EP4069873A1

EP4069873A1 - Nickel-chromium-aluminum alloy with good processability, creep resistance, and corrosion resistance, and use thereof

Info

Publication number: EP4069873A1
Application number: EP20839221.7A
Authority: EP
Inventors: Heike Hattendorf; Benedikt NOWAK
Original assignee: VDM Metals International GmbH
Current assignee: VDM Metals International GmbH
Priority date: 2019-12-06
Filing date: 2020-12-04
Publication date: 2022-10-12
Also published as: CN114787402B; US20230160040A2; JP7479472B2; DE102020132219A1; KR20220099565A; JP2023504562A; WO2021110218A1; US20230020446A1; CN114787402A

Abstract

The invention relates to a nickel-chromium-aluminum alloy comprising (in mass %) 12 to 30% chromium, 1.8 to 4.0% aluminum, 0.1 to 7.0% iron, 0.001 to 0.50% silicon, 0.001 to 2.0% manganese, 0.00 to 1.00% titanium, 0.00 to 1.10% niobium, 0.00 to 0.5% copper, 0.00 to 5.00% cobalt, in each case 0.0002 to 0.05% magnesium and/or calcium, 0.001 to 0.12% carbon, 0.001 to 0.050% nitrogen, 0.001 to 0.030% phosphorus, 0.0001 to 0.020% oxygen, max. 0.010% sulfur, max. 2.0% molybdenum, max. 2.0% tungsten, and a remainder of nickel with a minimum content of ≥ 50% and the usual process-related impurities for use in solar power towers, using chloride and/or carbonate salt melts as a heat transfer medium, wherein in order to ensure a good processability, the following condition must be met: Fv ≥ 0.9 mit Fv = 4.88050 - 0.095546*Fe - 0.0178784*Cr - 0.992452*AI - 1.51498*Ti - 0.506893*Nb + 0.0426004*AI*Fe, where Fe, Cr, AI, Ti, and Nb are the concentration of the respective elements in mass %.

Description

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

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

Due to their properties, nickel alloys with different nickel, chromium and aluminum contents have not only been used for a long time in the chemical and petrochemical industries as well as in furnace construction, but 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 and a heat exchanger system converts the heat 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 600 ° C, depending on the alloy used for the components [Kruizenga et al., 2013, Materials Corrosion of High Temperature Alloys Immersed in 600 ° C Binary Nitrate Salt, SANDIA report, SAND 2013-2526].

In order to increase the efficiency of the solar tower power plants, the maximum operating temperature of the salts must be increased. This is possible through the use of, for example, chloride and / or carbonate salts as a heat transfer medium [Mehos et al., 2017, Concentrating Solar Power Gen3 Demonstration Roadmap, National Renewable Energy Lab. (NREL)]. These do not decompose at temperatures above about 600 ° C like the sodium and potassium nitrate salts, but have a maximum operating temperature of up to 1000 ° C. The disadvantage of using these salt systems is the stronger corrosive attack at higher temperatures compared to the sodium and potassium nitrate salts, especially in the area of the solar receiver or the heat absorption pipes, which transport the salt and represent the hottest point in the salt cycle. As a result, materials that are usually used for nitrate salts, such as Alloy 800H (N08810) or Alloy 625 (N06625), cannot be used, as they are insufficient to withstand the corrosion attack by chloride and / or carbonate salts at temperatures> 600 ° C would withstand a long time [Shankar et al. , 2013, Corrosion of Ni-containing Alloys in Molten LiCI-KCI Medium, Corrosion, 69 (1), 48-57]. A minimum lifespan of 30 years for the absorber system is assumed [Gomez-Vidal et al., 2017, Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests, Solar Energy Materials and Solar Cells, 166, 222-233].

An essential prerequisite for using an alloy in the environment described above is good high-temperature corrosion resistance in chloride and / or carbonate molten salts. This is determined not only by a continuous chromium oxide layer (Cr ₂ O ₃ ), but in particular by an underlying as closed as possible, ie continuous and gapless aluminum oxide layer (Al ₂ O ₃ ). Under certain conditions, the chromium ions preferentially dissolve in the molten salt. A certain aluminum content in the alloy can slow down this process through the formation of a closed Al ₂ O ₃ layer [George Y. Lai, 1990, High-Temperature Corrosion of Engineering Alloys, ASM International, p. 100, Fig. 6.11]. In contrast, the chromium content should not be too low, since the absorber pipes are exposed to the salt on the inside but to the ambient air on the outside. This means that a certain chromium content is necessary to ensure oxidation resistance at the elevated temperatures of up to 1000 ° C in ambient air. 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 γ ′ and / or γ ”phase.

Another important parameter with regard to the fatigue load of the material due to the daily sun and cloud cycles calculated over the entire service life of the solar receiver is the alternating strength.

