JP7669364B2 - Nickel-chromium-iron-aluminum alloy having excellent workability, creep strength and corrosion resistance and use thereof - Google Patents

Description

The present invention relates to a nickel-chromium-iron-aluminum wrought alloy that exhibits excellent high-temperature corrosion resistance, good creep strength, and improved workability.

Austenitic nickel-chromium-iron-aluminium alloys with different nickel, chromium and aluminium contents have long been used in furnace construction and in the chemical and petrochemical industries. This application requires good high-temperature corrosion resistance and good hot/creep strength.

Nickel alloys with different nickel, chromium and aluminum contents are also of great interest for use in solar tower power plants due to their properties. The installation consists of a field of mirror bodies (heliostats) arranged around a tall tower. The mirror bodies concentrate the sunlight on an absorber (solar receiver) attached to the top. The absorber consists of a tube system, which heats up a heat transfer medium. This medium circulates in a circulation section with an intermediate storage tank. By means of a heat exchanger system, the thermal energy is converted into electricity in a secondary circuit using a generator. The heat transfer medium is mainly a mixture of molten sodium and potassium nitrate salts, the maximum operating temperature of the salt is about 700°C, depending on the alloy used in the components (Kruizenga et al., Materials Corrosion of High Temperature Alloys Immersed in 600°C Binary Nitrate Salt, Sandia Report, SAND 2013-2526, 2013).

Above 700°C, the melting and decomposition of potassium nitrate salts becomes evident, and the corrosion of the metal tubes progresses significantly. Therefore, the maximum use temperature is 600-700°C, depending on the material. Materials used in the absorber are especially Alloy 800H (material number 1.4876, UNS N08810) or Alloy 625 (material number 2.4856, UNS N06625) (see Table 1).

It should be noted that, in general, the high-temperature corrosion resistance of the alloys shown in Table 1 improves with increasing chromium content. Alloys containing Al form a chromium oxide film (Cr ₂ O ₃ ), under which there is a more or less closed aluminum oxide film (Al ₂ O ₃ ). The addition of small amounts of elements with strong oxygen affinity, such as yttrium and cerium, improves the oxidation resistance. The chromium content is slowly consumed during use in the area of use to form a protective film. Thus, the higher the content of the chromium element forming the protective film, the longer the life of the material will be, since the time when the chromium content falls below a critical value and oxides other than Cr ₂ O ₃ , such as oxides containing iron and/or nickel, are delayed. The high-temperature corrosion resistance can be further improved by adding aluminum and/or silicon. Above a certain minimum content, the consumption of chromium is reduced, since these elements form a closed film under the chromium oxide film.

In particular, a high carbon content improves hot strength or creep strength at a given temperature. However, high contents of solid solution strengthening elements such as chromium, aluminum, silicon, molybdenum and tungsten also improve hot strength. In the range of 500°C to 900°C, the addition of aluminum, titanium and/or niobium can improve strength by precipitating gamma-min and/or gamma-inch phases.

Prior art examples of these alloys are shown in Table 1. Alloys such as Alloy 602 CA (N06025), Alloy 693 (N06693), Alloy 603 (N06603), or Alloy 214 (N07208) are known to exhibit superior corrosion resistance compared to Alloy 600 (N06600) and Alloy 601 (N06601) due to their high aluminum content of over 1.8%. Alloy 214, due to its high aluminum content, exhibits superior durability in a 60% sodium nitrate/40% potassium nitrate salt melt (Kruizenga et al., Materials Corrosion of High Temperature Alloys Immersed in 600°C Binary Nitrate Salt, Sandia Report, SAND 2013-2526, 2013). On the other hand, alloys such as Alloy 602 CA (N06025), Alloy 693 (N06693), Alloy 603 (N06603) and Alloy 214 (N07208) have high carbon and aluminum contents and therefore excellent hot or creep strength in the temperature range where molten nitrates are used. Alloy 602 CA (N06025) and Alloy 603 (N06603) still have excellent hot or creep strength at temperatures above 1000°C. However, the workability is impaired by, for example, the high aluminum content, and the higher the aluminum content, the greater the impairment (for example, in the case of Alloy 693 (N06693) and Alloy 214 (N07208)). The same is true to a large extent for silicon, which forms a low-melting intermetallic phase with nickel. In particular, Alloy 602 CA (N06025) and Alloy 603 (N06603) have a high proportion of primary carbides, limiting their cold workability.

WO 2019/075177 discloses a solar tower system including an absorber tube, a storage tank, and a heat exchanger, all of which contain a molten salt at a temperature above 650°C as a heat transfer medium, wherein the disclosure discloses that at least one of the absorber tube, storage tank, and heat exchanger components is made of (by weight%) 25-45% Ni, 12-32% Cr, 0.1-2.0% Nb, up to 4% Ta, up to It is made from an alloy containing 1% V, max 2% Mn, max 1.0% Al, max 5% Mo, max 5% W, max 0.2% Ti, max 2% Zr, max 5% Co, max 0.1% Y, max 0.1% La, max 0.1% Cs, max 0.1% other rare earths, max 0.20% C, max 3% Si, 0.05-0.50% N, max 0.02% B, and the balance Fe and impurities.

EP 0 508 058 A1 discloses an austenitic nickel-chromium-iron alloy consisting of (by weight) 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, balance Ni and unavoidable dissolution impurities.

U.S. Patent No. 4,882,125 discloses a high chromium nickel alloy characterized by excellent resistance to sulfidation and oxidation at temperatures above 1093°C, excellent creep strength of over 200 at a stress of 2000 PSI at temperatures above 983°C, good tensile strength, and good ductility (both at room temperature and at high temperatures), and consisting of (by weight percent) 27-35% Cr, 2.5-5% Al, 2.5-6% Fe, 0.5-2.5% Nb, max 0.1% C, max 1% each of Ti and Zr, max 0.05% Ce, max 0.05% Y, max 1% Si, max 1% Mn, and balance Ni.

EP 0 549 286 A1 discloses a high temperature durable 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 the sum of at least one of the elements of the group Mg, Ca, Ce, <0.5% of the sum of Mg + Ca, <1% Ce, 0.0001-0.1% B, 0-0.5% Zr, 0.0001-0.2% N, 0-10% Co, balance iron and impurities.

