EP3578676A1 - Austenitic alloy with high aluminum content and associated design process - Google Patents

Abstract

The invention relates to an austenitic alloy based on nickel, chromium and iron, and with a high aluminum content, intended to be used at a given operating temperature (Ts) between 900 ° C and 1200 ° C, the alloy comprising the following compounds in mass percentages: • chromium between 20% and 32%, • nickel between 30% and 60%, • aluminum between 3.5% and 6%, • carbon between 0.4% and 0.7%, • titanium between 0.05% and 0.3%, • niobium and / or tantalum between 0.6% and 2%, • an element, composed of at least one rare earth and / or hafnium, between 0.002% and 0.1%, • silicon between 0 and 0.5%, • manganese between 0 and 0.5%, • tungsten between 0 and 2%, • iron for balance the alloying compounds. The alloy has less than 1% by volume of a B2-NiAl intermetallic phase and less than 1% by volume of a chromium-rich alpha prime phase, after the temperature of service (Ts) has been applied to it.

Description

FIELD OF THE INVENTION

The present invention relates to the field of austenitic alloys requiring good mechanical and environmental resistance, at high temperatures, in particular for use in steam cracking furnaces in the petrochemical industry. In particular, it relates to a high aluminum content austenitic alloy which exhibits excellent corrosion and creep resistance at temperatures above 900 ° C.

BACKGROUND OF THE INVENTION

Austenitic alloys based on nickel, chromium and iron called "refractory" have been known for many years for their applications at very high temperatures (see in particular the document FR2333870 ). To increase their resistance to the environment, and in particular to carburization and oxidation, it has been proposed to add aluminum as disclosed in the document US4248629 . Due to the formation of an aluminum oxide layer on its surface, the alloy then has excellent resistance to carburization and oxidation in an environment at very high temperatures.

It has nevertheless been observed that the increase, beyond 2-3% by weight, of the aluminum content adversely affects the creep resistance of the alloy.

The current performance of refractory alloys in a harsh environment at higher temperatures at 900 ° C, limit the achievable yields in the applications for which they are used, as well as the service life of the equipment in question (such as a steam cracker tube, for example). On the one hand, environmental constraints involving coking, carburizing, oxidation or nitriding limit the duration or maximum operating temperature applicable; on the other hand, the mechanical stresses cause deformation and damage to the alloys, which must be contained at deformation rates sufficiently low to ensure sufficiently long equipment life.

It would therefore be desirable to have an austenitic alloy having a high aluminum content to ensure high environmental resistance (corrosion by carburization, oxidation or nitriding) while guaranteeing a creep resistance at least as high as the alloys currently known. , containing little (typically less than 3%) or no aluminum.

OBJECT OF THE INVENTION

The present invention proposes a solution for achieving the aforementioned objectives. The invention relates to a high aluminum austenitic alloy which has excellent environmental and creep resistance at temperatures of 900 ° C or higher. The invention also relates to a method for designing such an alloy.

BRIEF DESCRIPTION OF THE INVENTION

• du chrome entre 20% et 32%,
• du nickel entre 30% et 60%,
• de l'aluminium entre 3,5% et 6%,
• du carbone entre 0,4% et 0,7%,
• du titane entre 0,05% et 0,3%,
• du niobium et/ou du tantale entre 0,6% et 2%,
• un élément, composé d'au moins une terre rare et/ou d'hafnium, entre 0,002% et 0,1%,
• du silicium entre 0 et 0,5%,
• du manganèse entre 0 et 0,5%,
• du tungstène entre 0 et 2%,
• du fer pour faire la balance des composés de l'alliage ;

En outre, l'alliage présente moins de 1% en volume d'une phase intermétallique B2-NiAl et moins de 1% en volume d'une phase alpha prime riche en chrome, après que la température de service lui ait été appliquée.The invention relates to an austenitic alloy based on nickel, chromium and iron, and with a high aluminum content, intended for use at a given operating temperature between 900 ° C and 1200 ° C, the alloy comprising the following compounds in percentages by weight:

• chromium between 20% and 32%,
• nickel between 30% and 60%,
• aluminum between 3.5% and 6%,
• carbon between 0.4% and 0.7%,
• titanium between 0.05% and 0.3%,
• niobium and / or tantalum between 0.6% and 2%,
• an element composed of at least one rare earth and / or hafnium between 0.002% and 0.1%,
• silicon between 0 and 0.5%,
• manganese between 0 and 0.5%,
• tungsten between 0 and 2%,
• iron to balance the compounds of the alloy;

In addition, the alloy has less than 1% by volume of a B2-NiAl intermetallic phase and less than 1% by volume of a chromium-rich alpha prime phase after the service temperature has been applied thereto.

• les pourcentages massiques de l'aluminium x_Al, du nickel x_Ni, du chrome x_Cr, du titane x_Ti, du carbone x_C, du niobium x_Nb, du tantale x_Ta, du silicium x_Si et du manganèse x_Mn respectent les deux relations (R3,R4) suivantes : $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+455,4x_{\mathit{Al}}-0,32x_{\mathit{<figure-callout\; id="2"\; label="Ni"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Ni</figure-callout>}}^2+15,{3x}_{\mathit{Ni}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Si}}+27x_{\mathit{Mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Ta}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="Ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">Ts</figure-callout>}$$
  
  $$1,8x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+38,3x_{\mathit{Al}}+0,42x_{\mathit{<figure-callout\; id="2"\; label="Ni"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Ni</figure-callout>}}^2-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572\le \mathrm{Ts}$$
  
  où Ts représente la température de service ;
• le pourcentage massique de nickel x_Ni est défini à partir de la résolution d'équations du second degré (E3,E4), issues des relations (R3,R4) liant les pourcentages massiques des composés de l'alliage et la température de service ;
• le pourcentage massique de nickel x_Ni est compris entre une valeur (X), supérieure à 30%, consistant en la valeur la plus grande entre les solutions (X',X") des équations (E3,E4), et une valeur majorée de dix points (X+10) ;
• la somme des pourcentages du niobium et du tantale, lorsque ces deux composés sont présents, est supérieure à 0,6% et inférieure ou égale à 2% ;
• le pourcentage massique d'aluminium dans l'alliage est supérieur à 3,8%, voire supérieur à 4% ;
• le pourcentage massique de chrome dans l'alliage est inférieur à 30%, voire inférieur à 28% ;
• le pourcentage massique total de terre(s) rare(s) et/ou d'hafnium dans l'alliage est compris entre 0,002% et 0,05%.

According to other advantageous and nonlimiting features of the invention, taken alone or in any technically feasible combination:

• the weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , silicon x _Si and manganese x _Mn meet the two following relations (R3, R4): $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">ts</figure-callout>}$$
  
  $$1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$
  
  where Ts represents the service temperature;
• the mass percentage of nickel x _Ni is defined from the resolution of equations of the second degree (E3, E4), resulting from the relations (R3, R4) linking the mass percentages of the compounds of the alloy and the operating temperature;
• the mass percentage of nickel x _Ni is between a value (X), greater than 30%, consisting of the largest value between the solutions (X ', X ") of the equations (E3, E4), and an increased value ten points (X + 10);
• the sum of the percentages of niobium and tantalum, when these two compounds are present, is greater than 0.6% and less than or equal to 2%;
• the weight percentage of aluminum in the alloy is greater than 3.8%, or even greater than 4%;
• the mass percentage of chromium in the alloy is less than 30%, or even less than 28%;
• the total mass percentage of rare earth (s) and / or hafnium in the alloy is between 0.002% and 0.05%.

