Patent Application
20240117471
This application claims the benefit of the filing date of French Patent Application Serial No. FR2210006, filed Sep. 30, 2022, for “FE—CR—NI—AL HIGH NICKEL CONTENT REFRACTORY AUSTENITIC STEEL.”
The present disclosure relates to the field of austenitic alloys requiring good mechanical strength and environmental resistance at high temperatures, in particular for use in reforming furnaces for the direct reduction of iron ore or more generally as a structural material for very high temperature applications such as in heat treatment furnaces. It particularly relates to an austenitic alloy with a high nickel content, which has excellent resistance to corrosion and creep at operating temperatures above 1100° C.
Austenitic alloys based on nickel, chromium and iron, called “refractory” alloys, have been known for many years for their applications at very high temperatures (see in particular document FR2333870).
To increase their environmental resistance, and in particular to carburization and oxidation, it was proposed to add aluminum as disclosed in document U.S. Pat. No. 4,248,629. Due to the formation of a layer of aluminum oxide on its surface, the resistance to carburization and oxidation in a very high temperature environment is improved.
In alloys that are subjected to extreme temperatures (typically between 1100° C. and 1185° C.), oxidized and/or decarburized internal areas appear close to the surface of the parts. Such damage appears in particular in “chromia-former” refractory austenitic steels due to the regeneration of the protective layer of Cr2O3 in operation. In the case of “alumina-former” refractory austenitic steels, decarburized areas and internal oxidation and nitriding may appear if the layer of alumina formed is not protective or is discontinuous. This damage to the microstructure, near the surface, linked to the ability of the alloy to self-protect from the environment, negatively impacts the creep resistance.
The current performance of refractory alloys limits the achievable yields in particular applications, in particular in the context of reformers for the direct reduction of iron ore where the operating temperatures conventionally go up to 1175° C. This extreme temperature combined with the mechanical stresses (stresses related to the weight of the parts themselves or to pressures of a few bars in operation) applied to the parts (for example, tubes) consisting of these alloys results in a very high creep stress, which limits the service life of the parts in question (and associated equipment).
It is therefore important to further improve the properties of refractory austenitic alloys with high chromium and nickel contents in order to achieve high performance, both in terms of resistance to environment and oxidation, as well as in terms of creep resistance, in particular for applications requiring operating temperatures greater than or equal to 1100° C.
The present disclosure proposes a solution for achieving the aforementioned aims. The present disclosure relates to an “alumina-former” refractory austenitic alloy with high chromium and nickel contents, which exhibits excellent resistance to the environment and creep at temperatures greater than or equal to 1100° C., typically between 1100° C. and 1185° C.
More particularly, the present disclosure relates to a refractory austenitic alloy intended to be used at an operating temperature greater than or equal to 1100° C., comprising all the following elements in percentages by weight:
−17.64+19.61xAl−1.29xAl2−101.46xN+450.65xN2−5.8368xN3+9.68xV+43.12xTi+30.02xSi+11.42xNi−0.18xNi2+35.05xNb+47.92xCr−0.34xCr2+13.97xMn−239.66xC≥1070.
According to advantageous features of the present disclosure, taken alone or in any feasible combination:
Other features and advantages of the present disclosure will emerge from the following detailed description of the present disclosure with reference to the appended figures, in which:
The present disclosure relates to a refractory austenitic alloy intended to be used at an operating temperature greater than or equal to 1100° C. In particular, the present alloy can be used for reforming furnaces, which are subjected to refractory brick temperatures typically between 1100° C. and 1185° C.
The austenitic alloy according to the present disclosure comprises all the following elements in percentage by weight:
Throughout the remainder of the description, the expressions “content,” “quantity” or “percentage” in terms of a compound of the alloy should be interpreted as relating to the “percentage by weight” of the compound. When a percentage by weight is indicated “between X and Y,” X and Y constituting the limits of the composition range, the limits should be considered as included in the range unless otherwise specified.
The refractory austenitic alloy according to the present disclosure is mainly composed of nickel (between 50.0% and 61.0%), chromium (between 25.0% and 32.0%), iron (between 4.0% and 18.0%) and aluminum (between 1.0% and 6.0%).
