WO2021110217A1

WO2021110217A1 - Nickel-chromium-iron-aluminum alloy having good processability, creep resistance and corrosion resistance, and use thereof

Info

Publication number: WO2021110217A1
Application number: PCT/DE2020/101025
Authority: WO
Inventors: Heike Hattendorf; Benedikt NOWAK
Original assignee: Vdm Metals International Gmbh
Priority date: 2019-12-06
Filing date: 2020-12-04
Publication date: 2021-06-10
Also published as: US20230002861A1; DE102020132193A1; JP2023504842A; KR20220098789A; EP4069874A1; CN114746565A

Abstract

The invention relates to a nickel-chromium-iron-aluminum alloy containing (in wt.%) >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, 0.0002 to 0.05% each of 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, not more than 0.010% sulfur, not more than 2.0% molybdenum, not more than 2.0% tungsten, the remainder nickel with nickel ≥ 50% and the usual process-related impurities, for use in solar power tower plants using nitrate salt melts as the heat transfer medium, wherein the following relations must be satisfied: Fp ≤ 39.9 (2a) with 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 is the concentration of the respective elements in % by weight.

Description

Nickel-chromium-iron-aluminum alloy with good processability, creep resistance and corrosion resistance, as well as their use

The invention relates to a wrought nickel-chromium-iron-aluminum alloy with excellent high-temperature corrosion resistance, good creep resistance and improved processability.

Austenitic nickel-chromium-iron-aluminum alloys with different nickel, chromium and aluminum contents have long been used in furnace construction and in the chemical and petrochemical industries. For this use, good high-temperature corrosion resistance and good heat resistance / creep resistance are required.

Due to their properties, nickel alloys with different nickel, chromium and aluminum contents are also of great interest with regard to use in solar tower power plants. These systems consist of an array of mirrors (heliostats) that are arranged around a high tower. The mirrors concentrate the sunlight on the absorber (solar receiver) mounted in the tip. The absorber consists of a pipe system in which a heat transfer medium is heated. This medium circulates in a circuit with intermediate storage tanks. A heat exchanger system converts the thermal energy into electricity in a secondary circuit with the help of a generator. The heat transfer medium is primarily a salt mixture of sodium and potassium nitrate salt melts, which results in a maximum operating temperature of the salt of around 700 ° C, 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 noticeably, which greatly increases the corrosion of the metallic pipes. Therefore, depending on the material, the maximum operating temperature is between 600 and 700 ° C. The materials usually used in the absorber include Alloy 800H (material number 1.4876, UNS N08810) or Alloy 625 (material number 2.4856, UNS N06625) (see table 1).

In general, it should be noted that the high-temperature corrosion resistance of the alloys given in Table 1 increases with increasing chromium content. The Al-containing alloys form a chromium oxide layer (Cr203) with a more or less closed aluminum oxide layer (Al2O3) underneath. Small additions of elements with a strong affinity for oxygen such as B. yttrium or cerium improve the oxidation resistance. The chromium content is slowly consumed in the course of use in the application area to build up the protective layer. Therefore, a higher chromium content increases the service life of the material, since a higher content of the element chromium, which forms the protective layer, delays the point in time at which the chromium content is below the critical limit and oxides other than Cr2O3 are formed, such as iron and / or nickel-containing oxides. A further increase in high-temperature corrosion resistance can be achieved by adding aluminum and / or silicon. Above a certain minimum content, these elements form a closed layer below the chromium oxide layer and thus reduce the consumption of chromium.

The heat resistance or creep resistance at the specified temperatures is improved, among other things, by a high carbon content. However, high contents of solid solution strengthening elements such as chromium, aluminum, silicon, molybdenum and tungsten also improve the heat resistance. In the range from 500 ° C. to 900 ° C., additions of aluminum, titanium and / or niobium can improve the strength by eliminating the g and / or g ”phase.

Prior art examples of these alloys 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 compared to Alloy 600 (N06600) or Alloy 601 (N06601) due to a high aluminum content known by more than 1.8%. Due to its high aluminum content, Alloy 214 shows excellent resistance in 60% sodium nitrate / 40% potassium nitrate molten salts (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), due to the high carbon or aluminum content, show excellent heat resistance or creep resistance in the temperature range in which nitrate salt melts are used become. Alloy 602 CA (N06025) and Alloy 603 (N06603) still have excellent heat resistance and creep resistance even at temperatures above 1000 ° C. However, z. B. the processability is impaired by the high aluminum content, the impairment being the greater the higher the aluminum content (for example with Alloy 693 (N06693) and Alloy 214 (N07208)). The same applies to a greater extent to 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 absorber tubes, a storage tank and a heat exchanger, all of which contain a molten salt as a heat transfer medium at temperatures> 650 ° C., the disclosure including that at least one of the components absorber tubes, storage tank and heat exchanger , is made from an alloy containing (in% by 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 Contains 3% Si, 0.05-0.50% N, up to 0.02% B and the remainder Fe and impurities.

EP 0 508 058 A1 discloses an austenitic nickel-chromium-iron alloy consisting of (in% by mass) 0.12-0.3% C, 23-30% Cr, 8-11% Fe, 1,8 - 2.4% AI, 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 , Remainder Ni including unavoidable impurities caused by the melting process.

US 4882125 B1 discloses a nickel alloy with a high chromium content, which has excellent creep resistance of more than 200 hours at temperatures above 983 ° C. at a voltage of 2000 PSI due to excellent resistance to sulphurization and oxidation at temperatures greater than 1093 ° C. , good tensile strength and good elongation (both at room temperature and elevated temperatures), consisting of (in% by weight) 27 - 35% Cr, 2.5 - 5% Al, 2.5 - 6% Fe, 0 , 5 - 2.5% Nb, up to 0.1% C, each 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 balance.

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 at least one of the elements of the group containing Mg, Ca, Ce, <0.5% in Total Mg + Ca, <1% Ce, 0.0001-0.1% B, 0-0.5% Zr, 0.0001-0.2% N, 0-10% Co, remainder iron and impurities.

