WO2024235867A1

WO2024235867A1 - A nickel-base alloy, an object made thereof and the use thereof

Info

Publication number: WO2024235867A1
Application number: PCT/EP2024/062976
Authority: WO
Inventors: Mats Lundberg; Guocai Chai; Rohit OJHA
Original assignee: Alleima Emea Ab
Priority date: 2023-05-12
Filing date: 2024-05-10
Publication date: 2024-11-21

Abstract

The present disclosure relates to a nickel-based alloy. More specifically, the present disclosure relates to a medium to high entropy nickel-based alloy having the following composition, in percent by weight (wt-%): Cr 20.0 to 26.0; Fe less than 8.5; Mn less than 4.0; Si 0.5 to 2.0; Al 1.0 to 4.0; Ti 0.50 to 4.0; C less than 0.05; N less than 0.05; Ta less than 2.0; Nb less than 4.0; Co 3.0 to 6.0; Cu 2.1 to 6.0; Mo less than 2.0; W less than 1.0; optionally B less than 0.005; optionally P less than 0.005; optionally S less than 0.005; optionally Ca, Mg and Ce in a total amount of less than 0.5; Ni balance and incidental impurities; wherein the nickel-based alloy fulfils the following requirements: a) Cu - (0.1*Fe + 2.1) ≥ 0.0, wherein the numerical values of the alloying elements are given in weight%; and b) Formula (I), wherein C_i is the mole fraction and R is the gas constant. The nickel-based alloy according to the present disclosure is primarily intended for solution annealed objects adapted to be exposed to atmospheres where metal dusting easily occurs.

Description

A nickel-base alloy, an object made thereof and the use thereof

TECHNICAL FIELD

The present disclosure relates to a nickel-based alloy. More specifically, the present disclosure relates to a medium to high entropy nickel-based alloy and an object made thereof suitable for use in a metal dusting environment.

BACKGROUND ART

Metal dusting corrosion is a complex form of high temperature corrosion caused by carbon supersaturated gas and this corrosion will lead to a rapid degradation of metals and metal alloys. Usually, metal dusting corrosion is associated with temperatures between 500 and 800°C in reaction gases containing H₂, CO, CO₂, H₂O and/or hydrocarbons, such as methane. The corrosion attacks seem to occur randomly over the surface of the metal or metal alloy object and take place locally in the form of pits or holes. In the immediate vicinity of the pits or holes, dust-like corrosion products are present in the form of a dark grey to black powder.

The risk of metal dusting increases with increased carbon concentration in a reaction gas or on a surface of a metal or metal alloy object. It has been shown that catalytically controlled decomposition of carbon compounds to elemental carbon plays a central role in the process. Additionally, the pressure and the flow velocity of the gases and the exposure time will also play a role for the risk of metal dusting corrosion.

One material which can be used in metal dusting applications is disclosed in for example EP1403392 B1 , wherein the alloy comprises, in mass %, C: not more than 0.2%, Si: 0.01- 4%, Mn: 0.05-2%, P: not more than 0.04%, S: not more than 0.015%, Cr: 10-35%, Ni: 30- 78%, Al: not less than 0.005% but less than 4.5%, N: 0.005-0.2%, and one or both of Cu: 0.015-3% and Co: 0.015-3%, with the balance substantially being Fe of over 0% but not more than 10%. Another example of a material which is suitable for metal dusting is disclosed in EP1717330 B1 , wherein a metal tube having a Cu-enriched surface layer and comprises an alloy composition, in mass %, C: 0.01 - 0.6%, Si: 0.01 - 5%, Mn: 0.01 - 10%, P: at most 0.08%, S: at most 0.05%, Cr: 15 - 35%, Ni: 30 - 75%, Cu: 0.01 - 10%, N: 0.001 - 0.25%, Al: 0.001 - 10%, O: at most 0.02%, balanced by Fe and impurities.

