HEAT-RESISTANT AUSTENITIC FE-CR-NI-AL STEEL WITH HIGH NICKEL CONTENT

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of austenitic alloys requiring good mechanical and environmental resistance at high temperatures, in particular for use in reforming furnaces for the direct reduction of iron ore or more generally as a structural material for very high temperature applications such as in heat treatment furnaces. It relates in particular to a high nickel austenitic alloy, which has excellent resistance to corrosion and creep at service temperatures greater than or equal to 1100°C. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Austenitic alloys based on nickel, chromium and iron known as “refractory” have been known for many years for their applications at very high temperatures (see in particular document FR2333870).

[0003] To increase their resistance to the environment, and in particular to carburization and oxidation, it has been proposed to add aluminum as disclosed in document US4248629. Due to the formation of an aluminum oxide layer on its surface, the resistance to carburization and oxidation in a very high temperature environment is improved.

[0004] US5997809A provides a high temperature carburization and oxidation resistant alloy consisting essentially, by weight percent, of about 27 to 35 chromium, about 0 to 7 iron, about 3 to 4.4 aluminum, about 0 to 0.14 titanium, about 0.2 to 3 niobium, about 0.12 to 0.5 carbon, about 0 to 0.05 zirconium, about 0.002 to 0.05 total cerium and yttrium, about 0 to 1 manganese, about 0 to 1 silicon, about 0 to 0.5 calcium plus magnesium, about 0 to 0.1 boron and the balance nickel plus incidental impurities.

[0005] In alloys subjected to extreme temperatures (typically between 1100°C and 1185°C), internal oxidized and / or decarburized zones appear near the surface of the parts. Such damage appears particularly in "chrominoforming" refractory austenitic steels due to the regeneration of the protective Cr 2 O 3 layer in service. In the case of "aluminoforming" refractory austenitic steels, decarburized zones and internal oxidation and nitriding can appear if the alumina layer formed is not protective or is discontinuous. This damage to the microstructure, near the surface, linked to the alloy's ability to self-protect from the environment, negatively impacts creep resistance.

[0006] The current performance of refractory alloys limits the yields achievable in specific applications, particularly in the context of reformers for the direct reduction of iron ore, where operating temperatures typically reach up to 1175°C. This extreme temperature combined with the mechanical constraints (constraints linked to the own weight of the parts or to pressures of a few bars in service) applied to the parts (for example tubes) made of these alloys results in very high creep stress which limits the service life of the parts in question (and associated equipment).

[0007] It is therefore important to further improve the properties of refractory austenitic alloys with high chromium and nickel contents, to achieve high performance, both in terms of resistance to the environment and oxidation, and in terms of creep resistance, particularly for applications requiring service temperatures greater than or equal to 1100°C. SUBJECT OF THE INVENTION

[0008] The present invention provides a solution for achieving the above-mentioned objectives. The invention relates to a refractory austenitic "aluminoforming" alloy, with high chromium and nickel contents, which has excellent resistance to the environment and to creep, at temperatures greater than or equal to 1100°C, typically between 1100°C and 1185°C. BRIEF DESCRIPTION OF THE INVENTION

[0009] The present invention relates to a refractory austenitic alloy, intended to be used at a service temperature greater than or equal to 1100°C, comprising all of the following compounds in mass percentage: 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 element(s) in a total content of between 0.010% and 0.060%, a reactive element being defined as one of rare earths 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 compounds of the alloy, zirconium, tungsten and sulfur being absent from the alloy, or in the form of impurities respectively at less than 0.030%, at less than 0.010% and at less than 0.0060%, the alloy also meeting two criteria linking the mass percentages (x Cr, x Al, x C, x Si, x Mn, x Ti, x Nb, x N, x V, x S, x Ni) of all or part of the compounds of said alloy: a first criterion defined by: 1 − K Al − K S × 2 e 26 − x Cr 0 , 26 + 1 + K Al × 2 e 2 − x Al 0 , 4 + 1 + K S × 2 e x S − 0 , 003 0 , 1 × x S + 1 ≥ 1 avec K Al = 0 , 1728 + 0 , 1293 × ln x Al et K S = 0 , 3089 × e 64 x S ; and a second criterion defined by:

[0010] According to advantageous characteristics of the invention, taken alone or in any feasible combination: the mass percentage of vanadium is greater than or equal to 0.010%, preferably greater than or equal to 0.10%; the mass percentage of aluminum is greater than or equal to 2.0%, preferably greater than or equal to 2.50%; the mass percentage of sulfur is less than 0.0050%, preferably less than 0.0020%, or even more preferably less than 0.00050%; the mass percentage of nitrogen is greater than or equal to 0.045%, even more preferably greater than or equal to 0.048%, even more preferably greater than or equal to 0.060%, even more preferably greater than or equal to 0.10%, or even more preferably greater than or equal to 0.12%; the mass percentage of chromium is between 26% and 31%; the mass percentage of carbon is greater than or equal to 0.16%, preferably greater than or equal to 0.25% or even preferably greater than or equal to 0.35%; the total mass percentage of reactive elements is greater than or equal to 0.020%. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Other characteristics and advantages of the invention will emerge from the detailed description which follows with reference to the appended figures in which: There Figure 1 presents a table of composition of the alloy in accordance with the invention; The Figure 2 presents a table comprising nine examples of refractory austenitic alloys, the alloys numbered 1 to 4 being part of the alloys in accordance with the present invention; The Figure 3 presents eight cross-sectional images by optical microscopy of alloys 1 to 8, after they have undergone an accelerated aging heat treatment at 1150°C for 125 h; The Figure 4 presents two cross-sectional images by scanning electron microscopy of alloys 1 and 6, after they have undergone heat treatment at 1150°C for 125h, and two EDS analyses (chemical analysis by energy dispersive spectroscopy) of these two alloys; The Figure 5apresents the mass gain of alloys 2 and 5, having undergone respectively 20 and 10 oxidation cycles of 45 min at 1150°C; The Figure 5b presents two cross-sectional images by scanning electron microscopy of alloys 2 and 5, having undergone respectively 20 oxidation cycles of 45 min at 1150°C (alloy 2, a and c) and 10 cycles (alloy 5, b and d); The Figure 6a presents the creep performance of the alloys described in the Figure 2 , in the form of an LMP representation (Larson-Miller parameter); The Figure 6b presents the high LMP creep performances of alloys 1 to 8 described in the table of the Figure 2 ; the grades were tested at low stress (9 MPa) and at temperatures of 1150°C and 1175°C. DETAILED DESCRIPTION OF THE INVENTION

[0012] The invention relates to a refractory austenitic alloy, intended to be used at a service 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.

[0013] The austenitic alloy according to the invention comprises all of the following compounds in mass percentage: 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 element(s) at 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%, to balance the alloy compounds.

[0014] In the remainder of the description, the expressions "content", "amount" or "percentage", in relation to a compound of the alloy, will be used interchangeably and must be interpreted as relating to the "mass percentage" of said compound. Where a mass percentage is indicated "between X and Y", X and Y constituting the limits of the composition range, said limits must be considered as included in the range, unless expressly stated otherwise.

[0015] The refractory austenitic alloy according to the invention 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%).

[0016] A minimum of 25.0% chromium is required to ensure good corrosion resistance (oxidation) and to allow the formation of chromium carbides, which have a positive impact on the alloy's creep resistance. The maximum mass percentage of chromium is limited to 32.0%, in particular to limit excessive integration of alphagenic elements that tend to destabilize the austenitic structure of the alloy. Advantageously, the Cr content is defined between 26.0% and 31.0%, to further promote the alloy's environmental protection and creep resistance.

[0017] The minimum nickel content is set at 50.0% to maintain a refractory alloy with an austenitic structure, as the alloy contains at least 25.0% chromium and other alphagenic elements that tend to destabilize the austenitic structure in favor of a ferritic structure. The amount of nickel is limited to 61.0%, or even limited to 57.0%, or even 55.0% for economic reasons, as nickel is a major cost contributor.

