WO2024231541A1

WO2024231541A1 - Nickel-based alloy

Info

Publication number: WO2024231541A1
Application number: PCT/EP2024/062928
Authority: WO
Inventors: Karin HANSSON ANTONSSON; Christina HARALDSSON; Mats HÄTTESTRAND; Ulf KIVISÄÄK
Original assignee: Alleima Tube Ab
Priority date: 2023-05-11
Filing date: 2024-05-10
Publication date: 2024-11-14

Abstract

Nickel-based alloy having the following composition, in percent by weight (wt.-%): C equal to or less than 0.05, Si equal to or less than 1.0, Mn 0.5 - 1.5, P equal to or less than 0.03, S equal to or less than 0.03, Cr 31.5-36.0, Mo 7.0-10.0, N 0.08-0.18, Cu equal to or less than 0.4, Fe 2.0- 6.5, optionally Al up to 0.30, optionally one of Ca or Mg each up to 0.05 or REM up to 0.5, optionally up to 0.0050 of B, balance Ni and normally occurring impurities up to at most 1.5 wt.-% in total; wherein the composition fulfils the requirement of Equation 1: E_Ni ≥ 1.864* E_cr -24.0 (Equation 1) wherein E_Cr = [wt.-% Cr]+[wt.-% Mo] + 1.5*[wt.-% Si], and E_Ni = [wt.-% Ni]+30*[wt.-% C]+30*[wt.-% N]+0.5*[wt.-% Mn]+0.5*[wt.-% Cu],

Description

NICKEL-BASED ALLOY

TECHNICAL FIELD

The present disclosure relates in general to a nickel-based alloy intended for use in highly corrosive environments. The present disclosure further relates in general to a method for producing an object of the nickel-based alloy.

BACKGROUND

Nickel-based alloys are important to modern industry because of their ability to withstand a wide variety of severe operating conditions involving corrosive environments, high temperatures, high stresses, and combinations of these factors. Thus, nickel-based alloys may be used in applications where the properties of today's austenitic stainless steels are insufficient to meet the requirements.

However, nickel-based alloys with high Cr and Mo content often tend to form unwanted intermetallic phases even though to lesser extent then in austenitic stainless steels. These phases may be formed already during solidification, primarily due to microsegregation. Intermetallic phases may negatively affect both the mechanical properties and the corrosion resistance of a nickel-based alloy. It is known that the content of intermetallic phases in nickel-based alloys can be reduced by certain methods, such as remelting and soaking. Common remelting techniques include vacuum arc remelting (VAR) and electroslag remelting (ESR). However, such processes are very expensive. In order to reduce the manufacturing costs, it would be desirable to be able to use conventional metallurgical methods which are used for producing austenitic stainless steels.

One example of a well-known nickel-based alloy is Alloy 625 (UNS N06625), which is used in a large variety of applications due to its corrosion resistance and mechanical properties. Alloy 625 typically comprises <0.5 wt.% Mn, 20.0-23.0 wt.-% Cr, <1.0 wt.-% Co, 8.0-10.0 wt.-% Mo, 3.15-4.15 wt.-% Nb+Ta, <5.0 wt.-% Fe, and at least 58 wt.-% Ni. However, said alloy does not have sufficient properties to meet more aggressive environments. Another example of a previously known nickel-based alloy is G-35 (UNS N06035) which was developed to resist "wet process" phosphoric acid, which is used in the production of fertilizers. This alloy has also a high resistance to chloride-induced localized attack. G-35 typically comprises <0.5 wt.% Mn, 32.25-34.25 wt.-% Cr, <1.0 wt.-% Co, 7.6-9.0 wt.-% Mo, <0.60 wt.-% W, <0.30 wt.-% Cu, <0.20 wt.-% V, and <2.0 wt.-% Fe. However, said alloy is difficult to produce by conventional metallurgical processes in a melt shop where stainless steel is also melted and where iron contamination of the melt is likely to occur.

WO 2019/224287 Al discloses an austenitic alloy comprising 25.0-33.0 wt.-% Cr, 42.0-52.0 wt.-% Ni, 6.0-9.0 wt.-% and 0.07-0.11 wt.-% N, and which fulfils the condition of ENi > 1.864* ECr - 19.92. Said alloy has a good corrosion resistance, demonstrates a low content of intermetallic phases after solidification, and may be produced by conventional metallurgical methods. Although said alloy works very well in many applications, there may be applications where even higher resistance to localized corrosion may be desired.

