High Nickel Austenitic Steel (Fe-Cr-Ni-Al)

Abstract

The invention relates to a refractory austenitic alloy, intended for use at a service temperature greater than or equal to 1100°C, comprising all of the following compounds in mass percentages: 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 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%, 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 also meeting two criteria relating the mass percentages (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: with and ;- and a second criterion defined by: . No Figure,

Description

Title of the invention: High Nickel Fe-Cr-Ni-Al Refractory Austenitic Steel FIELD OF INVENTION

[0001] The present invention relates to the field of austenitic alloys requiring good mechanical and environmental resistance at high temperatures, particularly 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 that exhibits excellent resistance to corrosion and scorching at service temperatures of 1100°C or higher. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Austenitic alloys based on nickel, chromium and iron, known as "refractory" alloys, 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, resistance to carburization and oxidation in a very high temperature environment is improved.

[0004] 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 occurs particularly in refractory austenitic "chrominoforming" steels due to the regeneration of the protective Cr2O3 layer during service. In the case of refractory austenitic "aluminoforming" 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 the strength in the finishing process.

[0005] The current performance of refractory alloys limits the achievable yields in specific applications, particularly in reformers for the direct reduction of iron ore, where operating temperatures typically reach up to 1175°C. This extreme temperature, combined with mechanical stresses (stresses related to the weight of the parts themselves or to pressures) (of a few bars in service) applied to parts (for example tubes) made of these alloys results in very high stress in finishing which limits the life of the parts in question (and associated equipment).

[0006] It is therefore important to further improve the properties of high chromium and nickel content refractory austenitic alloys to achieve high performance, both in terms of resistance to the environment and oxidation, and in terms of resistance to finishing, in particular for applications requiring service temperatures greater than or equal to 1100°C. SUBJECT OF THE INVENTION

[0007] The present invention proposes a solution to achieve the aforementioned objectives. The invention relates to a refractory austenitic "aluminoforming" alloy, with high chromium and nickel content, which exhibits excellent resistance to the environment and to refining, at temperatures greater than or equal to 1100°C, typically between 1100°C and 1185°C. BRIEF DESCRIPTION OF THE INVENTION

[0008] The present invention relates to a refractory austenitic alloy, intended for use at a service temperature greater than or equal to 1100°C, comprising all of the following compounds in mass percentage:

[0009] - 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) with 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%, 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 also meeting two criteria relating the mass percentages (xCr, xAb, xc, xSi, xMn, xTi, xNb, xN, xv, xs, xNi) of all or part of the compounds of said alloy: - a first criterion defined by: (lK M -K s} x + K M X ] +K s x ] a 1 withKÆ=04728 + 04293xln(xJetKs=0J089xe(6^ ; - and a second criterion defined by: -17.64--19.61v,..-1.29..,--101.46^+456.65., / -5.8868., / +9.68^1+4 3.12.^,-30.02...:+11.42.^,-0.18.,. / +35,0.+.,.,,+47.92..-,-0.34.,

[0010] According to advantageous features of the invention, taken alone or in any feasible combination: • the mass percentage of vanadium is greater than 0.0010%, preferably greater than or equal to 0.010%, again preferably greater than or equal to 0.10%; • the mass percentage of aluminium is greater than or equal to 2.0%, preferably greater than or equal to 2.50%; • the mass percentage of sulfur is less than 0.0020%, preferably less than 0.00050%; • the mass percentage of nitrogen is greater than or equal to 0.040%, preferably greater than or equal to 0.060%, 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 0.35%; • the total mass percentage of reactive elements is greater than or equal to 0.010%. Brief description of the drawings

[0011] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying figures in which:

[0012] [Fig.1] Fig.1 presents a composition table of the alloy according to the invention;

[0013] [Fig.2] Figure [Fig.2] presents a table comprising eight examples of alloys austenitic refractory alloys, the alloys numbered 1 to 4 being part of the alloys conforming to the present invention;

