FE-CR-NI-AL HIGH NICKEL CONTENT REFRACTORY AUSTENITIC STEEL

Abstract

A refractory austenitic alloy suitable for use at temperatures greater than or equal to 1100? C., comprises the following elements in percent by weight: chromium between 25.0% and 32.0%, nickel between 50.0% and 61.0%, aluminum between 1.0% and 6.0%, niobium between 0.15% and 1.50%, carbon between 0.05 and 0.60%, one or more reactive elements in a total content of 0.060% or less, silicon at 0.30% or less, manganese at 0.30% or less, titanium at 0.40% or less, nitrogen at 0.20% or less, vanadium at 1.0% or less, iron between 4.0% and 18.0%, for balancing the elements of the alloy, zirconium, tungsten and sulfur being absent from the alloy, or in the form of impurities. The alloy also satisfies two criteria connecting the percentages by weight of at least some of the elements of the alloy.

Claims

1. A refractory austenitic alloy, comprising the following elements in percentage by weight: chromium between 25.0% and 32.0%, nickel between 50.0% and 61.0%, aluminum between 1.0% and 6.0%, niobium between 0.15% and 1.50%, carbon between 0.05% and 0.60%, one or more reactive elements in a total content of between 0.010% and 0.060%, a reactive element being defined as one of rare earth or hafnium, silicon at 0.30% or less, manganese at 0.30% or less, titanium at 0.40% or less, nitrogen between 0.015% and 0.20%, vanadium between 0.005% and 1.0%, iron between 4.0% and 18.0%, to balance the alloy elements, zirconium, tungsten and sulfur being absent from the alloy, or in the form of impurities, respectively, at less than 0.030% zirconium, less than 0.010% tungsten, and less than 0.0060% sulfur, the alloy further satisfying a first criterion and a second criterion connecting the percentages by weight (x.sub.Cr, x.sub.Al, x.sub.C, x.sub.Si, x.sub.Mn, x.sub.Ti, x.sub.Nb, x.sub.N, x.sub.V, x.sub.S, x.sub.Ni) of at least some of the elements of the alloy; the first criterion defined by: $\left(1-K_{Al}-K_S\right)?\left[\frac{2}{e^{\left(\frac{26-x_{\mathrm{Cr}}}{0.26}\right)}+1}\right]+K_{Al}?\left[\frac{2}{e^{\left(\frac{2-x_{\mathrm{Al}}}{0.4}\right)}+1}\right]+K_S?\left[\frac{2}{e^{\left(\frac{x_{S^{-0.003}}}{0.1?x_S}\right)}+1}\right]?1\mathrm{with}{K}_{Al}=0.1728+0.1293?\ln \left(x_{Al}\right)\mathrm{and}{K}_S=0.3089?e^{\left(64x_S\right)};$ and the second criterion defined by:
?17.64+19.61x.sub.Al?1.29x.sub.Al.sup.2?101.46x.sub.N+450.65x.sub.N.sup.2?5.8368x.sub.N.sup.3+9.68x.sup.V+43.12x.sub.Ti+30.02x.sub.Si+11.42x.sub.Ni?0.18x.sub.Ni.sup.2+35.05x.sub.Nb+47.92x.sub.Cr?0.34x.sub.Cr.sup.2+13.97x.sub.Mn?239.66x.sub.C?1070.

2. The refractory austenitic alloy of claim 1, wherein the percentage by weight of vanadium is greater than or equal to 0.010%.

3. The refractory austenitic alloy of claim 2, wherein the percentage by weight of vanadium is greater than or equal to 0.10%.

4. The refractory austenitic alloy of claim 1, wherein the percentage by weight of aluminum is greater than or equal to 2.0%.

5. The refractory austenitic alloy of claim 4, wherein the percentage by weight of aluminum is greater than or equal to 2.50%.

