Patent Application
20190106770
The present invention relates to a thermostable and corrosion-resistant cast nickel-chromium alloy.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
High-temperature processes, for example those used in the petrochemical industry, require materials which are not only heat-resistant but also sufficiently corrosion-resistant and in particular are able to withstand the loads imposed by hot product and combustion gases. For example, the tube coils used in cracking and reformer furnaces are externally exposed to strongly oxidizing combustion gases with a temperature of up to 1100° C. and above, whereas a strongly carburizing atmosphere at temperatures of up to 1100° C. prevails in the interior of cracking tubes, and a weakly carburizing, differently oxidizing atmosphere prevails in the interior of reformer tubes at temperatures of up to 900° C. and a high pressure. Moreover, contact with the hot combustion gases leads to nitriding of the tube material and to the formation of a layer of scale, which is associated with an increase in the external diameter of the tube by a few percent and a reduction in the wall thickness by up to 10%.
By contrast, the carburizing atmosphere inside the tube causes carbon to diffuse into the tube material, where, at temperatures of over 900° C., it leads to the formation of carbides, such as M23C6, and, with increasing carburization, to the formation of the carbon-rich carbide M7C3. The consequence of this is internal stresses resulting from the increase in volume associated with the carbide formation or transformation and a decrease in the strength and ductility of the tube material. Furthermore, graphite or dissociation carbon may form in the interior of the tube material, which can, in combination with internal stresses, lead to the formation of cracks, which in turn cause more carbon to diffuse into the tube material.
Consequently, high-temperature processes require materials with a high creep strength or limiting rupture stress, microstructural stability and resistance to carburization and oxidation. This requirement is—within limits—satisfied by alloys which, in addition to iron, contain 20 to 35% of nickel, 20 to 25% of chromium and, to improve the resistance to carburization, up to 1.5% of silicon, such as for example the nickel-chromium steel alloy 35Ni25Cr-1.5Si, which is suitable for centrifugally cast tubes and is still resistant to oxidation and carburization even at temperatures of 1100° C. The high nickel content reduces the diffusion rate and the solubility of the carbon and therefore increases the resistance to carburization.
On account of their chromium content, at relatively high temperatures and under oxidizing conditions the alloys form a covering layer of Cr2O3, which acts as a barrier layer preventing the penetration of oxygen and carbon into the tube material beneath it. However, at temperatures over 1050° C., the Cr2O3 becomes volatile, and consequently the protective action of the covering layer is rapidly lost.
Under cracking conditions, carbon deposits are inevitably also formed on the tube inner wall and/or on the Cr2O3 covering layer, and at temperatures of over 1050° C. in the presence of carbon and steam, the chromium oxide is converted into chromium carbide. To reduce the associated adverse effect on the resistance to carburization, the carbon deposits in the tube have to be burnt from time to time with the aid of a steam/air mixture, and the operating temperatures generally have to be kept below 1050° C.
The resistance to carburization and oxidation is further put at risk by the limited creep rupture strength and ductility of the conventional nickel-chromium alloys, which lead to the formation of creep cracks in the chromium oxide covering layer and to the penetration of carbon and oxygen into the tube material via the cracks. In particular in the event of a cyclical temperature loading, covering layer cracks may form and also the covering layer may become partially detached.
Tests have revealed that microstructural phase reactions, in particular at higher silicon contents, for example of over 2.5%, evidently lead to a loss of ductility and to a reduction in the short-time strength.
It would therefore be desirable and advantageous to inhibit the damage mechanism of carburization—reduction in the creep rupture strength or limiting rupture stress—internal oxidation, with the further result of increased carburization and oxidation, and to provide an improved casting alloy which still has a reasonable service life even under extremely high operating temperatures in a carburizing and/or oxidizing atmosphere.
According to one aspect of the present invention, a nickel-chromium casting alloy having defined aluminum and yttrium contents and comprising, in weight percent,
The total content of nickel, chromium and aluminum combined in the alloy should be from 80 to 90%.
