Alumina forming austenite-ferrite stainless steel alloy

Abstract

The present disclosure relates to an alumina forming austenite-ferrite stainless steel alloy. The present disclosure also relates to a method of producing an alumina forming austenite-ferrite stainless steel object. Further the disclosure relates to a product and the use in the temperature range of 500 to 900°C.

Description

[0001] Alumina forming austenite-ferrite stainless steel alloy

[0002] Technical field

[0003] The present disclosure relates to an austenite-ferrite stainless steel alloy. More specifically, the present disclosure relates to an alumina forming austenite-ferrite stainless steel alloy. The present disclosure also relates to a method of producing an alumina forming austenite-ferrite stainless steel object, a product comprising said alumina forming austenite-ferrite stainless steel, and the use of the object in specific environment.

[0004] Background

[0005] Prior art discloses examples of duplex stainless steels comprising austenite and ferrite phase which are able of forming a protective alumina layer on a surface thereof when being subjected to an oxygen-containing atmosphere at high temperatures. High contents of nickel, chromium and aluminum are typical for such duplex stainless steel.

[0006] 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, pages 297-302 discloses a ferrite-austenite duplex alloy tested in air at 1250°C. However, the drawback with these alloys is that their oxidation resistance is not sufficient at temperatures between 500 to 900°C. In addition, some of these alloys are shown to be too brittle and thus the ductility will be too low for a conventional manufacturing route.

[0007] 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, pages 1-14 discloses high aluminum content (>5 wt.%) duplex stainless steels tested for corrosion resistance in 1200°C steam and simulated pressurized water reactor (PWR) operating conditions. However, the oxidation resistance for the disclosed compositions are not sufficient at temperatures between 500 to 900°C.

[0008] Hence, there still exist a need in this technical field for an optimized austenite-ferrite stainless steel alloy that will allow an object comprising the stainless steel to be manufactured by using a conventional manufacturing route and that will provide an object with excellent oxidation resistance when used in the temperature range of 500 to 900°C. Summary

[0009] The present disclosure therefore provides an improved alumina forming austenite-ferrite stainless steel alloy composition having an optimized microstructure comprising austenite and ferrite, to be used in a temperature range of 500 to 900°C.

[0010] The austenite-ferrite stainless steel according to the present disclosure is characterized in that the stainless steel comprises the following composition (in weight%);

[0011] Cr 11.0 to 16.0;

[0012] Ni 11.5 to 15.0;

[0013] A1 3.5 to 5.0;

[0014] C 0.01 to 0.15;

[0015] Nb 0.01 to 2.0;

[0016] Mn 0.01 to 3.5;

[0017] Si 0.01 to 0.8;

[0018] Cu 0 to 5.5;

[0019] Zr 0 to 0.3;

[0020] Mo+W 0 to 3.0; balance is Fe and unavoidable impurities; wherein the austenitic-ferritic stainless steel has a microstructure comprising of ferrite which is more than 15 volume% and less than 45 volume% and the remainder being austenite.

[0021] In the present disclosure, the ferrite content of the austenitic-ferritic stainless steel is within the range of more than 15 volume% to less than 45 volume%, the remainder being austenite as it has been found that this microstructure is of critical importance for the oxidation resistance properties in the temperature range of 500 up to 900 °C. If the amount of ferrite is lower than 15 volume %, it has been shown that the oxidation resistance as well as the mechanical strength will be reduced. If the amount of ferrite is higher than 45 volume%, it has been shown that the present stainless steel will have problems forming a protective oxide layer at temperatures above approximately 650°C.

[0022] The present disclosure furthermore provides a method of producing an object comprising the austenite-ferrite stainless steel composition with the microstructure as defined hereinabove or hereinafter. The method of producing the alloy is a conventional smelt metallurgy manufacturing route as it has surprisingly been found that the present stainless steel will have a hot ductility high enough to make this possible.

