ES2967219T3 - Oxidation resistant alloy - Google Patents

Abstract

The present invention relates to alloys used to prepare steel pipes, i.e., pipes for use in chemical engineering applications. In particular, the invention relates to low-carbon steel-aluminum alloys and pipes made from such alloys. They can be used in plants such as ethylene cracking furnaces that need to be able to withstand elevated oxidation and carburization temperatures for extended periods of time; the alloy can develop a pure, stable, and continuous aluminum oxide layer on its surface when in service, which is protective and anti-coking. (Automatic translation with Google Translate, no legal value)

Description

DESCRIPTION

Oxidation resistant alloy

The present invention relates to alloys used to prepare steel tubes, that is, tubes for use in chemical engineering applications. In particular, the invention relates to low carbon steel and aluminum alloys and pipes manufactured from such alloys. They can be used in plants such as ethylene cracking furnaces that must be able to withstand elevated temperatures for long periods of time.

Alloy materials for use at high temperatures, for example alloy tubes used in ethylene pyrolysis cracking furnaces and to some extent in steam methane reforming, often suffer from insufficient resistance to oxidation and carburization. The industry continues to pursue improved materials and other technologies to enable more efficient ethylene production under increasingly stringent pyrolysis/cracking conditions (higher temperatures, shorter residence times, and lower product partial pressures), leading to higher ethylene yields. Current alloys have specific problems related to their corrosion resistance, leading to failure at increasingly higher design procedure temperatures. This is currently the case for both castable alloy tubes and forged alloy tubes.

It is necessary to heat the tubes to very high temperatures (above 900°C) to allow steam cracking to occur continuously. This imposes strict design requirements on the reactor and the piping used in its construction. A particular problem concerns the oxidation and/or corrosion of pipes as well as problems caused by coking of pipes during use.

Pipes for steam cracking furnaces can be prepared by a centrifugal casting process. Centrifugal casting is a well-established procedure used to cast thin-walled cylinders, pipes, and other axially symmetrical objects. A benefit of this procedure 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 aluminum, copper and nickel alloys. The centrifugal casting procedure employs a permanent mold that rotates around its axis at high speeds, typically 300 to 3,000 rpm, as molten metal is poured. The molten metal is centrifugally thrown toward the inner wall of the mold, where it can solidify after cooling. The resulting cast cylinder, that is, a tube, has fine grain, and the surface roughness of the outer surface of the cylinder is relatively low.

Document JP64-031931 describes the manufacture of a curved tube of a heat resistant alloy. The tube is prepared by centrifugal casting and JP64-031931 alloy is made of high-strength and heat-resistant cast steel containing 15-30% chromium, 20-40% nickel, as well as the optional inclusion of small amounts of manganese and molybdenum. Small amounts of niobium and titanium are also added to the alloy. The cast tube is then subjected to the additional step of aging treatment at a temperature of 700 to 1100 °C to deposit secondary carbide within the grain structure. This patent does not attempt to control the formation of primary carbide or to control the relative amounts of niobium and titanium, or carbon and nitrogen. It is then subjected to another processing step involving high frequency bending or die cutting at a temperature in the range of 550 to 1100 °C.

Document WO2012/121389 discloses an alloy intended for use in nuclear applications such as heat exchanges in pressurized water reactors. The material is said to have excellent fine workability and corrosion resistance. This material is based on a nickel-chromium-iron alloy and contains small amounts of manganese, titanium and, optionally, aluminum as alloying elements.

Document WO2000/22068 discloses anti-coking coatings for refractory alloys used in the petroleum industry. The coatings are obtained by a method that involves subjecting the surface of refractory alloys to the action of oxygen and/or nitrogen gas plasma at low frequency and depositing a coating based on silicon oxide, nitride or oxynitride on the treated surface by means of deposition. plasma-enhanced vapor chemistry at low frequency.

US 2002192494 discloses a method for protecting low carbon steel and stainless steel from coking and corrosion at elevated temperatures in corrosive environments such as during ethylene production by hydrocarbon pyrolysis. The coating is an alloy of chromium, aluminum and silicon together with three other components, one of which is selected from nickel, cobalt, iron or a mixture of these elements, the second of which is selected from yttrium, hafnium, zirconium, lanthanum, scandium or a combination of those elements, and the third is selected from tantalum, titanium, platinum, palladium, rhenium, molybdenum, tungsten, niobium, boron or a combination of those elements. The mixed powder composition of these alloys is applied to the surface and heat treated to bond the coating to the surface.

US 2001001399 discloses steel-based austenitic nickel-chromium steel alloys that are particularly suitable for use as heat-resistant and high-strength materials for parts, such as pipes, in petrochemical cracking furnaces that are intended for the production of ethylene or synthesis gas. The alloy is a low carbon steel containing 0.3 to 1.0% carbon.

In addition to all the usual technical issues associated with preparing a steel pipe for use in a chemical plant, there are two particular problems that must be addressed when manufacturing pipe for this type of application. These problems arise due to the harsh working environment that the steel tubes will be exposed to and the fact that any "downtime" in plant operation is very costly in terms of lost production. Pipes must be resistant to rust and/or corrosion over extended periods of use. The pipes must also be able to be in service for long periods of time without becoming coked or fouled. Decoking reactor tubes is traditionally done by passing air and steam through the tubes to burn the coke. However, this is an operation of at least one or two days and therefore results in significant downtime and loss of production with the attendant costs of this downtime.

