Alumina-forming austenitic-ferritic stainless steel alloy

Abstract

The present disclosure relates to an alumina forming austenitic-ferritic stainless steel alloy. The present disclosure also relates to a method of making an alumina-forming austenitic-ferritic stainless steel object. Furthermore, the present disclosure relates to a product and use in the temperature range of 500 to 900 DEG C.

Description

Technical Field

[0001] The present disclosure relates to an austenitic-ferritic stainless steel alloy. More specifically, the present disclosure relates to an alumina-forming austenitic-ferritic stainless steel alloy. The present disclosure also relates to a method of making an alumina-forming austenitic-ferritic stainless steel object, a product comprising the alumina-forming austenitic-ferritic stainless steel, and the use of the object in specific environments. Background Art

[0002] The prior art discloses examples of duplex stainless steels comprising austenite and ferrite phases, which are capable of forming a protective aluminum oxide layer on their surface when subjected to an oxygen-containing atmosphere at high temperatures. Such duplex stainless steels typically contain high levels of nickel, chromium, and aluminum.

[0003] Wang et al., “Effects of carbon and chromium on the solidification structure and properties of ferrite-austenite duplex heat-resistant alloy,” Science and Technology of advanced materials, Elsevier Science, Vol. 2, No. 1, July 30, 2001, pp. 297-302, discloses a ferrite-austenite duplex alloy tested in air at 1250° C. However, a disadvantage of these alloys is that their oxidation resistance is insufficient at temperatures between 500 and 900° C. In addition, some of these alloys appear too brittle, and therefore have too low a ductility for conventional manufacturing methods.

[0004] Hyunmyung et al., “Development of alumina-forming duplex stainless steels as accident tolerant fuel cladding materials for light water reactors,” Journal of Nuclear Materials, Elsevier Science, Vol. 507, April 21, 2018, pp. 1-14, discloses high aluminum content (>5 wt%) duplex stainless steels that were tested for corrosion resistance under 1200°C steam and simulated pressurized water reactor (PWR) operating conditions. However, the oxidation resistance of the disclosed compositions is insufficient at temperatures between 500 and 900°C.

[0005] Therefore, there remains a need in the art for an optimized austenitic-ferritic stainless steel alloy that will enable the manufacture of objects comprising stainless steel using conventional manufacturing routes and that will provide objects with excellent oxidation resistance when used in the temperature range of 500 to 900°C. Summary of the Invention

[0006] Thus, the present disclosure provides an improved alumina forming austenitic-ferritic stainless steel alloy composition having an optimized microstructure comprising austenite and ferrite to be used in the temperature range of 500 to 900°C.

[0007] The austenitic-ferritic stainless steel according to the present disclosure is characterized in that the stainless steel comprises the following composition (in weight %):

[0008]

[0009] The balance is Fe and unavoidable impurities;

[0010] The austenitic-ferritic stainless steel has a microstructure comprising greater than 15 volume % and less than 45 volume % ferrite with the remainder being austenite.

[0011] In the present disclosure, the austenitic-ferritic stainless steel has a ferrite content in the range of greater than 15 volume % to less than 45 volume %, with the remainder being austenite, as this microstructure has been found to be crucial for oxidation resistance in the temperature range of 500 to 900°C. If the amount of ferrite is less than 15 volume %, oxidation resistance and mechanical strength will be reduced. If the ferrite content is greater than 45 volume %, the stainless steel of the present invention may have problems forming a protective oxide layer at temperatures above approximately 650°C.

[0012] The present disclosure also provides a method of making an object comprising an austenitic-ferritic stainless steel composition having a microstructure as defined above or below. The method of making the alloy is a conventional smelting metallurgical manufacturing route, as it has been surprisingly found that the stainless steel of the present invention will have sufficiently high hot ductility to make this possible.

[0013] The present disclosure also provides an object comprising an austenitic-ferritic stainless steel composition having a microstructure as defined above or below. The stainless steel of the present invention will enable the formation of an aluminum oxide layer on the object, which will enable the object to be used in an atmosphere having a wide range of oxygen concentrations within a temperature range of 500-900°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1a -b discloses the results of oxidation tests at 800 and 900°C, respectively.

