Patent Application
20230002861
The invention relates to a nickel-chromium-iron-aluminum wrought alloy having excellent high-temperature corrosion resistance, good creep resistance and improved processability.
Austenitic nickel-chromium-iron-aluminum alloys having different nickel, chromium and aluminum contents have long been used in furnace construction and in the chemical and petrochemical industry. For this service, a good high-temperature corrosion resistance and a good hot strength/creep resistance are required.
Due to their properties, nickel alloys having different nickel, chromium and aluminum contents are also of great interest with respect to the use in solar tower power plants. These plants consist of a field of mirrors (heliostats), which are disposed around a high tower. Due to the mirrors, the sunlight is concentrated on the absorber (solar receiver) mounted at the tip. The absorber consists of a tube system, in which a heat-transfer medium is heated. This medium circulates in a loop having temporary storage tanks. Due to a heat-exchanger system, the thermal energy is converted by means of a generator into electricity in a secondary loop. The heat-transfer medium is especially a salt mixture of sodium and potassium nitrate salt melts, whereby a maximum service temperature of the salt of around 700° C. is obtained, depending on the alloy used for the components (Kruizenga et al., Materials Corrosion of High Temperature Alloys Immersed in 600° C. Binary Nitrate Salt, Sandia Report, SAND 2013-2526, 2013).
At temperatures above 700° C., the potassium nitrate salt melts decompose markedly, which greatly increases the corrosion of the metallic tubes. Therefore the maximum service temperature lies between 600 and 700° C., depending on material. The materials usually used in the absorber are, among others, Alloy 800H (material number 1.4876, UNS N08810) or Alloy 625 (material number 2.4856, UNS N06625) (see Table 1).
In general, it must be pointed out that the high-temperature corrosion resistance of the alloys listed in Table 1 increases with increasing chromium content. The Al-containing alloys form a chromium oxide layer (Cr2O3) with an underlying aluminum oxide layer (Al2O3), which is more or less closed. Small additions of strongly oxygen-affine elements such as, for example, yttrium or cerium, improve the oxidation resistance. In the course of the service in the area of application for establishment of the protective layer, the chromium content is slowly consumed. The useful life of the material is therefore prolonged by a higher chromium content, since a higher content of the element chromium, which forms the protective layer, delays the time at which the chromium content goes below the critical limit and oxides other than Cr2O3 are formed that are, for example, iron-containing and/or nickel-containing oxides. A further increase of the high-temperature corrosion resistance can be achieved by additions of aluminum and/or silicon. Starting from a certain minimum content, these elements form a closed layer underneath the chromium oxide layer and in this way reduce the consumption of chromium.
The hot strength and creep strength at the indicated temperatures are improved by a high carbon content among other possibilities. However, even high contents of solid-solution-strengthening elements such as chromium, aluminum, silicon, molybdenum and tungsten improve the hot strength. In the range of 500° C. to 900° C., additions of aluminum, titanium and/or niobium may improve the strength by precipitation of the γ′ and/or γ″ phase.
Examples of these alloys according to the prior art are listed in Table 1.
Alloys such as Alloy 602 CA (N06025), Alloy 693 (N06693), Alloy 603 (N06603) or Alloy 214 (N07208) are known for their excellent corrosion resistance in comparison with Alloy 600 (N06600) or Alloy 601 (N06601), on the basis of a high aluminum content of more than 1.8%. On the basis of its high aluminum content, Alloy 214 has an excellent resistance in 60% sodium nitrate/40% potassium nitrate salt melts (Kruizenga et al., Materials Corrosion of High Temperature Alloys Immersed in 600° C. Binary Nitrate Salt, Sandia Report, SAND 2013-2526, 2013). At the same time, alloys such as Alloy 602 CA (N06025), Alloy 693 (N06693), Alloy 603 (N06603) or Alloy 214 (N07208) exhibit, on the basis of the high carbon content or aluminum content, an excellent hot strength or creep strength in the temperature range in which nitrate salt melts are used. Alloy 602 CA (N06025) and Alloy 603 (N06603) still have an excellent hot strength or creep strength even at temperatures above 1000° C. However, due to the high aluminum contents, for example, the processability is impaired, wherein the impairment becomes greater with increasing aluminum content (for example, in Alloy 693 (N06693) and Alloy 214 (N07208)). The same is true to a greater degree 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 a high proportion of primary carbides.
WO 2019/075177 A1 discloses a solar tower system that includes the absorber tube, a storage tank and a heat exchanger, all of which contain a molten salt at temperatures of >650° C. as heat-transfer medium, wherein the disclosure states that at least one of the components (absorber tube, storage tank and heat exchanger) is made from an alloy that contains (in mass-%) 25-45% Ni, 12-32% Cr, 0.1-2.0% Nb, up to 4% Ta, up to 1% V, up to 2% Mn, up to 1.0% Al, up to 5% Mo, up to 5% W, up to 0.2% Ti, up to 2% Zr, up to 5% Co, up to 0.1% Y, up to 0.1% La, up to 0.1% Cs, up to 0.1% other rare earths, up to 0.20% C, up to 3% Si, 0.05-0.50% N, up to 0.02% B and the rest Fe and impurities.