From DE 600 04 737 A1 a heat-resistant nickel-based alloy is known which contains ≦0.1% C, 0.01-2% Si, ≦2% Mn, ≦0.005% S, 10-25% Cr, 2.1-<4.5% Al, ≦0.055% N and at least one of the elements B, Zr, Hf in total 0.001-1%, whereby the aforementioned elements may be present in the following contents: B≦0.03%, Zr≦0.05%. 2%, Hf<0.8%, 0.01-15% Mo, 0.01-9% W, but the total content of Mo+W may be 2.5-15%, 0-3% Ti, 0-0.01% Mg, 0-0.01% Ca, 0-10% Fe, 0-1% Nb, 0-1% V, 0-0.1% Y, 0-0.1% La, 0-0.01% Ce, 0-0.1% Nd, 0-5% Cu, 0-5% Co, balance Ni. For Mo and W, the following formula must be satisfied:
2.5≦Mo+W≦15 (1)

DE 10 2015 200 881 A1 describes a tubular body made of austenitic steel for molten salts, in particular an absorption tube of a solar receiver with molten salt as heat transfer medium or another conduit for conveying molten salts, which has the following properties based on weight:
0-0.025% C, preferably 0.0095-0.024% C;
0.05-0.16% N;
2.4-2.6% Mo;
0.4-0.7% Si;
0.5-1.63% Mn;
0-0.0375% P;
0-0.0024% S;
17.15-18.0% Cr;
12.0-12.74% Ni;
0.0025-0.0045% B;
with the balance being Fe, and optionally normal impurities.

German Patent Application Publication No. 102012002514 describes a chromium alloy containing (by weight): 12-28% chromium, 1.8-3.0% aluminium, 1.0-15% iron, 0.01-0.5% silicon, 0.005-0.5% manganese, 0.01-0.20% yttrium, 0.02-0.60% titanium, 0.01-0.2% zirconium, 0.0002-0.05% magnesium, 0.0001-0.05% calcium, 0.03-0.11% carbon, 0.003-0.05% nitrogen, 0. A nickel-chromium-aluminum-iron alloy is described containing 0.0005-0.008% boron, 0.0001-0.010% oxygen, 0.001-0.030% phosphorus, max. 0.010% sulfur, max. 0.5% molybdenum, max. 0.5% tungsten, balance nickel and normal process impurities, where the following relationship must hold: 7.7C-x*a<1.0, where a=PN if PN>0 and a=0 if PN≦0, where x=(1.0Ti+1.06Zr)/(0.251Ti+0.132Zr), PN=0.251Ti+0.132Zr-0.857N, where Ti, Zr, N, C are the concentrations of the elements in weight percent.

German Patent No. 102012013437 describes a chromium alloy containing (by weight): >25-28% chromium, >2-3.0% aluminum, 1.0-11% iron, 0.01-0.2% silicon, 0.005-0.5% manganese, 0.01-0.20% yttrium, 0.02-0.60% titanium, 0.01-0.2% zirconium, 0.0002-0.05% magnesium, 0.0001-0.05% calcium, 0.03-0.11% carbon, 0.003-0.05% nitrogen, 0.0005-0.008% boron, 0.0001-0.010% oxygen, 0.001-0.030% phosphorus, max 0.010% sulfur, max 0.5% molybdenum, max 0.5% tungsten, balance nickel and normal process impurities, is described for use in the manufacture of seamless tubes, where the following relationship is satisfied: 0<7.7C-x*a<1.0 (2), if PN>0, then a=PN (3a), if PN≦0, then a=0 (3b), x=(1.0Ti+1.06Zr)/(0.251Ti+0.132Zr) (3c), PN=0.251Ti+0.132Zr-0.857N (4) must hold, where Ti, Zr, N, and C are the concentrations of the elements in weight percent.

DE 10 20120111161 A1 describes a nickel-chromium-aluminium alloy containing (in % by weight) 24-33% chromium, 1.8-4.0% aluminium, 0.10-7.0% iron, 0.001-0.50% silicon, 0.005-2.0% manganese, 0.00-0.60% titanium, 0.0002-0.05% magnesium and/or calcium, 0.005-0.12% carbon, 0.001-0.050% nitrogen, 0.0001-0.020% oxygen, 0.001-0.030% phosphorus, max. 0.010% sulphur, max. 2.0% molybdenum, max. 2.0% tungsten, the balance nickel and the usual process impurities, where the following relationship is satisfied: Cr+Al>28 (2a), Fp≦39.9 (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) must be true, where Cr, Fe, Al, Si, Ti, Mo, W, and C are the concentrations of the elements in weight percent.

US Patent No. 5,862,800 discloses a solar tower power plant for introducing solar energy into molten salt, using tubes of the same diameter and wall thickness made of alloy Alloy 625. The composition of Alloy 625 is given 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%, Ni balance (≧58%).

The present invention relates to a material having a sufficiently high chromium and aluminum content,
The objective was to design a nickel alloy that exhibits good phase stability, good workability, good atmospheric corrosion resistance similar to Alloy 602 CA (N06025), and good hot strength/creep strength, and thus could be used for other applications.

The object is to provide a nickel-chromium-iron-aluminium alloy containing (by weight): >17-33% chromium, 1.8-<4.0% aluminium, 0.10-15.0% iron, 0.001-0.50% silicon, 0.001-2.0% manganese, 0.00-0.60% titanium, 0.0002-0.05% each of magnesium and/or calcium, 0.005-0.12% carbon, 0.001-0.050% nitrogen, 0.0001-0.020% oxygen, 0.001-0.030% phosphorus, max 0.010% sulphur, max 2.0% molybdenum, max 2.0% tungsten, balance nickel and normal process impurities, with nickel > 50%, for use in solar tower power plants using molten nitrates as heat transfer media, the alloy having the following relationship:
Fp≦39.9 (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)
must hold, where:
Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in weight percent;
Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C are resolved by the alloy, which is the concentration in weight percent of the element.

Further advantageous embodiments of the subject matter of the present invention can be taken from the relevant dependent claims.

All alloy content data is in weight percent unless otherwise noted.