[0018] The iron mass percentage balances the alloy components, so that the sum of the mass percentages of said components reaches 100%. A content between 4.0% and 18.0% makes the balance on the other components more advantageous. Preferably, an iron content greater than or equal to 13.0% is desirable in order to reduce the costs of the grade.

[0019] 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) on the surface of the alloy, in a wide range of oxygen partial pressure (from less than 5 particles per million to 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 very resistant and effective barrier to corrosion (oxidation, carburization, nitriding) of the alloy, at high temperatures, typically 1100°C and above.

[0020] Advantageously, the mass percentage 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 allows access to a larger aluminum "reservoir" and thus to preserve the properties of the alloy over longer periods, in very harsh environments where the aluminum oxide layers are consumed.

[0021] It may be advantageous to keep the mass percentage of aluminum at or below 4%, to limit the precipitation of B2-NiAl intermetallic phases, which may adversely impact creep properties. As a reminder, B2 according to Strukturbericht notation describes a phase comprising two types of atoms (here, Ni and Al) in equal proportion and whose crystallographic structure is "interpenetrated primitive cubic", that is to say that each of the two types of atom forms a simple body-centered cubic lattice, with an atom of one type at the center of each cube of the other type.

[0022] Carbon must be present in the alloy for its hardening effect, by precipitation and by solid solution. The range of carbon mass percentage 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 significant volume fraction of carbides and improves the castability of the alloy.

[0023] The niobium content of the alloy is defined between 0.15% and 1.50% to fix the carbon in the form of carbonitrides rich in niobium and / or titanium. Advantageously, niobium, in combination with titanium, prevents the formation of the G phase, a phase rich in silicon, which is unfavorable to creep properties. Preferably, the niobium content is greater than or equal to 0.2%, 0.4%, 0.5%, 0.8%, or even 1%; and the niobium content is less than or equal to 1.4%, 1.3%, or even 1.2%.

[0024] A reactive element within the meaning of the present invention is defined as one of the rare earths or hafnium. The addition of at least one reactive element (such as for example cerium, yttrium, etc., or hafnium) is beneficial to the growth, adhesion and protective character of the alumina layer. This or these element(s) promote the fragmentation of the chromium carbide network and yet 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 any additional effect while it implies a strong impact on the cost and on the eco-responsible character of the material. A minimum total content of 0.010% is required to obtain the aforementioned benefits. Advantageously, the total mass percentage of reactive elements is chosen to be greater than or equal to 0.020%.

[0025] The alloy also contains silicon to improve castability and increase corrosion resistance. The amount of this element is nevertheless limited to 0.30%, or even 0.25%, in order to avoid the presence of G and σ phases (intermetallic phase comprising Fe, Cr, Ni and Si), which are detrimental to creep. Advantageously, the Si content is between 0.01% and 0.20%, or even between 0.05% and 0.20%.

[0026] Manganese is also present in the alloy, to improve weldability and for its beneficial effect in oxidation because it acts as a trap for sulfur. It also has a beneficial effect on creep because it increases the solubility of nitrogen in austenite and promotes the stability of the austenitic structure. However, its content is limited to 0.30% to limit the formation of the B2-NiAl intermetallic phase, which negatively impacts 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%.

[0027] The alloy contains vanadium, up to a mass percentage of 1.0%. This compound is known to improve the creep properties of austenitic stainless steels by its impact on the precipitation of chromium carbides, increasing their volume fraction. Vanadium also aids in the precipitation of carbonitrides rich in niobium, titanium and / or vanadium, during aging, and it also has a solid solution hardening effect. Its content must be limited to 1.0% to maintain its beneficial effects and avoid degradation of the oxidation behavior of the grade. Advantageously, the vanadium content is between 0.005% and 0.5%; it can optionally be greater than or equal to 0.010%, or even greater than or equal to 0.1%.