DE 600 04 737 T2 has made a heat-resistant nickel-based alloy known, containing <0.1% C, 0.01 - 2% Si, <2% Mn, <0.005% S, 10 - 25% Cr, 2.1 - <4.5% AI, <0.055% N, a total of 0.001 - 1% of at least one of the elements B, Zr, Hf, whereby the elements mentioned can be present in the following contents: B <0.03%, Zr <0.2 %, Hf <0.8%. 0.01 - 15% Mo, 0.01 - 9% W, where a total Mo + W content of 2.5-15% can be given, 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 , Remainder nickel. For Mo and W the following formula must be fulfilled:

2.5 <Mo + W <15 (1)

DE 102015200881 A1 describes a tubular body made of austenitic steel for a molten salt, in particular an absorber pipe of a solar receiver with a molten salt as a heat transfer medium or other pipeline for conveying a molten salt, with a steel composition which, on a weight basis, comprises:

0% to 0.025% C, preferably 0.0095% to 0.024% C;

0.05% to 0.16% N;

2.4% to 2.6% Mo;

0.4% to 0.7% Si;

0.5% to 1.63% Mn;

0% to 0.0375% P;

0% to 0.0024% S;

17.15% to 18.0% Cr;

12.0% to 12.74% Ni;

0.0025% to 0.0045% B; the remainder being Fe and possibly customary impurities.

In DE 102012002514 a nickel-chromium-aluminum-iron alloy, with (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 remainder nickel and the usual process-related impurities are described, with the following Relationships must be fulfilled: 7.7C - x ^* a <1, 0 with a = PN, if PN> 0, or a = 0, if PN <0. Where x = (1, 0Ti + 1, 06Zr) / (0.251 Ti + 0.132Zr) and PN = 0.251 Ti + 0.132Zr - 0.857N and Ti, Zr, N, C are the concentration of the respective elements in% by mass.

DE 102012013437B3 describes the use of a nickel-chromium-aluminum-iron alloy with (in% by mass)> 25 to 28% chromium,> 2 to 3.0% aluminum, 1.0 to 11% iron, 0.01 up 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 up to 0.030% phosphorus, max. 0.010% sulfur, max. 0.5% molybdenum, max. 0.5% tungsten, the remainder nickel and the usual process-related impurities, whereby the following relationships must be met: 0 <7.7 C - x ^* a <1, 0 (2) with a = PN, if PN> 0 (3a) or a = 0, if PN <0 (3b) and x = (1, 0 Ti + 1, 06 Zr) / (0.251 Ti + 0.132 Zr) (3c) where PN = 0.251 Ti + 0.132 Zr - 0.857 N (4) and Ti, Zr, N, C are the concentration of the relevant elements in% by mass, for the production of seamless tubes.

DE 1020120111161 A1 describes a nickel-chromium-aluminum alloy with (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, each 0.0002 to 0.05% magnesium and / or calcium, 0.005 to 0.12% carbon, 0.001 to 0.050% Nitrogen, 0.0001-0.020% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 2.0% molybdenum, max. 2.0% tungsten, the remainder nickel and the usual process-related impurities are described, with the following Relationships must be fulfilled: Cr + AI> 28 (2a) and Fp <; 39.9 with (3a) mp = Cr + 0.272-Fe + 2.36-Al + 2.22-Si + 2.48-Ti + 0.374-Mo + 0.538-W - 11.8-C (4a), where Cr, Fe, Al, Si, Ti, Mo, W and C are the concentration of the elements in question in% by mass.

The US 5,862,800 A discloses a solar tower power plant for the introduction of solar energy in molten salts, with pipes of the same diameter and of the same wall thickness, which are made of the alloy Alloy 625. The composition of Alloy 625 is specified 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%, remainder Ni (> 58%).

The object on which the invention is based is to design a nickel alloy which has sufficiently high chromium and aluminum contents so that it

• good phase stability,

• good processability,

• good corrosion resistance in air similar to that of Alloy 602 CA (N06025)

• and has a good heat resistance / creep resistance in order to use it for a different application.

This object is achieved by a nickel-chromium-iron-aluminum alloy with (in% by 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, each 0.0002 to 0.05% magnesium and / or calcium, 0.005 to 0.12% carbon, 0.001 up to 0.050% nitrogen, 0.0001 - 0.020% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 2.0% molybdenum, max. 2.0% tungsten, remainder nickel with nickel> 50% and the common process-related impurities for use in solar tower power plants using molten nitrate salts as a heat transfer medium, whereby the following relationship must be met: 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) where Cr, Fe, Al, Si , Ti, Mo, W and C are the concentrations of the respective elements in% by mass, with Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C being the concentrations of the respective elements in% by mass. Advantageous further developments of the subject matter of the invention can be found in the associated subclaims.

Unless otherwise stated, all specifications of alloy contents are in% by mass.

The spread range for the element chromium is between> 17 and 33%, whereby preferred ranges can be set as follows:

> 18 to 33%

20 to 33%

22 to 33%

24 to 33%

25 to 33%

26 to 33%

27 to 32%

28 to 32%

> 28 to 32%

29 to 31%

The aluminum content is between 1.8 and <4.0%, although here too, depending on the area of application of the alloy, preferred aluminum contents can be set as follows:

1.8 to 3.8%

1.8 to 3.2% 2.0 to 3.2% 2.0 to <3.0% 2.0 to 2.8% 2.2 to 2.8% 2.2 to 2.6% 2 .5 to <4.0%> 3.0 - <4.0%> 3.2 - <4.0% > 3.2 - 3.8%> 3.0 - <3.5%

The iron content is between 0.1 and 15.0%, whereby, depending on the area of application, preferred contents can be set within the following spread ranges:

0.1-12.0%

0.1-10.0%

0.1 -7.5%

0.1 -4.0%

0.1 -3.0%

0.1 - <2.5%

0.1 -2.0%

0.1 - <2.0%

0.1 -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 between 0.001 and 0.50%. Preferably, Si can be set in the alloy within the spread range as follows:

0.001 - <0.40%

0.001 - <0.25%

0.001 - 0.20%

0.001 - <0.10% 0.001 - <0.05%

The same applies to the element manganese, which can be contained in the alloy at 0.001 to 2.0%. Alternatively, the following spread range is also conceivable:

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 between 0.00 and 0.60%. Preferably, Ti can be set in the alloy within the spread range as follows:

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 are also contained in levels of 0.0002 to 0.05%. There is preferably the option of setting these elements in the alloy as follows:

0.0002 - 0.03%

0.0002 - 0.02%

0.0005 - 0.02%

The alloy contains 0.005 to 0.12% carbon. This can preferably be set in the alloy within the spread range as follows:

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%

This also applies to the element nitrogen, which is contained in levels between 0.001 and 0.05%. Preferred contents can be given as follows: 0.003 - 0.04%

The alloy also contains phosphorus in contents between 0.001 and 0.030%. Preferred contents can be given as follows:

0.001 - 0.020%

The alloy also contains oxygen in contents between 0.0001 and 0.020%, in particular contains 0.0001 to 0.010%.