A minimum of 25.0% of chromium is required to ensure good resistance to corrosion (oxidation) and to allow the formation of chromium carbides, which favorably impact the creep resistance of the alloy. The maximum percentage of chromium is limited to 32.0%, in particular in order to limit the excessively strong integration of an alphagenic element tending to destabilize the austenitic structure of the alloy. Advantageously, the Cr content is defined between 26.0% and 31.0% in order to further promote creep resistance and protection of the alloy from the environment.
The minimum content of nickel is defined at 50.0% so as to retain a refractory alloy of austenitic structure, since the alloy contains at least 25.0% chromium as well as other alphagenic elements that tend to destabilize the austenitic structure in favor of a terrific structure. The quantity of nickel is limited to 61.0%, or even limited to 57.0%, or even 55.0% for economic reasons, the nickel being a high cost contributor.
The iron percentage by weight balances the elements of the alloy, so that the sum of the percentage by weight of the elements reaches 100%. A content of between 4.0% and 18.0% balances the other more advantageous elements. Preferentially, an iron content greater than or equal to 13.0% is desirable in order to reduce the costs of the grade.
Aluminum is present in the alloy at a medium-to-high content between 1.0% and 6.0%. Such a content allows the formation of a continuous layer of aluminum oxide (alumina), at the surface of the alloy, in a wide range of partial pressure of oxygen (ranging from less than 5 particles per million at high partial pressures such as in air), and a wide range of temperatures (typically, temperatures above 1000° C.). The surface layer of aluminum oxide then forms a barrier that is very resistant and effective against corrosion (oxidation, carburization, nitriding) of the alloy, at high temperatures, typically 1100° C. and above.
Advantageously, the percentage by weight of aluminum is greater than or equal to 2.0%, or even greater than or equal to 2.5%. A higher aluminum content ensures the formation of an aluminum oxide layer in a wider range of environmental conditions. It also makes it possible to have access to a larger “reservoir” of aluminum and thus to preserve the properties of the alloy over longer durations, in very severe environments where the layers of aluminum oxides are consumed.
It may be advantageous to maintain the percentage by weight of aluminum at or below 4%, to limit the precipitation of intermetallic phases B2-NiAl, likely to adversely affect the creep properties. As a reminder, B2 according to the Strukturbericht notation describes a phase comprising two types of atoms (here, Ni and Al) in an equal proportion and whose crystallographic structure is “interpenetrated primitive cubic,” that is, each of the two types of atoms forms a simple centered cubic network, with an atom of one type at the center of each cube of the other type.
The carbon must be present in the alloy for its hardening effect, by precipitation and by solid solution. The range of carbon in percentage by weight is defined between 0.05% and 0.60%. Advantageously, a percentage greater than or equal to 0.16%, or even 0.25%, or even 0.35% allows the formation of a large volume fraction of carbides and improves the flowability of the alloy.
The niobium content of the alloy is defined between 0.15% and 1.50% in order to establish the carbon in the form of niobium- and/or titanium-rich carbonitrides. Advantageously, the niobium, in combination with the titanium, prevents the formation of phase G, silicon-rich phase, unfavorable to creep properties. Preferentially, the niobium content is greater than or equal to 0.2%, to 0.4%, to 0.5%, to 0.8%, or even to 1%; and the niobium content is less than or equal to 1.4%, to 1.3%, or even to 1.2%.
A reactive element within the meaning of the present disclosure is defined as one of rare earth or hafnium. Adding at least one reactive element (such as cerium, yttrium, etc., or hafnium) is beneficial to the growth, adhesion and protective character of the alumina layer. Although this or these element(s) promote the fragmentation of the network of chromium carbides, nevertheless they have a beneficial effect with respect to creep resistance. A total content (sum of the contents of all the reactive elements introduced) greater than 0.060% does not provide an additional effect, whereas it involves a strong impact on the cost and on the eco-responsible nature of the material. A minimum content of 0.010% is required to obtain the aforementioned benefits. Advantageously, the total percentage by weight of reactive elements is chosen greater than or equal to 0.020%.
