal deformation, the creep strength is defined as good. In other words, a material is said to have higher creep strength than another if it exhibits a lower creep rate than the other material at a given applied stress.

[0057] The principles and advantages of the present FeCrAl alloy and the selection of the ranges of the alloying elements which renders the unexpected superiority may be described as follows below. The present disclosure is however not limited to the exemplifying embodiments discussed but may be varied within the scope of the appended claims. Upper and lower limits of the individual elements of the composition can be freely combined within the broadest limits set out in the claims, unless explicitly disclosed otherwise. Additionally, when ranges are disclosed in the present disclosure, such ranges include the respective end values of the range, unless explicitly disclosed otherwise. Similarly, when an open range is disclosed, the open range also include the single end value of the open range, unless explicitly disclosed otherwise.

[0058] In the present disclosure, the terms “wt%” and “weight%” are used interchangeably and means percent by weight.

[0059] Carbon (C)

[0060] Carbon may be included in the inventive alloy to increase strength by precipitation hardening. Carbon may also be present as an unavoidable impurity resulting from the production process. At too high levels, carbon may result in difficulties in forming the alloy to different objects and also a negative effect on the corrosion resistance. Therefore, the content of carbon in the inventive alloy is equal to or less than 0.3 wt%. According to embodiments, the content of C may be equal to or less than 0.25 wt%, such as equal to or less than 0.2 wt%. According to embodiments, the carbon content is in the range of 0.005 to 0.25 wt%, such as of 0.05 to 0.2 wt%. Nitrogen (N)

[0061] Nitrogen may be included in the inventive alloy to increase strength by precipitation hardening. Nitrogen may also be present as an unavoidable impurity resulting from the production process. At too high levels, nitrogen may have a negative effect on the corrosion resistance. Therefore, the maximum amount of nitrogen is equal to or less than 0.2 wt%. According to the present alloy, the N content may be equal to or less than 0.1 wt%, such as equal to or less than 0.05 wt%. According to embodiments, the content of N is of 0.001 to 0.05 wt%.

[0062] Chromium (Cr)

[0063] Chromium promotes the formation of the AI2O3 layer on the inventive alloy, through the so- called third element effect, by formation of chromium oxide in the transient oxidation stage. Chromium shall be present in the inventive alloy in an amount of at least 9 wt%. However, since the inventive alloy is a ferritic alloy, chromium should not exceed 12 wt% to avoid embrittlement when subjected to a temperature in the range of 450 - 500 °C over longer periods. Most FeCrAl alloys rely on a high level of Cr to ensure a good oxidation resistance by aiding the formation of a protective alumina film on the surface. The high Cr however also causes embrittlement when the material is used at around 450 to 500°C. The inventive alloy has a good oxidation resistance despite having sufficiently low Cr level to avoid this embrittlement.

[0064] According to the present alloy, the content of chromium is therefore of 9 to 12 wt%, such as of 10 to 12 wt% or such as 9.5 to 11.5 wt%. or such as 9 to 11 wt%.

[0065] Aluminium (Al)

[0066] Aluminium is an important element in the inventive alloy since aluminium, when exposed to oxygen at high temperature, forms a dense and thin oxide of AI2O3 protecting the underlying alloy surface from further oxidation. The amount of aluminium in the present FeCrAl alloy should be at least 3 wt% to ensure that the AI2O3 layer is formed, and that sufficient aluminium is present to heal the AI2O3 layer when damaged. However, aluminium has a negative impact on the formability and high amounts of aluminum may result in the formation of cracks during mechanical working. Consequently, the amount of aluminium should not exceed 6 wt% in the inventive alloy. The content of aluminum is therefore in the range of 3 to 6 wt%, such as of 3 to 5 wt%, such as 3.5 to 4.5 wt%.

[0067] Silicon (Si)

[0068] Silicon may be present as an impurity in the inventive alloy up to 0.7 wt% or may be deliberately added to the present alloy in higher levels to further enhance the oxidation resistance. Silicon has however a negative impact on the formability and high amounts of silicon may result in formation of cracks in the alloy during mechanical working. Consequently, the amount of silicon should be equal or less than 1.5 wt%, or example, silicon may be equal or less than 1.2 wt%, such as 1.0 wt%.