• du chrome entre 20% et 32%,
• du nickel entre 30% et 60%,
• de l'aluminium entre 3,5% et 6%,
• du carbone entre 0,4% et 0,7%,
• du titane entre 0,05% et 0,3%,
• du niobium et/ou du tantale entre 0,6 et 2%,
• un élément, composé d'au moins une terre rare et/ou d'hafnium, entre 0,002% et 0,1%,
• du silicium entre 0 et 0,5%,
• du manganèse entre 0 et 0,5%,
• du tungstène entre 0 et 2%,
• du fer pour faire la balance des composés de l'alliage ;

The invention also relates to a process for the design of an austenitic alloy based on nickel, chromium and iron and having a high aluminum content for use in a operating temperature given between 900 ° C and 1200 ° C; the alloy comprising the following compounds in percentages by weight:

• chromium between 20% and 32%,
• nickel between 30% and 60%,
• aluminum between 3.5% and 6%,
• carbon between 0.4% and 0.7%,
• titanium between 0.05% and 0.3%,
• niobium and / or tantalum between 0.6 and 2%,
• an element composed of at least one rare earth and / or hafnium between 0.002% and 0.1%,
• silicon between 0 and 0.5%,
• manganese between 0 and 0.5%,
• tungsten between 0 and 2%,
• iron to balance the compounds of the alloy;

The method comprises the choice of the respective weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , silicon x _Si and manganese x _Mn , so that the alloy has less than 1% by volume of a B2-NiAl intermetallic phase and less than 1% by volume of a chromium rich alpha prime phase, after the service temperature has been applied to him.

• les pourcentages massiques de l'aluminium x_Al, du nickel x_Ni, du chrome x_Cr, du titane x_Ti, du carbone x_C, du niobium x_Nb, du tantale x_Ta, du silicium x_Si et du manganèse x_Mn respectent les deux relations (R3,R4) suivantes : $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+455,4x_{\mathit{Al}}-0,32{\mathrm{<figure-callout\; id="2"\; label="x\; Ni"\; filenames="imgf0001.png"\; state="\{\{state\}\}">x</figure-callout>}}_{\mathit{Ni}}^{\mathit{}}+15,{3x}_{\mathit{Ni}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Si}}+27x_{\mathit{Mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Ta}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="Ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">Ts</figure-callout>}$$
  
  $$1,8x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+38,3x_{\mathit{Al}}+0,42{\mathrm{<figure-callout\; id="2"\; label="x\; Ni"\; filenames="imgf0001.png"\; state="\{\{state\}\}">x</figure-callout>}}_{\mathit{Ni}}^{\mathit{}}-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572\le \mathrm{Ts}$$
  
  où Ts représente la température de service ;
• le pourcentage massique de nickel x_Ni est défini à partir de la résolution d'équations du second degré (E3,E4), issues des relations (R3,R4) liant les pourcentages massiques des composés de l'alliage et la température de service ;
• le pourcentage massique de nickel x_Ni est compris entre une valeur (X), supérieure à 30%, consistant en la valeur la plus grande entre les solutions (X',X") des équations (E3,E4), et une valeur majorée de dix points (X+10) ;
• le pourcentage massique d'aluminium dans l'alliage est supérieur à 3,8%, voire supérieur à 4%.

According to other advantageous and nonlimiting features of the invention, taken alone or in any technically feasible combination:

• the weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , silicon x _Si and manganese x _Mn meet the two following relations (R3, R4): $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">ts</figure-callout>}$$
  
  $$1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$
  
  where Ts represents the service temperature;
• the mass percentage of nickel x _Ni is defined from the resolution of equations of the second degree (E3, E4), resulting from the relations (R3, R4) linking the mass percentages of the compounds of the alloy and the operating temperature;
• the mass percentage of nickel x _Ni is between a value (X), greater than 30%, consisting of the largest value between the solutions (X ', X ") of the equations (E3, E4), and an increased value ten points (X + 10);
• the mass percentage of aluminum in the alloy is greater than 3.8%, or even greater than 4%.

BRIEF DESCRIPTION OF THE DRAWINGS

• le tableau 1 présente la composition d'alliages testés dans le cadre de l'invention ;
• les figures la à 1d présentent les diagrammes de phase, issues de simulations, pour quatre alliages testés dans le cadre de l'invention ;
• les figures 2a à 2d présentent des images par microscopie électronique à balayage (MEB) de quatre alliages testés dans le cadre de l'invention ;
• la figure 3 présente des courbes de fluage à 1050°C sous une contrainte de 17MPa, pour quatre alliages testés dans le cadre de l'invention ;
• la figure 4 présente les fractions molaires des carbures primaires M₇C₃ et M₂₃C₆ après solidification de l'alliage, en fonction de la teneur en chrome.

Other features and advantages of the invention will emerge from the detailed description which follows with reference to the appended figures in which:

• Table 1 shows the composition of alloys tested in the context of the invention;
• FIGS. 1a to 1d present the phase diagrams, resulting from simulations, for four alloys tested in the context of the invention;
• the Figures 2a to 2d present images by scanning electron microscopy (SEM) of four alloys tested in the context of the invention;
• the figure 3 presents creep curves at 1050 ° C. under a stress of 17 MPa, for four alloys tested within the scope of the invention;
• the figure 4 shows the molar fractions of the primary carbides M ₇ C ₃ and M ₂₃ C ₆ after solidification of the alloy, depending on the chromium content.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an austenitic alloy based on nickel, chromium and iron, with a high aluminum content, intended to be used at a service temperature Ts between 900 ° C. and 1200 ° C. Ts can typically be set to 1000 ° C.

Note that the austenitic alloy according to the invention could be used at operating temperatures below 900 ° C, but would not have, in these temperature ranges, significant advantage over a standard alloy containing little or no 'aluminum.

• du chrome entre 20% et 32%,
• du nickel entre 30% et 60%,
• de l'aluminium entre 3,5% et 6%,
• du carbone entre 0,4% et 0,7%,
• du titane entre 0,05% et 0,3%,
• du niobium et/ou du tantale entre 0,6 et 2%,
• un élément, composé d'au moins une terre rare et/ou d'hafnium, entre 0,002% et 0,1%,
• du silicium entre 0 et 0,5%,
• du manganèse entre 0 et 0,5%,
• du tungstène entre 0 et 2%,
• du fer pour faire la balance des composés de l'alliage.