The present invention aims to provide pipes that are resistant to oxidation. Furthermore, the invention aims for the surfaces of these pipes to be resistant to the formation of coke with the intention of reducing the frequency of decoking operations. An objective of the invention is that when the alloy is manufactured in pipes, the surface of these has a higher resistance to the formation of coke compared to conventional pipes. Similarly, it is an objective of the invention that once formed into pipes and used in a chemical reactor or similar environment that the surface of the pipes is stable and resistant to chemical attack, oxidation, corrosion, etc. Furthermore, it is an objective of the invention that the pipes also have the appropriate mechanical properties in terms of creep resistance and ductility, etc., than existing pipes, so that they are suitable for use in high temperature chemical reactors.

It is also an object of the present invention to prepare a pipe that can be produced by a process that is convenient to execute, so that the manufacturing process is relatively simple. It is also an objective to provide a procedure that is applicable to the large scale production of alloy steel pipes. The invention aims to provide a more economical production method and/or that is also economical when considering interruptions in use and maintenance throughout life. It is also an object of the present invention to provide a steel alloy that is economical to manufacture and that avoids or reduces the need for expensive alloy components.

It is also a goal to have pipelines that can be prepared without the need for additional post-processing steps.

The invention satisfies some or all of the above objectives.

According to a first aspect of the present invention, there is provided an alloy comprising:

from 0.15 to 0.35% by weight of carbon,

from 2.5 to 5.0% by weight of aluminum,

from 40 to 45% by weight of nickel,

from 25 to 35% by weight of chromium,

from 0.50 to 1.50% by weight of niobium and/or vanadium,

from 0.01% by weight to 0.25% by weight of yttrium,

from 0.01% by weight to 0.25% by weight of tungsten and/or tantalum,

from 0.01% by weight to 0.25% by weight in total of one or more of titanium and/or zirconium and/or hafnium,

up to 0.9% by weight of manganese,

up to 0.9% by weight of silicon, and

up to 0.10% by weight of nitrogen,

the rest of the composition being iron and incidental impurities.

In one embodiment, the alloy comprises:

from 0.15 to 0.35% by weight of carbon,

from 2.5 to 5.0% by weight of aluminum,

from 40 to 45% by weight of nickel,

from 25 to 35% by weight of chromium,

from 0.50 to 1.50% by weight of niobium and/or vanadium,

from 0.01% by weight to 0.05% by weight of yttrium,

from 0.05% by weight to 0.25% by weight of tungsten and/or tantalum,

from 0.04% by weight to 0.15% by weight in total of one or more of titanium and/or zirconium and/or hafnium,

an amount of up to 0.9% by weight of manganese,

an amount of up to 0.6% by weight of silicon, and

an amount of up to 0.10% by weight of nitrogen,

the rest of the composition being iron and incidental impurities.

The alloys of the invention can be used to manufacture steel tubes, particularly steel tubes intended for use in chemical engineering applications such as chemical reactors and ethylene crackers.

It is the careful control of the metallurgical composition that contributes to improving the anti-coking properties of these alloys. Each of the elemental components described in the steel compositions above plays an important role in the properties of the steel and its resistance to coking and oxidation. Without wishing to be bound by any theory, it is believed that the particular combination of elements gives rise to the superior properties of the alloys of the invention due to the formation of a thin but continuous layer of alumina that can form on the surface of the alloy. at an early stage of use. This layer is stable and contributes to a reduced level of coke formation because the alumina is effectively non-catalytic and does not promote the formation of carbon deposits on the surface. In contrast, the alumina layer that forms during use generally provides an environment that is unfavorable for carbon deposition on the pipe surface.

Furthermore, due to the particular chemical composition of the alloys of the invention, it is possible that this protective oxide layer forms under what would be considered very reducing conditions, such as in situ in the ethylene cracker. In fact, the alloys of the present invention are capable of forming a stable protective surface layer even at oxygen partial pressures as low as 10-27 atmospheres. This means that pipes manufactured from the alloys of the invention can be used directly without the need for additional treatment and without the need for additional coatings. Accordingly, an additional benefit of steel tubes of the invention (i.e., pipes manufactured using steel alloys of the invention) is that they do not require post-production post-treatment in order to be used in chemical engineering applications. This is a significant advantage both in terms of time and cost over pipes made from existing alloys.

The alloys of the invention also benefit from the fact that they contain little or no nickel, chromium or iron at the alloy surface/cracking gas interface. These elements normally behave as catalytic elements when in contact with the cracking gas and would promote the formation and deposition of coke under normal conditions of use. The ability of the alloys of the present invention to be able during service to avoid any concentration or the complete presence of these elements on their surface and to form an alumina oxide layer containing little or none of these elements represents a substantial advantage over the alloys of the prior art. The retention of these particular elements, which would otherwise be problematic in terms of carbon deposition, within the bulk matrix of the alloy is therefore an important characteristic of the alloys of the present invention and arises as a result of carefully controlled metallurgy. Therefore, it can be seen that the combination of the thin surface layer of alumina and the absence of elemental components such as nickel, chromium and iron on the surface together lead to substantial improvements in resistance to coking, coke formation and the deposition.

Surprisingly, the relative proportions of the elements necessary to guarantee the anti-coking effect do not negatively affect the mechanical properties of the alloys. This is important because good mechanical properties of components are required in chemical engineering applications. Surprisingly, the inventors discovered that the combination of elements used in the alloys of the present invention also contributes to the high temperature resistance of the steel tube.

The individual elemental components of the alloy perform the functions described below.

