BRPI0922060B1 - Precipitation hardenable nickel-based alloy, powder of this alloy, solid component comprising this powder and the use of said alloy - Google Patents

Abstract

PRECIPITATION HARDENABLE NICKEL BASED ALLOY, POWDER OF THIS ALLOY, SOLID COMPONENT INCLUDING THIS POWDER AND THE USE OF SUCH ALLOY. The present invention relates to a nickel-based alloy, for use at high temperatures, wherein the alloy comprises in % by weight: - C: 0.05-0.2; - Si: maximum of 1.5; - Mn: maximum of 0.5; - Cr: 15-20; - Al: 4-6; - Fe: 15-25; - Co: maximum of 10; - N: 0.03-0.15; - O: maximum of 0.5; - one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb: 0.25-2.2; - one or more elements selected from the group consisting of rare earth metals (REM): maximum 0.5; - Ni balance and normally occurring impurities.

Description

The present invention relates to a nickel-based alloy, designed for use at high temperatures, for example, above 900°C. Specifically, the present invention relates to a precipitation hardenable nickel-based alloy, having as another component aluminum, which enables the formation of a stable aluminum oxide on the surface, thereby providing the alloy with satisfactory resistance to corrosion. oxidation. Furthermore, the present invention relates to a powder of said nickel-based alloy and the use of said alloy.

Background of the Technique

Aluminium-alloyed nickel-based alloys are used in a variety of high-temperature applications, such as heat treating furnaces, as they form a stable aluminum oxide shield on the surface. Aluminum oxide normally has very good adhesion and does not tend to crack or fall off the surface. In addition, aluminum oxide has a low growth rate at high temperatures. Therefore, this type of alloy normally presents a satisfactory resistance to oxidation.

Aluminum oxide-forming nickel-based alloys are known to be difficult to manufacture, especially when hot processed. A factor that strongly contributes to this is the intermetallic phase Y' (NiβAl), which is formed at temperatures below approximately 900°C, during slow cooling and/or heating, such as, for example, during heat treatments or during processing at warm. This intermetallic phase makes the alloy hard and brittle, consequently difficult to process. The precipitation of y' also reduces the activity of aluminum in the alloy, thereby making the formation of protective aluminum oxide more difficult.

An example of an aluminum oxide-forming nickel-based alloy is disclosed in US patent document 4,882,125. The alloy comprises 27-35% Cr, 2.55% Al and 2.5-6% Fe. It is disclosed that high aluminum contents reduce the toughness of the material and that the Al content must be at least 2.75% in order to generate satisfactory oxidation protection, but preferably not to exceed 4% in order to not deteriorate ductility. The patent further teaches that high Fe contents deteriorate oxidation properties, for which reason the iron content must not exceed 6%.

Another example of an aluminum oxide-forming nickel-based alloy is disclosed in US patent document 4,460,542. The alloy comprises 14-18% Cr, 4-6% Al and 1.5-8% Fe. That patent teaches that additions of 46% Al provide superior oxidation properties compared to nickel-based alloys that form chromium oxide on the surface. Also, in that patent, it is disclosed that Fe has a negative effect on the oxidation properties, for which reason the iron content must be at most 8%.

WO 2004/067788 A1 discloses yet another example of an aluminum oxide-forming nickel-based alloy. In this case, the alloy comprises 15-40% Cr, 1.5-7% Al and 0.5-13% Fe. Best results are obtained when the alloy comprises a maximum of 26.5% Cr, a maximum of 11% Fe and 3-6% Al.

WO 00/34541 A1 discloses a nickel-based alloy comprising 19-23% Cr, 3-4.4% Al and 18-22% Fe.

The alloy is designed for use at high temperatures. WO 00/34541 A1 discloses that the combination of 19-23% Cr and 3-4% Al is critical for the formation of protective scales of Al2O3-Cr2O3. The nickel-based alloy is hardened by precipitating 1 to 5 mol% of granulated Cr7C3, which is obtained by heat treatment for 24 hours. The alloy is produced by melting, e.g. by vacuum melting, casting and processing into standard construction formats such as rods, bars, etc. This alloy shows satisfactory oxidation resistance up to a temperature of 1000°C.

Also, iron-based aluminum oxide forming alloys are known. However, this type of alloy normally has low mechanical strength at high temperatures. Therefore, small particles are usually added to increase the flow resistance of the material. This is described, for example, in the publication “Metals Handbook”, 10a. edition, volume 2, page 943. Another problem that this type of alloy presents is that its ductility at room temperature is normally very low, which makes soldering more difficult. In order to obtain a reliable weld on a ferritic material, preheating of the material to be welded to a temperature of at least 200°C is normally required. In many cases, stress relief annealing at a temperature of 750850°C is also required after welding.

Summary of the Invention

The object of the present invention is to obtain an alloy with excellent oxidation resistance under high temperatures, specifically from about 900°C to at least about 1250°C, and which still has satisfactory hot machinability and, furthermore, a satisfactory flow resistance.

The aforementioned objective is achieved by means of a precipitation-hardening nickel-based alloy, comprising in percentage by weight: - C: 0.05-0.2; - Si: maximum of 1.5; - Mn: maximum of 0.5; - Cr: 15-20; - Al: 4-6; - Fe: 15-25; - Co: maximum of 10; - N: 0.03-0.15; - O: maximum of 0.5; one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb, wherein the total amount of that group is 0.25-2.2; one or more elements selected from the group consisting of rare earth metals (REM), wherein the total amount of this group is a maximum of 0.5; - balance of Ni and normally occurring impurities.