The alloy comprises the following compounds, their amount in the alloy being expressed as a percentage by mass:

• chromium between 20% and 32%,
• nickel between 30% and 60%,
• aluminum between 3.5% and 6%,
• carbon between 0.4% and 0.7%,
• titanium between 0.05% and 0.3%,
• niobium and / or tantalum between 0.6 and 2%,
• an element composed of at least one rare earth and / or hafnium between 0.002% and 0.1%,
• silicon between 0 and 0.5%,
• manganese between 0 and 0.5%,
• tungsten between 0 and 2%,
• iron to balance the compounds of the alloy.

In the remainder of the description, the terms "content", "quantity" or "percentage" with respect to a compound of the alloy will have to be interpreted as relating to the "mass percentage" of said compound.

In the alloy according to the invention, a minimum of 20% chromium is required to ensure good resistance to corrosion and to allow the formation of chromium carbides, which favorably impact the creep resistance of the alloy. The maximum mass percentage of chromium is constrained to 32% in particular to limit the integration of alphagene element tending to destabilize the austenitic structure of the alloy.

In addition, it should be noted that the type of primary carbides (M ₇ C ₃ or M ₂₃ C ₆ ), predominant after solidification of the alloy, varies depending on the chromium content, as illustrated in FIG. figure 4 . It is observed that the molar fraction of the primary carbides M ₇ C ₃ passes through an optimum for a chromium content between 23% and 28%, then decreases, whereas the molar fraction of the primary carbides M ₂₃ C ₆ increases significantly beyond a chromium content of the order of 30%. Advantageously, the weight percentage of chromium is thus kept below 30%, or even less than 28%, so as to guarantee a majority fraction of M ₇ C ₃ primary carbides after solidification of the alloy, which make it possible to obtain a thin and homogeneous dispersion of M ₂₃ C ₆ secondary carbides (from the transformation of primary carbides M ₇ C ₃ during high temperature cycles). Such a dispersion of secondary precipitates M ₂₃ C ₆ (thinner and uniformly distributed than the primary carbides M ₂₃ C ₆ ) provides improved creep properties to the alloy.

The minimum mass percentage of nickel is defined at 30% so as to maintain a refractory alloy of austenitic structure, the alloy containing at least 20% of chromium as well as other alphagenic elements tending to destabilize the austenitic structure in favor of a ferritic structure. The amount of nickel is limited to 60%, or even 55% for economic reasons, nickel being a strong contributor of costs.

Advantageously, the range of nickel content can be defined for a purpose just necessary, to prevent the formation of harmful phases at the service temperature Ts while maintaining controlled costs, as will be described later.

The mass percentage of carbon is defined at a minimum of 0.4% to allow formation in the alloy of a large volume fraction of carbides, said carbides reinforcing the creep resistance of the alloy. The maximum percentage is set at 0.7% in order to maintain sufficient ductility in the use of the material, carbide reinforcement also having a reduced ductility effect.

Titanium has a strong impact on the formation of finer and uniformly distributed carbides in the alloy: it is particularly effective at low levels, called micro additions. It is included in the alloy in a mass percentage ranging from 0.05% to 0.3%.

Niobium and / or tantalum, each in proportions ranging from 0.6% to 2%, are added to the alloy. These two compounds also contribute to the formation of carbides. Advantageously, when the two compounds are present in the alloy, the sum of the mass percentages of niobium and tantalum is greater than 0.6% and less than or equal to 2%.

Aluminum is present in the alloy at a high content, between 3.5% and 6%. Such a content allows the formation of a continuous aluminum oxide layer on the surface of the alloy in a wide range of oxygen partial pressure (ranging from less than 5 particles per million to high partial pressures such as under air), and a wide temperature range (intermediate temperatures around 800 ° C to temperatures above 1200 ° 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 900 ° C. and above.

Advantageously, the weight percentage of aluminum is greater than or equal to 3.8% or even 4%. A high aluminum content ensures the formation of an aluminum oxide layer over a wider range of environmental conditions. It also allows 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 aluminum oxide layers are consumed.

The addition of an element composed of at least one rare earth (such as, for example, yttrium, cerium) and / or hafnium is beneficial to the growth and adhesion of the oxide layer. aluminum on the surface of the alloy. The total amount of this element is set at a minimum of 0.002%. An amount greater than 0.1% does not bring any additional effect whereas it implies a strong impact on the cost; it can even be harmful to mechanical properties, including high mechanical resistance temperatures. Advantageously, the total content of rare earth (s) and / or hafnium is limited to 0.05%, or even limited to 0.01%.

The alloy may optionally contain silicon, to promote flow during casting of the alloy and enhance its resistance to corrosion. The amount of silicon is nevertheless limited to 0.5% to avoid negatively impacting the creep resistance of the alloy.

The alloy may also contain manganese, but in a mass percentage of less than 0.5% to avoid or limit the formation of spinel oxide of manganese and chromium which has a very fast formation kinetics but is less stable and protective than chromium oxide and even more so than aluminum oxide.

The alloy may optionally contain tungsten, which plays a minor role in improving the mechanical properties at high temperatures, by processing the tungsten enriched chromium carbides and by solid solution hardening. This element is limited to 2% because too much tungsten in the chromium carbides will make them lose their stability and their role of hardening at high temperatures.

Finally, the alloy comprises iron, in a percentage complementing the composition of the alloy, so that the sum of the mass percentages of the compounds reaches 100%.

Of course, the alloy may also comprise low levels of other conventional elements of steels found in particular in the raw materials or in the manufacturing steps. At low levels, these elements have little impact or special need. We thus find at levels less than 0.5% of elements such as molybdenum or copper. The alloy can possibly be polluted by trace impurities of the order of the particle per million, to the hundreds of particles per million, such as phosphorus, sulfur, lead, tin, etc. .

As mentioned in the introduction, it is usual for an austenitic alloy with a high aluminum content, correlatively to an excellent resistance to corrosion, to show a degradation of the creep resistance, and this above a mass percentage of aluminum of the order of 3%.

Thus, going beyond the role of each individual compound of the alloy, the Applicant has studied the connection between the microstructure of the alloy and its mechanical properties at the service temperature or beyond. The operating temperature is the temperature at which the alloy is intended to be subjected during its use: for example, for an alloy forming a steam cracking furnace tube, the operating temperature may be between 950 ° C and 1150 ° C.

These studies, notably based on scanning electron microscopy (SEM) or transmission (TEM) characterizations and on creep tests, made it possible to demonstrate that the creep properties of the alloy with a high content of aluminum (greater than or equal to 3.5%) are directly impacted by the precipitation of a B2-NiAl intermetallic phase and / or an alpha prime phase (of crystalline cubic crystalline structure) rich in chromium, at the service temperature ts.

As a reminder, B2 according to the notation Strukturbericht qualifies 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 atoms forms a simple centered cubic network, with one atom of one type in the center of each cube of the other type. Note that here the B2-NiAl phase is not necessarily stoichiometric, the Al sites may possibly be replaced by Cr or Fe atoms.

Thus, the Applicant has been able to determine that, in a high aluminum austenitic alloy, the creep resistance, at the service temperature Ts, decreases with the increase of the volume fraction of the B2-NiAl phase in the alloy. brought to said temperature. It is the same with the increase of the volume fraction of alpha prime phases.