Carbon

Carbon is required in an amount from 0.15 to 0.35% by weight in the alloys of the invention. The amount is carefully controlled because carbon has several different functions. For example, carbon is often an important component of steel to provide tensile strength and resistance to creep rupture. This is because carbon is an essential component in the formation of carbides that normally provide strength to steel due to the precipitation of primary and secondary carbides. At the same time, the interaction between the amount of aluminum added and the amount of carbon must be closely controlled. Aluminum is generally incompatible with carbon since aluminum rejects carbon in the sense of decreasing its solubility in the austenitic matrix. This has the effect of carbon migrating to the grain boundaries during solidification with the consequence that the alloy precipitates an excessive amount of primary chromium carbide, forming a thick network that makes it brittle. According to the above, 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 0.15% by weight 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 interacts negatively with the added aluminum. For this reason, the upper limit of carbon is 0.35% by weight. In a preferred embodiment, the amount of carbon is from 0.20 to 0.35%, and in a more preferred embodiment the amount of carbon is from 0.25 to 0.30% by weight.

Aluminum

Aluminum is another very important component of the alloys of the invention. Aluminum is present in an amount from 2.5 to 5.0% by weight. Aluminum alloys are generally difficult to cast and therefore can be hard and complicated work. The steels of the present invention are relatively easy to work despite the presence of aluminum.

Aluminum is necessary for the formation of the protective alumina layer on the surface. At the same time, aluminum and carbon compete for solubility and this can be a problem. On the other hand, as mentioned above, aluminum also decreases the solubility of carbon but, reciprocally, carbon decreases the solubility of aluminum in the matrix, which leads to brittleness. Not only because of the precipitation of thick chromium carbide at the grain boundary during solidification, but also because of the possibility of additionally precipitating thick nickel aluminide intermetallic. In fact, aluminum has also been rejected like carbon during solidification by the austenitic matrix. This is the undesirable precipitation of nickel aluminide, known for its embrittlement properties, during solidification.

Therefore, it is important to ensure that the upper limit of aluminum does not exceed 5% by weight so that the alloy does not become brittle and form an uncontrolled excess of intermetallic nickel aluminide in the interdental space and grain boundaries. At the same time, a minimum amount of aluminum of 2.5% by weight is required to ensure that a substantially continuous coating of alumina can form on the surface of the alloy during use.

Therefore, aluminum is added to the alloy to allow the formation of a continuous, dense, homogeneous and adherent thin protective layer of alumina (aluminum oxide) on the surface of the alloy during service. This aluminum oxide layer is resistant to oxidation (slow growth of the oxide layer during service prevents further reaction or oxidation of the alloy). Aluminum oxide is a stable surface barrier that stops carbon diffusion in the alloy. It serves to facilitate anti-coking (without catalytic reaction that involves the formation of graphite/coke during cracking), protecting the alloy and allowing its surface to be neutral when exposed to the gas to be cracked.

The higher the aluminum content, the greater the aluminum deposition/stock, which is important in case the aluminum oxide layer needs to be re-formed. Conditions under which this could be a problem include spallation of aluminum oxide due to over-decoking, a variation in service conditions, or an uncontrolled event in the cracking furnace. Alumina also provides resistance to carburization as described above and therefore it is important to ensure that there is sufficient aluminum in the alloy to provide an effective coating. This is the reason why the lower limit of aluminum is specified. The minimum of 2.5 wt% is important to ensure layer formation at an adequate rate during service.

In the case of the alloys of the present invention, the aluminum content may exceed the usual levels normally included in aluminum steels specifically due to the deliberately low level of carbon that is added to the alloys of the invention. Therefore, it is possible for the amount of aluminum to reach levels much higher than 3.5% by weight due to the relatively low carbon content, and amounts of aluminum of up to 5.0% by weight can be achieved without damaging the properties of the alloy. Above this level there is a risk that the alloy becomes brittle, so that, for example, it may break during handling or during subsequent manufacturing steps, such as straightening the pipe after casting.

The aluminum content is preferably in the range of 3.0% by weight to 5.0% by weight, and more preferably is in the range of 3.2% by weight to 4.6% by weight, such as in the range of 3.5% by weight to 4.5 % in weigh.

It is important to note that the maximum and minimum aluminum content is considered in relation to the carbon content. This is also relevant to the increase in thermal expansion of the alloy at high temperature. The relative proportions of carbon and aluminum, calculated in terms of their weight % in the alloy, range from approximately 1:33.3 to 1:7.1. Preferably, the relative amounts are in the range from about 1:17 to 1:22.

Chrome

Chromium is present in an amount from 25% by weight to 35% by weight. Chromium forms a primary carbide network during solidification (as described for carbon) that imparts primary strength to the alloy and also forms secondary carbides during service with good creep resistance properties. In the event that the aluminum has been completely exhausted after a long service of the alloy, or after a spectacular decoking of the furnace, the chromium may form an oxide layer that may delay the carburization of the alloy. The chromium content of the matrix is capable of delaying the carbonization/carburization of the alloy by trapping the carbon when it is in the diffusion process, thus forming chromium carbides.

Chromium is not anti-coking, but provides an effective and well-documented oxidation and corrosion resistance effect, as well as acting as a carbide former. The formation of carbides ensures creep and strengthens precipitations in the alloy. The lower limit of 25 wt% chromium is required to ensure sufficient oxidation resistance and the upper limit of 35 wt% is determined by the fact that above this level it is difficult to obtain a stable austenite phase. Additionally, too high a level of chromium makes steel unworkable. In some embodiments, chromium is present in the range of 26% by weight to 31% by weight, more particularly in the range of 28% by weight to 31% by weight, such as in the range of 28% by weight to 30 % in weigh. In certain cases, the amount of chromium is in the range of 29% by weight to 30% by weight. In some alternative embodiments, chromium is present in the range from 30 wt% to 33 wt%, and more preferably in the range from 31 wt% to 32 wt%.

