FIELD OF THE INVENTION The present invention relates to a Cu-base alloy, which is resistant or immune to carburization, metal dusting, coking, and resistant to oxidation at elevated temperatures and a method for its production. The invention relates also to the use of said alloy in construction components in CO-containing atmospheres, and/or hydrocarbon-containing atmospheres or solid-carbon-containing processes or processes that contain ammonia and/or other reactive nitrogen- compounds as well as products formed from such alloys.

BACKGROUND OF THE INVENTION The increasing demands on efficiency in the petrochemical industry lead to the development of new processes that require higher temperatures. These processes give rise to e. g. carburization, metal dusting, coking, and oxidation.

When exposed to increased temperatures i. e. temperatures above approximately 700°C presently used alloys are exposed for severe corrosion.

A possible solution for an alloy for use in environments which can give rise to the above-mentioned corrosion mechanisms, copper and its alloys could be an alternative, but a strong limitation for the use of copper or its alloys is their low melting point.

SUMMARY OF THE INVENTION It is therefor an object of the invention to provide a copper-based alloy with an increased melting point, more specificly a copper-base alloy with a melting point above 1000°C.

It is another object of the invention to provide a method for the production of a copper-based alloy with a melting point above 1000°C.

It is another object of the invention to provide a copper-based alloy for use as construction components in CO-containing atmospheres, and/or hydrocarbon- containing atmospheres or solid-carbon-containing processes or processes that contain ammonia and/or other reactive nitrogen-compounds as well as products formed from such alloys.

SHORT DESCRIPTION OF THE DRAWINGS Figure 1 shows the liquidus temperature (1) and solidus temperature (2) as a function of the molybdenum content for a Cu-8. 7AI bronze, which in the pure Cu-AI-system is called ß.

Figure 2 shows the liquidus temperature (1) and solidus temperature (2) as a function of the tungsten content for a Cu-8. 7AI bronze, which in the pure Cu-AI-system is called (3.

DETAILED DESCRIPTION OF THE INVENTION These objects are fulfilled with an alloy as described in the following having the following composition (in weight-%): Al 4-15 Si 0.1-6 Mo 0.5-40 W 0-40 Where the total of Mo and W does not exceed 40%, One or more of the group of Rare Earth Metals (REM), such as yttrium, hafnium, zirconium, lanthanum and/or cerium up to 0.3 weight-% of each element, or a total of maximum 3.0 weight-%.

Cu and normally occurring alloying additions and impurities balancing the alloy.

Below, the effects of different alloying elements in the corrosion resistant alloy are described and specified.

Aluminum Aluminum should be added for its capacity to form a protective alumina layer on the surface of the alloy in the temperature range of 300°C to 1300°C even in environments that solely contain trace of oxygen. Aluminum should be added in an amount up to 15 weight-%, preferably up to 13 weight-%, but not less than 4 weight-%.

Silicon Silicon can be used in order to promote the protective effect of aluminum in this type of alloy by forming aluminumsilicate, which has a higher formation rate compared to that of pure alumina. In this type of alloy the lower starting temperature for the formation of a protective oxide is favorable. Therefore silicon can be added to the alloy in order to improve the oxide formation at low temperatures. Thus, it is especially favorable for material which should be used in the temperature range of 300-900°C to be alloyed with silicon in a content of up to 6 weight-%, preferably up to 4 weight-%, most preferably between 1.5 weight-% and 4 weight-% and not less than 0. 1 %. If the alloy will be used at temperatures above 900°C, the content of silicon is favourable for the oxidation resistance, but also an alloy which does not contain silicon, forms a protective alumina and therefore the content of silicon should be up to 6 weight-%, preferably 0.1-3 weight-%.

Nickel, Iron, Cobalt and Manganese Transition metals, especially iron, nickel and cobalt, are known to have a strong catalytic effect on the formation of coke. The protecting capacity of the alumina layer, which will be formed on the surface of the alloy, however, allows that

proportionately high levels of these elements could be permitted, but not more than in a total of 5 weight-% of iron, manganese, nickel and cobalt.

Molybdenum Molybdenum can be used to stabilize the high temperature beta-phase in an aluminum-bronze to improve its mechanical strength at increased temperatures from approximately 600°C and increases respectively raises the melting point up to 1300°C, depending on the molybdenum content. Dependent on the content of Al a pure Cu-AI-alloy with the Al-content in the preferred range as mentioned above has a melting point between 1040° and 1070°C. Therefore, a Cu-based aluminum bronze in the beta phase, containing 0.5 to 40 weight-% of molybdenum will have a melting point between 1040°C to 1300°C depending on the molybdenum and the aluminum content. In order to increase the melting point of the alloy-system a molybdenum addition of e. g. 10 wt-% will give a melting point of 1100°C, while an addition of molybdenum of e. g. 22 wt-% will increase the melting point of a beta phase aluminum-bronze up to 1200°C, while an addition of molybdenum of e. g. 40 wt-% will result in a melting point of 1300°C. Molybdenum can partially or completely be replaced by tungsten. The total of Mo+W should not exceed 40 wt-%.