6. The refractory austenitic alloy of claim 1, wherein the percentage by weight of sulfur is less than 0.0050%.

7. The refractory austenitic alloy of claim 6, wherein the percentage by weight of sulfur is less than 0.0020%.

8. The refractory austenitic alloy of claim 7, wherein the percentage by weight of sulfur is less than 0.00050%.

9. The refractory austenitic alloy of claim 1, wherein the percentage by weight of nitrogen is greater than or equal to 0.060%.

10. The refractory austenitic alloy of claim 9, wherein the percentage by weight of nitrogen is greater than or equal to 0.10%.

11. The refractory austenitic alloy of claim 10, wherein the percentage by weight of nitrogen is greater than or equal to 0.12%.

12. The refractory austenitic alloy of claim 1, wherein the percentage by weight of chromium is between 26% and 31%.

13. The refractory austenitic alloy of claim 1, wherein the percentage by weight of carbon is greater than or equal to 0.16%.

14. The refractory austenitic alloy of claim 13, wherein the percentage by weight of carbon is greater than or equal to 0.25%.

15. The refractory austenitic alloy of claim 14, wherein the percentage by weight of carbon is greater than or equal to 0.35%.

16. The refractory austenitic alloy of claim 1, wherein the total percentage by weight of reactive elements is greater than or equal to 0.020%.

17. The refractory austenitic alloy of claim 2, wherein the percentage by weight of aluminum is greater than or equal to 2.0%.

18. The refractory austenitic alloy of claim 17, wherein the percentage by weight of sulfur is less than 0.0050%.

19. The refractory austenitic alloy of claim 18, wherein the percentage by weight of nitrogen is greater than or equal to 0.060%.

20. The refractory austenitic alloy of claim 19, wherein the percentage by weight of chromium is between 26% and 31%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Other features and advantages of the present disclosure will emerge from the following detailed description of the present disclosure with reference to the appended figures, in which:

[0035] FIG. 1 shows a composition table of the alloy according to the present disclosure;

[0036] FIG. 2 shows a table comprising nine examples of refractory austenitic alloys, the alloys numbered 1 to 4 forming part of the alloys in accordance with the present disclosure;

[0037] FIG. 3 shows eight images in cross-section by optical microscopy of alloys 1 to 8, after they have been subjected to a heat treatment of accelerated aging at 1150? C. for 125 h;

[0038] FIG. 4 shows two images in cross-section by scanning electron microscopy of alloys 1 and 6, after they have been subjected to a heat treatment at 1150? C. for 125 h, and two EDS analyses (energy-dispersive spectroscopy chemical analysis) of these two alloys;

[0039] FIG. 5A shows the weight gain of alloys 2 and 5, having been subjected, respectively, to 20 and 10 oxidation cycles of 45 min at 1150? C.;

[0040] FIG. 5B shows two images in cross-section by electron microscopy of alloys 2 and 5, having, respectively, undergone 20 cycles of oxidation of 45 min at 1150? C. (alloy 2, a and c) and 10 cycles (alloy 5, b and d);

[0041] FIG. 6A shows the creep performance of the alloys described in FIG. 2, in the form of an LMP representation (Larson-Miller parameter); and

[0042] FIG. 6B shows the creep performance at high LMP of alloys 1 to 8 described in the table of FIG. 2; the grades were tested at low stress (9 MPa) and at temperatures of 1150? C. and 1175? C.

DETAILED DESCRIPTION

[0043] The present disclosure relates to a refractory austenitic alloy intended to be used at an operating temperature greater than or equal to 1100? C. In particular, the present alloy can be used for reforming furnaces, which are subjected to refractory brick temperatures typically between 1100? C. and 1185? C.

[0044] The austenitic alloy according to the present disclosure comprises all the following elements in percentage by weight: [0045] chromium between 25.0% and 32.0%, [0046] nickel between 50.0% and 61.0%, [0047] aluminum between 1.0% and 6.0%, [0048] niobium between 0.15% and 1.50%, [0049] carbon between 0.05% and 0.60%, [0050] one or more reactive elements at 0.060% or less, [0051] silicon at 0.30% or less, [0052] manganese at 0.30% or less, [0053] titanium at 0.40% or less, [0054] nitrogen at 0.20% or less, [0055] vanadium at 1.0% or less, [0056] iron between 4.0% and 18.0%, to balance the alloy elements.