The chromium content is determined in equilibrium with the nickel content of the alloy so that the final alloy has an austenitic base matrix stable at elevated temperature. It is important that a stable austenitic matrix exists at each expected service temperature to which the alloy is likely to be exposed. Therefore, it is important to consider the amount of chromium to be used in the context of the amount of nickel that is also present in the alloy. The relatively high amount of chromium is intended to provide its availability to form chromium oxide on the surface and to allow the chromium to trap carbon in the form of chromium carbide in case of carburization not contained in the fine alumina oxide.

Nickel

Nickel is present in an amount from 40% by weight to 45% by weight. Nickel provides the stable austenitic matrix base of the alloy. Nickel is an essential element to obtain a stable austenite structure and improves the stability of austenite and suppresses the generation of sigma phase. Nickel is the austenitic stabilizing element, allowing the alloy to be generally strong above 800°C. Therefore it forms a stable matrix with iron that allows the possible precipitation of carbides/nitrides. The lower limit of nickel content is chosen simply because it is a sufficient amount to improve the stability of austenite relative to the lower limits of the other elements. The lower limit of nickel content is chosen simply because it is a sufficient amount to improve the stability of austenite relative to the lower limits of the other elements.

Nickel is also very important for cracking furnace applications, as it provides better resistance to carburization of the alloy, due to its slow carbon diffusion kinetics. In this sense, the kinetics of nickel in terms of carburization is much slower than in the case of iron. On the other hand, nickel can be harmful in some circumstances because it favors the formation of catalytic coke when it is present on the surface of the alloy, and more precisely at the alloy/cracking gas interface. Therefore, the upper limit of nickel is necessary to contribute to the carburization resistance of the alloy but at the same time should not be exceeded due to its known property of promoting catalytic coking if it appears on the surface of the alloy. Therefore, it is important to ensure that there is not too much nickel in the alloy. Furthermore, nickel is an expensive material and, consequently, avoiding using nickel content also has an economic advantage. The lower limit of nickel is governed by the need to provide a suitable austenitic matrix. In some embodiments, the nickel is present in an amount from 42% by weight to 45% by weight, and more preferably in an amount from 42% by weight to 44% by weight. In some particularly preferred embodiments, the nickel is present in an amount of from 42.5% by weight to 43.5% by weight.

In some alternative embodiments of the invention, it is possible to replace part of the nickel content with cobalt. Cobalt is capable of performing the same function as nickel in the alloys of the invention. In such embodiments, the total amount of cobalt and nickel is from 40% by weight to 45% by weight. Up to 75% by weight of nickel can be replaced by cobalt and, in principle, any amount from 1% by weight to 75% by weight of nickel can be replaced by cobalt; However, in practice it is more common to replace between 5% by weight and 15% by weight of nickel with cobalt due to the higher cost of cobalt. The amount of nickel (and cobalt, when present) that is included in the alloy of the present invention is carefully controlled in relation to the amount of chromium that is present to ensure that the alloy is truly a stable austenitic matrix at any temperature. service that is used.

Niobium

Niobium is present in an amount from 0.50 to 1.50% by weight. The function of niobium is to form thin primary carbides during solidification, these primary carbides have the particularity in this specific alloy of re-dissolving at high temperature (therefore during service) and as a consequence of facilitating the precipitation of the secondary carbides and ensure its dispersion. This contributes to the creep resistance of the alloy.

Therefore, the primary niobium carbides act as a carbon reservoir, which can then be released at service temperature to create more creep-efficient carbides. This feature is essential as carbon is not present in the usual excess that would be typical in a high temperature alloy having a carbon content greater than 0.40 wt%. In the alloys of the present invention, the release of carbon by niobium compensates for this and therefore provides carbon when necessary. Furthermore, during solidification, niobium carbides form mainly in the interdendritic space or grain boundary, thus predominantly participating in the primary carbide network. The amount of niobium used is chosen based on its effectiveness in altering the primary chromium carbide network. In some embodiments, the niobium is present in an amount from 0.80 wt% to 1.10 wt%, and preferably from 0.80 wt% to 0.90 wt% or from 0.90 wt% to 1.10 wt%. In other embodiments, the niobium is present in an amount of from 1.10% by weight to 1.30% by weight.

In some alternative embodiments of the invention, it is possible to replace niobium with vanadium. Vanadium is a carbide former of the same periodic group as niobium and can be used in the same quantities as those described above for niobium. Additionally, a mixture of the two can be employed as long as the ranges mentioned above for niobium are met for the mixture.

Titanium, zirconium and hafnium

The titanium and/or zirconium may be present independently of each other in a total amount of 0.01% by weight to 0.25% by weight, and preferably in an amount from 0.04% by weight to 0.15% by weight. In other words, within the indicated range titanium may be present, without zirconium being present; or zirconium may be present within the indicated range, without titanium being present; or both titanium and zirconium may be present and in this case each may be present in any quantity within the indicated range, subject to not exceeding the total limit. Both elements work in a similar way forming carbides. It is generally preferred to have titanium present. In some embodiments, the amount of the element(s) (typically, but not necessarily, titanium alone) is from 0.08 wt% to 0.15 wt%, and more preferably is from 0.08 wt% to 0.13 wt%. In some embodiments, only titanium is present in this amount and substantially no zirconium or hafnium is present.

Titanium and zirconium are stable carbide-forming elements and are used alone or together to improve the creep properties of the alloy. Either element individually or both elements together do this by precipitating secondary carbides during service. On the other hand, as carbide-forming elements, they not only form their respective carbides but are also capable of forming titanium/zirconium-niobium double carbide precipitates that improve creep resistance.

