Patent Application
20150050182
The invention relates to a nickel-chromium-aluminum alloy with excellent high-temperature corrosion resistance, good creep resistance and improved processability.
Austenitic nickel-chromium-aluminum alloys with different nickel, chromium and aluminum contents have long been used in furnace construction and in the chemical as well as petrochemical industry. For this use, a good high-temperature corrosion resistance even in carburizing atmospheres and a good heat resistance/creep resistance are necessary.
In general, it may be remarked that the high-temperature corrosion resistance of the alloys listed in Table 1 increases with increasing chromium content. All these alloys form a chromium oxide layer (Cr2O3) with an underlying, more or less closed Al2O3 layer. Small additions of strongly oxygen-affine elements such as, e.g. Y or Ce improve the oxidation resistance. The chromium content is slowly consumed for build-up of the protecting layer in the course of use in the application zone. Therefore the lifetime of the material is prolonged by a higher chromium content, since a higher content of the element chromium forming the protective layer extends the time at which the Cr content lies below the critical limit and oxides other than Cr2O3 are formed, which are, e.g. iron-containing and nickel-containing oxides. A further increase of the high-temperature corrosion. resistance could be achieved by additions of aluminum and silicon. Starting from a certain minimum content, these elements form a closed layer under the chromium oxide layer and thus reduce the consumption of chromium.
In carburizing atmospheres (CO, H2, CH4, CO2, H2O mixtures), carbon may penetrate into the material, and so the formation of internal carbides may take place. These cause a loss of notch impact toughness. Also, the melting point may sink to very low values (down to 350° C.) and transformation processes may occur due to chromium depletion of the matrix.
A high resistance to carburization is achieved by materials with low solubility for carbon and low rate of diffusion of the carbon. In general, therefore, nickel alloys are more resistant to carburization than iron-base alloys, since both the diffusion of carbon and also the solubility of carbon in nickel are smaller than in iron. An increase of the chromium content brings about a higher carburization resistance by formation of a protecting chromium oxide layer, unless the oxygen partial pressure in the gas is not sufficient for the formation of this protecting chromium oxide layer. At very low oxygen partial pressure, it is possible to use materials that form a layer of silicon oxide or of the even more stable aluminum oxide, both of which are still able to form protecting oxide layers at much lower oxygen contents.
In the case that the carbon activity is >1, the so-called “metal dusting” may occur in alloys based on nickel, iron or cobalt. In contact with the supersaturated gas, the alloys may absorb large amounts of carbon. The segregation processes taking place in the alloy supersaturated with carbon leads to material destruction. In the process, the alloy decomposes into a mixture of metal particles, graphite, carbides and/or oxides. This type of material destruction takes place in the temperature range from 500° C. to 750° C.
Typical conditions for the occurrence of metal dusting are strongly carburizing CO, H2 or CH4 gas mixtures, such as occur in the synthesis of ammonia, in methanol plants, in metallurgical processes but also in hardening furnaces.
The resistance to metal dusting tends to increase with increasing nickel content of the alloy (Grabke, H. J., Krajak, R., Müller-Lorenz, E. M., Strauss, S.: Materials and Corrosion 47 (1996), p. 495), although even nickel alloys are not generally resistant to metal dusting.
The chromium and the aluminum content have a distinct influence on the corrosion resistance under metal dusting conditions (see
The heat resistance or creep resistance at the indicated temperatures is improved by a high carbon content among other factors. However, high contents of solid-solution-strengthening elements such as chromium, aluminum, silicon, molybdenum and tungsten improve the heat resistance. In the range of 500° C. to 900° C., additions of aluminum, titanium and/or niobium can improve the resistance, and specifically by precipitation of the γ′ and/or γ″ phase.
Examples according to the prior art are listed in Table 1.
Alloys such as Alloy 602 CA (N06025), Alloy 693 (N06693) or Alloy 603 (N06603) are known for their excellent corrosion resistance in comparison with Alloy 600 (N06600) or Alloy 601 (N06601) by virtue of the high aluminum content of more than 1.8%. Alloy 602 CA (N06025), Alloy 693 (N06693), Alloy 603 (N06603) and Alloy 690 (N06690) exhibit excellent carburization resistance or metal dusting resistance by virtue of their high chromium and/or aluminum contents. At the same time, by virtue of the high carbon or aluminum content, alloys such as Alloy 602 CA (N06025), Alloy 693 (N06693) or Alloy 603 (N06603) have excellent heat resistance or creep resistance in the temperature range in which metal dusting occurs. Alloy 602 CA (N06025) and Alloy 603 (N06603) still have excellent heat resistance or creep resistance even at temperatures above 1000° C. Because of, for example, the high aluminum content, however, the processability is impaired, and the impairment becomes all the greater the higher the aluminum content is (For example, in Alloy 693-N06693). The same is true to a greater extent for silicon, which forms low-melting intermetallic phases with nickel. In Alloy 602 CA (N06025) or Alloy 603 (N06603), the cold formability in particular is limited by the high proportion of primary carbides.