The nickel-based alloy according to the present invention is austenitic and exhibits satisfactory oxidation resistance, especially at high temperatures, for example above 900°C. Oxidation resistance is high even at temperatures of around 1100°C. Since the present alloy forms a stable aluminum oxide on the surface, it can still be used at temperatures above those where chromium oxide forming materials undergo intense oxidation, i.e. above approximately 1150°C.

It has been found that by adding relatively high Fe contents to a nickel-based aluminum oxide-forming alloy, it is possible to reduce the stability of the y' intermetallic phase, which in turn makes the alloy easier to manufacture. and process. A reduced stability of y' provides for a slower formation of such precipitations at a given cooling rate, which facilitates hot processing of the alloy. This also leads to a reduced risk of reduced Al activity, which in turn ensures that a stable, oxidation-resistant aluminum oxide can be formed on the surface of the alloy.

The nickel-based alloy according to the invention is more ductile at room temperature than known aluminum oxide-forming ferritic alloys. Therefore, preheating or holding the heated alloy prior to soldering is unnecessary and subsequent stress relief annealing can be avoided. The nickel-based alloy according to the invention, therefore, allows for an easier welding procedure, compared with ferritic aluminum oxide-forming alloys.

The nickel-based alloy according to the invention is precipitation hardenable. This is achieved by adding one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb. These elements form a precipitation hardenable particle with C and/or N and optionally added oxygen. The dispersion contributes to the mechanical strength and provides the alloy with excellent flow resistance, even at high temperatures, without impairing the hot machinability of the alloy.

The nickel-based alloy is produced using a powder metallurgy process. The metallurgical process of manufacturing the powder results in a material that quickly solidifies, in which the brittle phase does not have time to form and large variations in composition by segregation do not develop. A rapidly solidifying powder mixture will therefore provide a body of material with an essentially homogeneous composition and an essentially uniform distribution of very small particles of dispersion.

A powder produced from a nickel-based alloy will comprise precipitation-hardened particles as described above, which will provide a powder-produced product of excellent mechanical properties, especially at high temperatures. Furthermore, a nickel-based alloy powder makes it possible not only to manufacture traditional shapes such as tubes, rods, wire, plates and strips, but also to manufacture solid components with complex geometry. In addition, composite materials in which the nickel-based alloy is incorporated can be readily manufactured, if desired, for example, to produce a final product with a first load bearing component and a second corrosion resistant component.

The nickel-based alloy according to the invention is especially suitable for use at high temperatures, such as above 900°C and up to at least 1250°C, especially in applications where the mechanical load on the material can be become high. Furthermore, the alloy according to the invention is suitable for use in environments with high requirements for satisfactory oxidation resistance. Examples of suitable applications include building materials for heat treatment furnaces, in rolling mills for rolling hearth furnaces, in muffle tubes for annealing in a protective atmosphere, in building material for heating elements, in turbine combustion chamber material. in gas-gas type heat exchangers, e.g. in the glass making industry or gas turbines, in woven wire conveyor belts designed for heat treatment furnaces, in radiation tubes for heating furnaces heat treatment, or in thermocouple protection tubes.

Brief Description of Drawings

Figure 1a shows the result of a simulation of the effect of Ni content on phase stability at different temperatures.

Figure 1b shows the influence of varying levels of Al and Fe on the minimum stability of y'•

Figure 1c shows the influence of varying levels of Al and Cr on the minimum stability of y'•

Figure 2 shows the result of a simulation of the effect of Fe content on the stability of nickel aluminides•

Figure 3 shows the result of a simulation of the effect of Al content on the stability of nickel aluminides•

Figure 4 shows the result of a simulation of the effect of the Co content on the stability of nickel aluminides•

Figure 5 shows the result of tensile tests of examples of alloys according to the invention.

Figure 6 shows the deformation resistance of six different processing runs, according to the invention, at room temperature, at a temperature of 500° and at a temperature of 600°•

Figure 7 shows the tensile strength of six different processing runs according to the invention, at room temperature, at 500° and at 600°•

Figure 8 shows the elongation at fracture of six different processing runs according to the invention, at room temperature, at a temperature of 500° and at a temperature of 600°•

Figure 9 shows the results of oxidation tests in air, at a temperature of 1000°C, of eight different processing runs according to the invention, and two comparative materials.

Figure 10 shows the results of oxidation tests in air, at a temperature of 1100°C, of eight different processing runs according to the invention, and two comparative materials.

Figure 11 shows a photograph of the microstructure of the processing run (A) taken in SEM.

Figure 12a shows the size distribution of carbonitride precipitates in the processing run (A).

Figure 12b shows the size distribution of precipitates in the processing runs (A-D).

Figure 13 shows the result of flow tests of compositions that are not precipitation hardenable.

Figure 14 shows the result of air oxidation tests at 1100°C of four compositions which are not precipitation hardenable. Detailed Description of the Invention

As mentioned above, nickel-based alloys alloyed with aluminum are generally considered difficult to hot process. An important factor is that there is only a limited temperature interval between the melting of the alloy and the precipitation of unwanted intermetallic phases, such as nickel aluminides. Al and Cr alloying elements are beneficial for oxidation resistance, however, they make the nickel-based alloy difficult to process as it increases the stability of nickel aluminides and therefore reduces the temperature range for processing to league hot. The hot machinability of the alloy is a very important factor in enabling products from the alloy to be easily and economically produced. It has been found that the alloy of the present invention has an increased temperature range for hot processing, as a function of its composition, which provides the alloy with satisfactory hot machinability.