The distribution range of chromium element is >17-33%, and the preferred range can be set as follows:
->18-33%
- 20 to 33%
- 22 to 33%
- 24 to 33%
- 25 to 33%
- 26 to 33%
- 27 to 32%
- 28 to 32%
-> 28-32%
- 29 to 31%

The aluminum content is 1.8 to <4.0%, but again depending on the application of the alloy, the preferred aluminum content can be set as follows:
- 1.8 to 3.8%
- 1.8 to 3.2%
- 2.0 to 3.2%
-2.0~<3.0%
- 2.0 to 2.8%
- 2.2 to 2.8%
- 2.2 to 2.6%
-2.5~<4.0%
->3.0~<4.0%
- >3.2~<4.0%
- >3.2-3.8%
- >3.0~<3.5%

The iron content is in the range of 0.1-15.0%, and the preferred content can be set within the following distribution range according to the application scope:
-0.1~12.0%
-0.1~10.0%
- 0.1 to 7.5%
- 0.1 to 4.0%
- 0.1 to 3.0%
- 0.1~<2.5%
- 0.1 to 2.0%
- 0.1~<2.0%
- 0.1 to 1.0%
- 0.1~<1.0%
- 1.0~15.0%
- 1.25~15.0%
->2.5~15.0%
->4.0~15.0%
->4.0~12.0%
->7.0~15.0%
->7.0~10.5%
-7.5-10.5%

The silicon content is 0.001-0.50%. Preferably, Si can be set in the alloy within the following distribution range:
- 0.001~<0.40%
- 0.001~<0.25%
-0.001~0.20%
- 0.001~<0.10%
- 0.001~<0.05%

Similarly, manganese may be present in the alloy at 0.001-2.0% or in the following range:
-0.001~0.50%
- 0.001~<0.40%
-0.001~0.20%
-0.001~0.10%
- 0.001~<0.05%
- 0.005~<0.05%

The titanium content is 0.00-0.60%. Preferably, Ti is set in the alloy within the following distribution range:
-0.001~0.60%
-0.001~0.50%
-0.001~0.30%
-0.001~0.10%
-0.01~0.30%
-0.01~0.25%
- 0.00~<0.02%

Magnesium and/or calcium may also be included with a content of 0.0002 to 0.05%. Preferably, these elements may be set in the alloy as follows:
-0.0002~0.03%
-0.0002~0.02%
-0.0005~0.02%

The alloy contains 0.005-0.12% carbon, which can preferably be set within the following distribution ranges in the alloy:
- 0.01~<0.12%
-0.005~0.10%
- 0.005~<0.08%
- 0.005~<0.05%
-0.01~0.03%
- 0.01~<0.03%
-0.02~0.10%
-0.03~0.10%

The same applies to the nitrogen element, which is present with a content of 0.001 to 0.05%. The preferred content can be indicated as follows:
-0.003~0.04%

The alloy further comprises phosphorus with a content of 0.001 to 0.030%. The preferred contents can be indicated as follows:
-0.001~0.020%

The alloy further contains oxygen in a content of 0.0001-0.020%, in particular 0.0001-0.010%.

The elemental sulfur is present in the alloy as follows:
- Sulfur max 0.010%

Molybdenum and tungsten, either alone or in combination, are present in the alloy in a maximum content of 2.0% each. The preferred contents can be shown as follows:
- Mo max 1.0%
- W max 1.0%
- Mo max <0.50%
- W max<0.50%
- Mo max <0.10%
- W max <010%
- Mo max <0.05%
- W max<0.05%

In order to obtain sufficient phase stability, the following relationship is satisfied:
Fp≦39.9 (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)
must hold, where:
Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in weight percent.

Preferred ranges can be set as follows:
Fp≦38.4 (2b)
Fp≦36.6 (2c)

The nickel content is greater than or equal to 50%. The nickel content may preferably be set as follows:
- ≧55% or >55%
- ≧60% or >60%
- ≧65% or >65%
- ≧68% or >68%

Optionally, the yttrium element can be set in the alloy at a content of 0.001-0.20%. Preferably, Y can be set in the alloy in the following distribution range:
-0.001~0.15%
-0.001~0.10%
-0.001~0.08%
- 0.001~<0.045%
- 0.01~<0.045%

Optionally, the lanthanum element can be set in the alloy at a content of 0.001-0.20%. Preferably, La can be set in the alloy in the following distribution range:
-0.001~0.15%
-0.001~0.10%
-0.001~0.08%
-0.001~0.04%
-0.01~0.04%

Optionally, the cerium element may be set in the alloy at a content of 0.001-0.20%. Preferably, Ce may be set in the alloy in the following distribution range:
-0.001~0.15%
-0.001~0.10%
-0.001~0.08%
-0.001~0.04%
-0.01~0.04%

Optionally, when cerium and lanthanum are added simultaneously, cerium mischmetal (a mixture of about 50% Ce, about 25% La, about 15% Pr, about 5% Nd, Sm, Tb and Y) can be used in a content of 0.001-0.20%. Preferably, the cerium mischmetal can be set in the alloy within the following distribution range:
-0.001~0.15%
-0.001~0.10%
-0.001~0.08%
-0.001~0.04%
-0.01~0.04%

Optionally, the niobium element may be set in the alloy at a content of 0.00-1.10%. Preferably, the niobium may be set in the alloy within the following distribution range:
- 0.001~<1.10%
- 0.001~<0.70%
- 0.001~<0.50%
-0.001~0.30%
- 0.001~<0.30%
- 0.001~<0.20%
-0.01~0.30%
-0.10~1.10%
-0.20~0.70%

If niobium is present in the alloy, then an additional term for niobium must be added to equation (3a) as follows:
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)
where Cr, Fe, Al, Si, Ti, Nb, Mo, W and C are the concentrations of the respective elements in weight percent.

Optionally, the zirconium content is 0.001-0.20%. Preferably, the zirconium is set within the following range of distribution in the alloy:
-0.001~0.15%
- 0.001~<0.10%
-0.001~0.07%
-0.001~0.04%
-0.01~0.15%
- 0.01~<0.10%

Optionally, the hafnium content is 0.001-0.20%. Preferably, the hafnium is set within the following distribution range in the alloy:
-0.001~0.15%
- 0.001~<0.10%
-0.001~0.07%
-0.001~0.04%
-0.01~0.15%
- 0.01~<0.10%

Optionally, the alloy may contain 0.001-0.60% tantalum.

Preferably, Ta can be set in the alloy within the following distribution range:
-0.001~0.60%
-0.001~0.50%
-0.001~0.30%
-0.001~0.10%
- 0.001~<0.02%
-0.01~0.30%
-0.01~0.25%

Optionally, elemental boron may be included in the alloy as follows:
-0.0001~0.008%

The preferred content can be shown as follows:
-0.0005~0.008%
-0.0005~0.004%

Additionally, the alloy may optionally contain 0.0-5.0% Cobalt, which may be further limited as follows:
-0.001~5.0%
-0.01~5.0%
- 0.01~<5.0%
-0.01~2.0%
- 0.1 to 2.0%
- 0.1~<2.0%
-0.001~0.5

Additionally, the alloy may contain up to 0.5% copper.