U.S. Pat. No. 6,623,869 B1 discloses a metallic material that consists of not more than 0.2% C, 0.01-4% Si, 0.05-2.0% Mn, not more than 0.04% P, not more than 0.015% S, 10-35% Cr, 30-78% Ni, 0.005-<4.5% A1, 0.005-0.2% N and at least one of the elements 0.015-3% Cu or 0.015-3% Co, with the rest up to 100% iron. Therein the value of 40Si+Ni+5Al+40N+10(Cu+Co) is not smaller than 50, where the symbols of the elements denote the fractional content of the corresponding elements. The material has an excellent corrosion resistance in an environment in which metal dusting can occur and it may therefore be used for furnace pipes, pipe systems, heat-exchanger tubes and the like in petroleum refineries or petrochemical plants, and it can markedly improve the lifetime and safety of the plant.
EP 0 508 058 A1 discloses an austenitic nickel-chromium-iron alloy consisting of (in % by weight) C 0.12-0.3%, Cr 23-30%, Fe 8-11%, Al 1.8-2.4%, Y 0.01-0.15%, Ti 0.01-1.0%, Nb 0.01-1.0%, Zr 0.01-0.2%, Mg 0.001-0.015%, Ca 0.001-0.01%, N max. 0.03%, Si max. 0.5%, Mn max. 0.25%, P max. 0.02%, S max. 0.01%, Ni the rest, including unavoidable smelting-related impurities.
U.S. Pat. No. 4,882,125 B1 discloses a high-chromium-containing nickel alloy, which is characterized by an outstanding resistance to sulfurization and oxidation at temperatures higher than 1093° C., an outstanding creep resistance of longer than 200 h at temperatures above 983° C. and a stress of 2000 PSI, a good tensile strength and a good elongation, both at room temperature and elevated temperature, consisting of (in % by wt) 27-35% Cr, 2.5-5% Al, 2.5-6% Fe, 0.5-2.5% Nb, up to 0.1% C, respectively up to 1% Ti and Zr, up to 0.05% Ce, up to 0.05% Y, up to 1% Si, up to 1% Mn and Ni the rest.
EP 0 549 286 B1 discloses a high-temperature-resistant Ni—Cr alloy containing 55-65% Ni, 19-25% Cr, 1-4.5% Al, 0.045-0.3% Y, 0.15-1% Ti, 0.005-0.5% C, 0.1-1.5% Si, 0-1% Mn and at least 0.005%, of at least one of the elements of the group that contains Mg, Ca, Ce, <0.5% in total of Mg+Ca, <1% Ce, 0.0001-0.1% B, 0-0.5% Zr, 0.0001-0.2% N, 0-10% Co, 0-0.5% Cu, 0-0.5% Mo, 0-0.3% Nb, 0-0.1% V, 0-0.1% W, the rest iron and impurities.
From DE 600 04 737 T2 there has become known a heat-resisting nickel-base alloy containing ≦0.1% C, 0.01-2% Si, ≦2% Mn, ≦0.005% S, 10-25% Cr, 2.1-<4.5% Al, 0.055% N, in total 0.001-1% of at least one of the elements B, Zr, Hf, wherein the said elements may be present in the following contents: B≦0.03%, Zr≦0.2%, Hf<0.8%, Mo 0.01-15%, W 0.01-9%, wherein a total Mo+W content of 2.5-15% may be specified, Ti 0-3%, Mg 0-0.01%, Ca 0-0.01%, Fe 0-10%, Nb 0-1%, V 0-1%, Y 0-0.1%, La 0-0.1%, Ce 0-0.01%, Nd 0-0.1%, Cu 0-5%, Co 0-5%, the rest nickel. For Mo and W, the following formula must be satisfied:
2.5≦Mo+W≦15 (1)
The task underlying the invention consists in designing a nickel-chromium-aluminum alloy which, with sufficiently high chromium and aluminum contents, assures an excellent metal dusting resistance, but which at the same time exhibits
This task is accomplished by a nickel-chromium-aluminum alloy with (in % by wt) 24 to 33% chromium 1.8 to 4.0% aluminum, 0.10 to 7.0% iron, 0.001 to 0.50% silicon, 0.005 to 2.0% manganese, 0.00 to 0.60% titanium, respectively 0.0002 to 0.05% magnesium and/or calcium, 0.005 to 0.12% carbon, 0.001 to 0.050% nitrogen, 0.0001-0.020% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 2.0% molybdenum, max. 2.0% tungsten, the rest nickel and the usual process-related impurities, wherein the following relationships must be satisfied:
Cr+Al≧28 (2a)
and Fp≦39.9 with (3a)
Fp=Cr+0.272*Fe+2.36*A1+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (4a)
where Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in % by mass.