[0014] [Fig.3] Figure [Fig.3] presents eight cross-sectional optical microscopy images of the alloys 1 to 8, after they have undergone an accelerated aging heat treatment at 1150°C for 125h;

[0015] [Fig.4] The figure shows two cross-sectional images obtained by electron microscopy at scanning of alloys 1 and 6, after they have undergone heat treatment at 1150°C for 125h, and two EDS (energy-dispersive X-ray spectroscopy) analyses of these two alloys;

[0016] [Fig. 5a] Figure 5a shows the mass gain of alloys 2 and 5, having undergone respectively 20 and 10 oxidation cycles of 45 min at 1150°C;

[0017] [Fig. 5b] Figure 5b shows two cross-sectional electron microscopy images scanning of alloys 2 and 5, having respectively undergone 20 oxidation cycles of 45 min at 1150°C (alloy 2, a and c) and 10 cycles (alloy 5, b and d);

[0018] [Fig. 6a] Figure 6a shows the finishing performance of the alloys described in the [Fig.2], in the form of an LMP representation (Larson-Miller parameter);

[0019] [Fig. 6b] Figure [Fig. 6b] shows the high LMP finishing performance of the alloys described in the table in [Fig.2]; the grades were tested at low stress (9 MPa) and at temperatures of 1150°C and 1175°C. DETAILED DESCRIPTION OF THE INVENTION

[0020] The invention relates to a refractory austenitic alloy, intended for use 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.

[0021] The austenitic alloy according to the invention comprises all of the following compounds in mass percentage:

[0022] - 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 compounds of the alloy.

[0023] In the remainder of this description, the terms "content," "quantity," or "percentage," when referring to a component of the alloy, shall be used interchangeably and shall be interpreted as referring to the "mass percentage" of said component. When a mass percentage is indicated "between X and Y," with X and Y constituting the limits of the composition range, said limits shall be considered as included within the range, unless expressly stated otherwise.

[0024] 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 aluminium (between 1.0% and 6.0%).

[0025] A minimum of 25.0% chromium is required to ensure good corrosion (oxidation) resistance and to allow the formation of chromium carbides, which favorably affect the alloy's resistance to machining. The maximum mass percentage of chromium is limited to 32.0%, notably to limit excessive integration of alpha-stabilizing 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 enhance the alloy's environmental protection and its resistance to machining.

[0026] The minimum nickel content is set at 50.0% in order to maintain a refractory alloy with an austenitic structure, since the alloy contains at least 25.0% chromium as well as other alpha-stabilizing 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 to 57.0%, or even 55.0% for economic reasons, as nickel is a significant cost contributor.

[0027] The iron mass percentage balances the alloy's 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 with respect to the other components more advantageous. Preferably, an iron content greater than or equal to 13.0% is desirable in order to reduce the cost of the alloy grade.

[0028] 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, over a wide range of oxygen partial pressures (from less than 5 parts per million to high partial pressures such as those found in air), and a wide range of temperatures (typically, temperatures above 1000°C). The surface layer of aluminum oxide then forms a highly resistant and effective barrier against corrosion (oxidation, carburization, nitriding) of the alloy at high temperatures, typically 1100°C and above.

[0029] 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 under a wider range of environmental conditions. It also allows access to a larger "reservoir" of aluminum and thus preserves the alloy's properties for longer periods, even in very harsh environments where aluminum oxide layers are consumed.

[0030] It may be advantageous to maintain the aluminum mass percentage at or below 4% to limit the precipitation of B2-NiAl intermetallic phases, which could adversely affect the finishing properties. As a reminder, B2, according to the Strukturbericht notation, describes a phase comprising two types of atoms (here, Ni and Al) in equal proportions and whose crystallographic structure is "primitive interpenetrating cubic," that is, each of the two types of atoms forms a body-centered simple cubic lattice, with one atom of one type at the center of each cube of the other type.