Copper content may be further limited as follows:
- Maximum 0.20%
- Maximum 0.10%
- Maximum 0.05%
- < 0.05%
- Maximum 0.015%
- < 0.015%

If copper is present in the alloy, then a copper term must be added to equation (3a) 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 (3c)
where Cr, Fe, Al, Si, Ti, Cu, Mo, W and C are the concentrations of the respective elements in weight percent.

If niobium and copper are present in the alloy, then it is necessary to add terms for niobium and copper to equation (3a) 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 (3d)
where Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the respective elements in weight percent.

Additionally, the alloy may contain up to 0.5% vanadium.

The vanadium content may be further limited as follows:
- Maximum 0.20%
- Maximum 0.10%
- Maximum 0.05%

Finally, the impurities may further include the elements lead, zinc and tin in the following contents:
Pb max. 0.002%
Zn maximum 0.002%
Sn max. 0.002%

Furthermore, optionally, the following relationship, which indicates particularly good processability:
Fa≦60 (4a)
Fa=Cr+20.4*Ti+201*C (5a)
can be satisfied, where:
Cr, Ti and C are the concentrations of the elements in weight percent. Preferred ranges are as follows:
Fa≦54 (4b)
It can be set as follows.

If niobium is present in the alloy, then an additional term for niobium must be added to equation (5a) as follows:
Fa=Cr+6.15*Nb+20.4*Ti+201*C (5b)
where Cr, Nb, Ti and C are the concentrations of the respective elements in weight percent.

Optionally, furthermore, the following relationship, which expresses particularly good hot strength or creep strength:
F≧47 (6a)
Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (7a)
can be satisfied, where:
Cr, Ti, Al, Si and C are the concentrations of the elements in weight percent.

The preferred ranges are as follows:
F≧49 (6b)
F≧53 (6c)
It can be set as follows.

If niobium and/or boron are present in the alloy, then it is necessary to add terms for niobium and/or boron to equation (7a) as follows:
Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (7b)
where Cr, Ti, Nb, Al, Si, C and B are the concentrations of the respective elements in weight percent.

The alloy according to the invention is preferably processed in a VOD (vacuum oxygen decarburization) or VLF (vacuum ladle furnace) plant after open melting. However, melting and casting in a vacuum (VIM) is also possible. After ingot casting or continuous casting, the alloy is annealed, if necessary, at temperatures between 900 ° C and 1270 ° C for 0.1 h to 100 h to the desired semi-finished product form, followed by hot forming, if necessary with intermediate annealing at 900 ° C to 1270 ° C for 0.05 h to 100 h. The surface of the material can be chemically and/or mechanically removed if necessary (and several times) and cleaned in between and/or at the end. After the end of the hot forming, if necessary, an intermediate annealing can be carried out at 700 ° C to 1250 ° C under a protective gas, for example argon or hydrogen, for 0.1 min to 70 h, followed by cooling in air, in a stirred annealing atmosphere or in a water bath and cold forming to the desired semi-finished product form to a degree of forming of up to 98%. Then, solution treatment is carried out at a temperature range of 700°C to 1250°C for 0.1 minutes to 70 hours, optionally under a protective gas such as argon or hydrogen, followed by cooling in air, in a stirred annealing atmosphere or in a water bath. If necessary, chemical and/or mechanical cleaning of the material surface can be carried out during and/or after the final annealing.

The alloys of the present invention can be readily manufactured and used in product forms such as strip, sheet, rod, wire, longitudinal welded pipe, and seamless pipe.

These product forms are manufactured with an average particle size of 5 μm to 600 μm. The preferred range is 20 μm to 200 μm.

The alloy according to the invention is preferably used in solar tower power plants that use molten nitrates as the heat transfer medium. The alloy according to the invention can be used in all components that come into contact with the molten salt.

The alloy according to the invention can be used in particular in the absorbers (solar receivers) of the towers of solar power plants and/or in heat exchangers of the power generation circuits (e.g. via steam turbines) and/or in storage tanks and transport pipes.

The nitrate may preferably be a mixture of sodium nitrate and potassium nitrate.

This mixture may preferably be of the following composition:
- 50-70% sodium nitrate and 50-30% potassium nitrate - 55-65% sodium nitrate and 45-35% potassium nitrate - 58-62% sodium nitrate and 42-38% potassium nitrate

Alternatively, a mixture of sodium nitrate, potassium nitrate and sodium nitride may be used.

If desired, the salt mixture can also be used under a pure _CO2 atmosphere.

The maximum use temperature is 800° C. The maximum use temperature may be limited as follows:
- Maximum 750°C
- Maximum 700°C
- Maximum 680°C
- Maximum 650°C
- < 650 ° C.
- Maximum 620°C
- Max. 600°C
- < 600 ° C.

Tests carried out
The resulting phases in phase stability equilibrium were calculated for the different alloy variants with the program JMatPro from Thermotech Inc. As database for the calculations, the database TTNI7 for nickel-based alloys from Thermotech Inc. was used.

The formability is determined in a tensile test at room temperature according to DIN EN ISO 6892-1, in which the yield strength R _p0.2 , the tensile strength R _m and the elongation at break A are measured. The elongation A is calculated from the elongation of the broken test specimen at the original gauge length L ₀ and at the gauge length L _U after break:
A=(L _U - _{L 0} )/L ₀ 100%=ΔL/L ₀ 100%
The elongation at break is indexed according to the gauge length:
For example, for A ₅ , the gauge length L ₀ =5·d ₀ , where d ₀ =the initial diameter of the circular specimen.

The tests were carried out on circular specimens with a gauge range diameter of 6 mm and a gauge length L ₀ of 30 mm. The specimens were taken transverse to the forming direction of the semi-finished product. The forming speed was 10 ^-3 1/s (40%/min) for R _p0.2 =10 MPa/s and R _m =6.7.

The measured elongation A in a tensile test at room temperature can be used as an indicator of formability. A material with good processability has an elongation of at least 50%.

The hot strength is determined by a hot tensile test according to DIN EN ISO 6892-2, in which the yield strength R _p0.2 , the tensile strength R _m and the elongation at break A are determined analogously to the tensile test at room temperature (DIN EN ISO 6892-1).

The tests were carried out on circular specimens with a gauge range diameter of 6 mm and an initial gauge length L ₀ of 30 mm. The specimens were taken transverse to the forming direction of the semi-finished product. The forming speed was 10 ⁻⁵ 1/s (0.5%/min) for R _p0.2 =8.33 and 10 ⁻⁴ 1/s (5%/min) for R _m =8.33 (DIN EN ISO 6892-2).