The alloy further contains silicon, in order to improve flowability and increase resistance to corrosion. The quantity of this element is nevertheless limited to 0.30%, or even to 0.25%, in order to avoid the presence of G and σ phases (intermetallic phase comprising Fe, Cr, Ni and Si), harmful to creep. Advantageously, the Si content is between 0.01% and 0.20%, or even between 0.05% and 0.20%.
Manganese is also present in the alloy, to improve weldability and for its beneficial effect on oxidation because it acts as a trap for sulfur. It also has a beneficial effect on creep since it increases the solubility of the nitrogen in the austenite and promotes the stability of the austenitic structure. However, its content is limited to 0.30% in order to limit the formation of the B2-NiAl intermetallic phase, which negatively impacts the creep resistance. Advantageously, the manganese content is between 0.05% and 0.25%, or even between 0.05% and 0.20%, or even between 0.01% and 0.20%.
The alloy comprises vanadium, up to a percentage by weight of 1.0%. This compound is known to improve the creep properties of stainless austenitic steels by its impact on the precipitation of chromium carbides, by increasing their volume fraction. Vanadium also helps the precipitation of carbonitrides rich in niobium, titanium and/or vanadium, during aging, and it also has a hardening effect by solid solution. Its content must be limited to 1.0% in order to maintain its beneficial effects and to avoid degradation of the behavior in oxidation of the grade. Advantageously, the vanadium content is between 0.005% and 0.5%; it may optionally be greater than or equal to 0.010%, or even greater than or equal to 0.1%.
The titanium promotes the formation of intra-granular fine carbonitrides and their subsequent evolution during aging (favorable to creep resistance). It may be included in the alloy as a percentage by weight ranging up to 0.40%. Advantageously, the percentage by weight of titanium is greater than 0.05%.
The alloy also contains nitrogen, which, by its gamma-phase producing character (stabilizes the austenitic structure), improves the creep properties. Its presence in the alloy also contributes to the formation of carbonitrides rich in niobium, titanium and/or vanadium, which reinforce the creep properties. Its content is limited to 0.20% in order to avoid the formation of phases unfavorable to the creep and oxidation properties. Advantageously, the percentage by weight of nitrogen is greater than or equal to 0.015%, preferentially greater than or equal to 0.040%, to 0.045%, to 0.048%, to 0.060%, even more preferentially greater than or equal to 0.10%, or even preferentially greater than or equal to 0.12%.
Sulfur is an undesirable element in the alloy, but may be in trace form (impurity) in the grade. It is desirable to limit the presence of this element in order to degrade as little as possible the protective character of the alumina layer. Sulfur can therefore be present in the alloy but at contents strictly less than 0.0060% (that is <60 ppm). Advantageously, the sulfur content is less than 0.0050% (<50 ppm), or even less than 0.0020% (<20 ppm), preferentially less than 0.00050% (<5 ppm).
Other elements may optionally be found in trace form in the alloy, such as, for example, zirconium (<0.03%), tungsten (<0.01%), cobalt (<0.08%), molybdenum (<0.2%), copper (<0.05%) or tantalum (<0.02%), but they are not deliberately introduced into the alloy; their potential presence being related to the fact that these elements can be found as impurities in the fillers incorporated during the manufacture of the alloy.
The alloy possibly may be polluted by trace impurities such as phosphorus, lead, tin, boron, magnesium or arsenic, the content of which is of the order of one particle per million (ppm) and is strictly less than 200 ppm.
It should be noted that the composition of the alloy can be measured by spark spectrometry.
The table of
The first criterion is an oxidation criterion, determined empirically. It connects the chromium, aluminum and sulfur contents of the alloy. The equation is constructed around acceptable values of these three elements (26% for Cr, 2% for Al and 30 ppm for sulfur). This equation grants a different weight to each element according to the impact of its content on the resistance to oxidation at high temperature. For simplicity, the criterion was normalized and it must be greater than 1 in order to guarantee good oxidation behavior.
The first criterion is defined by:
with KAl=0.1728+0.1293×ln(xAl) and KS=0.3089×e(64x
The second criterion relates to the solvus temperature of a certain type of carbides, namely M23C6 carbides. A relationship was established between the percentages by weight of certain elements that are linked to the solvus temperature of the M23C6 carbides. This temperature must be high (namely greater than or equal to 1070° C.) in order to promote the secondary precipitation of the Cr (M23C6) carbides at operating temperatures and to guarantee optimal mechanical performance (creep resistance).