The present invention is based on the discovery that a relatively high addition of Fe to a nickel-based alloy with 4-6% Al and high Cr content reduces the stability of the intermetallic phase Y' • Precipitation of the Y' phase improves the low temperature yield strength, but makes production more difficult as the alloy becomes hard and brittle with too high Y' contents • In addition, the Y' phase reduces the Al activity in the alloy, which makes the formation of protective aluminum oxide on the surface more difficult. For an alloy designed for use at high temperatures, for example above 900°C, it is therefore important to reduce the content of the y' phase, which is obtained according to the alloy composition of the present invention.

Furthermore, y'-phase precipitations in previously known aluminum oxide-forming nickel-based alloys are not stable above temperatures of about 1000°, so their influence on yield strength is interrupted during the use of these above alloys. of that temperature. The alloy according to the present invention comprises a minimal content of y'-phase and is still primarily intended for use at high temperatures, where, consequently, there is a risk of dissolution of the y'-phase. In order to maintain yield strength, the alloy is therefore precipitation hardenable. This is mainly achieved by selected levels of carbon and nitrogen, and possibly even oxygen, in combination with selected levels of Ta, Zr, Hf, Ti and Nb. It is possible to produce the alloy using a conventional melt production process, but in this case precipitation hardening will be insufficient to obtain. Therefore, the alloy is produced using a powder metallurgy process. The solid components thereafter can be manufactured from the powder produced by compaction, in accordance with previously known techniques, such as hot isostatic pressing (HIP) or cold isostatic pressing (CIP). If necessary, the manufactured solid component can thereafter be further processed, for example by rolling, extrusion or drawing, in order to obtain the desired product shape. Also, it is possible to produce complex geometries directly from the powder through sintering.

The composition of the present alloy and the fact that it is precipitation hardenable resulted in a nickel-based alloy that has excellent oxidation resistance, even at high temperatures, such as at least 1100°C, being relatively easy to heat process and having satisfactory flow resistance.

According to a preferred embodiment of the precipitation hardenable nickel-based alloy according to the invention, the dispersed particles have an average diameter of less than 1 µm, preferably less than 500 nm. Best results are obtained when the dispersed particles have an average diameter of 50-200 nm.

According to a further preferred embodiment of the precipitation hardenable nickel-based alloy according to the invention, more than 85% of the dispersed particles must have a diameter of 300 nm or less.

The effect of the various elements on the properties of the alloy will be discussed below, where all grades indicated are expressed in percent by weight. Carbon

Free carbon will take up interstitial locations in the crystal structure and thereby block dislocation mobility at temperatures up to about 400-500°C. Carbon also forms carbides with other alloying elements such as Ta, Ti, Hf, Zr and Nb. In a microstructure of finely dispersed carbides, these carbides provide obstacles to dislocation motion and have an effect even at higher temperatures. Carbon is an essential element to improve the yield strength of the alloy, since dislocation mobility is the mechanism that generates yield elongation. However, very high carbon contents make the alloy difficult to cold process, due to the ductility deteriorated at lower temperatures, for example, below 300°C. Thus, the alloy comprises 0.05-0.2% carbon.

Silicon

Silicon may be present in the alloy with contents of up to 1.5%. Silicon with excessively high contents in nickel-based alloys can lead to an increased risk of nickel silicide precipitation, which causes the material to become brittle in this type of alloy. Flow test results of similar alloys showed that the yield life, ie the time to yield fracture, is reduced with silicon contents close to 1.5%. However, the reason for this is not known. Because of this, the Si content should preferably be at most 1%. According to a preferred embodiment, the alloy comprises only silicon impurity contents, i.e. up to 0.3%.

Manganese

Manganese is present in the alloy as an impurity. It is likely that up to 0.5% can be allowed without negatively influencing the properties of the alloy, so the alloy comprises a maximum of 0.5% manganese. According to a preferred embodiment, the alloy comprises only Mn impurity contents, i.e. up to 0.2%.

Chrome

Chromium is an element that for a long period of time has been the main element when creating a dense, protective oxide scale. Less than 15% Cr in an austenitic structure tends to provide an oxide that does not entirely cover the surface and is not dense, consequently providing insufficient oxidation resistance for the alloy. Also, there is a risk that the material closest to the oxide will be depleted of Cr, so that possible damage to the oxide may not be recovered, since there is not enough Cr to form a new oxide.

A nickel-based alloy comprising 4% Al should, however, comprise no more than about 20% Cr, as higher contents increase the risk of y' and β-phase formation. example, below, with reference to figure 1c, calculated for an alloy comprising about 19% Fe.

Therefore, in order to minimize the presence of y' and β phases, the alloy comprises a maximum of 20% Cr. Also, there may be the risk of formation of other undesired phases, such as ferrite rich in a and chromium phase, with excessively high levels of Cr. In addition, Cr can also stabilize nickel aluminides at high temperatures.

Therefore, the alloy comprises 15-20% Cr, preferably 17-20% Cr. Best results are achieved when the alloy comprises 17-19% Cr.

