ALLOY COMPOSITION

Description

BACKGROUND

Alloy materials for use at high temperatures, for example, alloy tubes used in furnaces, reformers, and process gas heaters, must display one or more properties selected from the group consisting of: high creep strength, oxidation resistance, carburisation resistance and metal dusting resistance.

High temperature alloys need to withstand very high temperatures (e.g. above 900° C.). This places stringent design requirements on the tubes used in its construction. A particular problem concerns rupturing of the alloy, as well as corrosion of the pipes after prolonged exposure to carburization and creep environment.

Alloy tubes can be prepared by a centrifugal casting process. Centrifugal casting is a well-established process that is used to cast thin-walled cylinders, tubes and other axially symmetric objects that are not readily extruded. One benefit of this process is that it allows precise control of the metallurgy and crystal structure of the alloy product. It is generally used for casting iron, steel, stainless steels and alloys of aluminium, copper and nickel. The centrifugal casting process employs a permanent mould which is rotated about its axis at high speeds of typically 300 to 3000 rpm as the molten metal is poured. The molten metal is centrifugally thrown towards the inside mould wall where it is able to solidify after cooling. The resulting cast cylinder, i.e. tube, has a fine grain and the surface roughness of the outer surface of the cylinder is relatively rough.

WO2019/034845 discloses alloys which are resistant to oxidation and/or resistant to coke formation. In addition, WO2019/034845 discloses alloys which also have acceptable mechanical properties in terms of creep resistance and ductility etc for their intended purpose (i.e. use in high-temperature chemical reactors).

The alloy composition disclosed in WO2019/034845 comprises:

• • from 0.15 wt. % to 0.35 wt. % carbon,
  • from 2.5 wt. % to 5.0 wt. % aluminium,
  • from 40 wt. % to 45 wt. % nickel,
  • from 25 wt. % to 35 wt. % chromium,
  • from 0.50 wt. % to 1.50 wt. % niobium and/or vanadium,
  • from 0.01 wt. % to 0.25 wt. % yttrium,
  • from 0.01 wt. % to 0.25 wt. % tungsten and/or tantalum,
  • from 0.01 wt. % to 0.25 wt. % in total of one or more of titanium and/or zirconium and/or hafnium,
  • up to 0.9 wt. % manganese,
  • up to 0.9 wt. % silicon, and
  • up to 0.10 wt. % nitrogen,
  • with the balance of the composition being iron and incidental impurities.

It is also an aim of the present invention to prepare an alloy composition that has improved creep strength and/or rupture strength relative to conventional alloy compositions.

The invention satisfies some or all of the above aims.

BRIEF SUMMARY OF THE DISCLOSURE

According to a first aspect, there is provided an alloy composition comprising:

• • from 0.30 wt. % to 0.55 wt. % carbon,
  • from 42 wt. % to 52 wt. % nickel,
  • from 23 wt. % to 28 wt. % chromium,
  • from 3.2 wt. % to 4.5 wt. % aluminium,
  • from 1.1 wt. % to 1.6 wt. % tungsten,
  • from 0.79 wt. % to 1.05 wt. % niobium,
  • from 0.1 wt. % to 0.15 wt. % molybdenum,
  • from 0.08 wt. % to 0.25 wt. % titanium,
  • from 0.001 wt. % to 0.10 wt. % zirconium,
  • up to 0.003 wt. % rare earth metal,
  • up to 0.003 wt. % tantalum,
  • up to 0.6 wt. % silicon,
  • up to 0.6 wt. % manganese,
  • up to 0.08 wt. % nitrogen, and
  • up to 0.01 wt. % oxygen,
  • with the balance of the composition being iron and incidental impurities.

According to a second aspect, there is provided a steel tube made from an alloy composition according to the first aspect.

According to a third aspect, there is provided the use of an alloy composition according to the first aspect, or a steel tube according to the second aspect, in high temperature chemical reactors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plot of creep strain against life for Sample 1 and Sample 3 of alloys according to the present invention, at 1150° C. and 6.4 MPa. The creep strain analysis is described in Example 7 and Samples 1 and 3 are described in Example 6.

FIG. 2A shows an SEM image of a cross section of Sample 3 of Example 6, taken after rupture at 3,360 hours of 1150° C. and 5.3 MPa. The white box shows the area of the surface imaged in FIG. 2B.

FIG. 2B shows an SEM image of a cross section of Sample 3 of Example 6, taken after rupture at 3,360 hours of 1150° C. and 5.3 MPa, at the region shown in the white box of FIG. 2A. It is clear from the cross section that an aluminium oxide layer (shown by the white dashed box) is still present on the surface of the sample.

FIG. 3A shows the percentage weight of each of carbon, oxygen, aluminium, and chromium, along a cross section of the sample, as determined by EDX mapping of the cross section imaged in FIG. 2B.

FIGS. 3B, 3C, and 3D show the EDX mapping of Al, O, and Cr, respectively, for the cross section imaged in FIG. 2B. The white dashed box in each figure corresponds to the white dashed box in the SEM image of FIG. 2B.

FIG. 4A shows an SEM image of Example 6, sample 3, as-cast.

FIG. 4B shows an SEM image of Example 6, Sample 3, after rupture at 2,445 hours of 1150° C. and 6.4 MPa.

FIG. 5 shows bar chart representations of the elemental composition (in weight percentage) in the primary chromium carbide regions of Example 6, sample 3, (as-cast) and also in Example 6, Sample 3, after 1 week of aging and after 3,360 hours of aging at 1150° C. and 5.3 MPa. The numbers over each bar chart represent the weight percentage of tungsten found in the primary chromium carbide region of each sample.

FIG. 6 shows bar chart representations of the elemental composition (in weight percentage) in the niobium carbide regions of Example 6, sample 3, (as-cast) and also in Example 6, Sample 3, after 1 week of aging and after 3,360 hours of aging at 1150° C. and 5.3 MPa. The numbers over each bar chart represent the weight percentage of tungsten found in the niobium carbide region of each sample.

DETAILED DESCRIPTION

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 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. The invention is not restricted to the details of any foregoing embodiments. The invention 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.

The reader'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.

It is to be understood that reference to a weight percentage for any elemental component of the alloys described herein represents that weight percentage within normal mathematical rounding convention. For example, where a particular element is specified as being present in an amount of from 1 wt. % to 10 wt. %, it is intended that this range encompasses ≥0.5 wt. % to <10.5 wt. %.

The alloys of the present invention may be used to fabricate steel pipes, particularly steel pipes which are intended to be used in chemical engineering applications such as chemical reactors, ethylene crackers, steam reformers, and DRI systems.

It is the careful control of the metallurgical composition which contributes to the improved balance of anti-coking, oxidation resistance, creep rupture strength properties of these alloys. Each of the elemental components described in the above steel compositions plays an important role in the properties of the steel, such as creep rupture strength, and its resistance to coking and oxidation.

Without wishing to be bound by theory, we consider that the particular combination of elements in alloys of the present invention gives rise to improved creep rupture strength, particularly at high operating temperatures.