The element sulfur is given in the alloy as follows:

Sulfur max. 0.010%

Molybdenum and tungsten are contained individually or in combination in the alloy with a maximum content of 2.0% each. Preferred contents can be given as follows:

Mon 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 addition, the following relationship must be fulfilled so that there is sufficient phase stability:

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) where Cr, Fe, Al, Si, Ti, Mo, W and C are the concentration of the elements in question in% by mass.

Preferred areas can be set with:

Fp <38.4 (2b)

Fp <36.6 (2c)

The nickel content is greater than or equal to 50% or greater than 50%. It can preferably be set as follows:

> 55% or> 55%

> 60% or> 60%

> 65% or> 65%

> 68% or> 68%

Optionally, the element yttrium can be set in the alloy in contents of 0.001 to 0.20%. Preferably, Y can be set in the alloy within the spread range as follows:

0.001 - 0.15%

0.001 -0.10%

0.001 - 0.08%

0.001 - <0.045%

0.01 - <0.045%

Optionally, the element lanthanum can be set in the alloy in contents of 0.001 to 0.20%. Preferably, La can be set in the alloy within the spread range as follows:

0.001 - 0.15% 0.001 - 0.10% 0.001 - 0.08% 0.001 - 0.04% 0.01 - 0.04%

Optionally, the element cerium can be set in the alloy with a content of 0.001 to 0.20%. Preferably, Ce can be set in the alloy within the spread range as follows:

0.001 - 0.15%

0.001 -0.10%

0.001 - 0.08%

0.001 - 0.04%

0.01 - 0.04%

Optionally, with the simultaneous addition of cerium and lanthanum, cerium mischmetal (a mixture of approx. 50% Ce, approx. 25% La, approx. 15% Pr, approx. 5% Nd, Sm, Tb and Y) can be used in contents from 0.001 to 0.20%. Cerium mischmetal can preferably be set in the alloy within the spread range as follows:

0.001 - 0.15%

0.001 -0.10%

0.001 - 0.08%

0.001 - 0.04%

0.01 - 0.04%

Optionally, the element niobium can be set in the alloy in contents of 0.00 to 1.10%. Niobium can preferably be adjusted in the alloy within the spread range as follows:

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 the alloy contains niobium, the formula (3a) must be supplemented by a term with niobium as follows:

Mp = Cr + 0.272 ^* 2.36 ^* Fe + AI + 2.22 ^* 2.48 ^* Si + Ti + 1, 26 + Nb ^* 0.374 ^* 0.538 ^* Mo + W - 11 8 ^* C (3b) wherein Cr , Fe, Al, Si, Ti, Nb, Mo, W and C are the concentration of the element in question in% by mass.

The zirconium content is optionally between 0.001 and 0.20%. Zirconium can preferably be adjusted in the alloy within the spread range as follows: 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 flafnium content is between 0.001 and 0.20%. Flafnium can preferably be adjusted in the alloy within the spread range as follows:

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 can also contain 0.001 to 0.60% tantalum.

Preferably, Ta can be set in the alloy within the spread range as follows:

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, the element boron can be contained in the alloy as follows:

0.0001 - 0.008%

Preferred contents can be given as follows:

0.0005 - 0.008%

0.0005 - 0.004%

Furthermore, the alloy can optionally contain between 0.0 to 5.0% cobalt, which can also be restricted as follows:

0.001 to 5.0%

0.01 to 5.0%

0.01 to <5.0%

0.01 to 2.0%

0.1 to 2.0%

0.1 to <2.0%

0.001 to 0.5%

Furthermore, the alloy can contain a maximum of 0.5% copper.

The copper content can also be restricted as follows: max. 0.20% max. 0.10% max. 0.05%

<0.05% max. 0.015%

<0.015%

If the alloy contains copper, a term with copper must be added to formula (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 concentration of the element in question in% by mass.

If the alloy contains niobium and copper, the formula (3a) must be supplemented with a term with niobium and a term with copper as follows:

Mp = Cr + 0.272 ^* 2.36 ^* Fe + AI + 2.22 ^* 2.48 ^* Si + Ti + 1, 26 + Nb ^* 0.477 ^* 0.374 ^* Cu + Mo + 0.538 ^* W - 11 8 ^* C ( 3d) where Cr, Fe, Al, Si, Ti, Nb, Cu, Mo, W and C are the concentration of the element in question in% by mass.

Furthermore, the alloy can contain a maximum of 0.5% vanadium.

The vanadium content can also be limited as follows: max. 0.20% max. 0.10% max. 0.05%

Finally, the elements lead, zinc and tin can also be present in levels of impurities as follows:

Pb max. 0.002%

Zn max. 0.002% Sn 0.002% or less.

Furthermore, the following relationship, which describes particularly good processability, can optionally be fulfilled:

Fa <60 with (4a)

Fa = Cr + 20.4 ^* Ti + 201 ^* C (5a) where Cr, Ti and C are the concentration of the relevant elements in% by mass. Preferred areas can be set with:

Fa <54 (4b)

If the alloy contains niobium, the formula (5a) must be supplemented by a term with niobium as follows:

Fa = Cr + 6.15 ^* Nb + 20.4 ^* Ti + 201 ^* C (5b) where Cr, Nb, Ti and C are the concentration of the elements in question in% by mass.

Furthermore, the following relationship can optionally be fulfilled, which describes a particularly good heat resistance or creep resistance:

Fk> 47 with (6a)

Fk = Cr + 19 ^* Ti + 10.2 ^* Al + 12.5 ^* Si + 98 ^* C (7a) where Cr, Ti, Al, Si and C are the concentration of the relevant elements in% by mass.