[0057] Throughout the remainder of the description, the expressions content, quantity or percentage in terms of a compound of the alloy should be interpreted as relating to the percentage by weight of the compound. When a percentage by weight is indicated between X and Y, X and Y constituting the limits of the composition range, the limits should be considered as included in the range unless otherwise specified.

[0058] The refractory austenitic alloy according to the present disclosure is mainly composed of nickel (between 50.0% and 61.0%), chromium (between 25.0% and 32.0%), iron (between 4.0% and 18.0%) and aluminum (between 1.0% and 6.0%).

[0059] A minimum of 25.0% of chromium is required to ensure good resistance to corrosion (oxidation) and to allow the formation of chromium carbides, which favorably impact the creep resistance of the alloy. The maximum percentage of chromium is limited to 32.0%, in particular in order to limit the excessively strong integration of an alphagenic element tending to destabilize the austenitic structure of the alloy. Advantageously, the Cr content is defined between 26.0% and 31.0% in order to further promote creep resistance and protection of the alloy from the environment.

[0060] The minimum content of nickel is defined at 50.0% so as to retain a refractory alloy of austenitic structure, since the alloy contains at least 25.0% chromium as well as other alphagenic elements that tend to destabilize the austenitic structure in favor of a terrific structure. The quantity of nickel is limited to 61.0%, or even limited to 57.0%, or even 55.0% for economic reasons, the nickel being a high cost contributor.

[0061] The iron percentage by weight balances the elements of the alloy, so that the sum of the percentage by weight of the elements reaches 100%. A content of between 4.0% and 18.0% balances the other more advantageous elements. Preferentially, an iron content greater than or equal to 13.0% is desirable in order to reduce the costs of the grade.

[0062] Aluminum is present in the alloy at a medium-to-high content between 1.0% and 6.0%. Such a content allows the formation of a continuous layer of aluminum oxide (alumina), at the surface of the alloy, in a wide range of partial pressure of oxygen (ranging from less than 5 particles per million at high partial pressures such as in air), and a wide range of temperatures (typically, temperatures above 1000? C.). The surface layer of aluminum oxide then forms a barrier that is very resistant and effective against corrosion (oxidation, carburization, nitriding) of the alloy, at high temperatures, typically 1100? C. and above.

[0063] Advantageously, the percentage by weight of aluminum is greater than or equal to 2.0%, or even greater than or equal to 2.5%. A higher aluminum content ensures the formation of an aluminum oxide layer in a wider range of environmental conditions. It also makes it possible to have access to a larger reservoir of aluminum and thus to preserve the properties of the alloy over longer durations, in very severe environments where the layers of aluminum oxides are consumed.

[0064] It may be advantageous to maintain the percentage by weight of aluminum at or below 4%, to limit the precipitation of intermetallic phases B2-NiAl, likely to adversely affect the creep properties. As a reminder, B2 according to the Strukturbericht notation describes a phase comprising two types of atoms (here, Ni and Al) in an equal proportion and whose crystallographic structure is interpenetrated primitive cubic, that is, each of the two types of atoms forms a simple centered cubic network, with an atom of one type at the center of each cube of the other type.

[0065] The carbon must be present in the alloy for its hardening effect, by precipitation and by solid solution. The range of carbon in percentage by weight is defined between 0.05% and 0.60%. Advantageously, a percentage greater than or equal to 0.16%, or even 0.25%, or even 0.35% allows the formation of a large volume fraction of carbides and improves the flowability of the alloy.