[0028] Titanium promotes the formation of fine intragranular carbonitrides and their subsequent evolution during aging (favorable to creep resistance). It can be included in the alloy in a mass percentage of up to 0.40%. Advantageously, the mass percentage of titanium is greater than 0.05%.

[0029] The alloy also contains nitrogen which, due to its gammagenic character (stabilizes the austenitic structure), improves creep properties. Its presence in the alloy also contributes to the formation of carbonitrides rich in niobium, titanium and / or vanadium which reinforce creep properties. Its content is limited to 0.20% to avoid the formation of phases unfavorable to creep and oxidation properties. The mass percentage of nitrogen is greater than or equal to 0.015%, preferably greater than or equal to 0.040%, 0.045%, 0.048%, 0.060%, still preferably greater than or equal to 0.10%, or even still preferably greater than or equal to 0.12%.

[0030] Sulfur is an undesirable element in the alloy, but can be found in trace form (impurity) in the grade. It is desirable to limit the presence of this element in order to degrade the protective character of the alumina layer as little as possible. Sulfur can therefore be present in the alloy but at contents strictly lower than 0.0060% (i.e. < 60 ppm). Advantageously, the sulfur content is lower than 0.0050% (< 50 ppm), or even lower than 0.0020% (< 20 ppm), preferably lower than 0.00050% (< 5 ppm).

[0031] Other compounds may possibly be found in trace amounts in the alloy, such as 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 is linked to the fact that these elements may be found as impurities in the fillers incorporated during the manufacture of the alloy.

[0032] The alloy may possibly be polluted by other trace impurities whose content is in the order of one part per million (ppm), and strictly less than 200 ppm, such as phosphorus, lead, tin, boron, magnesium or arsenic.

[0033] Note that the composition of the alloy can be measured by spark spectrometry.

[0034] The table of the Figure 1presents the composition of the austenitic alloy in accordance with the present invention. The austenitic alloy according to the invention further complies with two criteria linking the mass percentages (x Cr, x Al, x C, x Si, x Mn, x Ti, x Nb, x N, x V, x S, x Ni) of all or part of the compounds of said alloy.

[0035] The first criterion is an oxidation criterion, determined empirically. It links the chromium, aluminum, and sulfur contents of the alloy. The equation is built around acceptable values ​​for these three compounds (26% for Cr, 2% for Al, and 30 ppm for sulfur). This equation gives a different weight to each element depending on the impact of its content on high-temperature oxidation resistance. For simplicity, the criterion has been standardized and must be greater than 1 to ensure good oxidation behavior.

[0036] The first criterion is defined by: 1 − K Al − K S × 2 e 26 − x Cr 0 , 26 + 1 + K Al × 2 e 2 − x Al 0 , 4 + 1 + K S × 2 e x S − 0 , 003 0 , 1 × x S + 1 ≥ 1 with K Al= 0.1728 + 0.1293 × In( x Al ) And KS = 0.3089 × e (64 x S)<

[0037] The second criterion concerns the solvus temperature of a certain type of carbides, namely M 23 C 6 carbides. A relationship has been established between the mass percentages of certain elements which are related to the solvus temperature of M 23 C 6 carbides. This temperature must be high (i.e. greater than or equal to 1070°C) to promote the secondary precipitation of Cr carbides (M 23 C 6 ) at operating temperatures and to ensure optimal mechanical performance (creep resistance).

[0038] The second criterion is defined by:

[0039] As mentioned in the introduction, it is common for a refractory austenitic alloy to form a decarburized layer and / or an internal oxidation layer, a consequence of the evolution of the microstructure near the surface due to the very high service temperatures. This phenomenon, linked to the ability of the alloy to self-protect from the environment, has a significant impact on the service life of these alloys at these temperatures.

[0040] Thus, going beyond the role of each individual compound of the alloy, the applicant studied the link between the microstructure of the alloy, its resistance to oxidation and its mechanical properties at service temperatures typically greater than or equal to 1100°C. The service temperature is the temperature to which the alloy is intended to be subjected, during its use: for example, for an alloy forming a reformer tube in a direct iron ore reduction installation, the service temperature could be between 1050°C and 1175°C.