Aluminum

Aluminum is an element that generates a denser and more protective oxide scale compared to chromium. However, aluminum cannot replace Cr, since the formation of aluminum oxide is slower than that of chromium oxide at lower temperatures. The alloy comprises at least 4% Al, preferably more than 4% Al, which guarantees sufficient resistance to oxidation at high temperatures and that the oxide covers the surface entirely. The relatively high Al content provides excellent oxidation resistance, even at temperatures of around 1100°C. With Al contents above 6%, there is a risk of the formation of intermetallic phases in a nickel-based matrix, whereby the ductility of the material is considerably deteriorated (this will also be discussed below with reference to Figure 3). Therefore, the alloy should comprise 4-6% Al, preferably >4-5.5%, more preferably >4-5.2% Al.

Iron

It has been shown in accordance with the present invention that relatively high Fe contents in a nickel-based aluminum oxide-forming alloy can provide positive effects. The additions of Fe generate a metallic structure that is energetically unfavorable for the formation of the brittle phase y', which, in turn, considerably reduces the risk of the alloy becoming hard and brittle. Consequently, machinability is improved. Therefore, the alloy comprises at least 15% Fe. However, high Fe contents can lead to the formation of unwanted phases. Therefore, the alloy should not comprise more than 25% Fe.

Furthermore, with Fe contents of approximately 21-22%, there is a risk of the formation of a β (NiAl) phase, which, in some cases, can increase the property of the same becoming brittle. This, for example, is shown below with reference to figure 1b and figure 2.

Preferably, the alloy should comprise 16-21.5% Fe. According to a preferred embodiment, the alloy comprises 17-21% Fe.

Nickel

The alloy according to the invention is a nickel-based alloy. Nickel is an element that stabilizes an austenitic structure in alloys and, in this way, counterbalances the formation of some brittle intermetallic phases, such as the a-phase. The austenitic structure of the alloy is beneficial, for example, when it is used in welding. The austenitic structure also contributes to the satisfactory yield strength of the alloy at high temperatures. This could be a result that the diffusion rate is lower in an austenitic structure than in a ferritic structure.

In one embodiment, the alloy comprises 52-62% Ni, preferably 52-60% Ni.

Cobalt

In some commercial alloys, a part of the nickel is replaced by cobalt, in order to increase the mechanical strength of the alloy, which can be done in the alloy according to the invention. A part of the Ni in the alloy can be replaced by an equal amount of Co. This increases the stability of the BCC aluminide, NiAl, which then grows at the expense of the Y' phase, which can be advantageous in certain temperature ranges. This addition of Co, however, must be balanced against the oxidation properties, since the presence of NiAl will reduce the activity of Al and thereby deteriorate the ability to form aluminum oxide. The addition of Co will also affect the melting point of the alloy. Thus, for example, an addition of 10% Co will provide an alloy with NiAl precipitations, which are stable up to a temperature of 950°C, but below the melting point of approximately 20°C. In accordance with one embodiment of the present invention, nickel is therefore partially replaced by cobalt. However, the cobalt content must not exceed 10%. Nitrogen

Like carbon, free nitrogen takes up interstitial locations in the crystal structure and thereby blocks dislocation mobility at temperatures up to approximately 400-500°C. Nitrogen also forms nitrides and/or carbonitrides with other alloying elements such as Ta, Ti, Hf, Zr and Nb. In a microstructure where these particles are finely dispersed, they can provide obstacles to dislocation mobility, especially at higher temperatures. Therefore, nitrogen is added in order to improve the yield strength of the alloy. However, when nitrogen is added to aluminum alloys, the risk of secondary aluminum nitride formation is high and therefore the present nickel-based alloy has a very limited nitrogen content. The alloy comprises 0.03-0.15% N, preferably 0.05-0.15% N, more preferably 0.05-0.10% N. Oxygen

Oxygen may be present in the alloy of the invention, either as an impurity or as an active addition of up to 0.5%. Oxygen can contribute to increase the alloy's yield strength, forming small oxide dispersions, together with Zr, Hf, Ta and Ti, which when finely distributed in the alloy, improve its yield strength. These oxide dispersions have a higher dissolution temperature than the corresponding carbides and nitrides, so oxygen is a preferred addition for use at high temperatures. Oxygen can also form dispersions with Al, with the elements of Group 3 of the Periodic Table, Sc, Y and La, as well as with the fourteen lanthanides and in the same way as the elements mentioned above, contribute to a greater resistance to flow. of the league. According to a preferred embodiment, the alloy comprises 200-2000 ppm oxygen, preferably 400-1000 ppm oxygen. Tantalum, Hafnium, Zirconium, Titanium and Niobium

The elements of the group consisting of Ta, Hf and Zr form very small and stable particles with carbon and nitrogen. It is these particles that, if finely dispersed in the structure, help to block dislocation movement, thereby increasing the flow resistance, that is, providing precipitation hardening v. It is also possible to obtain this effect with the addition of Ti. Ti additions, however, can sometimes cause problems, especially during metallurgical production of alloy powder, as carbides and nitrides form already in the melt, prior to atomization, which in turn can clog the orifice during the atomization.

Niobium also forms stable dispersions with C and/or N and can therefore suitably be added to the alloy according to the invention.

The alloy comprises one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb, in a total amount from that group of 0.25-2.2%, preferably 0.3-1.5%, plus preferably 0.6-1.5%.

The alloy preferably comprises such an amount of the elements Ta, Zr, Hf, Ti and Nb that essentially all carbon and nitrogen are bonded to these elements. This ensures, for example, that the risk of the formation of chromium carbides during use at high temperatures of the alloy is significantly reduced.