[0069] According to one embodiment, silicon is an impurity and present in the alloy in an amount of equal to or less than 0.7 wt%.

[0070] According to one embodiment, silicone is present in the alloy in a content of higher than 0.7 to 1.5 wt%, such as higher than 0.7 to 1.2 wt%.

[0071] Manganese may be present as an impurity in the inventive alloy up to 0.5 wt%. The content of Mn should be as low as possible due to its reduction of oxidation resistance.

[0072] Molybdenum (Mo)

[0073] Molybdenum may be present in the present alloy as an impurity and then the limit is equal to or less than 0.50 wt%, such as equal to or less than 0.20 wt%.

[0074] According to embodiments, Mo may also be added deliberately in a range of more than 0.5 to 3.5 wt% as it will have positive effects on the hot-strength and on the corrosion resistance. In order to ensure the effects of hot strength due to the solid solution strengthening and / or precipitation strengthening, Mo may be present in the range of 1.0 to 4.0 wt%. According to one embodiment, Mo is deliberately added, and the content is then in the range of more than 0.5 to 3.5 wt%, such as 1.0 to 3.5 wt%.

[0075] Tantalum (Ta)

[0076] The inventors have surprisingly, as mentioned above, found that a Ta content of at least 0.6 wt% will have a significant impact on the creep strength by increasing it. It has been shown that by having a Ta content in the range of 0.6 to 4.5 wt%, such as in the range of 0.6 to 4 wt%, the creep strength will be greatly improved which means that the objects containing the FeCrAl alloy as defined hereinabove or hereinafter will be able to carry load even in high temperature applications when used in different applications.

[0077] Without being bound to any theory, it is believed that the increase in creep strength is due to precipitation strengthening by formation of Ta-rich phases. The precipitation ideally occurs as fine particles both along the grain boundaries and within the grains.

[0078] The precipitate phase will remain stable up to at least 1200°C, and these precipitates will have a grain boundary pinning effect which limits grain coarsening. The common general knowledge states that a coarse grained microstructure is beneficial for creep strength, it is therefore very surprising that significant increase in creep strength will be achieved despite fine grain size.

[0079] However, too much Ta will cause very large intragranular precipitates, which will not contribute to creep strength but instead facilitate oxygen transport into the material which will have a negative effect on the oxidation resistance. Above 4.0 wt% Ta, this effect will become severe and have a negative effect on oxidation resistance as the precipitates will become too large and plentiful to remain discrete. At this point, internal oxidation can lead to heavy spallation of the protective oxide scale.

[0080] Hence, according to embodiments, the content of Ta is in the range of 0.6 to 4.0 wt%. However, in order to achieve the best creep strength, the content of Ta is in the range of 0.6 to 2.0 wt%, such as 0.8 to 2.0 wt%, such as 0.8 to 1.5 wt%.

[0081] Titanium (Ti\ Zirconium (Zr). and Hafnium (Hf)

[0082] One or more of the alloying elements titanium, zirconium and hafnium may be included in the present alloy in order to bind any free nitrogen in the alloy, which otherwise will affect corrosion resistance negatively. The content of each element may be equal to or less than 1.0 wt%, such as equal to or less than 0.8 wt%, such as equal to or less than < 0.5 wt%. According to embodiments, the content of Zr may be up to or equal to 0.5 wt%. According to embodiments, from 0.01 to 0.5 wt%. According to embodiments, the content of Hafnium may be up to or equal to 1.0 wt%. According to embodiments, the content of Hafnium may be from 0.001 to 1.0 wt%.

[0083] Niobium (Nb)

[0084] Niobium may be added to the present alloy for binding any free carbon, which otherwise affects corrosion resistance negatively. If present, Nb is present in an amount of equal to or less than 2.5 wt%, such as equal to or less than 2.0 wt%, equal to or less than < 1.0 wt%, equal to or less than < 0.5 wt%. According to embodiment, Nb is present in an amount of more than or equal to 0.001 wt%.