[0041] The studies carried out, notably based on characterizations by optical microscopy, scanning electron microscopy (SEM) and on creep tests, have made it possible to highlight 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 carbides rich in chromium of type M 23 C 6 , at the service temperature.

[0042] Thus, the applicant was able to determine that, in an austenitic alloy with a high nickel content, the creep resistance, at the service temperature, can achieve exceptional performance when it has not only a microstructure "favorable" for creep resistance but also very good resistance to oxidation at said temperature, hence the definition of the two criteria previously stated. This synergistic effect is particularly true for the very high service temperatures targeted and represents the heart of this invention. A microstructure optimized for creep resistance is a necessary but not sufficient condition for high creep resistance at very high temperatures (>1100°C); it turns out that the ability of the grade to self-protect from the environment plays a crucial role and is also necessary (criterion 1).

[0043] A "favorable" microstructure in this case means that, at the service temperature, the chemical composition of the alloy must be such that the solvus temperature of the M 23 C 6 carbides is equal to or greater than 1070°C, to favor the secondary precipitation of said carbides from the M 7 C 3 carbides present in the as-cast alloy.

[0044] From correlations between physical characterizations and CALPHAD simulations (phase diagram calculations, allowing the phases present in the alloy at temperature equilibrium to be predicted, depending on its composition), an R2 relationship was established between the mass percentages of certain compounds in the alloy and the maximum temperature. T max M 23 C 6 of the stability domain of the chromium carbide phase M 23 C 6: T max M 23 C 6 ° C = − 17 , 64 + 19 , 61 x Al − 1 , 29 x Al 2 − 101 , 46 x N + 450 , 65 x N 2 − 5 , 8368 x N 3 + 9 , 68 x V + 43 , 12 x Ti + 30 , 02 x Si + 11 , 42 x Ni − 0 , 18 x Ni 2 + 35 , 05 x Nb + 47 , 92 x Cr − 0 , 34 x Cr 2 + 13 , 97 x Mn − 239 , 66 x C with x Al , x N , x V , x Ti , x Si , x Ni , x Nb , x Cr , x Mn , x C are the mass percentages respectively of Al, N, V, Ti, Si, Ni, Nb, Cr, Mn and C in the alloy.

[0045] The said maximum temperature of the stability domain can be seen as the limiting temperature below which there is a transformation in the alloy of the M 7 C 3 carbides (present in the alloy in the as-cast state) into M 23 C 6 carbides; this transformation leads to a secondary precipitation of desired chromium carbides, which improves the creep performance of the alloy. Such a transformation takes place over a temperature range corresponding to the stability domain of the M 23 C 6 phase.

[0046] According to the invention, the maximum temperature T max M 23 C 6 must be greater than or equal to 1070°C in order to favor secondary precipitation in the alloy subjected to the service temperature, during its use. This condition corresponds to the second criterion. Advantageously, the maximum temperature T max M 23 C 6 can be defined as greater than or equal to 1100°C, or even greater than or equal to 1150°C.

[0047] As stated previously, this relationship R2 is only valid and relevant for an alloy having main compounds (Cr, Ni, Al, Nb, C, Si, Mn, Ti, Fe, N, V) in the mass percentage ranges defined according to the invention.

[0048] Validation of the second criterion, linked to the maximum temperature of the stability domain of secondary carbides M 23 C 6, is however not sufficient to guarantee optimal creep performance at the service temperature.

[0049] The alloy must also have excellent resistance to oxidation. Three elements, chromium, aluminum, and sulfur, play a crucial role in the alloy's ability to self-protect. Based on correlations between physical characteristics and chemical composition, an R1 relationship was established: f oxy = 1 − K Al − K S × 2 e 26 − x Cr 0 , 26 + 1 + K Al × 2 e 2 − x Al 0 , 4 + 1 + K S × 2 e x S − 0 , 003 0 , 1 × x S + 1 with K Al = 0.1728 + 0.1293 × In(x Al ) and KS = 0.3089 × e (64xS< )

[0050] The term f oxy is an oxidation function and x Cr , x Al and x S are the mass percentages of Cr, Al and S respectively in the alloy.