[0066] The niobium content of the alloy is defined between 0.15% and 1.50% in order to establish the carbon in the form of niobium- and/or titanium-rich carbonitrides. Advantageously, the niobium, in combination with the titanium, prevents the formation of phase G, silicon-rich phase, unfavorable to creep properties. Preferentially, the niobium content is greater than or equal to 0.2%, to 0.4%, to 0.5%, to 0.8%, or even to 1%; and the niobium content is less than or equal to 1.4%, to 1.3%, or even to 1.2%.

[0067] A reactive element within the meaning of the present disclosure is defined as one of rare earth or hafnium. Adding at least one reactive element (such as cerium, yttrium, etc., or hafnium) is beneficial to the growth, adhesion and protective character of the alumina layer. Although this or these element(s) promote the fragmentation of the network of chromium carbides, nevertheless they have a beneficial effect with respect to creep resistance. A total content (sum of the contents of all the reactive elements introduced) greater than 0.060% does not provide an additional effect, whereas it involves a strong impact on the cost and on the eco-responsible nature of the material. A minimum content of 0.010% is required to obtain the aforementioned benefits. Advantageously, the total percentage by weight of reactive elements is chosen greater than or equal to 0.020%.

[0068] The alloy further contains silicon, in order to improve flowability and increase resistance to corrosion. The quantity of this element is nevertheless limited to 0.30%, or even to 0.25%, in order to avoid the presence of G and ? phases (intermetallic phase comprising Fe, Cr, Ni and Si), harmful to creep. Advantageously, the Si content is between 0.01% and 0.20%, or even between 0.05% and 0.20%.

[0069] Manganese is also present in the alloy, to improve weldability and for its beneficial effect on oxidation because it acts as a trap for sulfur. It also has a beneficial effect on creep since it increases the solubility of the nitrogen in the austenite and promotes the stability of the austenitic structure. However, its content is limited to 0.30% in order to limit the formation of the B2-NiAl intermetallic phase, which negatively impacts the creep resistance. Advantageously, the manganese content is between 0.05% and 0.25%, or even between 0.05% and 0.20%, or even between 0.01% and 0.20%.

[0070] The alloy comprises vanadium, up to a percentage by weight of 1.0%. This compound is known to improve the creep properties of stainless austenitic steels by its impact on the precipitation of chromium carbides, by increasing their volume fraction. Vanadium also helps the precipitation of carbonitrides rich in niobium, titanium and/or vanadium, during aging, and it also has a hardening effect by solid solution. Its content must be limited to 1.0% in order to maintain its beneficial effects and to avoid degradation of the behavior in oxidation of the grade. Advantageously, the vanadium content is between 0.005% and 0.5%; it may optionally be greater than or equal to 0.010%, or even greater than or equal to 0.1%.

[0071] The titanium promotes the formation of intra-granular fine carbonitrides and their subsequent evolution during aging (favorable to creep resistance). It may be included in the alloy as a percentage by weight ranging up to 0.40%. Advantageously, the percentage by weight of titanium is greater than 0.05%.

[0072] The alloy also contains nitrogen, which, by its gamma-phase producing character (stabilizes the austenitic structure), improves the creep properties. Its presence in the alloy also contributes to the formation of carbonitrides rich in niobium, titanium and/or vanadium, which reinforce the creep properties. Its content is limited to 0.20% in order to avoid the formation of phases unfavorable to the creep and oxidation properties. Advantageously, the percentage by weight of nitrogen is greater than or equal to 0.015%, preferentially greater than or equal to 0.040%, to 0.045%, to 0.048%, to 0.060%, even more preferentially greater than or equal to 0.10%, or even preferentially greater than or equal to 0.12%.

[0073] Sulfur is an undesirable element in the alloy, but may be in trace form (impurity) in the grade. It is desirable to limit the presence of this element in order to degrade as little as possible the protective character of the alumina layer. Sulfur can therefore be present in the alloy but at contents strictly less than 0.0060% (that is <60 ppm). Advantageously, the sulfur content is less than 0.0050% (<50 ppm), or even less than 0.0020% (<20 ppm), preferentially less than 0.00050% (<5 ppm).