Preferred ranges can be set with: Fk> 49 (6b) Fk> 53 (6c)

If the alloy contains niobium and / or boron, the formula (7a) must be supplemented by a term with niobium and / or boron 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 concentration of the elements in question in% by mass. The alloy according to the invention is preferably melted openly, followed by a treatment in a VOD (vacuum oxygen decarburization) or VLF (vacuum ladle furnace) system. Melting and pouring in a vacuum (VIM) is also possible. After casting in blocks or as a continuous casting, the alloy is annealed into the desired form, if necessary at temperatures between 900 ° C and 1270 ° C for 0.1 hours to 100 hours, then hot-formed, if necessary with intermediate anneals between 900 ° C and 1270 ° C for 0.05 hours to 100 hours. If necessary, the surface of the material can be removed chemically and / or mechanically in between and / or at the end for cleaning (also several times). After the end of hot forming, cold forming with degrees of deformation of up to 98% into the desired semi-finished product shape, if necessary with intermediate annealing between 700 ° C and 1250 ° C for 0.1 minutes to 70 hours, if necessary under protective gas, such as. B. argon or hydrogen, followed by cooling in air, in the agitated annealing atmosphere or in a water bath. Thereafter, a solution annealing takes place in the temperature range between 700 ° C to 1250 ° C for 0.1 minutes to 70 hours, if necessary under protective gas, such as. B. argon or hydrogen, followed by cooling in air, in the agitated annealing atmosphere or in a water bath. If necessary, chemical and / or mechanical cleaning of the material surface can take place in between and / or after the last annealing.

The alloy according to the invention can be easily manufactured and used in the product forms strip, sheet metal, rod, wire, longitudinally welded tube and seamless tube.

These product forms are produced with an average grain size of 5 gm to 600 gm. The preferred range is between 20 pm and 200 pm.

The alloy according to the invention should preferably be used in solar tower power plants using nitrate salt melts as the heat transfer medium. It can be used for all components that are in contact with the molten salt.

It can 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 electricity-generating circuit (for example via a steam turbine) and / or for the storage unit and / or the transport pipes

The nitrate salts can preferably be a mixture of sodium and potassium nitrate salts.

The mixture can preferably consist of the following compositions: 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 can be used.

If necessary, the salt mixtures can also be used under a pure CO2 atmosphere.

The maximum operating temperature is 800 ° C. It can be restricted as follows: max. 750 ° C max. 700 ° C max. 680 ° C max. 650 ° C

<650 ° C max. 620 ° C max. 600 ° C

<600 ° C Tests carried out

Phase stability

The phases occurring in equilibrium were calculated for the various alloy variants with the JMatPro program from Thermotech. The database TTNI7 for nickel-based alloys from Thermotech was used as the database for the calculations.

The formability is determined in a tensile test according to DIN EN ISO 6892-1 at room temperature. The yield strength R _{Po, 2} , the tensile strength R _m and the elongation A up to break are determined. The elongation A is determined on the broken specimen from the extension of the original measuring distance L _o and the measuring length after the break L _u :

A = (L _u -L _o ) / L _o 100% = AU L _o 100%

Depending on the measuring length, the elongation at break is provided with indices:

For example, for As the measuring length L _o = 5-d _o with d _o = starting diameter of a round specimen.

The tests were carried out on round specimens with a diameter of 6 mm in the measuring range and a measuring length L _o of 30 mm. Samples were taken across the direction of deformation of the semi-finished product. The deformation rate was at R _{Po, 2} 10 MPa / s and at R _m 6.7 10 ^-3 1 / s (40% / min).

The measured value of the elongation A in the tensile test at room temperature can be used as a measure of the deformability. A material that is easy to process should have an elongation of at least 50%.

The heat resistance is determined in a hot tensile test according to DIN EN ISO 6892-2. The yield strength is R _{Po, 2} , the tensile strength R _m and the elongation A determined up to breakage analogously to the tensile test at room temperature (DIN EN ISO 6892-1).

The tests were carried out on round specimens with a diameter of 6 mm in the measuring range and an initial measuring length L _o of 30 mm. Samples were taken across the direction of deformation of the semi-finished product. The deformation speed was 8.33 10 ^'5 1 / s (0.5% / min) _{for R Po,} and 8.33 10 ^{' 4} 1 / s (5% / min) _{for R m (DIN EN ISO 6892-} 2).

The sample is installed in a tensile testing machine at room temperature and heated to the desired temperature with no tensile force. After the test temperature has been reached, the temperature is equalized for one hour (600 ° C) or two hours (700 ° C to 1100 ° C). The sample is then loaded with a tensile force in such a way that the desired expansion rates are maintained, and the test begins.

The creep resistance of a material improves with increasing heat resistance. This is why the heat resistance is also used to assess the creep resistance of the various materials.

The corrosion resistance at higher temperatures was determined in an oxidation test at 1000 ° C. in air, the test being interrupted every 96 hours and the changes in mass of the samples due to the oxidation being determined. During the experiment, the samples were placed in ceramic crucibles so that any flaking oxide was collected and the mass of the flaked oxide could thus be determined. The sum of the mass change of a sample (net mass change) and the mass of the flaked oxide is the gross mass change of the sample. The specific change in mass is the change in mass related to the surface of the specimen. In the following, these are referred to as m _net for the specific net mass change, m _gross for the specific gross mass change, m _spall for the specific mass change of the flaked oxides. The tests were carried out on samples with a thickness of approx. 5 mm. It 3 samples of each batch were removed from storage, the values given being the mean values of these 3 samples.

Description of properties

- Corrosion resistance in molten salts

In Kruizenga et al. (2013, Materials Corrosion of High Temperature Alloys Immersed in 600 ° C Binary Nitrate Salt) include the nickel alloys Alloy 625 (N06625), Alloy 120 (N08120), Alloy 230 (N02230), Alloy 242 (N10242), Alloy 214 (N07208) (Table 1) have been examined for their corrosion resistance in a molten salt made of 60% sodium nitrate salt and 40% potassium nitrate salt at 600 ° C. through which air flows. The analyzes of the alloys used are shown in Table 2. 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 sample before the test, after the test and after the oxide layer had been removed. This is used to determine the weight loss (loss of descaling) based on the surface of the sample before the test.

