Alumina-forming austenitic-ferritic stainless steel alloys

Abstract

The present disclosure relates to alumina-forming austenitic-ferritic stainless steel alloys, methods of making alumina-forming austenitic-ferritic stainless steel bodies, and products and uses in the temperature range of 500-900°C.

Description

[Technical field]

[0001] The present disclosure relates to austenitic-ferritic stainless steel alloys. More specifically, the present disclosure relates to alumina-forming austenitic-ferritic stainless steel alloys. The present disclosure also relates to methods of making alumina-forming austenitic-ferritic stainless steel bodies, products comprising said alumina-forming austenitic-ferritic stainless steels, and uses of the bodies in certain environments. [Background technology]

[0002] The prior art discloses examples of duplex stainless steels containing austenite and ferrite phases that can form a protective alumina layer on their surface when exposed to oxygen-containing atmospheres at high temperatures. High contents of nickel, chromium and aluminum are typical for such duplex stainless steels.

[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. l, 30 July 2001, pp. 297-302, disclose ferrite-austenite duplex alloys tested in air at 1250°C. However, a drawback of these alloys is that their oxidation resistance is not sufficient at temperatures between 500°C and 900°C. Furthermore, some of these alloys have been shown to be very brittle and therefore the ductility is too low for conventional manufacturing routes.

[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, 21 April 2018, pp. 1-14, discloses high aluminum content (>5 wt.%) duplex stainless steels that were tested for corrosion resistance at 1200°C steam and simulated pressurized water reactor (PWR) operating conditions. However, the oxidation resistance of the disclosed compositions is not sufficient at temperatures between 500 and 900°C.

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

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

[0007] An austenitic-ferritic stainless steel according to the present disclosure comprises a stainless steel having the following composition in weight percent: Cr 11.0-16.0; Ni 11.5-15.0; A l 3.5~5.0; C 0.01-0.15; Nb 0.01-2.0; Mn 0.01-3.5; Si 0.01~0.8; Cu 0-5.5; Zr 0~0.3; Mo+W 0~3.0; The balance is Fe and unavoidable impurities; Austenitic ferritic stainless steels are characterized as having a microstructure containing greater than 15% and less than 45% by volume ferrite, with the remainder being austenite.

[0008] In this disclosure, the ferrite content of austenitic ferritic stainless steels is in the range of more than 15% to less than 45% by volume, the remainder being austenite, and this microstructure has been found to be crucial for oxidation resistance properties in the temperature range of 500-900°C. It has been shown that when the ferrite amount is less than 15% by volume, oxidation resistance and mechanical strength are reduced. When the ferrite amount is more than 45% by volume, the stainless steels have been shown to have problems forming a protective oxide layer at temperatures above about 650°C.

[0009] The present disclosure further provides a method of producing an object comprising an austenitic-ferritic stainless steel composition having a microstructure as defined above or below. The method of producing the alloy is a conventional melt metallurgical production route, since it has surprisingly been found that the present stainless steels have a high enough hot ductility to make this possible.

[0010] The present disclosure further provides an article comprising an austenitic-ferritic stainless steel composition having a microstructure as defined above or below. The stainless steel of the present invention allows for the formation of an aluminum oxide layer on the article, which allows for the article to be used in atmospheres having a wide range of oxygen concentrations at temperatures ranging from 500 to 900°C. [Brief description of the drawings]

[0011] [Figure 1a] The results of oxidation tests at 800°C are disclosed. [Figure 1b] The results of oxidation tests at 900°C are disclosed. [Diagram 2]A graph showing yield strength as a function of different heat treatments is disclosed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present disclosure relates to an alloy having the following composition in weight percent: Cr 11.0-16.0; Ni 11.5-15.0; A l 3.5~5.0; C 0.01-0.15; Nb 0.01-2.0; Mn 0.01-3.5; Si 0.01~0.8; Cu 0-5.5; Zr 0~0.3; Mo+W 0~3.0; The balance is Fe and commonly occurring impurities; The austenitic ferritic stainless steel relates to an alumina-forming austenitic-ferritic stainless steel characterized by having a microstructure containing greater than 15 volume percent and less than 45 volume percent ferrite, with the remainder being austenite.

[0013] 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. Also, the list of properties or contributions mentioned for certain elements should not be considered exhaustive.