[0074] Other elements may optionally be found in trace form in the alloy, such as, for example, zirconium (<0.03%), tungsten (<0.01%), cobalt (<0.08%), molybdenum (<0.2%), copper (<0.05%) or tantalum (<0.02%), but they are not deliberately introduced into the alloy; their potential presence being related to the fact that these elements can be found as impurities in the fillers incorporated during the manufacture of the alloy.

[0075] The alloy possibly may be polluted by trace impurities such as phosphorus, lead, tin, boron, magnesium or arsenic, the content of which is of the order of one particle per million (ppm) and is strictly less than 200 ppm.

[0076] It should be noted that the composition of the alloy can be measured by spark spectrometry.

[0077] The table of FIG. 1 shows the composition of the austenitic alloy according to the present disclosure. The austenitic alloy according to the present disclosure also respects two criteria connecting the percentages by weight (x.sub.Cr, x.sub.Al, x.sub.C, x.sub.Si, x.sub.Mn, x.sub.Ti, x.sub.Nb, x.sub.N, x.sub.V, x.sub.S, x.sub.Ni) of all or part of the elements of the alloy.

[0078] The first criterion is an oxidation criterion, determined empirically. It connects the chromium, aluminum and sulfur contents of the alloy. The equation is constructed around acceptable values of these three elements (26% for Cr, 2% for Al and 30 ppm for sulfur). This equation grants a different weight to each element according to the impact of its content on the resistance to oxidation at high temperature. For simplicity, the criterion was normalized and it must be greater than 1 in order to guarantee good oxidation behavior.

[0079] The first criterion is defined by:

[00002]$\left(1-K_{Al}-K_S\right)?\left[\frac{2}{e^{\left(\frac{26-x_{\mathrm{Cr}}}{0.26}\right)}+1}\right]+K_{Al}?\left[\frac{2}{e^{\left(\frac{2-x_{\mathrm{Al}}}{0.4}\right)}+1}\right]+K_S?\left[\frac{2}{e^{\left(\frac{x_{S^{-0.003}}}{0.1?x_S}\right)}+1}\right]?1$

with K.sub.Al=0.1728+0.1293?ln(x.sub.Al) and K.sub.S=0.3089?e.sup.(64x.sup.S.sup.)

[0080] The second criterion relates to the solvus temperature of a certain type of carbides, namely M.sub.23C.sub.6 carbides. A relationship was established between the percentages by weight of certain elements that are linked to the solvus temperature of the M.sub.23C.sub.6 carbides. This temperature must be high (namely greater than or equal to 1070? C.) in order to promote the secondary precipitation of the Cr (M.sub.23C.sub.6) carbides at operating temperatures and to guarantee optimal mechanical performance (creep resistance).

[0081] The second criterion is defined by:

?17.64+19.61x.sub.Al?1.29x.sub.Al.sup.2?101.46x.sub.N+450.65x.sub.N.sup.2?5.8368x.sub.N.sup.3+9.68x.sup.V+43.12x.sub.Ti+30.02x.sub.Si+11.42x.sub.Ni?0.18x.sub.Ni.sup.2+35.05x.sub.Nb+47.92x.sub.Cr?0.34x.sub.Cr.sup.2+13.97x.sub.Mn?239.66x.sub.C>1070? C.

[0082] As mentioned in the introduction, it is normal for a refractory austenitic alloy to form a decarburized layer and/or an internal oxidation layer, as a result of the evolution of the microstructure near the surface due to the very high operating temperatures. This phenomenon, linked to the capacity of the alloy to be self-protected from the environment, has a significant impact on the service life of these alloys at these temperatures.