Table 3 shows the corrosion rate after 3000 hours: once as a descaling loss in mg / cm ² and once converted as a metal loss in pm / year. Alloy 214 has the lowest corrosion rate with 5.7 pm / year with an aluminum content of 4.5%, followed by Alloy 224 with a corrosion rate of 8.3 pm / year with an aluminum content of 3.8%. All other nickel alloys examined (Alloy 625, Alloy 120, Alloy 242 and Alloy 230) have a significantly higher corrosion rate of 16.8 pm / year and higher with aluminum contents of less than 0.5%. Alloy 214 and Alloy 224 form an aluminum oxide layer that provides good protection against molten nitrate salts. If the aluminum content is too low, as in the case of the alloys Alloy 625, Alloy 120, Alloy 242 and Alloy 230, no aluminum oxide layer can form, which leads to an increased corrosion rate. It is therefore advantageous that an alloy that is formed in nitrate salt melts is to be used, has an aluminum content that is large enough that a closed aluminum oxide layer is formed.

In addition to excellent corrosion resistance in molten nitrate salts, the alloy according to the invention also has the following properties:

• good phase stability

• good processability

• good corrosion resistance in air similar to that of Alloy 602CA (N06025)

• good heat resistance / creep resistance

• Phase stability

In the nickel-chromium-iron-aluminum system with additions of Ti and / or Nb, various embrittling TCP (topologically close packed) phases, such as B. Laves phases, sigma phases or m phases or also the embrittling h or e phase (see e.g. Ralf Bürgel, Handbook of High Temperature Materials Technology, 3rd edition, Vieweg Verlag, Wiesbaden, 2006, page 370 - 374). The calculation of the equilibrium phase proportions as a function of the temperature for z. B. N06690 for batch 111389 (see Table 4 for typical compositions) mathematically show the formation of α-chromium (BCC phase in Figure 1) below 720 ° C (T _S BCC) in significant proportions. The formation of this phase is very difficult because it is analytically very different from the basic material. If, however, the solvus temperature T _S BCC of this phase is very high, it can certainly occur, as is the case, for example, in E. Slevolden, JZ Albertsen. U. Fink, "Tjeldbergodden Methanol Plant: Metal Dusting Investigations," Corrosion / 2011, paper no. 11144 (Houston, TX: NACE 2011), p. 15 “for a variant of Alloy 693 (UNS 06693). Figure 2 and Figure 3 show the phase diagrams of the Alloy 693 variants (from US Pat. No. 4,88,125 Table 1) for Alloy 3 and Alloy 10 from Table 4. This phase is brittle and thus leads to undesired embrittlement of the material. Alloy 3 has a formation _{temperature T s} BCC of 1079 ° C, Alloy 10 of 939 ° C. In “E. Slevolden, JZ Albertsen. U. Fink, Tjeldbergodden Methanol Plant: Metal Dusting Investigations, “Corrosion / 2011, paper no. 11144 (Houston, TX: NACE 2011), p. 15 “the exact analysis of the alloy in which a-chromium (BCC phase) occurs is not described. It can be assumed, however, that among the examples given in Table 4 for Alloy 693, in the analyzes that mathematically have the highest solvus temperatures T _s BCC (such as, for example, Alloy 10), α-chromium can form. In a corrected analysis (with reduced solvus temperature T _{S BCC} ), "E. Slevolden, JZ Albertsen. U. Fink, Tjeldbergodden Methanol Plant: Metal Dusting Investigations, “Corrosion / 2011, paper no. 11144 (Houston, TX: NACE 2011), p. 15 “a-chromium then only detected near the surface. In order to avoid the occurrence of such an embrittling phase, the solvus temperature T _s BCC in the alloy according to the invention should be less than or equal to 939 ° C., which is the lowest solvus temperature T _s BCC among the examples for alloy 693 in Table 4 (from US Pat 1) corresponds.

This is especially the case when the following formula is met:

Fp <39.9 with (2a)

Mp = Cr + 0.272 ^* 2.36 ^* Fe + AI + 2.22 ^* 2.48 ^* Si + 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 concentration of the relevant elements in% by mass. Table 4 with the alloys according to the prior art shows that Fp for Alloy 8, Alloy 3 and Alloy 2 is greater than 39.9 and for Alloy 10 it is exactly 39.9. For all other alloys with T _{s BCC} less than 939 ° C, F _p <39.9.

- workability

Formability is considered here as an example for processability.

An alloy can be hardened by several mechanisms so that it has a high heat resistance or creep resistance. The addition of another element, depending on the element, has a greater or lesser effect great increase in strength (solid solution hardening). Much more effective is an increase in strength through fine particles or precipitates (particle hardening). This can e.g. B. be done by the y'-phase, which is when adding AI and other elements, such as. B. Ti to form a nickel alloy or by carbides that are formed by adding carbon to a chromium-containing nickel alloy (see e.g. Ralf Bürgel, Flandbuch der Flochtemperaturwerkstofftechnik, 3rd edition, Vieweg Verlag, Wiesbaden, 2006, page 358-369).

The increase in the content of y’-phase-forming elements or the C content increases the high-temperature strength, but increasingly affects the deformability, even in the solution-annealed state.

For a very easily deformable material, elongations As in tensile tests at room temperature of greater than or equal to 50%, but at least greater than or equal to 45% are aimed for.

This is achieved in particular when the following relationship is met between the carbide-forming elements Cr, Nb, Ti and C:

Fa <60 with (4a)

- Heat resistance / creep resistance

At the same time, the yield strength or tensile strength at higher temperatures should at least reach the values 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) It would be desirable for the yield strength or tensile strength to be in the range of the tensile strength of Alloy 602 CA (see Table 6). At least 3 of the 4 following relations should be fulfilled:

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 achieved in particular if the following relationship between the mainly hardening elements is met:

Fk> 47 with (6a)

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 concentration of the elements in question in% by mass.

- Corrosion resistance in air:

The alloy according to the invention is said to have good corrosion resistance in air, similar to that of Alloy 602 CA (N06025).