SUMMARY

The aspect of the present disclosure is to provide a nickel-based alloy which can be produced by conventional metallurgical methods (i.e., which can be produced without the need of special process steps, such as remelting), that has good corrosion resistance in highly corrosive environments, and which forms only a low amount of intermetallic phase during solidification.

In accordance with the present disclosure, a nickel-based alloy is provided having the following composition, in percent by weight (wt.-%):

C equal to or less than 0.05,

Si equal to or less than 1.0,

Mn 0.5 - 1.5,

P equal to or less than 0.03,

S equal to or less than 0.03,

Cr 31.5 - 36.0,

Mo 7.0 - 10.0,

N 0.08- 0.18, Cu equal to or less than 0.4,

Fe 2.0 - 6.5, optionally Al up to 0.30, optionally one of Ca or Mg each up to 0.05 or REM up to 0.5, optionally up to 0.0050 of B, balance Ni and normally occurring impurities up to at most 1.5 wt.-% in total; wherein the composition fulfils the requirement of Equation 1:

E_Ni > 1.864* E_& - 24.0 (Equation 1) wherein

E& = [wt.-% Cr]+[wt.-% Mo] + 1.5*[wt.-% Si], and

E_Ni = [wt.-% Ni]+30*[wt.-% C]+30*[wt.-% N]+0.5*[wt.-% Mn]+0.5*[wt.-% Cu],

It has surprisingly been found that a high corrosion resistance, in particular a high pitting corrosion resistance, may be achieved while at the same time achieving a low amount of intermetallic phase during solidification by means of the herein described nickel-based alloy. Furthermore, the herein described nickel-based alloy demonstrates a high proof stress as well as high ductility. Moreover, the herein described nickel-based alloy may be produced by conventional metallurgical processes.

The herein described nickel-based alloy is suitable for use in a variety of applications involving highly corrosive environments, for example the chemical process industry (including petrochemical industry). By way of example, the nickel-based alloy as described herein may be used in heat exchangers, process tubing, and/or piping within the chemical process industry. In view of its excellent properties, it may also be used in such components when the components are subjected to seawater as a cooling medium. The herein described nickel-based alloy may also be used in other components configured to be subjected to seawater cooling, and optionally one or more aggressive solutions present on a process side of the component.

Furthermore, the present disclosure provides a method for producing an object of the herein described nickel-based alloy. The method comprises: casting a melt having the composition as described above, thereby obtaining a cast alloy, optionally heat treating the cast alloy, hot working of the cast (and optionally heat treated) alloy, optionally heat treating the hot worked alloy, optionally cold working the hot worked (and optionally heat treated) alloy, and optionally heat treating the cold worked alloy.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 illustrates the amount of intermetallic phase formed during solidification for experimental alloys as a function of 1.864* E_Cr - E_Ni,

Fig. 2 illustrates critical pitting temperature (CPT) of tested alloys, determined according to ASTM G150 with 4.5M MgCh pH 5 as electrolyte and a potential of 600 mV vs. SCE, and

Fig. 3 illustrates the general corrosion rate of tested alloys, determined according to ISO 18069 with 20% H2SO4and at a temperature of 93°C.

DETAILED DESCRIPTION

The invention will 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 appended claims.

When ranges are disclosed in the present disclosure, such ranges include the respective end values of the range, unless explicitly disclosed otherwise. Similarly, when an open range is disclosed, the open range also include the single end value of the open range, unless explicitly disclosed otherwise.

The present disclosure provides an austenitic nickel-based alloy. The austenitic nickel-based alloy may be present in any possible product form without departing from the present disclosure. For example, the austenitic nickel-based alloy may be present as an ingot, a billet, a bloom, a bar, a tube, a pipe, a rod, a strip, a plate, a sheet, a hollow, a fitting, a flange or a wire (such as a welding wire), although other product forms are also plausible.

The present austenitic nickel-based alloy will provide for that the product forms will have acceptable properties, such as corrosion resistance and mechanical properties, even though they will contain iron.