In addition, and again without wishing to be bound by theory, we believe that the particular combination of elements in alloys of the present invention gives rise to the superior oxidation resistance properties of the alloys of the invention, e.g. because of the formation of a thin but continuous layer of alumina which forms on the surface of the alloy at an early stage in use. An alumina layer may contribute to a reduced level of coke formation because alumina is effectively non-catalytic and does not promote the formation of carbon deposits on the surface. The alumina layer which forms in use thereby provides an environment which is unfavourable to the deposition of carbon on the surface of the pipes.

In addition, because of the particular chemical composition of the alloys of the present invention, it is possible for a protective oxide layer to form under what would be considered to be very reducing conditions such as in situ in an ethylene cracker reactor. Indeed, the alloys of the present invention may be able to form a stable protective surface layer even at partial pressures of oxygen as low as 10⁻²⁷ atmospheres. This means that pipes made from alloys of the invention could be put into use directly without the need for further treatment and without any additional coatings being required. Hence a further benefit of the alloy compositions of the present invention (e.g., tubes made using steel alloys of the invention) is that they may require no subsequent treating after production in order to be used in chemical engineering applications. This would be a significant advantage both in terms of time and cost relative to pipes made from existing alloys.

The alloys of the invention may also benefit from the fact that they contain little or no nickel, chromium or iron at the interface between the alloy surface and reactants. These elements ordinarily behave as catalytic elements when in contact with e.g., the cracking gas, and would promote coke formation and deposition under normal conditions of use. The possibility for the alloys of the present invention to be able during service to avoid any concentration of or the full presence of these elements on its surface and to form an alumina oxide layer containing little or none of these elements could avoid coking on the surface of the alloy. The retention of these particular elements, which would otherwise be problematic in terms of carbon deposition, within the bulk matrix of the alloy would therefore be a benefit of the alloys of the present invention. It can therefore be seen that the combination of the thin surface layer of alumina and the absence of elemental components such as nickel, chromium and iron at the surface could together lead to substantial improvements in resistance to coking, coke formation and deposition.

The following embodiments may apply to each of the first, second, or third aspects of the invention. These embodiments are independent and interchangeable. Any one embodiment may be combined with any other embodiment, where technically possible. In other words, any of the features described in the following embodiments may (where technically possible) be combined with the features described in one or more other embodiments.

In an embodiment, there is provided an alloy composition comprising:

• • from 0.30 wt. % to 0.55 wt. % carbon,
  • from 42 wt. % to 52 wt. % nickel,
  • from 23 wt. % to 28 wt. % chromium,
  • from 3.2 wt. % to 4.5 wt. % aluminium,
  • from 1.1 wt. % to 1.6 wt. % tungsten,
  • from 0.79 wt. % to 1.05 wt. % niobium,
  • from 0.1 wt. % to 0.15 wt. % molybdenum,
  • from 0.08 wt. % to 0.25 wt. % titanium,
  • from 0.001 wt. % to 0.10 wt. % zirconium,
  • up to 0.003 wt. % rare earth metal,
  • up to 0.003 wt. % tantalum,
  • up to 0.6 wt. % silicon,
  • up to 0.6 wt. % manganese,
  • up to 0.08 wt. % nitrogen,
  • up to 0.01 wt. % oxygen,
  • from 11.504 wt. % to 28.429 wt. % iron,
  • and incidental impurities.

The individual elemental components in the alloy perform the roles discussed below.

Carbon

Carbon is required in an amount of 0.30 wt. % to 0.55 wt. % in the alloys of the invention. The amount is carefully controlled because carbon has several different functions.

For example, carbon is usually an important component of steel for providing tensile strength and resistance to creep rupture. This is because carbon is an essential component in the formation of carbides which normally provide steel with its strength due to the precipitation of the primary and secondary carbides.

However, existing steel alloy compositions (e.g. mild steel and low alloy steel) comprising ‘high’ amounts of carbon (e.g. 0.30 wt. % or more) are generally regarded as having poor oxidation resistance. The particular chemical composition of the alloys of the present invention, which contain higher amounts of carbon (e.g. 0.30 wt. % or more), may maintain oxidation resistance.

The interplay between the amount of aluminium which is added and the amount of carbon must also be closely controlled. Aluminium is generally incompatible with carbon as aluminium decreases the solubility of carbon in the austenitic matrix. This reduced solubility of carbon results in carbon migrating to the grain boundaries during solidification and the precipitation of an excessive quantity of primary chromium carbide. The precipitated primary chromium carbide can form a thick network that makes the alloy become brittle.

Accordingly, it is necessary to have sufficient carbon in the alloy to ensure sufficient strength in the resulting alloy and this is the reason for the requirement to have 0.30 wt. % or more carbon in the alloy. At the same time it is important to ensure that the upper limit of the amount of carbon is not too high so that it adversely interacts with the added aluminium and/or negatively impacts on oxidation resistance. For this reason, the upper limit of carbon is 0.55 wt. %. As discussed herein, the present inventors have surprisingly found that this higher carbon content is tolerated and still results in an oxidation resistant alloy.

In an embodiment, the amount of carbon is from greater than 0.30 wt. % to 0.55 wt. %. In an embodiment, the amount of carbon is from 0.32 wt. % to 0.55 wt. %, 0.33 wt. % to 0.55 wt. %, 0.33 wt. % to 0.55 wt. %, 0.34 wt. % to 0.55 wt. %, 0.35 wt. % to 0.55 wt. %, 0.36 wt. % to 0.55 wt. %, 0.37 wt. % to 0.55 wt. %, 0.38 wt. % to 0.55 wt. %, 0.39 wt. % to 0.55 wt. %, or 0.40 wt. % to 0.55 wt. %. In an embodiment, the amount of carbon is from 0.41 wt. % to 0.55 wt. %, 0.42 wt. % to 0.55 wt. % or 0.43 wt. % to 0.55 wt. %.

In an embodiment, the amount of carbon is from 0.30 wt. % to 0.54 wt. %, 0.30 wt. % to 0.53 wt. %, 0.30 wt. % to 0.52 wt. %, 0.30 wt. % to 0.51 wt. %, 0.30 wt. % to 0.50 wt. %, 0.30 wt. % to 0.49 wt. %, 0.30 wt. % to 0.48 wt. %, or 0.30 wt. % to 0.47 wt. %. In an embodiment, the amount of carbon is from 0.30 wt. % to 0.46 wt. %, 0.30 wt. % to 0.45 wt. %, 0.30 wt. % to 0.44 wt. %, 0.30 wt. % to 0.43 wt. %, 0.30 wt. % to 0.42 wt. %, 0.30 wt. % to 0.41 wt. % or 0.30 wt. % to 0.40 wt. %. In an embodiment, the amount of carbon is from 0.30 wt. % to 0.39 wt. %.

In an embodiment, the amount of carbon is from 0.31 wt. % to 0.54 wt. %, 0.31 wt. % to 0.53 wt. %, 0.31 wt. % to 0.52 wt. %, 0.31 wt. % to 0.51 wt. %, 0.31 wt. % to 0.50 wt. %, 0.31 wt. % to 0.49 wt. %, 0.31 wt. % to 0.48 wt. %, or 0.31 wt. % to 0.47 wt. %.