[0014] Iron (Fe) balance The main function of Fe in austenitic and ferritic stainless steels is to balance the steel composition or alloying element composition of the object. The balance also includes unavoidable impurities, which will be described later.

[0015] Chromium (Cr) 11.0~16.0wt% Cr is an important element as it is a ferrite stabilizer, thus helping to maintain a proper microstructure containing more than 15% by volume and less than 45% by volume ferrite with the remainder being austenite.

[0016] Cr also promotes the formation of an alumina, i.e. an aluminum oxide, layer on the manufactured object via the so-called third element effect due to the formation of chromium oxide in the transient oxidation phase. Especially at temperatures in the range of 500-600 ° C, if the amount of chromium is insufficient, the formation of the alumina layer on the object can be impaired. Chromium is also an important element for improving the corrosion resistance. Therefore, the minimum content of chromium in the present steel is 11.0 wt.%.

[0017] According to one embodiment, the minimum chromium content is 12.0 wt%.

[0018] However, too much Cr leads to too high a ferrite content, which leads to a decrease in oxidation resistance, especially at high temperatures such as 800-900°C. Too high a Cr content also leads to the formation of secondary phases such as sigma phases, which cause embrittlement. Therefore, the maximum content of chromium is 16.0wt%, e.g. up to 15.5wt%.

[0019] According to an embodiment, the Cr content is 11.0 to 16.0 wt%, for example, 12.0 to 16.0 wt%, for example, 12.0 to 15.5 wt%.

[0020] Nickel (Ni) 11.5-15.0wt% Ni is an important element because it is an austenite stabilizer and therefore helps to maintain the proper microstructure. Too little Ni runs the risk of too much ferrite content, resulting in a loss of oxidation resistance, especially at temperatures above 800°C. In addition, too little Ni also leads to an austenite to martensite transformation at room temperature. Therefore, the minimum content of nickel is 11.5wt%.

[0021] On the other hand, if the amount of Ni is too large, the amount of ferrite is too small, resulting in poor mechanical properties such as low tensile strength.

[0022] Ni should be balanced with the amount of aluminum added, since it also binds aluminum as nickel aluminide, thereby inhibiting to some extent the formation of the alumina layer. However, nickel aluminide may provide improved mechanical properties in the manufactured object, such as increased hardness and improved yield strength. Nickel may be substituted to some extent with cobalt (Co). However, Ni is preferred, since Co is less preferred from an environmental point of view. Thus, the maximum content of Ni is 15.0 wt%. According to an embodiment, the content of Ni is 11.5-15.0 wt%, for example 12.0-14.5 wt%.

[0023] Aluminum (Al) 3.5-5.0wt% 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, so A l is also an important element in this steel. If the Al content is too low, the formation of a sufficiently thick protective alumina layer is limited or not formed when the steel is exposed to oxygen-containing atmospheres at high temperatures, such as 500-900 °C. Furthermore, Al forms nickel aluminides together with Ni, thereby contributing to an increase in its hardness. Therefore, the minimum content of aluminum is 3.5 wt%.

[0024] Furthermore, A l A is a ferrite stabilizer and therefore helps maintain the proper microstructure. l If the amount of A is too high, the ferrite content will be too high, and the oxidation resistance will decrease, especially at temperatures such as 800-900°C. Therefore, the maximum content of alumina is 5.0 wt%. According to an embodiment, A l The content is 3.5 to 5.0 wt%, for example, 3.7 to 4.9 wt%.

[0025] Carbon (C)0.01~0.15wt% C forms carbides with some elements present in austenitic-ferritic stainless steels, thereby contributing to an increase in the hardness and strength of the steel (e.g. creep properties). In addition, C is also an austenite stabilizer. The minimum content of carbon is therefore 0.01wt%, for example 0.03wt%. If the carbon content o is too high, the risk of forming too many carbides, for example M23C6 and / or M7C3 carbides, increases and the oxidation resistance decreases. The maximum content of carbon is therefore 0.15wt%, for example 0.13wt%. According to an embodiment, the content of C is between 0.01 and 0.15wt%, for example 0.03 and 0.13wt%.

[0026] Niobium (Nb) 0.01-2.0wt% Nb is a ferrite stabilizer and therefore helps maintain the proper microstructure.