[0015] Figure 2 A graph showing yield strength as a function of different heat treatments is disclosed. DETAILED DESCRIPTION

[0016] The present disclosure relates to an alumina-forming austenitic-ferritic stainless steel characterized in that the alloy has the following composition (in weight %):

[0017]

[0018]

[0019] The balance is Fe and commonly present impurities;

[0020] And wherein the austenitic-ferritic stainless steel has a microstructure comprising greater than 15 volume % and less than 45 volume % ferrite with the remainder being austenite.

[0021] The alloying elements of the steel according to the present disclosure will now be described in more detail. The terms "weight %" and "wt %" are used interchangeably. Furthermore, a list of properties or contributions mentioned for a particular element should not be considered exhaustive.

[0022] Iron (Fe) balance

[0023] The primary role of iron in austenitic-ferritic stainless steels is to balance the composition of the steel composition or object with the remaining alloying elements. The balance also includes unavoidable impurities, which will be discussed below.

[0024] Chromium (Cr) 11.0 to 16.0 wt%

[0025] Cr is an important element because it is a ferrite stabilizer and therefore helps maintain a suitable microstructure comprising greater than 15 volume % and less than 45 volume % ferrite with the remainder being austenite.

[0026] Cr also promotes the formation of alumina, or an aluminum oxide layer, on the manufactured object through the so-called third element effect, which forms chromium oxide during a transient oxidation phase. In particular, at temperatures in the range of 500-600°C, insufficient chromium content can hinder the formation of the aluminum oxide layer on the object. Furthermore, chromium is an important element because it improves corrosion resistance. Therefore, the minimum chromium content in the steel of the present invention is 11.0% by weight. According to one embodiment, the minimum chromium content is 12.0% by weight.

[0027] However, if the Cr content is too high, the ferrite content will become too high, which will lead to reduced oxidation resistance, especially at higher temperatures such as 800 to 900°C. Excessive Cr content can also lead to the formation of secondary phases, such as sigma phase, which will cause embrittlement. Therefore, the maximum chromium content is 16.0% by weight, such as a maximum of 15.5% by weight.

[0028] According to an embodiment, the content of Cr is 11.0 to 16.0 wt %, such as 12.0 to 16.0 wt %, such as 12.0 to 15.5 wt %.

[0029] Nickel (Ni) 11.5 to 15.0 wt%

[0030] Nickel is an important element because it stabilizes austenite and thus helps maintain a suitable microstructure. If the nickel content is too low, there is a risk of excessive ferrite phase formation, which can lead to a loss of oxidation resistance, particularly at temperatures above 800°C. Furthermore, too little nickel can cause austenite to transform into martensite at room temperature. Therefore, the minimum nickel content is 11.5% by weight.

[0031] On the other hand, if the amount of Ni is too high, the amount of ferrite will be too low, which will result in poor mechanical properties such as low tensile strength.

[0032] Ni should also be balanced with the amount of aluminum added, because it will combine with aluminum in the form of nickel aluminide, thereby suppressing the formation of aluminum oxide layer to a certain extent. However, nickel aluminide can provide improved mechanical properties in the manufactured object, such as increased hardness and improved yield strength. To a certain extent, nickel can be replaced by cobalt (Co). However, since Co is less preferred from an environmental point of view, Ni is preferred. Therefore, the maximum content of Ni is 15.0 wt %. According to an embodiment, the content of Ni is 11.5 to 15.0 wt %, such as 12.0 to 14.5 wt %.

[0033] Aluminum (Al) 3.5 to 5.0 wt%

[0034] Al is also a critical element in the steel of this invention because, when exposed to oxygen at high temperatures, Al forms a dense, thin layer of aluminum oxide on the fabricated object, protecting the underlying surface from further oxidation. If the Al content is too low, the formation of a sufficiently thick protective aluminum oxide layer will be limited or impossible when the steel is subjected to an oxygen-containing atmosphere at elevated temperatures, such as 500-900°C. Furthermore, Al forms nickel aluminides with Ni, contributing to its hardness. Therefore, the minimum aluminum content is 3.5% by weight.