Advantageous improvements of the subject matter of the invention are to be inferred from the associated dependent claims.
The spread for the element chromium lies between 24 and 33%, wherein preferred ranges may be adjusted as follows:
The aluminum content lies between 1.8 and 4.0%, wherein here also preferred aluminum contents may be adjusted as follows depending on the field of use of the alloy:
The iron content lies between 0.1 and 7.0%, wherein defined contents may be adjusted within the following spread depending on the area of application:
The silicon content lies between 0.001 and 0.50%. Preferably Si may be adjusted in the alloy within the spread as follows:
The same is true for the element manganese, which may be contained in proportions of 0.005 to 2.0% in the alloy. Alternatively, the following spread is also conceivable:
The titanium content lies between 0.0 and 0.60%. Preferably Ti may be adjusted within the spread as follows in the alloy:
Magnesium and/or calcium is also contained in contents of 0.0002 to 0.05%. Preferably the possibility exists of adjusting these elements as follows in the alloy:
The alloy contains 0.005 to 0.12% carbon. Preferably this may be adjusted within the spread as follows in the alloy:
This is true in the same way for the element nitrogen, which is contained in contents between 0.001 and 0.05%. Preferred contents may be stated as follows:
The alloy further contains phosphorus in contents between 0.001 and 0.030%. Preferred contents may be stated as follows:
The alloy further contains oxygen in contents between 0.0001 and 0.020%, containing especially 0.0001 to 0.010%.
The element sulfur is specified as follows in the alloy:
Molybdenum and tungsten are contained individually or in combination in the alloy in a content of respectively at most 2.0%. Preferred contents may be stated as follows:
The following relationship between Cr and Al must be satisfied, so that a sufficient resistance to metal dusting is achieved:
Cr+Al≧28 (2a)
where Cr and Al are the concentrations of the elements in question in % by mass.
Preferred ranges may be adjusted with
Cr+Al≧29 (2b)
Cr+Al≧30 (2c)
Cr+Al≧31 (2d)
Furthermore the following relationship must be satisfied, so that a sufficient phase stability is achieved:
Fp≦39.9 with (3a)
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (4a)
where Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in % by mass.
Preferred ranges may be adjusted with:
Fp≦38.4 (3b)
Fp≦36.6 (3c)
Optionally the element yttrium may be adjusted in contents of 0.01 to 0.20% in the alloy. Preferably Y may be adjusted within the spread as follows in the alloy:
Optionally the element lanthanum may be adjusted in contents of 0.001 to 0.20% in the alloy. Preferably La may be adjusted within the spread as follows in the alloy:
Optionally the element Ce may be adjusted in contents of 0.001 to 0.20% in the alloy. Preferably Ce may be adjusted within the spread as follows in the alloy:
Optionally, in the case of simultaneous addition of Ce and La, cerium mixed metal may also be used, and specifically in contents of 0.001 to 0.20%. Preferably cerium mixed metal may be adjusted within the spread as follows in the alloy:
Optionally, the element Nb may be adjusted in contents of 0.0 to 1.10% in the alloy. Preferably Nb may be adjusted within the spread as follows in the alloy:
If Nb is contained in the alloy, Formula 4a must be supplemented with a term for Nb as follows:
Fp=Cr+0.272*Fe+2.36*A1+2.22*Si+2.48*Ti+1.26*Nb+0.374*Mo+0.538*W−11.8*C (4b)
where Cr, Fe, Al, Si, Ti, Nb, Mo, W and C are the concentrations of the elements in question in % by mass.
If necessary, zirconium may be used in contents between 0.01 and 0.20%. Preferably Zr may be adjusted within the spread as follows in the alloy:
Optionally, zirconium may also be replaced completely or partly by
Optionally, 0.001 to 0.60% tantalum may also be contained in the alloy.
Optionally, the element boron may be contained as follows in the alloy:
Preferred contents may be stated as follows:
Furthermore, the alloy may contain between 0.0 and 5.0% cobalt, which furthermore may be limited even more as follows:
Furthermore, at most 0.5% Cu may be contained in the alloy.