In an embodiment, the amount of carbon is from 0.32 wt. % to 0.54 wt. %, 0.32 wt. % to 0.53 wt. %, 0.32 wt. % to 0.52 wt. %, 0.32 wt. % to 0.51 wt. %, 0.32 wt. % to 0.50 wt. %, 0.32 wt. % to 0.49 wt. %, 0.32 wt. % to 0.48 wt. %, or 0.32 wt. % to 0.47 wt. %.

In a preferred embodiment, the amount of carbon is from 0.32 wt. % to 0.47 wt. %.

Aluminium

Aluminium is another very important component of the alloys of the invention. Aluminium is present in an amount of from 3.2 wt. % to 4.5 wt. %.

Aluminium and carbon compete for solubility in the alloy composition. Furthermore, as mentioned above, aluminium also decreases the solubility of carbon but reciprocally the carbon decreases the solubility of the aluminium in the matrix leading to brittleness. Thus, brittleness can result from the precipitation of thick chromium carbide in the grain boundaries during solidification as discussed above, and also because of precipitation of nickel-aluminide resulting from the reduced solubility of aluminium.

It is therefore important to ensure that the upper limit of aluminium does not exceed 4.5 wt. % so that the alloy does not become brittle due to the uncontrolled formation of excess intermetallic nickel-aluminide at the inter-dendritic space and grain boundaries.

Aluminium is also necessary for the formation of the alumina protective layer at the surface. Therefore, a minimum amount of aluminium of 3.2 wt. % is required in order to ensure that a substantially continuous, dense coating of alumina is formed on the surface of the alloy when in use. This aluminium oxide layer is oxidation resistant (slow growth of the oxide layer during service prevents further reaction or oxidation of the alloy). The aluminium oxide is a surface stable barrier stopping the diffusion of carbon in the alloy. It serves to facilitate anti-coking/carburisation resistance (no catalytic reaction involving formation of graphite/coke during cracking), protecting the alloy, and allows the surface of the alloy to be case neutral when exposed to the gas to be cracked.

The presence of at least 3.2 wt. % aluminium also provides a reservoir/stock of aluminium, which is important in case the aluminium oxide layer needs to be re-formed. Conditions under which this might be an issue include aluminium oxide spallation due to over-decoking, or variation in the service conditions, or an uncontrolled event in the cracking furnace.

Due to the conflict between carbon and aluminium within the alloy matrix, and particularly the reciprocal detrimental impact on the solubility of both elements, it is generally accepted that higher aluminium contents are only possible in alloys comprising relatively low levels of carbon. However, in the alloys of the present invention, the inventors have surprisingly found that alloys comprising a relatively high carbon content are able to also contain a relatively high aluminium content without detrimental effects on the properties of the alloy. Above 4.5 wt. % aluminium, there is a risk of the alloy becoming brittle, such that for example it can break during manipulation or during manufacturing processes, such as pipe straightening after the alloy has been cast.

In an embodiment, the amount of aluminium is from 3.2 wt. % to 4.4 wt. %, 3.2 wt. % to 4.3 wt. %, 3.2 wt. % to 4.2 wt. %, 3.2 wt. % to 4.1 wt. %, 3.2 wt. % to 4.0 wt. %, or 3.2 wt. % to 3.9 wt. %. In an embodiment, the amount of aluminium is from 3.5 wt. % to 4.5 wt. %, 3.6 wt. % to 4.5 wt. %, 3.7 wt. % to 4.5 wt. %, 3.8 wt. % to 4.5 wt. %, 3.9 wt. % to 4.5 wt. %, or 4.0 wt. % to 4.5 wt. %.

In a preferred embodiment, the aluminium content is in the range from 3.2 wt. % to 4.2 wt. %, and more preferably is in the range from 3.2 wt. % to 4.0 wt. %. Yet more preferably, aluminium is in the range from 3.2 wt. % to 3.9 wt. %.

Chromium

Chromium is present in an amount of from 23 wt. % to 28 wt. %. The chromium forms a primary carbide network during solidification (as described in the case of carbon) which give primary strength to the alloy, and also forms secondary carbides during service with good creep resistance properties. Chromium is not generally regarded as an anti-coking component, but in the situation in which the aluminium has been fully depleted after long service of the alloy or following dramatic decoking of the furnace, the chromium will be able to form an oxide layer which will be able to delay the carburisation of the alloy and/or provide corrosion resistance and/or provide oxidation resistance. The chromium content of the matrix is able to delay carbonisation/carburisation of the alloy by trapping the carbon when it is in the process of diffusing through the matrix, thereby forming chromium carbides.

The lower limit of 23 wt. % of chromium is required in order to ensure sufficient oxidation resistance and/or the formation of creep strengthening precipitations in the alloy and the upper limit of 28 wt. % is determined by the fact that above this level the steel is unworkable due to the higher amount of chromium carbide precipitation. Higher amounts of chromium carbide can result in the alloy becoming brittle and allowing an easy path for crack propagation, which is detrimental for long term service exposed alloy.

In some embodiments, the chromium is present in the range of from 23 wt. % to 27 wt. %, 23 wt. % to 26 wt. %, 23 wt. % to 25 wt. %, or 23 wt. % to 24 wt. %. In some embodiments, the chromium is present in the range of from 24 wt. % to 28 wt. %, 25 wt. % to 28 wt. %, 26 wt. % to 28 wt. %, or 27 wt. % to 28 wt. %.

In preferred embodiments, the chromium is present in the range of from 23.5 wt. % to 26.5 wt. %. In more preferred embodiments, the chromium is present in the range of from 23.9 wt. % to 26.2 wt. %.

Nickel

Nickel is present in an amount of from 42 wt. % to 52 wt. %. Nickel is an element which is essential in order to obtain a stable austenite structure and improves the stability of austenite and suppresses the generation of the sigma phase. Nickel is the austenitic stabiliser element, allowing the alloy to be generally strong at above 800° C. Therefore, it forms a stable matrix with the iron which allows the possible precipitation of the carbides/nitrides.

The lower limit of the nickel content is chosen because it provides a sufficient amount for improving the stability of austenite with respect to the lower limits of the other elements. For example, the nickel content is determined in balance with the chromium content for the alloy so that the ultimate alloy possesses a stable austenitic base matrix at elevated temperature. It is important that there is a stable austenitic matrix at every expected service temperature to which the alloy is likely to be exposed. It is therefore important to consider the amount of nickel to be used in the context of the amount of chromium that is also present in the alloy.

Nickel is also very important for cracking furnace applications as it gives better carburisation resistance to the alloy, because of its slow carbon diffusion kinetics. In this respect, the kinetics of nickel in terms of carburisation are much slower than in the case of iron. The lower limit of nickel is therefore also governed by the need to provide carburisation resistance of the alloy.

On the other hand, nickel can in some circumstances be deleterious because it promotes the catalytic formation of coke when nickel is present in the surface of the alloy, and more precisely at the interface between the surface of the alloy/cracking gas. Thus, the upper limit of nickel should not be exceeded because of its known property to promote catalytic coking.

In addition, nickel is an expensive material and consequently avoiding a too high nickel content also has an economic advantage.