EP 0 508 058 A1 discloses an austenitic nickel-chromium-iron alloy consisting of (in mass-%) 0.12-0.3% C, 23-30% Cr, 8-11% Fe, 1.8-2.4% Al, 0.01-0.15% Y, 0.01-1.0% Ti, 0.01-1.0% Nb, 0.01-0.2% Zr, 0.001-0.015% Mg, 0.001-0.01% Ca, max. 0.03% N, max. 0.5% Si, max. 0.25% Mn, max. 0.02% P, max. 0.01% S, the rest Ni 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 sulfidation and oxidation at temperatures above 1093° C., an outstanding creep resistance of more than 200 hours at temperatures above 983° C. and a stress of 2000 PSI, a good tensile strength and good elongation, both at room temperature and elevated temperatures, consisting of (in weight-%) 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 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% in total of at least one of the elements of the group that includes 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, the rest iron and impurities.
From DE 600 04 737 T2, a heat-resisting nickel-base alloy has become known, 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 content of Mo+W of 2.5-15% may be present, 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)
DE 102015200881A1 describes a tubular body of austenitic steel for a salt melt, especially absorber tube of a solar receiver having a salt melt as heat carrier or other tubing for conveying a salt melt, wherein the steel composition comprises, on a weight basis:
0% to 0.025% C, preferably 0.0095% to 0.024% C;
wherein the rest is Fe and possibly common impurities.
DE 102012002514 describes a nickel-chromium-aluminum-iron alloy containing (in mass-%) 12 to 28% chromium, 1.8 to 3.0% aluminum, 1.0 to 15% iron, 0.01 to 0.5% silicon, 0.005 to 0.5% manganese, 0.01 to 0.20% yttrium, 0.02 to 0.60% titanium, 0.01 to 0.2% zirconium, 0.0002 to 0.05% magnesium, 0.0001 to 0.05% calcium, 0.03 to 0.11% carbon, 0.003 to 0.05% nitrogen, 0.0005 to 0.008% boron, 0.0001-0.010% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 0.5% molybdenum, max. 0.5% tungsten, the rest nickel and the common process-related impurities, wherein the following relationships must be satisfied: 7.7 C−x*a<1.0 with a=PN, when PN>0, or a=0, when PN≥0. Therein x=(1.0 Ti+1.06 Zr)/(0.251 Ti+0.132 Zr) and PN=0.251 Ti+0.132 Zr−0.857 N and Ti, Zr, N, C are the concentrations of the elements in question in mass-%.
DE 102012013437B3 describes the use of a nickel-chromium-aluminum-iron alloy containing (in mass-%)>25 to 28% chromium, >2 to 3.0% aluminum, 1.0 to 11% iron, 0.01 to 0.2% silicon, 0.005 to 0.5% manganese, 0.01 to 0.20% yttrium, 0.02 to 0.60% titanium, 0.01 to 0.2% zirconium, 0.0002 to 0.05% magnesium, 0.0001 to 0.05% calcium, 0.03 to 0.11% carbon, 0.003 to 0.05% nitrogen, 0.0005 to 0.008% boron, 0.0001-0.010% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 0.5% molybdenum, max. 0.5% tungsten, the rest nickel and the common process-related impurities, wherein the following relationships must be satisfied: 0<7.7 C−x*a<1.0 (2) with a=PN, when PN>0 (3a), or a=0, when PN≤0 (3b) and x=(1.0 Ti+1.06 Zr)/(0.251 Ti+0.132 Zr) (3c), wherein PN=0.251 Ti+0.132 Zr−0.857 N (4) and Ti, Zr, N, C are the concentrations of the elements in question in mass-%, for the manufacture of seamless tubes.
DE 1020120111161A1 describes a nickel-chromium-aluminum alloy containing (in mass-%) 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 to 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 common process-related impurities, wherein the following relationships must be satisfied: Cr+Al≥28 (2a) and Fp 5; 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), wherein Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in mass-%.
U.S. Pat. No. 5,862,800 A discloses a solar tower power plant for introduction of solar energy into molten salts, wherein tubes of equal diameter and equal wall thickness are used that consist of Alloy 625. The composition of Alloy 625 is indicated as follows: Cr 20-23%, Al≥0.4%, Fe≥5%, Si≥0.5%, Mn≥0.5%, Ti≥0.4%, C 0.03-0.1%, P≥0.02%, S≥0.015%, Mo 8-10%, Nb 3.15-4.15%, the rest Ni (≥58%).