Also, recent material developments have provided an alloy, such as VDM® Alloy 699 XA, which is suitable to be used in metal dusting applications. Fe-Ni-Cr or austenitic aluminaforming steels may also be used for these applications. However, even though these alloys are used in metal dusting environments today, they will still be subjected to metal dusting corrosion and there is therefore a need for an improved metallic alloy which will be able to better withstand the corrosion which occurs in these environments and thus can be used for prolonged times in metal dusting environments. Additionally, the aforementioned alloys may be difficult to hot work and may be difficult to weld.

SUMMARY OF THE DISCLOSURE

An aspect of the present disclosure is thus to provide a nickel-base alloy with outstanding metal dusting resistance. This means that an object comprising this nickel-based alloy will be excellent to use in an environment in which metal dusting corrosion readily occurs.

Hence, the present disclosure therefore provides a nickel-based alloy having the following composition, in percent by weight (wt%):

Cr 20.0 to 26.0;

Fe less than 8.5;

Mn less than 4.0;

Si 0.5 to 2.0;

Al 1.0 to 4.0;

Ti 0.50 to 4.0;

C less than 0.05;

N less than 0.05;

Ta less than 2.0;

Nb less than 4.0;

Co 3.0 to 6.0;

Cu 2.1 to 6.0;

Mo less than 2.0;

W less than 1.0; optionally B less than 0.005; optionally P less than 0.005; optionally S less than 0.005; optionally Ca, Mg and Ce in a total amount of less than 0.5;

Ni balance and incidental impurities; wherein the nickel-based alloy additionally fulfils the requirements: a) Cu-(0.1*Fe+2.1) > 0.0, wherein the numerical values of the alloying elements are given in weight%; and b) ASm/x/-R > 1.36, AS_m.x = -R ^t^Ca in c;), wherein Cj is the mole fraction and R is the gas constant.

The present inventors have surprisingly found that if the present alloy fulfils the above requirements, i.e. , Cu - (0.1*Fe + 2.1) > 0.0 and ASm,x /-R > 1.36, the obtained alloy or an object made thereof will have great metal dusting resistance. It is believed, without being bound to any theory, that these two requirements will have an impact on the diffusion rates of carbon and by fulfilling these requirements, the deposition of carbon on the surface of nickel-based alloy is eliminated or at least reduced whereby the metal dusting degradation of the nickel-based alloy is decreased or even eliminated. Thus, as result of diligent research to solve the problems described above, the present inventors have been able to control the composition of present nickel-based alloy and the diffusion rate of carbon. Consequently, a metal dusting resistant nickel-based alloy has been obtained which is both hot workable and weldable.

Further, the present disclosure relates to an object comprising the nickel-based alloy as defined hereinabove or hereinafter.

The present disclosure also relates to the use of an object comprising the nickel-based alloy as defined hereinabove or hereinafter, in a metal dusting environment, such as an environment comprising or consisting of carbon supersaturated gas.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1. shows results from inventive and comparative samples being exposed to metal dusting testing;

Figure 2a. shows images of comparative samples after being metal dusting tested;

Figure 2b. shows images of comparative samples after being metal dusting tested; and

Figure 2c. shows images of inventive samples after being metal dusting tested.

DETAILED DESCRIPTION

The present disclosure relates nickel-based alloy having the following composition, in percent by weight (wt%): Cr 20.0 to 26.0; Fe less than 8.5;

Mn less than 4.0;

Si 0.5 to 2.0;

Al 1.0 to 4.0;

Ti 0.50 to 4.0;

C less than 0.05;

N less than 0.05;

Ta less than 2.0;

Nb less than 4.0;

Co 3.0 to 6.0;

Cu 2.1 to 6.0;

Mo less than 2.0;

W less than 1.0;

Ni balance and incidental impurities; optionally B less than 0.005; optionally P less than 0.005; optionally S less than 0.005; optionally Ca, Mg and Ce in a total amount of less than 0.5;

Ni balance and incidental impurities; wherein the nickel-based alloy fulfils the following requirements: a) Cu - (0.1 *Fe + 2.1) > 0.0, wherein the numerical values of the alloying elements are given in weight%; and b) ASm/x /-R > 1.36, AS_m.x = —R .i=i(.Ci In c;), wherein q is the mole fraction and R is the gas constant.