Examples:

- Manufacture:

Tables 5a and 5b show the analyzes of the batches melted on a laboratory scale together with some large-scale, state-of-the-art melted batches of Alloy 602CA (N06025), Alloy 690 (N06690), Alloy 601 (N06601) used for comparison. The batches according to the prior art are marked with a T, those according to the invention with an E. The batches melted on a laboratory scale are marked with an L, the large-scale batches with a G. The blocks of the alloys melted on a laboratory scale in a vacuum in Tables 5a and b were annealed between 900 ° C and 1270 ° C for 8 hours and by means of hot rolling and further intermediate anneals between 900 ° C and 1270 ° C for 0.1 to 1 hour Final thickness of 13 mm or 6 mm hot-rolled. The sheets produced in this way were solution annealed between 900 ° C. and 1270 ° C. for 1 hour. The samples required for the measurements were produced from these sheets.

In the case of large-scale melted alloys, a sample was taken from large-scale production from a sheet metal manufactured in-house with a suitable thickness. The samples required for the measurements were produced from these sheets.

All alloy variants typically had a grain size of 70 to 505 μm.

For the sample batches in Table 5a and b, the following properties are compared:

Corrosion resistance in molten nitrate salts. Phase stability

The formability based on the tensile test at room temperature The high-temperature strength / creep resistance using hot tensile tests The corrosion resistance in air using an oxidation test

- Corrosion resistance in nitrate melts:

For batches 2301 and 250129 to 250138 and 250147 to 250149 melted on a laboratory scale, as well as batches 250164, 250311 and 250526, aluminum is greater than or equal to 1.8%. This aluminum content is sufficiently high that a closed aluminum oxide layer can form under the chromium oxide layer. They thus meet the requirement that has been placed on the corrosion resistance in molten salts.

- phase stability: The phase diagrams were therefore calculated for the selected state-of-the-art alloys in Table 4 and all laboratory batches (Tables 5a and 5b) and the solvus temperature T _s BCC 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 is greater, the greater the solvus temperature T _s BCC. All examples of N06693 with a higher solvus temperature T _s BCC than that of Alloy 10 have an MP> 39.9. The requirement of Fp <39.9 (formula 2a) is therefore a good criterion for obtaining sufficient phase stability in an alloy. All laboratory batches in Tables 5a and b meet the criterion MP <39.9.

- Formability (processability):

In Table 6, the yield strength R _{Po, 2} , the tensile strength R _m and the elongation As for room temperature (RT) and for 600 ° C are entered, furthermore the tensile strength R _m for 800 ° C. The values for Fa and Fk are also entered.

The sample batches 156817 and 160483 of the alloy according to the state of the art Alloy 602 CA have in Table 6 a comparatively small elongation As at room temperature of 36 and 42%, respectively, which are below the requirements for good formability. Fa is greater than 60 and thus above the range that characterizes good formability. All alloys (E) according to the invention show an elongation of more than 50%. You thus meet the requirements. Fa is less than 60 for all alloys according to the invention. You are therefore in the range in which good formability is given. The elongation is particularly high when Fa is comparatively small.

- Heat resistance / creep resistance:

The sample batch 156656 of the alloy according to the prior art Alloy 601 in Table 6 is an example of the minimum requirements for yield strength and tensile strength at 600 ° C and 800 ° C, the sample batches 156817 and 160483 of the alloy according to the prior art Alloy 602 CA contrast examples of very good values for yield strength and tensile strength at 600 ° C and 800 ° C, respectively. Alloy 601 represents a material that shows the minimum requirements for high temperature strength or creep resistance, which are described in the relationships 8a to 8d. Alloy 602 CA represents a material that shows excellent heat resistance or creep resistance, which are described in the relationships 9a to 9d. The value for Fk is significantly greater than 47 for both alloys, and for Alloy 602 CA, it is also significantly greater than the value of Alloy 601, which reflects the increased strength values of Alloy 602 CA. The alloys (E) according to the invention all show a yield strength and tensile strength at 600 ° C. or 800 ° C. in the range or significantly above that of alloy 601 and thus meet the ratios 8a to 8d. They are in the range of the values of Alloy 602 CA and, apart from batch 250526 and batch 250311, also meet the desirable requirements, i.e. 3 of the 4 relations 9a to 9d. For all alloys according to the invention in the examples in Table 6, Fk is also greater than 47 or greater than 54 and thus in the range that is characterized by good heat resistance or creep resistance. Among the laboratory batches not according to the invention, batches 2297 and 2300 are an example that relations 8a to 8d are not fulfilled and for which Fk is also less 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. a total of 1056 hours. Table 7 shows the specific gross mass change, the specific net mass change and the specific mass change of the flaked oxide after 1056 hours. The sample batches of the state-of-the-art alloys Alloy 601 and Alloy 690 show a significantly higher gross mass change than Alloy 602 CA, whereby that of Alloy 601 is again significantly larger than that of Alloy 690. Both form a chromium oxide layer that grows faster as an aluminum oxide layer. Alloy 601 still contains approx. 1.3% Al. This content is too low to form an even partially closed aluminum oxide layer, which is why the aluminum inside the metallic material oxidizes below the oxide layer (internal oxidation). This causes an increased increase in mass compared to Alloy 690. Alloy 602CA has approximately 2.3% aluminum. With this alloy, a closed aluminum oxide layer can thus form below the chromium oxide layer. This noticeably reduces the growth of the oxide layer and thus also the specific increase in mass. All alloys (E) according to the invention contain at least 2% aluminum and thus have a similarly low or lower gross increase in mass as Alloy 602 CA. Similarly to the sample batches of Alloy 602 CA, all the alloys according to the invention also show flaking in the area of measurement accuracy, while Alloy 601 and Alloy 690 show large flaking.

The claimed limits for the alloy "E" according to the invention can therefore be justified in detail as follows:

Too low a chromium content means that the chromium concentration drops very quickly below the critical limit when the alloy is used in a corrosive atmosphere, so that a closed chromium oxide layer can no longer form. Therefore a content of> 17% chromium is the lower limit. Too high a chromium content worsens the phase stability of the alloy, especially with the high aluminum content of> 1.8%. Therefore 33% chromium is to be regarded as the upper limit.

The formation of an aluminum oxide layer beneath the chromium oxide layer reduces the rate of oxidation. Below 1.8% aluminum, the aluminum oxide layer is too patchy to fully develop its effect. Too high an aluminum content affects the workability of the alloy. Therefore, an aluminum content of <4.0% is the upper limit.