In addition, titanium and zirconium have a second function: they are very active oxygen scavengers and, consequently, help protect the aluminum present in the melt from oxidation. Therefore, these elements oxidize before aluminum. Titanium/zirconium are added to the melt as a deoxidizer.

The amounts of one or both elements used represent a balance between obtaining an improvement in the creep properties of the alloy (which are decreased due to the addition of aluminum) and the undesirable formation of oxides, carbides and nitrides when added. too much. Indeed, the addition of too high a quantity of titanium or zirconium can lead to the undesirable formation of oxides, carbides and nitrides, hardening the alloy too much.

Cost is also an issue as both items are expensive raw materials.

In some alternative embodiments of the invention, it is possible to replace one or both amounts of titanium and zirconium with hafnium.

Therefore, in certain embodiments of the invention, hafnium can be used in addition to or instead of one or both of titanium and zirconium. In other words, in some embodiments of the invention there may be no titanium or zirconium present in the alloy and hafnium may be present in place of these elements. In other embodiments, some or all of the titanium and/or zirconium is replaced by hafnium, with the consequence that all three elements are present. In this circumstance, the minimum amount of the three elements together would be 0.04% by weight and the maximum amount of the three elements together would be 0.15% by weight. Hafnium is effective in increasing the creep resistance properties of the alloy, but has the disadvantage of being an expensive raw material.

In certain embodiments of the invention, it is preferred that titanium be used in the alloys of the invention without zirconium or hafnium being present. In an alternative embodiment of the invention, both titanium and zirconium can be used without hafnium being present.

Tungsten

The tungsten is present in an amount from 0.01% by weight to 0.25% by weight, and preferably is present in an amount from 0.05% by weight to 0.25% by weight. Tungsten is a large element in terms of its atomic size and is also a carbide-forming element. Both factors contribute to an improvement in the alloy's creep properties when tungsten is added. The amount of tungsten added is a balance between improving creep properties and limiting the resulting high temperature plastic strain elongation involved by its presence. There are diminishing returns to this improvement, on the one hand, and the expense of adding tungsten, on the other hand. In some embodiments, the amount of tungsten is from 0.05% by weight to 0.15% by weight, and more preferably is from 0.05% by weight to 0.10% by weight. In certain embodiments, the amount of tungsten is preferably from 0.01% by weight to 0.05% by weight.

In certain alternative embodiments of the invention, the tungsten can be replaced by tantalum for the same purpose. In this circumstance, there would be no tungsten present in the alloy and the amount of tungsten would be replaced by the same amount of tantalum. In these alternative embodiments, the amount of tantalum would therefore be from 0.01% by weight to 0.25% by weight, with subranges similar to those described above in relation to tungsten being preferred for tantalum. However, it is preferable to use tungsten instead of tantalum. In certain embodiments, both tungsten and tantalum could be used in amounts such that the total amount of both elements is from 0.01% by weight to 0.25% by weight.

Yttrium

Yttrium is present in an amount from 0.01% by weight to 0.25% by weight, and preferably is present in an amount from 0.01% by weight to 0.05% by weight. A particularly preferred amount is from 0.03% by weight to 0.05% by weight. Yttrium plays several functions in the alloys of the invention. First of all, it serves to protect the addition of aluminum to the melt, since the melt is formed in air and not under a special protective atmosphere.

Yttrium is more reactive with oxygen than aluminum, so it acts as an oxygen scavenger.

Furthermore, yttrium, when present in small amounts, modifies the shape of the primary carbide network making the resulting carbides thinner and discontinuous and, consequently, less brittle. Yttrium also has the advantage of improving the adhesion of the surface oxide layer, acting as a "sink" for the cavities.

It is not necessary to have a very large amount of yttrium (another very expensive element) in the alloy to achieve these beneficial effects and an amount within the indicated range is sufficient for this purpose. In some embodiments, the amount of yttrium is from 0.01% by weight to 0.03% by weight.

Silicon

Silicon is present in an amount up to 0.9% by weight. In some embodiments, silicon is present in an amount up to 0.6% by weight. In certain embodiments, the amount of silicon is from 0.6% by weight to 0.9% by weight. Silicon provides the deoxidant function and is usually an essential component in an austenite stainless steel. Silicon can also help increase the stability of any surface oxide film. Silicon also provides some fluidity to the molten pool prior to the addition of aluminum to the melt. On the other hand, if the silicon content is too high, the workability of the steel is reduced. High Si content can also cause the formation of a harmful phase known as G phase, which is composed of nickel, silicon and niobium (Ni16Nb6Si7). Consequently, the amount of silicon must be carefully controlled. In some embodiments, the silicon is present in an amount from 0.3% by weight to 0.6% by weight, and more preferably from 0.5% by weight to 0.6% by weight. In certain embodiments, silicon may be absent or only a lower limit of 0.1 wt% silicon is needed.

Manganese

Manganese is present in an amount up to 0.9% by weight. Manganese can improve the workability of the alloy and is also an effective deoxidizer and contributes to the formation of austenite in steel. However, manganese's high diffusion coefficient at high temperature makes it compete with aluminum. On the other hand, the addition of too much manganese can result in a reduction in high temperature strength and also toughness over a prolonged period of time. Consequently, the amount of manganese should be limited to 0.9% by weight. In some embodiments, manganese is present in an amount of from 0.4 wt.% to 0.8 wt.%, and more preferably from 0.4 wt.% to 0.6 wt.%. In certain embodiments, manganese is present in an amount of from 0.7% by weight to 0.9% by weight. In certain embodiments, manganese may be absent or only a lower limit of manganese of 0.1% by weight is needed.