[0085] Yttrium (Y)

[0086] Yttrium may be added to improve the adherence of the AI2O3 layer. If yttrium is added, the content is equal to or less than 0.10 wt%, as above 0.10 wt% the formed yttrium oxides may cause embrittlement of the material. According to one embodiment, the content of yttrium is in impurity level such as less than 0.01 wt%. According to one embodiment, yttrium is purposively added and then in a range of 0.01 to 0.10 wt%.

[0087] Oxygen (O)

[0088] Oxygen may be added to achieve a precipitation hardening effect. The maximum content of oxygen is equal to or less than up to 0.05 wt%, such as of 0.02 - 0.05 wt%, such as 0.02 - 0.04 wt%. At too high levels, i.e., above 0.05 wt%, oxygen will result in brittleness. The balance of the FeCrAl alloy as defined hereinabove or hereinafter is iron (Fe) and unavoidable impurities.

[0089] The herein described alloy may, in addition to the elements already specified and discussed above, comprise up to at most 1.5 wt% in total of normally occurring impurities, such as less than or equal to 1.0 wt%, such as less than or equal to 0.8 wt% . In the present disclosure, normally occurring impurities are considered to be impurities resulting from the manufacturing process and / or the raw material used. Normally occurring impurities is herein intended to encompass both impurities and trace elements. Normally occurring impurities are for examples Cobalt (Co) and Nickel (Ni). Further, the herein described alloy may comprise normally occurring impurities of Vanadium (V). The content of V is limited to at most 0.1 wt%.

[0090] According to one embodiment, the present FeCrAl alloy consists or comprises of all the elements mentioned herein and in the different ranges as mentioned herein.

[0091] The present invention also relates to a manufacturing process for obtaining the alloy as defined hereinabove or hereinafter, the process comprises the steps of: a) melting; b) casting; c) hot working; examples of hot working are forging or hot rolling d) optionally cold working; examples of cold working are cold rolling or cold drawing; e) optionally heat treatment, example of heat treatment is annealing.

[0092] The present disclosure is further described in the following non-limiting examples.

[0093] EXAMPLES

[0094] The compositions in Table 1 were melted in a vacuum furnace, cast into ingots, and hot rolled into rods. These rods were then cold drawn to 04 mm and annealed prior sampling for creep testing.

[0095] Table 1. The compositions of the alloys. The balance is Fe and unavoidable impurities. The alloys with are within the present disclosure.

[0096] Figure la shows a SEM image of Example No 2, having 0.3 wt% Ta and Figure lb shows a SEM image of Example No. 3 having 0.6 wt% Ta and Figure 1c shows a SEM image of Example No. 11 having 1.1 wt% Ta. As can be seen from the images, the FeCrAl alloy with 1.1 wt% Ta, Example No. 11 will have grain boundaries decorated with precipitates, and an abundance of homogeneously distributed fine particles in the grains in addition to the coarse intragranular precipitates. The fine and coarse intragranular particles are of different phases but are rich in Ta. Thus, Figure la shows a microstructure of a FeCrAl alloy (Example 2) containing Ta but the Ta content is less than 0.6 wt%, i.e., outside the scope of the present disclosure and as is shown in Figure 2, this FeCrAl alloy has no increase in creep strength. Figure lb shows the microstructure of a FeCrAl alloy (Example 3) containing Ta within the claimed range and as can be seen from Figure 2, an increased creep strength can be observed. Precipitates are still predominantly intragranular and coarse, but the number density is higher. Figure 1c shows an FeCrAl alloy (Example 11) having a good microstructure for creep strength. A good microstructure is characterized by very fine particles which densely decorate the grain boundaries, and also a homogeneous and high density of fine intragranular particles in addition to the coarse intragranular particles. As shown in Figure 2, such an FeCrAl alloy has excellent creep strength.