In some embodiments, the nickel is present in an amount of from 42 wt. % to 51 wt. %, 42 wt. % to 50 wt. %, 42 wt. % to 49 wt. %, 42 wt. % to 48 wt. %, 42 wt. % to 47 wt. %, 42 wt. % to 46 wt. %, 42 wt. % to 45 wt. % or 42 wt. % to 44 wt. %. In some embodiments, the nickel is present in an amount of from 43 wt. % to 51 wt. %, 43 wt. % to 50 wt. %, 43 wt. % to 49 wt. %, 43 wt. % to 48 wt. %, 43 wt. % to 47 wt. %, 43 wt. % to 46 wt. %, 43 wt. % to 45 wt. %, or 43 wt. % to 44 wt. %. In some embodiments, the nickel is present in an amount of from 43 wt. % to 52 wt. %, 44 wt. % to 52 wt. %, 45 wt. % to 52 wt. %, 46 wt. % to 52 wt. %, 47 wt. % to 52 wt. %, 48 wt. % to 52 wt. %, 49 wt. % to 52 wt. %, 50 wt. % to 52 wt. %, or 51 wt. % to 52 wt. %.

In preferred embodiments, nickel is present in an amount of from 43 wt. % to 50 wt. %. In more preferred embodiments, nickel is present in an amount of from 43.9 wt. % to 49.9 wt. %.

Niobium

Niobium is present in an amount of from 0.79 wt. % to 1.05 wt. %. The function of the niobium is to form thin primary carbides during solidification, which are able to re-dissolve at high temperature (therefore during service) to facilitate the precipitation of secondary carbides and ensure their dispersion. This contributes to the creep resistance of the alloy.

The amount of niobium employed is chosen on the grounds of its efficiency at disrupting the primary chromium carbide network.

In some embodiments, the niobium is present in an amount of from 0.79 wt. % to 1.0 wt. %, 0.79 wt. % to 0.95 wt. %, 0.79 wt. % to 0.9 wt. %, or 0.79 wt. % to 0.85 wt. %.

In some embodiments, the niobium is present in an amount of from 0.8 wt. % to 1.05 wt. %.

In some embodiments, the niobium is present in an amount of from 0.8 wt. % to 1.0 wt. %, 0.8 wt. % to 0.95 wt. %, 0.8 wt. % to 0.9 wt. %, or 0.8 wt. % to 0.85 wt. %. In some embodiments, the niobium is present in an amount of from 0.85 wt. % to 1.05 wt. %, 0.9 wt. % to 1.05 wt. %, 0.95 wt. % to 1.05 wt. %, or 1.0 wt. % to 1.05 wt. %.

In preferred embodiments, the niobium is present in an amount of from 0.79 wt. % to less than 1.0 wt. %. In more preferred embodiments, the niobium is present in an amount of from 0.79 wt. % to 0.95 wt. %. In yet more preferred embodiments, the niobium is present in an amount of from 0.79 wt. % to 0.92 wt. %.

Tungsten

Tungsten is present in an amount of from 1.1 wt. % to 1.6 wt. %. Tungsten is a large element in terms of its atomic size and it is also a carbide-forming element. Both of these factors contribute to an improvement of creep properties of the alloy when tungsten is added. The amount of tungsten that is added is a balance between improving the creep properties, and limiting the high temperature plastic deformation elongation and/or ductility.

Tungsten is a ‘substitutional element’ and is therefore mostly retained in the alloy matrix. Tungsten partly moves into the carbide phase, thereby stabilising the primary chromium carbide and slowing diffusion of carbon within the matrix, thus contributing to the overall strength of the alloy. Tungsten thus stabilises the primary carbides reducing the transfer induced by aging which ordinarily allows carbon to migrate into the matrix, allowing high quantity of secondary carbide precipitates close to the primary carbides. The amount of tungsten present in the alloy is determined by the requirement to significantly increase the creep properties of the alloy at very high temperature, such as 1150° C., compared to existing alloys, but without increasing the quantity and size of carbides formed in the alloy. This is particularly the case for the chromium carbides, which, when present in too a high a quantity, can result in chromium oxide impurities being present in the aluminium oxide that is intended to be formed at the surface. The quantity of tungsten is therefore critical to the properties of the present alloys, to ensure excellent creep properties at high temperatures, but without significantly decreasing the ductility (ductility of minimum 5%) and therefore the weldability of the alloy.

In addition, and without wishing to be bound by theory, it is believed that the amount of tungsten present in the alloys of the present invention is able to stabilise primary chromium carbides within the alloy matrix, even after prolonged aging at high temperature (e.g. 1150° C.). In turn, this stabilisation of the primary chromium carbides imparts excellent creep strength properties.

In an embodiment, tungsten is present in an amount from 1.1 wt. % to 1.5 wt. %, 1.1 wt. % to 1.4 wt. %, 1.1 wt. % to 1.3 wt. %, or 1.1 wt. % to 1.2 wt. %. In an embodiment, tungsten is present in an amount from 1.2 wt. % to 1.6 wt. %, 1.3 wt. % to 1.6 wt. %, 1.4 wt. % to 1.6 wt. %, or 1.5 wt. % to 1.6 wt. %.

In preferred embodiments, tungsten is present in an amount from 1.1 wt. % to 1.45 wt. %. In more preferred embodiments, tungsten is present in an amount from 1.2 wt. % to 1.45 wt. %. In yet more preferred embodiments, tungsten is present in an amount from 1.21 wt. % to 1.44 wt. %. In even more preferred embodiments, tungsten is present in an amount from 1.22 wt. % to 1.43 wt. %. In further still preferred embodiments, tungsten is present in an amount from 1.23 wt. % to 1.42 wt. %.

Titanium and Zirconium

Titanium is present in an amount of from 0.08 wt. % to 0.25 wt. %. Zirconium is present in an amount of from 0.001 wt. % to 0.10 wt. %. Titanium and zirconium are stable carbide-forming elements and are used to improve the creep properties of the alloy. Either element individually or both elements together do this by precipitating secondary carbides during service. Furthermore, as carbide forming elements they not only form their respective carbides but also are able to form a titanium/zirconium-niobium double carbide precipitates which improves creep strength.

In addition, titanium and zirconium have a second function in that they are very active scavengers for oxygen and consequently they help to protect the aluminium present in the melt from oxidation. These elements are therefore oxidised in preference to the aluminium. Titanium and zirconium are added to the melt as a deoxidiser.

Titanium and zirconium also form oxides and nitrides, which can cause over-hardening of the alloy. The upper limit of titanium and zirconium therefore avoids the undesirable formation of oxides and nitrides.

Cost is also an issue since both of these elements are expensive raw materials.

The quantities of titanium and zirconium represent a balance between obtaining an improvement in the creep properties of the alloy by the formation of carbides and undesirable oxide and nitride formation when too much is added.

In embodiments, titanium is present in an amount of from 0.08 wt. % to 0.22 wt. %, 0.08 wt. % to 0.2 wt. %, 0.08 wt. % to 0.18 wt. %, 0.08 wt. % to 0.15 wt. %, 0.08 wt. % to 0.12 wt. %, or 0.08 wt. % to 0.1 wt. %. In embodiments, titanium is present in an amount of from 0.09 wt. % to 0.25 wt. %, 0.1 wt. % to 0.25 wt. %, 0.12 wt. % to 0.25 wt. %, 0.15 wt. % to 0.25 wt. %, 0.18 wt. % to 0.25 wt. %, 0.2 wt. % to 0.25 wt. %, or 0.22 wt. % to 0.25 wt. %.