It has surprisingly been found that, by means of the herein described austenitic nickel-based alloy, a very high corrosion resistance may be achieved while at the same time avoiding a high amount of intermetallic phase formation during solidification from (primary) melting. The austenitic nickel- based alloy further possesses mechanical properties making it a suitable candidate for replacing previously known nickel-based alloys. Furthermore, the herein described austenitic nickel-based alloy may be produced by conventional metallurgical processes and does therefore not require remelting to obtain the desired properties. By the term "conventional metallurgical processes" is meant a process containing the steps of melting, casting, hot working and cold working, but which does not include special steps, for example remelting and long diffusion heat treatment operations.

The austenitic nickel-based alloy according to the present disclosure has the following composition, in percent by weight (wt.-%):

C equal to or less than 0.05,

Si equal to or less than 1.0,

Mn 0.5 - 1.5,

P equal to or less than 0.03,

S equal to or less than 0.03,

Cr 31.5 - 36.0,

Mo 7.0 - 10.0,

N 0.08- 0.18,

Cu equal to or less than 0.4,

Fe 2.0 - 6.5, optionally Al up to 0.30, optionally one of Ca or Mg each up to 0.05 or REM up to 0.5, optionally up to 0.0050 of B, balance Ni and normally occurring impurities up to at most 1.5 wt.-% in total; wherein the composition fulfils the requirement of Equation 1: E_Ni > 1.864* E_& - 24.0 (Equation 1) wherein

E& = [wt.-% Cr]+[wt.-% Mo] + 1.5*[wt.-% Si], and

E_Ni = [wt.-% Ni]+30*[wt.-% C]+30*[wt.-% N]+0.5*[wt.-% Mn]+0.5*[wt.-% Cu],

As evident from the above, the herein described austenitic nickel-based alloy comprises a relatively high amount of iron compared to previously known nickel-based alloys designed for highly aggressive environments, such as G-35. It has surprisingly been found that, by selecting the composition so as to fulfil the requirement of Equation 1 described above, a good thermal stability is obtained as a result of a considerably reduced amount of intermetallic phases formed during solidification despite the relatively high iron content. Allowing a higher amount of iron in the alloy, compared to previously known nickel-based alloys designed for highly aggressive environments, has the advantage of enabling a considerable reduction in the manufacturing costs. For example, the alloy may be produced in a facility otherwise used for production of steels without requiring for example remelting or other specific process steps adding to the manufacturing costs.

According to embodiments, the composition of the austenitic nickel-based alloy may further fulfill the requirement of PRE being equal to or higher than 61.0, wherein PRE = [wt.-% Cr]+3.3*[wt.-% Mo]+ 16*[ wt.-% N], To further improve the pitting corrosion resistance, the composition of the austenitic nickel-based alloy may fulfil the requirement of PRE being equal to or greater than 63.0.

In the following, the importance of the different alloying elements of the austenitic nickel-based alloy will be briefly discussed. All percentages for the chemical composition are given in weight-% (wt.-%), unless explicitly disclosed otherwise. Upper and lower limits of the individual elements of the composition can be freely combined within the broadest limits set out in the claims, unless explicitly disclosed otherwise. Carbon (C): equal to or less than 0.05 %

Carbon is not a purposively added element in the herein described alloy but may be present as a result of the raw material and process used. Too high carbon contents may however lead to precipitation of chromium carbides in the grain boundaries, which in turn may lead to a reduction in corrosion resistance. Therefore, the carbon content of the herein described alloy is equal to or less than 0.05%. The risk for formation of chromium carbides is further reduced with reduced carbon content. Thus, carbon may alternatively be present in an amount of equal to or less than 0.03%.

In view of carbon merely constituting an impurity in the herein described alloy, there is no critical lower limit for the presence thereof. However, carbon may in some cases be present in an amount of at least 0.005%.

Silicon (Si): equal to or less than 1.0 %

Silicon is an element which is frequently added as a deoxidation agent. However, excessive contents of silicon may lead to precipitation of unwanted intermetallic phases, such as sigma phase, and may reduce hot workability. Therefore, the silicon content of the herein described alloy is equal to or less than 1.0%. In order to further reduce the risk for formation of intermetallic phase, the silicon content may be equal to or less than 0.50%, or even equal to or less than 0.30%.

Although lower contents are also possible, the silicon content of the herein described alloy may according to one alternative be equal to or more than 0.10%. According to embodiments, Si may be in the range of 0.10 - 0.50%, such as 0.10 - 0.30%.

