Abstract:
An improved nickel-chromium-iron alloy is provided, which comprises up to about 5% of hafnium-containing particles. In one embodiment, an improved creep resistant castable oxide dispersion strengthened nickel-chromium-iron alloy comprises up to about 5% of hafnium, with at least part of the hafnium being present as finely dispersed oxidized particles. Further embodiments of the improved alloy can comprise additionally up to about 15% by weight aluminum. The alloy is particularly useful in the production of creep resistant tubes and castings, for example, for the petrochemical market.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/533,850, filed Nov. 29, 2005, which is a national stage of International Application No. PCT/GB03/04754, filed Nov. 4, 2003, which claims priority to United Kingdom Patent Application No. GB0225648.5, filed Nov. 4, 2002. This application is also a continuation of U.S. patent application Ser. No. 10/533,034, filed Apr. 13, 2006, which is a national stage of International Application No. PCT/GB03/04665, filed Oct. 30, 2003, which claims priority to United Kingdom Patent Application Nos. GB0225648.5, filed Nov. 4, 2002; GB0228576.5, filed Dec. 9, 2002; and GB0324859.8, filed Oct. 24, 2003. The entire disclosures of each of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    This section provides background information related to the present disclosure which is not necessarily prior art. 
         [0003]    The present technology relates to high temperature alloys, and more particularly to oxide dispersion strengthened alloys having improved creep resistance and carburization resistance at high temperatures. 
         [0004]    Frequently high temperature alloys used, for example, in the manufacture of alloy tubes for steam methane reforming, suffer from insufficient creep resistance. In other applications of high temperature alloys, for example, alloy tubes used in ethylene pyrolysis, the alloys suffer from insufficient carburization resistance and, in consequence, insufficient creep resistance. 
         [0005]    The petrochemical industry continues to look for improved materials and other technologies capable of withstanding increasingly demanding process conditions to enable more efficient production and achieve enhanced yields. In the case of steam methane reforming, these conditions involve higher temperatures and higher gas pressures. In the case of ethylene pyrolysis, the conditions involve increasingly severe pyrolysis/cracking conditions (higher temperatures, shorter residence times, and lower partial pressures of product). Currently available alloys have specific deficiencies that cause relatively early failure under these process conditions. This is the case presently for both castable alloy tubes and wrought alloy tubes. 
         [0006]    An example of a known alloy material is INCOLOY (ÉD alloy 803 (UNS S 35045), which is an iron-nickel-chromium alloy specifically designed for use in petrochemical, chemical and thermal processing applications. The composition of INCOLOY 803, by weight, is 25% Cr, 35% Ni, 1% Mn, 0.6% Ti, 0.5% Al, 0.7% Si, 0.07% C and balance Fe. Relatively unsuccessful efforts have been made to improve the properties of this alloy by the addition of further alloying components and also by cladding. 
         [0007]    It has been known for about thirty years that alloy creep resistance can be considerably improved by adding a fine dispersion of oxide particles into a metallic matrix, yielding a so-called oxide dispersion strengthened (ODS) alloy. Such alloys exhibit a creep threshold, that is to say, below a certain stress their creep rate is very low. This behavior is commonly explained by interfacial pinning of the moving dislocations at the oxide particle; Bartsch, M., A. Wasilkowska, A. Czyrska-Filemonowicz and U. Messerschmidt Materials Science &amp; Engineering A 272, 152-162 (1999). It has recently been proposed to provide oxide dispersion strengthened clad tubes based on INCOLOY 803, but to date no entirely successful commercial product is available. 
         [0008]    The nickel-chromium-iron alloys in the ethylene pyrolysis market which have been produced to have good corrosion resistance and acceptable creep resistance mainly develop an oxide coating layer based on chromium oxide (with in some cases admixed silica). This layer under excessively carburizing service conditions (high temperature, high carbon activity, low oxygen pressure) can become destabilized and is then no longer a functional carbon diffusion barrier. Alumina is known to be a very stable oxide and ideally it would be desirable to create an alumina layer on the surface of the nickel-chromium-iron alloy, for example, by adding aluminum to the melt. However, aluminum has two highly detrimental effects on the mechanical properties of such alloys and especially on the creep resistance. Firstly, addition of aluminum to the melt can produce a dispersion of alumina in the alloy that can drastically reduce the creep resistance properties. Secondly, aluminum can form brittle Ni-Al phases in the alloy. 
         [0009]    It will be apparent that there is a need for new high temperature alloys with improved properties for a variety of high temperature applications. 
       SUMMARY 
       [0010]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0011]    According to the present technology there is provided a new class of improved nickel-chromium-iron alloys comprising hafnium and methods for their production. 
         [0012]    In a first aspect, the technology provides an improved creep resistant nickel-chromium-iron alloy comprising up to about 5% by weight of hafnium-containing particles. 
         [0013]    In a second aspect, the technology provides an improved oxide dispersion strengthened nickel-chromium-iron alloy which comprises up to about 5% by weight of hafnium, with at least part of the hafnium being present as finely divided oxidized particles. 
         [0014]    In a third aspect, the technology provides a corrosion resistant nickel-chromium-iron-aluminum alloy comprising up to about 15%, preferably up to about 10%, by weight of aluminum and up to about 5% by weight of hafnium-containing particles. 
         [0015]    The alloys of the technology are castable and can be formed into tubes and coils. 
         [0016]    In a further aspect, the present technology provides an oxide dispersion strengthened castable alloy comprising, by weight: 
