Fe-Cr-Al alloy tube

Abstract

The present disclosure relates to Fe-Cr-Al tubes. More specifically, the present disclosure relates to Fe-Cr-Al tubes having a specific powder composition (wt%): Cr 12.00~25.00; Al 3.50-6.50; Ti 0.20~1.10; N 0.06-0.20; Zr 0.05~0.20; Y 0.01~0.15; C ≤ 0.050; Si ≤ 0.50; Hf ≤ 0.30; Ta ≤ 0.30; Mn ≤ 0.40; Ni ≤ 0.60; O ≦600ppm; The balance is Fe and unavoidable impurities Here, TiN is present as an inoculant.

Description

[Technical field]

[0001] The present disclosure relates to Fe-Cr-Al alloy tubing, and more particularly, to tubing manufactured from specific Fe-Cr-Al powder compositions. [Background technology]

[0002] Fe-Cr-Al tubes have excellent heat resistance up to approximately 1450°C, while also having very good form stability and corrosion resistance. High temperature applications for such Fe-Cr-Al tubes range from oxidizing, sulfiding to carburizing environments. Fe-Cr-Al tubes offer several advantages over other tube materials, such as chromia-forming tube materials in these demanding environments. This is primarily due to the ability of Fe-Cr-Al tubes to form a dense and adherent alumina layer that protects the tube from corrosion and atmospheric attack.

[0003] China Patent Publication No. 110004367 (CN110004367) discloses that Fe-Cr-Al tubes with a composition (by weight, wt%) of 14-22% Cr, 3-5% Al, 0.15-0.5% Y, and the balance Fe can be manufactured by preparing powder, hot isostatic pressing (HIP) the powder into a billet, forging, heat treating, piercing, followed by a cold working step at room temperature, followed by a heat treatment step to reduce the diameter and / or wall thickness of the tube. However, the Fe-Cr-Al tubes mentioned therein have the problem of crack formation during operation.

[0004] Thus, there remains a need in the art for more crack resistant Fe-Cr-Al alloy tubes.

[0005] The present disclosure aims to solve or at least reduce the problems mentioned above.

[0006] Summary of the Invention The present disclosure thus provides iron-chromium-aluminum (Fe-Cr-Al) alloy tubes made from specific powder compositions optimized to provide tubes that are less brittle at room temperature and thus capable of being cold worked.

[0007] The Fe-Cr-Al tube has the following composition (by weight): Cr 12.00~25.00; Al 3.50-6.50; Ti 0.20~1.10; N 0.06-0.20; Zr 0.05~0.20; Y 0.01~0.15; C ≤ 0.050; Si ≤ 0.50; Hf ≤ 0.30; Ta ≤ 0.30; Mn ≤ 0.40; Ni ≤ 0.60; O ≦600ppm; The balance is Fe and unavoidable impurities wherein TiN is present as an inoculant.

[0008] In the present disclosure, TiN is present as an inoculant in the Fe-Cr-Al powder. The inoculant has been found to provide several advantages for the tube and during the process of manufacturing the tube. In particular, the TiN inoculant provides grain refinement. Tubes made from the powders defined above or below do not crack when cold worked or when exposed to stress or thermal shock.

[0009] Detailed Description The present disclosure relates to an Fe—Cr—Al tube having the following composition (in weight percent): Cr 12.0~25.0; Al 3.50-6.50; Ti 0.20~1.10; N 0.06-0.20; Zr 0.05~0.20; Y 0.01~0.15; C ≤ 0.050; Si ≤ 0.50; Hf ≤ 0.30; Ta ≤ 0.30; Mn ≤ 0.40; Ni ≤ 0.60; O ≦600ppm; The balance is Fe and unavoidable impurities wherein TiN is present as an inoculant.

[0010] The alloying elements of the powders according to the present disclosure are described in more detail below. The terms "weight %" and "wt%" are used interchangeably. Also, the listing of properties or contributions mentioned for specific elements should not be considered exhaustive.

