WO2025109334A1

WO2025109334A1 - Alloy composition

Info

Publication number: WO2025109334A1
Application number: PCT/GB2024/052956
Authority: WO
Inventors: Igor NOWAK
Original assignee: Paralloy Ltd
Current assignee: Paralloy Ltd
Priority date: 2023-11-24
Filing date: 2024-11-22
Publication date: 2025-05-30
Anticipated expiration: 2026-05-24

Abstract

The present invention relates to steel alloy compositions that may be suitable for use in forming the components of heat treatment furnaces, such as heat-resistant bottom plates in hydrogen heat treatment furnaces. The alloy compositions of the present invention may be heat-resistant alloy compositions that are able to withstand elevated temperatures for extended periods of time, whilst also being able to withstand repeated heating and cooling cycles.

Description

Alloy Composition

[0001] The present invention relates to steel alloy compositions. The alloy may be suitable for use in forming the components of heat treatment furnaces, such as heat resistant bottom plates in hydrogen heat treatment furnaces. Accordingly, the alloy compositions of the present invention may be heat-resistant alloy compositions that are able to withstand elevated temperatures for extended periods of time, whilst also being able to withstand repeated heating and cooling cycles.

BACKGROUND

[0002] Heat treatment furnaces, such as hydrogen heat treatment furnaces, or Bell annealing furnaces operate at temperatures of around 1200 °C. The temperature of the heat treatment furnace is ramped to such operating temperatures before being maintained at the operating temperature for a number of days, depending on the material being heat treated. After the heat treatment process, the furnace is cooled slowly to ambient temperature over a number of days. This temperature cycling is repeated throughout the lifetime of the furnace components. In applications such as hydrogen heat treatment furnaces, or Bell annealing furnaces, components within the furnace are exposed to atmospheres with a high hydrogen content, including 100% hydrogen atmospheres.

[0003] A typical heat-resistant alloy used in heat treatment furnaces is the FMBM alloy composition produced by FVC.

[0004] Steel alloy compositions can vary greatly depending on the properties required of the steel, including simple carbon steels, low alloys, stainless (martensitic stainless or austenitic stainless), super austenitic, and duplex (a combination of austenitic and ferritic). Depending on their chemical composition and subsequent heat treatment steps steel alloy compositions may comprise several phases, including austenite, ferrite, martensite and other phases. Sigma phase is a brittle phase that can form in some stainless steels. Sigma phase is therefore generally avoided to prevent the formation of brittle components. It may be that steel alloys are fully ferritic, fully austenitic, or a combination of both ferritic and austenitic phases.

[0005] It is generally understood that high temperature steel alloys are austenitic, with the inclusion of around 10% by volume of a ferritic phase in order to prevent hot cracking of the alloy during solidification and cooling. The theoretical ferrite content of a steel alloy composition can be calculated using known techniques, including those set out in ASTM [0006] The thermal cycling of steel alloy components within heat treatment furnace can result in failure of the components due to thermal expansion/contraction of the alloy associated with volumetric changes of particular phases within the alloy. This can lead to brittleness at ambient temperatures and premature failure of the component.

[0007] Figure 1 (a and b) shows a component (heat resistant bottom plate made of the FMBM alloy produced by FVC) of a heat treatment furnace after 84 temperature cycles. As is apparent from this figure, the component has failed due to the presence of multiple cracks.

[0008] It is an aim of the present invention to overcome some or all of the problems associated with known heat resistant alloys. Thus, it may be an aim of the invention to provide a heat-resistant alloy composition with improved ductility properties and/or an increased lifetime with respect to prior art alloys.

BRIEF SUMMARY OF THE DISCLOSURE

[0009] In accordance with a first aspect, the present invention provides an alloy composition comprising: from 0.17 wt.% to 0.25 wt.% carbon; from 0.5 wt.% to 1 .0 wt.% silicon; from 0.5 wt.% to 1 .5 wt.% manganese; from 13.0 wt.% to 15.0 wt.% nickel; from 19.0 wt.% to 23.0 wt.% chromium; from 0.5 wt.% to 1 .5 wt.% niobium; from 0.1 wt.% to 0.2 wt.% vanadium; up to 1 .0 wt.% molybdenum; up to 0.5 wt.% copper; up to 0.025 wt.% aluminium; up to 0.2 wt.% nitrogen; up to 0.03 wt.% sulphur; up to 0.03 wt.% phosphorus; with the balance of the composition being iron and incidental impurities.