         [0000]                                                Carbon   0.01-0.7%             Silicon   0.1-3.0%             Manganese   0-3.0%           Nickel   15-90%             Chromium   5-40%            Molybdenum   0-3.0%           Niobium   0-2.0%           Tantalum   0-2.0%           Titanium   0-2.0%           Zirconium   0-2.0%           Cobalt   0-2.0%           Tungsten   0-4.0%           Hafnium   0.01-4.5%             Aluminum   0-15%            Nitrogen   0.001-0.5%              Oxygen   0.001-0.7%                           
balance iron and incidental impurities, with the proviso, that at least one carbide forming element whose carbide is more stable than chromium carbide selected from niobium, titanium, tungsten, tantalum and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0017]    A preferred embodiment of an oxide dispersion strengthened nickel-chromium-iron castable alloy according to the technology comprises, by weight: 
         [0000]                                                Carbon   0.01-0.5%             Silicon   0.01-2.5%             Manganese   0-2.5%           Nickel   15-50%             Chromium   20-40%             Molybdenum   0-1.0%           Niobium   0-1.7%           Titanium   0-0.5%           Zirconium   0-0.5%           Cobalt   0-2.0%           Tungsten   0-1.0%           Hafnium   0.01-4.5%             Aluminum   0-15%                         
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0018]    Preferred alloy compositions according to the technology include the following: 
         [0000]                                                Carbon   0.3 to 0.7%           Silicon   0.1 to 2.5%           Manganese   2.5% max.           Nickel   30 to 40%           Chromium   20 to 30%           Molybdenum   3.0% max.           Niobium   2.0% max.           Hafnium   0.01 to 4.5%           Titanium   0.5% max.           Zirconium   0.5% max.           Cobalt   2.0% max.           Tungsten   1.0% max.           Nitrogen   0.001-0.5%           Oxygen   0.001-0.7%                        
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0000]                                                Carbon   0.03 to 0.2%           Silicon   0.1 to 0.25%           Manganese   2.5% max.           Nickel   30 to 40%           Chromium   20 to 30%           Molybdenum   3.0% max.           Niobium   1.7% max.           Hafnium   0.01 to 4.5%           Titanium   0.5% max.           Zirconium   0.5% max.           Cobalt   2.05% max.           Tungsten   1.0% max.           Aluminum   0-15.0%           Nitrogen   0.001-0.5%           Oxygen   0.001-0.7%                        
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0000]                                                Carbon   0.3 to 0.7%           Silicon   0.01 to 2.5%           Manganese   2.5% max.           Nickel   40 to 60%           Chromium   30 to 40%           Molybdenum   3.0% max.           Niobium   2.0% max.           Hafnium   0.01 to 4.5%           Titanium   1.0% max.           Zirconium   1.0% max.           Cobalt   2.0% max.           Tungsten   1.0% max.           Aluminum   0-15.0%           Nitrogen   0.001-0.5%           Oxygen   0.001-0.7%                        
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0000]                                                Carbon   0.03 to 0.2%           Silicon   0.1 to 2.5%           Manganese   2.5% max.           Nickel   40 to 50%           Chromium   30 to 40%           Molybdenum   3.0% max.           Niobium   2.0% max.           Hafnium   0.01 to 4.5%           Titanium   0.5% max.           Zirconium   0.5% max.           Cobalt   2.0% max.           Tungsten   1.0% max.           Aluminum   0-15.0%           Nitrogen   0.001-0.5%           Oxygen   0.001-0.7%                        
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0000]                                                Carbon   0.3 to 0.7%           Silicon   0.01 to 2.5%           Manganese   2.5% max.           Nickel   19 to 22%           Chromium   24 to 27%           Molybdenum   3.0% max.           Niobium   2.0% max           Hafnium   0.01 to 4.5%           Cobalt   2.0% max.           Tungsten   1.0% max.           Aluminum   0-15.0%           Nitrogen   0.001-0.5%           Oxygen   0.001-0.7%                        
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0000]                                                Carbon   0.03 to 0.2%           Silicon   0.1 to 2.5%           Manganese   2.5% max           Nickel   30 to 45%           Chromium   19 to 22%           Molybdenum   3.0% max.           Niobium   2.0% max.           Hafnium   0.01 to 4.5%           Titanium   0.5% max.           Zirconium   0.5% max.           Cobalt   2.0% max.           Tungsten   1.0% max.           Aluminum   0-15.0%           Nitrogen   0.001-0.5%           Oxygen   0.001-0.7%                        
balance iron and incidental impurities, with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0019]    Other preferred nickel-chromium-iron castable alloys according to the technology include the following compositions, where all percentages are given by weight: 
         [0000]                                                                                              Carbon   Silicon   Manganese   Nickel   Chromium   Molybdenum               A    0.3-0.5   0.1-2.5   2.5 max   30-40   20-30   1.0 max       B   0.03-0.2   0.1-2.5   2.5 max   30-40   20-30   1.0 max       C    0.3-0.6   0.1-2.5   2.5 max   40-60   30-40   1.0 max       D   0.03-0.2   0.1-2.5   2.5 max   40-60   30-40   1.0 max       E   0.30-0.5   0.1-2.5   2.5 max   19-22   24-27   1.0 max       F   0.03-0.2   0.1-2.5   2.5 max   30-45   19-22   1.0 max                        Niobium   Hafnium   Optional Aluminum   Titanium   Zirconium   Cobalt   Tungsten               A   2 max   0.025-4.5   6.0 max   0.5 max   0.5 max   2.0 max   1.0 max       B   2 max   0.025-4.5   6.0 max   0.5 max   0.5 max   2.0 max   1.0 max       C   2 max   0.025-4.5   6.0 max   0.5 max   0.5 max   2.0 max   1.0 max       D   2 max   0.025-4.5   6.0 max   0.5 max   0.5 max   2.0 max   1.0 max       E   2 max   0.025-4.5   6.0 max           2.0 max   1.0 max       F   2 max   0.025-4.5   6.0 max   0.5 max   0.5 max   2.0 max   1.0 max                    
balance iron and incidental impurities.
 