The test specimen is placed in the tensile testing machine at room temperature and heated to the desired temperature without applying tensile force. After reaching the test temperature, it takes 1 hour (600°C) or 2 hours (700°C to 1100°C) to reach temperature equilibrium. Then, a tensile force is applied to the specimen so that the desired strain rate is maintained, and the test begins.

The creep strength of a material improves with increasing hot strength. Therefore, hot strength is also used to evaluate the creep strength of various materials.

The corrosion resistance at high temperatures was investigated in an oxidation test at 1000 ° C in air, during which the test was interrupted every 96 hours to measure the weight change of the test specimens due to oxidation. During the test, the test specimens were placed in a ceramic crucible, the spalled oxide could be collected and the weight of the spalled oxide could be measured. The sum of the weight change of the test specimen (net weight change) and the weight of the spalled oxide is the total weight change of the test specimen. The specific weight change is the weight change per unit surface of the test specimen. In the following, the specific net weight change is referred to as m _Netto , the specific total weight change as m _Brutto and the specific weight change of the spalled oxide as m _spall . The test was carried out on test specimens with a thickness of about 5 mm. Three test specimens were taken from each batch, the values shown being the average values of these three test specimens.

Description of properties – Corrosion resistance in molten salt
In Kruizenga et al. (2013, Materials Corrosion of High Temperature Alloys Immersed in 600°C Binary Nitrate Salt), the corrosion resistance at 600°C in a molten salt consisting of 60% sodium nitrate and 40% potassium nitrate flowing with air was tested, among others, for the nickel alloys Alloy 625 (N06625), Alloy 120 (N08120), Alloy 230 (N02230), Alloy 242 (N10242), and Alloy 214 (N07208) (Table 1). The analysis results of the alloys used are shown in Table 2. After the test, the oxide film was removed from the metal surface and the weight of the test specimen was measured before the test, after the test, and after the oxide film was removed to determine the weight of the oxide film. From this, the weight loss (amount of scale removed) relative to the surface of the test specimen before the test was determined.

Table 3 shows the corrosion rates after 3000 hours, expressed on the one hand as scale removal (mg/ ^cm2 ) and on the other hand as metal loss (μm/year). Alloy 214 shows the lowest corrosion rate of 5.7 μm/year at an aluminum content of 4.5%, followed by Alloy 224 at an aluminum content of 3.8% with a corrosion rate of 8.3 μm/year. All other nickel alloys tested (Alloy 625, Alloy 120, Alloy 242 and Alloy 230) show much higher corrosion rates of more than 16.8 μm/year at aluminum contents below 0.5%. Alloy 214 and Alloy 224 form an aluminum oxide film that provides good protection against molten nitrates. If the aluminum content is too low, such as in alloys Alloy 625, Alloy 120, Alloy 242, and Alloy 230, the aluminum oxide film does not form and the corrosion rate increases. Therefore, it is advantageous for alloys used in molten nitrates to have sufficient aluminum content to allow the formation of a closed aluminum oxide film.

In addition to excellent corrosion resistance in molten nitrates, the alloy according to the invention further possesses the following properties:
- Good phase stability - Good workability - Good corrosion resistance in air similar to Alloy 602CA (N06025) - Good hot strength/creep strength - Phase stability

In the nickel-chromium-iron-aluminium system with the addition of Ti and/or Nb, various embrittled TCP (topologically close-packed) phases can form, such as, for example, Laves, σ or μ phases, or embrittled η or ε phases, depending on the alloy content (see, for example, Ralf Buergel, Handbuch der Hochtemperaturwerkstofftechnik, 3. Auflage, Vieweg Verlag, Wiesbaden, 2006, p.70-374). Calculation of the equilibrium phase fractions as a function of temperature, for example for N06690 of batch 111389 (see Table 4 for typical compositions), mathematically shows that below 720° C. (T _{S BCC} ), a significant proportion of α-chromium (BCC phase in FIG. 1) is formed. The formation of this phase is very difficult, since analytically it is very different from the base material. However, this may well occur if the solvus temperature T S BCC of this phase is very high, as described for example for a variant of Alloy 693 ( _UNS 06693) in "E. Slevolden, JZ Albertsen. U. Fink, Tjeldbergodden _Methanol Plant: Metal Dusting Investigations," Corrosion/2011, paper no. 11144 (Houston, TX: NACE 2011), p.15". Figures 2 and 3 show the phase diagrams of the variants of Alloy 693 in Table 4, Alloy 3 or Alloy 10 (from U.S. Pat. No. 4,882,125, Table 1). This phase is brittle and therefore causes undesirable embrittlement of the material. The formation temperature T _{S BCC} of Alloy 3 is 1079°C, and the formation temperature T _{S BCC} of Alloy 10 is 939°C. "E. Slevolden, JZ Albertsen. U. Fink, "Tjeldbergodden Methanol Plant: Metal Dusting Investigations," Corrosion/2011, paper no. 11144 (Houston, TX: NACE 2011), p.15" does not provide accurate analytical results for alloys in which α-chromium (BCC phase) occurs. However, among the examples shown for Alloy 693 in Table 4, it is presumed that α-chromium may occur in the analyzed material with the highest calculated solvus temperature T _{S BCC} (e.g., Alloy 10). In a corrected analysis (reducing the solvus temperature T _{S BCC} ), in "E. Slevolden, JZ Albertsen. U. Fink, Tjeldbergodden Methanol Plant: Metal Dusting Investigations," Corrosion/2011, paper no. 11144 (Houston, TX: NACE 2011), p.15," α-chromium was only detected near the surface. In order to avoid the occurrence of such embrittled phases, it is desirable for the alloy according to the present invention to have a solvus temperature T _{S BCC} of 939° C. or less, which corresponds to the lowest solvus temperature T _{S BCC} among the examples of Alloy 693 (from U.S. Pat. No. 4,882,125, Table 1) in Table 4.

This is especially true of the following formula:
Fp≦39.9 (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)
where:
Cr, Al, Fe, Si, Ti, Nb, Cu, Mo, W and C are the concentrations of the elements in weight percent. From Table 4, which contains the prior art alloys, it can be seen that Fp is greater than 39.9 for alloy 8, alloy 3 and alloy 2, and is just 39.9 for alloy 10. For all other alloys with _{T S BCC} less than 939°C, Fp≦39.9.

- Processability As an example of processability, formability will be considered here.