[0010] In accordance with a second aspect, the present invention provides a heat treatment furnace component made from the alloy composition of the first aspect. [0011] In accordance with a third aspect, the present invention provides a use of the alloy composition of the first aspect in a heat treatment furnace.

[0012] In accordance with a fourth aspect, the present invention provides a use of the component of the second aspect in a heat treatment furnace.

[0013] In accordance with a fifth aspect, the present invention provides a method comprising: a) heating an alloy composition of the first aspect or a component of the second aspect to a temperature above 1000 °C; b) cooling to ambient temperature; c) repeating steps a) and b) at least one time.

[0014] The following embodiments may apply to each of the first, second, third, fourth, and fifth 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.

FERRITE/AUSTENITE PHASE

[0015] In embodiments, the alloy composition comprises less than 10% ferrite phase, by volume. In embodiments, the alloy composition comprises less than 5% ferrite phase by volume. In embodiments, the alloy composition comprises less than 3% ferrite phase by volume. In embodiments, the alloy composition comprises less than 2% ferrite phase by volume. In embodiments, the alloy composition comprises less than 1% ferrite phase by volume. In embodiments, the alloy composition comprises less than 0.5% ferrite phase by volume. It may be that, where an alloy composition comprises ferrite, the remaining volume of the alloy composition is austenite. For example, where an alloy composition comprises 1% ferrite phase by volume, the alloy composition also comprises 99% austenite phase by volume.

[0016] In embodiments, the alloy composition may be substantially free of ferrite, i.e., has a ferrite content of 0% by volume. In embodiments, the alloy composition may be fully austenitic, i.e., has an austenite content of 100% by volume.

ELEMENTAL COMPONENTS

[0017] The individual elemental components in the alloy may perform the roles discussed herein. The individual elemental components in the alloy may, alternatively or in addition, perform roles not discussed below. It may be that two or more elemental components work together to perform a particular function.

CARBON

[0018] Carbon is required in an amount of 0.17 to 0.25 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. 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 0.17 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. Above 0.25 wt.%, the morphology of the carbide structure becomes undesirable, resulting in a reduction in the toughness of the steel. For this reason, the upper limit of carbon is 0.25 wt.%. The amount of carbon may be from 0.17 to 0.24 wt.%, 0.17 to 0.23 wt.%, 0.17 to 0.22 wt.%, 0.17 to 0.21 wt.%, 0.17 to 0.20 wt.%, 0.17 to 0.19 wt.%, 0.17 to 0.18 wt.%. In a preferred embodiment the amount of carbon is from 0.17 to 0.20 wt.%. In a more preferred embodiment, the amount of carbon is from 0.17 to 0.19 wt.%. The amount of carbon may be 0.18 wt.%.

SILICON

[0019] Silicon is present in an amount of from 0.5 wt.% to 1.0 wt.%. Silicon provides the function of a deoxidiser and is usually an essential component in an austenite stainless steel. In some embodiments, silicon is present in an amount from 0.6 wt.% to 1.0 wt.%. In certain embodiments, the amount of silicon is from 0.7 wt.% to 1 .0 wt.%. In certain embodiments, the amount of silicon is from 0.8 wt.% to 1.0 wt.%. In certain embodiments, the amount of silicon is from 0.9 wt.% to 1 .0 wt.%.

MANGANESE

[0020] Manganese is present in an amount of from 0.5 wt.% to 1 .5 wt.%. Manganese is an effective de-oxidant and contributes to austenite formation in the steel. 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 1.5 wt.%. In some embodiments, the manganese is present in an amount from 0.5 wt.% to 1 .4 wt.%, from 0.5 wt.% to 1.3 wt.%, from 0.5 wt.% to 1.2 wt.%, from 0.5 wt.% to 1.1 wt.%, from 0.5 wt.% to 1 .0 wt.%, or preferably from 0.5 wt.% to 0.9 wt.%. In some embodiments, the manganese is present in an amount from 0.6 wt.% to 1.5 wt.%, from 0.7 wt.% to 1.5 wt.%, or from 0.8 wt.% to 1.5 wt.%. In certain preferred embodiments, the manganese is present in an amount of from 0.8 wt.% to 1 .0 wt.%.