[0083] Thus, going beyond the role of each individual compound of the alloy, the link between the microstructure of the alloy, its resistance to oxidation and its mechanical properties at operating temperatures typically greater than or equal to 1100? C. has been studied. The operating temperature is the temperature at which the alloy is intended to be subjected, during use thereof: for example, for an alloy forming a reformer tube in an installation for direct reduction of iron ore, the operating temperature may be between 1050? C. and 1175? C.

[0084] The studies carried out, in particular based on characterizations by optical microscopy, scanning electron microscopy (SEM) and on creep tests, have made it possible to demonstrate the fact that the creep properties of the alloy with a high nickel content (greater than or equal to 50%) are directly impacted by its oxidation behavior and by the precipitation of secondary M.sub.23C.sub.6 type carbides rich in chromium at the operating temperature.

[0085] Thus, it was determined that, in an austenitic alloy with a high nickel content, the creep resistance, at the operating temperature, can achieve exceptional performance when it has not only a favorable microstructure for creep resistance but also a very good resistance to oxidation at the temperature, hence the definition of the two criteria stated above. This synergistic effect is particularly true for the very high operating temperatures targeted and represents an important feature of embodiments of the present disclosure. While a microstructure optimized for creep resistance is a necessary condition but not sufficient for high resistance to creep at very high temperatures (>1100? C.), it is found that the capacity of the grade to be self-protected from the environment plays a crucial role and is also necessary (criterion 1).

[0086] A favorable microstructure in this case means that, at the operating temperature, the chemical composition of the alloy must be such that the solvus temperature of the M.sub.23C.sub.6 carbides is equal to or greater than 1070? C., in order to promote the secondary precipitation of the carbides from the M.sub.7C.sub.3 carbides present in the alloy as a raw casting.

[0087] From correlations between the physical characterizations and CALPHAD simulations (calculations of phase diagrams, making it possible to predict the phases present in the alloy at equilibrium temperature, depending on its composition), an R2 relationship has been established between the percentages by weight of certain elements of the alloy and the maximum temperature T.sub.max.sup.M23C6 of the stability range of the M.sub.23C.sub.6 chromium carbides phase:

[0088] [R2]

[00003]$T_{\max }^{M23C6}\left(?C.\right)-17.64+19.61x_{Al}-1\text{.29}{x}_{\mathrm{Al}}^2-101.46x_N+450.65x_N^2-5.8368x_N^3+9.68x_V+43.12x_{Ti}+30.02x_{Si}+11.42x_{\mathrm{Ni}}-0\text{.18}{x}_{\mathrm{Ni}}^2+35.05x_{\mathrm{Nb}}+47.92x_{\mathrm{Cr}}-0.34x_{\mathrm{Cr}}^2+13.97x_{\mathrm{Mn}}-239.66x_C$

with x.sub.Al, x.sub.N, x.sub.V, x.sub.Ti, x.sub.Si , x.sub.Ni, x.sub.Nb, x.sub.Cr, x.sub.Mn, x.sub.C being the percentages by weight, respectively, of Al, N, V, Ti, Si, Ni, Nb, Cr, Mn and C in the alloy.

[0089] The maximum temperature of the stability domain can be seen as the limit temperature below which there is transformation in the alloy of the M.sub.7C.sub.3 carbides (present in the raw casting-state alloys) into M.sub.23C.sub.6 carbides; this transformation leads to a secondary precipitation of the desired chromium carbides, which improves the creep performance of the alloy. Such a transformation takes place over a range of temperatures corresponding to the stability domain of the M.sub.23C.sub.6 phase.

[0090] According to the present disclosure, the maximum temperature T.sub.max.sup.M23C6 must be greater than or equal to 1070? C. in order to promote the secondary precipitation in the alloy subjected to the operating temperature, during its use. This condition corresponds to the second criterion. Advantageously, the maximum temperature T.sub.max.sup.M23C6 may be defined as greater than or equal to 1100? C., or even greater than or equal to 1150? C.

[0091] As stated above, this R2 relationship is only valid and relevant for an alloy having main elements (Cr, Ni, Al, Nb, C, Si, Mn, Ti, Fe, N, V) in the ranges of percentages by weight defined according to the present disclosure.