         [0020]    The amount of hafnium in the alloy, by weight, is preferably from 0.05 to 3.0%, more preferably from 0.1% to 1.0% and most preferably from 0.2 to 0.5% for the high carbon alloy (0.3-0.6% carbon), and more than 1% for the low carbon alloy (0.03-0.2% carbon), preferably from 1% to 4.5%. 
         [0021]    Preferably the hafnium is present in the alloy in the form of finely divided oxidized particles having an average particle size of from 50 microns to 0.25 microns, or less, more preferably from 5 microns to 0.25 microns or less. 
         [0022]    Examples of particularly preferred alloy compositions according to the technology consist essentially of the following components, by weight: 
         [0000]                                                Carbon   0.45%           Silicon    1.3%           Manganese    0.9%           Nickel   33.8%           Chromium   25.7%           Molybdenum   0.03%           Niobium   0.85%           Hafnium   0.25%           Titanium    0.1%           Zirconium   0.01%           Cobalt   0.04%           Tungsten   0.01%           Nitrogen    0.1%           Iron   balance                        
with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0000]                                                Carbon   0.07%           Silicon    1.0%           Manganese   0.98%           Nickel   32.5%           Chromium   25.8%           Molybdenum   0.20%           Niobium   0.04%           Hafnium    1.1%           Titanium   0.12%           Zirconium   0.01%           Cobalt   0.04%           Tungsten   0.08%           Nitrogen    0.1%           Iron   balance                        
with the proviso, that at least one of niobium, titanium and zirconium is present and that at least part of the hafnium is present as finely divided oxide particles.
 