[0031] Carbon must be present in the alloy for its hardening effect, through precipitation and solid solution. The mass percentage range of carbon is defined between 0.05% and 0.60%. Advantageously, a percentage greater than or equal to 0.25%, or even 0.35%, allows the formation of a significant carbide volume fraction and improves the castability of the alloy.

[0032] The niobium content of the alloy is defined as being between 0.15% and 1.50% to fix the carbon in the form of niobium- and / or titanium-rich carbonitrides. Advantageously, the niobium, in combination with the titanium, prevents the formation of the G phase, a silicon-rich phase, which is detrimental to the finishing properties. Preferably, the niobium content is greater than or equal to 0.2%, 0.4%, 0.5%, or even 0.8%; and the niobium content is less than or equal to 1.4%, 1.3%, or even 1.2%.

[0033] A reactive element within the meaning of the present invention is defined as one of the rare earth elements 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 element or these elements promote the fragmentation of the chromium carbide network and yet have a beneficial effect on the strength of the layer. 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 has a significant impact on the cost and the eco-responsibility of the material. A minimum total content of 0.010% is required to obtain the aforementioned benefits.

[0034] The alloy also contains silicon to improve castability and increase corrosion resistance. However, the amount of this element is limited to 0.30% to avoid the presence of the G and o phases (intermetallic phase comprising Fe, Cr, Ni, and Si), which are detrimental to the finishing process. Advantageously, the Si content is between 0.05% and 0.20%.

[0035] Manganese is also present in the alloy to improve weldability and for its beneficial effect on oxidation, as it acts as a sulfur trap. It also has a beneficial effect on refining because it increases the solubility of Nitrogen in austenite 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 resistance to scouring. Advantageously, the manganese content is between 0.05% and 0.25%.

[0036] The alloy comprises vanadium, up to a mass percentage of 1.0%. This compound is known to improve the finishing 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 degrading the oxidation behavior of the alloy. Advantageously, the vanadium content is between 0.005% and 0.5%.

[0037] 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%.

[0038] The alloy also contains nitrogen which, by its gamma-stabilizing character (stabilizing the austenitic structure), improves creep properties. Its content is limited to 0.20% to avoid the formation of phases unfavorable to creep and oxidation properties. Advantageously, the mass percentage of nitrogen is greater than or equal to 0.040%, preferably greater than or equal to 0.060%, more preferably greater than or equal to 0.10%, or even more preferably greater than or equal to 0.12%.

[0039] Sulfur is an undesirable element in the alloy, but may be present in trace amounts (impurities) in the alloy. It is desirable to limit the presence of this element in order to degrade the protective properties of the alumina layer as little as possible. Sulfur may therefore be present in the alloy, but at levels strictly below 0.0060% (i.e., < 60 ppm). Advantageously, the sulfur content is less than 0.0020% (< 20 ppm), preferably less than 0.00050% (< 5 ppm).

[0040] 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 intentionally introduced into the alloy; their potential presence is related to the fact that these elements may be found as impurities in the fillers incorporated during the manufacture of the alloy.

[0041] The alloy may possibly be contaminated by other trace impurities with a content on the order of one part per million (ppm), and strictly less than 200 ppm, such as phosphorus, lead, tin, boron, magnesium or arsenic.

[0042] Note that the composition of the alloy can be measured by spark spectrometry.

[0043] The table in [Fig. 1] shows the composition of the austenitic alloy according to the present invention. The austenitic alloy according to the invention further complies with two criteria relating the mass percentages (xCr, xAb, xc, xSi, xMn, xTi, xNb, xN, xv, xs, xNi) of all or part of the compounds of said alloy.