Manganese (Mn): 0.5 - 1.5 %

Manganese is an element which may be used for binding sulfur by formation of MnS and thereby increasing hot ductility of the alloy. Mn is also an effective deoxidizing agent. Furthermore, Mn is an austenite stabilizing element. Mn may also contribute to increased deformation hardening during cold working. Therefore, the herein described alloy comprises equal to or more than 0.5% of Mn. To further increase the deformation hardening during cold working, Mn may according to embodiments be present in an amount of equal to or more than 0.60%, or even equal to or more than 0.85%. However, too high amounts of Mn may reduce the hot workability of the alloy as well as reduce the strength. Therefore, the herein described alloy comprises equal to or less than 1.5 % Mn. According to embodiments, Mn may be present in an amount of equal to or less than 1.30%, such as 1.20%.

Phosphorus (P): equal to or less than 0.03%

Phosphorus is not a purposively added element but may be present as an impurity. It is well known that P, if present in too high amounts, has a negative effect on hot workability and resistance to hot cracking. In the herein described alloy, P may therefore be present in amounts of equal to or less than 0.03% without substantially affecting the properties negatively.

Sulfur (S) : equal to or less than 0.03%

Like phosphorus, sulfur is an impurity element in the herein described alloy which may deteriorate the hot workability if present in too high amounts. S may be allowed in contents of equal to or less than 0.03% without substantially affecting the properties of the alloy negatively. Lower amounts of S are beneficial, and the S content may therefore alternatively be limited to equal to or less than 0.02%.

Chromium (Cr): 31.5 - 36.0 %

Chromium is a very important element as it contributes to the corrosion resistance, both in terms of pitting corrosion resistance and crevice corrosion resistance. Furthermore, Cr contributes to general corrosion resistance as well as stress corrosion cracking resistance. To achieve the desired corrosion resistance, the herein described alloy comprises equal to or more than 31.5% Cr. For the purpose of further increasing the corrosion resistance, the alloy may according to embodiments comprise equal to or more than 32.0% Cr, or equal to or more than 32.5% Cr.

However, too high contents of chromium increase the risk of formation of intermetallic phases, such as sigma phase, which in turn may lead to deterioration of corrosion resistance as well as reduced ductility. Therefore, the chromium content of the herein described alloy is equal to or less than 36.0%. According to embodiments, the chromium content is equal to or less than 35.0%. Molybdenum (Mo): 7.0 - 10.0 %

Molybdenum is an element which is effective in stabilizing the passive film formed on the surface of the austenitic nickel-based alloy, and which has a significant contribution to improved pitting corrosion resistance. Mo may also contribute to increased resistance to crevice corrosion. To obtain the desired corrosion resistance, the herein described alloy comprises equal to or more than 7.0% Mo. To further improve the corrosion resistance, the alloy may comprise equal to or more than 7.5% Mo, or even equal to or more than 8.0% Mo.

However, too high amounts of Mo may have a negative effect on hot workability. Furthermore, too high contents of molybdenum increase the risk of formation of intermetallic phases, such as sigma phase, which in turn may lead to deterioration of corrosion resistance as well as reduced ductility. Therefore, the herein described alloy comprises equal to or less than 10.0% Mo. According to embodiments, the herein described alloy may comprise equal to or less than 9.5% Mo, or equal to or less than 9.0% Mo.

Nitrogen (N): 0.08 - 0.18 %

Nitrogen is an effective element for increasing strength by solution hardening and may also improve structure stability. Furthermore, nitrogen also contributes to pitting corrosion resistance. Nitrogen may also contribute to deformation hardening during cold working of the alloy. In order to achieve the desired contribution to strength and pitting corrosion resistance, the alloy comprises equal to or higher than 0.08% of N. To further improve the strength and pitting corrosion resistance, the alloy may according to embodiments comprise equal to or higher than 0.10% of N.

However, if present in too high amount, nitrogen may have a negative impact on the hot workability and may also lead to formation of chromium nitrides, that adversely affects mechanical properties and corrosion properties. Therefore, the alloy comprises equal to or less than 0.18% of N. The present alloy may comprise equal to or less than 0.17% of N. The present alloy may comprise equal to or less than 0.15% of N. Copper (Cu): equal to or less than 0.4 %

Copper may be added to the herein described alloy for the purpose of improving corrosion resistance in for example sulfuric acid or phosphoric acid. Copper may alternatively be present as a result of the raw material used for making the alloy. Removing copper from austenitic alloys is extremely difficult once it is present, and if such alloys are used as raw material in the production of the herein described alloy, copper will inevitably be present. High amounts of Cu may risk leading to a reduced hot workability of the alloy. Therefore, the Cu content is equal to or less than 0.4%.