According to a preferred embodiment, the alloy comprises 0.1-0.5% Hf. According to another embodiment, the alloy comprises 0.05-0.35% Zr. In yet another embodiment, the alloy comprises 0.05-0.5% Ta. According to another embodiment, the alloy comprises 0.05-0.4% Ti. According to yet another embodiment, the alloy comprises 0.1-0.8% Nb.

Rare Earth Metals (REM)

Rare earth metals (REM) refer in the present context to the elements of Group 3 of the Periodic Table, Sc, Y, and La, as well as the fourteen lanthanides. Rare earth metals affect the oxidation properties by doping the oxide formed. Excess alloying of these elements can provide an oxide that tends to crack the surface and too little addition of these elements tends to provide an oxide that weakens adhesion to the metal surface. The alloy may comprise one or more elements from the group consisting of REM, with a content of up to 0.5% in total, preferably 0.05-0.25%. In a preferred embodiment, yttrium is added to the alloy in an amount of 0.05-0.25%.

The nickel-based alloy according to the invention may also comprise normally occurring impurities as a result of the raw material used or the selected manufacturing process. Examples of impurities include Ca, S and P.

The precipitation hardenable nickel-based alloy exhibits satisfactory oxidation resistance, among other things, as a result of the Al and Cr contents. Also, the dispersion has satisfactory mechanical properties, such as yield and tensile strength, as well as ductility. The dispersion has satisfactory machinability, which makes it easy to manufacture products, for example, by hot extrusion or hot rolling.

The nickel-based alloy identified above is primarily intended for use at high temperatures. Examples of applications where the alloy is especially suitable include building materials for heat treating furnaces, rolling mills for hearth rolling furnaces, muffle tubes for annealing in a protective atmosphere, building material for heating elements, in combustion of gas turbines, in gas-gas heat exchangers, e.g. in the glass making or gas turbine industry, in tubular reactors in high-temperature processes, in woven wire conveyor belts designed for heat treatment, in radiation tubes for heating heat treatment furnaces, or in protection tubes for thermocouples.

Simulation

The stability of the phases at different alloy compositions and temperatures was studied by thermodynamic simulations using Thermo-Calc software. A thermodynamic database for nickel-based alloys, called “NiFe-Super, version 4”, was used for the simulations. It is commonly known that this type of calculation, in most cases, corresponds exactly to reality.

The influence of iron on the stability of nickel aluminides β (NiAl) and y' (NÍ3AI), and the stability of α (chromium-rich ferrite) was studied. Calculations were made for a chromium content of 18% by weight and aluminum content of 4.5% by weight. The result for a simulation in which the temperature and nickel content were varied is shown in figure 1. Along the X axis, iron is replaced by nickel in the alloy.

These simulations showed that there is an area for an alloy with 4.5 wt% Al and 18 wt% Cr, where the stability of the y' phase has a minimum value. This minimum is 58% by weight of nickel and an iron content of approximately 19% by weight, being marked in the figure by the dotted circle. Lower levels of Fe increase the stability of y', while higher levels provide the formation of β-phase nickel aluminide (NiAl). Compositions close to this minimum value provide a wide temperature range between alloy melting and nickel aluminides precipitation, therefore facilitating hot machinability as explained above.

The influence of variations in Al and Cr contents on the minimum value identified above was also studied. By varying the Al content between 4 and 6% and at the same time adjusting the Fe content so that the minimum y' stability is achieved, figure 1b can be calculated. Figure 1b shows how the minimum value can be moved when the Fe and Al contents are varied. The minimum value is moved along the line in the figure as the temperature changes. It is evident from the figure that increasing the Al content reduces the amount of Fe needed to obtain the minimum. Furthermore, the temperature rises to the minimum, from 814°C at marking (1) to 953°C at marking (9).

Figure 1c shows the same type of calculation as Figure 1b, but where the Cr and Al contents were varied and the Fe content was maintained at approximately 19%. It is evident from the figure that increasing the Al content reduces the amount of Cr needed to obtain the minimum value. Furthermore, the temperature rises from 815°C at marking (1) to 951°C at marking (10).

In figure 2, the influence of different iron contents on the stability of nickel aluminides, ferrite and austenite is shown. The composition was in that case 18% by weight of Cr, 4.5% by weight of Al, balance of Ni, with three different contents of iron, respectively, 16% by weight, 19% by weight and 22% by weight. The lowest dissolution temperature for nickel aluminides was obtained for the Fe content of 19%. For the higher Fe content, the β phase is stable, while the lower Fe content increases the stability of the Y' phase, which results in a higher dissolution temperature.

In figure 3, the influence of different Al contents on the stability of nickel aluminides and ferrite is shown. In that case, the composition was 18% by weight of Cr, 19% by weight of Fe, balance of Ni, with four different contents of Al, respectively, 4% by weight, 4.5% by weight, 5% by weight and 6% by weight. The increase in Al contents increases the dissolution temperature for nickel aluminides. When the Al content is 6%, the β intermetallic phase is stable, up to temperatures of about 1100°C. The increase in Al contents increases the stability of the ferrite in lower temperature ranges, below approximately 800°C. Cobalt Addition Effect Simulation

In order to investigate what effect cobalt might have on the alloy, simulations were performed using Thermo-Calc software. A thermodynamic database for nickel-based alloys, called “NiFe-Super, version 4” was used for the simulations. The calculations were made with the starting composition, containing 18% Cr, 19% Fe, 4.5% Al, balance of Ni. Nickel was replaced by 5, 10 and 15% Co in the starting composition and the precipitation balance fraction was calculated as a function of temperature.