[0035] Furthermore, Al stabilizes ferrite and thus helps maintain a suitable microstructure. If the amount of Al is too high, the ferrite content becomes too high, resulting in reduced oxidation resistance, particularly at temperatures of, for example, 800 to 900°C. Therefore, the maximum aluminum content is 5.0% by weight. According to embodiments, the Al content is 3.5 to 5.0% by weight, such as 3.7 to 4.9% by weight.

[0036] Carbon (C) 0.01 to 0.15 wt%

[0037] C will form carbides with many elements present in the austenitic-ferritic stainless steel, thereby contributing to the improvement of the hardness and strength of the steel (e.g., creep performance). In addition, C is also an austenite stabilizer. Therefore, the minimum carbon content is 0.01 wt%, such as 0.03 wt%. Too high a carbon content will increase the risk of forming too many carbides, such as M 23 C6 and / or M7C3 carbides, which will reduce oxidation resistance. Therefore, the maximum carbon content is 0.15 wt%, such as 0.13 wt%. According to an embodiment, the C content is 0.01 to 0.15 wt%, such as 0.03 to 0.13 wt%.

[0038] Niobium (Nb) 0.01 to 2.0 wt%

[0039] Nb is a ferrite stabilizer and therefore helps maintain a proper microstructure.

[0040] In addition, Nb will form niobium carbides together with C, thereby inhibiting the excessive formation of chromium carbides, which may have a negative impact on the formation of the aluminum oxide layer. Therefore, the minimum content of Nb is 0.01% by weight. According to one embodiment, the minimum content of Nb is 0.05% by weight.

[0041] However, too much Nb will lead to the formation of excessive niobium carbides, which will make the steel brittle. Therefore, the maximum content of niobium is 2.0% by weight, such as a maximum of 1.50% by weight. According to an embodiment, the content of Nb is therefore 0.01 to 2.0% by weight, such as 0.05 to 1.60% by weight, such as 0.05 to 1.60% by weight.

[0042] Manganese (Mn) 0.01 to 3.5 wt%

[0043] Manganese (Mn) is an austenite stabilizer and can replace nickel to a certain extent without compromising oxidation resistance. Therefore, the maximum content of Mn is 3.5 wt %, such as a maximum of 3.2 wt %. According to an embodiment, the content of Mn is 0.01 to 3.5 wt %, such as 0.01 to 3.2 wt %, such as 0.05 to 3.1 wt %.

[0044] Silicon (Si) 0.01 to 0.8 wt%

[0045] Si is added to improve oxidation resistance. Therefore, the minimum Si content is 0.01 wt %. However, excessive Si may increase the risk of sigma phase formation. Therefore, the maximum Si content is 0.8 wt %. According to one embodiment, the maximum Si content is 0.7 wt %, such as 0.6 wt %. According to an embodiment, the Si content is 0.01 to 0.8 wt %, such as 0.01 to 0.7 wt %, such as 0.01 to 0.6 wt %.

[0046] Copper (Cu) 0 to 5.5 wt%

[0047] Cu can be optionally added or can be regarded as an impurity. If it is intentionally added, and in order to have the desired effect, the minimum content is 0.5 weight %. Cu can have a positive effect on the formation of nickel aluminides that can provide improved mechanical properties (such as improved hardness and yield strength). However, too much Cu will lead to excessive nickel aluminides, which will reduce the hot ductility during object manufacturing. For these reasons, the maximum content of Cu is 5.5 weight %, such as 5.3 weight %, such as 5.2 weight %. According to an embodiment, the content of Cu is 0 to 5.5 weight %. According to an embodiment, the content of Cu is 0 to less than 0.5 weight % or 0.5 to 5.5 weight %.

[0048] Zirconium (Zr) 0 to 0.3 wt%

[0049] Zr can be added optionally or can be considered an impurity. If added intentionally, the minimum content is 0.05% by weight. Zr forms zirconium carbonitride together with carbon and nitrogen, thereby inhibiting the formation of aluminum nitrides and chromium carbides, which can inhibit the formation of aluminum oxide layers. However, excessive Zr can lead to reduced hot ductility of the steel and difficulty in hot working. For this reason, the maximum Zr content is 0.3% by weight. Depending on the embodiment, the Zr content is 0 to less than 0.05% by weight or 0.05 to 0.3% by weight.