Moreover, high amounts of Cu are not desired since it may present a problem when seeking to recycle the herein described alloy due to the above-described difficulty in removing said element. Therefore, the upper limit of Cu may alternatively be equal to or less than 0.25%.

Copper may for example be present in an amount of at least 0.01%, such as at least 0.02%, such as at least 0.05%. The corrosion resistance of the alloy may be further improved if Cu is present in an amount of equal to or above 0.10%.

Iron (Fe): 2.0 - 6.5 %

Iron is not an important element for obtaining the desired properties of the alloy, such as corrosion resistance, and is normally considered as an unwanted element in nickel-based alloys intended for use in highly corrosive environments. However, in order to allow a cost-efficient manufacturing process while still obtaining a desired thermal stability and corrosion resistance, the herein described alloy comprises equal to or more than 2.0% of Fe. As previously mentioned, such a high presence of iron may be allowed in the herein described alloy in view of the composition fulfilling the requirements of Equation 1. Allowing higher amounts of Fe in the alloy would further facilitate the production. Therefore, the iron content of the alloy may according to embodiments be equal to 2.5%, or advantageously equal to or more than 3.0%.

Hence, the alloy as defined hereinabove or hereinafter contains iron primarily in order to be able to manufacture the present alloy in a cost-efficient manner. However, too high contents of iron would lead to a reduction of the alloying elements contributing to the desired corrosion resistance. Furthermore, too high contents of iron would increase the risk of formation of intermetallic phases, such as sigma phase, which in turn may lead to deterioration of corrosion resistance as well as reduced ductility. Such an increased risk of formation of intermetallic phases would be a result of higher amounts of iron inherently leading to a reduction of the amount of nickel present in the alloy. The amount of nickel in the alloy has a considerable impact on the risk of formation of intermetallic phases, as evident from the requirement of Equation 1 described above. Therefore, the herein described alloy comprises equal to or less than 6.5% Fe. According to embodiments, iron is present in an amount of equal to or less than 6.0%, or even equal to or less than 5.5%.

Aluminum (Al) optionally up to 0.30%

Aluminum may be added during a manufacturing process for producing austenitic alloys, for example for the purpose of deoxidation. The herein described alloy may for example comprise Al in an amount of up to 0.30%, such as up to 0.10%. However, Al may in some cases be present in an amount of at least 0.001%.

Calcium (Ca or Magnesium (Mg) or REM: optionally Ca or Mg up to 0.05% or REM up to 0.5 Calcium and magnesium and REM are also examples of elements that may be added during a manufacturing process, for example for the purpose of improving hot ductility or machinability. The herein described alloy may optionally comprise either Ca or Mg in an amount of up to 0.05%. Alternatively, the herein described alloy may optionally comprise REM in an amount of up to 0.5%. However, these elements may in some cases be present alone in an amount of at least 0.0005%.

Boron (B): optionally equal to or less than 0.0050 %

Yet another example of an element which may be added in the manufacturing process is boron (B) which is an element that may sometimes be used in austenitic alloys as a grain refiner or to improve the hot ductility. The herein described alloy may comprise B in amounts of equal to or less than 50 ppm, or equal to or less than 30 ppm. However, boron may in some cases be present in an amount of at least 0.0005%. Normally occurring impurities: up to at most 1.5 % in total

The herein described alloy may, in addition to the elements already specified and discussed above, comprise up to at most 1.5% in total of normally occurring impurities. In the present disclosure, normally occurring impurities are considered to be impurities resulting from the manufacturing process and/or the raw material used. Normally occurring impurities is herein intended to encompass both impurities and trace elements. The amount of normally occurring impurities may according to embodiments suitably be equal to or less than 1.0% in total.

