Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/05—Mixtures of metal powder with non-metallic powder
  • C22C1/059—Making alloys comprising less than 5% by weight of dispersed reinforcing phases
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0207—Using a mixture of prealloyed powders or a master alloy
  • C22C33/0228—Using a mixture of prealloyed powders or a master alloy comprising other non-metallic compounds or more than 5% of graphite
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
  • C22C33/0285—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5% with Cr, Co, or Ni having a minimum content higher than 5%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum

Definitions

• the present invention relates to to high temperature alloys, and more particularly to oxide dispersion strengthened alloys having improved creep resistance and carburisation resistance at high temperatures.
• high temperature alloys used for example, in the manufacture of alloy tubes for steam methane reforming, suffer from insufficient creep resistance.
• high temperature alloys for example, alloy tubes used in ethylene pyrolysis, the alloys suffer from insufficient carburisation resistance and, in consequence, insufficient creep resistance.
• INCOLOY® 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%AI, 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. 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) 5 alloy.
• ODS oxide dispersion strengthened
• Such alloys exhibit a creep threshold, that is to say, below a certain stress their creep rate is very low. This behaviour 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 & Engineering A 272, 152-162 (1999). It has recently been proposed 0 to provide oxide dispersion strengthened clad tubes based on INCOLOY 803, but to date no entirely successful commercial product is available (www.oit.doe.gov/imf/factsheets/mtu tubes).
• the nickel-chromium-iron alloys in the ethylene pyrolysis market which 5 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 carburising service conditions high temperature, high carbon activity, low oxygen pressure
• 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 aluminium to the melt.
• aluminium has two highly detrimental effects on the mechanical properties of such alloys and especially on the creep resistance. Firstly, addition of 5 aluminium to the melt can produce a dispersion of alumina in the alloy that can drastically reduce the creep resistance properties. Secondly, aluminium can form brittle Ni-AI phases in the alloy.
• the invention provides an improved creep resistant nickel-chromium-iron alloy comprising up to about 5% by weight of hafnium- containing particles.
• the invention 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 oxidised particles.
• the invention provides a corrosion resistant nickel- chromium-iron-aluminium alloy comprising up to about 15%, preferably up to about 10%, by weight of aluminium and up to about 5% by weight of hafnium- containing particles.
• the alloys of the invention are castable and can be formed into tubes and coils.
• the present invention provides an oxide dispersion strengthened castable alloy comprising, by weight:
• 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.
• a preferred embodiment of an oxide dispersion strengthened nickel- chromium-iron castable alloy according to the invention comprises, by weight:
• Aluminium 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.
• Preferred alloy compositions according to the invention include the following:
• Tungsten 1.0% max., Aluminium 0 - 15.0% Nitrogen 0.001 - 0.5% Oxygen 0.001 - 0.7% balance iron and incidental impurities,
• nickel-chromium-iron castable alloys according to the invention include the following compositions, where all percentages are given by weight:
• 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%.
• the hafnium is present in the alloy in the form of finely divided oxidised 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.
• alloy compositions according to the invention consist essentially of the following components, by weight:
• Incidental impurities in the alloys of the invention can comprise, for example, phosphorus, sulphur, vanadium, zinc, arsenic, tin, lead, copper and cerium, up to a total amount of about 1.0%.
• the invention 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.
• the alloys of the invention 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 aluminium) being taken up in the slag.
• the correct oxidising 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 oxidised 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 and the optimum free oxygen level necessary to react with the hafnium particles can readily be found by routine experimentation.
• any such micro-additions are made after the addition of hafnium.
• alloying amounts of titanium and/or zirconium may be added, up to the specified limits of 0.5% by weight in each case.
• 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.
• hafnium is added to the melt as finely divided particles and that it is oxidised jn situ.
• 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.
• 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 invention 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.
• 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 decarburisation (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.
• 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 1690C for nickel-chromium-iron-aluminium.
• Hafnium particles are preferably added to the melt just before pouring the molten alloy into the mould. 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.
• 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.
• the high carbon alloys of the invention (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 invention 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.
• the low carbon alloys of the invention (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.
• the invention provides for the formation of a hafnia / hafnium oxide dispersion (the hafnium can be oxidised to form HfO 2 , but it can be expected that there will also be formed an oxide HfO x with x as a variable).