The compositions of the alloys Alloy 800H, Alloy 600, Alloy 601, Alloy 690, Alloy 602CA, Alloy 603, Alloy 214, Alloy 625 and Alloy 702 according to the prior art are listed in Table 1.

Alloy 602CA (N06025) and Alloy 603 (N06603) have excellent heat resistance and creep resistance even at temperatures above 1000 ° C. However, the processability is impaired, among other things, by the high aluminum content, the impairment being the greater the higher the aluminum content (such as in Alloy 214 (UNS N07214)). In Alloy 602CA (N06025) or Alloy 603 (N06603), in particular, the cold formability is reduced due to a high content of primary carbides compared to alloys such as Alloy 601 (N06601). This means that the aluminum content must be high enough so that a closed Al ₂ O ₃ layer is formed, but it must not be too high either, since otherwise the processability is adversely affected. This requirement also applies to chrome. This makes it clear that an exact coordination of the aluminum or chromium content is of decisive importance for the corrosion resistance in chloride and / or carbonate salt melts at elevated temperatures.

WO 2019/075177 A1 discloses an improved solar tower system that has absorber tubes, a storage tank and a heat exchanger, all of which are as Heat transfer medium contain a molten salt at temperatures> 650 ° C, whereby the improvement includes that at least one of the components (absorber tubes, storage tank and heat exchanger) is made of an alloy that (in mass%) 25 - 45% Ni, 12 - 32% Cr, 0.1 - 2.0% Nb, up to 4% Ta, up to 1% Va, up to 2% Mn, up to 1, 0% AI, up to 5% Mo, up to 5% W, up to 0.2% Ti, up to 2% Zr, up to 5% Co, up to 0.1% Y, up to 0.1% La, up to 0.1% Cs, up to 0.1% other rare earths, up to 0.20% C, up to 3% Si, N 0.05 - 0.50%, up to 0.02% B and the remainder Fe and impurities.

EP 549286 discloses a Ni-Cr-Al alloy containing 0.005-0.5% C, 0.0001-0.1% N, 19-25% Cr, 55-65% Ni, 0-1% Mn, 0 , 1 - 1, 5% Si, 0.15 - 1% Ti, 1 - 4.5% Al, 0 - 0.5% Zr, 0 - 10% Co, 0.045 - 0.3% Y, 0.0001 - 0.1 % B and the remainder of Fe and impurities, among other things in the field of thermal

Process application, in the chemical and petrochemical industry, as well as for components in turbine engines.

WO 00/34540 discloses a high-temperature alloy containing 27-35% Cr, 0-7% Fe, 3-4.4% Al, 0-0.4% Ti, 0.2-3% Nb, 0.12 - 0.5% C, 0 - 0.05% Zr, 0.002 - 0.05% Ce and Y, as well as 0 - 1% Mn, 0 - 1% Si, 0 - 0.5% Ca + Mg, 0 - 0.1% B and the remainder of Ni and impurities, which is used, among other things, in the field of thermal process applications such as furnace construction, e.g. as a material for the production of furnace muffles and similar components for

High temperature applications.

The object on which the invention is based is to produce a nickel-chromium-aluminum alloy

• good phase stability,

• good processability,

• and good corrosion resistance in air, similar to that of Alloy 602CA (N06025) has to be supplied to another application.

Furthermore it is necessary that this alloy additionally

• good heat resistance,

• good creep resistance,

• and has good fatigue strength.

This problem is solved by a nickel-chromium-aluminum alloy, with (in mass%) 12 to 30% chromium, 1.8 to 4.0% aluminum, 0.1 to 7.0% iron, 0.001 to 0, 50% silicon, 0.001 to 2.0% manganese, 0.00 to 1.00% titanium, 0.00 to 1.10% niobium, 0.00 to 0.5% copper, 0.00 to 5.00% Cobalt, each 0.0002 to 0.05% magnesium and / or calcium, 0.001 to 0.12% carbon, 0.001 to 0.050% nitrogen, 0.001 to 0.030% phosphorus, 0.0001 to 0.020% oxygen, max. 0.010% sulfur , max. 2.0% molybdenum, max. 2.0% tungsten, the remainder nickel with a minimum content of ≥ 50% and the usual process-related impurities for use in solar tower power plants using chloride and / or carbonate molten salt as a heat transfer medium, whereby the following condition must be fulfilled to ensure good processability: F _v ≥ 0.9 with F _v = 4.88050 - 0.095546 * Fe - 0.0178784 * Cr - 0.992452 * AI - 1.51498 * Ti - 0.506893 * Nb + 0.0426004 * AI * Fe where Fe, Cr, Al, Ti and Nb are the concentration of the elements in question 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 12 and 30%, whereby preferred ranges can be set as follows:

14 to 30%

14 to 28%

14 to <27%

16 to 24%

18 to <24%

> 18 to 23%

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

2.0 to 4.0%

> 2.0 to <4.0%

2.2 to 4.0%

2.5 to 4.0%

2.5 to 3.5%

2.5 to 3.2%

2.7 to 3.7%

> 3.0 to 4.0%

> 3.2 - <4.0%

> 3.2 -3.8%

> 3.0 - <3.5%

2.0 to 3.0%

The iron content is between 0.1 and 7.0%, whereby, depending on the area of application, preferred contents can be set within the following spread ranges:

0.1 -4.0%

0.1 -3.0% 0.1 - <2.5%

0.1 -2.0%

0.1 - <2.0%

0.1 - <1.0%

The silicon content is between 0.001 and 0.50%. Preferably, Si can be adjusted in the alloy within the spread range as follows:

0.001 -0.40%

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: 0.001-1.0%

0.001 -0.50%

0.001 - <0.40%

0.001 - <0.30%

0.001 -0.20%

0.001 -0.10%

0.001 - <0.05%

The titanium content is between 0.00 and 1.0%. Preferably, Ti can be set in the alloy within the spread range as follows:

0.001 -0.60%,

0.001 - <0.50%

0.001 - <0.40%

0.001 - <0.20% 0.001 - <0.15%

0.001 - <0.10%

0.001 - <0.05%

0.00 - <0.02%

The Nb content is between 0.00 and 1.1%. Preferably, Nb can be adjusted in the alloy within the spread range as follows:

0.001 -0.7%

0.001 - <0.50%

0.001 -0.30%

0.001 - <0.20%

0.00 - <0.01%

0.01 - 0.30%

0.10-1.10%

0.20-0.70%

The alloy contains 0.00 to 0.50% copper. This can preferably be set in the alloy within the spread range as follows: a maximum of 0.20% a maximum of 0.10%

0.00-0.05%

0.00 - <0.05%

0.00-0.015%

0.00 - <0.015%

Furthermore, the alloy can contain between 0.0 and 5.00% cobalt. The cobalt content can also be restricted as follows:

0.001 to <5.0%

0.001 to 2.0%

0.001 to 1.0%

0.001 to <1.0% 0.001 to 0.50%

0.001 to <0.50%

0.001 to 0.10%

0.001 to <0.10%

0.01 to 2.0%

0.1 to 2.0%

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 0.001 to 0.12% carbon. This can preferably be set in the alloy within the spread range as follows: 0.001 - 0.10%

0.001 - <0.08%

0.001 - <0.05%

0.005 - <0.05%

0.01 - 0.03%

0.01 - <0.03%

0.02 - 0.10%

0.03 - 0.10%

This applies in the same way 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: 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:

Mo max. 1.0%

Mo max. <0.50%

Mo max. <0.10%

Mo max. <0.05%

Mo max. <0.01%

W max. 1.0%

W max. <0.50%

W max. <0.10%

W max. <0.05%

W max. <0.01%

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

≥ 55% or> 55%

≥ 60% or> 60%

≥ 65% or> 65%

≥ 70% or> 70%

≥ 75% or> 75%

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 -0.15%

Optionally, the element lanthanum can be set in the alloy in contents of 0.001 to 0.20%. Lanthanum can preferably be used within the

Spread range can be set in the alloy 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, cerium 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 can also be used in contents of 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%

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 hafnium content is between 0.001 and 0.20%. Hafnium 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.

Tantalum can preferably 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%

0.00 - <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 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:

Lead max. 0.002%

Zinc max. 0.002%

Tin max. 0.002%

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. But melting and pouring in a vacuum is also possible. After casting in blocks or as a continuous casting, the alloy is annealed in the desired semi-finished 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 the hot forming, cold forming with degrees of deformation 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 e.g. B. argon or hydrogen, followed by cooling in air, in the agitated annealing atmosphere or in a water bath. Thereafter, a solution heat treatment takes place in the temperature range from 700 ° C to 1250 ° C for 0.1 minutes to 70 hours, possibly 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.

The alloy according to the invention should preferably be used in solar tower power plants using chloride and / or carbonate molten salts 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 chloride and / or carbonate salt melts can preferably consist of the following mixtures:

MgCl ₂ -KCl

ZnCI ₂ -NaCI-KCI

NaCl-LiCl

KCI-NaCI KCI-LiCI

LiCI-KCI-CsCI

Na ₂ CO ₃ -K ₂ CO ₃ -Li ₂ CO ₃

Li ₂ CO ₃ -Na ₂ CO ₃

NaCl-Na ₂ CO ₃

The mixture can preferably consist of the following compositions: 60-75% zinc chloride, 2-15% sodium chloride and 15-30% potassium chloride 30-50% magnesium chloride and 50-70% potassium chloride 25-40% sodium carbonate, 25-40% potassium carbonate and 25 - 40% lithium carbonate

If necessary, the salt mixtures can also be used under a pure CO ₂ atmosphere.