The task underlying the invention consists in designing a nickel alloy that has sufficiently high chromium and aluminum contents, so that it has
This task is accomplished by a nickel-chromium-iron-aluminum alloy containing (in mass-%) >17 to 33% chromium, 1.8 to <4.0% aluminum, 0.10 to 15.0% iron, 0.001 to 0.50% silicon, 0.001 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 to 0.020% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 2.0% molybdenum, max. 2.0% tungsten, the rest nickel with nickel ≥50% and the common process-related impurities for the use in solar tower power plants with use of nitrate salt melts as heat-transfer medium, wherein the following relationships must be satisfied:
Fp≤39.9 with (2a)
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (3a)
wherein Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in mass-%,
wherein Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the elements in question in mass-%.
Advantageous further developments of the subject matter of the invention can be inferred from the associated dependent claims.
All values of alloy contents are in mass-% unless otherwise indicated.
The range of values for the element chromium lies between >17 and 33%, wherein preferred ranges may be adjusted as follows:
The aluminum content lies between 1.8 and <4.0%, wherein here also, depending on service area of the alloy, preferred aluminum contents may be adjusted as follows:
The iron content lies between 0.1 and 15.0%, wherein, depending on the area of application, preferred contents may be adjusted within the following ranges of values:
The silicon content lies between 0.001 and 0.50%. Preferably, Si can be adjusted within the range of values as follows in the alloy:
The same is true for the element manganese, which may be contained in proportions of 0.001 to 2.0% in the alloy.
Alternatively, the following range of values is also conceivable:
The titanium content lies between 0.00 and 0.60%. Preferably, Ti can be adjusted within the range of values as follows in the alloy:
Magnesium and/or calcium is also present in contents of 0.0002 to 0.05%. Preferably, the possibility exists of adjusting these elements in the alloy as follows:
The alloy contains 0.005 to 0.12% carbon. Preferably this may be adjusted within the range of values as follows in the alloy:
This is true in the same way for the element nitrogen, which is present in contents between 0.001 and 0.05%. Preferred contents may be obtained as follows:
Furthermore, the alloy contains phosphorus in contents between 0.001 and 0.030%. Preferred contents may be obtained as follows:
Furthermore, the alloy contains oxygen in contents between 0.0001 and 0.020%, especially 0.0001 to 0.010%
The element sulfur is present as follows in the alloy:
Molybdenum and tungsten are contained individually or in combination in the alloy with a content of respectively at most 2.0%. Preferred contents may be obtained as follows:
Beyond this, the following relationship must be satisfied in order that adequate phase stability is ensured:
Fp≤39.9 with (2a)
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (3a)
wherein Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in mass-%.
Preferred ranges may be adjusted as follows:
Fp≤38.4 (2b)
Fp≤36.6 (2c)
The nickel content is greater than or equal to 50% or greater than 50%. It may be preferably adjusted as follows:
Optionally, the element yttrium may be adjusted to contents of 0.001 to 0.20% in the alloy. Preferably, Y can be adjusted within the range of values as follows in the alloy:
Optionally, the element lanthanum may be adjusted to contents of 0.001 to 0.20% in the alloy. Preferably, La may be adjusted within the range of values as follows in the alloy:
Optionally, the element cerium may be adjusted to contents of 0.001 to 0.20% in the alloy. Preferably, Ce may be adjusted within the range of values as follows in the alloy:
Optionally, in case of simultaneous addition of cerium and lanthanum, cerium mixed metal (a mixture of around 50% Ce, around 25% La, around 15% Pr, around 5% Nd, Sm, Tb and Y) may also be used in contents of 0.001 to 0.20%. Preferably, cerium mixed metal may be adjusted within the range of values as follows in the alloy:
Optionally, the element niobium may be adjusted to contents of 0.00 to 1.10% in the alloy. Preferably, niobium may be adjusted within the range of values as follows in the alloy:
If niobium is contained in the alloy, the formula (3a) must be supplemented as follows by a term for niobium:
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+1.26*Nb+0.374*Mo+0.538*W−11.8*C (3b)
wherein Cr, Fe, Al, Si, Ti, Nb, Mo, W and C are the concentrations of the elements in question in mass-%.
Optionally, the zirconium content lies between 0.001 and 0.20%.
Preferably, zirconium may be adjusted within the range of values as follows in the alloy:
Optionally, the hafnium content lies between 0.001 and 0.20%.
Preferably, hafnium may be adjusted within the range of values as follows in the alloy:
Optionally, 0.001 to 0.60% tantalum may also be contained in the alloy.
Preferably, Ta may be adjusted within the range of values as follows in the alloy:
Optionally, the element boron may be contained as follows in the alloy:
Preferably contents may be obtained as follows:
Furthermore, the alloy may optionally contain between 0.0 and 5.0% cobalt, which beyond this may still be limited as follows:
Furthermore, at most 0.5% copper may be contained in the alloy.
Beyond this, the content of copper may be limited as follows:
If copper is contained in the alloy, the formula (3a) must be supplemented as follows by a term for copper:
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 (3c)
wherein Cr, Fe, Al, Si, Ti, Cu, Mo, W and C are the concentrations of the elements in question in mass-%.