• 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.
• 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 oxidised 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.
• the creep resistance of such high carbon alloys derives from the ability of the particle dispersion to delay the motion of the dislocations in the alloy lattice.
• 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 aluminium is absent).
• the invention 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 oxidised 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.
• the improved high temperature performance of the new low carbon alloys of this further aspect of the invention 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 aluminium is absent).
• the nickel-chromium-iron alloy of the invention also comprises aluminium
• the aluminium is preferably present in an amount of from 0.1% to 10% by weight, more preferably from 0.5% to 6% by weight and most preferably from 1.0 to 5% by weight.
• a method of manufacturing a carburisation resistant nickel-chromium-iron alloy which comprises adding sequentially finely divided hafnium particles and aluminium to a melt of the alloy before pouring.
• this method allows the addition of elements that are reactive with oxygen for example titanium, zirconium or aluminium, to the melt, before pouring the molten alloy into the mould.
• elements that are reactive with oxygen for example titanium, zirconium or aluminium
• the aluminium is added to the melt immediately before pouring the molten alloy into the mould.
• 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 aluminium additions do not form oxides, which could react detrimentally with the hafnium particles and reduce the yields of titanium, zirconium and aluminium present in the alloy.
• hafnium limits the amount of available oxygen in the alloy able to react with the aluminium and minimises or eliminates the formation of a detrimental dispersion of alumina particles.
• the alloys of the invention can be formed into tubes, for example, by rotational moulding, and such rotationally moulded tubes are a further aspect of the invention.
• the rotational moulding 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.
• 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.
• hafnium addition 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.
• 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 carburisation.
• 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.
• Figure 1 is a photomicrograph of a first alloy according to the invention with its composition by weight
• Figure 2 is a photomicrograph of a second alloy according to the invention with its composition by weight
• Figure 3 is a photomicrograph of a third alloy according to the invention with its composition by weight
• Figure 4 is a photomicrograph of a fourth alloy according to the invention with its composition by weight
• Figure 5 is a photomicrograph of a fifth alloy according to the invention.
• Figure 6 is a photomicrograph of a sixth alloy according to the invention.
• Figure 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 invention.
• Figure 8 is a chart showing comparison of creep strength between a prior art alloy and an alloy according to this invention.
• Figure 9 shows the results of a pack-carburisation test performed between a prior art alloy and an alloy according to this invention. Detailed Description of the Best Mode for Carrying Out the Invention
• melt composition is produced in a clean furnace:
• 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.
• 100kg 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 oxidised, and so to transform the hafnium in hafnium oxide.
• 0.18% titanium, in the form of FeTi is added to the ladle.
• the alloy in the ladle is stirred and immediately poured into a tube mould.
• the creep resistance properties of the alloy thus produced were compared with the properties of an otherwise identical commercial alloy without hafnium.
• Example 1 The procedure of Example 1 is repeated using the same melt composition except that the titanium addition is omitted.
• 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.
• 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 invention 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.
• This example describes the production of a low carbon oxide dispersion strengthened alloy according to the invention.
• melt composition is produced in a clean furnace:
• 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.
• 100kg 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 oxidised, and so to transform the hafnium in hafnium oxide dispersion.
• 0.25% titanium, in the form of FeTi is added to the ladle.
• the alloy in the ladle is stirred and immediately poured into a tube mould.
• the chemical composition of the tube alloy by spectrometer analysis is:
• Example 3 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:
• 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.
• This Example describes the production of an oxide dispersion strengthened nickel-chromium-iron alloy according to the invention comprising both hafnium and aluminium.
• a nickel-chromium-iron alloy melt having the following constituents by weight is formed in a clean furnace and brought to tapping temperature.
• Example 5 has been tested to confirm that aluminium can improve the carburisation resistance of a hafnium-containing alloy according to the invention.
• a very severe pack-carburisation test was performed, the results of which are shown in Figure 9.
• the creep resistance of the alloy was found to be substantially maintained compared to an identical alloy without hafnia and aluminium additions. Indeed only a decrease of maximum 20% in creep resistance was observed compared to an identical alloy without hafnium and aluminium additions.
• an identical alloy with an aluminium addition, but without hafnium showed a decrease in creep resistance of 80%.

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