The minimum application temperature of the salt mixture is 250 ° C. It can be restricted as follows: min. 290 ° C min. 300 ° C min. 350 ° C min. 400 ° C min. 450 ° C min. 500 ° C min. 550 ° C min. 600 ° C

> 600 ° C min. 650 ° C

> 650 ° C min. 680 ° C

> 680 ° C min. 700 ° C min. 750 ° C The maximum operating temperature of the salt mixture is 1000 ° C. It can be restricted as follows: max. 950 ° C max. 900 ° C max. 850 ° C max. 830 ° C <830 ° C max. 800 ° C

Evaluation of the properties:

Corrosion resistance

In the case of an alloy that contains aluminum, it is of decisive importance for the corrosion resistance in chloride and / or carbonate-containing melts at elevated temperatures whether a gapless, closed aluminum oxide layer is formed.

For this, minimum contents of the elements for oxide layer formation, here chromium and aluminum, are necessary [David Young, 2008, High Temp oxidation and corrosion of metals, Elsevier, pp. 334-341]. The minimum content of aluminum required for formation can be significantly reduced by adding chromium, which is also known as the “third element effect”. Giggins and Pettit have shown that at 1200 ° C a content between 3.5-11.0% Al and 0-40% Cr is necessary so that an external Al ₂ O ₃ layer forms and it does not lead to internal oxidation Aluminum comes [Giggins and Pettit, 1971, Oxidation of Ni - Cr - Al Alloys Between 1000 ° and 1200 ° C, J. Electrochem. Soc., 118 (11), 1782-1790]

However, the formation of a closed Al ₂ O ₃ layer for alloys with an Al content of <3%, such as Alloy 602CA, is possible at lower temperatures under certain atmospheres [Schiek et al., 2015, Scale Formation of Alloy 602 CA. Düring isothermal oxidation at 800-1100 ° C in different Types of Water Vapor Containing Atmospheres, Oxidation of Metals, 84 (5-6), 661- 694].

By adding reactive elements such as yttrium or cerium in small amounts, the oxidation resistance can be further improved.

A gap in the Al ₂ O ₃ layer offers a preferred point of attack for corrosion in environments containing chloride and / or carbonate salt. Chlorine and chloride ions lead to accelerated corrosion. The mechanism is described in detail by Grabke et al. [Grabke et al., 1995, Effects of chlorides, hydrogen chlorides, and sulfur dioxide in the oxidation of steels below deposits, Corrosion Science, 37 (7), 1023-1043]. According to [Kruizenga, 2012, Corrosion Mechanisms in Chloride and Carbonate Salts, SANDIA report, SAND 2012-7594], the chromium content should not be too high, since Cr ions preferentially migrate as a solution into the molten salt under certain conditions. A sufficiently high aluminum content can slow down this process by forming a closed Al ₂ O ₃ layer [George Y. Lai, 1990, High-Temperature Corrosion of Engineering Alloys, ASM International, p. 100, FIG. 6.11].

The chromium content should also not be too low, since the absorber pipes are exposed to the salt on the inside, but the ambient air on the outside. 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, as a higher content of the element chromium, which forms the protective layer, delays the point in time when the chromium content is below the critical limit and oxides other than Cr ₂ O ₃ are formed, which e.g. and / or oxides containing nickel. These oxides have a significantly lower resistance to oxidation than a protective passive layer made of Cr ₂ O ₃ . A sufficiently high chromium content is therefore necessary to ensure oxidation resistance at the elevated temperatures of up to 1000 ° C in ambient air. In some studies, alloys with increasing nickel content show improved corrosion resistance in chlorine-containing atmospheres, whereby the addition of molybdenum can lead to catastrophic corrosion [Indacochea et al., 2001, High-Temperature Oxidation and Corrosion of Structural Materials in Molten Chlorides , Oxidation of Metals, 55 (1), 1-16]. In other works it could be proven that a chromium content of <16% leads to extremely high corrosion rates in sodium / potassium / zinc-chlorine salts at 250 ° C and 500 ° C, since the alloy Hastelloy N with only 7% chromium clearly has a has a higher corrosion rate than C-276 and C-22 with> = 16% chromium [Vignarooban et al., 2014, Corrosion resistance of Hasteiloys in molten metal-chloride heat-transfer fluids for concentrating solar power applications, Solar Energy, 103, 62-69]. In contrast to the work by Indacochea et al. led a molybdenum content of 16% and 13% in the alloys C-276 and C-22 in the work of Vignarooban et al. not too catastrophic corrosion, which can possibly be attributed to the examined temperature range. This was in Indacochea et al. at 725 ° C or 650 ° C, it is significantly higher than that of Vignarooban et al. with 500 ° C or 250 ° C. The catastrophic corrosion in the work of Indacochea et al. at higher temperatures could thus be _{explained by the formation of oxychlorides MoO x} Cl _y , the formation of which is extremely temperature-dependent [Klöwer et al., 1999, Chloride-induced oxidation of nickel-base alloys 600H, 625 and 626 Si, Proc. Conf. Eurocorr]. This also applies to tungsten, but not to such an extreme extent.