The cost of the alloy increases as the iron content is reduced. Below 0.1%, the costs rise disproportionately because special starting material has to be used. That is why 0.1% is iron for cost reasons to be regarded as the lower limit. As the iron content increases, the phase stability decreases (formation of embrittling phases), especially with high chromium and aluminum contents. Therefore, 15% iron is a sensible upper limit in order to ensure the phase stability of the alloy according to the invention.

Silicon is required in 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 with high aluminum and chromium contents. The silicon content is therefore limited to 0.50%.

A minimum manganese content of 0.001% is necessary to improve processability. Manganese is limited to 2.0% as this element reduces oxidation resistance.

Titanium increases the high temperature strength. From 0.60% the oxidation behavior can be worsened, which is why 0.60% is the maximum value.

Even very low magnesium and / or calcium contents improve processing by setting sulfur, which prevents the occurrence of low-melting nickel-sulfur eutectics. A minimum content of 0.0002% is therefore required for magnesium and / or calcium. If the contents are too high, intermetallic nickel-magnesium phases or nickel-calcium phases can occur, which again significantly worsen the processability. The magnesium and / or calcium content is therefore limited to a maximum of 0.05%.

A minimum carbon content of 0.005% is necessary for good creep resistance. Carbon is limited to a maximum of 0.12%, since above this level this element reduces the workability due to the 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%, as the formation of coarse carbonitrides reduces the processability.

The oxygen content must be less than or equal to 0.020% in order to enable the alloy to be manufactured. Too low an oxygen content increases the costs. The oxygen content is therefore> 0.0001%.

The content of phosphorus should be less than or equal to 0.030%, since this surface-active element affects the oxidation resistance. Too low a phosphorus content increases the costs. The phosphorus content is therefore> 0.001%.

The sulfur content should be set as low as possible, since this surface-active element impairs the resistance to oxidation. A maximum of 0.010% sulfur is therefore specified.

Molybdenum is limited to a maximum of 2.0%, as this element is the

Oxidation resistance reduced.

Tungsten is limited to a maximum of 2.0%, as this element is the

Oxidation resistance also reduced.

Nickel is the remainder. Too low a nickel content reduces the phase stability, especially with high chromium contents. Nickel must therefore be greater than or equal to 50%.

In addition, the following relationship must be fulfilled so that there is sufficient phase stability:

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) where Cr, Fe, Al, Si, Ti, Mo, W and C are the concentration of the elements in question in% by mass. The limits for Fp and the possible inclusion of further elements have been explained in detail in the previous text.

If necessary, the oxidation resistance can be further improved by adding elements with an affinity for oxygen, such as yttrium, lanthanum, cerium, cerium, mischmetal. These elements are built into the oxide layer and block the diffusion paths of oxygen on the grain boundaries.

A minimum yttrium content of 0.001% is necessary in order to maintain the yttrium's oxidation resistance increasing effect. The upper limit is set at 0.20% for cost reasons.

A minimum content of 0.001% lanthanum is necessary in order to maintain the oxidation resistance-increasing effect of the lanthanum. The upper limit is set at 0.20% for cost reasons.

A minimum cerium content of 0.001% is necessary to achieve the

Oxidation resistance-increasing effect of cerium. The upper limit is set at 0.20% for cost reasons.

A minimum content of 0.001% cerium mischmetal is necessary in order to maintain the oxidation resistance-increasing effect of the cerium mischmetal. The upper limit is set at 0.20% for cost reasons.

If necessary, niobium can be added, since niobium also increases the high temperature strength. Higher contents increase the costs very much. The upper limit is therefore set at 1, 10%. If necessary, the alloy can also contain tantalum, since tantalum also increases the high temperature strength and the oxidation resistance. Higher contents increase the costs very much. The upper limit is therefore set at 0.60%. A minimum content of 0.001% is required to have an effect.

If necessary, the alloy can also contain zirconium. A minimum content of 0.001% zirconium is necessary in order to maintain the effect of the zirconium, which increases the high temperature strength and the oxidation resistance. For cost reasons, the upper limit is set at 0.20% zirconium.

If necessary, the alloy can also contain hafnium. A minimum content of 0.001% hafnium is necessary in order to maintain the effect of the hafnium, which increases the resistance to high temperatures and resistance to oxidation. For cost reasons, the upper limit is set at 0.20% hafnium.

If necessary, boron can be added to the alloy because boron improves creep resistance. Therefore a content of at least 0.0001% should be present. At the same time, this surface-active element worsens the resistance to oxidation. A maximum of 0.008% boron is therefore specified.

Cobalt can be contained in this alloy up to 5.0%. Higher contents noticeably reduce the resistance to oxidation.

Copper is limited to a maximum of 0.5%, as this element reduces the resistance to oxidation.

Vanadium is limited to a maximum of 0.5%, as this element is the

Oxidation resistance reduced.

Lead is limited to a maximum of 0.002%, as this element reduces the resistance to oxidation. The same goes for zinc and tin. Furthermore, the following relationship of the carbide-forming elements chromium, titanium and carbon can optionally be fulfilled, which describes particularly good processability:

Fa <60 with (4a)

Fa = Cr + 20.4 ^* Ti + 201 ^* C (5a) where Cr, Ti and C are the concentration of the relevant elements in% by mass. The limits for Fa and the possible inclusion of further elements have been explained in detail in the previous text.

Furthermore, the following relationship can optionally be fulfilled between the elements increasing the strength, which describes a particularly good heat resistance or creep resistance:

Fk> 47 with (6a)

Fk = Cr + 19 ^* Ti + 10.2 ^* Al + 12.5 ^* Si + 98 ^* C (7a) where Cr, Ti, Al, Si and C are the concentration of the relevant elements in% by mass. The limits for Fa and the possible inclusion of further elements have been explained in detail in the previous text.

belle 1: Alloys according to ASTM B 168-19 ¹⁾ , ASTM B167-18 ²⁾ ASTM B443-18 ³⁾ ASTM B 163-18 ⁴⁾ , ASTM B622-15 ⁵⁾ TM B409-06 ⁶⁾ . All data in% by mass, ⁷⁾ in any ASTM standard, from the UNS list.

Table 2: Composition of the alloys examined in (Kruizenga et al., Materials Corrosion of High Temperature Alloys Immersed in 600 ° C Binary Nitrate Salt, Sandia Report, SAND 2013-2526, 2013) in% by mass.