[0023] The present disclosure furthermore provides an object comprising the austenite-ferrite stainless steel composition with the microstructure as defined hereinabove or hereinafter. The present stainless steel will enable a formation of a layer of aluminium oxide on the object which will allow the object to be used in atmospheres with a wide range of oxygen concentration in the temperature range of from 500-900°C.

[0024] Brief description of figures

[0025] Figures la-b discloses the result of oxidation testing at 800 and 900 °C, respectively.

[0026] Figure 2 discloses a graph showing the yield strength as a function of different heat treatments.

[0027] Detailed description

[0028] The present disclosure relates to an alumina forming austenite-ferrite stainless steel characterized in that the alloy has the following composition (in weight%);

[0029] Cr 11.0 to 16.0;

[0030] Ni 11.5 to 15.0;

[0031] A1 3.5 to 5.0;

[0032] C 0.01 to 0.15;

[0033] Nb 0.01 to 2.0;

[0034] Mn 0.01 to 3.5;

[0035] Si 0.01 to 0.8;

[0036] Cu 0 to 5.5;

[0037] Zr 0 to 0.3;

[0038] Mo+W 0 to 3.0; balance being Fe and normally occurring impurities; and wherein the austenitic-ferritic stainless steel has a microstructure comprising of ferrite which is more than 15 volume% and less than 45 volume% and the remainder being austenite.

[0039] 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 a specific element should not be considered exhaustive.

[0040] Iron (Fe) balance

[0041] The main function for Fe in the austenitic-ferritic stainless steel is to balance the steel composition or the composition of alloying elements of the object. The balance also includes unavoidable impurities which will be discussed.

[0042] Chromium (Cr) 11.0 to 16.0 wt%

[0043] Cr is an important element as it is a ferrite stabilizer and thus assists to maintain the appropriate microstructure comprising of ferrite which is more than 15 volume% and less than 45 volume% and the remainder being austenite.

[0044] Cr also facilitates the formation of alumina, i.e. aluminium oxide layer, on a manufactured object through the so called third element effect by formation of chromium oxide in the transient oxidation stage. In particular at temperatures in the region of 500-600°C, the formation of an alumina layer on the object may suffer if the amount of chromium is insufficient. Further, chromium is an important element since it will improve the corrosion resistance. Thus, the minimum content of chromium in the present steel is 11.0 wt%. According to one embodiment, the minimum content of chromium is 12.0 wt%.

[0045] However, if the amount of Cr is too high, the ferrite content will become too high, which will lead to a reduction in oxidation resistance, especially at higher temperatures, such as 800 to 900°C. A too high Cr content will also result in the formation of secondary phases, such as sigma phase, which will cause embrittlement. Thus, the maximum content of chromium is 16.0 wt%, such as maximum 15.5 wt%.

[0046] According to embodiments, the content of Cr is from 11.0 to 16.0 wt%, such as from 12.0 to 16.0 wt%, such as from 12.0 to 15.5 wt%.

[0047] Nickel (Ni) 11.5 to 15.0 wt%

[0048] Ni is an important element since it is an austenite stabilizer and thus assists to maintain the appropriate microstructure. If the amount of nickel is too low, there is a risk of the ferrite phase content being too high, which will result in loss of oxidation resistance, especially in temperatures above 800°C. Further, too little Ni will also result in a transformation of the austenite to martensite at room temperature. Thus, the minimum content of nickel is 11.5 wt%.

[0049] 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 a low tensile strength.

[0050] Ni should also be balanced against the amount of added aluminum as it will bind aluminum as nickel aluminides, thereby suppressing to some extent the formation of the alumina layer. The nickel aluminides may however provide for improved mechanical properties in the manufactured object, such as increased hardness and improved yield strength. To some extent, nickel may be substituted with cobalt (Co). However, since Co is less preferable from an environmental point of view, Ni is preferred. Thus, the maximum content of Ni is 15.0 wt%. According to embodiments, the content of Ni is from 11.5 to 15.0 wt%, such as from 12.0 to 14.5 wt%.