The second criterion is defined by:
−17.64+19.61xAl−1.29xAl2−101.46xN+450.65xN2−5.8368xN3+9.68xV+43.12xTi+30.02xSi+11.42xNi−0.18xNi2+35.05xNb+47.92xCr−0.34xCr2+13.97xMn−239.66xC>1070° C.
As mentioned in the introduction, it is normal for a refractory austenitic alloy to form a decarburized layer and/or an internal oxidation layer, as a result of the evolution of the microstructure near the surface due to the very high operating temperatures. This phenomenon, linked to the capacity of the alloy to be self-protected from the environment, has a significant impact on the service life of these alloys at these temperatures.
Thus, going beyond the role of each individual compound of the alloy, the link between the microstructure of the alloy, its resistance to oxidation and its mechanical properties at operating temperatures typically greater than or equal to 1100° C. has been studied. The operating temperature is the temperature at which the alloy is intended to be subjected, during use thereof: for example, for an alloy forming a reformer tube in an installation for direct reduction of iron ore, the operating temperature may be between 1050° C. and 1175° C.
The studies carried out, in particular based on characterizations by optical microscopy, scanning electron microscopy (SEM) and on creep tests, have made it possible to demonstrate the fact that the creep properties of the alloy with a high nickel content (greater than or equal to 50%) are directly impacted by its oxidation behavior and by the precipitation of secondary M23C6 type carbides rich in chromium at the operating temperature.
Thus, it was determined that, in an austenitic alloy with a high nickel content, the creep resistance, at the operating temperature, can achieve exceptional performance when it has not only a “favorable” microstructure for creep resistance but also a very good resistance to oxidation at the temperature, hence the definition of the two criteria stated above. This synergistic effect is particularly true for the very high operating temperatures targeted and represents an important feature of embodiments of the present disclosure. While a microstructure optimized for creep resistance is a necessary condition but not sufficient for high resistance to creep at very high temperatures (>1100° C.), it is found that the capacity of the grade to be self-protected from the environment plays a crucial role and is also necessary (criterion 1).
A “favorable” microstructure in this case means that, at the operating temperature, the chemical composition of the alloy must be such that the solvus temperature of the M23C6 carbides is equal to or greater than 1070° C., in order to promote the secondary precipitation of the carbides from the M7C3 carbides present in the alloy as a raw casting.
From correlations between the physical characterizations and CALPHAD simulations (calculations of phase diagrams, making it possible to predict the phases present in the alloy at equilibrium temperature, depending on its composition), an R2 relationship has been established between the percentages by weight of certain elements of the alloy and the maximum temperature TmaxM23C6 of the stability range of the M23C6 chromium carbides phase:
[R2]
with xAl, xN, xV, xTi, xSi , xNi, xNb, xCr, xMn, xC being the percentages by weight, respectively, of Al, N, V, Ti, Si, Ni, Nb, Cr, Mn and C in the alloy.
The maximum temperature of the stability domain can be seen as the limit temperature below which there is transformation in the alloy of the M7C3 carbides (present in the raw casting-state alloys) into M23C6 carbides; this transformation leads to a secondary precipitation of the desired chromium carbides, which improves the creep performance of the alloy. Such a transformation takes place over a range of temperatures corresponding to the stability domain of the M23C6 phase.
According to the present disclosure, the maximum temperature TmaxM23C6 must be greater than or equal to 1070° C. in order to promote the secondary precipitation in the alloy subjected to the operating temperature, during its use. This condition corresponds to the second criterion. Advantageously, the maximum temperature TmaxM23C6 may be defined as greater than or equal to 1100° C., or even greater than or equal to 1150° C.
As stated above, this R2 relationship is only valid and relevant for an alloy having main elements (Cr, Ni, Al, Nb, C, Si, Mn, Ti, Fe, N, V) in the ranges of percentages by weight defined according to the present disclosure.
Validation of the second criterion, linked to the maximum temperature of the stability domain of the secondary M23C6 carbides is, however, not sufficient to ensure optimal creep performance at the operating temperature.