Alloys can be strengthened by several mechanisms, resulting in high hot and creep strength. The addition of other elements can increase the strength more or less depending on the element (solid solution strengthening). However, it is much more effective to increase the strength by particles or precipitates (particle strengthening). This can be achieved, for example, by the γ' phase that occurs when adding Al or other elements, for example Ti, to nickel alloys, or by the carbides that occur when adding carbon to nickel alloys containing chromium (see, for example, Ralf Buergel, Handbuch der Hochtemperaturwerkstofftechnik, 3. Auflage, Vieweg Verlag, Wiesbaden, 2006, p.358-369).

Increasing the content of γ' phase forming elements or the C content certainly improves hot strength, but formability is increasingly impaired even in the solution-treated state.

For a material with very good formability, an elongation _A5 in a tensile test at room temperature of 50% or more, and at least 45% or more, is targeted.

This is especially true for the carbide forming elements Cr, Nb, Ti and C, as expressed by the following relationship:
Fa≦60 (4a)
Fa=Cr+6.15*Nb+20.4*Ti+201*C (5b)
This is achieved when
Cr, Nb, Ti and C are the concentrations of the elements in weight percent.

- Hot strength/creep strength At the same time, the yield strength or tensile strength at high temperatures should reach at least the value of Alloy 601 (see Table 6).
600 ° C.: Yield strength R _p0.2 > 150 MPa; tensile strength R _m > 500 MPa (8a, 8b)
800° C.: Yield strength R _p0.2 >130 MPa; tensile strength R _m >135 MPa (8c, 8d)

The yield strength or tensile strength is preferably in the range of the tensile strength of Alloy 602 CA (see Table 6). At least three of the following four relationships should be true:
600 ° C.: Yield strength R _p0.2 > 250 MPa; tensile strength R _m > 570 MPa (9a, 9b)
800° C.: Yield strength R _p0.2 >180 MPa; tensile strength R _m >190 MPa (9c, 9d)

Requirements 8a, 8b, 8c and 8d are particularly related to the following relationships for the main strengthening elements:
F≧47 (6a)
Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (7b)
is satisfied if
Cr, Ti, Nb, Al, Si, C and B are the concentrations of the elements in weight percent.

- Corrosion resistance in the atmosphere:
The alloy according to the invention has good atmospheric corrosion resistance similar to that of Alloy 602 CA (N06025).

FIG. 1 shows the relationship between the amount of phases in thermodynamic equilibrium (wt%) and temperature for Alloy 690 (N06690) of a typical batch example 111389 (Tables 5a and 5b). FIG. 1 shows the relationship between the amount (wt%) of the phase in thermodynamic equilibrium and temperature for Alloy 693 (N06693), an example of Alloy 3 in Table 4. FIG. 1 shows the relationship between the amount (wt%) of the phase in thermodynamic equilibrium and temperature for Alloy 693 (N06693), an example of Alloy 10 in Table 4.

Example:
- Manufacturing:
Tables 5a and 5b show the analysis of the batches melted on a laboratory scale and, for comparison, of some prior art batches melted on an industrial scale: Alloy 602CA (N06025), Alloy 690 (N06690), Alloy 601 (N06601). The prior art batches are marked with a T and the batches according to the invention with an E. The batches melted on a laboratory scale are marked with an L and the batches melted on an industrial scale with a G.

Laboratory-scale vacuum melted ingots of the alloys of Tables 5a and 5b were annealed at 900°C-1270°C for 8 hours, hot rolled and then subjected to an intermediate annealing at 900°C-1270°C for 0.1-1 hour to a final thickness of 13 mm and 6 mm, respectively. The sheets thus produced were solution treated at 900°C-1270°C for 1 hour. The test specimens required for the measurements were prepared from these sheets.

For alloys melted on an industrial scale, samples were taken from in-house manufactured industrial sheets of appropriate thickness. From these sheets, the test specimens required for the measurements were prepared.

All alloy variations typically had grain sizes between 70 and 505 μm.

The following properties are compared for the batch examples in Tables 5a and 5b:
- Corrosion resistance in molten nitrates - Phase stability - Formability by tensile tests at room temperature - Hot strength/creep strength by hot tensile tests - Corrosion resistance in air by oxidation tests

- Corrosion resistance in molten nitrates:
For batches 2301, 250129-250138, and 250147-250149, as well as batches 250164, 250311, and 250526, which were melted on a laboratory scale, the aluminum content is equal to or greater than 1.8%. This aluminum content is high enough that a closed aluminum oxide film can occur under the chromium oxide film, and therefore they meet the requirements for corrosion resistance in molten salts.

Phase stability:
Phase diagrams were calculated for selected prior art alloys in Table 4 and all experimental batches (Tables 5a and 5b) and the solvus temperatures _TSBCC were entered in Tables 4 and 5a. Also, the values of Fp were calculated by Equation 3d for the compositions in Table 4 or Tables 5a and 5b. The higher the solvus temperature _{TSBCC ,} the higher _Fp . All the examples of N06693 _with solvus temperatures _TSBCC higher than Alloy 10 have Fp>39.9. Therefore, the requirement of Fp≦39.9 (Equation 2a) is a good criterion for obtaining sufficient phase stability in the alloy. All the experimental batches in Tables 5a and 5b meet the criterion of Fp≦39.9.

- Moldability (processability):
Table 6 gives the yield strength R _p0.2 , the tensile strength R _m and the elongation A ₅ at room temperature (RT) and at 600° C., as well as the tensile strength R _m at 800° C. Also given are the values of Fa and Fk.

In Table 6, the prior art alloy Alloy 602 CA batch examples 156817 and 160483 have relatively low room temperature elongations _A5 of 36% and 42%, respectively, which are below the requirement for good formability. Fa is above 60, and therefore outside the range characterizing good formability. The alloys according to the invention (E) all have elongations above 50%, thus meeting the requirement. The alloys according to the invention all have Fa below 60, thus falling within the range for good formability. The elongations are high, especially when Fa is relatively low.