         [0023]    Incidental impurities in the alloys of the technology can comprise, for example, phosphorus, sulphur, vanadium, zinc, arsenic, tin, lead, copper and cerium, up to a total amount of about 1.0%. 
         [0024]    In a still further aspect, the technology provides a method of manufacturing an oxide dispersion strengthened castable nickel-chromium-iron alloy which comprises adding finely divided hafnium particles to a melt of the alloy before pouring, under conditions such that at least part of the hafnium is converted to oxide in the melt. 
         [0025]    To manufacture the alloys of the technology, it is important to provide conditions in the melt which permit oxidation of the hafnium particles without allowing detrimental reactions which would result in the hafnium (with or without aluminum) being taken up in the slag. The correct oxidizing conditions can be achieved by appropriate adjustment or additions of the components, example, silicon and/or manganese, and by ensuring that unwanted contaminants are absent or kept to a minimum. If the slag is able to react with the oxidized hafnium particles this of course removes them detrimentally from the melt. The level of oxygen in the melt can be varied by additions of, for example, one or more of silicon, niobium, titanium, zirconium, chromium, manganese, calcium, CaSi and CaSiMn and the optimum free oxygen level necessary to react with the hafnium particles can readily be found by routine experimentation. In some embodiments, a method for manufacturing an oxide dispersion strengthened nickel-chromium-iron alloy comprises adding finely divided hafnium particles to a melt of the alloy and varying the level of oxygen in the melt by the addition of at least one substance chosen from the group of silicon, niobium, titanium, zirconium, chromium, manganese, calcium, CaSi and CaSiMn. 
         [0026]    In the manufacture of the castable nickel-chromium-iron alloys of the technology, it is often desirable to introduce micro-additions of certain components to obtain the desired alloy properties. Such components can be very reactive with oxygen, but in general less reactive than hafnium. Formation of oxides by these micro-additions should be avoided, and preferably the added components should form carbides, carbonitrides, or nitrides, or stay in solid solution. Preferably any such micro-additions are made after the addition of hafnium. For example, after the reaction of the hafnium particles with free oxygen, alloying amounts of titanium and/or zirconium may be added, up to the specified limits of 0.5% by weight in each case. 
         [0027]    The substantial removal of available free oxygen from the melt helps to ensure that any such titanium and/or zirconium and/or additions do not form oxides, which could react detrimentally with the hafnium particles and reduce the yields of titanium, zirconium and hafnium present in the alloy. 
         [0028]    It is important that the hafnium is added to the melt as finely divided particles and that it is oxidized in situ. In some embodiments, the hafnium particles described herein are added to the molten alloy in the furnace. 
         [0029]    We have discovered that hafnium added to nickel/chromium alloys in non-particulate form does not disperse, or reacts only with the carbon/nitrogen present resulting in a decrease of the alloy properties. Attempts to add large pieces of hafnium to nickel/chromium micro-alloys have revealed that the hafnium does not disperse, but settles to the bottom of the alloy melt, and so is not present in the final casting. Surprisingly, we have also found that the addition of hafnia (hafnium oxide) particles directly to the melt does not provide the desired dispersion strengthening either. Hafnia added in this way simply goes into the slag. According to the technology it has been found that it is necessary to carry out the oxidation of the hafnium particles in the melt in order to obtain the desired improvements. 
         [0030]    The charge make up can be a virgin charge (pure metals), a mixture of virgin charge and reverts, a mixture of virgin charge and ingots, or a mixture of virgin charge and reverts and ingots. The ingots can be made from argon/oxygen decarburization (AOD) revert alloy treatment or from in-house reverts treated, for example, by argon purging. In each case the chemical composition of the melt should be carefully monitored to avoid contaminants and the formation of unwanted slag. 
         [0031]    Special care should be taken to deslag the bath, and the maximum amount of slag is preferably removed from the surface of the bath. It is possible to improve slag removal by the use of a neutral deslag powder. If desired the melt can be maintained in an argon atmosphere, but this is not essential. 
         [0032]    The melt temperature is preferably in the range of from 1350° C. to 1700° C., preferably from 1610° C. to 1670° C. for nickel-chromium-iron, and 1630° C. to 1690° C. for nickel-chromium-iron-aluminum. 
         [0033]    Hafnium particles are preferably added to the melt just before pouring the molten alloy into the mold. If a ladle is used, the hafnium is preferably added in the ladle. To improve the hafnium dispersion, the molten alloy is preferably stirred before pouring. 