For example, the herein described alloy may comprise normally occurring impurities belonging to the group consisting of niobium (Nb), titanium (Ti), tantalum (Ta), zirconium (Zr) and vanadium (V). The content of element belonging to said group may in the present alloy be limited to at most 0.2 wt.-% each, and equal to or less than 0.5 wt.-% in total, to reduce the risk for formation of unintended intermetallic phases. To further reduce the risk of formation of unintended intermetallic phases, the elements belonging to the group consisting of niobium (Nb), titanium (Ti), tantalum (Ta), zirconium (Zr) and vanadium (V) may be present in an amount of at most 0.1% each, and equal to or less than 0.3% in total.

Furthermore, the herein described alloy may additionally, or alternatively, comprise cobalt (Co) and/or tungsten (W) as impurity elements. Co and W are examples of elements which are often present in austenitic alloys and may therefore be present in the raw material used for manufacturing the herein described alloy. The normally occurring impurities belonging to the group consisting of Co and W may be limited to at most 0.5% each, or at most 0.3% each.

One additional example of an impurity that may be present is tin (Sn), which may be present in an amount of equal to or less than 0.1%.

Nickel (Ni)

As previously mentioned, the herein described alloy comprises nickel as a balance. Nickel may for example be present in an amount of at least 50%. To further improve the properties, nickel may be present in an amount of at least 51.0%. Method of production

The herein described austenitic nickel-based alloy may be produced by conventional metallurgical methods without the need for e.g., remelting, and processed to desired product form depending on the intended application of use.

More specifically, the herein described nickel-based alloy may be produced by providing a melt having the composition described above followed by casting of said melt to a cast alloy. The melt having the intended composition may for example be produced in a process using EAF (electric arc furnace) followed by AOD (Argon Oxygen Decarburization) and optionally final adjustments. The melt may for example be cast into an ingot, a billet, or a bloom.

If desired, the cast alloy may thereafter be subjected to a heat treatment. For example, the cast alloy may optionally be subjected to homogenization.

The cast alloy may thereafter be subjected to hot working. Said hot working may in some cases be performed to final intended product form. However, in most cases hot working is performed to an intermediate product form. An intermediate product form is here intended to mean a product form which is intended to be subjected to cold working and/or other process steps to arrive at intended final product form. Hot working may be performed by forging and/or rolling and/or extrusion.

The hot worked alloy may thereafter optionally be subjected to a heat treatment. Such a heat treatment may for example be an annealing for the purpose of recrystallisation and/or solution treatment.

Thereafter, the hot worked (and optionally heat treated) alloy may be subjected to cold working. For example, cold working may be performed by pilgering, rolling or drawing. According to one alternative, cold working may be performed to final intended product form.

The cold worked alloy may optionally be subjected to a heat treatment, if desired. For example, the cold worked alloy may be subjected to solution annealing or stress relieving. Depending on the intended use of the herein described alloy, it may after cold working and, where applicable, the subsequent heat treatment be subjected to further steps. Examples of such steps include, but is not limited to, straightening and/or machining.

It should be noted that the term "heat treatment" as used above may comprise a single or a plurality of consecutive heat treatment steps without departing from the present disclosure.

A final intended product form may for example be a bar, a tube, a pipe, a rod, a strip, a plate, a sheet, a hollow, a fitting, a flange, or a wire, although other products forms are also plausible. For many of the intended applications of the herein described alloy, the product form may be selected from the group consisting of tube, bar and welding wire. Such a welding wire may for example be used in a welding process for joining other components comprising the herein described alloy.

Properties

The herein described austenitic nickel-based alloy comprises less than 0.8 vol-% of intermetallic phase after solidification. This is a result of the composition of the austenitic nickel-based alloy, including both the specified ranges for the individual elements as well as the composition fulfilling the requirement of Equation 1 described above. In fact, it has been found that an intermetallic phase content of equal to or less than 0.6 vol.-% may be achieved despite the high content of alloying elements of the herein described alloy.

Furthermore, the herein described alloy may demonstrate a critical pitting temperature (CPT) of at least 88 °C, such as at least 92 °C, determined according to ASTM G150 with 4.5 M MgCh pH 5 as electrolyte and a potential of 600 mV vs SCE (Saturated Calomel Electrode). The CPT typically increases with increasing PRE and it has been found that in case the composition of the herein described alloy is selected such that PRE is equal to or higher than 63.0, a CPT of equal to or higher than 98°C determined according to ASTM G150 with 4.5 M MgCh pH 5 as electrolyte and a potential of 600 mV vs SCE (Saturated Calomel Electrode) may be obtained. The pitting temperature was measured on samples which were wet ground on all surfaces with a P600 paper. Furthermore, the relatively high amount of iron of the herein described alloy allows for a considerably more cost-efficient manufacturing method compared to previously known nickel- based alloys designed for highly corrosive environments. In fact, it may be produced by conventional metallurgical processes and in a facility where iron contamination is likely to occur because the same facility is used for stainless steel production.