[0044] The first criterion is an oxidation criterion, determined empirically. It relates the chromium, aluminum, and sulfur contents of the alloy. The equation is constructed around acceptable values ​​for these three compounds (26% for Cr, 2% for Al, and 30 ppm for sulfur). This equation assigns a different weight to each element according to the impact of its content on high-temperature oxidation resistance. For simplicity, the criterion has been standardized and must be greater than 1 to guarantee good oxidation behavior.

[0045] The first criterion is defined by: (lK^-Ke) x F1 + K sx [......1 > 1 ■ ai *7 L dWl J u L ekU+l J * LJ

[0046] with K Æ =0.1728 + 0.1293xln(x Æ ) and Ks = 0.3089 xe(64*s)

[0047] The second criterion concerns the solvus temperature of a certain type of carbide, namely M23C6 carbides. A relationship has been established between the mass percentages of certain elements that are linked to the solvus temperature of M23C6 carbides. This temperature must be high (i.e., greater than or equal to 1070°C) to promote the secondary precipitation of Cr (M23C6) carbides at operating temperatures and to guarantee optimal mechanical performance (creep resistance).

[0048] The second criterion is defined by: -1~ 64-15.61.^,-1.2511,--101.4^+450.65^,.--5.0365.1^-^).60^. +43.12xr,--3O O2xs,+ n.42x.,-,-0 13.Y^ C

[0049] 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 very high service temperatures. This phenomenon is linked to the alloy's ability to self-protect the environment, has a significant impact on the lifespan of these alloys at these temperatures.

[0050] Thus, going beyond the role of each individual component 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 plant, the service temperature may be between 1050°C and 1175°C.

[0051] Studies carried out, in particular based on characterizations by optical microscopy, scanning electron microscopy (SEM) and on finishing tests, have shown that the finishing 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 chromium-rich secondary carbides of type M23C6, at the service temperature.

[0052] Thus, the applicant was able to determine that, in a high nickel-content austenitic alloy, the resistance to grinding, at the service temperature, can achieve exceptional performance when it not only has a "favorable" microstructure for resistance to grinding but also very good resistance to oxidation at said temperature, hence the definition of the two criteria previously stated.

[0053] A “favorable” microstructure in this case means that, at the temperature of service, the chemical composition of the alloy must be such that the solvus temperature of the M23C6 carbides is equal to or greater than 1070°C, in order to favour the secondary precipitation of said carbides from the M7C3 carbides present in the as-cast alloy.

[0054] Based on correlations between physical characterizations and CALPHAD simulations (calculations of phase diagrams, allowing prediction of the phases present in the alloy at temperature equilibrium, as a function of its composition), an R2 relationship was established between the mass percentages of certain compounds in the alloy and the maximum temperature of the phase stability domain of Chromium carbides M23C6:

[0055] [R2] [005^1] +,7 / / 1" 0=-1 / ,04 + 15,511.,. 1 3.1, / .1.1,40, + .50 .5., / .5 3 3..., + 0 03.+ +..17..+ 30 0.1.+ 11,4.1, -.1.1, / +.5 . 5..,.,+ 17,0 / .., .0,3... / +1. / 77. ,,-..+7

[0057] with xAi, xN, xv, xTi, xSi, xNi, xNb, xCr, xMn, xc are the mass percentages respectively of Ai, N, V, Ti, Si, Ni, Nb, Cr, Mn and C in the alloy.

[0058] This maximum temperature of the stability range can be seen as the limiting temperature below which the M7C3 carbides (present in the alloy in the as-cast state) are transformed into M23C6 carbides in the alloy; this transformation leads to a desired secondary precipitation of chromium carbides, which improves the finishing performance of the alloy. Such a transformation takes place over a temperature range corresponding to the stability range of the M23C6 phase.

[0059] According to the invention, the maximum temperature is greater than or equal to at 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 can be defined greater than or equal to 1100°C, or even greater than or equal to 1150°C.

[0060] As stated previously, this R2 relationship is valid and relevant only for an alloy having principal compounds (Cr, Ni, Al, Nb, C, Si, Mn, Ti, Fe, N, V) in the mass percentage ranges defined according to the invention.