[0092] Validation of the second criterion, linked to the maximum temperature of the stability domain of the secondary M.sub.23C.sub.6 carbides is, however, not sufficient to ensure optimal creep performance at the operating temperature.

[0093] The alloy must also exhibit excellent resistance to oxidation. Three elements, chromium, aluminum and sulfur, play a crucial role in the ability of the alloy to self-protect. On the basis of correlations between the physical characterizations and the chemical composition, an R1 relationship was established:

[0094] [R1]

[00004]$f_{\mathrm{oxy}}=\left(1-K_{Al}-K_S\right)?\left[\frac{2}{e^{\left(\frac{26-x_{\mathrm{Cr}}}{0.26}\right)}+1}\right]+K_{Al}?\left[\frac{2}{e^{\left(\frac{2-x_{\mathrm{Al}}}{0.4}\right)}+1}\right]+K_S?\left[\frac{2}{e^{\left(\frac{x_{S^{-0.003}}}{0.1?x_S}\right)}+1}\right]$

with K.sub.Al=0.1728+0.1293?ln(x.sub.Al) and K.sub.S=0.3089?e.sup.(64x.sup.s.sup.)

[0095] The term f.sub.oxy is an oxidation function and x.sub.Cr, x.sub.Al and x.sub.S are the percentages by weight, respectively, of Cr, Al and S in the alloy.

[0096] Advantageously, the oxidation function f.sub.oxy must be greater than 1 in order to guarantee good oxidation behavior of the alloy subjected to the operating temperature, and to synergistically optimize the creep resistance of the alloy during its use. The condition f.sub.oxy?1 corresponds to the first criterion according to the present disclosure.

[0097] Examples of alloys will now be presented, in order to show how the composition ranges according to the present disclosure, combined with the aforementioned two criteria make it possible to obtain a nickel-rich, alumina-forming refractory austenitic alloy, particularly efficient in terms of resistance to oxidation and creep resistance, at operating temperatures greater than or equal to 1100? C.

[0098] Performance tests relate to the resistance of the alloys to accelerated aging, cyclical oxidation, and creep resistance.

[0099] The table in FIG. 2 shows various alloys that have been studied. Alloys 1 to 4 are in accordance with the present disclosure. Alloys 5 to 9 are counterexamples that do not satisfy all the features of the present disclosure.

[0100] FIG. 3 shows images in optical microscopy cross-section of alloys 1 to 8 after they have been subjected to a heat treatment of accelerated aging at 1150? C. for 125 h. The scale on these images is 50 ?m.

[0101] A dendritic structure is observed with an array of M.sub.7C.sub.3 and/or M.sub.23C.sub.6 type chromium carbides located at the inter-dendritic spaces as well as on the surface of the samples. Note that the surface was protected with a deposition of copper in the cases of alloys 1, 2 and 6, this deposition having a clear contrast on the optical microscopy images and is observable in the form of islands spaced apart at the surface.

[0102] The chromium-rich carbide network is entirely present up to the surface of the samples of alloys 1, 2, 3, 4 and 7. On the contrary, a free layer of chromium carbides near the surface of alloys 5 and 8, as well as an internal oxidation layer, are observed. In the case of alloy 6, the width of the decarburized layer is such that, on the image, the network of chromium carbides is not observed; on the other hand, a significant internal oxidation layer is observed.

[0103] The large black contrast objects formed inside the sample of alloys 5, 6 and 8 are aluminum nitrides.

[0104] The microstructures of alloys 1 and 6, observed by scanning electron microscope, are shown in FIG. 4 ((a) and (b)) and were analyzed chemically by energy dispersive spectroscopy (EDS), with the results also illustrated in FIG. 4 ((c) and (d)).

[0105] It can be seen that alloy 1 has formed a protective alumina layer on the surface. The aluminum signal obtained by EDS shows a peak at the surface (see FIG. 4 (c)) and the chromium profile (FIG. 4 (d)) shows a monotonic nominal concentration with peaks that correspond to the presence of chromium carbides.