[0027] Furthermore, Nb forms niobium carbide together with C, thereby suppressing the excessive formation of chromium carbide, which may have a negative effect on the formation of the alumina layer. Therefore, the minimum content of Nb is 0.01 wt%. According to one embodiment, the minimum content of Nb is 0.05 wt%.

[0028] However, too much Nb will form an excessive amount of niobium carbide, making the steel brittle. Therefore, the maximum content of niobium is 2.0wt%, for example, up to 1.50wt%. Therefore, according to an embodiment, the content of Nb is 0.01-2.0wt%, for example, 0.05-1.60wt%, for example, 0.05-1.60wt%.

[0029] Manganese (Mn) 0.01-3.5wt% Manganese (Mn) is an austenite stabilizer and can replace nickel to some extent without compromising oxidation resistance. Therefore, the maximum content of Mn is 3.5wt%, for example, up to 3.2wt%. According to an embodiment, the content of Mn is 0.01-3.5wt%, for example, 0.01-3.2wt%, for example, 0.05-3.1wt%.

[0030] Silicon (Si) 0.01-0.8wt% Si is added to improve oxidation resistance. Therefore, the minimum content of Si is 0.01wt%. However, too much Si may increase the risk of sigma phase formation. Therefore, the maximum content of silicon is 0.8wt%. According to an embodiment, the maximum content of Si is 0.7wt%, for example 0.6wt%. According to an embodiment, the content of Si is 0.01-0.8wt%, for example 0.01-0.7wt%, for example 0.01-0.6wt%.

[0031] Copper (Cu) 0-5.5wt% Cu may be added optionally or may be considered as an impurity. If added intentionally and to obtain the desired effect, the minimum content is 0.5 wt%. Cu may have a positive effect on the formation of nickel aluminides, which may provide improved mechanical properties such as improved hardness and yield strength. However, too much Cu will result in an excessive amount of nickel aluminides, which will reduce the hot ductility properties during the manufacture of the object. For these reasons, the maximum content of Cu is 5.5 wt%, such as 5.3 wt%, for example 5.2 wt%. According to an embodiment, the content of Cu is 0-5.5 wt%. According to an embodiment, the content of Cu is 0-less than 0.5 wt% or 0.5-5.5 wt%.

[0032] Zirconium (Zr) 0-0.3wt% Zr may be added optionally or may be considered as an impurity. If intentionally added, the minimum content is 0.05 wt%. Zr forms zirconium carbonitride with carbon and nitrogen, thereby inhibiting the formation of aluminum nitride and chromium carbide, and may inhibit the formation of an alumina layer. However, too much Zr reduces the hot ductility and makes hot working of the steel difficult. For this reason, the maximum content of Zr is 0.3 wt%. According to an embodiment, the content of Zr is 0 to less than 0.05 wt% or 0.05 to 0.3 wt%.

[0033] Molybdenum (Mo) and / or tungsten (W) 0-3.0wt% Mo and W are considered as equivalent elements and may be added in some cases. Mo and / or W bind to carbon by forming the corresponding carbides, thereby reducing the amount of chromium carbide formed. However, too much Mo and W may increase the risk of introducing intermetallic phases such as Laves and sigma phases. Therefore, for these reasons, the total content of W and Mo should be limited to a maximum of 3.0 wt.%.

[0034] Rare earth metal (REM) 0~0.1% by weight REMs, such as La, Ce, Y, Pr and Sm, may optionally be added. These elements are strong sulfide formers, thereby cleaning the steel from sulfur (S) and thus improving hot ductility, and may be present in amounts up to 0.1 wt.%. Above this level, in combination with the Cu and Ni contents defined in this disclosure, the REMs tend to have a negative effect on hot ductility.

[0035] Additionally, hafnium (Hf), tantalum (Ta), titanium (Ti) are considered to be functionally equivalent to the elements Zr and Nb and therefore may be present in the same amounts as specified for those elements and may be substituted partially or in whole for those elements.

[0036] Phosphorus (P) and Sulfur (S) may be considered as normally present impurities in this context. Phosphorus P may be tolerated at low levels in the alloy. According to one embodiment, P is <60 ppm. Sulfur S may be tolerated at low levels in the alloy. According to one embodiment, S is <60 ppm. According to one embodiment, P+S is <60 ppm.

[0037] Nitrogen (N) should be considered as a normally present impurity. According to one embodiment, the content of N<0.02 wt%.