         [0034]    Any type of hafnium can be used, but electrolytic hafnium is preferred. The hafnium particles are preferably reduced in size as much as possible, for example, by grinding to a fine powder in a suitable mill. The hafnium particles preferably have a particle size of less than 5 mm, preferably less than 4 mm, with an average particle size of from 1 to 2 mm. When dispersed in the melt, the hafnium particles are further reduced in size. 
         [0035]    The high carbon alloys of the technology (0.3-0.6% carbon) have a primary carbide network similar to the corresponding alloys without the oxide dispersion. The primary carbides are mainly composed of chromium and/or iron carbo-nitrides, optionally with niobium, titanium and/or zirconium carbo-nitrides also present. The technology also provides the possibility of obtaining a dispersion of secondary carbides after the alloy has been brought to a high temperature. These secondary carbides are mainly chromium (or other elements such as iron) carbo-nitrides and optionally niobium, titanium (and/or zirconium) carbo-nitrides. 
         [0036]    The low carbon alloys of the technology (0.03-0.2% carbon) can contain a dispersion of carbides, carbo-nitrides, or nitrides, for example, titanium nitrides, titanium carbo-nitrides, niobium carbides, niobium carbo-nitrides, niobium nitrides, zirconium nitrides, zirconium carbo-nitrides, zirconium carbides, tantalum carbides, tantalum carbo-nitrides, tantalum nitrides, tungsten carbides, tungsten nitrides, and/or tungsten carbo-nitrides. 
         [0037]    In addition to these precipitates, the technology provides for the formation of a hafnia/hafnium oxide dispersion (the hafnium can be oxidized to form HfO 2 , but it can be expected that there will also be formed an oxide HfO x  with x as a variable). Furthermore, in alloys containing more than a trace of niobium and titanium, for example, high carbon nickel-chromium-iron alloys, hafnium/niobium/titanium carbo-nitrides and (rarely) oxides mixtures (wherein the quantities of niobium and titanium are variable as well as the quantities of nitrogen and oxygen) can be expected to be present. Also, more numerous titanium nitride (and/or carbide) dispersions may be observed in the alloy, some of which may also contain hafnia particles. It is also possible that some hafnium carbo-nitrides may be formed. 
         [0038]    According to another aspect of the technology, there is provided an oxide dispersion strengthened nickel-chromium-iron alloy which comprises up to about 5% by weight of hafnium, with at least part of the hafnium being present as finely dispersed oxidized particles, the alloy having a carbon content of from 0.3% to 0.5% by weight and having improved high temperature creep resistance, leading to an improved service life expectancy. Without wishing to be confined to any particular theory, it is believed that the creep resistance of such high carbon alloys, in the substantial absence of aluminum, derives from the ability of the particle dispersion to delay the motion of the dislocations in the alloy lattice. In the case of a micro-alloy, without the oxide dispersion, the motion of dislocations can be delayed by the presence of carbide (and/or nitride) precipitates, but the presence of the oxide dispersion provides a substantial unexpected extra improvement. An example of a high carbon oxide dispersion strengthened alloy is alloy A in Table 1 (wherein aluminum is absent). 
         [0039]    In a still further aspect, the technology provides an oxide dispersion strengthened nickel-chromium-iron alloy, which comprises up to about 5% of hafnium, with at least part of the hafnium being present as finely dispersed oxidized particles, the alloy having a carbon content of from 0.03%-0.2%, preferably 0.03%-0.1%, more preferably 0.03%-0.08%, for example, about 0.05%-0.07%, and a significantly increased service temperature, preferably greater than 1150° C. Without wishing to be confined to any particular theory, it is believed that the improved high temperature performance of the new low carbon alloys of this further aspect of the technology is due to the replacement of the strengthening carbide dispersion by a hafnia dispersion which is more stable than the carbide at high temperature. An example of a low carbon oxide dispersion strengthened alloy is alloy B in Table 1 (wherein aluminum is absent). 
         [0040]    Where the nickel-chromium-iron alloy of the technology also comprises aluminum, the aluminum is preferably present in an amount of from 0.1% to 10% by weight, more preferably from 05% to 6% by weight and most preferably from 1.0 to 5% by weight In a still further aspect of the technology, there is provided a method of manufacturing a carburization resistant nickel-chromium-iron alloy which comprises adding sequentially finely divided hafnium particles and aluminum to a melt of the alloy before pouring. 
         [0041]    In a still further aspect of the technology this method allows the addition of elements that are reactive with oxygen for example titanium, zirconium or aluminum, to the melt, before pouring the molten alloy into the mold. 
         [0042]    Preferably the aluminum is added to the melt immediately before pouring the molten alloy into the mold. 