Tungsten In this alloy-system tungsten has the same properties as molybdenum in that sense it would stabilize the beta-phase and, thus, increase the melting point of the alloy. However, tungsten can replace molybdenum, even though its stabilizing effect is somewhat weaker. It could therefore be added in a content of 0 to 40 wt-%. The total of Mo + W should not exceed 40 wt-%.

Reactive Additions In order to further increase the oxidation resistance at higher temperatures, it is common practice to add a certain amount of reactive elements, such as Rare Earth Metals (REM), e. g. yttrium, hafnium, zirconium, lanthanum and/or cerium.

One or more of this group of elements should be added in an amount not

exceeding 1.0 weight-% per element. The total content of those elements should not exceed 3.0 weight-%, preferably not exceed 0.5 weight-%.

Copper The main component, which amounts to the balance of the alloy of the present invention, is copper. Previously it has not been possible to use copper or copper rich alloys in applications facing higher temperatures (>200°C), due to its high oxidation rate when in contact with oxygen rich atmospheres. The present invention will form a protective aluminum oxide at elevated temperatures in oxygen containing atmospheres. Copper is known to be resistant or immune to catalytic activity and coking, and therefore, the copper content should be kept as high as possible.

The alloy comprises up to 96 weight-% Cu, but at least 38 weight-% Cu, preferably at least 47 weight-%, most preferably at least 63 weight-% Cu.

It is clear to the person skilled in the art that a substitution of some of the Cu for Zn will only result in minor property changes for the alloy.

Further, the alloy comprises normally occurring alloying additions and impurities. These are defined as follows : Alloying additions Elements added for process metallurgical reasons, in order to obtain melt purification from e. g. S or O or added in order to improve the workability of the cast material. Examples of such elements are B, Ca, Mg. In order for such elements not to have a harmful effect on the properties of the alloy, the levels of each individual element should be less than 0. 1%. In addition, several of the elements previously mentioned, e. g. Al, Si, Ce, Fe and Mn can also be added for process metallurgical or hot workability reasons. The allowable concentrations of these elements are as defined in the previous sections.

Impurities Impurities refer to unwanted additions of elements from contaminants in the scrap metal used for melting or contamination from process equipment.

The alloy of the present invention will be produced by alloying a Cu-AI alloy with Mo and/or W in the above-described way and with the above-described contents of said elements.

The alloy of the present invention can be used as construction components in CO-containing atmospheres, and/or hydrocarbon-containing atmospheres or solid-carbon-containing processes or processes that contain ammonia and/or other reactive nitrogen-compounds as well as products formed from such alloys.

Examples of such high temperature processes are: steam reforming of natural gas, steam cracking of hydrocarbons to produce e. g. ethylene and propylene, annealing processes where cracked ammonia is used as shielding gas.

The alloy according to the present invention can be machined to construction material in the shape of tubes, pipes, plates, strip and wire or be used in the shape of coating on one or more surfaces of other commonly used construction materials in said shapes.

DESCRIPTION OF PREFERRED EMBODIEMENTS OF THE INVENTION EXAMPLE 1 : Coke formation at 1000°C in 83vol-% CO + 17vol-% H2 A laboratory exposure was performed in a tube furnace in a highly carburizing atmosphere. The relative tendency to coke formation at 1000°C was evaluated between a standard grade stainless steel and several Cu-base alloys. The chemical compositions of the materials investigated are given in Table 1.

Table 1 shows the chemical composition of the investigated materials, where Alloy 800HT is a comparative example. The examples 2 to 7 are Cu-based materials according to the present invention. All contents are given in wt-%.

Table 1 Example Cr Ni Fe Mo N Si Mn Co REM Ti Al Cu no. Alloy 20.4 30.10 0.05 0.009 0.73 0.53 0.5 0.5 <0.5 800HT 2 0. 5 3. 5 10. 5 Bal. 3 1. 1 10. 2 Bal. 4 4. 9 8. 8 Bal. 5 2. 1 8. 1 Bal. 6 0. 021 0. 022 0. 012 0. 075 12. 2 Bal. 7-4, 95 <0. 01 9. 6 Bal.

The test material was taken from cast material and cut into rectangular shape with dimensions of approximately 10x15x 3 mm and finally prepared by grinding to 600 mesh. Prior to testing, the specimen were cleaned in acetone and then placed in the cold furnace. In order to reach a low oxygen partial pressure, pure hydrogen was flushed through the furnace for three hours before introducing the reaction gas and heating to test temperature. The laboratory exposure was conducted at 1000°C/100h in a quartz tube furnace with a diameter of 25mm.

The gas flow rate was 250 ml/min, which corresponds to a gas velocity over the specimen of 9 mm/s. The temperature was stabilized at 1000°C after 30 minutes heating. The reaction gas had an input composition of 83vol- % CO+17% vol-H2.