[0061] Validation of the second criterion, related to the maximum temperature of the stability field of secondary carbides M23C6, is however not sufficient to guarantee optimal finishing performance at the service temperature.

[0062] The alloy must also exhibit excellent oxidation resistance. Three elements, chromium, aluminum, and sulfur, play a crucial role in the alloy's self-protective capacity. Based on correlations between physical characteristics and chemical composition, an RI relationship was established:

[0063] [RI] foxy = (i - KM - Ks) x + KMx + KSX

[0064] with K A1 = 0.1728 + 04293xln(x A1 ) and K s = 0.3089

[0065] The term foxy is an oxidation function and xCr, xAi and xs are the mass percentages respectively of Cr, Ai and S in the alloy.

[0066] Advantageously, the oxidation function foxy must be greater than 1 to ensure good oxidation behavior of the alloy subjected to the service temperature, and to synergistically optimize the alloy's resistance to wear during use. The condition foxy > 1 corresponds to the first criterion according to the present invention.

[0067] Examples of alloys will now be presented to illustrate how the composition ranges according to the invention, combined with the two aforementioned criteria allow the production of a refractory austenitic "aluminoforming" alloy rich in nickel, particularly efficient in terms of oxidation resistance and resistance to finishing, at service temperatures greater than or equal to 1100°C.

[0068] The performance tests relate to the resistance of the alloys to accelerated aging, to cyclic oxidation, and to their resistance to fining.

[0069] The table in [Fig. 2] presents various alloys that have been studied by the applicant. Alloys 1 to 4 conform to the present invention. Alloys 5 to 8 are counterexamples that do not satisfy all the features of the present invention.

[0070] Figure 3 shows optical microscopy cross-sectional images of alloys 1 to 8 after they have undergone accelerated aging heat treatment at 1150°C for 125h. The scale on these images is 50 sqm.

[0071] A dendritic structure is observed with a network of M7C3 and / or M23C6 type chromium carbides located in the interdendritic spaces as well as on the surface of the samples. 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 as spaced islands on the surface.

[0072] 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 the chromium carbide network is not visible in the image; instead, a significant internal oxidation layer is observed.

[0073] The large black contrast objects formed inside the sample of alloys 5, 6 and 8 are aluminum nitrides.

[0074] The microstructures of alloys 1 and 6, observed by scanning electron microscopy, are shown in [Fig.4] (a and b) and were chemically analyzed by energy-dispersive spectroscopy (EDS) ([Fig.4], c and d).

[0075] 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.

[0076] Alloy 6, which does not meet the first criterion, foxy > 1, has formed a chromium oxide (Cr2O3) layer on the surface, resulting in chromium depletion in the near-surface area (see profile in [Fig. 4] (d)). Just below this chromium layer, a non-protective aluminum oxide layer can be observed ([Fig. 4] (c)).

[0077] Figure 5a shows the mass evolution of alloys 2 and 5 during oxidation cyclic. The graph shows the number of cycles on the x-axis, one cycle corresponding to the sequence: 45 min at 1150°C and 15 min at room temperature. In addition to the mass evolution, Figure 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.

[0078] It can be observed that the high sulfur content combined with limited chromium and aluminum content in a refractory alloy limits the alloy's ability to self-protect against oxidation. The mass gain of alloy 5 ([Fig. 5b]) is a consequence of internal oxidation (Figures 5b(b) and (d)). Beyond cycle 3, a slight mass loss, probably due to spalling, is observed in alloy 5. Alloy 2, according to the present invention, exhibits mass stability under cyclic oxidation, once the protective alumina layer has formed.