         [0043]    In some embodiments, finely divided hafnium particles are added to the alloy melt before pouring, under conditions such that the formation of detrimental oxide from the reactive elements titanium, zirconium and aluminum are reduced. The addition of the hafnium particles can be effected in different ways. For example, any one of the reactive elements titanium, zirconium and aluminum can be added after hafnium is added to the melt. In some embodiments, the reactive elements titanium, zirconium and aluminum can be added after hafnium is added to the melt, and just before the pouring of the alloy melt into the mold. 
         [0044]    Without wishing to be bound by any particular theory, it is believed that the addition of finely divided hafnium particles to a nickel-chromium-iron alloy melt controls the partial pressure of oxygen to permit the oxidation of the hafnium in situ. In addition, it is believed that the addition of finely divided hafnium particles to a nickel-chromium-iron alloy melt controls the free oxygen content to permit the oxidation of the hafnium in situ. It has been unexpectedly found that addition of finely divided hafnium particles to a nickel-chromium-iron alloy melt controls the partial pressure of not only oxygen, but also of carbon, nitrogen and hydrogen which permits the oxidation of the hafnium in situ. 
         [0045]    The removal of available free oxygen from the melt, involves the reaction of the hafnium particles with oxygen, this helps to ensure that any titanium and/or zirconium and/or aluminum additions do not form oxides, which could react detrimentally with the hafnium particles and reduce the yields of titanium, zirconium and aluminum present in the alloy. 
         [0046]    Without wishing to be confined by any particular theory, it is believed that the addition of hafnium limits the amount of available oxygen in the alloy able to react with the aluminum and minimizes or eliminates the formation of a detrimental dispersion of alumina particles. In addition, the addition of finely divided hafnium particles to a melt of an nickel-chromium-iron alloy permits the oxidation in situ of beneficial oxide dispersion as hafnium oxide, thus avoiding the formation of detrimental precipitates. The formation of the beneficial oxide dispersion containing hafnium oxide in situ thereby also avoids the reaction of hafnium with slag. 
         [0047]    The alloys of the technology can be formed into tubes, for example, by rotational molding, and such rotationally molded tubes are a further aspect of the technology. The rotational molding process can provide a non-uniform particle distribution in the tube wall, with the greater concentration of particles being towards the outer surface of the tube wall, and this can be beneficial in some cases. For example, in certain applications the internal bore of the tube is machined, removing 4-5 mm of material; this gradient of concentration ensures that the hafnium/hafnia reinforcement is kept in the useful part of the tube. Other components that can be manufactured from the new alloys include fittings, fully fabricated ethylene furnace assemblies, reformer tubes and manifolds. 
         [0048]    For high chromium content (more than 10%) alloys, a further advantage of the hafnium addition is that it can tend to improve the oxide layer adherence at the surface of an alloy tube. For example, where nickel-chromium-iron alloys are used in ethylene furnaces, they are able to develop an oxide layer on the surface that protects the alloy against corrosion by carburization. This protective oxide layer is formed ideally of chromium/manganese/silicon oxides, but can also include iron and nickel oxides. The oxide layer has a tendency to spall during the tube service life (because of differences of coefficients of expansion with the alloy, compressive stresses in the oxide, etc). Spalling leaves the alloy unprotected against corrosion from the gaseous and particulate reactants of the ethylene cracking process. It has surprisingly been found that the addition of hafnium as described herein can tend to delay the spalling of the protective oxide layer. 
         [0049]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0050]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0051]    Embodiments of alloys according to the technology are illustrated in the accompanying Drawings, by way of example only, in which: 
           [0052]      FIG. 1  is a photomicrograph of a first alloy according to the technology with its composition by weight; 
           [0053]      FIG. 2  is a photomicrograph of a second alloy according to the technology with its composition by weight; 
           [0054]      FIG. 3  is a photomicrograph of a third alloy according to the technology with its composition by weight; 
           [0055]      FIG. 4  is a photomicrograph of a fourth alloy according to the technology with its composition by weight; 
           [0056]      FIG. 5  is a photomicrograph of a fifth alloy according to the technology; 
           [0057]      FIG. 6  is a photomicrograph of a sixth alloy according to the technology; 
           [0058]      FIG. 7  is a chart showing the results of a Larson-Miller plot of stress-rupture properties of prior art alloys and an alloy according to this technology; 
           [0059]      FIG. 8  is a chart showing comparison of creep strength between a prior art alloy and an alloy according to this technology; and 
           [0060]      FIG. 9  shows the results of a pack-carburization test performed between a prior art alloy and an alloy according to this technology. 
       