The copper content may be further restricted as follows:
If Cu is contained in the alloy, Formula 4a must be supplemented with a term for Cu as follows:
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.477*Cu+0.374*Mo+0.538*W−11.8*C (4c)
where Cr, Fe, Al, Si, Ti, Cu, Mo, W and C are the concentrations of the elements in question in % by mass.
If Nb and Cu are contained in the alloy, Formula 4a must be supplemented with a term for Nb and a term for Cu as follows:
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+1.26*Nb+0.477*Cu+0.374*Mo+0.538*W−11.8*C (4d)
where Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the elements in question in % by mass.
Furthermore, at most 0.5% vanadium may be contained in the alloy.
Finally, the elements lead, zinc and tin may be stated as impurities in contents as follows:
Furthermore, the following relationship, which assures a particularly good processability, may be optionally satisfied:
Fa≦60 with (5a)
Fa=Cr+20.4*Ti+201*C (6a)
where Cr, Ti and C are the concentrations of the elements in question in % by mass.
Preferred ranges may be adjusted with:
Fa≦54 (5b)
If Nb is contained in the alloy, Formula 6a must be supplemented with a term for Nb as follows:
Fa=Cr+6.15*Nb+20.4*Ti+201*C (6b)
where Cr, Nb, Ti and C are the concentrations of the elements in question in % by mass.
Furthermore, the following relationship, which describes a particularly good heat resistance or creep resistance, may be optionally satisfied:
Fk≧45 with (7a)
Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (8a)
where Cr, Ti, Al, Si and C are the concentrations of the elements in question in % by mass.
Preferred ranges may be adjusted with:
Fk≦49 (7b)
Fk≦53 (7c)
If Nb and/or B is contained in the alloy, Formula 8a must be supplemented with a term for Nb and/or B as follows:
Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (8b)
where Cr, Ti, Nb, Al, Si, C and B are the concentrations of the elements in question in % by mass.
The alloy according to the invention is preferably smelted in an open system, followed by a treatment in a VOD or VLF system. However, a smelting and pouring in vacuum is also possible. Thereafter the alloy is cast in ingots or as continuous strand. If necessary, the ingot is then annealed for 0.1 h to 70 h at temperatures between 900° C. and 1270° C. Furthermore, it is possible to remelt the alloy additionally with ESU and/or VAR. Thereafter the alloy is worked into the desired semifinished product shape. For this it is annealed if necessary for 0.1 h to 70 h at temperatures between 900° C. and 1270° C., then hot-formed, if necessary with intermediate annealings for 0.05 h to 70 h between 900° C. and 1270° C. If necessary, the surface of the material may also be milled chemically and/or mechanically occasionally (even several times) and/or at the end for cleaning. After the end of hot shaping, cold shaping to the desired semifinished product shape with reduction ratios up to 98% may take place if necessary, with intermediate annealings for 0.1 min to 70 h between 700° C. and 1250° C. if necessary, under shielding gas, if necessary, such as argon or hydrogen, for example, followed by cooling in air, in the agitated annealing atmosphere or in the water bath. Thereafter a solution annealing takes place for 0.1 min to 70 h between 700° C. and 1250° C., under shielding gas, if necessary, such as argon or hydrogen, for example, followed by cooling in air, in the agitated annealing atmosphere or in the water bath. If necessary, chemical and/or mechanical cleanings of the material surface may take place occasionally and/or after the last annealing.
The alloy according to the invention can be readily manufactured and used in the product forms of strip, sheet, bar, wire, longitudinally seam-welded pipe and seamless pipe.
These product forms are manufactured with a mean grain size of 5 μm to 600 μm. The preferred range lies between 20 μm and 200 μm.
The alloy according to the invention will preferably be used in zones in which carburizing conditions prevail, such as, for example, in structural parts, especially pipes, in the petrochemical industry. Furthermore, it is also suitable for furnace construction.
The phases occurring at equilibrium were calculated for the different alloy variants with the JMatPro program of Thermotech. The TTNI7 database of Thermotech for nickel-base alloys was used as the database for the calculations.
The formability is determined in a tension test according to DIN EN ISO 6892-1 at room temperature. Therein the yield strength Rp0.2, the tensile strength Rm and the elongation A at break are determined. The elongation A is determined on the broken specimen from the elongation of the original gauge length L0:
A=(Lu−L0)/L0 100%=ΔL/L0100%
where Lu=measured length after break.
Depending on gauge length, the elongation at break is characterized by indices:
For example, for A5 the gauge length is L0=5·d0, where d0=initial diameter of a round specimen.