[0051] Aluminum (Al) 3.5 to 5.0 wt%

[0052] A1 is also an important element in the present steel as Al will, when exposed to oxygen at high temperatures, form a dense and thin aluminum oxide layer on the manufactured object, which will protect the underlying surface from further oxidation. If the content of Al is too low, there will be limited or no formation of a sufficiently thick and protecting alumina layer when the steel is subjected to an oxygen-containing atmosphere at elevated temperatures, such as 500-900°C. Furthermore, Al will together with Ni form nickel aluminide, thereby contributing to an increased hardness thereof. Thus, the minimum content of aluminum is 3.5 wt%.

[0053] Further, A1 is a ferrite stabilizer and thus assists in maintaining the appropriate microstructure. If the amount of A1 is too high, the ferrite content becomes too high, resulting in reduced oxidation resistance, especially at temperatures, such as 800 to 900°C. Thus, the maximum content of alumina is 5.0 wt%. According to embodiments, the content of A1 is 3.5 to 5.0 wt%, such as 3.7 to 4.9 wt%.

[0054] Carbon (C) 0.01 to 0.15 wt%

[0055] C will form carbides with a number of elements present in the austenite-ferrite stainless steel and thereby will contribute to an elevated hardness and strength of the steel (e.g. creep properties). Further, C is also an austenite stabilizer. Thus, the minimum content of carbon is 0.01 wt%, such as 0.03 wt%. Too high carbon content o will increase the risk of formation of too much carbides, such as e.g. M23C6 and / or M7C3 carbides, which will reduce the oxidation resistance. Thus, the maximum content of carbon is 0.15 wt%, such as 0.13 wt%. According to embodiments, the content of C is 0.01 to 0.15 wt%, such as 0.03 to 0.13 wt%.

[0056] Niobium (Nb) 0.01 to 2.0 wt%

[0057] Nb is a ferrite stabilizer and thus assists in maintaining the appropriate microstructure.

[0058] Further, Nb will together with C form niobium carbide and thereby suppresses excessive formation of chromium carbides, which could have negative impact on the formation of the alumina layer. Thus, the minimum content of Nb is 0.01 wt%. According to one embodiment the minimum content of Nb is 0.05 wt%.

[0059] However, too much Nb will result in the formation of an excessive amount of niobium carbide, which will result that the steel will become brittle. Thus, the maximum content of niobium is 2.0 wt%, such as maximum 1.50 wt%. According to embodiments, the content of Nb is therefore from 0.01 to 2.0 wt%, such as 0.05 to 1.60 wt%, such as 0.05 to 1.60 wt%.

[0060] Manganese (Mn) 0.01 to 3.5 wt%

[0061] Manganese (Mn) is an austenite stabilizer and can to some extent replace nickel without compromising the oxidation resistance. Thus, the maximum content of Mn is 3.5 wt%, such as maximum 3.2 wt%. According to embodiments, 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%.

[0062] Silicon (Si) 0.01 to 0.8 wt%

[0063] Si is added in order to improve the oxidation resistance. Thus, the minimum content of Si is 0.01 wt%. Too much Si may however increase the risk of formation of sigma phase. Thus, the maximum content of silicon is 0.8 wt%. According to one embodiment the maximum content of Si is 0.7 wt%, such as 0.6 wt%. According to embodiments, the content of Si is 0.01 to 0.8 wt%, such as 0.01 to 0.7 wt%, such as 0.01 to 0.6 wt%.