[0079] The forging resistance of alloys 1 to 8 was evaluated from forging tests at 1100°C, 1125°C, 1150°C and / or 1175°C, under stresses of 16.5, 13, 11.5 and 9 MPa, the tests being carried out on samples taken from parts produced in the different alloys. A time to failure tR is extracted from these tests, which is transformed into a Larson-Miller parameter (LMP) in combination with the test temperature according to the following expression:

[0080] LMP = 1000 / T x (log tR + C)

[0081] T being the test temperature expressed in kelvin, tR the time to rupture expressed in hours and C a constant characteristic of the alloy; in our case C = 20.22.

[0082] Representing the results of the finishing tests using the Larson-Miller formalism allows for comparison of the performance of tests carried out at different temperatures. Figure 6a shows the results of the finishing tests on alloys 1 to 8. The graph presents the applied stress on the ordinate and the Larson-Miller parameter on the abscissa. Typically, the test conditions at high LMP correspond to low stresses and high temperatures, while at low LMP, they correspond to high stresses and lower temperatures.

[0083] Superior performance can be observed (especially at high LMP) of alloys 1 to 4, according to the invention, compared to alloys 5 to 8.

[0084] Fig. 6b shows in detail the results of the finishing tests carried out at 9 MPa and at temperatures of 1150°C and 1175°C on alloys 1 to 8. Alloys 1 to 4 achieve an LMP value greater than 33.32, a relevant performance threshold for such a refractory austenitic alloy.

[0085] All these results highlight the performance differences between refractory austenitic alloys with high nickel content and very similar compositions, These alloys can exhibit differences in resistance to scouring at very high temperatures (alloys 1 to 4 versus alloys 5 to 8). In addition to specific composition ranges, the applicant has defined two important criteria that the alloy must meet to offer optimal creep performance combined with excellent resistance to cyclic oxidation, for service temperatures of 1100°C or higher. A unique aspect of this approach lies in considering two distinct phenomena (oxidation factor and solvus temperature of M23C6 carbides) that have a synergistic effect beneficial to the alloy's mechanical performance (creep) while ensuring outstanding corrosion protection.

[0086] The invention is not limited to the embodiments described and alternative embodiments may be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. A refractory austenitic alloy, intended for use at a service temperature of 1100°C or higher, comprising all of the following compounds in mass percentages: - chromium from 25.0% to 32.0%, - nickel from 50.0% to 61.0%, - aluminum from 1.0% to 6.0%, - niobium from 0.15% to 1.50%, - carbon from 0.05% to 0.60%, - one or more reactive element(s) in a total content from 0.010% to 0.060%, a reactive element being defined as one of the rare earth elements or hafnium, - silicon from 0.30% or less, - manganese from 0.30% or less, - titanium from 0.40% or less, - nitrogen from 0.040% 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 respecting two criteria relating the mass percentages (xCr, xAi, xc, xSi, xMn, xTi, xNb, xN, xv, xs, xNi) of all or part of the compounds of said alloy: - a first criterion defined by: ( x [ ] +^sX [ ] > 1 with = 0.1728 + 0.1293 x and Ks=0.3089xe(64xs); - and a second criterion defined by: -1A64+19 Six,,-1.201.. / -101.467,.,.-450.6¾...^. 8368^. / +9.687,+43.121^+30.02.^+11.427,.,-0.1^ / +.35.051^+47 92xc,-0.34x\-, '+10^9^^^^,

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

3. Refractory austenitic alloy according to any one of the preceding claims, wherein the mass percentage of aluminium is greater than or equal to 2.0%, preferably greater than or equal to 2.50%.

4. Refractory austenitic alloy according to any one of the preceding claims, wherein the mass percentage of sulfur is less than 0.0020%, preferably less than 0.00050%.

5. Refractory austenitic alloy according to any one of the preceding claims, wherein the mass percentage of nitrogen is greater than or equal to 0.060%, more preferably greater than or equal to 0.10%, or more preferably greater than or equal to 0.12%.

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

7. Refractory austenitic alloy according to any one of the preceding claims, wherein the mass percentage of carbon is greater than or equal to 0.35%.

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