The tests were performed on round specimens with a diameter of 6 mm in the measurement zone and a gauge length L0 of 30 mm. The sampling took place transversely relative to the forming direction of the semifinished product. The deformation rate was 10 MPa/s for Rp0.2 and 6.7 10−3 l/s (40%/min) for Rm.
The magnitude of the elongation A in the tension test at room temperature may be taken as a measure of the deformability. A readily processable material should have an elongation of at least 50%.
The heat resistance is determined in a hot tension test according to DIN EN ISO 6892-2. Therein the yield strength Rp0.2, the tensile strength Rm and the elongation A at break are determined by analogy with the tension test at room temperature (DIN EN ISO 6892-1).
The tests were performed on round specimens with a diameter of 6 mm in the measurement zone and an initial gauge length L0 of 30 mm. The sampling took place transversely relative to the forming direction of the semifinished product. The deformation rate was 8.33 10−5 l/s (0.5%/min) for Rp0.2 and 8.33 10−4 l/s (5%/min) for Rm.
The respective specimen is mounted at room temperature in a tension testing machine and heated without loading by a tensile force to the desired temperature. After reaching the test temperature, the specimen is held without loading for one hour (600° C.) or two hours (700° C. to 1100° C.) for temperature equilibration. Thereafter the specimen is loaded with tensile force in such a way that the desired strain rates are maintained, and the test begins.
The creep resistance of a material improves with increasing heat resistance. Therefore the heat resistance is also used for appraisal of the creep resistance of the various materials.
The corrosion resistance at elevated temperatures was determined in an oxidation test at 1000° C. in air, wherein the test was interrupted every 96 hours and the dimensional changes of the specimens due to oxidation were determined. The specimens were placed in ceramic crucibles during the test, so that any oxide that may have spalled was collected and the mass of the spalled oxide can be determined by weighing the crucible containing the oxides. The sum of the mass of the spalled oxide and of the change in mass of the specimens corresponds to the gross change in mass of the specimen. The specific change in mass is the change in mass relative to the surface area of the specimens. Hereinafter these are denoted by mnet for the specific change in net mass, mgross for the specific change in gross mass, mspall for the specific change in mass of the spalled oxides. The tests were carried out on specimens of approximately 5 mm thickness. Three specimens were extracted from each batch, and the reported values are the mean values of these 3 specimens.
In addition to an excellent metal dusting resistance, the alloy according to the invention should also have the following properties:
In the nickel-chromium-aluminum-iron system with additions of Ti and/or Nb, various embrittling TCP phases such as, for example, the Laves phases, sigma phases or the μ-phases or also the embrittling η-phase or ε-phases can be formed, depending on alloying contents (see, for example, Ralf Bürgel, Handbook of High-Temperature Materials Engineering [in German], 3rd Edition, Vieweg Verlag, Wiesbaden, 2006, page 370-374). The calculation of the equilibrium phase fractions as a function of temperature, for example of the batch 111389 of N06690, (see Table 2, typical compositions) shows theoretically the formation of α-chromium with a low content of Ni and/or Fe(BCC phase in
This is the case in particular when the following formula is satisfied:
Fp≦39.9 with (3a)
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (4a)
where Cr, Al, Fe, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the elements in question in % by mass.
The Table 2 with the alloys according to the prior art shows that Fp for Alloy 8, Alloy 3 and Alloy 2 is >39.9 and for Alloy 10 is exactly 39.9. For all other alloys with Ts BCC≦939° C., Fp is ≦39.9.
The formability will be considered here as an example of processability.
An alloy can be hardened by several mechanisms, so that it has a high heat resistance or creep resistance. Thus the alloying addition of another element brings about a more or less large increase of the strength (solid-solution hardening), depending on element. An increase of the strength by fine particles or precipitates (precipitation hardening) is far more effective. This may take place, for example, by the y′-phase, which is formed by additions of Al and further elements, such as, for example: Ti to a nickel alloy, or by carbides, which are formed by addition of carbon to a chromium-containing nickel alloy (see, for example, Ralf Burgel, Handbook of High-Temperature Materials Engineering, 3rd Edition, Vieweg Verlag, Wiesbaden, 2006, page 358-369).
The increase of the content of elements forming the γ′-phase, or of the C content, indeed increases the heat resistance, but increasingly impairs the formability, even in the solution-annealed condition.
For a very readily formable material, elongations A5 of ≧50% but at least 45% are desired in the tension test at room temperature.
This is achieved in particular when the following relationship between the elements Cr, Nb, Ti and C forming the carbide is satisfied:
Fa≦60 with (5a)
Fa=Cr+6.15*Nb+20.4*Ti+201*C (6b)
where Cr, Nb, Ti and C are the concentrations of the elements in question in % by mass.