If niobium and copper are contained in the alloy, the formula (3a) must be supplemented as follows by a term for niobium and a term for copper:
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 (3d)
wherein Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the elements in question in mass-%.
Furthermore, at most 0.5% vanadium may be contained in the alloy.
Beyond this, the content of vanadium may be limited as follows:
Finally, as impurities, the elements lead, zinc and tin may also be present in contents as follows:
Furthermore, the following relationship, which describes a particularly good processability, may optionally be satisfied:
Fa≤60 with (4a)
Fa=Cr+20.4*Ti+201*C (5a)
wherein Cr, Ti, and C are the concentrations of the elements in question in mass-%.
Preferred ranges may be adjusted with:
Fa≤54 (4b)
If niobium is contained in the alloy, the formula (5a) must be supplemented as follows by a term for niobium:
Fa=Cr+6.15*Nb+20.4*Ti+201*C (5b)
wherein Cr, Nb, Ti and C are the concentrations of the elements in question in mass-%.
Furthermore, the following relationship, which describes a particularly good hot strength or creep strength, may optionally be satisfied
Fk≥47 with (6a)
Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (7a)
wherein Cr, Ti, Al, Si and C are the concentrations of the elements in question in mass-%.
Preferred ranges may be adjusted with:
Fk≤49 (6b)
Fk≤53 (6c)
If niobium and/or boron is contained in the alloy, the formula (7a) must be supplemented as follows by a term for niobium and/or boron:
Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (7b)
wherein Cr, Ti, Nb, Al, Si, C and B are the concentrations of the elements in question in mass-%.
The alloy according to the invention is preferably smelted in an open-hearth process, followed by a treatment in a VOD (vacuum oxygen decarburization) or VLF (Vacuum Ladle Furnace) system. However, a smelting and casting in vacuum (VIM) is also possible. After casting in ingots or as continuous casting, the alloy is annealed in the desired semifinished product mold if necessary at temperatures between 900° C. and 1270° C. for 0.1 hours to 100 hours, then hot-formed, if necessary with intermediate annealings between 900° C. and 1270° C. for 0.05 hours to 100 hours. The surface of the material may if necessary be chemically or mechanically machined for cleaning intermediately (even several times) and/or at the end. After the end of the hot forming, a cold forming to the desired semifinished product shape may be carried out if necessary, with reduction ratios up to 98%, if necessary with intermediate annealings between 700° C. and 1250° C. for 0.1 minutes to 70 hours, if necessary under shield gas, such as, for example, argon or hydrogen, followed by a cooling in air, in the agitated annealing atmosphere or in the water bath. Thereafter, a solution annealing is carried out in the temperature range between 700° C. and 1250° C. for 0.1 minutes to 70 hours, if necessary under shield gas, such as, for example, argon or hydrogen, followed by a 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 be carried out intermediately and/or after the last annealing.
The alloy according to the invention may be readily manufactured and used in the product forms of strip, sheet, rod, wire, longitudinally welded tube and seamless tube.
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 is intended preferably to be used in solar tower power plants with use of nitrate salt melts as the heat-transfer medium.
It may be used for all components that are in contact with the molten salt.
It may be used in particular for the absorber (solar receiver) in the tower of the solar power plant and/or for the heat exchanger for the current-generating loop (for example via a steam turbine) and/or for the storage tank and/or the transport tubes.
The nitrate salts may preferably be a mixture of sodium and potassium nitrate salts.
The mixture may preferably consist of the following compositions:
Alternatively, a mixture of sodium nitrate, potassium nitrate and sodium nitride may be used.
If necessary, the salt mixtures may also be used under a pure CO2 atmosphere.
The maximum service temperature is 800° C. It may be limited as follows:
The phases occurring in equilibrium were calculated for the various alloying 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. In the process, the offset yield strength Rp0.2, the tensile strength Rm and the elongation to break A are determined. The elongation A is determined on the broken specimen from the elongation of the original gauge length L0 and the gauge length after break LU:
A=(LU−L0)/L0 100%=ΔL/L0 100%
Depending on gauge length, the elongation to break is provided with indices:
For example, for As, the gauge length L0=5·d0, where d0=starting diameter of a round specimen.
The tests were performed on round specimens having a diameter of 6 mm in the measurement region and a gauge length L0 of 30 mm.
The sampling took place transverse relative to the direction of forming of the semifinished product. The forming speed was 10 MPa/s for Rp0.2 and 6.7 10−3 s−1 (40%/min) for Rm.
The measured value 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 hot strength is determined in a hot tension test according to DIN EN ISO 6892-2. In the process, the offset yield strength Rp0.2, the tensile strength Rm and the elongation to break A are determined by analogy with the tension test at room temperature (DIN EN ISO 6892-1).