[0038] Other impurities may also be present in the austenitic-ferritic stainless steels as defined above or below. Typically, such impurities are unavoidable due to the manufacturing process, for example due to the fact that they are present in the scrap metal that is melted to produce the melt having the composition of the steel.

[0039] Alternatively, even if such elements could be technically removed from the melt, they would not impair the functionality of the finished steel to the extent that the efforts required to remove them are motivated from a technical or economic point of view. According to one embodiment, which can be combined with all other embodiments mentioned in this disclosure, the maximum content of said normally present impurities is less than or equal to 0.5 wt.%.

[0040] Furthermore, the austenitic ferritic stainless steels defined above or below may comprise the elements mentioned herein in any of the ranges mentioned herein. According to one embodiment, the austenitic ferritic stainless steels of the present invention consist of all the elements mentioned herein in any of the ranges mentioned herein.

[0041] The present disclosure also relates to a method of producing an austenitic-ferritic stainless steel body using an austenitic-ferritic stainless steel composition as defined above or below, the method comprising the steps of:

[0042] a) providing an alumina-forming austenitic-ferritic stainless steel melt having an alloying element composition as defined above or below.

[0043] b) cooling the alumina-forming austenitic-ferritic stainless steel melt to a solid.

[0044] c) hot working the solid at a temperature of greater than 1000°C to 1300°C into a workpiece of predetermined shape.

[0045] The hot working process must be carried out above 1000 ° C, otherwise intermetallic formations would reduce ductility. According to an embodiment, the hot working temperature is above 1100 ° C. Above 1300 ° C, there is a risk of incipient melting, which could cause cracks in the body.

[0046] According to one embodiment, the hot working step may be repeated several times to obtain a workpiece of the desired shape.

[0047] According to one embodiment, the hot working step may include forging or hot rolling.

[0048] d) heat treating the workpiece at a temperature in the range of 1050°C to 1200°C for about 2 to 120 minutes.

[0049] The time and temperature of the heat treatment process depend on the size and volume of the workpiece. However, to decompose the intermetallic phases, the temperature must be at least 1050°C, e.g. at least 1100°C. Also, the ferrite content increases, the amount of which depends on the heat treatment temperature, so to obtain the correct microstructure containing austenite and ferrite, the maximum temperature is 1200°C, e.g. 1170°C, e.g. 1150°C.

[0050] e) quenching the heat treated workpiece to about room temperature.

[0051] Quenching can be accomplished by cooling the workpiece to about room temperature using air, water, or an oil bath.

[0052] According to one embodiment, an optional cold working step may be carried out after at least one hot working step c) in order to obtain a workpiece of predetermined shape with closer tolerances.

[0053] According to one embodiment, the quenching step e) may be followed by an optional ageing step, which is carried out at a temperature above 500°C, for example at 650-850°C, for up to 240 hours, for example up to 100 hours, in order to obtain age hardening effects, such as an increase in the yield strength of the final object. During the ageing step, no deleterious secondary phases, such as sigma or Laves phases, are formed. However, there may be a slight decrease in room temperature (RT) ductility after the ageing step.

[0054] Furthermore, a manufactured object of austenitic-ferritic stainless steel as defined above or below may comprise the austenitic-ferritic stainless steel alloying elements referred to herein in any of the ranges referred to herein. According to one embodiment, this object of austenitic-ferritic stainless steel consists of all the alloying elements referred to herein in any of the ranges referred to herein.

[0055] The final object can be of any shape, including but not limited to, tube, strip, sheet, or wire, and has excellent oxidation resistance, good weldability, and also mechanical properties that allow it to be used in non-pressurized applications, including but not limited to, muffle tubes, recuperator tubes, and high temperature heat exchangers.

[0056] Thus, the present disclosure further 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 exposed to temperatures in the range of 500-900° and low-oxygen atmospheres. Examples of such applications are muffle tubes, recuperator tubes and high-temperature heat exchangers.

[0057] The present disclosure is further illustrated by the following non-limiting experiments. EXAMPLES

[0058] Sample preparation Fourteen alumina-forming austenitic-ferritic stainless steel melt heats were prepared having the compositions disclosed in Table 1. Heats marked with an "*" are comparative examples and are therefore outside the scope of the present invention.

[0059] All melts were prepared by melting in an open atmosphere induction furnace except for 068, which was prepared by induction melting in a vacuum (VIM) and cooling the melt to a solid.