Nitrogen

Nitrogen is inevitably present in the alloys of the invention. It is present in an amount of up to 0.10% by weight. Nitrogen can be found in the alloy because the alloy mixture is prepared under an atmosphere of air, 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 is insignificant and/or immeasurable and in these cases the effective amount of nitrogen is almost zero. The absence of measurable nitrogen is not harmful. Likewise, in certain cases there is a measurable amount of nitrogen and in these cases the upper limit is 0.10% by weight.

Nitrogen forms austenite together with carbon and contributes to high temperature strength. Nitrogen allows the dilution, dispersion and homogenization of carbon. However, careful control of the amount of nitrogen is important because it retards the precipitation of primary chromium carbides when added in an appropriate amount. In effect, nitrogen helps control the 'behavior' of carbon to control its various precipitations. Nitrogen participates in the precipitation of secondary niobium carbides, niobium carbido-nitrides and niobium nitrides during aging, which are more stable than carbides at high temperature and therefore have better creep properties. However, if the amount of nitrogen is too large, an excessive amount of nitrides such as chromium nitrides and/or aluminum nitrides are produced, which reduces the ductility and toughness of the alloy over a long period of time and prevents it from the "nitrided" elements play their role in the alloy as they should. Therefore, it is essential to keep the amount of nitrogen present at a maximum of 0.10% by weight. However, there is no practical lower limit for measurable nitrogen in an alloy sample of the invention.

The alloys according to the present invention are produced in a conventional furnace and without the need for a special atmosphere. The first stage of alloy preparation involves determining the relative proportions by weight of the various component minerals (which are the source of the various elements required in the final alloy) to achieve the desired amounts of the various elements required in the alloy. final. Solid minerals are added to the hot oven. Heating continues to melt all the mineral components and ensure complete mixing of the minerals in the furnace so that the elements are properly distributed within the matrix.

In circumstances where 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 may be present as an impurity in the adding agent for the first essential element), then the overall composition must be carefully controlled to ensure that all 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 of the alloys of the invention. Sometimes elements are added in the form of preformed alloys, for example ferrotitanium could be used as a source of titanium and consequently the elemental calculations are adapted as necessary so that the required amount of the element (titanium in that example) is added correctly. particular). The skilled person will be able to analyze and compensate as necessary for variations in the essential elemental components due to the presence of incidental impurities using known analytical techniques and varying the amounts of the usual adding agents for each essential component.

A number of elements will be present in the alloy as unavoidable impurities. Such incidental elements will have no discernible technical benefit or adverse effect on the alloys of the present invention. In some cases, the presence of elements such as nitrogen can be tolerated in relatively large quantities as long as they do not affect the desired properties of the alloy. For example, although not specifically provided for in the alloys of the present invention, it is conceivable that an element could arise as an incidental impurity as a consequence of its appearance as an impurity in one of the deliberately added elemental components. This is acceptable as long as the presence of said element does not have any detrimental effect on the alloy. In certain cases, deliberately added elemental components, such as titanium, may bring with them other incidental elements. They can generally be tolerated as incidental impurities at low levels. When analysis reveals that such impurities are unacceptable, an alternative source of the desired elemental component (free of harmful impurities) is used.

The minerals and elements of the alloy composition are carefully added in a suitable sequence to obtain the desired content for the final composition. It is also important to protect the aluminum when it is added to the melt and prevent its oxidation because the melting takes place in air.

Once fusion and mixing have been achieved, the slag is decanted from the furnace to remove impurities and clean the furnace's liquid alloy bath. A sample of the molten alloy is then removed from the furnace, allowed to cool, and analyzed by X-ray fluorescence to determine its elemental composition. Composition adjustment may or may not be necessary at this stage to account for any loss of elemental mass due to volatility. The composition is adjusted by adding additional minerals as necessary and optionally reanalyzed to ensure the desired composition has been achieved.

Once the desired composition is reached, the temperature is further raised above the melting temperature to an extraction temperature to ensure easy pouring of the melt. At the same time, the mold is prepared for centrifugal casting.

The mold is a conventional centrifugal casting mold and this type of mold is well known to the skilled person. The mold preparation procedure involves washing the mold with water/steam to clean it and remove any old wash or coating that may have been used in a previous casting procedure. The washed mold is then coated with an insulating/molding agent necessary to prevent the alloy from sticking to the mold after casting. A typical insulating/releasing agent is silica or alumina.

A ceramic disc is then added to the centrifugal casting mold in a manner known in the art to ensure that the mold is liquid tight and ready for casting. This prevents any alloy leakage during the casting procedure. The mold temperature is adjusted in preparation for casting and may be in the range of 200° to 300°C. The mold is then rotated at high speed to generally obtain the range of 80 g to 120 g, with a rotation providing 100 g being typical for a centrifugal casting speed.

A ladle is then taken to the furnace and the desired weight of molten alloy for casting purposes, including relevant additions such as titanium and yttrium, is removed to achieve the desired final amount of aluminum and other reactive elements in the alloy composition. . The ladle itself is preheated to a temperature in the range of 800° to 1000°C to minimize cooling of the alloy after pouring. The alloy is then transferred to the hot ladle. At this stage, further analysis of the alloy can be performed.