In preferred embodiments, titanium is present from 0.1 wt. % to 0.25 wt %.

In embodiments, zirconium is present in an amount of from 0.001 wt. % to 0.09 wt. %, 0.001 wt. % to 0.08 wt. %, 0.001 wt. % to 0.07 wt. %, 0.001 wt. % to 0.06 wt. %, or 0.001 wt. % to 0.05 wt. %. In embodiments, zirconium is present in an amount of from 0.001 wt. % to 0.045 wt. %, 0.001 wt. % to 0.040 wt. %, 0.0035 wt. % to 0.030 wt. % or 0.001 wt. % to 0.025 wt. %.

In preferred embodiments, zirconium is present from 0.001 wt. % to 0.025 wt %.

In embodiments, zirconium is present in an amount of from 0.001 wt. % to 0.02 wt. %, 0.001 wt. % to 0.015 wt. %, 0.001 wt. % to 0.01 wt. %, 0.001 wt. % to 0.005 wt. %, or 0.001 wt. % to 0.0025 wt. %. In embodiments, zirconium is present in an amount of from 0.002 wt. % to 0.025 wt. %, 0.0025 wt. % to 0.025 wt. %, 0.005 wt. % to 0.025 wt. %, 0.01 wt. % to 0.025 wt. %, 0.015 wt. % to 0.25 wt. %, or 0.02 wt. % to 0.025 wt. %.

In preferred embodiments, zirconium is present in an amount from 0.004 wt. % to 0.022 wt %.

Molybdenum

Molybdenum is present in an amount of from 0.1 wt. % to 0.15 wt. %. Molybdenum is added to increase the stability of the primary chromium carbides, specifically increasing the stability of these carbides at very high temperature, but only when added in the small quantities specified herein. Without wishing to be bound by theory, it is believed that the amount of molybdenum present in the alloys of the present invention is able to stabilise primary chromium carbides within the alloy matrix, even after prolonged aging at high temperature (e.g. 1150° C.). In turn, this stabilisation of the primary chromium carbides imparts excellent creep strength properties, in a similar manner to the role of tungsten.

Therefore, and again without wishing to be bound by theory, it is believed that the ratio of molybdenum to tungsten present in alloys of the present invention is such that the primary chromium carbides are afforded excellent stability within the alloy matrix at high temperatures.

In an embodiment, molybdenum is present in an amount from 0.11 wt. % to 0.15 wt. % or 0.11 wt. % to 0.14 wt. %.

In an embodiment, molybdenum is present in an amount from 0.12 wt. % to 0.15 wt. % or 0.12 wt. % to 0.14 wt. %.

Rare Earth Metal and Tantalum

Rare earth metals include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, or Hf.

In an embodiment, the rare earth metal is selected from the group consisting of: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, Y, and Hf, or combination thereof.

In an embodiment, the rare earth metal is Y. In an embodiment, the Y is present in an amount of at least 0.0001 wt. % and up to 0.003 wt. %. Y may promote adherence of the oxide layer to the base material. Y can also be useful as a deoxidant during the melting process.

Rare earth metals (other than Y) and tantalum are residual elements present in the material used to produce the alloy composition of the present invention and are therefore present in an amount of up to 0.003 wt. %. In an embodiment, the alloy contains up to 0.001 wt. % rare earth metals (other than Y). In an embodiment, the alloy contains substantially no rare earth metals (other than Y), e.g. 0.000 wt. % rare earth metals. In an embodiment, the alloy contains substantially no rare earth metals, e.g. 0.000 wt. % tantalum.

Silicon

Silicon is present in an amount up to 0.6 wt. %. Silicon provides the function of a deoxidiser and is usually an essential component in an austenite stainless steel. Silicon may also contribute to increasing the stability of any surface oxide film. Silicon also provides some fluidity to the melt bath before the addition of the aluminium to the melt. On the other hand, if the content of silicon is too high the workability of the steel is reduced. A high Si content can also cause the formation of a detrimental phase known as the G phase which is composed of nickel, silicon and niobium (Ni16Nb6Si7). Consequently, the amount of silicon must be carefully controlled.

In some embodiments, silicon is present in an amount of up to 0.55 wt. %, up to 0.5 wt. % or up to 0.4 wt %.

In some embodiments, silicon is present in an amount of from 0.1 wt. % to 0.6 wt. %. In some embodiments, silicon is present in an amount of from 0.1 wt. % to 0.55 wt. %, 0.1 wt. % to 0.5 wt. % or 0.1 wt. % to 0.4 wt. %. In some embodiments, silicon is present in an amount of from 0.2 wt. % to 0.6 wt. %, 0.3 wt. % to 0.6 wt. %, 0.4 wt. % to 0.6 wt. %, or 0.5 wt. % to 0.6 wt. %.

In preferred embodiments, silicon is present in an amount of from 0.35 wt. % to 0.55 wt. %.

Manganese

Manganese is present in an amount of up to 0.6 wt. %. Manganese can improve the weldability of the alloy and it is also an effective de-oxidant and contributes to austenite formation in the steel. However, the high coefficient of diffusion of the manganese at high temperature means that it competes with the aluminium. Furthermore, the addition of too much manganese can result in a reduction in high-temperature strength and also toughness over an extended period of time. Consequently, the amount of manganese must be limited to 0.6 wt. %.

In some embodiments, manganese is present in an amount of up to 0.5 wt. %, or up to 0.4 wt. %.

In some embodiments, manganese is present in an amount of from 0.1 wt. % to 0.6 wt. %. In some embodiments, manganese is present in an amount of from 0.1 wt. % to 0.5 wt. % or 0.1 wt. % to 0.4 wt. %. In some embodiments, manganese is present in an amount of from 0.2 wt. % to 0.6 wt. %, 0.3 wt. % to 0.6 wt. %, 0.4 wt. % to 0.6 wt. %, or 0.5 wt. % to 0.6 wt. %.

In preferred embodiments, manganese is present in an amount of from 0.4 wt. % to 0.6 wt. %.

Nitrogen

Nitrogen is inevitably present in the alloys of the invention. It is present in an amount of up to 0.08 wt. %. Nitrogen may be found in the alloy because the alloy mix is prepared under an air atmosphere so it is often the case that nitrogen can diffuse into the liquid alloy during its production. However, it is not always possible to determine the amount of nitrogen in the alloy because the amount might be unmeasurable (because the effective amount of nitrogen is almost zero).

Nitrogen may form nitrides together with carbon and can contribute to high-temperature strength. Nitrogen allows the dilution, dispersion, and homogenisation of the carbon. However, careful control of the amount of nitrogen is important because it slows the precipitation of primary chromium carbides when it is added in a suitable quantity. However, if the quantity of nitrogen is too large then an excessive amount of nitrides are produced as chromium nitrides and/or aluminium nitrides which reduces the ductility and the toughness of the alloy over an extended period of time and prevent the ‘nitrided’ elements from fulfilling their role in the alloy as they should. It is therefore essential to keep the amount of nitrogen present to a maximum of 0.08 wt. %. There is however no practical lower limit to the measurable nitrogen in an alloy sample of the invention.