[0050] Molybdenum (Mo) and / or tungsten (W) 0 to 3.0 wt%

[0051] Mo and W are considered equivalent elements and can be added optionally. Mo and / or W will combine with carbon by forming corresponding carbides, thereby reducing the amount of chromium carbides formed. However, excessive Mo and W may increase the risk of introducing intermetallic phases such as Laves and sigma phases. For these reasons, the total content of W and Mo should therefore be limited to a maximum of 3.0 wt%.

[0052] Rare earth metals (REM) 0 to 0.1 wt%

[0053] REMs such as La, Ce, Y, Pr, and Sm may optionally be added. These elements are strong sulfide formers, scavenging sulfur (S) from the steel, thereby improving hot ductility, and may be present in amounts up to 0.1 wt. %. Above this level, combined with the Cu and Ni contents defined in this disclosure, REMs tend to negatively impact hot ductility.

[0054] Additionally, hafnium (Hf), tantalum (Ta), and titanium (Ti) are considered functionally equivalent to the elements Zr and Nb and therefore may be present in the same amounts as specified for these elements and may replace these elements in part or in whole.

[0055] As used herein, phosphorus (P) and sulfur (S) may be considered as commonly present impurities. Phosphorus (P) may be tolerated in alloys at low levels. According to one embodiment, P is ≤ 60 ppm. Sulfur (S) may be tolerated in alloys at low levels. According to one embodiment, S is ≤ 60 ppm. According to one embodiment, P + S is ≤ 60 ppm.

[0056] Nitrogen (N) is considered as a generally present impurity. According to one embodiment, the N content is ≤ 0.02 wt%.

[0057] Other impurities may also be present in the austenitic-ferritic stainless steel as defined above or below. Typically, such impurities are unavoidable due to the manufacturing process, for example due to the presence of such impurities in the scrap metal that is melted to produce the melt having the steel composition of the present invention.

[0058] Alternatively, such elements, even if technically removable from the melt, do not impair the functionality of the finished steel, so that the work required for their removal is technically or economically motivated. According to one embodiment, which can be combined with all other embodiments mentioned in the present disclosure, the maximum content of said normally present impurities does not exceed 0.5 wt.-%.

[0059] Furthermore, the austenitic-ferritic stainless steel as defined above or below may comprise the elements mentioned herein within any ranges mentioned herein.According to one embodiment, the austenitic-ferritic stainless steel of the present invention consists of all elements mentioned herein within any ranges mentioned herein.

[0060] The present disclosure also relates to a method of manufacturing an austenitic-ferritic stainless steel object using an austenitic-ferritic stainless steel composition as defined above or below, the method comprising the steps of:

[0061] a) Providing an alumina-forming austenitic-ferritic stainless steel melt having an alloying element composition as defined above or below.

[0062] b) cooling the alumina-forming austenitic-ferritic stainless steel melt into a solid.

[0063] c) hot working the solid into a workpiece of a predetermined shape at a temperature between 1000 and 1300°C.

[0064] The hot working step must be performed at a temperature above 1000° C., otherwise the formation of intermetallic compounds will reduce ductility. According to an embodiment, the hot working temperature is above 1100° C. Above 1300° C., the risk of incipient melting may cause cracks in the body.

[0065] According to one embodiment, the hot working step may be repeated multiple times to obtain a workpiece of a desired shape.

[0066] According to one embodiment, the hot working step may include forging or hot rolling.

[0067] d) heat treating the workpiece at a temperature in a range between 1050° C. and 1200° C. for a time period of about 2 to 120 minutes.

[0068] The time and temperature of the heat treatment step will depend on the size and volume of the workpiece. However, in order to decompose any intermetallic phases, the temperature must be at least 1050°C, such as at least 1100°C. Furthermore, the ferrite content will increase, the extent of which will depend on the heat treatment temperature, and thus a maximum temperature of 1200°C, such as 1170°C, such as 1150°C, is sufficient to obtain a suitable microstructure comprising austenite and ferrite.