The influence of Co on the stability of nickel aluminides β (NiAl) and y' (NÍ3AI), α (chromium-rich ferrite) as well as the o-phase were studied. The result is shown in figure 4.

Calculations show that additions of Co increase the dissolution temperature for nickel aluminides. Co additions also increase the stability of β nickel aluminide with respect to the y' phase. At the two highest levels of Co, there is a risk of o-phase precipitation at temperatures below approximately 650°C. Up to 10% by weight of Co can be used in the alloy for use at temperatures above 950°C.

Tensile Tests

A number of alloy compositions according to the invention were produced by powder metallurgy, then compacted by isostatic pressing, followed by hot extrusion and subsequent water quenching. The compositions of different processing runs are shown in Table 1. Table 1

The tensile tests of the compositions were carried out in accordance with the SS-EN 10002-1 Standard, at room temperature. Three samples of each composition were tested and the tensile test results, in the form of the average of the three samples, are shown in Table 2. However, processing run 1 was also tested directly after the HIP procedure (i.e., before the extrusion procedure). Table 2

The results show that the alloy according to the invention shows satisfactory elongation at fracture at room temperature during cold processing. In addition, the alloy has a yield strength that is greater than that of many austenitic steels and nickel-based alloys, which generally have a yield strength of approximately 200-300 MPa. The results can, for example, be compared with an austenitic chromium-nickel steel, with a nominal composition of 0.07 wt% C, 1.6 wt% Si, 1.5 wt% Mn, 25 wt%. by weight of Cr, 35% by weight of Ni, 0.16% by weight of N, 0.05% by weight of Ce and balance of Fe (corresponding to UNS S35315), which has a deformation resistance Rp0.2 of about 260 MPa, a tensile strength Rm of about 600 MPa and an elongation at fracture of about 35%. The results can be compared with precipitation hardenable aluminum oxide forming ferritic steel, known by the trade name KANTHAL APMT®, which has a nominal composition comprising 21 wt% Cr, 5 wt% Al, 3 wt% Al. Mo, maximum 0.7% by weight of Si, maximum of 0.4% by weight of Mn, maximum of 0.08% by weight of C, and which has a deformation resistance Rp0.2 of about 550 MPa , a tensile strength Rm of about 750 MPa and an elongation at fracture of about 25%.

In addition, tensile tests were carried out at temperatures of 500°C and 600°C, respectively, of the A-F processing runs shown in Table 1, in accordance with Standard SS-EN 10002-5. Three samples of each composition were tested, and the result of the tensile tests as an average of the three samples is shown in Table 3. The results of the tensile test at temperatures of 500°C and 600°C indicate that the alloy according to with the invention exhibits satisfactory mechanical properties at high temperatures, and satisfactory elongation at fracture at these temperatures. This, together with the successful results of hot extrusion and hot rolling, indicates that the alloy has satisfactory hot machinability.

The tensile test results of processing runs 1 and 2 are shown in figure 5 and the tensile test results of processing runs AF are shown in figures 6 to 8.Table 3

Impact Tests

Impact tests were performed on the material produced from the metal powders from the processing runs shown in Table 1. The samples were produced by hot isostatic pressing (HIP) and then hot extrusion and water quenching. The tests were carried out in accordance with the SS-EN 10045-1 Standard, at room temperature, and were carried out on each of the three samples of the compositions. The results are shown in Table 4.Table 4

The impact strength for all runs is well above 27 Joule, which is the value commonly used as a boundary between ductile material and brittle material.

Oxidation Test at 1000°C

Samples in the form of strips were produced from the processing runs shown in Table 1. The strips were subjected to a separation procedure with 220 µm paper. In addition, a sample of a nickel-based alloy known under the trademark SANDVIK SANICRO® 80 (corresponding to UNS N06003) and a sample of precipitation hardening aluminum oxide forming ferritic steel known under the trademark KANTHAL APMT (which has a nominal composition comprising 21% by weight Cr, 5% by weight Al, 3% by weight Mo, maximum 0.7% by weight Si, maximum 0.4% by weight Mn, maximum 0.08% by weight of C), were produced for comparison.

The oxidation test was carried out at a temperature of 1000°C in the open air. The samples were removed from the oven and cooled to room temperature, after 24, 48, 95, 186, 500 and 1005 hours, respectively, and weighed. After weighing, the samples were inserted into the oven to continue heating and oxidation. The results of the oxidation test are shown in figure 9.

The results show that the alloy according to the invention has satisfactory oxidation resistance at a temperature of 1000°C. All processing runs, except run D, showed considerably better oxidation resistance than the product SANDVIK SANICRO 80. In addition, the alloys according to the invention show an oxidation resistance at this temperature that compares with that of the product. KANTHAL APMT, which is an alloy considered to have excellent oxidation resistance.

The alloys according to the present invention quickly form a protective oxide, which after formation has a very low growth rate. Negative effects due to the high iron content, which had previously been reported in US Patents 4,882,125 and US 4,460,542, were not observed. It can be seen that most austenitic chromium oxide-forming alloys commonly used at high temperatures exhibit an oxide growth rate that is 4-8 times higher at this temperature.

Oxidation Test at 1100°C

The samples were produced from the same compositions and in the same manner as in the case of the oxidation test at 1000°C. The oxidation test, in this case, was carried out at a temperature of 1100°C in the open air. Samples were removed after 24, 48, 95, 186, 500 and 1005 hours, respectively, and then weighed. The results of the oxidation test are shown in figure 10.