It has been found that the present nickel-based alloy will during operation in a metal dusting environment generate a passive layer comprising oxides on the surface. This surface layer will prevent contact between the carburizing or reducing atmosphere and the nickel-based alloy, and the time of forming metal dusting pits or general attack is therefore prolonged compared to prior art materials.

The present disclosure also provides a powder of the nickel-based alloy as defined hereinabove or hereinafter. Thus, the nickel-based alloy as defined hereinabove or hereinafter may be in the form of a powder. The powder may be obtained through atomization. The present disclosure also relates to an object comprising the nickel-based alloy as defined hereinabove or hereinafter. The object may be selected from, as example, but not limited to, a tube, a composite tube, a pipe, a bar, a hollow, a billet, a bloom, a strip, a wire, a plate or a sheet. In the present disclosure, the term “composite tube” is intended to mean a multilayer tube comprising at least two different alloys, wherein at least one of the alloys is the nickel- based alloy as defined hereinabove or hereinafter and this alloy is the inner layer, i.e. the layer which is exposed to metal dusting environment.

Hence, the nickel-based alloy according to the present disclosure is primarily intended for providing an object manufactured thereof with the ability to be exposed to atmospheres where metal dusting easily occur, such as in a gaseous carburizing or reducing atmospheres. The gaseous carburizing atmosphere may contain reaction gases such as H₂, CO, CO2, H2O and/or hydrocarbons, such as methane. Usually, metal dusting corrosion is associated with temperatures between 500 and 800°C.

The use of an object comprising the present nickel-based alloy, such as for example a tube in an ammonia plant, may require welding. Good weldability is achieved when the present nickel-based alloy or an object made thereof is in a solution annealed condition as this will ensure an austenitic matrix free from precipitates. Thus, when the present nickel-based alloy or an object composed thereof is in a solution annealed condition, it will have good weldability, high strength, and excellent metal dusting resistance.

Hence, the present disclosure also relates to a solution annealed object comprising the nickel-based alloy as defined hereinabove or hereinafter. The solution annealed object may be selected from a tube, a composite tube, a pipe, a bar, a hollow, a billet, a bloom, a strip, a wire, a plate or a sheet. A solution annealed object comprising the nickel-based alloy as defined hereinabove or hereinafter may have a crystal structure which is fully austenitic. According to embodiments, the solution annealed object as defined hereinabove or hereinafter may have a tensile strength above 800 MPa, when measured at 650°C according to SS-EN ISO 6892-1.

According to embodiments, the solution annealed object comprising the nickel-based alloy as defined hereinabove or hereinafter may be manufactured using conventional metallurgical manufacturing methods comprising the steps of casting a melt, hot working and/or cold working followed by a solution annealing step, which step is performed at a temperature in the range of 1100 to 1200°C for 1 to 8 hours, such as 1 to 5 hours. According to embodiments, the solution annealed object comprising the nickel-based alloy as defined hereinabove or hereinafter may also be manufactured using powder metallurgy manufacturing methods, e.g., hot isostatic pressing (HIP) or additive manufacturing (AM), followed by a solution annealing step performed at a temperature in the range 1100 to 1200°C for 1 to 8 hours, such as 1 to 5 hours.

The present disclosure also relates to a use of a solution annealed object as defined hereinabove or hereinafter in an environment comprising or consisting of carbon supersaturated gas in a temperature range of 500 °C to 800 °C. For example, in a gaseous carburizing or reducing atmospheres containing H₂, CO, CO₂, H₂O or hydrocarbons.

The alloying elements will now be described in more detail. The terms “weight%” and “wt%” are used interchangeably. Upper and lower limits of the individual elements of the composition may be freely combined within the broadest limits set out in the claims, unless explicitly disclosed otherwise. Also, the list of properties or contributions mentioned for a specific element should not be considered exhaustive.