On the basis of these observations, a characteristic of the austenitic alloy according to the invention is that it has less than 1% by volume of a B2-NiAl intermetallic phase and less than 1% by volume of a phase rich in chrome alpha prime, after the service temperature Ts has been applied to it for a few hours, typically for more than 10 hours. Preferably, the alloy according to the invention has less than 0.5% by volume of each of the B2-NiAl and alpha prime phases, or even less than 0.2% by volume. The fact that the austenitic alloy comprises a very small volume fraction of B2-NiAl and alpha prime phases makes it possible to ensure that the austenitic alloy with a high aluminum content has excellent creep resistance, in addition to excellent resistance to abrasion. environment (corrosion).

The presence of a very small volume fraction of B2-NiAl and alpha prime phases in the alloy or its absence, after the service temperature Ts has been applied to it, can be verified experimentally on a sample (for example by scanning electron microscopy or transmission) or, as proposed below, anticipated at the time of design of the alloy or verified from the composition of said alloy.

$$T_{\mathit{max}}^{\mathit{\alpha ʹ}}\left(\mathit{°C}\right)=1,8x_{\mathit{Al}}^2+38,3x_{\mathit{Al}}+0,42x_{\mathit{Ni}}^2-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572$$

From correlations between the physical characterizations and simulations CALPHAD (calculations of phase diagrams, making it possible to predict the phases present in the alloy with equilibrium in temperature, according to its composition), two relations R1, R2 were established between the mass percentages of certain compounds of the alloy and the maximum temperature of the stability domain respectively of the B2-NiAl intermetallic phase (R1) and of the alpha prime phase (R2): $$T_{\mathit{max}}^{B2-\mathit{NiAl}}\left(\mathit{°\; C}\right)=-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866$$

$$T_{\mathit{max}}^{\mathit{\alpha \text{'}}}\left(\mathit{°\; C}\right)=1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572$$

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}},$

les températures maximales du domaine de stabilité respectivement de la phase intermétallique B2-NiAl et de la phase alpha prime, dans l'alliage. Lesdites températures maximales du domaine de stabilité peuvent être vues comme les températures limites en-dessous desquelles il y a formation dans l'alliage des phases B2-NiAl et alpha prime, sur une plage de températures correspondant au domaine de stabilité de chaque phase.Mass percentages in the alloy of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta and (when present) silicon x _Si and manganese x _Mn , are linked to $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}},$

the maximum temperatures of the stability domain respectively of the B2-NiAl intermetallic phase and of the alpha prime phase, in the alloy. Said maximum temperatures of the stability range can be seen as the limiting temperatures below which there is formation in the alloy of the phases B2-NiAl and alpha prime over a temperature range corresponding to the stability range of each phase.

These relationships are valid for mass percentages of the compounds included in the ranges defined above for the alloy according to the invention.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

du domaine de stabilité des phases B2-NiAl et alpha prime (en d'autres termes, ils tendent à élargir son domaine d'existence vers les hautes températures) ; les composés Ni et C tendent à diminuer les températures maximales $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

du domaine de stabilité des phases B2-NiAl et alpha prime.It appears that the compounds Al, Cr, Si, Mn, Ti, Nb and Ta tend to increase the maximum temperatures $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

from the stability domain of the B2-NiAl and alpha prime phases (in other words, they tend to widen their field of existence towards high temperatures); the compounds Ni and C tend to decrease the maximum temperatures $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

the stability domain of the B2-NiAl and alpha prime phases.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

des domaines de stabilité de la phase B2-NiAl et de la phase alpha prime, de sorte que l'alliage, soumis à Ts lors de son utilisation, ne présente pas ou très peu de précipités de phases intermétallique B2-NiAl et/ou alpha prime, susceptibles de diminuer sa résistance au fluage.Advantageously, the service temperature Ts must be greater than the maximum temperatures $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

stability domains of the B2-NiAl phase and the alpha prime phase, so that the alloy, subjected to Ts during its use, exhibits no or very few intermetallic phase precipitates B2-NiAl and / or alpha premium, which may reduce its resistance to creep.

$$1,8x_{\mathit{Al}}^2+38,3x_{\mathit{Al}}+0,42x_{\mathit{Ni}}^2-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572\le \mathrm{Ts}$$

où Ts représente la température de service.According to an advantageous embodiment of the invention, the weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta and when they are present, silicon x _Si and manganese x _Mn , thus respect one and the other of the two following relations R3, R4: $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">ts</figure-callout>}$$

$$1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$

where Ts represents the service temperature.

In the field of steam cracking furnaces, the alloys forming the tubes can be subjected to temperatures ranging from 950 ° C to 1150 ° C. A service temperature Ts of 1000 ° C may for example be taken into account in the above relationships to cover a large part of industrial cases.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

du domaine de stabilité des phases B2-NiAl et alpha prime. Le nickel est aussi le centre de coût majoritaire et il est intéressant de limiter sa teneur au minimum. Ainsi, selon un autre mode de réalisation avantageux de l'invention, le pourcentage massique de nickel est défini à partir des relations R3, R4 précédentes, par la résolution des équations du second degré E3, E4 suivantes : $$\left[-0,32\times {\left(\mathit{Xʹ}\right)}^2+15,3\times \left(\mathit{Xʹ}\right)+\mathit{Cʹ}\right]=0,$$

où:

• $\mathit{Cʹ}=-28,3x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+455,4x_{\mathit{Al}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Si}}+27x_{\mathit{Mn}}+$
  
  16x_Ti + 12x_Nb + 16x_Ta - 45x_C - 866 - Ts
• x_Al, x_Cr, x_Si, x_Mn, x_Ti, x_Nb, x_Ta, x_C sont respectivement les pourcentages massiques de l'aluminium, du chrome, du silicium, du manganèse, du titane, du niobium, du tantale et du carbone,
• Ts représente la température de service.

In the alloy according to the invention, nickel is considered as the majority stabilizing element and it must be introduced in sufficient quantity to reduce the maximum temperatures. $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

the stability domain of the B2-NiAl and alpha prime phases. Nickel is also the major cost center and it is interesting to limit its content to a minimum. Thus, according to another advantageous embodiment of the invention, the mass percentage of nickel is defined from the previous relations R3, R4, by solving the following equations of the second degree E3, E4: $$\left[-0,32\times {\left(\mathit{X\; \text{'}}\right)}^2+15,3\times \left(\mathit{X\; \text{'}}\right)+\mathit{C\text{'}}\right]=0,$$

or:

• $\mathit{C\text{'}}=-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+$
  
  16 x _Ti + 12 x _Nb + 16 x _Ta - 45 x _C - 866 - Ts
• x _Al, x _Cr , x _Si , x _Mn , x _Ti , x _Nb , x _Ta , x _C are respectively the mass percentages of aluminum, chromium, silicon, manganese, titanium, niobium, tantalum and carbon,
• Ts represents the service temperature.

$$\left[0,42\times {\left(\mathit{Xʺ}\right)}^2-51,2\times \left(\mathit{Xʺ}\right)+\mathit{Cʺ}\right]=0,$$

où:

• $\mathit{Cʺ}=1,8x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+38,3x_{\mathit{Al}}+27,8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+$
  
  22x_Ta - 334x_C + 1572 - Ts
• x_Al, x_Cr, x_Si, x_Mn, x_Ti, x_Nb, x_Ta, x_C sont respectivement les pourcentages massiques de l'aluminium, du chrome, du silicium, du manganèse, du titane, du niobium, du tantale et du carbone,
• Ts représente la température de service.