[0011] Iron (Fe) The primary function of the iron in the Fe-Cr-Al powder is to provide the balance of the alloying element composition of the powder composition or tube.

[0012] Chromium (Cr) 12.0-25.0% by weight Chromium is an important element since it improves the corrosion resistance of the obtained tube and increases its tensile and yield strength. Chromium also facilitates the formation of Al2O3 in the final tube due to the so-called third element effect (i.e. chromium oxide formation in the transition oxidation stage). Too little chromium leads to loss of corrosion resistance. Thus, chromium is preferably present in an amount of at least 12.0 wt.%, such as at least 15.0 wt.%, for example at least 20.0 wt.%. Too much chromium leads to α-α' decomposition and 475°C embrittlement, and also to an increase in the solid solution hardening effect on the ferritic structure. Thus, the maximum chromium content is set at 25.0 wt.%, such as at most 24.0 wt.%, for example at most 23.50 wt.%, for example at most 23.0 wt.%, for example at most 22.50 wt.%, for example at most 22.0 wt.%, for example at most 21.50 wt.%. According to some embodiments, the chromium content is 12.0-25.0 wt.%, such as 18.0-24.0 wt.%, for example 20.0-23.50 wt.%.

[0013] Aluminum (Al) 3.50-6.50% by weight Aluminum is an important element because, when exposed to oxygen at high temperatures, it forms a dense and thin Al2O3 layer on the surface of the manufactured tube, which protects the underlying surface from further oxidation. Aluminum also increases the electrical resistance. Too little aluminum will result in the loss of the ability to form an Al2O3 layer, which will also reduce the electrical resistance. Aluminum is therefore preferably present in an amount of at least 3.50% by weight, such as at least 4.00% by weight, such as at least 4.50% by weight, such as at least 4.80% by weight. Too much aluminum leads to brittleness at low temperatures and promotes the formation of undesirable brittle aluminides. The maximum amount of aluminum is therefore set to 6.50% by weight, such as at most 6.00% by weight, such as at most 5.50% by weight, such as at most 5.40% by weight, such as at most 5.30% by weight, such as at most 5.20% by weight. According to some embodiments of the present disclosure, the aluminum content is 3.50 to 6.50 wt %, for example, 4.00 to 5.50 wt %, for example, 4.50 to 5.50 wt %.

[0014] Titanium (Ti) 0.20-1.10% by weight Titanium is an important element since it forms TiN with nitrogen. According to an embodiment, it is desirable for the ratio Ti / N (wt%) to be at least 3.3, such as at least 4.5, depending on the molar weight of Ti and N.

[0015] In addition, titanium also reduces the activity of carbon by the formation of TiC, which can further improve the high temperature creep strength. If the amount of Ti is too low, there will not be enough TiN in the powder for the nucleation of ferrite crystals during solidification in the additive manufacturing process. Furthermore, if the content of Ti is too low, there is a high risk of undesired chromium carbides and / or brittle aluminum nitrides being formed. Therefore, titanium is preferably present in an amount of at least 0.20 wt%, such as at least 0.25 wt%, such as at least 0.30 wt%. On the other hand, if the content of titanium is too high, there may be a negative effect on the formation of Al2O3, since TiO2 may be formed. For these reasons, the maximum content of Ti is set to 1.10 wt%, such as at most 1.00 wt%, such as at most 0.90 wt%, such as at most 0.8 wt%. According to several embodiments of the present disclosure, the content of Ti is 0.20-0.80 wt%, such as 0.20-0.70 wt%, such as 0.24-0.60 wt%.

[0016] Nitrogen (N) 0.06-0.20% by weight Nitrogen is an important element because it forms TiN particles with titanium. In the present disclosure, TiN is the desired particle because it acts as an inoculant. According to some embodiments, it is desirable for the ratio Ti / N (wt%) to be at least 3.3, such as at least 4.5, depending on the molar weight of Ti and N.