Chromium (Cr) 20.0 to 26.0 wt%

Cr is an alloying element added to provide sufficient corrosion resistance in high temperatures as it will influence the corrosion resistance by forming a stable passive layer on the surface. The passive layer formed will provide for good metal dusting resistance. In order to ensure this this effect, it is necessary that the content of Cr is at least 20.0 wt%. According to embodiments, to further ensure that this effect is obtained, the content of Cr may be at least 20.5 wt%. However, if the Cr content exceeds 26.0%, the toughness of the alloy will decrease, and the hot workability may deteriorate thereby making working difficult. Further, if the Cr content exceeds 26.0%, there is a risk of the formation of intermetallic phases, which may cause embrittlement. According to embodiments, the content of Cr may not exceed 25.5 wt%.

Iron (Fe) less than 8.5 wt%

Fe may optionally be added to the nickel-based alloy. However, it has been found that too high amounts of Fe in the present alloy provides a detrimental effect on the metal dusting resistance. It is not necessary to reduce the Fe content to 0 wt% as an excessive reduction of Fe content causes a large increase in manufacturing costs and therefore, the Fe content is 8.5 wt% or less, such as 6.5 wt% or less. According to embodiments, the content of Fe may be in the range of 0.5. to 8.5 wt%. Manganese (Mn) less than 4.0 wt%

Mn may optionally be added since it is an austenitic structure stabilizer and may be used to replace some of content of the expensive alloying element Ni. In addition, Mn is beneficial for deoxidation during melting and provides improvement of the workability of the nickel-based alloy. However, an excessive high Mn will reduce the formation of the passive layer comprising Cr. Hence, the Mn content is 4.0 wt% or less. According to embodiments, the content of Mn is 3.0 wt% or less. According to embodiments, the content of Mn may be in the range of 0.1 to 4.0 wt%, such as 0.1 to 3.0 wt% or less.

Silicon (Si) 0.5 to 2.0 wt%

Si is added to provide improved metal dusting resistance. In high temperature environments, Si influences the corrosion resistance by forming a stable passive layer on the surface thereby is good metal dusting resistance is ensured. In order to obtain this effect, it is necessary that the content of Si is at least 0.5 wt%. According to embodiments, the content of Si may be at least 0.6 wt%. However, if the Si content exceeds 2.0 wt%, the weldability and structure stability may be negatively affected. According to embodiments, the content of Si may not exceed 1.8 wt%, such as 1.6 wt%.

Aluminium (Al) 1.0 to 4.0 wt%

Al is an element added to provide both improved metal dusting resistance and creep strength. In high temperature environments, Al will have an impact on the corrosion resistance by forming a stable passive layer on the surface thereby ensuring good metal dusting resistance. To obtain this effect, it is necessary that the content of Al is at least 1.0 wt%. According to embodiments, the content of Al may be at least 2.0 wt%. On the other hand, if the Al content exceeds 4.0 wt%, the weldability and hot workability may be negatively influenced. According to embodiments, the Al content may not exceed 3.5 wt%.

Titanium (Ti) 0.5 to 4.0 wt%

Ti is an element added to provide improved metal dusting resistance, high temperature strength and creep strength. In high temperature environments, Ti will enhance the corrosion resistance by forming a stable passive layer on the surface thereby good metal dusting resistance is obtained. To obtain this effect, it is necessary that the content of Ti is at least 0.5 wt%. According to embodiments, the content of Ti may be at least 0.7 wt%. On the other hand, if the Ti content exceeds 4.0 wt%, the weldability and hot workability may be negatively influenced. According to embodiments, the Ti content may not exceed 3.0 wt%. Carbon (C) less than 0.05 wt%

The content of C should be as low as possible, and it is not deliberated added. C is an austenitic structure stabilizer and small addition of C may however increase high temperature strength. Too high amount of C will cause a poor toughness and for this reason, C should be less than 0.05 wt%. According to embodiments, the content of C may be in the range of 0.001 to 0.05 wt%.

Nitrogen (N) less than 0.05 wt%

The content of N should be as low as possible, and it is not deliberated added to the nickel- based alloy. N is an austenitic structure stabilizer and small addition of N may however increase high temperature strength. Too high amount of N may cause the formation of unwanted aluminium nitrides and for this reason, N should be less than 0.05 wt%. According to embodiments, the content of N may be in the range of 0.01 to 0.05, such as 0.02 to 0.05 wt%.