It is preferable for the alloy, individually or in combination with one another, to contain at most 0.7% of carbon, up to 30% of chromium, up to 12% of iron, 2.2 to 6% of aluminum, 0.1 to 2.0% of niobium, 0.01 to 1.0% of titanium, up to 0.15% of zirconium and—to achieve a high creep rupture strength—up to 10% of cobalt, at least 3% of molybdenum and up to 5% of tungsten, for example 4 to 8% of cobalt, up to 4% of molybdenum and 2 to 4% of tungsten, if the high resistance to oxidation is not the primary factor. Therefore, depending on the loads encountered in the specific circumstances, the cobalt, molybdenum and tungsten contents have to be selected within the content limits specified by the invention.
An alloy comprising at most 0.7% of carbon, at most 0.2, more preferably at most 0.1% of silicon, up to 0.2% of manganese, 18 to 30% of chromium, 0.5 to 12% of iron, 2.2 to 5% of aluminum, 0.4 to 1.6% of niobium, 0.01 to 0.6% of titanium, 0.01 to 0.15% of zirconium, at most 0.6% of nitrogen, at most 10% of cobalt, and at most 5% of tungsten, is particularly suitable.
Optimum results can be achieved if, in each case individually or in combination with one another, the chromium content is at most 26.5%, the iron content is at most 11%, the aluminum content is from 3 to 6%, the titanium content is over 0.15%, the zirconium content is over 0.05%, the cobalt content is at least 0.2%, the tungsten content is over 0.05% and the yttrium content is 0.019 to 0.089%.
The high creep rupture strength of the alloy according to the invention, for example a service life of 2000 hours under a load of from 4 to 6 MPa and a temperature of 1200° C., guarantees that a continuous, securely bonded oxidic barrier layer is retained in the form of an Al2O3 layer which has the effect of preventing carburization and oxidation, results from the high aluminum content of the alloy and continues to top itself up or grow. As tests have shown, this layer comprises α-Al2O3 and contains at most isolated spots of mixed oxides, which do not alter the essential nature of the α-Al2O3 layer; at higher temperatures, in particular over 1050° C., in view of the rapidly decreasing stability of the Cr2O3 layer of conventional materials at these temperatures, is increasingly responsible for protecting the alloy according to the invention from carburization and oxidation. On the Al2O3 barrier layer, there may also—at least in part—be a covering layer of nickel oxide (NiO) and mixed oxides (Ni(Cr,Al)2O4), the condition and extent of which, however, is not of great significance, since the Al2O3 barrier layer below is responsible for protecting the alloy from oxidation and carburization. Cracks in the covering layer and the (partial) flaking of the covering layer which occurs at higher temperatures are therefore harmless.
To ensure that the α-aluminum oxide layer is as pure as possible and substantially free of mixed oxides, the following condition should be satisfied:
9[% Al]≥[% Cr].
On account of its high aluminum content, the microstructure of the alloy according to the invention, at over 4% of aluminum, inevitably contains γ′ phase, which has a strengthening action at low and medium temperatures but also reduces the ductility or elongation at break. In individual cases, therefore, it may be necessary to reach a compromise between ductility and resistance to oxidation/carburization which is oriented according to the intended use.
The barrier layer according to the invention comprising α-Al2O3, which is the most stable Al2O3 modification, is able to withstand all oxygen concentrations.
Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which:
Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.
The invention is explained in more detail below on the basis of exemplary embodiments and the seven comparative alloys 1 to 7 and nine alloys 8 to 26 according to the invention listed in the table below, and also the diagrams shown in
The table includes, as an example for two wrought alloys which are not covered by the invention and have a comparatively low carbon content and a very fine-grained microstructure with a grain size of ≤10 μm, comparative alloys 5 and 7, whereas all the other test alloys are casting alloys.