The alloy must also exhibit excellent resistance to oxidation. Three elements, chromium, aluminum and sulfur, play a crucial role in the ability of the alloy to self-protect. On the basis of correlations between the physical characterizations and the chemical composition, an R1 relationship was established:
[R1]
with KAl=0.1728+0.1293×ln(xAl) and KS=0.3089×e(64x
The term foxy is an oxidation function and xCr, xAl and xS are the percentages by weight, respectively, of Cr, Al and S in the alloy.
Advantageously, the oxidation function foxy must be greater than 1 in order to guarantee good oxidation behavior of the alloy subjected to the operating temperature, and to synergistically optimize the creep resistance of the alloy during its use. The condition foxy≥1 corresponds to the first criterion according to the present disclosure.
Examples of alloys will now be presented, in order to show how the composition ranges according to the present disclosure, combined with the aforementioned two criteria make it possible to obtain a nickel-rich, alumina-forming refractory austenitic alloy, particularly efficient in terms of resistance to oxidation and creep resistance, at operating temperatures greater than or equal to 1100° C.
Performance tests relate to the resistance of the alloys to accelerated aging, cyclical oxidation, and creep resistance.
The table in
A dendritic structure is observed with an array of M7C3 and/or M23C6 type chromium carbides located at the inter-dendritic spaces as well as on the surface of the samples. Note that the surface was protected with a deposition of copper in the cases of alloys 1, 2 and 6, this deposition having a clear contrast on the optical microscopy images and is observable in the form of islands spaced apart at the surface.
The chromium-rich carbide network is entirely present up to the surface of the samples of alloys 1, 2, 3, 4 and 7. On the contrary, a free layer of chromium carbides near the surface of alloys 5 and 8, as well as an internal oxidation layer, are observed. In the case of alloy 6, the width of the decarburized layer is such that, on the image, the network of chromium carbides is not observed; on the other hand, a significant internal oxidation layer is observed.
The large black contrast objects formed inside the sample of alloys 5, 6 and 8 are aluminum nitrides.
The microstructures of alloys 1 and 6, observed by scanning electron microscope, are shown in
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
Alloy 6, which does not meet the first criterion, foxy≥1, formed a chromium oxide layer (Cr2O3) on the surface that led to depletion of chromium in the area near the surface (see profile in
It can be seen that the high sulfur content combined with limited contents of chromium and aluminum in a refractory alloy limits the ability of the alloy to self-protect from oxidation. The weight gain of alloy 5 (
The creep resistance of alloys 1 to 9 was evaluated from creep tests at 1050° C., 1100° C., 1125° C., 1150° C. and/or 1175° C., subject to stresses of 17, 16.5, 13, 11.5, 10 and 9 MPa, with the tests being carried out on samples taken from parts produced in the various alloys. A time to rupture tR is extracted from these tests, that is transformed into Larson-Miller (LMP) in combination with the temperature of the test according to the following expression:
LMP=1000/T×(log tR+C)
T being the temperature of the test expressed in kelvin, tR the time to rupture expressed in hours and C a constant characteristic of the alloy; in this case C=20.22.
The representation of the results of the creep tests under the Larson-Miller formalism makes it possible to compare the performance of the tests carried out at different temperatures.
It is possible to observe a higher performance (especially at high LMP) of alloys 1 to 4, in accordance with the present disclosure, compared to alloys 5 to 9.
All these results emphasize differences in performance that refractory alloys with high nickel content, with very close compositions, may have in terms of creep resistance at very high temperatures (alloys 1 to 4 versus alloys 5 to 9). In addition to precise ranges of composition, two important criteria have been defined that the alloy must respect so as to offer the best creep performance combined with excellent resistance to cyclical oxidation, for operating temperatures greater than or equal to 1100° C. One originality of this approach comes from taking into account two distinct phenomena (oxidation factor and solvus temperature of M23C6 carbides) having a beneficial synergistic action on the mechanical performance (creep) of the alloy while ensuring a remarkable corrosion protection.
Of course, the present disclosure is not limited to the embodiments described and it is possible to add alternative embodiments thereto without departing from the scope of the present disclosure as defined by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2210006 | Sep 2022 | FR | national |