If [(15.3) ² + 4 × 0.32 × C ' ]> 0, then X' is expressed as: $$\mathit{X\; \text{'}}=\left[15,3+\sqrt{\left[{\left(15,3\right)}^2+4\times 0,32\times \mathit{C\text{'}}\right]}\right]/\left[2\times 0,32\right]$$

$$\left[0,42\times {\left(\mathit{X"}\right)}^2-51,2\times \left(\mathit{X"}\right)+\mathit{C"}\right]=0,$$

or:

• $\mathit{C"}=1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+27,8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+$
  
  22 x _Ta - 334 x _C + 1572 - Ts
• x _Al, x _Cr , x _Si , x _Mn , x _Ti , x _Nb , x _Ta , x _C are respectively the mass percentages of aluminum, chromium, silicon, manganese, titanium, niobium, tantalum and carbon,
• Ts represents the service temperature.

If [(51.2) ² - 4 × 0.42 × C " ]> 0, then X" is expressed as: $$\mathit{X"}=\left[51,2-\sqrt{\left[{\left(51,2\right)}^2-4\times 0,42\times \mathit{C"}\right]}\right]/\left[2\times 0,42\right]$$

• 30%, teneur minimale en Ni de l'alliage
• X'
• et X".

The percentage of nickel x _Ni is then chosen between X and X + 10, with X the maximum between:

• 30%, minimum Ni content of the alloy
• X '
• and X ".

X is a minimum value for the alloy to have very little or no B2-NiAl phases and alpha prime at the service temperature Ts. An upper bound is defined at X plus 10 points (X + 10), to leave an industrial latitude over the control of the composition. More nickel does not bring additional benefits and unnecessarily increases the costs of the alloy. Alternatively, the upper bound could be set to X + 8 or even X + 6.

• X' = 44,6
• et X" = 41,2

By way of example, for a service temperature Ts of 950 ° C. and for an alloy comprising the following mass percentages of carbon, manganese, silicon, chromium, niobium, aluminum, titanium and yttrium: Compound in% mass> C mn Yes Or Cr Nb al Ti Y Fe Alloy 0.45 0.2 0.2 xNi 25 0.8 4 0.1 0005 Ball. we obtain :

• X ' = 44.6
• and X " = 41.2

• un minimum de X = 44,6% (la valeur X, supérieure à 30%, consistant en la valeur la plus grande entre les solutions X' et X" des équations E3 et E4)
• et un maximum de X+10 = 54,6%.

The mass percentage of nickel x _Ni according to this embodiment is therefore chosen between:

• a minimum of X = 44.6% (the X value, greater than 30%, consisting of the largest value between solutions X 'and X "of equations E3 and E4)
• and a maximum of X + 10 = 54.6%.

The invention also relates to a method for designing an austenitic alloy with a high aluminum content and having excellent resistance to both corrosion and creep at a service temperature greater than or equal to 900 ° C.

• du chrome entre 20% et 32%,
• du nickel entre 30% et 60%,
• de l'aluminium entre 3,5% et 6%,
• du carbone entre 0,4% et 0,7%,
• du titane entre 0,05% et 0,3%,
• du niobium et/ou du tantale entre 0,6 et 2%,
• un élément, composé d'au moins une terre rare et/ou d'hafnium, entre 0,002% et 0,1%,
• du silicium entre 0 et 0,5%,
• du manganèse entre 0 et 0,5%,
• du tungstène entre 0 et 2%,
• du fer pour faire la balance des composés de l'alliage.

The design process according to the invention applies to an austenitic alloy based on nickel, chromium and iron, and high aluminum content, for use at a service temperature Ts between 900 ° and 1200 ° C, and comprising the following compounds in percentages by weight:

• chromium between 20% and 32%,
• nickel between 30% and 60%,
• aluminum between 3.5% and 6%,
• carbon between 0.4% and 0.7%,
• titanium between 0.05% and 0.3%,
• niobium and / or tantalum between 0.6 and 2%,
• an element composed of at least one rare earth and / or hafnium between 0.002% and 0.1%,
• silicon between 0 and 0.5%,
• manganese between 0 and 0.5%,
• tungsten between 0 and 2%,
• iron to balance the compounds of the alloy.

The design process includes the choice of the respective weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , and they are present, silicon x _Si , manganese x _Mn and tungsten x _W so that the alloy has less than 1%, even less than 0.5%, or even less than 0.2% by volume of a B2-NiAl intermetallic phase and / or an alpha prime phase, after the service temperature Ts has been applied to it for a few hours.

The presence of a very small volume fraction of B2-NiAl and alpha prime phases in the alloy or its absence, after the service temperature Ts has been applied thereto, can be verified experimentally on a sample (for example by scanning electron microscopy or transmission).

$$1,8x_{\mathit{Al}}^2+38,3x_{\mathit{Al}}+0,42x_{\mathit{Ni}}^2-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572\le \mathrm{Ts}$$

où Ts représente la température de service.According to an advantageous embodiment of the design process, the respective weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , and if they are present, silicon x _Si and manganese x _Mn , are chosen so as to respect one of the two following relations R3, R4: $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">ts</figure-callout>}$$

$$1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$

where Ts represents the service temperature.

où:

• $\begin{array}{c}\mathit{Cʹ}=-28,3x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+455,4x_{\mathit{Al}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Si}}+27x_{\mathit{Mn}}+\\ {16x}_{\mathit{\tau i}}+{12x}_{\mathit{Nb}}+{16x}_{\mathit{\tau a}}-{46x}_c-866-\mathit{Ts}\end{array}$
  
• x_Al, x_Cr, x_Si, x_Mn, x_Ti, x_Nb, x_Ta, x_C sont respectivement les pourcentages massiques de l'aluminium, du chrome, du silicium, du manganèse, du titane, du niobium, du tantale et du carbone,
• Ts représente la température de service.