Incidental Impurities

Alloys according to the present invention are produced in an air melted induction furnace and without the need for a special atmosphere. The first stage of preparing the alloy involves working out the relative proportions by weight of the various component minerals (which are the source of the various elements required in the final alloy) in order to achieve the desired amounts of the various elements which are required in the final alloy. The solid minerals are added to the furnace. Heating is continued in order to melt all of the mineral components together and ensure a thorough mixing of the minerals in the furnace so that the elements are properly distributed within the matrix.

In circumstances in which the addition of a particular essential element in the alloy composition of the invention also results in the addition of another essential element (because the other essential element is perhaps present as an impurity in the addition agent for the first essential element) then the overall composition must be carefully monitored to ensure that all of the essential components remain within the desired parameters. If necessary, this can be compensated for by adjusting the relative proportions of additional materials used for each of the essential components in the alloys of the invention. Sometimes elements are added in the form of preformed alloys. The skilled person will be able to analyse and compensate as necessary for variations in the essential elemental components due to the presence of incidental impurities using known analytical techniques and by varying the amounts of the usual addition agents for each essential component. Where analysis reveals that such impurities are unacceptable, an alternative source of the desired elemental component (free of damaging impurities) is used.

A number of elements will be present in the alloy as inevitable impurities. Such impurities may include S, P, Sn, Zn, Sb, As, Pb, Bi, Ca, Te, Se, and/or B. For example, the amount of S and P should each be kept to a maximum of 0.02 wt. %. For example, the amount of Sn, Zn, Sb, As, Pb, Bi, Te, Se, and B should each be kept to a maximum of 0.005 wt. %. For example, the amount of Ca should be kept to a maximum of 0.10 wt. %.

Elemental Relationships

Alloys according to the present invention provide several advantages over existing alloy compositions, including improved creep strength and increased time to rupture, whilst maintaining oxidation resistance.

In embodiments, the careful control of the relative amounts of several elemental components gives rise to the beneficial properties of the alloy compositions of the present invention. These elemental relationships are set out as parameters ‘a’, ‘b’, and ‘c’, below, where the presence of an element's symbol represents the wt. % of that element in the alloy composition:

a = 0.1 × C + 0.1 × Cr + 20 × Nb + 0.5 × W + Ti - 6 × Mo b   = 20 × W - ( 40 × Ta + 60 × Re ) c   = N ⁢ i A ⁢ l

In embodiments,

a < b

In embodiments,

5 ⁢ 3 < 3 ⁢ a < 7 ⁢ 0

In embodiments,

1 ⁢ 0 < c < 1 ⁢ 3 . 5

In embodiments, the alloy satisfies the requirements of all three of these conditions, i.e.:

a < b ; 53 < 3 ⁢ a < 70 ; and 10 < c < 1 ⁢ 3 . 5

Components

Alloy compositions of the invention may be used to form any steel component where high creep strength is required when operating at high temperatures (e.g. >900° C., >950° C., >1000° C., >1050° C., >1100° C., >1150° C.), including chemical reactors. Alloy compositions of the invention may be used to form components of chemical reactors. It may be that the component is a steel tube. Accordingly, the alloy compositions or steel tubes of the present invention may be used in high temperature chemical reactors.

EXAMPLES

General Procedures

The minerals and elements of the alloy composition are carefully added in an appropriate sequence in order to obtain the desired content for the final composition. It is also important to protect the aluminium (and other reactive elements) when added to the melt and avoid oxidation because the melting is performed in air.

Once melting and mixing has been achieved, any slag is decanted from the furnace in order to remove impurities and clean the bath of liquid alloy in the furnace. A sample of the molten alloy is then removed from the furnace, allowed to cool and analysed by Optical Emission Spectrometry (OES) in order to determine its elemental composition. An adjustment to the composition may or may not be required at this stage to accommodate for any elemental mass loss due to volatility. The composition is adjusted by the addition of further minerals as necessary, and optionally re-analysed to ensure that the desired composition has been achieved.

After the desired composition has been achieved, the temperature is further raised above the melting temperature to a tapping temperature in order to provide super heat needed to ensure easy casting of the melt. At the same time, the mould is prepared for centrifugal casting.

The mould is a conventional centrifugal casting mould and this type of mould is well known to the skilled person. The process of preparing the mould involves washing the mould with water/steam to clean it and to remove any old mould wash or coating that might have been used in a previous casting process. The washed mould is then coated with an insulating/release agent which is required to prevent the alloy from sticking to the mould after casting. A typical insulating/release agent is silica or alumina.

A disc of ceramic is then added to the centrifugal casting mould in the manner known in the art in order to ensure that the mould is liquid tight and ready for casting. This prevents any alloy leakage during the casting process. The mould temperature is adjusted in preparation for the casting. The mould is then rotated at high speed to e.g. a range of 80 g to 120 g. 100 g being typical for a centrifugal casting speed.

A ladle is then brought to the furnace and a desired weight of molten alloy is tapped off for the purposes of casting including the relevant additions (titanium, zirconium, yttrium and aluminium) in order to reach the final desired quantity of reactive elements of the alloy composition. The ladle itself is preheated to a temperature in the region of 800° to 1000° C. in order to minimise cooling of the alloy after pouring. Alloy is then transferred to the hot ladle. At this stage, a further analysis of the alloy composition may be performed.

The molten alloy in the ladle is then transferred to a pouring cup. The nose of the pouring cup has previously been adjusted to ensure that it mates with and properly fits the size of the input tube for the centrifugal casting mould. The level of molten alloy in the pouring cup is maintained in order to maintain adequate flow of alloy into the mould which is in effect fed by gravity. This provides a continuous flow of alloy into the mould until all of the weight of the alloy has been poured into the mould. The mould is rotated at high speed i.e. maintained at the centrifugal casting speed during the process and whilst the alloy is molten. The length of time the casting process takes depends ultimately on the desired thickness of the tube required and the skilled person is able to determine a suitable rotation time for a particular thickness of tube and weight of alloy. The mould is gradually slowed down as the alloy cools from its solidification point. Generally speaking, a “fast” solidification process is one in which the alloy is cast and then cools at a rate of more than about 100° C. per minute and a “slow” solidification process is one in which the alloy is cast and then cools at a rate of about 50° C. or greater per minute. The casting process is usually completed in less than about 10 minutes. The tube is extracted after the mould stops rotating and the process may be repeated again.

The alloy compositions and steel tubes of the present invention show excellent high-temperature strength and high creep resistance. The tubes are expected to also display exceptional corrosion resistance at elevated temperatures over an extended period of time. Consequently, these steels are particularly suited to use in chemical plant under demanding environments such as steam cracking. In addition, it is expected that steel tubes according to the invention may be used in a variety of other applications such as steam reformers and in nuclear applications in heat exchanges and the like, such as those found in pressurised water reactors.