[0069] e) Quenching the heat-treated workpiece to approximately room temperature.

[0070] The quenching can be performed by cooling the workpiece to approximately room temperature using an air, water, or oil bath.

[0071] According to one embodiment, the at least one hot working step c) may be followed by an optional cold working step in order to obtain a workpiece of a predetermined shape with finer tolerances.

[0072] According to one embodiment, an optional aging step can be performed after the quenching step e). The aging step is performed at a temperature above 500°C, such as 650 to 850°C, for up to 240 hours, such as up to 100 hours, to achieve an age-hardening effect, such as increasing the yield strength of the final body. During the aging step, no detrimental secondary phases, such as sigma or Laves phases, are formed. However, after the aging step, room temperature (RT) ductility may be slightly reduced.

[0073] Furthermore, the manufactured object of austenitic-ferritic stainless steel as defined above or below may comprise the austenitic-ferritic stainless steel alloying elements mentioned herein within any ranges mentioned herein. According to one embodiment, the austenitic-ferritic stainless steel object of the present invention consists of all the alloying elements mentioned herein within any ranges mentioned herein.

[0074] The final object can be any shape, such as, but not limited to, a tube, strip, sheet, or wire. The object will have excellent oxidation resistance, good weldability, and also have mechanical properties that enable it to be used in non-pressurized applications, such as, but not limited to, muffle tubes, recuperator tubes, and high temperature heat exchangers.

[0075] Therefore, in addition, the present disclosure relates to the use of an object comprising an alumina-forming austenitic-ferritic stainless steel as defined above or below in applications where the object is subjected to temperatures in the range of 500-900° C. and a low oxygen atmosphere. Examples of such applications are muffle tubes, recuperator tubes and high temperature heat exchangers.

[0076] The following non-limiting experiments further illustrate the present disclosure.

[0077] Example

[0078] Sample preparation

[0079] Fourteen alumina forming austenitic-ferritic stainless steel melt heats were prepared having the compositions disclosed in Table 1. Melt heats marked with "*" are comparative examples and therefore outside the scope of the present invention.

[0080] All melts except 068 were prepared by melting in an induction furnace in open atmosphere, 068 was prepared by vacuum induction melting (VIM), and the melt was allowed to cool to a solid.

[0081] The solids were cast into 9-inch ingots and then forged at temperatures between 1180 and 1280° C. The samples had dimensions of 50*120 mm.

[0082] The forged ingot was then heat treated at 1100°C for 20 minutes and quenched in water to approximately room temperature.

[0083] The samples were mechanically prepared by hot rolling from 50 mm to 15 mm.

[0084] After hot rolling, the samples were annealed at about 1130 °C for 20 min.

[0085] The ferrite and austenite contents of the annealed samples were measured according to ASTM E562 using 30 fields and a grid with 100 points. The measurements were performed using an optical microscope and the results can be seen in Table 1.

[0086] High temperature oxidation

[0087] Samples in the form of corrosion coupons (KO-5, 15 mm × 10 mm × 3 mm) were machined from different hot melt machines. The coupons were polished to 600 mesh, cleaned in ethanol / acetone and distilled water, and placed in a horizontal tube furnace where they were exposed to temperatures ranging from 500°C to 900°C for up to 500 hours in a well-controlled simulated air atmosphere (with 2.5% water vapor by volume).

[0088] After 24 h, 48 h, 96 h, 192 h and 500 h, the samples were removed from the furnace for gravimetric measurements. Gravimetric data were collected on a Sartorius balance with a precision of five decimal places.

[0089] Figures 1a and 1b show the mass change (g / m²) as a function of time at different temperatures. 2 ). If you can Figure 1bAs can be seen in the graph, the comparative samples (840, 841, 843) with high ferrite content and low alumina content are losing their ability to form a protective alumina layer, as evidenced by the increase in mass variation. The comparative example (961) with high ferrite content and high Al content, as well as the samples of the present invention, perform well at 800°C.

[0090] Figure 1b It is shown that only the samples according to the invention have good oxidation properties at 900° C. Therefore, the combination of ferrite content and alumina content is very important to obtain excellent oxidation properties in these temperature ranges.