[0051] Advantageously, the oxidation function f oxy must be greater than 1 in order to guarantee good oxidation behavior of the alloy subjected to the service temperature, and to optimize, in a synergistic manner, the creep resistance of the alloy during its use. The condition f oxy ≥ 1 corresponds to the first criterion according to the present invention.

[0052] Examples of alloys will now be presented to illustrate how the composition ranges according to the invention, combined with the two aforementioned criteria, make it possible to obtain a nickel-rich “aluminoforming” refractory austenitic alloy, which is particularly effective in terms of oxidation resistance and creep resistance, at service temperatures greater than or equal to 1100°C.

[0053] Performance tests focus on the resistance of alloys to accelerated aging, cyclic oxidation, and their creep resistance.

[0054] The table of the Figure 2 presents different alloys which have been studied by the applicant. Alloys 1 to 4 are in accordance with the present invention. Alloys 5 to 9 are counter-examples which do not satisfy all of the characteristics of the present invention.

[0055] There Figure 3shows optical microscopy cross-sectional images of alloys 1 to 8 after they have undergone accelerated aging heat treatment at 1150°C for 125 h. The scale on these images is 50 µm.

[0056] A dendritic structure with a network of chromium carbides of type M 7 C 3 and / or M 23 C 6 located at the inter-dendritic spaces as well as at the surface of the samples is observed. Note that the surface was protected with a copper deposit in the cases of alloys 1, 2 and 6, this deposit has a clear contrast on the optical microscopy images and is observable in the form of spaced islands at the surface.

[0057] The chromium-rich carbide network is fully present up to the surface of the samples of alloys 1, 2, 3, 4 and 7. In contrast, a free layer of chromium carbides is observed near the surface of alloys 5 and 8, as well as an internal oxidation layer. In the case of alloy 6, the width of the decarburized layer is such that, in the image, the chromium carbide network is not observed; on the other hand, a significant internal oxidation layer is observed.

[0058] The large black contrast objects formed inside the sample of alloys 5, 6 and 8 are aluminum nitrides.

[0059] The microstructures of alloys 1 and 6, observed by scanning electron microscope, are presented on the Figure 4 (a and b) and were chemically analyzed by energy dispersive spectroscopy (EDS) ( Figure 4, c and d ).

[0060] 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 Figure 4 (c) ) and the chrome profile ( figure 4 (d) ) shows a monotonous nominal concentration with peaks corresponding to the presence of chromium carbides.

[0061] Alloy 6, which does not meet the first criterion, f oxy ≥ 1, formed a layer of chromium oxide (Cr 2 O 3 ) on the surface which led to a depletion of chromium in the area near the surface (see profile on the figure 4 (d) ). Just below this chromine layer, we can observe a non-protective layer of aluminum oxide ( Figure 4 (c) ).

[0062] There Figure 5ashows the mass evolution of alloys 2 and 5, during cyclic oxidation. The graph shows the number of cycles on the abscissa, one cycle corresponding to the sequence: 45 min at 1150°C and 15 min at room temperature. In addition to the mass evolution, the Figures 5b presents cross-sectional images of these same alloys, having undergone 20 oxidation cycles in the case of alloy 2 ( Figure 5b (a) And (c)) and 10 cycles in the case of alloy 5 ( Figure 5b (b) And (d) ), at two different magnifications.

[0063] It can be seen that the high sulfur content combined with limited chromium and aluminum contents in a refractory alloy limits the alloy's ability to self-protect from oxidation. The mass gain of alloy 5 ( Figure 5b ) is the consequence of internal oxidation ( Figures 5b (b) And (d)). Beyond cycle 3, a slight mass loss, probably due to flaking, is observed in alloy 5. Alloy 2, according to the present invention, shows mass stability with cyclic oxidation, once the protective alumina layer has formed.