Example Alloy Compositions

Examples 1 and 2

Alloy compositions 1 and 2 were prepared using the materials set out in the below table in the quantities given:

Stage	Material	Used kg

1	Base Iron	26
2	Ferro Molybdenum	0.5
3	Nickel	135
4	High Carbon Chrome	13.6
5	Pure Chrome	16.1
6	0.3% Carbon Chrome	54.5
7	Ferro Niobium	3.8
8	Paralloy Revert	45
9	Ferro Tungsten	5.9
10	Manganese	1
11	Ferro Titanium	3.301
12	Ferro Zirconium	0.45
13	Nickel Yttrium	1.5
14	Aluminium	11.4

All materials in stages 1-9 are combined and melted completely. Materials in stages 10-14 are then added prior to casting.

The materials set out in the above Tables have the following elemental composition in wt %:

Ele-	Ferro	Nick-	0.03	Alu-	Base	Man-	Ferro
ment	Ti	el	Chrom	minium	Iron	ganese	Mo

C	0.15	0.008	0.026	0.01	0.01	0.01	0.02
Si	0.6	0.01	0.7	0.6	0.005		0.68
Mn	0.5			0.3	0.1	Bal
Ni	0.2	Bal
Cr	0.3		69.5
Mo					0.002		72.8
S	0.01	0.005	0.01		0.003	0.005	0.005
P	0.01	0.004	0.01		0.008	0.007	0.005
Al	0.6		0.1	Bal
N	0.18		0.03
Sn	0.001	0.002	0.003	0.001	0.001
Pb	0.001	0.001	0.001	0.005			0.001
Zn	0.001	0.001	0.001	0.04			0.001
Ti	73
Fe	Bal	0.05	Bal	0.3	Bal		Bal
Co		0.05			0.01
Cu		0.01	0.02	0.05	0.01		0.14
As		0.002	0.002
Sb		0.001			0.001
Mg				0.2
Te					0.001
W
Nb
Y
Zr

Ele-	Ferro	Ferro	Pure	HC		Paralloy
ment	Nb	Zr	chrome	Chrome	Ni Y	Revert	FeW

C	0.04	0.05	0.035	6.9	0.02	0.4	0.15
Si	1.2	0.07	0.78	0.9	0.06	1	0.35
Mn			0.38			1
Ni					Bal	45
Cr			85.6	64.5		35
Mo			0.04
S	0.003	0.007		0.002	0.005		0.003
P	0.005	0.01		0.004	0.002		0.005
Al	0.32	0.05	0.09
N	0.04	0.05	0.03				0.04
Sn	0.002	0.01	0.002		0.002		0.001
Pb	0.004	0.003	0.004		0.004		0.001
Zn		0.002					0.001
Ti	0.43
Fe	Bal	Bal	Bal	Bal	0.2	Bal	Bal
Co			0.06
Cu							0.09
As		0.002	0.003				0.001
Sb							0.001
Mg
Te
W							81.2
Nb	68.1					1
Y					52.3
Zr		78.1

Examples 3 and 4

Alloy compositions 3 and 4 were prepared using the materials set out in the below table in the quantities given:

Stage	Material	Used kg

1	Base Iron	39
2	Ferro Molybdenum	0.5
3	Nickel	116
4	High Carbon Chrome	20
5	Pure Chrome	22
6	0.3% Carbon Chrome	47.9
7	Ferro Niobium	3.5
8	Revert	45
9	Ferro Tungsten	5.6
10	Manganese	1.5
11	Ferro Titanium	3.5
12	Ferro Zirconium	0.5
13	Nickel Yttrium	1.5
14	Aluminium	10.5

All materials in stages 1-9 are combined and melted completely. Materials in stages 10-14 are then added prior to casting.

The materials set out in the above Tables have the following elemental composition in wt %:

	Ferro		0.03		Base
Element	Ti	Nickel	Chrom	Aluminium	Iron	Manganese

C	0.15	0.008	0.026	0.01	0.01	0.01
Si	0.6	0.01	0.7	0.6	0.005
Mn	0.5			0.3	0.1	Bal
Ni	0.2	Bal
Cr	0.3		69.5
Mo					0.002
S	0.01	0.005	0.01		0.003	0.005
P	0.01	0.004	0.01		0.008	0.007
Al	0.6		0.1	Bal
N	0.18		0.03
Sn	0.001	0.002	0.003	0.001	0.001
Pb	0.001	0.001	0.001	0.005
Zn	0.001	0.001	0.001	0.04
Ti	73
Fe	Bal	0.05	Bal	0.3	Bal
Co		0.05			0.01
Cu		0.01	0.02	0.05	0.01
As		0.002	0.002
Sb		0.001			0.001
Mg				0.2
Te					0.001
W
Nb
Y
Zr

Ele-	Ferro	Ferro	Pure	HC		Paralloy
ment	Nb	Zr	chrome	Chrome	Ni Y	Revert	FeW

C	0.04	0.05	0.035	6.9	0.02	0.4	0.15
Si	1.2	0.07	0.78	0.9	0.06	1	0.35
Mn			0.38			1
Ni					Bal	45
Cr			85.6	64.5		35
Mo			0.04
S	0.003	0.007		0.002	0.005		0.003
P	0.005	0.01		0.004	0.002		0.005
Al	0.32	0.05	0.09
N	0.04	0.05	0.03				0.04
Sn	0.002	0.01	0.002		0.002		0.001
Pb	0.004	0.003	0.004		0.004		0.001
Zn		0.002					0.001
Ti	0.43
Fe	Bal	Bal	Bal	Bal	0.2	Bal	Bal
Co			0.06
Cu							0.09
As		0.002	0.003				0.001
Sb							0.001
Mg
Te
W							81.2
Nb	68.1					1
Y					52.3
Zr		78.1

Comparative Example 1 (“Optim-AI”-Corresponding to WO2019/034845)

The following alloy composition was used as comparative example 1. The elemental composition was as follows:

C	Si	Mn	Ni	Cr	Nb	W
0.3	0.4	0.4	44	27	1	0.03

				Rare
	Ti	Al	Zr	Earth	Ta	Fe
	minor	4	minor	minor	—	Bal

Example 5

The alloy compositions of Examples 1-4 were tested by cutting a section from the “hot end” of the centrifuged steel tube and analysing the weight percentage of each element by OES.

Sample	C	Si	Mn	Ni	Cr	Mo	Nb	W

1	0.459	0.534	0.58	44.16	25.739	0.13	0.834	1.235
2	0.466	0.54	0.586	43.96	25.827	0.132	0.839	1.251
3	0.335	0.402	0.422	49.67	24.034	0.133	0.89	1.391
4	0.344	0.386	0.422	49.48	23.906	0.134	0.92	1.413

					Rare
Sample	Ti	Al	N	Zr	Earth	Ta	O	Fe

1	0.148	3.615	0.034	0.01	0.0001	0.0001	0.01	Bal
2	0.12	3.599	0.04	0.005	0.0001	0.0001	0.01	Bal
3	0.222	3.81	0.028	0.022	0.0001	0.0001	0.01	Bal
4	0.19	3.892	0.032	0.019	0.0001	0.0001	0.01	Bal

Example 6

The alloy compositions of Examples 1-4 were tested by cutting a section from the “cold end” of the centrifuged steel tube and analysing the weight percentage of each element by OES.