In further work it could be shown that a reduced iron content in the alloy Alloy 625 compared to Alloy 800H leads to a lower corrosion rate in sodium / lithium / chlorine salts at 650 ° C or 700 ° C [Gomez et al., 2016 , Corrosion of alloys in a Chloride molten salt (NaCI-LiCI) for solar thermal technologies, Solar Energy Materials and Solar Cells, 157, 234-244].

Shankar et al. have investigated Alloy 600, Alloy 625, Alloy 690 and Alloy 800H and found that an increased nickel and reduced iron content is advantageous in a lithium / potassium-chlorine atmosphere at 400, 500 and 600 ° C [Shankar et al., 2013, Corrosion of Ni-containing Alloys in Molten LiCI-KCI Medium, Corrosion, 69 (1), 48-57]. In addition, Alloy 625 showed a higher corrosion rate compared to Alloy 600 and Alloy 690, which could be explained by the significant molybdenum content (11% by mass in Alloy 625), since there is no molybdenum in Alloy 600 and Alloy 690.

These results with regard to an increased nickel and reduced iron content could be found in the work of Gomez-Vidal et al. are also observed [Gomez-Vidal et al., 2017, Corrosion resistance of alumina-forming alloys against molten chlorides for energy production: Pre-oxidation treatment and isothermal corrosion tests, Solar Energy Materials and Solar Cells, 166, 222-233]. Surprisingly, it was also found that an aluminum content of> 2% and <4% in Alloy 702 increases the corrosion resistance through the formation of an Al ₂ O ₃ layer.

In summary, it can be said that an increased nickel and reduced chromium content are very advantageous with regard to corrosion from chlorine and / or carbonate salt melts, as is an increased aluminum content. The addition of iron, molybdenum and tungsten should be avoided.

Processability

To coordinate or optimize the chromium, iron and aluminum contents and to investigate the influence of the addition of other alloying elements such as titanium and niobium, thermodynamic calculations were carried out with the JMatPro version 11 simulation program from Thermotech. With the help of the program, time-temperature-conversion (TTT) diagrams were calculated for various alloy contents (see Figure 1). This diagram shows the time after which a certain content (here 0.1%) of a phase is eliminated at a certain temperature. This is calculated for each phase without taking into account the consumption of alloying elements by other phases. In the ZTU diagram, each phase has a minimum time until this phase is eliminated. This minimum time t _γ'1 became for the γ'-phase (GAMMA_PRIME in Figure 1) intended for different alloy compositions. The γ'-phase causes a strong increase in solidification and hardness with a simultaneous reduction in elongation. If it forms too quickly in too large quantities, this can lead to cracks during processing (e.g. hot forming, welding). In the following, therefore, the minimum time t _{γ'1 is} taken as the parameter for the processability. For the alloy in Figure 1 it is t _γ'1 = 107 min.

In order to ensure good processability, the time t _{γ'1 should be} greater than or equal to 8 minutes. This is particularly the case when the following condition is met: F _v ≥ 0.9 with (1) F _v =

4.88050 - 0.095546 ^* Fe - 0.0178784 ^* Cr - 0.992452 ^* AI - 1.51498 ^* Ti - 0.506893 ^* Nb + 0.0426004 ^* AI ^* Fe

(2) where Fe, Cr, Al, Ti and Nb are the concentration of the respective elements in mass%. Formula (2) is only valid for compositions with a content of Cr <31%, AI <5%, Fe <18%, Ti <1, 5% and Nb <4%.

Table 2 shows the composition, the time t _γ'1 and F _v of alloys according to the prior art (T) together with the alloy compositions according to the invention (E) and those not according to the invention.

_{In Table 2a, the time t γ'1 was} calculated using JMatPro for the compositions A001 to A018 and the alloys according to the state of the art (see penultimate column), as well as the value for F _v calculated using formula (2) (see last column) . Table 2b is the continuation of Table 2a with regard to the composition of the alloys according to the prior art and the compositions A001 to A018. Table 2c shows the continuation of Table 2a with the compositions A019 to A032. Table 2d is the continuation of Table 2c with regard to the compositions A019 to A032.