The molten alloy in the ladle is then transferred to a pouring cup. The tip of the pouring cup has been pre-adjusted to ensure it correctly matches and fits the size of the inlet tube for the centrifugal casting mold. The level of molten alloy in the pouring cup is maintained to maintain adequate flow of alloy into the mold which is, in effect, fed by gravity. This provides a continuous flow of alloy into the mold until the entire weight of the alloy has been poured into the mold. The mold rotates at high speed, that is, it is maintained at the centrifugal casting speed during the procedure and while the alloy is melted. The time the casting procedure takes ultimately depends on the desired thickness of tube required and the expert is able to determine an appropriate rotation time for a particular tube thickness and alloy weight. The mold gradually slows down as the alloy cools from its solidification point. Generally speaking, a "fast" solidification procedure is one in which the alloy is melted and then cooled at a rate of more than about 100 °C per minute and a "slow" solidification procedure is one in which the alloy is melted and then cooled at a rate of approximately 50°C or more per minute. The casting procedure is usually completed in less than 10 minutes. The tube is removed once the mold stops and the procedure can be repeated again.

The steel tubes of the present invention show excellent high temperature resistance and high creep resistance. The tubes also exhibit exceptional corrosion resistance at elevated temperatures over an extended period of time. Consequently, these steels are particularly suitable for use in chemical plants in demanding environments such as steam cracking. Furthermore, it is expected that the steel tubes according to the invention can 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 pressurized water reactors.

The alloys of the invention were tested to demonstrate the effective formation of alumina (aluminum oxide) on the surface in the hostile environment of a bulk carburizing test at 1100°C over a period of 200 hours for an alloy containing 3.86% by weight of aluminum. The alloy composition is shown in table 1 below:

Table 1

The alloy was analyzed by E.D.X (energy dispersive X-ray). EDX cross-sectional surface mapping identifies oxide layer elements in the composition throughout its thickness and allows comparison with the alloy composition immediately below. Each of the elements in the alloy composition possesses a characteristic X-ray line such as Ka, and on the EDX map a greater amount of a particular element in an area is indicated by a greater brightness in the area.

Some of the key technical benefits of the invention can be seen in the following figures in which:

Figure 1 shows an alloy according to the invention that has been subjected to EDX cross-sectional surface mapping at the relatively low magnitude of 500X.

It can be seen in Figure 1 that aluminum and oxygen appear in the alloy sample according to the invention as a very bright and continuous line. This indicates two things. Firstly, aluminum and oxygen are present in a very high concentration, as alumina, in exactly the same position where the surface oxide layer is observed by SEM (scanning electron microscope) photography. Secondly, there is a continuous layer of alumina on the surface of the material.

This analysis demonstrates that there is a dense, adherent, thin and continuous layer of alumina (aluminum oxide) on the surface of the alloy even after bulk carburizing at 1100 °C for a period of 200 hours. It can also be seen that the alumina layer remains very stable and therefore does not transform into aluminum carbide, and acts as a protective layer, that is, a barrier against carburization.

Figure 2 shows the EDX surface mapping at the highest magnification of 2000X for an alloy according to the invention having the composition shown in Table 1. In Figure 2, it can be seen that aluminum and oxygen appear as the components brighter and are therefore present in very high concentrations. Furthermore, it can be seen that these elements appear in exactly the same position where the surface oxide layer is observed by SEM. An additional important feature is the dark areas of the figure that indicate the absence of elements such as nickel, iron and chromium. Figure 2 also illustrates the thin, dense, adherent, continuous alumina surface layer and indicates that it is pure and free of highly catalytic elements such as nickel and iron. The continuous surface layer and the absence of catalytic elements contribute to the anti-coke effect of the alloy when exposed to an external carbon source. It can be seen from Figure 2 that the alumina layer is anti-coking and does not bond or react catalytically with carbon during bulk carburizing at 1100 °C over a period of 200 hours.

Figure 3 shows a high magnification SEM photograph of the carburized surface of the alloy according to the invention having the composition of Table 1 (an alloy containing 3.8% by weight of aluminum). The photograph clearly illustrates a continuous protective layer of alumina on the surface. The thickness of the alumina layer was determined to be 4.28 ± 1.21 pm based on 100 measurements.

Therefore, it can be seen that the alloys of the invention demonstrate superior properties in terms of resistance to carburization and also exhibit resistance to oxidation due to the alumina layer that can form in situ on the alloys in use.

The alloys of the present invention can be tested using a thermal oxidation cycle test. This test involves circulating the alloy through high and low temperatures for an extended period of time to expose the alloy to oxidative stress. This test is frequently used to study the oxidation characteristics of materials such as superalloys/superalloy coatings used in gas turbines that are intended for high temperature use and/or use in extreme conditions.

The testing machine consists of an oven brought to the test temperature and a sample handling apparatus that can introduce and remove the sample from the oven very quickly to facilitate rapid heating and cooling. The alloys of the present invention were tested at 1150 °C. The higher the temperature, the faster the oxide layer grows, so a thick oxide layer is likely to form.

The sample handling apparatus is a mechanism for quickly introducing and removing the test sample from the oven. The higher the oven temperature, the more extreme the thermal shock will be to the sample as it is introduced and removed from the oven. Repeated exposure of the sample under these conditions leads to possible spallation of the oxide layer. The adhesion and thickness of the resulting oxide layer on the surface can then be investigated and used as a basis for establishing the performance of the alloys of the invention relative to those of the prior art.