[0064] Copper (Cu) 0 to 5.5 wt%

[0065] Cu may be optionally added or may be seen as an impurity. If added on purpose, and in order to have the desired effects, the minimum content is 0.5 wt%. Cu may have a positive effect on the formation of nickel aluminides which may provide for improved mechanical properties, such as improved hardness and yield strength. However, too much Cu results in excessive amounts of nickel aluminides, which will reduce the hot ductility properties during manufacturing of objects. For these reasons the maximum content of Cu is 5.5 wt%, such as 5.3 wt%, such as 5.2 wt%. According to embodiments, the content of Cu is 0 to 5.5 wt%. According to embodiments, the content of Cu is 0 to below 0.5 wt% or 0.5 to 5.5 wt%.

[0066] Zirconium (Zr) 0 to 0.3 wt%

[0067] Zr may be optionally added or may be regarded as an impurity. If added on purpose, the minimum content is 0.05 wt%. Zr forms together with carbon and nitrogen zirconium carbonitrides and thereby suppresses the formation of aluminium nitrides and chromium carbides, which may suppress the formation of an alumina layer. However, too much Zr will result in reduced hot ductility and the difficulty to hot work the steel. For this reason, the maximum content of Zr is 0.3 wt%. According to embodiments, the content of Zr is 0 to below 0.05 wt% or 0.05 to 0.3 wt%.

[0068] Molybdenum (Mo) and / or Tungsten (W) 0 to 3.0 wt %

[0069] Mo and W are regarded as equivalent elements and may be added optionally.. Mo and / or W will bind to carbon by the formation of corresponding carbides thereby reducing the amount of formed chromium carbides. However, too much of Mo and W may increase the risk of introducing intermetallic phases, such Laves and sigma phases. For these reasons, the total content of W and Mo should therefore be limited to a maximum of 3.0 weight%.

[0070] Rare earth metals (REM) 0 to 0.1 weight%

[0071] REMs, e.g. La, Ce, Y, Pr and Sm may optionally be added. These elements are strong sulfide formers, thereby cleaning the steel from sulphur (S) and thus improving the hot ductility and may be present up to an amount of 0.1 weight%. Above that level, in combination with the contents of Cu and Ni as defined in this disclosure, the REMs tend to have a negative impact on the hot ductility.

[0072] In addition, Hafnium (Hf), tantalum (Ta), titanium (Ti) is considered to be functionally equivalent with the elements Zr and Nb and may therefore be present in the same amounts as is specified for those elements, and may, partially or totally, replace these elements. Phosphor (P) and Sulphur (S) may be regarded as normally occurring impurities in this context. Phosphor, P, may be accepted at low levels in the alloy. According to one embodiment, P is <60 ppm. Sulphur, S, may be accepted at low levels in the alloy. According to one embodiment S is <60ppm. According to one embodiment P+S is <60 ppm.

[0073] Nitrogen (N) is to be regarded as a normally occurring impurity. According to one embodiment, the content of N<0.02 wt%.

[0074] Other impurities may also be present in the austenite-ferrite stainless steel as defined hereinabove or hereinafter. Typically, such impurities are unavoidable due to the manufacturing process, for example due to the fact that they are present in the scrap metal which is molten in order to generate a melt having the composition of the present steel. Alternatively, such elements, even though they could technically be removed from the melt, do not harm the functionality of the finished steel to such extent that the work needed for removing them would be motivated from a technical or economical point of view. According to one embodiment, combinable with all other embodiments mentioned in this disclosure, the maximum content of said normally occurring impurities is not more than 0.5 wt%.

[0075] Additionally, the austenitic-ferritic stainless steel as defined hereinabove or hereinafter may comprise the elements mentioned herein in any of the ranges mentioned herein. According to an embodiment, the present austenitic-ferritic stainless steel consists of all the elements mentioned herein in any of the ranges mentioned herein.