Heat resistance/creep resistance
At the same time, the yield strength or the tensile strength at higher temperatures should reach at least the values of Alloy 601 (see Table 4).
600° C.: yield strength Rp0.2>150 MPa; tensile strength Rm>500 MPa (9a, 9b)
800° C.: yield strength Rp0.2>130 MPa; tensile strength Rm>135 MPa (9c, 9d)
It would be desirable for the yield strength or the tensile strength to lie at least in the range of the values Alloy 602CA (see Table 4). At least 3 of the 4 following relationships should be satisfied:
600° C.: yield strength Rp0.2>230 MPa; tensile strength Rm>550 MPa (10a, 10b)
800° C.: yield strength Rp0.2>180 MPa; tensile strength Rm>190 MPa (10c, 10d)
This is achieved in particular when the following relationship between the mainly hardening elements is satisfied:
Fk≦45 with (7a)
Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (8b)
where Cr, Ti, Nb, Al, Si, C and B are the concentrations of the elements in question in % by mass.
The alloy according to the invention should have a good corrosion resistance in air similar to that of Alloy 602CA (N06025).
Tables 3a and 3b show the analyses of the batches smelted on the laboratory scale together with some industrially smelted batches, cited for comparison, according to the prior art, of Alloy 602CA (N06025), Alloy 690 (N06690), Alloy 601 (N06601). The batches according to the prior art are marked with a T, those according to the invention with an E. The batches corresponding to the laboratory scale are marked with an L, those smelted industrially with a G.
The ingots of the alloys smelted in vacuum on the laboratory scale in Table 3a and b were annealed for 8 h between 900° C. and 1270° C. and hot-rolled to a final thickness of 13 mm or 6 mm by means of hot rolls and further intermediate annealings for 0.1 to 1 h between 900° C. and 1270° C. The sheets produced in this way were solution-annealed for 1 h between 900° C. and 1270° C. The specimens needed for the measurements were prepared from these sheets.
For the industrially smelted alloys, a sample from the industrial production was taken from a commercially produced sheet of suitable thickness. The specimens needed for the measurements were taken from this sample.
All alloy variants typically had a grain size of 70 to 300
For the exemplary batches in Table 3a and 3b, the following properties were compared.
For the batches 2297 to 2308 and 250060 to 250149 smelted on the laboratory scale, but especially for the batches according to the invention marked with E (2301, 250129, 250132, 250133, 250134, 250137, 250138, 250147, 250148), the Formula (2a) Al+Cr≧28 is satisfied. They therefore satisfy the requirement that has been imposed on the metal dusting resistance.
For the selected alloys according to the prior art in Table 2 and for all laboratory batches (Tables 3a and 3b), the phase diagrams were therefore calculated and the formation temperature Ts BCC was entered in Tables 2 and 3a. For the compositions in Tables 2 as well as 3a and 3b, the value for Fp according to Formula 4a was also calculated. Fp is larger the higher the formation temperature Ts BCC. All examples of N06693 with a formation temperature Ts BCChigher than that of Alloy 10 have an Fp>39.9. The requirement Fp≦39.9 (Formula 3a) is therefore a good criterion for obtaining an adequate phase stability in an alloy. All laboratory batches in Table 3a and 3b satisfy the criterion Fp≦39.9.
The yield strength Rp0.2, the tensile strength Rm and the elongation A5 for room temperature RT and for 600° C. are entered in Table 4, as is the tensile strength Rm for 800° C. The values for Fa and Fk are also entered.
Exemplary batches 156817 and 160483 of the alloy according to the prior art, Alloy 602 CA in Table 4, have a comparatively small elongation A5 at room temperature of 36 or 42%, which fall short of the requirements for good formability. Fa is >60 and therefore above the range that characterizes good formability. All alloys according to the invention (E) exhibit an elongation >50%. Thus they satisfy the requirements. Fa is <60 for all alloys according to the invention. They therefore lie in the range of good formability. The elongation is particularly high when Fa is comparatively small.