[0097] The minimum creep rates for several compositions containing a variation of Ta levels are shown in Figure 2. The creep tests were performed on cold drawn and annealed wire at 1000°C in air, with 2.50 MPa applied in uniaxial tension using dead weights. As can be seen from Figure 2, the alloy containing 0.6 wt% Ta or more show lower creep rates than normal. The creep rate drops drastically with increasing Ta up to around 1.0 wt%, to a creep rate of two to three orders of magnitude lower than what is typical for a similar alloy without Ta-addition.

[0098] The creep strength of conventionally manufactured FeCrAl alloys is often a limiting material property. It limits the design space in some cases, whereas other cases must resort to FeCrAl alloys manufactured using an alternate process to achieve sufficient form stability, e.g., via powder metallurgy. The inventive alloy improves the creep strength of conventionally manufactured FeCrAl significantly, which allows for expanded design space as well as new applications. Specifically, it opens up the use of conventionally manufactured FeCrAl as construction material for high temperature applications.

[0099] Oxidation testing

[0100] Oxidation testing was performed by exposing specimens of Example 11 and other alloy namely 253 SMA an alloy of tradename Nikothal® 80, and an alloy of tradename Kanthal® D to air at a temperature of 1200 °C in 100 h cycles. The specimens were cuboids with dimensions 20x10x2 [mm]. To ensure that all samples had a comparable starting surface condition, all surfaces were ground using 500 grit SiC abrasive paper, and all samples were cleaned using ultrasonic cleaning and ethanol rinsing shortly before the test. The samples were put in individual alumina crucibles and put into a pre-heated furnace together and taken out together at the end of each 100-h cycle. The samples were weighed together with their respective crucible using a precise scale before the test and after each exposure cycle. The gross mass gain, shown in FIGURE 3, thus represents the accumulated oxide formation during the test. Lower values mean better oxidation resistance and as can be seen from Figure 3, the inventive alloy had as good oxidation resistance as alloys which are known to have good oxidation resistance, namely the alloy sold under tradename Nikothal® 80 and Kanthal® D.

Claims

CLAIMS1. An iron chromium aluminium, FeCrAl, alloy comprising in weight %, wt%,C equal to or less than 0.3;N equal to or less than 0.2 ;Ta 0.6 - 4.5;Cr 9 - 12;Al 3 - 6;Si equal to or less than 1.5;Mn equal to or less than 0.5;Mo equal to or less than 3.5;Nb equal to or less than 2.5;Ti equal to or less than 1.0;Zr equal to or less than 1.0;Hf equal to or less than 1.0;Balance Fe and unavoidable impurities.

2. The FeCrAl alloy according to claim 1, wherein the content of C may be equal to or less than 0.25 wt%.

3. The FeCrAl alloy according to claim 1 or claim 2, wherein the content of N may be equal to or less than 0.1 wt%.

4. The FeCrAl alloy according to anyone of claims 1 to 3, wherein the content of Cr is of 9 to 11 wt%.

5. The FeCrAl alloy according to anyone of claims 1 to 4, wherein the content of Al is of 3 to 5 wt%.

6. The FeCrAl alloy according to anyone of claims 1 to 5, wherein the Si is present as animpurity.

7. The FeCrAl alloy according to anyone of claims 1 to 5, wherein the Si is purposively added in an amount of above 0.7 to 1.5 wt%.

8. The FeCrAl alloy according to anyone of claims 1 to 7, wherein Mo is present as an impurity.

9. The FeCrAl alloy according to anyone of claims 1 to 7, wherein Mo is in the range of more than 0.5 to 3.5 wt%,10. The FeCrAl alloy according to claims 1 to 9, wherein the content of Ta is in the range of 0.8 to 2.0 wt%, such as 0.8 to 1.5 wt%.

11. A conventional process used for manufacturing the FeCrAl alloy according to anyone of claims 1 to 10, comprising the steps of: a) melting; b) casting; c) hot working; d) optionally cold working; and e) optionally heat treatment.

12. An object comprising the FeCrAl alloy according to anyone of claims 1 to 11.

13. A coating comprising the FeCrAl alloy according to anyone of claims 1 to 11.

14. Use of an object or a coating according to claim 12 or claim 13 in molten lead.

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