- Hot strength/creep strength:
Prior art alloy Alloy 601 example batch 156656 in Table 6 is an example of the minimum requirements for yield strength and tensile strength at 600°C or 800°C, while prior art alloy Alloy 602 CA example batches 156817 and 160483 are examples of very good values for yield strength and tensile strength at 600°C or 800°C. Alloy 601 is representative of a material that meets the minimum requirements for hot or creep strength as described by relationships 8a-8d. Alloy 602 CA is representative of a material that exhibits excellent hot or creep strength as described by relationships 9a-9d. Both alloys have Fk values well above 47, with Alloy 602 CA also significantly exceeding the values for Alloy 601, indicating increased strength values for Alloy 602 CA. All alloys (E) according to the invention have yield strengths at 600°C or 800°C that are within or significantly exceed those of Alloy 601, thus satisfying relations 8a-8d. They are within the range of values for Alloy 602 CA, and, except for batches 250526 and 250311, also satisfy the desirable requirements, i.e., three of the four relations 9a-9d. Fk is also greater than 47 or greater than 54 for all alloys according to the invention in the examples of Table 6, thus within the ranges characterized by good hot or creep strength. Among the experimental batches not according to the invention, batches 2297 and 2300 are examples where relations 8a-8d do not hold and Fk is also less than 47.

- Corrosion resistance in the atmosphere:
Table 7 shows the specific weight change after 11 cycles of 96 hours at 1100°C in air, i.e., a total of 1056 hours. Table 7 shows the specific gross weight change, specific net weight change, and specific weight change of the spalled oxide after 1056 hours. The prior art alloy batch examples Alloy 601 and Alloy 690 show significantly higher total weight change than Alloy 602 CA, with the total weight change of Alloy 601 being even more significantly higher than that of Alloy 690. Both form chromium oxide films that grow faster than aluminum oxide films. Alloy 601 still contains about 1.3% Al. This content is too low to form even a partially closed aluminum oxide film, so aluminum is oxidized inside the metal material under the oxide film (internal oxidation). This results in an increase in weight compared to Alloy 690. Alloy 602CA contains about 2.3% aluminum. Therefore, in this alloy, a closed aluminum oxide film can form under the chromium oxide film. This significantly suppresses the growth of the oxide film and also suppresses the specific weight gain. Alloy (E) according to the invention contains at least 2% aluminum, so it shows a low or lower total weight gain like Alloy 602 CA. Also, like the batch example of Alloy 602 CA, all of the alloys according to the invention show spalling within the measurement accuracy, but Alloy 601 and Alloy 690 show significant spalling.

Thus, the claimed limits of alloy "E" according to the present invention can be justified in detail as follows:
If the chromium content is too low, the chromium concentration falls below the critical limit too quickly when the alloy is used in a corrosive atmosphere, and a closed chromium oxide film cannot be formed. Therefore, a chromium content of more than 17% is the lower limit. If the chromium content is too high, the phase stability of the alloy decreases, especially at high aluminum contents of 1.8% and above. Therefore, a chromium content of 33% is the upper limit.

The oxidation rate slows down due to the formation of an aluminum oxide film under the chromium oxide film. If the aluminum content falls below 1.8%, the aluminum oxide film is incomplete and the effect is not fully exerted. If the aluminum content is too high, the workability of the alloy is impaired. Therefore, the upper limit for the aluminum content is less than 4.0%.

Reducing the iron content increases the cost of the alloy. Below 0.1%, special starting materials must be used, which increases the cost disproportionately. Therefore, an iron content of 0.1% is set as the lower limit due to cost considerations. Increasing the iron content reduces the phase stability (brittle phases are formed), especially at high chromium and aluminum contents. Therefore, 15% iron is a reasonable upper limit to ensure phase stability of the alloy according to the invention.

Silicon is required for the production of the alloy. Therefore, a minimum content of 0.001% is required. Workability and phase stability are impaired, especially at higher aluminum and chromium contents. Therefore, the silicon content is limited to 0.50%.

A minimum manganese content of 0.001% is required to improve workability. Manganese element reduces oxidation resistance and is therefore limited to 2.0%.

Titanium increases high temperature strength. Above 0.60%, oxidation behavior may deteriorate, so the maximum is 0.60%.

Even with very low magnesium and/or calcium contents, the solidification of sulfur improves workability, thereby avoiding the formation of low-melting nickel-sulfur eutectic. Therefore, a minimum content of 0.0002% for magnesium and/or calcium is required. If the content is too high, the intermetallic nickel-magnesium or nickel-calcium phases are formed, which also significantly reduces workability. Therefore, the magnesium and/or calcium content is limited to a maximum of 0.05%.

A minimum carbon content of 0.005% is necessary to obtain good creep strength. Carbon is limited to a maximum of 0.12%, because above this content this element reduces workability due to excessive formation of primary carbides.

A minimum nitrogen content of 0.001% is required, which improves the workability of the material. Nitrogen is limited to a maximum of 0.05% because the formation of coarse carbonitrides reduces workability.

The alloy cannot be manufactured unless the oxygen content is 0.020% or less. If the oxygen content is too low, the cost increases. Therefore, the oxygen content is 0.0001% or more.

Since phosphorus is a surface active element and impairs oxidation resistance, it is desirable for the phosphorus content to be 0.030% or less. If the phosphorus content is too low, the cost will increase. Therefore, the phosphorus content is 0.001% or more.

Because sulfur is a surface active element and impairs oxidation resistance, it is desirable to set the content as low as possible. For this reason, the maximum sulfur content is set at 0.010%.

Molybdenum element is limited to a maximum of 2.0% as it reduces oxidation resistance.

Tungsten element also reduces oxidation resistance and is therefore limited to a maximum of 2.0%.

Nickel is the balance element. Too little nickel reduces phase stability, especially at high chromium contents. Therefore, nickel must be at least 50%.

In order to obtain sufficient phase stability, the following relationship is satisfied:
Fp≦39.9 (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)
must hold, where:
Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations of the elements in weight percent. The limitations on Fp and the possible inclusion of other elements have been justified in detail above.

If necessary, oxidation resistance can be further improved by adding oxygen-affinity elements such as yttrium, lanthanum, cerium, and cerium mischmetal. These elements are incorporated into the oxide film and block oxygen diffusion paths at the grain boundaries.

To obtain the oxidation resistance-improving effect of yttrium, a minimum yttrium content of 0.001% is required. Due to cost considerations, the upper limit is set at 0.20%.

To obtain the oxidation resistance-improving effect of lanthanum, a minimum lanthanum content of 0.001% is required. Due to cost considerations, the upper limit is set at 0.20%.

To obtain the oxidation resistance-improving effect of cerium, a minimum cerium content of 0.001% is required. Due to cost considerations, the upper limit is set at 0.20%.

To obtain the oxidation resistance-improving effect of cerium mischmetal, a minimum cerium mischmetal content of 0.001% is required. Due to cost considerations, the upper limit is set at 0.20%.

Niobium also increases high temperature strength, so it can be added if necessary. Increasing the content makes the cost very high. Therefore, the upper limit is set at 1.10%.