[0106] Alloy 6, which does not meet the first criterion, f.sub.oxy?1, formed a chromium oxide layer (Cr.sub.2O.sub.3) on the surface that led to depletion of chromium in the area near the surface (see profile in FIG. 4 (d)). Just below this layer of chromia, a non-protective layer of aluminum oxide (FIG. 4 (c)) can be observed.

[0107] FIG. 5A shows the evolution in weight of alloys 2 and 5 during cyclical oxidation. The graph shows the number of cycles on the abscissa, with a cycle corresponding to the sequence: 45 min at 1150? C. and 15 min at ambient temperature. In addition to the evolution in weight, FIG. 5B shows cross-sectional images of these same alloys, having undergone 20 oxidation cycles in the case of alloy 2 (FIG. 5B (a) and (c)) and 10 cycles in the case of alloy 5 (FIG. 5B (b) and (d)), at two different magnifications.

[0108] It can be seen that the high sulfur content combined with limited contents of chromium and aluminum in a refractory alloy limits the ability of the alloy to self-protect from oxidation. The weight gain of alloy 5 (FIG. 5B) is the consequence of internal oxidation (FIG. 5B (b) and (d)). Beyond cycle 3, a slight loss of weight, probably due to flaking, is observed in alloy 5. Alloy 2, in accordance with the present disclosure, shows a stability of weight with cyclical oxidation, once the protective alumina layer has formed.

[0109] The creep resistance of alloys 1 to 9 was evaluated from creep tests at 1050? C., 1100? C., 1125? C., 1150? C. and/or 1175? C., subject to stresses of 17, 16.5, 13, 11.5, 10 and 9 MPa, with the tests being carried out on samples taken from parts produced in the various alloys. A time to rupture t.sub.R is extracted from these tests, that is transformed into Larson-Miller (LMP) in combination with the temperature of the test according to the following expression:

LMP=1000/T?(log t.sub.R+C)

[0110] T being the temperature of the test expressed in kelvin, t.sub.R the time to rupture expressed in hours and C a constant characteristic of the alloy; in this case C=20.22.

[0111] The representation of the results of the creep tests under the Larson-Miller formalism makes it possible to compare the performance of the tests carried out at different temperatures. FIG. 6A shows the results of the creep tests on alloys 1 to 9. The graph shows the applied stress on the ordinate and the Larson-Miller parameter on the abscissa. Typically, the conditions of the high LMP tests correspond to low stresses and high temperatures, whereas at low LMP, they correspond to significant stresses and lower temperatures.

[0112] It is possible to observe a higher performance (especially at high LMP) of alloys 1 to 4, in accordance with the present disclosure, compared to alloys 5 to 9.

[0113] FIG. 6B shows in detail the results of the creep tests carried out at 9 MPa and at temperatures of 1150? C. and 1175? C. on alloys 1 to 8. Alloys 1 to 4 reach an LMP value greater than 33.32, a relevant performance threshold for such a refractory austenitic alloy.

[0114] All these results emphasize differences in performance that refractory alloys with high nickel content, with very close compositions, may have in terms of creep resistance at very high temperatures (alloys 1 to 4 versus alloys 5 to 9). In addition to precise ranges of composition, two important criteria have been defined that the alloy must respect so as to offer the best creep performance combined with excellent resistance to cyclical oxidation, for operating temperatures greater than or equal to 1100? C. One originality of this approach comes from taking into account two distinct phenomena (oxidation factor and solvus temperature of M.sub.23C.sub.6 carbides) having a beneficial synergistic action on the mechanical performance (creep) of the alloy while ensuring a remarkable corrosion protection.

[0115] Of course, the present disclosure is not limited to the embodiments described and it is possible to add alternative embodiments thereto without departing from the scope of the present disclosure as defined by the claims.