NICKEL

[0021] Nickel is present in an amount of from 13.0 wt.% to 15.0 wt.%. The nickel provides the stable austenitic matrix base of the alloy. Nickel is an element which is essential in order to obtain a stable austenite structure and improves the stability of austenite and supresses 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 simply for the reason that this is a sufficient amount for improving the stability of austenite with respect to the lower limits of the other elements. The lower limit of nickel is governed by the need to provide an adequate austenitic matrix. In some embodiments, the nickel is present in an amount of from 13.0 wt.% to less than 15 wt.%, and more preferably in an amount of from 13.0 wt.% to 14.5 wt.%. In some embodiments, the nickel is present in an amount of from 13.1 wt.% to less than 15 wt.%. In some embodiments, the nickel is present in an amount of from 13.2 wt.% to less than 15 wt.%. In some embodiments, the nickel is present in an amount of from 13.3 wt.% to less than 15 wt.%. In some embodiments, the nickel is present in an amount of from 13.4 wt.% to less than 15 wt.%. In some embodiments, the nickel is present in an amount of from 13.2 wt.% to 13.6 wt.%.

CHROMIUM

[0022] Chromium is present in an amount of from 19.0 wt.% to 23.0 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.

[0023] Carbide formation ensures creep strengthening precipitations in the alloy. The lower limit of 19.0 wt.% of chromium is required to ensure sufficient mechanical strength and the upper limit of 23.0 wt.% is determined by the fact that above this level it is difficult to obtain a stable austenite phase without the need to also increase the nickel content much further. In some embodiments, the chromium is present in the range of from 19.0 wt.% to 22.0 wt.%, from 19.0 wt.% to 21.5 wt.%, or from 19.0 wt.% to 21.0 wt.%. In certain cases, the amount of chromium is in the range from 19.0 wt.% to 20.0 wt.%. In some alternative embodiments, the chromium is present in the range of from 19.5 wt.% to 20.5 wt.%, and more preferably in the range of from 19.5 wt.% to 20.0 wt.%. [0024] The chromium content is determined in balance with the nickel 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 chromium to be used in the context of the amount of nickel that is also present in the alloy.

NIOBIUM

[0025] Niobium is present in an amount of from 0.5 to 1.5 wt.%. In some embodiments, the niobium is present in an amount of from 0.8 wt.% to 1.5 wt.%, from 0.9 wt.% to 1.5 wt.% or preferably from 1.0 wt.% to 1.5 wt.%. In other preferred embodiments, the niobium is present in an amount of from 1.0 wt.% to 1.4 wt.%, from 1.0 wt.% to 1.3 wt.%, from 1.0 wt.% to 1.2 wt.%, or from 1.0 wt.% to 1.1 wt.%. In some embodiments, niobium is present in an amount of from 0.8 wt.% to 1.2 wt.%. In some embodiments, niobium is present in an amount of from 0.9 wt.% to 1 .1 wt.%.

VANADIUM

[0026] Vanadium is present in an amount of from 0.1 wt.% to 0.2 wt.%. Vanadium is a carbide former from the same periodic group as niobium. Small additions of vanadium are thought to be beneficial to form finely dispersed carbide morphology of the chromium carbides. Similarly to niobium carbides, vanadium can form carbides and nitro-carbides within the matrix and on interdendritic boundaries for improved creep resistance. Without wishing to be bound by theory, the addition of higher amounts of vanadium may reverse these beneficial effects by forming large precipitates. In some embodiments, vanadium is present in an amount of from 0.1 wt.% to 0.15 wt.%.

MOLYBDENUM

[0027] Molybdenum is present in an amount of up to 1.0 wt.%. Molybdenum is a solid solution strengthener and a carbide former. However, and without wishing to be bound by theory, the strengthening effect by both mechanisms of molybdenum is believed to be marginal in comparison to the combined strengthening mechanisms of chromium and vanadium carbides. The addition of greater amounts of molybdenum is therefore considered to be costly.

[0028] In some embodiments, molybdenum is present in an amount of up to 0.9 wt.%, up to 0.8 wt.%, up to 0.7 wt.%, up to 0.6 wt.%. In some embodiments, molybdenum is present in an amount of from 0.3 wt.% to 1.0 wt.%, from 0.4 wt.% to 1.0 wt.%, or from 0.5 wt.% to 1 .0 wt.%. In some embodiments, molybdenum is present in an amount of from 0.3 wt.% to 0.9 wt.%, from 0.3 wt.% to 0.8 wt.%, or from 0.3 wt.% to 0.7 wt.%. COPPER, ALUMINIUM, NITROGEN, SULPHUR, PHOSPHORUS

[0029] Copper, Aluminium, Nitrogen, Sulphur, and Phosphorus may be inadvertently present in the alloys of the invention.