Sample	C	Si	Mn	Ni	Cr	Mo	Nb	W

1	0.452	0.507	0.561	43.98	26.175	0.127	0.811	1.236
2	0.438	0.504	0.565	43.94	26.023	0.127	0.793	1.233
3	0.342	0.377	0.408	49.85	23.964	0.128	0.892	1.391
4	0.321	0.375	0.408	49.68	24.052	0.127	0.859	1.383

					Rare
Sample	Ti	Al	N	Zr	Earth	Ta	O	Fe

1	0.156	3.297	0.044	0.009	0.0001	0.0001	0.01	Bal
2	0.107	3.431	0.051	0.004	0.0001	0.0001	0.01	Bal
3	0.245	3.841	0.047	0.02	0.0001	0.0001	0.01	Bal
4	0.181	3.704	0.037	0.014	0.0001	0.0001	0.01	Bal

Example 7

Samples 1 and 3 of Example 6 and Comparative Example 1 were machined in a lathe to the 5 correct dimensions for testing in an Applied Test Systems, Inc. series 2510 Lever Arm Creep Testing System. Testing was carried out to ASTM E139 standard. The test was set up to measure (1) stress to rupture and (2) creep testing. Additional strain measurement kit was fitted to monitor creep in the tested sample.

	Comparative Example 1

Temperature (° C.)	Stress (MPa)		Rupture Life (hours)

1150	5.3	Sample A	875
		Sample B	953
		Sample C	486
		Sample D	785
1150	6.4	Sample A	555
		Sample B	507
		Sample C	856

	Example 6

Temperature (° C.)	Stress (MPa)		Rupture Life (hours)

1150	5.3	Sample 1	>5,000*
		Sample 3	3,360
1150	6.4	Sample 1	2,915
		Sample 3	2,445
*As at the filing date Sample 1 has not ruptured under the 1150° C. and 5.3 MPa conditions. The Rupture Life mentioned here is the current value as at the filing date.

Alloy Analysis

SEM images were obtained using a TESCAN VEGA scanning electron microscope. Operating conditions and magnifications were as described alongside the relevant SEM image in the figures.

EDX analysis was performed using a TESCAN VEGA scanning electron microscope equipped with an Oxford Instruments Ultimax 170 EDS detector.

Example 8

SEM images of Sample 3 of Example 6 were taken after rupture occurred at 3,360 hours following exposure to conditions of 1150° C. and 5.3 MPa (see FIGS. 2A and 2B). The white box in FIG. 2A shows the area of the surface imaged in FIG. 2B. It is clear from the cross section that an aluminium oxide layer (shown by the white dashed box in FIG. 2B) is still present on the surface of the sample.

Sample 3 of Example 6 was also characterised using EDX mapping after rupture occurred at 3,360 hours following exposure to conditions of 1150° C. and 5.3 MPa (see FIGS. 3A-3D). EDX mapping was used to the to determine the weight percentage of elements (particularly, O, Al, and Cr) across a cross section of the sample.

It is clear from FIGS. 3A-C that a layer of pure aluminium oxide is present on the surface of the alloy. This confirms that the alloy is able to form a protective layer of aluminium oxide during service and that this can be maintained for a duration equivalent to at least the rupture life.

It is also clear from FIG. 3A that aluminium remains present in the alloy beyond the oxide layer. This suggests that the alloy is able to replenish the aluminium oxide layer, even after prolonged oxidation (3,360 hrs) at high temperatures (1150° C.) in air.

FIGS. 3A-C also show a lack of chromium at the surface, indicating the absence of chromium carbide on the surface of the alloy. The absence of chromium carbide at the surface prevents the generation of chromium oxide at the surface. The presence of chromium oxide would reduce the purity of the aluminium oxide layer and therefore reduce the efficiency of the aluminium oxide layer in preventing oxidation, carburisation and coking.

Example 9

SEM images were obtained of Example 6, sample 3, as-cast, and of Example 6, Sample 3, after rupture at 2,445 hours of 1150° C. and 6.4 MPa.

FIG. 4A shows the presence of primary chromium carbides within the as-cast alloy matrix. The dark region ‘s5’ indicated by the arrow in FIG. 4A shows an exemplary primary chromium carbide region within the matrix. The indicated region clearly shows the elongated morphology and size of such carbides in the as-cast alloy.

FIG. 4B again indicates the presence of primary chromium carbides within the alloy matrix after prolonged heating at 1150° C. It is clear from FIG. 4B that the primary chromium carbides retained their elongated morphology and size following prolonged heating at 1150° C.

The skilled person will appreciate that, during the aging of existing alloys at high temperature, primary chromium carbides are known to agglomerate within the matrix, becoming larger in size, and exhibiting a more ‘rounded’ morphology, indicating structural instability within the matrix. Conversely, the entirely surprising result depicted in FIG. 4B, shows that alloys of the present invention are able to maintain the size and morphology of the primary chromium carbides after aging at high temperatures, giving rise to the superior creep properties and rupture life described herein.

Example 10

SEM images were obtained of Example 6, sample 3, as-cast, and of Example 6, Sample 3, after rupture at 2,445 hours of 1150° C. and 6.4 MPa.

FIG. 4A shows the presence of niobium carbides within the as-cast alloy matrix. The light region ‘s2’ indicated by the arrow in FIG. 4A shows an exemplary niobium carbide region within the matrix. The indicated region clearly shows the elongated morphology and size of such carbides in the as-cast alloy.

FIG. 4B shows the presence of the niobium carbides within the alloy matrix after prolonged heating at 1150° C. It is clear from FIG. 4B that the niobium carbides retained their morphology and size following prolonged heating at 1150° C., demonstrating the structural stability of alloys of the invention.

Example 11

Quantitative EDX analysis of Example 6, sample 3, was performed on the as-cast alloy, and also on Example 6, Sample 3, after 1 week and after 3,360 hours of aging at 1150° C. and 5.3 MPa. EDX analysis was performed on a region of the sample corresponding to either a primary chromium carbide region (FIG. 5) or a niobium carbide region (FIG. 6) and the weight percentage of each detected element was recorded.

The results of the analysis performed on the primary chromium carbide regions are shown in FIG. 5, where the amount of tungsten present in the primary chromium carbide region was recorded as 2.61 wt. % (as-cast), 7.21 wt. % (aged 1 week), and 7.39 wt. % (aged 3,360 hours).

These results demonstrate the migration of tungsten into the primary chromium carbide regions within the matrix during aging and service. The stability of the primary chromium carbides is demonstrated from the above results from the migration and then retention of tungsten in these regions.

The results of the analysis performed on the niobium carbide regions are shown in FIG. 6, where the amount of tungsten present in the niobium carbide regions was recorded as 5.8 wt. % (as-cast), 6.16 wt. % (aged 1 week), and 5.05 wt. % (aged 3,360 hours).

These results demonstrate that the amount of tungsten in the niobium carbide regions remains relatively high and constant during service, suggesting that the stability of the niobium carbides demonstrated above results from the presence of tungsten in these regions.