The tests were performed on round specimens having a diameter of 6 mm in the measurement region and a starting gauge length L0 of 30 mm. The sampling took place transverse relative to the direction of forming of the semifinished product. The forming speed was 8.33 10−5 s−1 (0.5%/min) for Rp0.2 and 8.33 10−3 s−1 (5%/min) for Rm (DIN EN ISO 6892-2).
The specimen is mounted at room temperature in a tension-testing machine and heated without loading by a tensile force to the desired temperature. After attainment of the test temperature, a temperature equilibration is carried out for one hour (600° C.) or for two hours (700° C. to 1100° C.). Then the specimen is so loaded with a tensile force that the desired elongation rates are maintained and the test begins.
The creep strength of a material is improved with increasing high-temperature strength. Therefore the hot strength is also used for assessment of the creep strength of the various materials.
The corrosion resistance at higher temperatures was determined in an oxidation test at 1000° C. in air, wherein the test was interrupted every 96 hours and the changes in mass of the specimens due to oxidation were determined. During the test, the specimens were placed in ceramic crucibles, so that any oxide spalled off was collected and thus it was possible to determine the mass of the spalled oxide. The sum of the change in mass of a specimen (net change in mass) and of the mass of the spalled oxide is 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. These are denoted in the following as mnet for the specific net change in mass, mgross for the specific gross change in mass, mspall for the specific change in mass of the spalled oxides. The tests were performed on specimens having a thickness of approximately 5 mm. For each batch, 3 specimens were aged, wherein the indicated values are the mean values of these 3 specimens.
Corrosion Resistance in Salt Melts
In Kruizenga et al. (2013, Materials Corrosion of High Temperature Alloys Immersed in 600° C. Binary Nitrate Salt) the nickel alloys Alloy 625 (N06625), Alloy 120 (N08120), Alloy 230 (N02230), Alloy 242 (N10242), Alloy 214 (N07208) among others (Table 1) were investigated for their corrosion resistance in a salt melt, through which air was being passed, of 60% sodium nitrate salt and 40% potassium nitrate salt at 600° C. Table 2 shows the analysis of the alloys used. After the end of the test, the weight of the oxide layer was determined by removing it from the surface of the metal and weighing the specimen before the test, after the test and after the removal of the oxide layer. From this, the weight loss (descaling loss) relative to the surface area of the specimen before the test was determined.
Table 3 shows the corrosion rate after 3000 hours: on the one hand as scaling loss in mg/cm2 and on the other hand converted to metal loss in μm/year. The smallest corrosion rate was found for the alloy named Alloy 214 with 5.7 μm/year at an aluminum content of 4.5%, followed by Alloy 224 with a corrosion rate of 8.3 μm/year at an aluminum content of 3.8%. All other nickel alloys investigated (Alloy 625, Alloy 120, Alloy 242 and Alloy 230) have a much higher corrosion rate of 16.8 μm/year and greater at aluminum contents smaller than 0.5%. Alloy 214 and Alloy 224 form an aluminum oxide layer, which develops a good protection against nitrate salt melts. If the content of aluminum is too low, such as in the alloys named Alloy 625, Alloy 120, Alloy 242 and Alloy 230, no aluminum oxide layer can be formed, which leads to an increased corrosion rate.
Accordingly, it is advantageous that an alloy to be used in nitrate salt melts have an aluminum content that is sufficiently high that a closed aluminum oxide layer is formed.
The alloy according to the invention has not only an excellent corrosion resistance in nitrate salt melts but at the same time also the following properties;
In the nickel-chromium-iron-aluminum system having additions of Ti and/or Nb, various embrittling TCP (topologically close packed) phases can be formed, depending on alloy contents, such as, for example, Laves phases, sigma phases or μ-phases or even the embrittling η- or ε-phase (see, for example, Ralf Burgel, Handbuch der Hochtemperaturwerkstofftechnik [Handbook of High-Temperature Materials Engineering], 3rd edition, Vieweg Verlag, Wiesbaden, 2006, pages 370-374). The calculation of the equilibrium phase proportions as a function of temperature for N06690 in batch 111389, for example (see Table 4 for typical compositions), theoretically shows the formation of α-chromium (BCC phase in
This is the case in particular when the following formula is satisfied:
Fp≤39.9 with (2a)
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 (3d)
wherein Cr, Al, Fe, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the elements in question in mass-%. Table 4 containing the alloys according to the prior shows that Fp is greater than 39.9 for Alloy 8, Alloy 3 and Alloy 2 and is exactly 39.9 for Alloy 10. For all other alloys having Ts BCC lower than 939° C., Fp is ≤39.9.
Processability
The formability will be considered here as an example for the processability.
An alloy may be hardened by several mechanisms, so that it has a high hot strength or creep resistance. Thus the alloying of a different element leads, depending on element, to a more or less large increase of the strength (solution hardening). An increase of the strength by fine particles or precipitates (particle hardening) is much more effective. This may be done, for example, by the γ′ phase, which is formed with 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, Handbuch der Hochtemperaturwerkstofftechnik [Handbook of High-Temperature Materials Engineering, 3rd edition, Vieweg Verlag, Wiesbaden, 2006, pages 358-369).