[0030] Copper should be restricted to the upper limit described herein to prevent formation of undesirable phases that can degrade the mechanical properties of the steel, such as making the material more brittle at elevated temperatures. The presence of too much copper may also degrade protective oxide scale integrity. In embodiments, copper is present in an amount of up to 0.4 wt.%, up to 0.3 wt.%, up to 0.2 wt.%, or up to 0.1 wt.%.

[0031] The presence of too much aluminium can make the alloy susceptible to intergranual corrosion and reduce its high-temperature strength. Aluminium can also affect the machinability and formations of inclusions, as well as increasing brittleness via formation of aluminium nitrides. Accordingly, aluminium should be restricted to the upper limit described herein. In embodiments, aluminium is present in an amount of up to 0.02 wt.% or up to 0.01 wt.%.

[0032] Nitrogen is known to increase creep life in austenitic stainless steels via delaying the rate of coalescence of strengthening carbides, as well as lowering the stacking fault energy. However, it's presence has to be limited within alloys of the present invention due to excessive precipitation of nitrides that can degrade the material toughness, which may resulting in cracking of the steels at elevated temperatures. Accordingly, nitrogen should be restricted to the upper limit described herein. In embodiments, nitrogen is present in an amount of up to 0.15 wt.%, up to 0.1 wt.%, or up to 0.05 wt.%.

[0033] Sulphur and phosphorus are considered tramp elements (i.e. those elements that are not easily removed from the composition) and can deleteriously affect mechanical properties. The amount of sulphur and phosphorus should therefore be kept to a minimum, however these elements may be intentionally added in some applications to aid machinability, e.g. free-cutting steels. Accordingly, sulphur and phosphorus should each be restricted to the upper limits described herein. In embodiments, each of sulphur and phosphorous may be independently present in an amount of up to 0.03 wt.%, up to 0.02 wt.%, or up to 0.01 wt.%. In embodiments, sulphur may be substantially absent. In embodiments, phosphorus may be substantially absent.

INCIDENTAL IMPURITIES

[0034] Alloys according to the present invention are produced in a conventional 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.

[0035] 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.

[0036] A number of elements will be present in the alloy as inevitable impurities. Such impurities may include Sn, Zn, Sb, As, Ca, Te, Se, and/or B. These elemental impurities should each be kept to a maximum of 0.02 wt.%. Ti, W, Zr, and/or Hf may also be present as incidental impurities. These elemental impurities should each be kept to a maximum of 0.1 wt.%.

[0037] Such incidental elements will not have any discernible technical benefit or adverse effect on the alloys of the present invention. In some cases, the presence of such elements, as the nitrogen can be tolerated in relatively large amounts provided that they do not affect the desired properties of the alloy. For example, although not specifically envisaged in the alloys of the present invention, it is conceivable that an element may arise as an incidental impurity as a consequence of its occurrence as an impurity in one of the deliberately added elemental components. This is acceptable provided that the presence of such an element does not have any deleterious effects on the alloy. In certain cases, deliberately added elemental components such as chromium may bring with them other incidental elements. These can be generally tolerated as incidental impurities at low levels. Where analysis reveals that such impurities are unacceptable, an alternative source of the desired elemental component (free of damaging impurities) is used.

[0038] In an embodiment, the alloy composition comprises: from 0.17 wt.% to 0.2 wt.% carbon; from 0.5 wt.% to 1 .0 wt.% silicon; from 0.5 wt.% to 0.9 wt.% manganese; from 13.2 wt.% to less than 15.0 wt.% nickel; from 19.0 wt.% to 23.0 wt.% chromium; from 1.0 wt.% to 1.3 wt.% niobium; from 0.1 wt.% to 0.2 wt.% vanadium; from 0.3 wt.% to 1.0 wt.% molybdenum; up to 0.5 wt.% copper; up to 0.025 wt.% aluminium; up to 0.2 wt.% nitrogen; up to 0.03 wt.% sulphur; up to 0.03 wt.% phosphorus; with the balance of the composition being iron and incidental impurities.