[0091] Mechanical testing

[0092] The yield strength of the samples of the present invention (953, 954, 958) under annealing and aging conditions was measured according to standard ISO6892-1. The results are shown in Figure 2 It can be clearly seen that aging the sample in the temperature range of 650°C to 850°C will increase the yield strength. This strengthening mechanism is most likely due to the formation of nickel aluminides during aging.

[0093] It is therefore apparent that the alumina-forming austenitic-ferritic stainless steel alloy of the present invention exhibits excellent oxidation resistance in the temperature range of 500 to 900°C and also has good mechanical properties.

[0094] Table 1 shows 14 alumina-forming austenitic-ferritic stainless steel hot melts prepared. These values ​​are in weight percent, with the remainder being Fe and unavoidable impurities, excluding ferrite, whose value is in volume percent. Hot melts marked with "*" are comparative example hot melts.

[0095]

[0096] Table 1 continued

[0097] Hot Melt 960 068 171 172 174 175 178 C 0.011 0.085 0.1 0.12 0.15 0.058 0.13 Si 0.04 0.0l 0.24 0.52 0.26 0.09 0.25 Mn 0.02 0.05 2.48 3.06 1.54 0.74 0.22 P 0.004 -- -- -- -- -- -- S 0.0018 0.0006 -- 0.0007 0.0007 -- -- Cr 14.98 15.18 13.31 12.21 14.73 14.97 12.91 Ni 12.16 14.09 14.28 12.25 13.78 12.32 12.47 Mo -- -- 0.03 0.01 0.01 0.01 0.01 V 0.005 -- -- -- 0.006 0.005 0.005 Ti -- -- -- -- 0.003 -- -- Cu 5.1 -- 0.54 3.88 2.43 1.37 1.39 Nb 0.01 0.99 1.01 1.01 1.45 0.65 1.5 N 0.01 0.006 0.008 0.01 0.01 0.016 0.009 B 0.0006 -- -- -- -- -- -- Al 4.1 3.8 4.8 4.3 4.5 3.5 4.11 Ce 0.01 0.01 0.05 0.02 0.05 0.01 0.05 Zr 0.002 0.28 0.12 0.057 0.11 0.046 0.13 La 0.005 -- -- -- -- -- -- Ferrite (volume %) 31 23 36 26 41 17 26

Claims

1. An alumina-forming austenitic-ferritic stainless steel comprising the following composition in weight percent: Optional maximum level of 0.1 wt. % of one or more elements selected from the group consisting of rare earth metals (REM); The balance is Fe and commonly present impurities; and The austenite-ferrite stainless steel has a microstructure comprising more than 15 volume % and less than 45 volume % ferrite and the remainder austenite.

2. The austenitic-ferritic stainless steel alloy of claim 1, wherein the composition comprises 0.5 to 5.5 wt. % Cu.

3. Austenitic-ferritic stainless steel alloy according to claim 1 or 2, wherein the composition comprises 0.05 to 0.3 wt. % Zr.

4. Austenitic-ferritic stainless steel alloy according to any one of claims 1 to 3, wherein the composition comprises 0.05 to 1.6 wt. % Nb.

5. A method of manufacturing an austenitic-ferritic stainless steel object, the method comprising the steps of: a) providing an alumina-forming austenitic-ferritic stainless steel melt having a composition according to any one of claims 1 to 4; b) cooling the alumina-forming austenitic-ferritic stainless steel melt into a solid; c) hot working the solid into a workpiece of a predetermined shape at a temperature between 1000 and 1300° C.; d) heat treating the workpiece at a temperature in a range between 1050° C. and 1200° C. for a time period of about 2 to 120 minutes; e) Quenching the heat-treated workpiece to approximately room temperature.

6. The method according to claim 5, further comprising a cold working step after the hot working step c).

7. The method according to claims 5 and 6, further comprising an aging step after the quenching step e).

8. An object comprising an alumina forming austenitic-ferritic stainless steel according to any one of claims 1 to 4 or manufactured according to any one of claims 1 to 7.

9. Use of an object according to claim 8 in an application wherein the product is subjected to temperatures in the range 500-900°C.

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