[0076] The present disclosure also relates to a method of producing an austenite-ferrite stainless steel object using the austenitic-ferritic stainless steel composition as defined hereinabove or hereinafter, said method comprising the steps of: a) providing an alumina forming austenite-ferrite stainless steel melt having the alloying element composition as defined hereinabove or hereinafter. b) cooling the alumina forming austenite-ferrite stainless steel melt to a solid body. c) hot working said solid body at temperatures between above 1000 to 1300°C to a work piece of predetermined shape. The hot working step must be performed above 1000°C as otherwise the formation of intermetallic will reduce the ductility. According to embodiments, the hot working temperature is above 1100°C. Above 1300°C, the risk of incipient melting may cause cracks in the body.

[0077] According to an embodiment, the hot working step may be repeated several times in order to obtain the desired shape of the work piece.

[0078] According to an embodiment, the hot working step may include forging or hot rolling. d) heat treating said piece at a temperature in the range of between 1050°C to 1200°C for a time of about 2 to 120 minutes.

[0079] The time and temperature of the heat-treating step will be dependent on the size and volume of the work piece. However, the temperature must be at least 1050°C, such as at least 1100°C in order to resolve any intermetallic phases. Also, the ferrite content will be increased and how much will depend on the heat treating temperature, thus the maximum temperature is 1200°C, such as 1170°C, such as 1150°C in order to obtain the right microstructure comprising austenite and ferrite. e) quenching the heat-treated work piece down to approximately room temperature.

[0080] The quenching may be performed by cooling the work piece to approximately room temperature using air, water, or oil bath.

[0081] According to an embodiment, an optional cold working step may be performed after the at least one hot working step, c) in order to obtain the work piece of predetermined shape with finer tolerances.

[0082] According to an embodiment, an optional aging step may be performed after the quenching step, e). The aging step is performed at a temperature above 500°C, such as 650 to 850°C up to 240 hours, such as up to 100 hours, in order to obtain an aging hardening effect, such as an increase in the yield strength of the final object. During the ageing step , no harmful secondary phases, such as sigma phase or Laves phase, will be formed. However, there may be a slight decrease in room temperature (RT) ductility after an aging step. Moreover, the manufactured object of the austenite-ferrite stainless steel as defined hereinabove or hereinafter may comprise the austenite-ferrite stainless steel alloying elements mentioned herein in any of the ranges mentioned herein. According to one embodiment, the present object of the austenite-ferrite stainless steel consists of all the alloying elements mentioned herein in any of the ranges mentioned herein.

[0083] The final object can be in any shape, such as, but not limited to, a tube, a strip, a sheet, or a wire. The object will have excellent oxidation resistance, good weldability and also have mechanical properties which will enable it to be used in non-pressurized applications, such as but not limited to, muffle tube, a recuperator tube and a high temperature heat exchanger.

[0084] Thus, additionally the present disclosure relates to the use of an object comprising the alumina forming austenite-ferrite stainless steel as defined hereinabove or hereinafter in applications in which the object is subjected to a temperature in the range of 500-900° and to a low oxygen atmosphere. Example of such application are a muffle tube, a recuperator tube and a high temperature heat exchanger.

[0085] The present disclosure is further illustrated by the following non-limiting experiments.

[0086] Examples Sample preparation

[0087] 14 alumina forming austenite-ferrite stainless steel melt heats were prepared having compositions as disclosed in Table 1. The heats marked with “*” is comparative examples and are thus outside the scope of this invention.

[0088] All melts except 068 were prepared by melting in an induction furnace in open atmosphere, 068 was prepared by induction melting in vacuum (VIM) and the melts were allowed to cool into solid bodies.

[0089] The solid bodies were casted to a 9 inch ingots and then forged at temperatures between 1180 to 1280°C. The samples had a dimension of 50*120 mm.

[0090] The forged ingots were then heat treated during 20 min in 1100°C and quenched in water to approximately room temperature. Machine preparation of the samples by hot rolling from 50 mm down to 15 mm.

[0091] After hot rolling, the samples were annealed at about 1130 °C for 20 min.