[0039] In an embodiment, the alloy composition comprises:

0.18 wt.% carbon;

0.99 wt.% silicon;

0.88 wt.% manganese;

13.4 wt.% nickel;

19.6 wt.% chromium;

1.05 wt.% niobium;

0.12 wt.% vanadium;

0.5 wt.% molybdenum;

0.24 wt.% copper;

0.06 wt.% nitrogen;

0.02 wt.% sulphur;

0.02 wt.% phosphorus; with the balance of the composition being iron and incidental impurities.

[0040] In an embodiment, the alloy composition comprises:

0.18 wt.% carbon;

0.99 wt.% silicon; 0.88 wt.% manganese;

13.4 wt.% nickel;

19.6 wt.% chromium;

1.05 wt.% niobium;

0.12 wt.% vanadium;

0.5 wt.% molybdenum;

0.24 wt.% copper;

0.06 wt.% nitrogen;

0.02 wt.% sulphur;

0.02 wt.% phosphorus;

62.8 wt.% iron; with the balance of the composition being incidental impurities.

COMPONENTS

[0041] Alloy compositions of the invention may be used to form any steel component where high creep strength is required when operating at high temperature, including in atmospheres rich in hydrogen. Alloy compositions of the invention may be used to form components of a heat treatment furnace. It may be that the component is a component of a hydrogen heat treatment furnace. It may be that the component is a component of a Bell annealing furnace. It may be that the component is a component of a direct iron reduction (DRI) apparatus. It may be that the component is a plate, for example a heat resistant bottom plate. It may be that the component is a heat resistant bottom plate for a hydrogen heat treatment furnace.

METHODS

[0042] The present invention provides a method comprising: a) heating an alloy composition of the first aspect or a component of the second aspect to a temperature above 1000 °C; b) cooling to ambient temperature; c) repeating steps a) and b) at least one time.

[0043] It may be that step a) comprises heating the composition or component to a temperature of approximately 1100 °C; optionally to a temperature above 1100 °C. It may be that step a) comprises heating the composition or component to a temperature of approximately 1200 °C; optionally to a temperature above 1200 °C.

[0044] The term “ambient temperature” encompasses temperatures of from 5 °C to 35 °C. Thus, it may be that the alloy composition or component is cooled to a temperature of from 5 °C to 35 °C in step b). It may be that the alloy composition or component is cooled to a temperature of from 10 °C to 30 °C in step b). It may be that the alloy composition or component is cooled to a temperature of from 15 °C to 25 °C in step b).

[0045] It may be that steps a) and b) are repeated at least 10 times. It may be that steps a) and b) are repeated at least 25 times. It may be that steps a) and b) are repeated at least 50 times. It may be that steps a) and b) are repeated at least 75 times. It may be that steps a) and b) are repeated at least 100 times. It may be that steps a) and b) are repeated at least 10, 25, 75, or 100 times without failure of the component occurring, e.g. without cracking of the component occurring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 (a and b) show photographs of a heat resistant bottom plate for a furnace produced from FMBM after being subjected to 84 heating cycles, at which point the component failed.

Figure 2a and 2b show optical microscopy images of the as-cast alloy composition of comparative example 1 (2a) and example 1 (2b).

Figures 3a and 3b show photographs of heat resistant bottom plates according to comparative example 1 and example 1 , respectively, after 28 heat treatment cycles.

Figures 4a and 4b show photographs of heat resistant bottom plates according to comparative example 1 and example 1 , respectively, after 57 heat treatment cycles.

Figure 5a shows an optical microscopy image of the as-cast alloy composition of comparative example 1 .

Figure 5b shows an optical microscopy image of the as-cast alloy composition of example 1 .

Figures 5c, 5d, and 5e show optical microscopy images of the alloy composition of comparative example 1 after being subject to thermal cycling. Figures 6a and 6b show optical microscopy images of the fracture face in failed components produced from the alloy composition of comparative example 1 after being subject to thermal cycling.

Figure 7 shows an optical microscopy image of the alloy composition of example 1 after being subject to a heat treatment process (aged at 1150°C for 48 hours).

Figures 8a, 8b, and 8c show graphs of constant load (creep) stress to rupture at 1000 °C and 23 MPa.

Figure 9a shows the three support plates as cast without having been subjected to any heat treatment cycles. The left support plate was cast using the alloy composition of Example 1. The right support plate was cast using the alloy composition of Comparative Example 1.

Figures 9b and 9c respectively compare the Comparative Example 1 vs Example 1 after 42 cycles.