The increase of the content of elements that form γ′-phase or of the C content indeed increases the hot strength, but increasingly impairs the deformability, even in the solution-annealed condition.
For a very readily formable material, elongations A5 of greater than or equal to 50% but at least greater than or equal to 45% in the tension test at room temperature are desirable.
This is achieved in particular when the following relationship between the carbide-forming elements Cr, Nb, Ti and C is satisfied:
Fa≤60 with (4a)
Fa=Cr+6.15*Nb+20.4*Ti+201*C (5b)
wherein Cr, Nb, Ti and C are the concentrations of the elements in question in mass-%.
Hot strength/creep strength
At the same time, the offset yield strength or the tensile strength at higher temperatures should attain at least the values of Alloy 601 (see Table 6).
600° C.: offset yield strength Rp0.2>150 MPa; tensile strength Rm>500 MPa (8a,8b)
800° C.: offset yield strength Rp0.2>130 MPa; tensile strength Rm>135 MPa (8c, 8d)
It would be desirable for the offset yield strength or the tensile strength to lie in the range of the tensile strength of Alloy 602 CA (see Table 6). At least 3 of the 4 following relationships should be satisfied:
600° C.: offset yield strength Rp0.2>250 MPa; tensile strength Rm>570 MPa (9a, 9b)
800° C.: offset yield strength Rp0.2>180 MPa; tensile strength Rm>190 MPa (9c, 9d)
The requirements 8a, 8b, 8c and 8d are met in particular when the following relationship between the principally hardening elements is satisfied:
Fk≤47 with (6a)
Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (7b)
wherein Cr, Ti, Nb, Al, Si, C and B are the concentrations of the elements in question in mass-%.
Corrosion Resistance in Air:
The alloy according to the invention is intended to have a good corrosion resistance in air, similar to that of Alloy 602 CA (N06025).
Manufacture:
Tables 5a and 5b show the analyses of the batches smelted on the laboratory scale together with some batches of Alloy 602CA (N06025), Alloy 690 (N06690), Alloy 601 (N06601) smelted on the industrial scale according to the prior art and used for comparison. The batches according to the prior art are identified with a T and those according to the invention with an E. The batches smelted on the laboratory scale are marked with an L, the batches smelted on the industrial scale with a G.
The ingots of the alloys in Table 5a and b, smelted on the laboratory scale in vacuum, were annealed between 900° C. and 1270° C. for 8 hours and hot-rolled to a final thickness of 13 mm and 6 mm by means of hot rolling and further intermediate annealings between 900° C. and 1270° C. for 0.1 to 1 hour. The sheets produced in this way were solution-annealed between 900° C. and 1270° C. for 1 hour. The specimens needed for the measurements were manufactured from these sheets.
For the alloys smelted on the industrial scale, a sample was taken from the industrial-scale fabrication of a commercially fabricated sheet having appropriate thickness. The specimens needed for the measurements were manufactured from these sheets.
All alloy variants typically had a grain size of 70 to 505 m.
For the exemplary batches in Table 5a and b, the following properties were compared:
In the batches 2301 and 250129 to 250138 and 250147 to 250149, smelted on the laboratory scale, as well as the batches 250164, 250311 and 250526, aluminum is greater than or equal to 1.8%.
This aluminum content is sufficiently high, so that a closed aluminum oxide layer is able to form underneath the chromium oxide layer. Thus they meet the requirement that was imposed on the corrosion resistance in salt melts.
Phase stability:
For the chosen alloys according to the prior art in Table 4 and for all laboratory batches (Tables 5a and 5b), the phase diagrams were therefore calculated and the solvus temperature Ts BCC was entered in Tables 4 and 5a. For the compositions in Tables 4 and 5a and b, the value for Fp was also calculated according to formula 3d. Fp becomes greater with increasing solvus temperature Ts BCC. All examples of N06693 with a higher solvus temperature Ts BCC higher than that of Alloy 10 have an Fp >39.9. The requirement Fp≤39.9 (formula 2a) is therefore a good criterion for achieving an adequate phase stability for an alloy. All laboratory batches in Tables 5a and b meet the criterion Fp≤39.9.
Formability (processability):
Offset yield strength Rp0.2, the tensile strength Rm and the elongation A5 for room temperature (RT) and for 600° C. are entered in Table 6, as is further the tensile strength Rm for 800° C.
Moreover, the values for Fa and Fk are entered.
In Table 6, the exemplary batches 156817 and 160483 of the alloy according to the prior art, Alloy 602 CA, have a relatively small elongation A5 at room temperature of 36 and 42% respectively, which lie below the requirements for a good formability. Fa is greater than 60 and thus above the range that characterizes a good formability. All alloys according to the invention (E) exhibit an elongation greater than 50%. Thus they meet the requirements. Fa is smaller than 60 for all alloys according to the invention. Thus they lie in the range in which a good formability is ensured. The elongation is particularly high when Fa is relatively small.