The alloys of the invention were tested by repeating thermal shock exposure for 50 cycles. Figure 4 of the drawings illustrates the temperature profile in the thermal oxidation test. The samples were previously oxidized at 875 °C, for a period of 48 hours. Subsequently, the samples were exposed to 50 cycles in and out of the oven and the temperature profile is shown in Figure 4. Each cycle consists of heating the tested sample from room temperature by introducing it into the oven at 1150 °C , keep the sample in the oven for 45 minutes at that temperature in an atmosphere of air, and then remove the sample from the oven and cool it to room temperature over a period of 15 minutes. In the graph in Figure 4, the vertical axis represents both the sample temperature in degrees Celsius and also the air pressure in millibar. From the graph you can see that the air pressure in millibar is slightly less than 1000 millibar (approximately 975 mbar). It can also be seen from the graph that the maximum temperature reached in the heating cycles is 1150 °C and that the minimum temperature, represented by the inverse peaks in the graph, is effectively the ambient temperature. The horizontal axis of the graph indicates the time in hours during the entire test.

The repetitive nature of the test effectively exposes the sample to both mechanical fatigue and thermal fatigue, so that effects at the interface between the bulk sample and its oxide surface can be investigated. The growth rate of the oxide layer is measured when the sample is exposed to high temperatures. Therefore, the amount of stress and thermal cracking of the oxide layer can be observed as the oxide layer develops.

The mass change of the sample is recorded before and after the test to evaluate the stability of the oxide layer. The mass of any sample exposed to these harsh conditions is expected to change because the test is so rigorous. However, a smaller variation in mass before and after testing is indicative of a more stable and adherent oxide layer. This in turn is indicative of the stability of the alloy over a long period, such as during long service. The results obtained with the alloys according to the invention are very impressive.

Figure 5 shows the compositions of three alloys according to the invention that were subjected to the thermal oxidation test.

Figure 6 shows photographs of the three samples after the thermooxidation test and the mass variation for each of the samples. It can be seen from Figure 6 that the mass change after testing is negligible (all show a mass change of less than 1.15 mg) and, consequently, that the alloys of the invention demonstrate excellent stability under these conditions. This also shows that the alloys will be stable over a long period of use.

Figure 7 illustrates the cross section of the surface of sample C of Figure 5 after 50 cycles of thermooxidation. This is investigated by X-ray spectroscopy using copper Ka radiation. It can be seen that there is a continuous layer of oxide that is present on most of the sample. The oxide thickness is 3.61 pm ± 0.61 pm (based on 137 measurements). Figure 7 also highlights the scale of the aluminum and oxygen components of the alloy, as well as the scale of the nickel plating in the alloy.

The results of the thermooxidation test demonstrate that the alloys of the invention have good resistance to oxidation under extreme conditions. These results also show that the alloys will have a long service life.

Claims (15)

1. An alloy comprising: from 0.15 to 0.35% by weight of carbon, from 2.5 to 5.0% by weight of aluminum, from 40 to 45% by weight of nickel, from 25 to 35% by weight of chromium, from 0.50 to 1.50% by weight of niobium and/or vanadium, from 0.01% by weight to 0.25% by weight of yttrium, from 0.01% by weight to 0.25% by weight of tungsten and/or tantalum, from 0.01% by weight to 0.25% by weight in total of one or more of titanium and/or zirconium and/or hafnium, up to 0.9% by weight of manganese, up to 0.9% by weight of silicon, and up to 0.10% by weight of nitrogen, the rest of the composition being iron and incidental impurities, optionally where up to 75% by weight of the nickel is replaced with cobalt. 2. An alloy according to claim 1, comprising: from 0.15 to 0.35% by weight of carbon, from 2.5 to 5.0% by weight of aluminum, from 40 to 45% by weight of nickel, from 25 to 35% by weight of chromium, from 0.50 to 1.50% by weight of niobium, and/or vanadium from 0.01% by weight to 0.05% by weight of yttrium, from 0.05% by weight to 0.25% by weight of tungsten and/or tantalum, from 0.04% by weight to 0.15% by weight in total of one or more of titanium and/or zirconium and/or hafnium, an amount of up to 0.9% by weight of manganese, an amount of up to 0.6% by weight of silicon, and an amount of up to 0.10% by weight of nitrogen, the rest of the composition being iron and optionally incidental impurities where up to 75% by weight of the nickel is replaced with cobalt. 3. An alloy according to claim 1 or 2, wherein carbon is present in an amount from 0.20% by weight to 0.35% by weight. 4. An alloy according to claim 1, 2 or 3, wherein the aluminum is present in an amount from 3.5% by weight to 4.5% by weight. 5. An alloy according to claim 1, 2, 3 or 4, wherein the nickel is present in an amount from 42% by weight to 45% by weight. 6. An alloy according to any of the preceding claims, wherein the chromium is present in an amount from 28% by weight to 30% by weight. 7. An alloy according to any of the preceding claims, wherein the niobium is present in an amount from 0.80% by weight to 1.50% by weight. 8. An alloy according to any of the preceding claims, wherein the silicon is present in an amount from 0.3% by weight to 0.6% by weight. 9. An alloy according to any of the preceding claims, wherein the manganese is present in an amount from 0.4% by weight to 0.8% by weight. 10. An alloy according to any of the preceding claims, wherein the tungsten is present in an amount from 0.05% by weight to 0.15% by weight. 11. An alloy according to any of the preceding claims, wherein the titanium is present in an amount from 0.08% by weight to 0.15% by weight. 12. An alloy according to any of the preceding claims, wherein yttrium is present in an amount from 0.01% by weight to 0.03% by weight. 13. An alloy according to any of the preceding claims, wherein the nitrogen is present in an amount from 0.03% by weight to 0.06% by weight. 14. An alloy according to any of the preceding claims, in which between 1% by weight and 75% by weight of the nickel is replaced by cobalt, optionally in which between 5% by weight and 15% by weight of the Nickel is replaced by cobalt. 15. A steel tube manufactured from an alloy according to any of claims 1 to 14.