Figures 9d and 9e respectively compare the Comparative Example 1 vs Example 1 after 66 cycles.

Figures 9f and 9g respectively compare the Comparative Example 1 vs Example 1 after 86 cycles.

Figure 10 - LMP plot of Example 1 vs Comparative Example 2.

Figure 11 - LMP plot of Example 1 vs Comparative Example 3.

DETAILED DESCRIPTION

Examples

Comparative Example 1

[0047] An alloy having the following elemental composition was prepared and is referred to herein as FMBM:

[0048] The alloy was prepared from a charge make up comprising virgin and foundry revert materials to form the composition described above. The composition was melted in an air induction furnace. [0049] The alloy composition was cast as a bottom plate for a heat treatment furnace using a mould with a centre feederhead. On casting, the mould was rotated such that the centrifugal force distributed the metal around the plate mould.

Comparative Example 2

[0050] An alloy having the following elemental composition is referred to herein as Paralloy CR32W:

Comparative Example 3

[0051] An alloy having the following elemental composition is referred to herein as HK40,

ASMT A351 :

Example 1

[0052] An alloy having the following elemental composition was prepared:

[0053] The alloy composition and bottom plate for a heat treatment furnace was prepared analogously to comparative example 1.

Example 2

[0054] The ferrite content of the alloy of comparative example 1 and example 1 were calculated using the method described in ASTM 800, the results of which are shown in Table 1 :

Table 1

[0055] It is clear from Table 1 that the alloy composition of example 1 of the present invention are calculated to have zero ferrite. [0056] This reduction in ferrite in alloys of the present invention was confirmed with optical microscopy, which can be seen in Figure 2. Figure 2a depicts the as-cast alloy of comparative example 1 and Figure 2b depicts the as-cast alloy of example 1. It is clear from these images that the alloy of example 1 lacks the ferrite/sigma phase that is present in the as-cast alloy of comparative example 1 .

Example 3

[0057] Comparative example 1 and example 1 were subjected to thermal cycling experiments. The heat treatment furnace bottom plate of comparative example 1 and example 1 were placed into a hydrogen heat treatment furnace at room temperature in air. The temperature was gradually increased. As the temperature was increased, the air was replaced with nitrogen before itself being replaced with hydrogen. The furnace was heated to 1230°C for up to 7 days, after which the temperature was gradually decreased to room temperature over a period of 2 days. For comparative example 1 , this cycle was repeated 84 times until failure. The thermal cycling experiments for example 1 are ongoing. To date, the bottom plate of example 1 has been subjected to 57 cycles without mechanical failure being observed.

[0058] Figures 3a and 3b show photographs of heat resistant bottom plates according to comparative example 1 and example 1 , respectively, after 28 heat treatment cycles. It is clear from Figure 3a that comparative example 1 is already showing signs of failure after 28 heat treatment cycles, while Figure 3b shows that the heat treatment plate of example 1 shows no such signs of failure.

[0059] Figures 4a and 4b show photographs of heat resistant bottom plates according to comparative example 1 and example 1 , respectively, after 57 heat treatment cycles. It is clear from Figure 4a that comparative example 1 shows further signs of failure after 57 heat treatment cycles, while Figure 4b shows that the heat treatment plate of example 1 still shows no such signs of failure, even after 57 cycles.

[0060] Figure 5a shows an optical microscopy image depicting the microstructure of comparative example 1 as-cast. It is clear from Figure 5a that the as-cast alloy comprises a microstructure consisting of pools of ferrite.

[0061] Figure 5b shows an optical microscopy image depicting the microstructure of example 1 as-cast. Similar to Figure 2b, it is clear from Figure 5b that the as-cast alloy of example 1 lacks the ferrite/sigma phase that is present in the as-cast alloy of comparative example 1 .

[0062] Figures 5c, 5d, and 5e show optical microscopy images depicting the microstructure of comparative example 1 which has been subjected to the thermal cycling process described above, after failure of the bottom plate at 84 cycles. It is clear from Figures 5c, 5d, and 5e that the ferrite present in the as-cast alloy is transformed into sigma phase by the cooling step of the thermal cycling of the alloy.

[0063] Figure 6a shows an optical microscopy image depicting the fracture face of a crack in the component formed from the alloy of comparative example 1 , which has been subjected to 84 cycles of the thermal cycling process described above. It is clear from Figure 6a that the fracture face has visible cracks along the sigma phase precipitates.