[0017] Nitrogen is also an important element since it allows the precipitation of other metal nitrides (e.g. ZrN). ZrN improves high temperature creep resistance. However, if the nitrogen content is too low, too few nitrides will be formed. Nitrogen is therefore preferably present in an amount of at least 0.06 wt%, such as at least 0.07 wt%, such as at least 0.08 wt%, such as at least 0.09 wt%. Furthermore, if the nitrogen content is too high relative to the titanium content, there may be a risk of AlN being formed (which has a detrimental effect on oxidation resistance). For these reasons, the maximum N content is set to 0.20 wt%, such as at most 0.15 wt%, such as at most 0.10 wt%. According to several embodiments of the present disclosure, the N content is 0.60-0.20 wt%, such as at least 0.07-0.15 wt%, such as at least 0.07-0.12 wt%.

[0018] TiN inoculant The tubes containing the powdered Fe-Cr-Al alloys defined above and below have uniformly distributed TiN inoculants, which are desirable inoculants that provide grain refinement to the tubes, and the resulting grain structure of the tubes has a significantly smaller average grain size than typical conventional Fe-Cr-Al tubes lacking these TiN inoculants.

[0019] Thus, the homogeneous and finely distributed TiN inoculant in the Fe-Cr-Al powder according to the invention leads to a tube with a finer grained Fe-Cr-Al alloy, which reduces the cracking behavior during and after the cold working of the tube according to the invention, which also makes the tube according to the invention more strain tolerant.

[0020] Zirconium (Zr) 0.05-0.20% by weight Zirconium is an important element in the powder composition since it reduces the activity of C and N by the formation of ZrC or ZrN precipitates. Zirconium can also improve the high temperature creep strength of the produced tube. Too little Zr increases the risk of undesired chromium carbide and / or aluminum nitride formation. Zirconium is therefore preferably present in an amount of at least 0.05 wt%, such as at least 0.07 wt%, for example at least 0.10 wt%. On the other hand, too high a content of zirconium may adversely affect the formation of Al2O3. For these reasons, the maximum content of zirconium is set to 0.20 wt%, for example up to 0.15 wt%. According to several embodiments of the present disclosure, the content of zirconium is 0.05-0.20 wt%, for example 0.07-0.20 wt%, for example 0.070-0.10 wt%.

[0021] Yttrium (Y) 0.01 to 0.15% by weight The addition of yttrium improves the oxidation resistance of the produced tube. If too little yttrium is added, the oxidation resistance will be reduced. For this reason, yttrium must be added in an amount of at least 0.01% by weight, such as at least 0.02% by weight, such as at least 0.04% by weight, such as 0.05% by weight, such as 0.06% by weight. However, too much yttrium added will lead to hot embrittlement. As a result, the maximum yttrium content is set at 0.15% by weight, such as 0.10% by weight, such as 0.08% by weight.

[0022] Carbon (C)≦0.050% by weight Carbon is not an intentionally added element, but is an unavoidable element due to powder processing. This element can lead to a decrease in hot ductility and the formation of metal carbides. Therefore, in order to limit the presence of too many metal carbide precipitates, the carbon content must be ≦0.050 wt.%, such as ≦0.040 wt.%, for example ≦0.030 wt.%.

[0023] Silicon (Si)≦0.50% by weight Silicon may be present at levels up to 0.50% by weight to increase electrical resistivity and improve high temperature corrosion resistance, but above this level there is increased hardness and brittleness at low temperatures.

[0024] Tantalum (Ta)≦0.30% by weight Tantalum is an optional addition and when added, it improves high temperature creep strength. Tantalum can also reduce the activity of carbon by forming TaC precipitates, so the maximum tantalum content is set at 0.30 wt.%.

[0025] Hafnium≦0.30% by weight Hafnium may be optionally added. The addition of hafnium improves high temperature creep strength. However, hafnium may reduce carbon activity by forming HfC precipitates. Therefore, the maximum hafnium content is set to ≦0.30 wt.%.