Niobium (Nb) 0.01 to 4.0 wt%

Nb is an element added to provide improved metal dusting resistance, high temperature strength and creep strength. In order to obtain this effect, it is necessary that the content of Nb is at least 0.01 wt%. According to embodiments and to improve these effects further, the content of Nb may be at least 0.5 wt%, such as at least 1.0 wt%, such as at least 2.0 wt%. However, if the Nb content exceeds 4.0 wt%, the hot workability may be negatively affected and there may be a risk of formation of unwanted Laves phases. According to embodiments, the content of Nb may not exceed 3.5 wt%.

Tantalum (Ta) less than 2.0 wt%

Ta may be added to provide improved metal dusting resistance, the entropy value, high temperature strength and creep strength. According to embodiments, the content of Ta may be at least 0.03 wt%, such as at least 0.5 wt%, such as at least 0.75 wt%, such as at least 0.85 wt%. However, if the Ta content exceeds 2.0 wt%, the hot workability may be negatively affected and there may also be a risk for formation of unwanted Laves phases. According to embodiments, the Ta content may not exceed 1.5 wt%. According to embodiments, the Ta content may be in the range of 0.03 to 2.0 wt%, such as 0.5 to 2.0 wt%.

Cobalt (Co) 3.0 to 6.0 wt%

Co is an effective austenitic structure stabilizer and is added to provide increased entropy value, high temperature strength and creep strength. In order to ensure this effect, it is necessary that the content of Co is at least 3.0 wt%. According to embodiments, the content of Co is at least 3.2 wt%, such as at least 3.4 wt%. On the other hand, if the Co content exceeds 6.0 wt%, the hot workability may be negatively affected.

Copper (Cu) 2.1 to 6.0 wt%

Cu is an effective austenitic structure stabilizer and is added to provide improved metal dusting resistance. In order to ensure this effect, it is necessary that the content of Cu is at least 2.1 wt%, such as at least 2.5 wt%. On the other hand, if the Cu content exceeds 6.0 wt% the weldability and hot workability may be negatively influenced. According to embodiments, the highest content of Cu may be 5.0 wt%.

Molybdenum (Mo) less than 2.0 wt%

Mo may optionally be added to the nickel-based alloy since it increases the entropy value. Thus, if added, the Mo content is 2.0 wt% or less, such as 1.0 wt% or less. According to embodiments, the content of Mo may be in the range of 0.01 to 0.05 wt%.

Tungsten (W) less than 1.0 wt%

W may optionally be added to the nickel-based alloy since it increases the entropy value. Thus, if added, the W content is 1.0 wt% or less, such as 0.5 wt% or less. According to embodiments, the content of Mo may be in the range of 0.01 to 1.0 wt%.

Boron (B) less than 0.005 wt%

B is an element that may be added to provide improved hot workability and creep strength. However, if the B content exceeds 0.005 wt%, the weldability may be negatively affected. According to embodiments, the content of B may be in the range of 0.0001 to 0.005 wt%.

Phosphorus (P) less than 0.005 wt%

P is effective in suppressing the reactions between the carburizing gases and the nickel- based alloy but is strongly detrimental to hot workability and weldability. For at least these reasons, the content of P is less than 0.005 wt%. According to embodiments, the content of P may be in the range of 0.0001 to 0.005 wt%.

Sulphur (S) less than 0.005 wt%

S is effective in suppressing the reactions between the carburizing gases and the nickel- based alloy but is strongly detrimental to hot workability and weldability. For at least these reasons, the content of S is less than 0.005 wt%. According to embodiments, the content of S may be in the range of 0.0001 to 0.005 wt%. Calcium (Ca), Magnesium (Mg) and Cerium (Ce) less than 0.5 wt%

Ca, Mg and Ce are elements that may be added to provide improved hot workability. If added, the total amount less than 0.5 wt%. According to embodiments, the total content of Ca, Mg and Ce may be in the range of 0.01 to 0.5 wt%.

In the present nickel base alloy, the balance is Nickel (Ni) and incidental impurities.