According to an advantageous embodiment, the design method according to the invention makes it possible to define the mass percentage of nickel x _Ni : said percentage is defined from the aforementioned relations R3, R4, by solving the equations E3, E4 of second degree: $$\left[-0,32\times {\left(\mathit{X\; \text{'}}\right)}^2+15,3\times \left(\mathit{X\; \text{'}}\right)+\mathit{C\text{'}}\right]=0,$$

or:

• $\begin{array}{c}\mathit{C\text{'}}=-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+\\ {16x}_{\mathit{\tau i}}+{12x}_{\mathit{Nb}}+{16x}_{\mathit{\tau a}}-{46x}_c-866-\mathit{ts}\end{array}$
  
• x _Al, x _Cr , x _Si , x _Mn , x _Ti , x _Nb , x _Ta , x _C are respectively the mass percentages of aluminum, chromium, silicon, manganese, titanium, niobium, tantalum and carbon,
• Ts represents the service temperature.

$$\left[0,42\times {\left(\mathit{Xʺ}\right)}^2-51,2\times \left(\mathit{Xʺ}\right)+\mathit{Cʺ}\right]=0,$$

où:

• $\mathit{Cʺ}=1,8x_{\mathit{<figure-callout\; id="2"\; label="Al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Al</figure-callout>}}^2+38,3x_{\mathit{Al}}+27,8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+$
  
  22x_Ta - 334x_C + 1572 - Ts
• x_Al, x_Cr, x_Si, x_Mn, x_Ti, x_Nb, x_Ta, x_C sont respectivement les pourcentages massiques de l'aluminium, du chrome, du silicium, du manganèse, du titane, du niobium, du tantale et du carbone,
• Ts représente la température de service.

When [(15.3) ² + 4 × 0.32 × C ' ]> 0, X' is then expressed as: $$\mathit{X\; \text{'}}=\left[15,3+\sqrt{\left[{\left(15,3\right)}^2+4\times 0,32\times \mathit{C\text{'}}\right]}\right]/\left[2\times 0,32\right]$$

$$\left[0,42\times {\left(\mathit{X"}\right)}^2-51,2\times \left(\mathit{X"}\right)+\mathit{C"}\right]=0,$$

or:

• $\mathit{C"}=1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+27,8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+$
  
  22 x _Ta - 334 x _C + 1572 - Ts
• x _Al, x _Cr , x _Si , x _Mn , x _Ti , x _Nb , x _Ta , x _C are respectively the mass percentages of aluminum, chromium, silicon, manganese, titanium, niobium, tantalum and carbon,
• Ts represents the service temperature.

When [(51.2) ² - 4 × 0.42 × C "]> 0, X" is then expressed as: $$\mathit{X"}=\left[51,2-\sqrt{\left[{\left(51,2\right)}^2-4\times 0,42\times \mathit{C"}\right]}\right]/\left[2\times 0,42\right]$$

• 30%, teneur minimale en Ni de l'alliage
• X'
• et X".

The percentage of nickel x _Ni is then chosen between X and X + 10, with X the maximum between:

• 30%, minimum Ni content of the alloy
• X '
• and X ".

X is a minimum value for the alloy to have very little or no B2-NiAl phases and alpha prime at the service temperature Ts. An upper bound is defined at X + 10 because more nickel does not bring additional benefits and unnecessarily increases the costs of the alloy; the upper limit could possibly be set to X + 8 or even X + 6.

As stated above, in the field of steam cracking furnaces, the alloys forming the tubes are usually subjected to temperatures ranging from 950 ° C to 1150 ° C. Service temperatures Ts of 950 ° C, 1000 ° C or 1050 ° C may be the most commonly considered.

The invention also relates to a method for validating the compatibility of a high aluminum austenitic alloy with a service temperature Ts defined between 900 ° C and 1200 ° C. Compatible alloy means an alloy having excellent resistance to corrosion or creep, said service temperature Ts or above.

• du chrome entre 20% et 32%,
• du nickel entre 30% et 60%
• de l'aluminium entre 3,5% et 6%,
• du carbone entre 0,4% et 0,7%,
• du titane entre 0,05% et 0,3%,
• du niobium et/ou du tantale entre 0,6 et 2%,
• un élément, composé d'au moins une terre rare et/ou d'hafnium, entre 0,002% et 0,1%,
• du silicium entre 0 et 0,5%,
• du manganèse entre 0 et 0,5%,
• du tungstène entre 0 et 2%,
• du fer pour faire la balance des composés de l'alliage.

The validation process according to the invention applies to an austenitic alloy based on nickel, chromium and iron, and with a high aluminum content intended to be used at a service temperature Ts or above, and comprising the following compounds in percentages by mass:

• chromium between 20% and 32%,
• nickel between 30% and 60%
• aluminum between 3.5% and 6%,
• carbon between 0.4% and 0.7%,
• titanium between 0.05% and 0.3%,
• niobium and / or tantalum between 0.6 and 2%,
• an element composed of at least one rare earth and / or hafnium between 0.002% and 0.1%,
• silicon between 0 and 0.5%,
• manganese between 0 and 0.5%,
• tungsten between 0 and 2%,
• iron to balance the compounds of the alloy.

The validation process includes verifying that the alloy is free or less than 1%, or even less than 0.5%, or even less than 0.2% by volume B2-NiAl intermetallic phase and alpha prime after the temperature service Ts was applied to him for a few hours (typically 10 hours).

The presence of a very small volume fraction of the B2-NiAl and alpha prime phases in the alloy or its absence, after the service temperature has been applied to it, can be verified experimentally on a sample (for example by microscopic analysis electronic scanning or transmission).

$$1,8x_{\mathit{Al}}^2+38,3x_{\mathit{Al}}+0,42x_{\mathit{Ni}}^2-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572\le \mathrm{Ts}$$

According to an advantageous embodiment of the validation process, the respective weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta and if present, silicon x _Si and manganese x _Mn , in the alloy are measured (for example by spark spectrometry); the following relations are then applied so as to check the compatibility of the alloy with a determined service temperature Ts: $$-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{<figure-callout\; id="1"\; label="ts"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">ts</figure-callout>}$$

$$1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$

If the inequality is respected for one and the other of the two relations R3, R4, the alloy is compatible with the determined service temperature Ts. If at least one inequality is not respected, the alloy is identified as not compatible with the determined service temperature Ts; said alloy may potentially be identified compatible with a higher service temperature Ts.

Tests and examples:

The examples of alloys described below have a high aluminum content (greater than 3.5%), their high environmental resistance has been verified and is considered assured.

Examples of service temperatures Ts of 950 ° C, 1000 ° C and 1050 ° C are used to demonstrate the variation that can be seen in this range and to control the most common service temperatures used in the application of steam cracking.

The creep resistance of the alloys presented as examples (Table 1) was evaluated using creep tests at 1050 ° C. under a constant stress of 17 MPa, the tests being carried out on samples taken from parts made in the various alloys. . From these tests, a deformation curve (percentage of deformation of the sample) as a function of time is extracted, and a time at break t _R , to arrive at the rupture of the sample.

The time at break t _R of the different samples is compared with the time at break t _Rref of an alloy based on nickel, chromium and iron known and used for applications petrochemical steam crackers, whose trade name is Manaurite® XTM.

The composition of the alloys numbered from 1 to 8 is detailed in Table 1. The composition of the reference alloy Manaurite XTM, denoted "Ref", is also described in Table 1. The time at break t _Rref of the Reference alloy under the creep test conditions considered is 1095 hours. The resistance of an alloy in the context of the present examples is therefore considered very good if the time at break t _R is in the same range of values, greater than or equal to 1000h.