The results show that the alloy according to the invention has satisfactory oxidation resistance at a temperature of 1100°C. The reference alloys used in this work, SANDVIK SANICRO 80 and KANTHAL APMT, are known to have excellent oxidation resistance for chromium oxide-forming and ferritic alumina-forming compounds, respectively. The oxidation test of the alloys according to the present invention shows, in general, better oxidation resistance than that of the SANDVIK SANICRO 80 product and even slightly better than that of the KANTHAL APMT product. All alloys tested show substantially better oxidation resistance than that of the alloy disclosed by patent document WO 00/34541. Tentative oxidation studies at a temperature of 1200°C indicate that the alloy according to the present invention shows a superior degree of oxidation resistance compared to the chromium oxide forming alloys SANDVIK SANICRO 80 and the aforementioned UNS S35315. This demonstrates that the addition of aluminum to the developed alloy increases oxidation resistance, especially at temperatures above 1100°C. microstructure

An example of the microstructure in a material with the composition according to the processing run (A), produced from metal powder compacted according to the HIP procedure, hot extruded and subjected to quenching with water, is shown in figure 11. The photograph was taken on a scanning electron microscope (SEM) with an amplitude of 30,000 times. The luminous precipitates seen in the microstructure are carbonitrides, containing mainly Hf, Ta and Zr.

An image analysis of about 10,000 carbonitride precipitates from the material shown in Figure 11 was performed using the SEM procedure. The mean diameter of the precipitates was about 130 nm. The frequency of carbonitride precipitates in different size ranges relative to image analysis is shown in figure 12a.

In addition, the size of precipitation hardenable precipitates in processing runs (B) to (D) was investigated. Figure 12b shows the relative frequency of particle diameter from processing runs (A) to (D). It is evident that the dispersions in all runs generally have a diameter of less than 300 nm.

Flow Tests of Processing Runs 1 and 2

Flow resistance tests for processing runs 1 and 2 were performed as shown in Table 1. Test samples were produced from metallic powder, which was compacted by the HIP procedure. During the flow test, threaded samples with a length of 35 mm and a diameter of 5 mm at the thinnest part were used. The tests were carried out at a temperature of 1200°C and a load of 4 MPa. The test was performed for double samples. Processing run 1, which comprises only a small content of precipitation hardenable particles, due to the low C content (0.05%) and only 0.395% Hf (without additions of Nb, Ti, Zr and Ta), showed a time to fracture of 358 and 387 hours, respectively, for the samples. However, processing run 2, which has a relatively high content of precipitation hardenable particles due to the relatively high content of C (0.14%) and a total of 1.148% of Zr, Ta and Hf, showed a time to fracture of 3064 and 4576 hours, respectively. The beneficial effect of precipitation hardening thus becomes clear from the results presented.

Flow Test of A-F Processing Runs

The test samples for the flow tests were produced from metallic powder, which was compacted by the HIP procedure, with subsequent hot extrusion, from 77 mm to 25 mm in diameter, followed by quenching with water. During the flow test, threaded samples with a length of 35 mm and a diameter of 5 mm at the thinnest part were used. The tests were carried out at a temperature of 1200°C, with a load of 5 MPa, and a temperature of 1000°C with a load of 15 MPa. Breakthrough time for different materials is shown in Table 5.Table 5

The results show how the material according to the invention has a higher yield strength compared to commercially available nickel-based alloys. Also, it is shown how the material according to the invention exhibits sufficient yield strength and oxidation resistance for practical use at temperatures exceeding 1200°C, unlike the vast majority of commercially available nickel-based alloys.

The high yield strength of the processing run (D) is believed to be a result of the high carbon content, as well as the high levels of Ti, Nb, Ta, HF and Zr.

Run Flow Tests that are not precipitation hardenable

A number of runs of experimental samples of approximately 1 kg size were produced by induction melting and casting under a protective argon atmosphere for comparison purposes. The processing runs were not precipitation hardenable as they were not produced using a powder metallurgical procedure. The compositions are shown in Table 6.

The materials produced were then transformed into rods, with a diameter of 15 mm and, after that, laminated at a temperature of 1200°C. The samples for the flow tests were produced from blanks that had been rolled with a cross section of 10 mm2. During the flow test, threaded samples with a length of 35 mm and a diameter of 5 mm at the thinnest part were used. Table 6

The flow test was carried out at a temperature of 1200°C and a load of 4 MPa. The result is shown in figure 13.

A comparison of the times to fracture, as shown in Figure 13, with those of the processing run 2 tests above, shows the beneficial effect on yield strength when the material was produced by a powder metallurgy procedure. Run 2 was tested under the same load and temperature as the comparative batches shown in Table 6 and showed times to fracture in excess of 3000 hours, while the comparative batches all fractured at times well below 500 hours.

Processing run 4249, which had a high content of C (0.13%) and a relatively high content of Ta+Zr+Hf (0.96%), still has a yield strength for fractures below 500 hours, while run 2, comprising approximately the same amount of C (0.14%) and a slightly higher content of precipitation hardening elements (1.148%) showed a time to fracture more than 6 times longer.

Oxidation Tests at 1100°C Temperature of Races that are not precipitation hardenable

Samples in the form of strips were produced from runs 4249, 4251, 4257, and 4258 and subjected to separation with 220 μm paper. The samples were tested for oxidation at 1100°C in open air. Samples were removed after 24, 48, 96, 186, 500 and 1000 hours, respectively, and then weighed. The results of the oxidation tests are shown in figure 14.