Hot strength/creep strength
The exemplary batch 156656 of the alloy according to the prior art, Alloy 601 in Table 6, is an example of the minimum requirements of offset yield strength and tensile strength at 600° C. and 800° C.; in contrast, the exemplary batches 156817 and 160483 of the alloy according to the prior art, Alloy 602 CA, are examples of very good values of offset yield strength and tensile strength at 600° C. and 800° C. Alloy 601 represents a material that exhibits the minimum requirements of hot strength and creep strength that are described in relationships 8a to 8d. Alloy 602 CA represents a material that exhibits an outstanding hot strength and creep strength that are described in relationships 9a to 9d. For both alloys, the value for Fk is much larger than 47 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 an offset yield strength and tensile strength at 600° C. and 800° C. in the range of or clearly above that of Alloy 601, and therefore satisfy the relationships 8a to 8d. They lie in the range of the values of Alloy 602 CA and, with the exception of batch 250526 and batch 250311, also meet the desirable requirements, i.e. 3 of the 4 relationships 9a to 9d. Fk also is larger than 47 for all alloys according to the invention in the examples in Table 6, or larger than 54 and thus in the range that is characterized by a good hot strength and creep resistance. Among the laboratory batches that are not according to the invention, batches 2297 and 2300 are an example that does not satisfy the relationships 8a to 8d and also has an Fk smaller than 47.
Corrosion resistance in air:
Table 7 shows the specific changes in mass after an oxidation test at 1100° C. in air after 11 cycles of 96 hours, i.e. in total 1056 hours. In Table 7, the specific gross change in mass, the specific net change in mass and the specific change in mass of the spalled oxides after 1096 hours are indicated. The exemplary batches of the alloys according to the prior art, Alloy 601 and Alloy 690, exhibit a much higher gross change in mass than Alloy 602 CA, wherein that of Alloy 601 is in turn much larger than that of Alloy 690. Both form a chromium oxide layer that grows more rapidly than an aluminum oxide layer. Alloy 601 still contains approximately 1.3% Al. This content is too small in order to form an even only partly closed aluminum oxide layer, for which reason the aluminum in the interior of the metallic material is oxidized underneath the oxide layer (internal oxidation). This causes a large increase in mass in comparison with Alloy 690. Alloy 602CA contains approximately 2.3% aluminum. For this alloy, therefore, a closed aluminum oxide layer is able to form underneath the chromium oxide layer. 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 similarly small or smaller gross increase in mass than Alloy 602 CA. Also, all alloys according to the invention exhibit spalling in the range of the measurement accuracy, similarly to the exemplary batches of Alloy 602 CA, whereas Alloy 601 and Alloy 690 exhibit great spalling.
The claimed limits for the alloy “E” according to the invention can therefore be justified individually as follows: Too low chromium contents mean that the chromium concentration during use of the alloy in a corrosive atmosphere decreases very rapidly below the critical limit, so that a closed chromium oxide layer can no longer be formed. Therefore a content of >17% chromium is the lower limit. Too high chromium contents worsen the phase stability of the alloy, especially at the high aluminum contents of ≥1.8%. Therefore 33% chromium is to be regarded as the upper limit.
The formation of an aluminum oxide layer underneath the chromium oxide layer reduces the oxidation rate. Below 1.8% aluminum, the aluminum oxide layer is too incomplete to develop its effect fully. Too high aluminum contents impair the processability of the alloy. Therefore an aluminum content of <4.0% forms the upper limit.
The costs for the alloy increase with the reduction of the iron content. Below 0.1%, the costs rise disproportionally, since special primary material must be used. For cost reasons, therefore, 0.1% iron is to be regarded as the lower limit. With increase of the iron content, the phase stability is reduced (formation of embrittling phases), especially at high chromium and aluminum contents. Therefore 15% Fe is a practical upper limit in order to ensure the phase stability of the alloy according to the invention.
Silicon is needed for the manufacture of the alloy. A minimum content of 0.001% is therefore necessary. Too high contents in turn impair the processability and the phase stability, especially at high aluminum and chromium contents. The silicon content is therefore restricted to 0.50%.
A minimum content of 0.001% manganese is necessary for improvement of the processability. Manganese is limited to 2.0%, since this element reduces the oxidation resistance.
Titanium increases the high temperature strength. At 0.60% and above, the oxidation behavior may be impaired, which is why 0.60% is the maximum value.
Even very low magnesium contents and/or calcium contents improve the processing by the binding of sulfur, whereby the occurrence of low-melting nickel-sulfur eutectics is avoided. For magnesium and/or calcium, therefore, a minimum content of 0.0002% is necessary. At too high contents, intermetallic nickel-magnesium phases or nickel-calcium phases may occur, which again greatly worsen the processability. The magnesium content and/or calcium content is therefore limited to at most 0.05%.