The alloys referenced 1 to 8 of Table 1 comprise mass percentages of aluminum ranging from 3.5% to 5.6%. The other compounds of each alloy 1 to 8 have mass percentages included in the foregoing ranges for an alloy according to the invention, as can be seen in Table 1.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

de domaine de stabilité des phases intermétallique B2-NiAl et alpha prime, c'est-à-dire les températures en-dessous desquelles lesdites phases apparaissent, peuvent être calculées à partir des pourcentages massiques des composés aluminium, nickel, chrome, titane, carbone, niobium, tantale et quand ils sont présents, silicium et manganèse, d'après les relations R1,R2 établies par la demanderesse : $$T_{\mathit{max}}^{B2-\mathit{NiAl}}\left(\mathit{°C}\right)=-28,3x_{\mathit{Al}}^2+455,4x_{\mathit{Al}}-0,32x_{\mathit{Ni}}^2+15,{3x}_{\mathit{Ni}}-0,22x_{\mathit{Cr}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Si}}+27x_{\mathit{Mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Ta}}-45x_C-866$$

$$T_{\mathit{max}}^{\mathit{\alpha ʹ}}\left(\mathit{°C}\right)=1,8x_{\mathit{Al}}^2+38,3x_{\mathit{Al}}+0,42x_{\mathit{Ni}}^2-51.2x_{\mathit{Ni}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Si}}+8x_{\mathit{Mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Ta}}-334x_C+1572$$

Maximum temperature values $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

B2-NiAl and alpha prime intermetallic phase stability domain, that is to say the temperatures below which said phases appear, can be calculated from the mass percentages of the compounds aluminum, nickel, chromium, titanium, carbon , niobium, tantalum and when present, silicon and manganese, according to the relations R1, R2 established by the plaintiff: $$T_{\mathit{max}}^{B2-\mathit{NiAl}}\left(\mathit{°\; C}\right)=-28,3x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{<figure-callout\; id="2"\; label="Cr"\; filenames="imgf0001.png"\; state="\{\{state\}\}">Cr</figure-callout>}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866$$

$$T_{\mathit{max}}^{\mathit{\alpha \text{'}}}\left(\mathit{°\; C}\right)=1,8x_{\mathit{<figure-callout\; id="2"\; label="al"\; filenames="imgf0001.png"\; state="\{\{state\}\}">al</figure-callout>}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572$$

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

apparaissent également sur les diagrammes de phase issus de simulations CALPHAD présentés sur les figures la à 1d : le domaine de stabilité de la phase B2-NiAl est représenté par la courbe à symboles ronds évidés, le domaine de stabilité de la phase alpha-prime est représenté par la courbe à symboles en croix noires.These maximum temperatures $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

also appear on the phase diagrams resulting from CALPHAD simulations presented in FIGS. 1a to 1d: the stability domain of the B2-NiAl phase is represented by the recessed round symbol curve, the stability domain of the alpha-prime phase is represented by the black cross-symbols curve.

de 822.1°C, 906°C, 1079.6°C, 961.9°C, 1127.8°C, 1175.2°C, 988.2°C, 1255.2°C et respectivement une température maximale $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

de 878.6°C, 895°C, 1158.3°C, 907.1°C, 1098.4°C, 1120.1°C, 858.7°C, 961.2°C.The alloys 1 to 8 respectively have a maximum temperature $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

of 822.1 ° C, 906 ° C, 1079.6 ° C, 961.9 ° C, 1127.8 ° C, 1175.2 ° C, 988.2 ° C, 1255.2 ° C and respectively a maximum temperature $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

878.6 ° C, 895 ° C, 1158.3 ° C, 907.1 ° C, 1098.4 ° C, 1120.1 ° C, 858.7 ° C, 961.2 ° C.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}\le \mathit{Ts},$

ils ne présentent donc pas de phases B2-NiAl et alpha prime à la température de service Ts et sont conformes à l'invention. Les alliages 3, 4, 5, 6, 7 et 8 ne respectent les deux relations précitées, et ne sont donc pas conformes à l'invention, pour une température de service de 950°C.For a service temperature of 950 ° C, alloys 1 and 2 respect both relationships $T_{\mathit{max}}^{B2-\mathit{NiAl}}\le \mathit{ts}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}\le \mathit{ts},$

they therefore do not have phases B2-NiAl and alpha prime at the service temperature Ts and are in accordance with the invention. The alloys 3, 4, 5, 6, 7 and 8 do not respect the two relations mentioned above, and therefore do not comply with the invention, for an operating temperature of 950 ° C.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}\le \mathit{Ts},$

et sont conformes à l'invention.For an operating temperature of 1000 ° C or 1050 ° C, alloys 1, 2, 4 and 7 respect both relationships $T_{\mathit{max}}^{B2-\mathit{NiAl}}\le \mathit{ts}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}\le \mathit{ts},$

and are in accordance with the invention.

The figure 2a shows that the alloy 4 (sample from the creep test at 1050 ° C, characterized physically post mortem, for example), has no B2-NiAl intermetallic phase, or alpha prime phase, after the temperature 1050 ° C has been applied to it. Only the classical phases are observed: carbides M ₂₃ C ₆ in an austenitic matrix. The initial M ₇ C ₃ primary interdendritic carbides were converted into M ₂₃ C ₆ secondary carbides, accompanied by a fine precipitation of M ₂₃ C ₆ secondary carbides (black zones).

Alloys 1, 2, 4 and 7 have break times t _R between 1000h and 1351h (Table 1), which corresponds to excellent creep resistance. The figure 3 presents the deformation of a sample of the alloy 4 during the creep test, as a function of time. The alloy 4 according to the invention for a service temperature Ts of 1050 ° C. undergoes only a very small deformation at 1050 ° C. under stress for at least the first 1000 hours.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}\le \mathit{Ts},$

et ne sont pas conformes à l'invention pour une température de service de 1000°C ou de 1050°C.Alloys 3, 5, 6 and 8 do not respect one or both of the two relationships $T_{\mathit{max}}^{B2-\mathit{NiAl}}\le \mathit{ts}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}\le \mathit{ts},$

and are not in accordance with the invention for a service temperature of 1000 ° C or 1050 ° C.

The Figures 2b, 2c and 2d show respectively that the alloys 5, 6 and 8 (samples from the creep test at 1050 ° C, characterized physically post mortem, by way of example) comprise B2-NiAl precipitates after the temperature of 1050 ° C was applied. This B2-NiAl intermetallic phase could be identified as such thanks to fine characterizations carried out by transmission electron microscopy (TEM). The B2-NiAl phase appeared under two different types in alloys 6 ( Figure 2c ) and 8 ( figure 2d ): a type I having a flat shape in the austenitic matrix, formed by homogeneous germination; and a type II present between the carbide precipitates and the austenitic matrix, formed by heterogeneous germination. It should be noted that the alpha prime phase rich in chromium has also been identified by TEM, essentially precipitating at the B2-NiAl / matrix interfaces and in the form of nano-precipitates in the B2-NiAl phase.

It may be noted that the alloys 3, 5, 6 and 8 have break times t _R between 47h and 500h, which corresponds to a mechanical resistance well below the reference referred to. The figure 3 presents the deformation of a sample of each of the alloys 5, 6 and 8 during the creep test as a function of time. They undergo significant deformation at 1050 ° C under stress during the first 250 hours.