The compositions A001 to A004 have an increasing aluminum content, starting from 2.0 to 4.0%. The others

Alloy components are constant. The time t _γ'1 becomes smaller with increasing aluminum _{concentration and the value for F v also} becomes smaller. In the case of the composition A004, F _v = 0.53 and thus condition (1) is not fulfilled. This composition is therefore not easy to process. This means that the higher the aluminum content, the more difficult it is to process the composition, since the time t _{γ'1 is} too short, which is characterized by condition (1) and formula (2).

The compositions A005 to A007 have an increasing chromium content (18, 20 and 23%). The time t _γ'1 'nd F _v also decrease with increasing chromium content, but the effect is far less than with aluminum.

The compositions A008 to A011 have a falling iron content, as a result of which the time t _γ'1 and the value for F _{v become} smaller. That is, the addition of iron improves the workability because the time t _{γ'1 becomes} longer.

The compositions A012 and A018 have a different titanium content, the compositions A012 to A014 having an increasing titanium content. The increasing titanium concentration leads to the time t _γ'1 and the value for F _v becoming shorter, so that the composition A014 with Ti = 1.2% can no longer be processed well, since F _v = 0.61 and therefore the condition ( 1) is no longer fulfilled. This also applies to the composition A015, which is no longer easy to process due to an increased titanium content (Ti = 0.8%) in combination with an aluminum concentration of 2.5%, since F _v = 0.81. The compositions A016 to A018 have an increasing titanium content with an iron content of 10%. The increasing titanium concentration leads to that the time t _γ'1 and the value for F _{v become} smaller, so that the composition A018 with Ti = 1.00% is no longer easy to process, since F _v = 0.82 and therefore condition (1) is no longer met .

The compositions A019 and A025 in Table 2c have a different niobium content, the compositions A019 to A021 having an increasing niobium content. The increasing niobium concentration leads to the time t _γ'1 and the value for F _v becoming shorter, so that the composition A021 with Nb = 3.0% can no longer be processed well, since F _v = 0.53 and thus the condition ( 1) is no longer fulfilled. This also applies to the composition A022, which, due to a niobium content of Nb = 1.0% in combination with an aluminum concentration of 3.5%, is no longer easy to process, since F _v = 0.72. The compositions A023 to A025 have an increasing niobium content at a chromium concentration of 23%. The increasing niobium concentration leads to the fact that the time t _γ'1 and the value for F _{v become} smaller, so that the composition A025 with Nb = 3.0% is no longer easy to process, since F _v = 0.85 and thus the condition ( 1) is no longer fulfilled.

The compositions A026 to A031 have a different content of yttrium, lanthanum, hafnium and tantalum. _{Since F v} > 0.9 applies to all compositions, they are easy to process, since the time t _{γ'1 is} sufficiently long. The composition A032 has a high chromium content of 29.6% with an aluminum content of 2.2% and an F _v = 2.07 and is therefore easy to process.

_{In summary, it can be stated that the alloy compositions (E) according to the invention are within the analytical limits worked out above} , meet condition (1) and formula (2) with regard to the value for F _v and thus have a sufficiently long time t γ'1 and are therefore good are processable. These are the compositions A001 to A003, A005 to A007, A009 to A013, A019 to A020 and A026 to A032. In the case of the state-of-the-art alloys in Table 2a, the alloys H214 and IN702 have a value for F _v <0.9. In the case of the alloy H214, this can be attributed to the increased aluminum content (4.5%) and in the case of IN702 to the increased aluminum content (3.25%) in combination with the increased titanium content (0.38%). This means that the alloys H214 and IN702 have too short a time t _γ'1 and thus poor processability.

The other alloys according to the prior art have a value of F _v > 0.9 and are therefore easy to process, since t _{γ'1 is} sufficiently large. This can be attributed to the lower aluminum content (2.78% for Alloy 603, or even lower for the others) and the limited titanium and niobium content in combination with aluminum.

It was not possible to _{calculate a value for F v} for Alloy 800 H, since the composition, with its relatively high iron content of 46.8%, is outside the validity range of formula (1) and condition (2).

Further improvements in processability result in particular when F _v ≥ 1.0.

Heat resistance, creep resistance and fatigue strength

An important point is the setting of a suitable grain size in order to achieve the highest possible strength, both in terms of fatigue strength and creep strength.