    
    
       [0061]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0062]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0063]    Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
         [0064]    The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0065]    The technology is further illustrated by the following Examples, in which all percentages are by weight: 
       Example 1 
       [0066]    The following melt composition is produced in a clean furnace: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Nickel 
                 35% 
               
               
                   
                 Chromium 
                 25% 
               
               
                   
                 Carbon 
                 0.4%  
               
               
                   
                 Niobium 
                 0.8-0.9% 
               
               
                   
                 Silicon 
                 1.6-1.8% 
               
               
                   
                 Manganese 
                 1.1-1.3% 
               
               
                   
                 Iron 
                 balance. 
               
               
                   
                   
               
             
          
         
       
     
         [0067]    The temperature of the melt is raised to a tap temperature of from 1640° C. to 1650° C. and the silicon content checked. The furnace is then de-slaged, removing as much slag as possible. 100 kg of alloy are then tapped into a ladle and 0.35% hafnium particles of particle size maximum 5 mm, average 1 to 2 mm, are added to the tap stream in order to get oxidized, and so to transform the hafnium in hafnium oxide. After the hafnium addition, 0.18% titanium, in the form of FeTi is added to the ladle. 
         [0068]    The alloy in the ladle is stirred and immediately poured into a tube mold. 
         [0069]    The creep resistance properties of the alloy thus produced were compared with the properties of an otherwise identical commercial alloy without hafnium. 
         [0070]    The results of a Larson-Miller plot of the stress-rupture properties of the commercial alloy derived from the regression analysis of numerous creep tests gave a typical figure of 16.7 MPa at a temperature of 1100° C. ( FIG. 7 ). The commercial alloy is expected to fail after a minimum of 100 hours, with a mean value failure of 275 hours. The alloy according to the technology had a minimum failure time of rupture of 370 hours and a mean value failure of 430 hours. The creep strength comparison is shown in  FIG. 8 . 
         [0071]    The results of a 100,000 hour creep rupture stress test for the alloy of Example 1 are given in Table 1: 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 CREEP RUPTURE STRESS 100,000H LIFE FOR ALLOY EXAMPLE 1: 
               
             
          
           
               
                   
                 N/mm2 (psi) 
               
             
          
           
               
                   
                 900 
                 950 
                 1000 
                 1050 
                 1100 
               
               
                   
                   
               
             
          
           
               
                 Mean 
                 33.86 (4929) 
                 23.15 (3374) 
                 14.70 (2148) 
                 8.67 (1273) 
                 4.75 (704) 
               
               
                 Minimum 
                 31.37 (4567) 
                 21.44 (3126) 
                 13.62 (1991) 
                 8.03 (1180) 
                 4.40 (653) 
               
               
                   
               
             
          
         
       
     
       Example 2 
       [0072]    The procedure of Example 1 is repeated using the same melt composition except that the titanium addition is omitted. 
         [0073]    The creep resistance properties of the alloy thus produced were compared with the properties of an otherwise identical commercial alloy from which the hafnium addition was omitted. 
         [0074]    The results of a Larson-Miller plot of the stress-rupture properties of the commercial alloy derived from the regression analysis of numerous creep tests gave a typical figure of 16.2 MPa at a temperature of 1100° C. The commercial alloy is expected to fail after a minimum of 100 hours, with a mean value failure of 202 hours. The alloy according to the technology had a minimum failure time of rupture of 396 hours, a mean value failure of 430 hours and a maximum failure time of rupture of 629 hours. 
         [0075]    The results of Examples 1 and 2 show the dramatic improvement in creep properties that can be obtained using the alloys and method of the technology. 
       Example 3 
       [0076]    This Example describes the production of a low carbon oxide dispersion strengthened alloy according to the technology. 
         [0077]    The following melt composition is produced in a clean furnace: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Nickel 
                 33%-35% 
               
               
                   
                 Chromium 
                 24%-26% 
               
               
                   
                 Carbon 
                 0.04%-0.08% 
               
               
                   
                 Silicon 
                 1.0%-1.2% 
               
               
                   
                 Manganese 
                 1.0%-1.2% 
               
               
                   
                 Molybdenum 
                 0.14%-0.3%  
               
               
                   
                 Iron 
                 balance. 
               