[0064] 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, under stresses of 17, 16.5, 13, 11.5, 10 and 9 MPa, the tests being carried out on samples taken from parts made from the different alloys. From these tests a time to rupture t R is extracted, which is transformed into a Larson-Miller parameter (LMP) in combination with the test temperature according to the following expression: LMP = 1000 / T × log t R + C

[0065] T being the test temperature expressed in kelvin, t R the time to rupture expressed in hours and C a constant characteristic of the alloy; in our case C = 20.22.

[0066] Representing the results of creep tests using the Larson-Miller formalism allows for comparison of the performance of tests carried out at different temperatures. Figure 6a groups together the results of 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, high LMP test conditions correspond to low stresses and high temperatures, while low LMP conditions correspond to high stresses and lower temperatures.

[0067] A superior performance (especially at high LMP) of alloys 1 to 4, in accordance with the invention, can be observed compared to alloys 5 to 9.

[0068] There Figure 6bshows in detail the results of creep tests carried out at 9MPa and 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.

[0069] All of these results highlight the performance differences that high nickel refractory austenitic alloys with very similar compositions can exhibit in terms of creep resistance at very high temperatures (alloys 1 to 4 versus alloys 5 to 9). In addition to precise composition ranges, the applicant has defined two important criteria that the alloy must meet in order to offer the best creep performance combined with excellent resistance to cyclic oxidation, for service temperatures greater than or equal to 1100°C. An originality of this approach comes from the consideration of two distinct phenomena (oxidation factor and solvus temperature of the M 23 C 6 carbides) having a synergistic action beneficial to the mechanical performance (creep) of the alloy while ensuring remarkable corrosion protection.

[0070] The invention is not limited to the embodiments described and variant embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

1. Refractory austenitic alloy, intended to be used at an operating temperature greater than or equal to 1100°C, comprising all the following compounds 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 the rare earths 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 compounds, zirconium, tungsten and sulfur being absent from the alloy, or in the form of impurities respectively at less than 0.030%, less than 0.010% and less than 0.0060%, the alloy further satisfying two criteria connecting the percentages by weight (xCr, xAl, xC, xSi, xMn, xTi, xNb, xN, xV, xS, xNi) of all or part of the compounds of said alloy: - a first criterion defined by: 1 − K Al − K S × 2 e 26 − x Cr 0.26 + 1 + K Al × 2 e 2 − x Al 0.4 + 1 + K S × 2 e x S − 0.003 0.1 × x S + 1 ≥ 1 where K Al = 0.1728 + 0.1293 × ln x Al and K S = 0.3089 × e 64 x S ; - and a second criterion defined by:

2. Refractory austenitic alloy according to the preceding claim, wherein the percentage by weight of vanadium is greater than or equal to 0.010%, more preferentially greater than or equal to 0.10%.

3. Refractory austenitic alloy according to either of the preceding claims, wherein the percentage by weight of aluminum is greater than or equal to 2.0%, preferentially greater than or equal to 2.50%.

4. Refractory austenitic alloy according to any of the preceding claims, wherein the percentage by weight of sulfur is less than 0.0050%, preferentially less than 0.0020%, or more preferentially less than 0.00050%.

5. Refractory austenitic alloy according to any of the preceding claims, wherein the percentage by weight of nitrogen is greater than or equal to 0.060%, more preferentially greater than or equal to 0.10%, even more preferentially greater than or equal to 0.12%.

6. Refractory austenitic alloy according to any of the preceding claims, wherein the percentage by weight of chromium is between 26% and 31%.

7. Refractory austenitic alloy according to any of the preceding claims, wherein the percentage by weight of carbon is greater than or equal to 0.16%, preferentially greater than 0.25% or more preferentially greater than 0.35%.

8. Refractory austenitic alloy according to any of the preceding claims, wherein the total percentage by weight of reactive elements is greater than or equal to 0.020%.