Fine-grain hardening (see Hall-Petch relationship) results in an increase in the high-temperature strength as well as an increase in ductility and toughness in the lower temperature range. At higher temperatures, however, this proves to be an unsuitable measure for increasing the strength, since in the creep range (T> 0.4

T _m , with T _m = melting point) a coarse-grained structure is advantageous [R. Bürgel, Handbook of High Temperature Materials Technology, 5th revised edition, 2015, p. 83/84]. The influence of the grain size on the creep strength or the creep rate can also be described mathematically in the case of grain boundary sliding, the grain size diameter being inversely proportional to the creep rate. This means, the larger the average grain size, the lower the creep rate during grain boundary sliding, which results in a higher creep strength [R. Bürgel, Handbook of High Temperature Materials Technology, 5th revised edition, 2015, p. 111]. In the case of diffusion creep, ie at higher temperatures and lower applied stresses compared to grain boundary sliding, the dependence of the creep rate results in a quadratic or cubic inversely proportional dependency. In this case, a coarser structure is even more important than with grain boundary sliding, which occurs with higher applied stresses than with diffusion creep [R. Bürgel, Handbook of High Temperature Materials Technology, 5th revised edition, 2015, pp. 117/119]

The dependence of the fatigue strength or HCF strength (HCF = High Cycle Fatigue) on the grain size is described by Haigh diagrams [R. Bürgel, Handbook of High Temperature Materials Technology, 5th revised edition, 2015, pp. 206/207]. At lower temperatures, it turns out that a finer-grain structure is advantageous. In general, the fatigue strength in the HCF (HCF = High Cycle Fatigue) as well as in the LCF (LCF = Low Cycle Fatigue) range increases with decreasing grain size at all temperatures [R. Bürgel, Handbook of High Temperature Materials Technology, 5th revised edition, 2015, p. 238].

The previous considerations show that a coarse-grained structure is advantageous with regard to creep strength, but this has a negative effect on fatigue strength.

The setting of a suitable grain size is therefore of crucial importance in order to ensure a reasonable compromise with regard to creep and fatigue strength. The alloys according to the invention therefore typically have an average grain size of 10 to 500 μm.

The grain size can preferably be within the following limits:

10 to 300 μm 10 to 200 μm 10 to 100 μm 10 to 50 μm 50 to 300 μm 60 to 300 μm 70 to 300 μm 75 to 280 μm 80 to 250 μm 20 to 250 μm 20 to 150 μm 20 to 100 μm 30 to 100 μm 40 to 100 μm

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 at the oxide-metal interface falls very quickly below the critical limit when the alloy is used in a corrosive atmosphere, so that if the oxide layer is damaged, a closed, pure chromium oxide layer can no longer form, but also may form other less protective oxides (e.g. nickel and / or iron oxides). Therefore 12% is the lower limit for chromium. This is particularly important because the absorber tubes are exposed to the salt on the inside but to the ambient air on the outside. This means that a certain chromium content is necessary to ensure oxidation resistance at the elevated temperatures of up to 1000 ° C in ambient air. Too high Chromium contents worsen the phase stability of the alloy, especially with the high aluminum contents of ≥ 1.8%. Therefore, 30% 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 that forms is too patchy to fully develop its effect. Too high an aluminum content affects the workability of the alloy. An aluminum content of 4.0% is therefore 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. Therefore 0.1% iron is the lower limit for reasons of cost. As the iron content increases, the phase stability decreases (formation of embrittling phases), especially with high chromium and aluminum contents. Therefore 7% 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 1.00% the oxidation behavior can be worsened, which is why 1.00% is the maximum value.

Even very low magnesium and / or calcium contents improve processing by setting sulfur, which causes the occurrence of low melting nickel-sulfur eutectics is avoided. 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 0.001% 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 this element reduces the processability through the formation of coarse carbonitrides.

The oxygen content must be ≤ 0.020% in order to guarantee the manufacturability of the alloy. Too low an oxygen content increases the costs. The oxygen content is therefore ≥ 0.0001%.

The phosphorus content should be less than or equal to 0.030%, since this surface-active element impairs the resistance to oxidation. 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.

If necessary, the oxidation resistance can be further improved by adding elements with an affinity for oxygen, such as, for example, yttrium, lanthanum, cerium, cerium, mischmetal. They do this by being built into the oxide layer and blocking 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 lanthanum's oxidation resistance increasing effect. The upper limit is set at 0.20% for cost reasons.

A minimum content of 0.001% cerium is necessary in order to maintain the effect of cerium, which increases the resistance to oxidation. 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. 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 hafnium's high-temperature strength and oxidation resistance-enhancing effect. 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 also reduces the resistance to oxidation.

Lead is limited to a maximum of 0.002%, as this element reduces the resistance to oxidation. The same goes for zinc and tin. Table 1: Alloys according to ASTM B 168-19 ¹⁾ / ASTM B167-18 ²⁾ , ASTM B 163-18 ³⁾ , ASTM B409-06 ⁴⁾ , ASTM B443-18 ⁵⁾ and ⁶⁾ in none of the ASTM and ASME standards , ⁷⁾ from the US list, AMS 5550 ⁸⁾ . All data in% by mass.