[0092] The ferrite and austenite content of the annealed samples were measured according to ASTM E562 with 30 fields and a grid with 100 points. A light optical microscope was used for the measurement and the results can be seen in Table 1.

[0093] High temperature oxidation

[0094] Samples in the form of corrosion coupons (KO-5, 15mm x 10mm x 3mm) were machined from the different heats. 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 of between 500°C and 900°C under a well-controlled atmosphere simulating air with 2.5% water vapor, by volume, for up to 500 hours.

[0095] The samples were removed from the furnace for gravimetric measurements after 24h, 48h, 96h, 192h and after 500h. The sampling of the gravimetric data was made on a Sartorius scale with five decimals accuracy.

[0096] Figure la and lb shows the mass change (g / m2) at different temperatures as a function of time. As can be seen from Figure lb, the comparative samples with high ferrite content and low alumina contents (840, 841, 843) are losing their ability of forming a protective alumina layer, as can be seen by that the mass change increases. The comparative example with high ferrite content and high A1 content (961) as well as the inventive samples works well at 800°C.

[0097] Figure lb shows that only the inventive samples have good oxidation properties at 900°C. Thus, the combination of ferrite content and alumina content is of great importance in order for excellent oxidation properties in these temperature range.

[0098] Mechanical testing

[0099] The yield strength of the inventive samples (953, 954958) in annealed and aged conditions were measured by the standard IS06892-1 and the results are shown in Figure 2. It can be clearly seen that aging the samples at a temperature range of 650°C and 850°C will increase the yield strength. This strengthening mechanism is most likely due to the formation nickel aluminides during aging.

[0100] Thus, it is evident that the inventive alumina forming austenite-ferrite stainless steel alloy shows excellent oxidation resistance in a temperature range of 500 to 900°C and also have good mechanical properties.

[0101] Table 1 shows the 14 alumina forming austenite-ferrite stainless steel melt heats that were prepared. The values are in wt% and the balance is Fe and unavoidable impurities except for the Ferrite which values are in vol%. The heats marked with a “*” are comparable heats.

[0102] Table 1 cont.

Claims

Claims1. An alumina forming austenite-ferrite stainless steel, comprising the following composition in weight%;Cr 11.0 to 16.0;Ni 11.5 to 15.0;A1 3.5 to 5.0;C 0.01 to 0.15;Nb 0.01 to 2.0;Mn 0.01 to 3.5;Si 0.01 to 0.8;Cu 0 to 5.5;Zr 0 to 0.3;Mo+W 0 to 3.0; optionally one or more elements selected from the group consisting of rare earth metals (REM) to a maximum level of 0.1 weight%; balance being Fe and normally occurring impurities; and wherein the austenitic-ferritic stainless steel has a microstructure comprising of ferrite which is more than 15 volume% and less than 45 volume% and the remainder being austenite.

2. The austenite-ferrite stainless steel alloy according to claim 1, wherein the composition comprises Cu 0.5 to 5.5 in weight%.

3. The austenite-ferrite stainless steel alloy according to claims 1 or 2, wherein the composition comprises Zr 0.05 to 0.3 in weight%.

4. The austenite-ferrite stainless steel alloy according to any one of claims 1 to 3, wherein the composition comprises Nb 0.05 to 1.6 weight%.

5. A method of producing an austenite-ferrite stainless steel object, said method comprising the following steps; a) providing an alumina forming austenite-ferrite stainless steel melt having a composition according to any one of claims 1 to 4;15b) cooling the alumina forming austenite-ferrite stainless steel melt to a solid body; c) hot working said solid body at temperatures between above 1000 to 1300°C to a work piece of predetermined shape; d) heat treating said work piece at a temperature in the range of between 1050°C to 1200°C for a time of about 2 to 120 minutes; e) quenching the heat-treated work piece down 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 the alumina forming austenite-ferrite 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 the object according to claim 8, in applications in which said product is subjected to a temperature in the range of 500-900°C.16

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