[0064] Similarly, Figure 6b shows secondary cracks along the sigma phase precipitates.

[0065] The microstructure of the alloys of the present invention are characterised by interdendritic chromium carbides and fine niobium rich particles in a matrix of austenite.

[0066] Without wishing to be bound by theory, it is believed that the formation of sigma phase in the alloy, which occurs as the alloy is cooled slowly through 800 to 500 °C, results in the failure of the alloy components through the formation of cracks in the structure.

Example 4

[0067] The alloy composition of Example 1 was subjected to a heat treatment process. The heat treatment furnace bottom plate of example 1 was placed into a heat treatment furnace at room temperature in air. The temperature was gradually increased at a rate of 200°C/hr. The furnace was heated to 1150°C for 48 hours, after which the temperature was gradually decreased to room temperature over a period of 2 days.

[0068] Figure 7 shows an optical microscopy image depicting the microstructure of example 1 which has been subjected to the process described above. The figure demonstrates that after treatment at high temperatures, a very fine distribution of carbides and nitro-carbides develop. In addition, it is evident that the primary network of carbides has not undergone significant change in morphology. This is important as it signifies coalescence rates have been very slow and these finely dispersed and uniform distribution of secondary precipitates provide the necessary high temperature strength.

Example 5

[0069] Samples of the as-cast alloy compositions of comparative example 1 and example 1 were subjected to stress to rupture analysis and creep strain tests. These are shown in Figures 8a, 8b, and 8c. These tests were carried out in accordance with ASTM E139-11 .

[0070] Figure 8a shows the constant load (creep) time to rupture (at 1000 °C and 23 MPa) for a sample of both comparative example 1 and example 1. It is clear from this graph that creep strength is vastly improved for alloys of the invention compared to prior art alloy compositions. [0071] Figure 8b plots the creep curve from the constant load (creep) strain tests (at 1000 °C and 23 MPa) for a sample of both comparative example 1 and example 1. Again, it is clear from this graph that creep strength is vastly improved for alloys of the invention compared to prior art alloy compositions.

[0072] Figure 8c shows the constant load (creep) strain (at 1000 °C and 23 MPa) for a sample of both comparative example 1 and example 1 . Again, it is clear from this graph that creep strength is vastly improved for alloys of the invention compared to prior art alloy compositions.

[0073] The microstructure of the alloys of the present invention, as shown in example 4, is believed to give rise to the beneficial strength properties at high temperature service in a constant load condition, as described in Example 5. This is evident from creep strength experiments undertaken with a constant load of 23 MPa at 1000 °C.

Example 6

[0074] For the purpose of this example, three support plates were cast, two support plates using the alloy composition of Example 1 and one support plate using the alloy composition of Comparative Example 1 . Figure 9a shows the three support plates. The left support plate was cast using the alloy composition of Example 1 . The right support plate was cast using the alloy composition of Comparative Example 1.

[0075] The three support plates were placed in a heat treatment furnace to compare their performance. The three plates have been exposed to the same number of heat treatment cycles and conditions for a period of time.

[0076] The heat treatment furnace conditions were variable at each cycle (because the heat treatment furnace was being used for real-life projects). The heat treatment conditions ranged from 1 hour at 400°C to 12 hours at 1170°C, and other temperatures and time periods within these ranges, e.g. 6 hours at 745°C.

[0077] A visual comparison was made between plates placed on the opposite side of the furnace (indicated with arrows in Figure 9a).

[0078] Figures 9b and 9c respectively compare the Comparative Example 1 vs Example 1 after 42 cycles - a small crack is shown in Figure 9b, but no cracks are evident in Figure 9c;

[0079] Figures 9d and 9e respectively compare the Comparative Example 1 vs Example 1 after 66 cycles - three cracks have developed in Figure 9d, but no cracks are evident in Figure 9e; [0080] Figures 9f and 9g respectively compare the Comparative Example 1 vs Example 1 after 86 cycles - three larger cracks have developed in Figure 9f, but no cracks are evident in Figure 9g.

[0081] To date the plates have been subjected to 95 heat treatment cycles. The support plates cast using the alloy of Example 1 still has no evident cracks.

[0082] Example 7

[0083] A sample of Example 1 was machined in a lathe to the 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 displacement transducers were fitted to monitor creep in the tested sample.

[0084] Stress rupture and creep data were also obtained for Comparative Examples 2 and 3.