If desired, the alloy may also contain tantalum, as this also increases high temperature strength and oxidation resistance. Increasing the content would be very costly, so the upper limit is set at 0.60%. A minimum content of 0.001% is required to be effective.

Optionally, the alloy may also contain zirconium . A minimum zirconium content of 0.001% is required to obtain the high temperature strength and oxidation resistance enhancing effects of zirconium . Cost considerations set an upper limit of 0.20% zirconium .

Optionally, the alloy may also contain hafnium. A minimum hafnium content of 0.001% is required to obtain the high temperature strength and oxidation resistance enhancing effects of hafnium. Cost considerations set an upper limit of 0.20% hafnium.

Boron can be added to the alloy if necessary, since it improves creep strength. Therefore, a content of at least 0.0001% is desirable. At the same time, this surface active element reduces oxidation resistance. Therefore, a maximum of 0.008% boron is set.

This alloy may contain up to 5.0% cobalt. Higher contents significantly reduce oxidation resistance.

Copper element reduces oxidation resistance and is therefore limited to a maximum of 0.5%.

Vanadium element is limited to a maximum of 0.5% as it reduces oxidation resistance.

The element lead reduces oxidation resistance and is therefore limited to a maximum of 0.002%. The same is true for zinc and tin.

Furthermore, optionally, the following relationship, which indicates particularly good workability for the carbide-forming elements chromium, titanium and carbon:
Fa≦60 (4a)
Fa=Cr+20.4*Ti+201*C (5a)
can be satisfied, where:
Cr, Ti and C are the concentrations of the elements in weight percent. The limitations on Fp and the possible inclusion of other elements have been thoroughly justified above.

Optionally, the strength-increasing elements may further satisfy the following relationship, which represents particularly good hot strength or creep strength:
F≧47 (6a)
Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (7a)
can be satisfied, where:
Cr, Ti, Al, Si and C are the concentrations of the elements in weight percent. The limitations on Fp and the possible inclusion of other elements have been thoroughly justified above.

Claims (22)

2. A molten sintered body containing (by weight ): > 28-33 % chromium, 1.8-< 4.0 % aluminium, 0.10-15.0% iron, 0.001-0.50% silicon, 0.001-2.0% manganese, 0.00- <0.02 % titanium, 0.0002-0.05% each of magnesium and/or calcium, 0.005- 1. A nickel-chromium-iron-aluminum alloy consisting of less than 0.03% carbon, 0.001-0.050% nitrogen, 0.0001-0.020% oxygen, 0.001-0.030% phosphorus, maximum 0.010% sulfur, maximum 2.0% molybdenum, maximum 2.0% tungsten, balance nickel and normal process impurities, and greater than 50% nickel, wherein the nitrate is a mixture of sodium and potassium nitrate salts in a composition of 50-70% sodium nitrate and 50-30% potassium nitrate, and has the following relationship:
Fp≦39.9 (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)
must hold, where:
Cr, Fe, Al, Si, Ti, Mo, W and C are the concentrations in weight percent of the elements, said alloy being used in all components that come into contact with said molten nitrates, and said alloy being usable up to a maximum temperature of 800°C. The alloy of claim 1, wherein the aluminum content is 1.8-3.8%. An alloy according to claim 1 or 2, having an iron content of 0.1 to 12.0%. The alloy of any one of claims 1 to 3, wherein the silicon content is from 0.001 to less than 0.40 %. An alloy according to any one of claims 1 to 4, having a manganese content of 0.001 to 0.50%. The alloy of any one of claims 1 to 5 , further comprising a yttrium content of 0.001 to 0.20%. The alloy of any one of claims 1 to 6 , further comprising a lanthanum content of 0.001 to 0.20%. The alloy of any one of claims 1 to 7 , further comprising a cerium content of 0.001 to 0.20%. The alloy of any one of claims 1 to 8 , further comprising a cerium mischmetal content of 0.001 to 0.20%. Furthermore, the niobium content is 0.0 to 1.1%, and the Nb term is added to the formula (3a) to obtain:
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)
10. The alloy of any one of claims 1 to 9, wherein Cr, Fe, Al, Si, Ti, Nb, Mo, W and C are the concentrations in weight percent of the elements. The alloy of any one of claims 1 to 10 , further comprising a zirconium content of 0.001 to 0.20%. The alloy of any one of claims 1 to 11 , further comprising a hafnium content of 0.001 to 0.20%. The alloy of any one of claims 1 to 12 , further comprising a boron content of 0.0001 to 0.008%. The alloy of any one of claims 1 to 13, further comprising 0.0 to 5.0% cobalt. Further, it contains up to 0.5% copper, and a term for Cu is added to the formula (3a) to give:
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)
2. The alloy of claim 1 , wherein Cr, Fe, Al, Si, Ti, Cu, Mo, W, and C are the concentrations in weight percent of the elements. An alloy according to any one of claims 1 to 15, further comprising up to 0.5% vanadium. An alloy according to any one of claims 1 to 16, wherein the impurities are set to a maximum content of 0.002% lead, 0.002% zinc, and 0.002% tin. 11. The alloy of claim 10, wherein for Nb-free alloys the following formulas hold: Fa≦60 (4a), Fa=Cr+20.4*Ti+201*C (5a), where Cr, Ti and C are the concentrations in weight percent of the elements, thereby achieving particularly good workability, or for Nb -containing alloys Fa=Cr+6.15*Nb+20.4*Ti+201*C (5b), where Cr, Nb, Ti and C are the concentrations in weight percent of the elements, 2. The alloy according to claim 1, wherein for the alloy without B and Nb, the following formulas hold: Fk≧47 (6a), Fk=Cr+19*Ti+10.2*Al+12.5*Si+98*C (7a), where Cr, Ti, Al, Si and C are the concentrations in weight percent of the elements, thereby achieving particularly good hot strength/creep strength. 14. The alloy according to claim 10 or 13, wherein for alloys containing B and/or Nb the following formulas hold: Fk≧47 (6a), Fk=Cr+19*Ti+34.3*Nb+10.2*Al+12.5*Si+98*C+2245*B (7b) hold, whereby particularly good hot strength/creep strength is achieved, where Cr, Ti, Nb, Al, Si, C and B are the concentrations in weight percent of the elements. 21. Use of the alloy according to any one of claims 1 to 20 as strip, sheet, wire, rod, longitudinal welded tube and seamless tube. 21. Use of the alloy according to any one of claims 1 to 20 for the production of strip, sheet, wire, rod, longitudinal welded tubes and seamless tubes.

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