According to embodiments, the content of Ni may be at least 39 wt%, such as at least 44 wt%, such as at least 49 wt%. According to an embodiment the content of Ni may be less than 76 wt%, such as less than 71 wt%, such as less than 66 wt%, such as less than 61 wt%. According to an embodiment the content of Ni may be in the range between 50 to 60 wt%.

According to embodiments, the austenitic nickel-based alloy comprises or consists of all the alloying elements mentioned hereinabove or hereinafter in the ranges mentioned hereinabove or hereinafter. According to embodiments, the austenitic nickel-based object comprises or consists of all the alloying elements mentioned hereinabove or hereinafter in the ranges mentioned hereinabove or hereinafter.

The term "incidental impurities" as referred to herein means substances that will contaminate the nickel-based alloy when it is industrially produced, due to the raw materials, such as ores and scraps, and due to various other factors in the production process and are allowed to contaminate within the ranges mentioned hereinafter without adversely affecting the properties of the nickel-based alloy as defined hereinabove or hereinafter. Examples of incidental impurities may be Oxygen (O), Germanium (Ge), Arsenic (As), Antimony (Sb), Tellurium (Te), Tinn (Sn), Lead (Pb), Bismuth (Bi) and Yttrium (Y). According to embodiments, the total content of incidental impurities may be up to at most 0.05 wt% in total, such as up to at most 0.025 wt%, such as up to at most 0.010 wt%.

In addition, as stated hereinabove or hereinafter, the nickel-based alloy must apart from the alloying elements ranges also fulfil two requirements:

The first requirement, Cu - (0.1*Fe + 2.1) > 0.0, wherein the numerical values are in weight% describes the relationship between Cu and Fe as it has been found that these elements have a significant impact on the metal dusting resistance. The inventors have surprisingly found that alloys fulfilling this equation will have great metal dusting resistance. According to embodiments, if Cu - (0.1*Fe + 2.1) > 1.0, the nickel-based alloy may have even greater metal dusting resistance. According to embodiments, if Cu - (0.1*Fe + 2.1) is > 2.0, the metal dusting resistance may further be improved.

The second requirement, ASm,x/-R > 1.36, AS_m.x = -R l iCc; Inct), wherein q is the mole fraction and R is the gas constant, relates to the entropy of the alloy. The entropy of an alloy will have an impact on the metal dusting resistance and the inventors have surprisingly found that by fulfilling this equation, the alloy will have great metal dusting resistance. Thus, the metal dusting degradation of the nickel-based alloy is thereby heavily reduced in comparison to known materials. According to embodiment, for even more improved metal dusting resistance, ASm,x/-R is > 1.48. Thus, by fulfilling both these requirements, the nickel- based alloy as defined hereinabove or hereinafter will have great metal dusting resistance.

The present invention will now be described in more detail below with reference to exemplifying embodiments. The invention is however not limited to the exemplifying embodiments discussed but may be varied within the scope of the disclosed ranges.

EXAMPLES

Eleven nickel-based sample heats having chemical compositions as specified in Table 1 , were produced, and tested accordingly.

Table 1 Composition of the samples, * indicates comparative examples.

Balance is Ni and incidental impurities.

The melts were produced on a laboratory scale by vacuum induction melting (VIM), casted and machined into bars having 24 mm in diameter. The bars were then subjected to hot rolling down to about 7*7 mm at a temperature of about 1180°C, and thereafter solution annealed at a temperature of about 1150°C for 4 hours.

The solution annealed samples were subjected to various tests as described below. Tensile strength (Rm) and elongation (A) were determined through tensile testing according to SS- EN ISO 6892-1 at room temperature. The samples were longitudinal to the rolling direction. The results are shown in Table 2. From the results, the highest tensile strengths in the solution annealed condition measured at room temperature were achieved for samples 3, 4 and 5, having a tensile strength above 800 MPa.

Table 2 Mechanical properties measured at room temperature, * indicates comparative examples.

In addition, tensile strength (Rm) and elongation (A) were determined through tensile testing according to SS-EN ISO 6892-1 at 650°C. The results are presented in Table 3. From the results, the highest tensile strengths in the solution annealed condition measured at 650°C was obtained for Heat Nos. 3 and 4, all having a tensile strength above or about 800 MPa. Table 3 Mechanical properties measured at 650°C, * indicates comparative examples.