Yttrium has a strong oxide-forming action which, in the alloy according to the invention, considerably improves the formation conditions and bonding of the α-Al2O3 layer.
The aluminum content of the alloy according to the invention has an important role in that aluminum leads to the formation of a γ′ precipitation phase, which significantly increases the tensile strength. As can been seen from the diagrams presented in
The limiting rupture strength of alloys according to the invention with different aluminum contents is presented in the Larson-Miller diagram shown in
LMP=T·(C+log10(tB)).
According to the illustration presented in
In the range around 1200° C., i.e. with greater Larson-Miller parameters, there are no known service life data for conventional centrifugally cast materials, whereas limiting rupture stresses of from 5.8 to 8.5 MPa are still observed for the alloys according to the invention for service lives of 1000 h, depending on the composition.
Further tests, in which the resistance to carburization of various specimens was tested in a slightly oxidizing atmosphere comprising hydrogen and 5% by volume of CH4, reveal the superiority of the alloy according to the invention compared to four standard alloys at a temperature of 1100° C. The long-time performance is of particular importance. The test results are presented in graph form in the diagram shown in
To simulate practical conditions, cyclical carburization tests were carried out, in which the specimens were alternatively held at a temperature of 1100° C. for 45 min and then at room temperature for 15 min in an atmosphere comprising hydrogen together with 4.7% by volume of CH4 and 6% by volume of steam. The results of the tests, which each comprise 500 cycles, are shown in the diagram presented in
The results of further tests, in which the specimens were subjected to cyclical thermal loading at 1150° C. in dry air, are presented in the diagram shown in
In a test carried out under conditions close to those encountered in practice, a number of specimens were subjected to cyclical carburization and decarburization in accordance with the NACE standard. Each cycle comprised carburization for three hundred hours in an atmosphere comprising hydrogen and 2% by volume of CH4, followed by decarburization for twenty-four hours in an atmosphere comprising air and 20% by volume of steam at 770° C. The test comprised four cycles. It can be seen from the diagram presented in
The diagram presented in
9[% Al]≥[% Cr].
The straight line in the diagram shown in
The diagram illustrated in
To illustrate the influence of the aluminum within the content limits according to the invention, the diagrams presented in
The diagram shown in
Overall, the two diagrams reveal that as the aluminum content increases, the service life until fracture in the limiting rupture stress test is reduced. Furthermore, as the temperature increases and the duration of loading increases and/or the loading level decreases, the negative influence of the aluminum on the limiting rupture stress life decreases. In other words: the alloys with a high aluminum content are particularly suitable for long-term use at temperatures for which it has hitherto been impossible to use cast or centrifugally cast materials.
In view of their superior strength properties and their excellent resistance to carburization and oxidation, the casting alloy according to the invention is particularly suitable for use as a material for furnace parts, radiant tubes for heating furnaces, rollers for annealing furnaces, parts of continuous-casting and strip-casting installations, hoods and muffles for annealing furnaces, parts of large diesel engines, containers for catalysts and for cracking and reformer tubes.
While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein:
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 02 989.3 | Jan 2003 | DE | national |
The present application is a continuation of copending U.S. application Ser. No. 12/169,229, filed Jul. 8, 2008, which is in turn a continuation of U.S. application Ser. No. 10/945,859, filed Sep. 21, 2004, now abandoned, which in turn is a continuation of prior filed PCT International Application No. PCT/EP2004/000504, filed Jan. 22, 2004, which claims the priority of German Patent Application, Serial No. 103 02 989.3, filed Jan. 25, 2003, the entire contents of each of which are incorporated herein by reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12169229 | Jul 2008 | US |
| Child | 16055645 | US | |
| Parent | 10945859 | Sep 2004 | US |
| Child | 12169229 | US | |
| Parent | PCT/EP2004/000504 | Jan 2004 | US |
| Child | 10945859 | US |