Table 3: Corrosion after 3000 hours in 60% sodium nitrate / 40% potassium nitrate salt melt of the alloys investigated in [Kruizenga et al., 2013, Materials Corrosion of High Temperature Alloys Immersed in 600 ° C Binary Nitrate Salt].

Table 4: Typical compositions of some alloys according to ASTM B 168-11, and Table 2 (prior art). All data in% by mass ^* ) Alloy composition from patent US 4,88,125 Table 1

Table 5a: Composition of the laboratory batches, part 1. All data in% by mass (T: alloy according to the state of the art, E: alloy according to the invention, L: melted on a laboratory scale, G: melted on an industrial scale)

Table 5b: Composition of the laboratory batches, part 2. All data in% by mass (applies to all alloys: Pb: max. 0.002%, Zn: max. 0.002%, Sn: max. 0.002%) (meaning of T, E, G, L, see table 5a)

Table 6: Results of the tensile tests at room temperature (RT), 600 ° C and 800 ° C. The deformation rate was 8.33 10 ^-5 1 / s (0.5% / min) _{for R Po, 2} and 8.33 10 ^-4 1 / s (5% / min) _{for R m;} KG = grain size.

Table 7: Results of the oxidation tests at 1000 ° C. in air after 1056 hours.

Claims

1.Nickel-chromium-iron-aluminum alloy with (in% by 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, each 0.0002 to 0.05% magnesium and / or calcium, 0.005 to 0.12% carbon, 0.001 to 0.050% nitrogen , 0.0001 - 0.020% oxygen, 0.001 to 0.030% phosphorus, max. 0.010% sulfur, max. 2.0% molybdenum, max. 2.0% tungsten, the remainder nickel with nickel> 50% and the usual process-related impurities for Use in solar tower power plants using molten nitrate salts as a heat transfer medium, whereby the following relationship must be met:

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) where Cr, Fe, Al, Si , Ti, Mo, W and C are the concentrations of the respective elements in% by mass.

2. Alloy according to claim 1, in particular for all components which are in contact with the molten salt, are used.

3. Alloy according to claim 1 or 2, wherein the alloy can be used up to a maximum temperature of 800 ° C.

4. Alloy according to one of claims 1 to 3, with a chromium content of> 18 to 33%.

5. Alloy according to one of claims 1 to 4, with an aluminum content of 1.8 to 3.8%.

6. Alloy according to one of claims 1 to 5, with an iron content of 0.1 to 12.0%.

7. Alloy according to one of claims 1 to 6, with a silicon content of 0.001 - <0.40%

8. Alloy according to one of claims 1 to 7 with a manganese content of 0.001 to 0.50%.

9. Alloy according to one of claims 1 to 8 with a titanium content of 0.001 to 0.50%.

10. Alloy according to one of claims 1 to 9, with a carbon content of 0.01 to 0.10%.

11. Alloy according to one of claims 1 to 10 optionally with one

Yttrium content from 0.001 to 0.20%.

12. Alloy according to one of claims 1 to 11 optionally with one

Lanthanum content from 0.001 to 0.20%.

13. Alloy according to one of claims 1 to 12, optionally with a cerium content of 0.001 to 0.20%.

14. Alloy according to one of claims 1 to 13, optionally with a cerium mischmetal content of 0.001 to 0.20%.

15. Alloy according to one of claims 1 to 14 optionally with a niobium content of 0.0 to 1.1%, the formula (3a) being supplemented by a term with Nb:

Mp = Cr + 0.272 ^* 2.36 ^* Fe + AI + 2.22 ^* 2.48 ^* Si + Ti + 1, 26 + Nb ^* 0.374 ^* 0.538 ^* Mo + W - 11 8 ^* C (3b) and Cr , Fe, Al, Si, Ti, Nb, Mo, W and C are the concentration of the element in question in% by mass.

16. Alloy according to one of claims 1 to 15, optionally with a zirconium content of 0.001 to 0.20%

17. Alloy according to one of claims 1 to 16, optionally with a hafnium content of 0.001 to 0.20%.

18. An alloy according to any one of claims 1 to 17 optionally with a boron content of 0.0001 to 0.008%.

19. The alloy of any one of claims 1 to 18, optionally further comprising 0.0 to 5.0% cobalt.

20. Alloy according to one of claims 1 to 19, furthermore containing a maximum of 0.5% copper, the formula (3a) being supplemented by a term with Cu:

Mp = 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). and Cr, Fe, Al, Si, Ti, Cu, Mo, W and C are the concentration of the element in question in% by mass.

21. Alloy according to one of claims 1 to 20, further containing a maximum of 0.5% vanadium.

22. Alloy according to one of Claims 1 to 21, in which the impurities are set in contents of a maximum of 0.002% lead, a maximum of 0.002% zinc, and a maximum of 0.002% tin.

23. Alloy according to one of claims 1 to 22, in which the following formula is fulfilled and particularly good processing is achieved: Fa <60 (4a) with Fa = Cr + 20.4 ^* Ti + 201 ^* C (5a) for an alloy without Nb, where Cr, Ti and C are the concentration of the elements in question in% by mass, or with Fa = Cr + 6.15 ^* Nb + 20.4 ^* Ti + 201 ^* C (5b) for an alloy with Nb, where Cr, Nb, Ti and C is the concentration of the respective elements in% by mass.

24. Alloy according to one of claims 1 to 23, in which the following formula is fulfilled and a particularly good heat resistance / creep resistance is achieved: Fk> 47 (6a) with Fk = Cr + 19 ^* Ti + 10.2 ^* Al + 12.5 ^* Si + 98 ^* C (7a) for an alloy without B and Nb, where Cr, Ti, Al, Si and C are the concentration of the relevant elements in% by mass, or with Fk = Cr + 19 ^* Ti + 34.3 ^* Nb + 10.2 ^* Al + 12.5 ^* Si + 98 ^* C + 2245 ^* B (7b) for an alloy with B and / or Nb, where Cr, Ti, Nb, Al, Si , C and B are the concentration of the respective elements in% by mass.

25. Use of the alloy according to one of claims 1 to 24 as a strip, sheet metal, wire, rod, longitudinally welded pipe and seamless pipe.

26. Use of the alloy according to one of claims 1 to 24 for the production of strip, sheet metal, wire, rod, longitudinally welded pipe and seamless pipes.