The results show that the alloy has a satisfactory oxidation resistance at a temperature of 1100°C. Since the oxidation properties of the material must be independent of precipitation hardening, the results indicate that precipitation hardenable alloys produced by the powder metallurgy procedure have the same compositions, i.e. with the alloy composition according to the present invention, should exhibit equally satisfactory oxidation resistance at that temperature.

Claims (19)

1. Precipitation hardenable nickel-based alloy characterized in that it comprises, in % by weight: - C: 0.05% - 0.2%; - Si: 0.04% - 1.5%; - Mn: 0.06% - 0.5%; - Cr: 15% - 20%; - Al: 4% - 6%; - Fe: 15% - 25%; - Co: 0.02% - 10%; - N: 0.03-0.15; - O: 200 ppm - 0.5%; one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb, wherein the total amount of this group is 0.25% - 2.2%; one or more elements selected from the group consisting of rare earth metals, consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb and Lu, in that the total amount of this group is 0.05% - 0.5%; Ni balance and normally occurring impurities. 2. Nickel-based alloy, according to claim 1, characterized in that the alloy comprises 16% - 21.5% by weight of Fe. 3. Nickel-based alloy according to any one of claims 1 and 2, characterized in that the alloy comprises 17% - 20% by weight of Cr, preferably 17% - 19% by weight of Cr. 4. Nickel-based alloy according to any one of claims 1 to 3, characterized in that the alloy comprises from 0.04% - 1% by weight of Si, preferably from 0.04% - 0, 3% by weight of Si. 5. Nickel-based alloy, according to any one of claims 1 to 4, characterized in that the alloy comprises one or more elements selected from the group consisting of rare earth metals, in a total content of 0.05 % - 0.25% by weight, preferably 0.05% - 0.25% by weight of Y. 6. Nickel-based alloy, according to any one of claims 1 to 5, characterized in that the alloy comprises one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb, in a total of 0.3% - 1.5%, preferably 0.6% - 1.5%. 7. Nickel-based alloy, according to any one of claims 1 to 6, characterized in that the alloy comprises 0.1% -0.5% by weight of Hf. 8. Nickel-based alloy, according to any one of claims 1 to 7, characterized in that the alloy comprises 0.05% - 0.35% by weight of Zr. 9. Nickel-based alloy, according to any one of claims 1 to 8, characterized in that the alloy comprises 0.05% - 0.5% by weight of Ta. 10. Nickel-based alloy, according to any one of claims 1 to 9, characterized in that the alloy comprises 0.05% - 0.4% by weight of Ti. 11. Nickel-based alloy, according to any one of claims 1 to 10, characterized in that the alloy comprises 0.1% - 0.8% by weight of Nb. 12. Nickel-based alloy according to any one of claims 1 to 11, characterized in that the alloy comprises > 4% - 5.5% by weight of Al, preferably > 4% - 5.2% in weight of Al. 13. Nickel-based alloy according to any one of claims 1 to 12, characterized in that the alloy comprises 200 - 2000 ppm O, preferably 400 - 1000 ppm O. 14. Nickel-based alloy according to any one of claims 1 to 13, characterized in that the alloy comprises 52% - 62% by weight of Ni, preferably 52% - 60% by weight of Ni. 15. Nickel-based alloy, according to any one of claims 1 to 14, characterized in that the alloy is essentially free of carbides of the form M7C3, where M is a metal. 16. Powder of a precipitation-hardening nickel-based alloy, as defined in claim 1, characterized in that it comprises, in % by weight: - C: 0.05% - 0.2%; - Si: 0.04% - 1.5%; - Mn: 0.06% - 0.5%; - Cr: 15% - 20%; - Al: 4% - 6%; - Fe: 15% - 25%; - Co: 0.02% - 10%; - N: 0.03-0.15; - O: 200 ppm - 0.5%; one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb, wherein the total amount of this group is 0.25% - 2.2%; one or more elements selected from the group consisting of rare earth metals, consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb and Lu, in that the total amount of this group is 0.05% - 0.5%; Ni balance and normally occurring impurities. 17. Solid component comprising an aluminum oxide-forming nickel-based alloy, wherein the nickel-based alloy is compacted from a powder, as defined in claim 16, characterized in that it comprises, in % by weight: - C : 0.05% - 0.2%; - Si: 0.04% - 1.5%; - Mn: 0.06% - 0.5%; - Cr: 15% - 20%; - Al: 4% - 6%; - Fe: 15% - 25%; - Co: 0.02% - 10%; - N: 0.03-0.15; - O: 200 ppm - 0.5%; one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb, wherein the total amount of this group is 0.25% - 2.2%; one or more elements selected from the group consisting of rare earth metals, consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb and Lu, in that the total amount of this group is 0.05% - 0.5%; Ni balance and normally occurring impurities. 18. Use of a nickel-based alloy, as defined in claim 1, characterized in that it is in products in the form of tubes, rods, strips, plates or wires. 19. Use of a nickel-based alloy, as defined in claim 1, characterized in that it is in construction material for heat treatment furnaces, for rolling mills for rolling hearth furnaces, in muffle tubes for annealing in a protective atmosphere, in construction material for heating elements, in gas turbine combustion chamber material, in gas-gas type heat exchangers, in high temperature process tubular reactors, in woven wire conveyor belts designed for heat treatment furnaces , in radiation tubes for heating heat treatment furnaces, or in protection tubes for thermocouples.

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