A minimum content of 0.005% carbon is necessary for a good creep resistance. Carbon is limited to at most 0.12%, since above such a content this element reduces the processability by the excessive formation of primary carbides.
A minimum content of 0.001% nitrogen is necessary, whereby the processability of the material is improved. Nitrogen is limited to at most 0.05%, since the processability is reduced due to the formation of coarse carbonitrides.
The oxygen content must be smaller than or equal to 0.020%, in order to ensure the manufacturability of the alloy. A too low oxygen content increases the costs. The oxygen content is therefore ≥0.0001%.
The content of phosphorus should be smaller than or equal to 0.030%, since this surface-active element impairs the oxidation resistance. A too low phosphorus content increases the costs.
The phosphorus content is therefore ≥0.001%.
The content of sulfur should be adjusted as low as possible, since this surface-active element impairs the oxidation resistance. Therefore at most 0.010% sulfur is specified.
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 likewise reduces the oxidation resistance.
Nickel is the residual element. A too low nickel content reduces the phase stability, especially at high chromium contents.
Nickel must therefore be larger than or equal to 50%.
Beyond this, the following relationship must be satisfied in order that adequate phase stability is ensured:
Fp≤39.9 with (2a)
Fp=Cr+0.272*Fe+2.36*Al+2.22*Si+2.48*Ti+0.374*Mo+0.538*W−11.8*C (3a)
wherein Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in question in mass-%. The limits for Fp and the possible incorporation of further elements have been justified in detail in the foregoing text.
If necessary, the oxidation resistance may be further improved with additions of oxygen-affine elements, such as, for example, yttrium, lanthanum, cerium, cerium mixed metal. These elements are incorporated in the oxide layer, where they block the paths of diffusion of the oxygen at the grain boundaries.
A minimum content of 0.001% yttrium is necessary to obtain the effect of the yttrium that increases the oxidation resistance.
For cost reasons, the upper limit is set to 0.20%.
A minimum content of 0.001% lanthanum is necessary to obtain the effect of the lanthanum that increases the oxidation resistance. For cost reasons, the upper limit is set to 0.20%.
A minimum content of 0.001% cerium is necessary to obtain the effect of the cerium that increases the oxidation resistance. For cost reasons, the upper limit is set to 0.20%.
A minimum content of 0.001% cerium mixed metal is necessary to obtain the effect of the cerium mixed metal that increases the oxidation resistance. For cost reasons, the upper limit is set to 0.20%.
If necessary, niobium may be added, since niobium also increases the high-temperature strength. Higher contents very greatly increase the costs. The upper limit is therefore set at 1.10%.
If necessary, the alloy may also contain tantalum, since tantalum also increases the high-temperature strength and the oxidation resistance. Higher contents very greatly increase the costs. 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 zirconium. A minimum content of 0.001% zirconium is necessary to obtain the effect of the zirconium that increases the high-temperature strength and the oxidation resistance. For cost reasons, the upper limit is set to 0.20% zirconium.
If necessary, the alloy may also contain hafnium. A minimum content of 0.001% hafnium is necessary to obtain the effect of the hafnium that increases the high-temperature strength and the oxidation resistance. For cost reasons, the upper limit is set to 0.20% hafnium.
If necessary, boron may be added to the alloy, since boron improves the creep resistance. Therefore a content of at least 0.0001% should be present. At the same time, this surface-active element worsens the oxidation resistance. Therefore at most 0.008% boron is specified.
Cobalt up to 5.0% may be contained in this alloy. Higher contents markedly reduce the oxidation resistance.
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 reduces the oxidation resistance.
Lead is limited to at most 0.002%, since this element reduces the oxidation resistance. The same is true for zinc and tin.
Furthermore, optionally the following relationship, which describes a particularly good processability, may be satisfied for the carbide-forming elements chromium, titanium and carbon:
Fa≤60 with (4a)
Fa=Cr+20.4*Ti+201*C (5a)
wherein Cr, Ti, and C are the concentrations of the elements in question in mass-%, The limits for Fa and the possible incorporation of further elements have been justified in detail in the foregoing text.
Furthermore, optionally the following relationship, which describes a particularly good hot strength and creep strength, may be satisfied between the elements that increase the strength:
Fk≤47 with (6a)
Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (7a)
wherein Cr, Ti, Al, Si and C are the concentrations of the elements in question in mass. The limits for Fa and the possible incorporation of further elements have been justified in detail in the foregoing text.
1)nominal compositions of alloys,
2)actual composition tested from heat
1)nach 3000 Stunden;
a)Dichte angenommen.
1)After 3000 hours;
a)Density assumed.
indicates data missing or illegible when filed
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 133 292.3 | Dec 2019 | DE | national |
| 10 2020 132 193.7 | Dec 2020 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DE2020/101025 | 12/4/2020 | WO |