The exemplary batch 156658 of the alloy according to the prior art, Alloy 601 in Table 4, is an example of the minimum requirements on yield strength and tensile strength at 600° C. and 800° C., whereas the exemplary batches 156817 and 160483 of the alloy according to the prior art, Alloy 602 CA, are examples of very good values of yield strength and tensile strength at 600° C. and 800° C. Alloy 601 represents a material that exhibits the minimum requirements on heat resistance and creep resistance, which are described in Formulas 9a to 9d, Alloy 602 CA a material that exhibits an outstanding heat resistance and creep resistance, which are described in the Formulas 10a to 10d. For both alloys, the value of Fk is much larger than 45, and for Alloy 602 CA it is additionally even much higher than the value of Alloy 601, which reflects the elevated strength values of Alloy 602 CA. The alloys according to the invention (E) all exhibit a yield strength and tensile strength at 600° C. and 800° C. in the range of or considerably above that of Alloy 601, and have therefore satisfied the Formulas 9a to 9d. They lie in the range of the values of Alloy 602 CA and also satisfy the desirable requirements, in other words 3 of the 4 Formulas 10a to 10d. Fk is also greater than 45 for all alloys according to the invention in the examples in Table 4, and in fact is even mostly greater than 54 and thus in the range which is characterized by a good heat resistance and creep resistance. Among the laboratory batches not according to the invention, batches 2297 and 2300 are an example wherein the Formulas 9a to 9d are not satisfied and also an Fk<45 is obtained.
Table 5 shows the specific changes in mass after an oxidation test at 1100° C. in air after 11 cycles of 96 h, i.e. a total of 1056 h. The specific gross change in mass, the specific net change in mass and the specific change in mass of the spalled oxides after 1056 h are indicated in Table 5. The exemplary batches of the alloys according to the prior art, Alloy 601 and Alloy 690, exhibited a much higher gross change in mass than Alloy 602 CA, that of Alloy 601 being even many times greater than that of Alloy 690. Both form a chromium oxide layer that grows faster than an aluminum oxide layer. Alloy 601 still contains approximately 1.3% Al. This content is too low yet to form an aluminum oxide layer that is even only partly closed, and so the aluminum in the interior of the metallic material underneath the oxide layer oxidizes (internal oxidation), which results in a greater mass increase in comparison with Alloy 690.
Alloy 602 CA has approximately 2.3% aluminum. Thus an at least partly closed aluminum oxide layer can form underneath the chromium oxide layer in this alloy. This reduces the growth of the oxide layer markedly and thus also the specific increase in mass. All alloys according to the invention (E) contain at least 2% aluminum and therefore have a gross increase in mass that is small, similar to that of Alloy 602 CA, or smaller. Also, all alloys according to the invention, similarly to the exemplary batches of Alloy 602 CA, exhibit spallings in the range of the measurement accuracy, while Alloy 601 and Alloy 690 exhibit great spallings.
The claimed limits for the alloy “E” according to the invention can therefore be substantiated in detail as follows:
Too low Cr contents mean that the Cr concentration at the oxide-metal interface sinks very rapidly below the critical limit during use of the alloy in a corrosive atmosphere, and so a closed pure chromium oxide can no longer be formed in case of a damage to the oxide layer, although other less protective oxides can form. Therefore 24% Cr is the lower limit for chromium. Too high Cr contents impair the phase stability of the alloy, especially at the high aluminum contents of ≧1.8%. Therefore 33% Cr must be regarded as the upper limit.
The formation of an aluminum oxide layer underneath the chromium oxide layer reduces the rate of oxidation. Below 1.8% Al, the aluminum oxide layer being formed has too many gaps in order to develop its effect completely. Too high Al contents impair the processability of the alloy. Therefore an Al content of 4.0% constitutes the upper limit.
The costs for the alloy rise with the reduction of the iron content. Below 0.1%, the costs rise disproportionately, since special raw material must be used. For cost reasons, therefore, 0.1% Fe must be regarded as the lower limit. With increase of the iron content, the phase stability decreases (formation of embrittling phases), especially at high chromium and aluminum contents. Therefore 7% Fe is a practical upper limit for ensuring the phase stability of the alloy according to the invention.
Si is needed during the manufacture of the alloy. Thus a minimum content of 0.001% is necessary. Too high contents again impair the processability and the phase stability, especially at high aluminum and chromium contents. The Si content is therefore limited to 0.50%.
A minimum content of 0.005% Mn is necessary for the improvement of the processability. Manganese is limited to 2.0%, since this element reduces the oxidation resistance.
Titanium increases the high-temperature resistance. From 0.60%, the oxidation behavior can be greatly impaired, and so 0.60% is the maximum value.
Even very low Mg and/or Ca contents improve the processability by binding sulfur, whereby the occurrence of low-melting NiS eutectics is prevented. Therefore a minimum content of respectively 0.0002% is necessary for Mg and or Ca. At too high contents, intermetallic Ni—Mg phases or Ni—Ca phases may form, which again greatly impair the processability. The Mg and/or Ca content is therefore limited to at most 0.05%.
A minimum content of 0.005% C is necessary for a good creep resistance. C is limited to a maximum of 0.12%, since above that content this element reduces the processability due to the excessive formation of primary carbides.