Moreover, the herein described austenitic nickel-based alloy has a proof stress (Rp0.2) of more than 320 MPa and an elongation of more than 35%, determined according to ISO 6892-1 at room temperature, in solution annealed condition. In fact, it has been found that it is possible to obtain a proof stress of more than 350 MPa and an elongation of more than 50% in solution annealed condition.

Experimental results

Ten different experimental alloys, numbered A-J below, were produced by melting in a high frequency induction furnace to obtain melts of 270 kg each. The melts were cast into ingots in 9" diameter molds. After solidification of the melts, the molds were removed, and the ingots were quenched in water. For the purpose of comparison, an Alloy X, intended to correspond to G-35 (UNS N06035), was produced in the same manner. The compositions of the produced experimental alloys A-J and Alloy X are given in Table 1.

For the purpose of investigating the amount of intermetallic phase formed during solidification, samples were taken from the upper part of the ingots and were metallography prepared and etched using Murakami etchant. Light optical microscopy (LOM) and image analysis were used to investigate the occurrence of intermetallic phase. The percentage (%) of intermetallic phase were measured with image analysis applied on a total of 20 randomly chosen image fields in magnification 200 times.

The results of the investigation of amount of intermetallic phases formed during solidification are presented in Table 1 and shown in Figure 1. As evident from the results, the amount of intermetallic phase rapidly increases when 1.864*Ecr-E_Ni is above 25. Therefore, in order to obtain an amount of intermetallic phase of less 0.8 vol.-%, 1.864*1.864*Ec_r-ENi should be equal to or less than 24.0. In fact, when the criterion of Equation 1 is fulfilled, the amount of intermetallic phase formed during solidification may be equal to or less than 0.6 vol.-% as demonstrated by the results.

Table 1 Chemical composition of the heats

* Comparative, outside claimed scope

After sample removal for investigating amount of intermetallic phase formed during solidification, the ingots were hot forged, hot rolled and thereafter cold rolled to plates of a final thickness of 7 mm. The cold rolled 7 mm plates were thereafter solution annealed at a temperature between 1175 °C and 1200 °C.

Forthe purpose of comparison of corrosion resistance, the commercially available Alloy 625 was also obtained and tested in the corrosion tests described below. Pitting corrosion resistance of Alloys A-G and J as well as Alloy X, all in solution annealed condition, was tested. More specifically, critical pitting temperature (CPT) was determined according to ASTM G150 with 4.5M MgCh pH 5 as electrolyte and a potential of 600 mV vs. SCE (Saturated Calomel Electrode). For comparison, also Alloy 625 (UNS N06625) was tested in solution annealed condition according to the same procedure.

The resistance to general corrosion in sulfuric acid, in solution annealed condition, was tested for the experimental alloys No. A-G and J. Also, Alloy 625 and Alloy X, in solution annealed condition, were tested according to the same procedure for comparison. General corrosion was determined according to ISO 18069 with 20% H2SO4 and at a temperature of 93°C where activation of the specimens in 26% hydrochloric acid at room temperature prior each test period was used. The results of the corrosion resistance testing described above are shown in Table 2. Furthermore, the results are illustrated in Figure 2 showing the critical pitting temperatures for the tested alloys, and Figure 3 showing the general corrosion for the tested alloys.

Table 2 Results of the corrosion resistance testing

* Comparative, outside claimed scope As evident from the results presented in Table 2, the experimental alloys according to the present disclosure demonstrate a CPT of above 92°C. This is considerably higher than achieved for the tested Alloy 625 as well as for Alloy X (which corresponds to G-35).

Furthermore, it can be realized that the resistance to pitting corrosion is higher for the alloy according to the present disclosure compared to the experimental results disclosed in WO 2019/224287 in view of being tested with a higher concentration of MgCh in the electrolyte while still demonstrating a similar or higher critical pitting temperature.