In order to have good creep resistance at a given service temperature Ts, the austenitic alloy with a high aluminum content according to the invention must comprise the compounds stated, in percentages by weight included in the stated ranges, and contain only one low volume fraction (less than or equal to 1%) or not at all of the B2-NiAl and alpha prime intermetallic phases, after the determined service temperature Ts has been applied thereto.

et $T_{\mathit{max}}^{\mathit{\alpha ʹ}}$

des domaines de stabilité respectivement de la phase intermétallique B2-NiAl et de la phase alpha prime et les pourcentages massiques des composés aluminium, nickel, chrome, titane, carbone, niobium, tantale, silicium et manganèse, permettent avantageusement de prévoir, pour une composition donnée de l'alliage (dans la limite des fourchettes énoncées), si ce dernier pourra présenter une haute résistance au fluage à une température de service Ts déterminée.The relations R1, R2, R3, R4 established by the applicant between the maximum temperatures $T_{\mathit{max}}^{B2-\mathit{NiAl}}$

and $T_{\mathit{max}}^{\mathit{\alpha \text{'}}}$

stability domains respectively of the B2-NiAl intermetallic phase and of the alpha prime phase and the mass percentages of the aluminum, nickel, chromium, titanium, carbon, niobium, tantalum, silicon and manganese compounds advantageously make it possible to provide, for a composition given the alloy (within the limits of the stated ranges), if the latter can exhibit a high creep resistance at a given service temperature Ts.

The relationships established by the applicant also advantageously make it possible to choose the mass percentage of nickel as a function of the other alloy compounds and the service temperature Ts, in a range ensuring the high resistance to creep of the alloy while limiting unnecessary costs of too much of this compound.

The austenitic alloys according to the invention can find applications in the field of petrochemicals (steam-cracking furnaces), in any other high-temperature application, typically greater than or equal to 900 ° C. combining problems of resistance to the environment and the environment. creep.

Of course, the invention is not limited to the embodiments described and variants can be made without departing from the scope of the invention as defined by the claims.

Claims (12)

Austenitic alloy based on nickel, chromium and iron, and with a high aluminum content, intended to be used at a given service temperature (Ts) between 900 ° C. and 1200 ° C., the alloy comprising the following compounds in mass percentages: • chromium between 20% and 32%, • nickel between 30% and 60%, • aluminum between 3.5% and 6%, • carbon between 0.4% and 0.7%, • titanium between 0.05% and 0.3%, • niobium and / or tantalum between 0.6% and 2%, An element composed of at least one rare earth and / or hafnium, between 0.002% and 0.1%, Silicon between 0 and 0.5%, • manganese between 0 and 0.5%, • tungsten between 0 and 2%, • iron to balance the compounds of the alloy; and having less than 1% by volume of a B2-NiAl intermetallic phase and less than 1% by volume of a chromium-rich alpha prime phase after the service temperature (Ts) has been applied thereto. Austenitic alloy according to the preceding claim, wherein the weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , silicon x _Si and manganese x _Mn respect the following two relations (R3, R4): $$-28,3x_{\mathit{al}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{Cr}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{ts}$$

$$1,8x_{\mathit{al}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$

where Ts represents the service temperature. Austenitic alloy according to the preceding claim, in which the mass percentage of nickel x _Ni is defined from the resolution of equations of the second degree (E3, E4) resulting from the relationships (R3, R4) linking the mass percentages of the compounds of the alloy and service temperature Ts. Austenitic alloy according to the preceding claim, wherein the mass percentage of nickel x _Ni is between a value (X), greater than 30%, consisting of the largest value between the solutions (X ', X ") of the equations ( E3, E4), and a value increased by ten points (X + 10). Austenitic alloy according to one of the preceding claims, wherein the sum of the percentages of niobium and tantalum, when these two compounds are present, is greater than 0.6% and less than or equal to 2%. Austenitic alloy according to one of the preceding claims, wherein the weight percentage of aluminum in the alloy is greater than 3.8%, or even greater than 4%. Austenitic alloy according to one of the preceding claims, wherein the mass percentage of chromium in the alloy is less than 30%, or even less than 28%. Austenitic alloy according to one of the preceding claims, wherein the total mass percentage of rare earths and / or hafnium in the alloy is between 0.002% and 0.05%. A process for the design of an austenitic alloy based on nickel, chromium and iron and having a high aluminum content for use at a given service temperature (Ts) of between 900 ° C. and 1200 ° C., and comprising following compounds in percentages by mass: • chromium between 20% and 32%, • nickel between 30% and 60%, • aluminum between 3.5% and 6%, • carbon between 0.4% and 0.7%, • titanium between 0.05% and 0.3%, • niobium and / or tantalum between 0.6 and 2%, An element composed of at least one rare earth and / or hafnium, between 0.002% and 0.1%, Silicon between 0 and 0.5%, • manganese between 0 and 0.5%, • tungsten between 0 and 2%, • iron to balance the compounds of the alloy; the method comprising the choice of the respective weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , silicon x _Si and manganese x _Mn , so that the alloy has less than 1% by volume of a B2-NiAl intermetallic phase and less than 1% by volume of a chromium rich alpha prime phase, after the service temperature (Ts) has been applied to him. A method of designing an austenitic alloy according to the preceding claim, wherein the weight percentages of aluminum x _Al, nickel x _Ni , chromium x _Cr , titanium x _Ti , carbon x _C , niobium x _Nb , tantalum x _Ta , silicon x _Si and manganese x _Mn respect the following two relations (R3, R4): $$-28,3x_{\mathit{al}}^2+455,4x_{\mathit{al}}-0,32x_{\mathit{Or}}^2+15,{3x}_{\mathit{Or}}-0,22x_{\mathit{Cr}}^2+20,7x_{\mathit{Cr}}+121x_{\mathit{Yes}}+27x_{\mathit{mn}}+16x_{\mathit{Ti}}+12x_{\mathit{Nb}}+16x_{\mathit{Your}}-45x_C-866\le \mathit{ts}$$

$$1,8x_{\mathit{al}}^2+38,3x_{\mathit{al}}+0,42x_{\mathit{Or}}^2-51.2x_{\mathit{Or}}+27.8x_{\mathit{Cr}}+34x_{\mathit{Yes}}+8x_{\mathit{mn}}+89x_{\mathit{Ti}}+39x_{\mathit{Nb}}+22x_{\mathit{Your}}-334x_C+1572\le \mathrm{ts}$$

where Ts represents the service temperature. A method of designing an austenitic alloy according to the preceding claim, wherein the mass percentage of nickel x _Ni is defined from the resolution of equations of the second degree (E3, E4) resulting from the relations (R3, R4) linking the mass percentages of the alloy compounds and the service temperature Ts, and wherein the mass percentage of nickel x _Ni is between a value (X), greater than 30%, consisting of the largest value between the solutions ( X ', X ") equations (E3, E4), and a value increased by ten points (X + 10). A method for designing an austenitic alloy according to one of the three preceding claims, wherein the weight percentage of aluminum in the alloy is greater than 3.8%, or even greater than 4%.

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