               
                   
                   
               
             
          
         
       
     
         [0078]    The temperature of the melt is raised to a tap temperature of from 1640° C. to 1650° C. and the silicon content checked. The furnace is then de-slaged, removing as much slag as possible. 100 kg of alloy are then tapped into a ladle and 0.75% hafnium particles of particle size maximum 5 mm, average 1-2 mm, are added to the tap stream in order to get oxidized, and so to transform the hafnium in hafnium oxide dispersion. After the hafnium addition, 0.25% titanium, in the form of FeTi is added to the ladle. 
         [0079]    The alloy in the ladle is stirred and immediately poured into a tube mold. The chemical composition of the tube alloy by spectrometer analysis is: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                   
               
             
             
               
                 C 
                 Si 
                 Mn 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 Hf 
                 Ti 
               
               
                   
               
               
                 0.07 
                 1.0 
                 0.91 
                 32.9 
                 25.5 
                 0.20 
                 0.03 
                 0.30 
                 0.17 
               
               
                   
               
             
          
           
               
                 Zr 
                 Co 
                 W 
               
               
                   
               
               
                 0.01 
                 0.03 
                 0.06 
               
               
                   
               
             
          
         
       
     
         [0000]      Traces(P+S+V+Zn+As+N+Sn+Pb+Cu+Ce)=0.24 
         [0080]    A photomicrograph of the alloy is shown in  FIG. 5 . The dispersed oxidized particles can clearly be seen. 
       Example 4 
       [0081]    The procedure of Example 3 is repeated using the same melt composition except that the hafnium addition is 0.5%. The chemical composition of the tube alloy by spectrometer analysis is: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
             
           
               
                   
               
             
             
               
                 C 
                 Si 
                 Mn 
                 Ni 
                 Cr 
                 Mo 
                 Nb 
                 Hf 
                 Ti 
               
               
                   
               
               
                 0.07 
                 1.00 
                 0.98 
                 32.5 
                 25.8 
                 0.02 
                 0.04 
                 0.50 
                 0.12 
               
               
                   
               
             
          
           
               
                 Zr 
                 Co 
                 W 
               
               
                   
               
               
                 0.01 
                 0.04 
                 0.08 
               
               
                   
               
             
          
         
       
     
         [0000]      Traces(P+S+V+Zn+As+N+Sn+Pb+Cu+Ce)=0.23 
         [0082]    A photomicrograph of the alloy is shown in  FIG. 6 . The dispersed oxidized particles can clearly be seen. 
         [0083]    Examples 3 and 4 show a higher solidus than the high carbon alloys of Examples 1 and 2, indeed their solidus is 1344° C. instead of 1260° C. for the high carbon alloys. 
       Example 5 
       [0084]    This Example describes the production of an oxide dispersion strengthened nickel-chromium-iron alloy according to the technology comprising both hafnium and aluminum. 
         [0085]    A nickel-chromium-iron alloy melt having the following constituents by weight is formed in a clean furnace and brought to tapping temperature. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Nickel 
                 35% 
               
               
                   
                 Chromium 
                 25% 
               
               
                   
                 Carbon 
                 0.4%  
               
               
                   
                 Niobium 
                 0.8-0.9% 
               
               
                   
                 Silicon 
                 1.6-1.8% 
               
               
                   
                 Manganese 
                 1.1-1.3% 
               
               
                   
                 Iron 
                 balance. 
               
               
                   
                   
               
             
          
         
       
     
         [0086]    100 Kg of the melted alloy is tapped into a ladle, whilst adding hafnium particles to the tap stream to give a hafnium level of 0.15% to 0.30% by weight in the alloy and in order to get it oxidized, and so to transform the hafnium in hafnium oxide dispersion. Immediately before pouring, aluminum is added to the melt to give an aluminum level of 1.5% to 1.8%. 
         [0087]    The alloy of Example 5 has been tested to confirm that aluminum can improve the carburization resistance of a hafnium-containing alloy according to the technology. A very severe pack-carburization test was performed, the results of which are shown in  FIG. 9 . The creep resistance of the alloy was found to be substantially maintained compared to an identical alloy without hafnia and aluminum additions. Indeed only a decrease of maximum 20% in creep resistance was observed compared to an identical alloy without hafnium and aluminum additions. On the other hand, an identical alloy with an aluminum addition, but without hafnium, showed a decrease in creep resistance of 80%. 
         [0088]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
         [0089]    All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0090]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0091]    The technology is not restricted to the details of any foregoing embodiments. The technology extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0092]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the technology. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the technology, and all such modifications are intended to be included within the scope of the technology.