Furthermore, the results presented in Table 2 demonstrate that when the composition is selected so that PRE is equal to or above 63.0, a CPT of at least 98°C may be achieved.

It can also be seen from the results presented in Table 2 that the experimental alloys according to the present disclosure has considerable higher resistance to general corrosion in sulfuric acid compared to Alloy 625. The resistance to general corrosion is also considerably better than for Alloy X (corresponding to G-35), exceptfor Alloy A which demonstrate a corrosion resistance comparable with that of Alloy X.

Moreover, tensile testing of the Alloys A-J and X, in solution annealed condition, was performed according to ISO 6892-1 at room temperature. The result of the tensile testing is presented in Table 3.

Table 3 The result of the tensile testing

* Comparative, outside claimed scope

As evident from the results presented in Table 3, all experimental alloys demonstrate a proof stress Rp0.2 of more than 320 MPa and an elongation well above 35 % in the solution annealed condition. This demonstrates that the alloy further possesses mechanical properties making it a suitable candidate for replacing previously known nickel-based alloys adapted for use in highly corrosive environments. In fact, the results demonstrate that it is possible to obtain a proof stress of more than 350 MPa and an elongation of more than 50% in solution annealed condition.

Claims

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

C equal to or less than 0.05,

Si equal to or less than 1.0,

Mn 0.5 - 1.5,

P equal to or less than 0.03,

S equal to or less than 0.03,

Cr 31.5 - 36.0,

Mo 7.0 - 10.0,

N 0.08 - 0.18,

Cu equal to or less than 0.4,

E_Ni > 1.864* E_& -24.0 (Equation 1) wherein

E& = [wt.-% Cr]+[wt.-% Mo] + 1.5*[wt.-% Si], and

E_Ni = [wt.-% Ni]+30*[wt.-% C]+30*[wt.-% N]+0.5*[wt.-% Mn]+0.5*[wt.-% Cu],

2. The nickel-based alloy according to claim 1, wherein Ni is present in an amount of at least 50 wt.-%.

3. The nickel-based alloy according to any one of claims 1 or 2, wherein Cr is present in an amount of 32.0 - 35.0 wt.-%.

4. The nickel-based alloy according to any one of the preceding claims, wherein Mo is present in an amount of 7.5 - 9.5 wt.-%.

5. The nickel-based alloy according to any one of the preceding claims, wherein N is present in an amount of 0.10 - 0.17 wt.-%.

6. The nickel-based alloy according to any one of the preceding claims, wherein Fe is present in an amount of 2.5 - 6.0 wt.-%.

7. The nickel-based alloy according to any one of the preceding claims, wherein Mn is present in an amount of 0.60 - 1.30 wt.-%.

8. The nickel-based alloy according to any one of the preceding claims, wherein Cu is present in an amount of 0.01 - 0.25 wt.-%.

9. The nickel-based alloy according to any one of the preceding claims, wherein Si is present in an amount of equal to or less than 0.50 wt.-%.

10. The nickel-based alloy according to any one of the preceding claims, wherein normally occurring impurities belonging to the group consisting of Nb, Ti, Ta, Zr and V are limited to at most 0.2 wt.-% each, and equal to or less than 0.5 wt.-% in total; and/or normally occurring impurities belonging to the group consisting of Co and W are limited to at most 0.5 wt.-% each, or at most 0.3 wt.-% each.

11. The nickel-based alloy according to any one of the preceding claims, wherein the composition fulfills the requirement of PRE being equal to or higherthan 61.0, wherein PRE = [wt.-% Cr]+3.3*[wt.-% Mo]+ 16*[ wt.-% N],

12. Use of the nickel-based alloy according to any one of the preceding claims in a component used within the chemical process industry and/or a component subjected to seawater cooling; optionally wherein said component is a heat exchanger component, process tubing component or a piping component.

13. A method for producing an object of the nickel-based alloy according to any one of claims 1 to 11, said method comprising: casting a melt having the composition, thereby obtaining a cast alloy, optionally heat treating the cast alloy, - hot working of the cast alloy,

- optionally heat treating the hot worked alloy, optionally cold working the hot worked alloy, and optionally heat treating the cold worked alloy.

14. The method according to claim 13, wherein the object is selected from the group consisting of a bar, a tube, a pipe, a rod, a strip, a plate, a sheet, a hollow, a fitting, a flange, or a wire.