[0085] The Larson-Miller method is a most widely accepted method of evaluating longterm creep strength from relatively short-term creep data. The LMP value can be calculated using the following formula:

(log(t_r) 4- Cj]

[0086]

[0087] where T is the absolute temperature, tr is the time to rupture and C is 20 (F.E. Larson, J. Miller, Transactions ASM E, 74 (1952) 765-771).

[0088] The LMP plots for Example 1 vs Comparative Example 2 is show in Figure 10.

[0089] The LMP plots for Example 1 vs Comparative Example 3 is show in Figure 11.

[0090] As shown in Figures 10 and 11: (1) for a given stress Example 1 can withstand higher temperature or has a longer time to rupture in comparison to Comparative Examples 2 and 3; and (2) for a given temperature and time to rupture, Example 1 achieves higher stresses than Comparative Examples 2 and 3 (in other words Example 1 has a higher rupture strength than Comparative Examples 2 and 3).

[0091] 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.

[0092] 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.

[0093] 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.

Claims

1. An alloy composition comprising: from 0.17 wt.% to 0.25 wt.% carbon; from 0.5 wt.% to 1.0 wt.% silicon; from 0.5 wt.% to 1.5 wt.% manganese; from 13.0 wt.% to 15.0 wt.% nickel; from 19.0 wt.% to 23.0 wt.% chromium; from 0.5 wt.% to 1.5 wt.% niobium; from 0.1 wt.% to 0.2 wt.% vanadium; up to 1.0 wt.% molybdenum; up to 0.5 wt.% copper; up to 0.025 wt.% aluminium; up to 0.2 wt.% nitrogen; up to 0.03 wt.% sulphur; up to 0.03 wt.% phosphorous; with the balance of the composition being iron and incidental impurities.

2. An alloy composition of claim 1, wherein carbon is present in an amount of from 0.17 wt.% to 0.2 wt.%.

3. An alloy composition of claim 1 or claim 2, wherein manganese is present in an amount of from 0.5 wt.% to 1.0 wt.%.

4. An alloy composition of any of claims 1 to 3, wherein nickel is present in an amount of from 13.2 wt.% to 14.5 wt.%.

5. An alloy composition of any preceding claim, wherein nickel is present in an amount of from 13.2 wt.% to 13.6 wt.%.

6. An alloy composition of any preceding claim, wherein niobium is present in an amount of from 1.0 wt.% to 1.5 wt.%.

7. An alloy composition according to claim 1, wherein the alloy composition comprises: from 0.17 wt.% to 0.2 wt.% carbon; from 0.5 wt.% to 1.0 wt.% silicon; from 0.5 wt.% to 0.9 wt.% manganese; from 13.2 wt.% to less than 15.0 wt.% nickel; from 19.0 wt.% to 23.0 wt.% chromium; from 1.0 wt.% to 1.3 wt.% niobium; from 0.1 wt.% to 0.2 wt.% vanadium; from 0.3 wt.% to 1.0 wt.% molybdenum; up to 0.5 wt.% copper; up to 0.025 wt.% aluminium; up to 0.2 wt.% nitrogen; up to 0.03 wt.% sulphur; up to 0.03 wt.% phosphorus; with the balance of the composition being iron and incidental impurities.

8. An alloy composition according to claim 1, wherein the alloy composition comprises:

0.18 wt.% carbon;

0.99 wt.% silicon;

0.88 wt.% manganese;

13.4 wt.% nickel;

19.6 wt.% chromium;

1.05 wt.% niobium;

0.12 wt.% vanadium;

0.5 wt.% molybdenum;

0.24 wt.% copper;

0.06 wt.% nitrogen;

0.02 wt.% sulphur;

9. A heat treatment furnace component made from the alloy composition according to any one of claims 1 to 8.

10. A heat treatment furnace component according to claim 9, wherein the component is a bottom plate. 11. Use of the alloy composition of any one of claims 1 to 8, or the component of claim

9 or claim 10, in a heat treatment furnace.

12. A method comprising: a) heating an alloy composition according to any one of claims 1 to 8, or a component according to claim 9 or claim 10, to a temperature above 1000 °C; b) cooling the composition or component to ambient temperature; c) repeating steps a) and b) at least one time.

13. The method of claim 12, wherein the composition or component is heated to a temperature above 1200 °C in step a).

14. The method of claim 12 or claim 13, wherein steps a) and b) are repeated at least 100 times.