[0060] The solid was cast into a 9-inch ingot and then forged at a temperature of 1180-1280°C. The dimensions of the specimen were 50*120mm.

[0061] The forged ingot was then heat treated at 1100°C for 20 minutes and quenched in water to approximately room temperature.

[0062] Mechanical preparation of specimens by hot rolling from 50 mm to 15 mm.

[0063] After hot rolling, the samples were annealed at about 1130°C for 20 min.

[0064] The ferrite and austenite contents of the annealed samples were measured according to ASTM E562 with 30 fields and a grid of 100 points. An optical microscope was used for the measurements and the results can be seen in Table 1.

[0065] High Temperature Oxidation Specimens in the form of corrosion coupons (KO-5, 15 mm × 10 mm × 3 mm) were machined from different heats. The coupons were polished to 600 mesh, cleaned with ethanol / acetone and distilled water, and placed in a horizontal tube furnace and exposed to temperatures ranging from 500°C to 900°C for up to 500 hours in a well-controlled atmosphere simulating air containing 2.5% water vapor by volume.

[0066] Samples were removed from the furnace for gravimetric measurements after 24, 48, 96, 192 and 500 hours. Gravimetric data sampling was performed on a Sartorius scale with 5 orders of magnitude accuracy.

[0067] Figures 1a and 1b show the mass change (g / m 2) as a function of time. As can be seen in FIG. 1b, the comparative samples (840, 841, 843) with high ferrite content and low alumina content lose the ability to form a protective alumina layer as shown by the increasing mass change. l The comparative example (961) with the content as well as the inventive sample perform well at 800°C.

[0068] Figure 1b shows that only the samples of the present 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.

[0069] Mechanical testing The yield strength of the inventive samples (953, 954958) in annealed and aged conditions was measured as per standard IS06892-1 and the results are shown in Figure 2. It can be clearly seen that the yield strength increases when the samples are aged in the temperature range of 650°C to 850°C. This strengthening mechanism is most likely due to the formation of nickel aluminides during aging.

[0070] 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 500-900°C and also has good mechanical properties.

[0071] Table 1 shows the heats of 14 alumina-forming austenitic-ferritic stainless steel melts that were prepared. Values ​​are in wt%, the balance being inevitable impurities excluding Fe and ferrite, and values ​​are in vol%. Heats marked with "*" are equivalent heats. JPEG0007675859000001.jpg77111Table 1 continued JPEG0007675859000002.jpg77111

Claims

1. 1. An alumina-forming austenitic-ferritic stainless steel having the following composition in weight percent: Cr 11.0-16.0; Ni 11.5-15.0; Al 3.5-5.0; C 0.01-0.15; Nb 0.01-2.0; Mn 0.01-3.5; Si 0.01-0.8; Cu 0-5.5; Zr 0-0.3; Mo+W 0-3.0; Optionally, one or more elements selected from the group consisting of rare earth metals (REM), up to a maximum level of 0.1 wt.%; The balance is Fe and normally present impurities; The austenitic-ferritic stainless steel has a microstructure containing more than 15 volume percent and less than 45 volume percent ferrite, the remainder being austenite. Austenitic-ferritic stainless steel.

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

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

4. 3. The austenitic-ferritic stainless steel alloy of claim 1 or 2, wherein the composition comprises 0.05-1.6 wt. % Nb.

5. 1. A method for producing an austenitic-ferritic stainless steel article, comprising the steps of: a) providing an alumina-forming austenitic-ferritic stainless steel melt having the composition of claim 1; b) cooling the alumina-forming austenitic-ferritic stainless steel melt to a solid; c) hot working the solid at a temperature above 1000°C to 1300°C into a workpiece of predetermined shape; d) heat treating the workpiece at a temperature in the range of 1050° C. to 1200° C. for 2 to 120 minutes; e) quenching the heat treated workpiece to room temperature; 1. A method comprising: an austenitic-ferritic stainless steel body comprising greater than 15 volume percent and less than 45 volume percent ferrite, with the remainder being austenite.

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

7. 7. The process according to claim 5 or 6, further comprising an aging step after the quenching step e).

8. 10. An object comprising the alumina-forming austenitic-ferritic stainless steel of claim 1.

9. Use of the body according to claim 8 in an application in which the body is exposed to temperatures in the range of 500 to 900°C.

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