Metal dusting testing

The solution annealed samples were grinded on the front and back with 320, 800 and finally 1200 grit and the sides with 800 and 1200 grit. The samples were cleaned with acetone and isopropanol to remove all residuals.

Metal dusting testing was performed by quasi-isothermal exposure tests at 620°C in a tube furnace, with 18 bar and a carburizing gas environment of 47 % CO, 47 % H₂, 2 % H₂O, 4 % CO₂ while heating in static argon, cooling in flowing argon, both at 1 bar.

Gravimetric measurements have been performed on the exposed samples after a thorough cleaning process and the results are shown in Figure 1. The figure shows the mass change of the tested samples wherein a positive slope indicates a mass increase, such as the formation of a protective oxide. A negative slope of the curve indicates a loss of mass, such as the formation of metal dusting pits.

The samples of the present invention are illustrated by the dashed lines, showing a positive slope even after 1960 hours of exposure. The comparative samples are illustrated by solid lines, all starting to show negative mass change values before 1500 hours of exposure.

In addition, the visual appearance of the samples can be divided into three typical groups after exposure: Figure 2a shows Group 1, containing comparative examples having metal dusting pits on all flat surfaces and on edges and corners. Figure 2b shows Group 2, containing comparative examples having metal dusting attacks on the edges. Figure 2c shows Group 3, containing samples of the present invention having no indication of metal dusting on flat surfaces or edges.

Thus, as is shown by the Figures, the nickel-base sample alloys falling within the scope of the present invention have outstanding metal dusting resistance compared with the comparative samples as the inventive samples have no indication of metal dusting on their flat surfaces or on their edges.

Claims

1 . A nickel-based alloy having the following composition, in percent by weight (wt-%):

Cr 20.0 to 26.0;

Fe less than 8.5;

Mn less than 4.0;

Si 0.5 to 2.0;

Al 1.0 to 4.0;

Ti 0.50 to 4.0;

C less than 0.05;

N less than 0.05;

Ta less than 2.0;

Nb less than 4.0;

Co 3.0 to 6.0;

Cu 2.1 to 6.0;

Mo less than 2.0;

Ni balance and incidental impurities; wherein the nickel-based alloy fulfils the following requirements: a) Cu - (0.1*Fe + 2.1) > 0.0, wherein the numerical values of the alloying elements are given in weight%; and b) ASm/x /(-R) > 1 .36, AS_m.x = —R Si iCc; In C;), wherein Cj is the mole fraction and R is the gas constant.

2. The nickel-base alloy according to claim 1 , wherein the content of Fe is less than 6.5 wt%.

3. The nickel-base alloy according to any one of preceding claims, wherein the content of Al is 2.0 to 4.0 wt%.

4. The nickel-base alloy according to any one of preceding claims, wherein the content of Ti is 0.5 to 2.0 wt%.

5. The nickel-base alloy according to any one of preceding claims, wherein the content of Nb is 2.0 to 3.5 wt%.

6. The nickel-base alloy according to any one of preceding claims, wherein the content of Co is 3.0 to 5.0 wt%.

7. The nickel-base alloy according to any one of preceding claims, wherein the content of Cu is 3.0 to 5.0 wt%.

8. The nickel-base alloy according to any one of preceding claims, wherein Cu - (0.1*Fe + 2.1) is > 1.0 such as Cu - (0.1*Fe + 2.1) is > 2.0 .

9. The nickel-base alloy according to any one of preceding claims, wherein ASm/x/-R is > 1.48.

10. The nickel-base alloy according to any one of preceding claims, wherein the nickel-base alloy is in the form of a powder.

11. An object comprising the nickel-base alloy according to any one of preceding claims.

12. The object according to claims 11 , wherein the object is selected from a tube, a composite tube, a pipe, a bar, a hollow, a billet, a bloom, a strip, a wire, a plate or a sheet.

13. The object according to claim 11 or 12, wherein said object is solution annealed.

14. Use of an object according to claims 11 or 12, in an environment wherein metal dusting corrosion occurs.