Table 2a: Alloy compositions according to the invention (E) and alloy compositions according to the prior art (T). Element content in mass%; Chg. = Batch number, or in the case of JMatPro calculations, it corresponds to the composition number; all batches have Pb <0.002%, Sn <0.002% and Zn <0.002%; *) Nominal composition. **) no formation of the γ'-phase.

Table 2b: Continuation of Table 2a: Alloy compositions according to the invention (E) and alloy compositions according to the prior art (T). Element content in mass%; Chg. = Batch number, or in the case of JMatPro calculations, it corresponds to the composition number; all batches have Pb <0.002%, Sn <0.002% and Zn <0.002%; *) Nominal composition. **) not taken into account in JMatPro calculations.

Table 2c: Alloy compositions according to the invention (E) and alloy compositions according to the prior art (T). Element content in mass%; Chg. = Batch number, or in the case of JMatPro calculations, it corresponds to the composition number; all batches have Pb <0.002%, Sn <0.002% and Zn <0.002%; *) Nominal composition. **) no formation of the γ'-phase.

Table 2d: Continuation of Table 2c: Alloy compositions according to the invention (E) and alloy compositions according to the prior art (T). Element content in mass%; Chg. = Batch number, or in the case of JMatPro calculations, it corresponds to the composition number; all batches have Pb <0.002%, Sn <0.002% and Zn <0.002%; *) Nominal composition. **) not taken into account in JMatPro calculation.

Claims

1. Nickel-chromium-aluminum alloy, with (in% by mass) 12 to 30% chromium, 1.8 to 4.0% aluminum, 0.1 to 7.0% iron, 0.001 to 0.50% silicon, 0.001 to 2.0% manganese, 0.00 to 1.00% titanium, 0.00 to 1.10% niobium, 0.00 to 0.5% copper, 0.00 to 5.00% cobalt, 0 each , 0002 to 0.05% magnesium and / or calcium, 0.001 to 0.12% carbon, 0.001 to 0.050% nitrogen, 0.001 to 0.030% phosphorus, 0.0001 to 0.020% oxygen, max. 0.010% sulfur, max. 2 , 0% molybdenum, max. 2.0% tungsten, the remainder nickel with a minimum content of ≥ 50% and the usual process-related impurities for use in solar tower power plants using chloride and / or carbonate molten salts as

Heat transfer medium, whereby the following condition must be met to ensure good processability: F _v ≥ 0.9 with F _v = 4.88050 - 0.095546 * Fe - 0.0178784 * Cr - 0.992452 * AI - 1.51498 * Ti - 0.506893 * Nb + 0.0426004 * AI * Fe where Fe, Cr, Al, Ti and Nb are the concentration of the elements in question in% by mass.

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

3. Alloy according to claim 1 or 2 at temperatures of 250 ° C to a maximum of 1000 ° C.

4. Alloy according to one of claims 1 to 3, with a chromium content of 14 to 30%.

5. Alloy according to one of claims 1 to 4, with an aluminum content of 2.0 to 4%.

6. Alloy according to one of claims 1 to 5, with an iron content of 0.1 to 4.0%.

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

8. Alloy according to one of claims 1 to 7, with a manganese content of 0.001 to 1.0%.

9. Alloy according to one of claims 1 to 8, with a titanium content of 0.001 to 0.60%.

10. Alloy according to one of claims 1 to 9, with a niobium content of 0.00 to 0.7%.

11. Alloy according to one of claims 1 to 10, with a carbon content of 0.001 to 0.10%.

12. Alloy according to one of claims 1 to 11, optionally with an yttrium content of 0.001 to 0.20%.

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

14. An alloy according to any one of claims 1 to 13, optionally including cerium at a content of 0.001 to 0.20%.

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

16. Alloy according to one of claims 1 to 15, optionally containing zircon with a 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 including boron at a content of 0.0001 to 0.008%.

19. Alloy according to one of claims 1 to 18, further optionally containing 0.00 to <5.0% cobalt.

20. Alloy according to one of claims 1 to 19, further optionally containing a maximum of 0.5% vanadium.

21. Alloy according to one of Claims 1 to 20, in which the impurities are set in contents of a maximum of 0.002% Pb, a maximum of 0.002% Zn, and a maximum of 0.002% Sn.

22. Use of the alloy according to one of claims 1 to 21 as a strip, sheet metal, wire, rod, longitudinally welded pipe and seamless pipe.

23. Use of the alloy according to one of claims 1 to 21 for the production of strip, sheet metal, wire, rod, longitudinally welded pipe and seamless pipes.