The results are presented in Table 2, which shows the weight gain due to coke/graphite formation at 1000°C on the surface of the specimen after 100h.

As comparative example a specimen of Alloy 800HT was tested.

Table 2 Material Coke formation at 1000°C : [mg/cm211 00h] 1 Alloy 800HT 5.2 2 Cu-1 0. 5AI-3. 5Fe 1. 0 3 Cu-1 OAI-1 Mo 0.3 4 Cu-9AI-5Mo 0. 3 5 Cu-8AI-2Si 0 6 Cu-12AI-Si-La 0 7 Cu-10AI-5Co 0. 5

As shown in Table 2 the CuAISi alloys, alloy 5 and 6, do not show any coke formation, which could be detected by the naked eye, but these type of alloys are restricted to moderate temperatures due to their low melting points (< 960°C). The molybdenum alloyed aluminum bronzes have higher melting points, maintaining a low coking rate compared to commercial steel alloys as Alloy 800HT.

EXAMPLE 2 The person skilled in the art realizes that the alloy according to the present invention could need a load carrier at elevated temperatures, i. e. temperatures above approximately 400°C. For this purpose, the alloy of the present invention can be machined to a component in a composite or bimetallic composite solution, which will be used as construction material in the different shapes as mentioned above. The later is especially valid if the alloy has low contents of molybdenum and tungsten, in the compositions with high molybdenum and/or tungsten contents, the highest temperature, where the alloy can be used without any load carrier is considerably higher.

The alloy according to the present invention can be machined to construction material in the shape of tubes, pipes, plate, strip and wire.

If so required, a stronger alloy can be produced in the shape of tubes or plate or strip, where the load-carrying alloy is coated on one or more surfaces with the alloy according to the present invention. Some of the methods, which can be used in order to produce a composite solution of the alloy and a load carrier are co-extrusion, co-welding or co-drawing and shrinkage of one tube on the load carrying component and one outer and/or inner tube of the alloy according to the invention, possibly followed by a heat treatment in order to obtain a metallurgical binding between the components. A similar method at the production of plate or strip is to hot-or cold-roll two or more plates or strips together. Composite plates or-tubes can also be produced by explosion welding of two or more different plates or tubes of a load carrier and the alloy according to the invention. An outer-and/or inner-component can also be applied on a load carrier by help of a powder metallurgical technique, such as HIP (Hot isostatic Pressing) or CIP (Cold Isostatic Pressing). In these cases the load carrier could be in the shape of tubes, pipes, plate, strip or wire or other suitable product form. After pressing, the formed composite will be further machined by e. g. hot extrusion and/or welding, drawing and forging.

Other methods for the production of composite material are gas phase deposition of copper, aluminum, molybdenum and/or tungsten by e. g. vaporization, pack cementation, sputtering, chemical vapor deposition (CVD), physical vapour deposition (PVD) or other methods. Aluminum-molybdenum bronze can also be deposited on the load carrier e. g. by dipping in a melt or by overlay welding. These methods are possible to use in order to produce all of the above-mentioned product forms. Different coating methods can be used in order to supply copper, molybdenum and aluminum to the alloy. In such cases, a final heat-treatment is required in order to homogenize the alloy with the purpose to keep its corrosion properties.

Composite strip or composite plates, produced as above described can be welded together to longitudinal welded or helical welded composite tubes with the alloy according to the invention on the inner and/or outside of the tube.

Suitable load carriers in the above mentioned product forms are such high temperature alloys, which today are used in the actual temperature range. This concerns for temperatures lower than 700°C martensitic or bainitic or ferritic iron alloys with additions of e. g. chromium, molybdenum, vanadium, niobium, tungsten, carbon and/or nitrogen in order to obtain mechanical strength at high temperature. At temperatures above approximately 500°C it is usual to use austenitic iron-chromium-nickel alloys, which are possibly mechanically strengthened as load carrier by alloying with e. g. molybdenum, vanadium, niobium, tungsten, carbon and/or nitrogen. In both of those groups of alloys chromium and sometimes aluminium and/or silicon are used in order to give the load carrier an improved corrosion resistance. In those cases, where the alloy according to the invention is deposited on both surfaces of such load carrier, the alloy according to the invention will deliver the corrosion resistance that is required. But that means, alloys whose maximum temperature of use in other applications is limited by the corrosion resistance being able to be used as load carriers at higher temperatures than otherwise. In those cases, where the alloy according to the invention is only deposited at one surface of the load carrier, it is necessary that the load carrier itself has a sufficient corrosion resistance in the environment its untreated surface is exposed for.

EXAMPLE 3 In Figure 1 a section of a phase diagram for the system Cu-Mo-W-AI calculated with Thermo-calc for a given Al-content of 8.7% is presented. Figure 1 shows the solidus/liquidus temperatures calculated as functions of the content of molybdenum. Figure 2 shows the solidus/liquidus temperatures calculated as functions of tungsten content. It is clear from the Figures that Mo and W partly or completely can replace each other in the alloy, regarding the effects on the melting point of the alloy.