A minimum content of 0.001% N is necessary, whereby the processability of the material is improved. N is limited to at most 0.05%, since this element reduces the processability by the formation of coarse carbonitrides.
The oxygen content must be 0.020%, in order to ensure manufacturability of the alloy. A too low oxygen content increases the costs. The oxygen content is therefore 0.001%.
The content of phosphorus should be lower than or equal to 0.030%, since this surface-active element impairs the oxidation resistance. A too low P content increases the costs. The P content is therefore 0.0001%.
The contents of sulfur should be adjusted as low as possible, since this surface-active element impairs the oxidation resistance. Therefore 0.010% S is set as the maximum.
Molybdenum is limited to at most 2.0%, since this element reduces the oxidation resistance.
Tungsten is limited to at most 2.0%, since this element also reduces the oxidation resistance.
The following relationship between Cr and Al must be satisfied, in order that sufficient resistance to metal dusting is achieved:
Cr+Al≧28 (2a)
where Cr and Al are the concentrations of the elements in question in % by mass. Only then is the content of oxide-forming elements high enough to ensure a sufficient metal dusting resistance.
Furthermore, the following relationship must be satisfied, in order that sufficient phase stability is achieved:
Fp≦39.9 with (3a)
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (4a)
where Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in % by mass. The limits for Fp as well as possible incorporation of further elements have been substantiated in detail in the foregoing text.
If necessary, the oxidation resistance may be further improved with additions of oxygen-affine elements. They achieve this by being incorporated in the oxide layer and blocking the diffusion paths of the oxygen at the grain boundaries therein.
A minimum content of 0.01% Y is necessary, in order to obtain the oxidation-resistance-increasing effect of the Y. For cost reasons, the upper limit is set at 0.20%.
A minimum content of 0.001% La is necessary, in order to obtain the oxidation-resistance-increasing effect of the La. For cost reasons, the upper limit is set at 0.20%.
A minimum content of 0.001% Ce is necessary, in order to obtain the oxidation-resistance-increasing effect of the Ce. For cost reasons, the upper limit is set at 0.20%.
A minimum content of 0.001% cerium mixed metal is necessary, in order to obtain the oxidation-resistance-increasing effect of the cerium mixed metal. For cost reasons, the upper limit is set at 0.20%.
If necessary, niobium may be added, since niobium also increases the high-temperature resistance. Higher contents raise the costs very greatly. The upper limit is therefore set at 1.10%.
If necessary, the alloy may also contain tantalum, since tantalum also increases the high-temperature resistance. Higher contents raise the costs very greatly. The upper limit is therefore set at 0.60%. A minimum content of 0.001% is necessary in order to achieve an effect.
If necessary, the alloy may also contain Zr. A minimum content of 0.01% Zr is necessary, in order to obtain the high-temperature-resistance-increasing and oxidation-resistance-increasing effect of the Zr. For cost reasons, the upper limit is set at 0.20% Zr.
If necessary, Zr may be replaced completely or partly by Hf, since this element, just as Zr, increases the high-temperature resistance and the oxidation resistance. The replacement is possible starting from contents of 0.001%. For cost reasons, the upper limit is set at 0.20% Hf.
If necessary, boron may be added to the alloy, since boron increases the creep resistance. Therefore a content of at least 0.0001% should be present. At the same time, this surface-active element impairs the oxidation resistance. Therefore 0.008% boron is set as the maximum.
Cobalt may be present in this alloy up to 5.0%. Higher contents reduce the oxidation resistance markedly.
Copper is limited to at most 0.5%, since this element reduces the oxidation resistance.
Vanadium is limited to at most 0.5%, since this element likewise reduces the oxidation resistance.
Pb is limited to at most 0.002%, since this element reduces the oxidation resistance. The same is true for Zn and Sn.
Furthermore, the following relationship, which describes a particularly good processability, may be optionally satisfied for carbide-forming elements Cr, Ti and C:
Fa≦60 with (5a)
Fa=Cr+20.4*Ti+201*C (6a)
where Cr, Ti and C are the concentrations of the elements in question in % by mass. The limits for Fa and the possible incorporation of further elements have been substantiated in detail in the foregoing text.
Furthermore, the following relationship, which describes a particularly good heat resistance or creep resistance, with respect to the strength-increasing elements may be optionally satisfied:
Fk≧45 with (7a)
Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (8a)
where Cr, Ti, Al, Si and C are the concentrations of the elements in question in % by mass. The limits for Fa and the possible incorporation of further elements have been substantiated in detail in the foregoing text.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 011 161.4 | Jun 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DE2013/000268 | 5/15/2013 | WO | 00 |