[0026] Manganese (Mn)≦0.40% by weight Manganese may be present as an impurity, which may have a detrimental effect on the oxidation resistance since it may interfere with the formation of the Al2O3 layer, so the maximum manganese content is ≦0.40 wt.%, for example ≦0.20 wt.%.

[0027] Nickel (Ni)≦0.6% by weight Nickel may be present as an impurity, but it can increase the hardness and brittleness at low temperatures, so the maximum content of nickel is ≦0.60% by weight, for example ≦0.5% by weight.

[0028] Oxygen (O)≦600ppm Oxygen may be present in the form of oxides. The maximum permitted content is ≦600 ppm.

[0029] According to embodiments, the powder may also contain trace fractions of one or more of the following impurity elements (by way of example only and not limitation): magnesium (Mg), cerium (Ce), calcium (Ca), phosphorus (P), tungsten (W), cobalt (Co), sulfur (S), molybdenum (Mo), niobium (Nb), vanadium (V), and copper (Cu), in amounts up to 0.2% by weight.

[0030] Furthermore, the Fe-Cr-Al powders defined above or below may contain the alloying elements mentioned herein, in any of the ranges mentioned herein. According to one embodiment, the Fe-Cr-Al tube according to the invention consists of all the alloying elements mentioned herein, in any of the ranges mentioned herein.

[0031] The Fe-Cr-Al powders defined above or below can be produced by various methods, for example Directly by gas atomization heating a powder containing any alloying element in the ranges mentioned above or below (but with a low nitrogen content) in a nitrogen-rich atmosphere, i.e. nitriding the powder; Mixing a powder containing any alloying element in the range mentioned above or below, but with a low nitrogen content, with a powder containing fine particles of less stable nitrides, Fine / small particles of TiN are mixed with the Fe-Cr-Al powder so that the resulting powder has the same alloying element composition as specified above or below. The present invention can be produced by, but is not limited to, the following methods.

[0032] According to an embodiment, the Fe-Cr-Al alloy tube as defined above or below may be manufactured by a method comprising the steps of: i) producing an Fe-Cr-Al powder having the composition specified above or below; ii) The Fe-Cr-Al powder is hot isostatically pressed (HIPed) into a billet. iii) The HIPed billet is machined to produce an extrusion billet. iv) Extruding the billet into a blank tube. v) Cold working the blank tube to final dimensions.

[0033] According to another embodiment, a tube of Fe-Cr-Al alloy as defined above or below may be manufactured by a method comprising the following steps: i) producing an Fe-Cr-Al powder having the composition specified above or below; ii) Hot isostatic pressing (HIP) of Fe-Cr-Al powder into blooms iii) hot working the HIPed blooms by hot rolling and / or hot forging into round bars; iv) Machining the round bar into an extrusion billet v) extruding the billet into a blank tube; vi) Cold working the blank tube to final dimensions.

[0034] According to yet another embodiment, a tube of Fe-Cr-Al alloy as defined above or below may be manufactured by a method comprising the steps of: i) producing an Fe-Cr-Al powder having the composition specified above or below; ii) The hollow capsule is filled with Fe-Cr-Al powder. iii) Capsules containing Fe-Cr-Al powder are subjected to cold isostatic pressing (CIP). iv) Heating the capsule containing the Fe-Cr-Al powder v) Extrusion of the capsule containing the Fe-Cr-Al powder into a blank tube. vi) Removing capsule residues by acid washing to obtain blank tubes vii) Cold working the blank tube to final dimensions.

[0035] Tubes of Fe-Cr-Al alloys as defined above and below are more easily weldable without the addition of filler materials, as a result of the grain refining effect brought about by the TiN inoculant according to the invention.

[0036] The Fe-Cr-Al alloy tubes as defined above or below operate well at temperatures up to 1350°C. Furthermore, the Fe-Cr-Al alloy tubes according to the invention have good high-temperature corrosion resistance and high resistance to oxidation, sulfidation and carburization. In addition, the tubes according to the invention have very high high-temperature creep strength and dimensional stability and high electrical resistance. The tubes according to the invention are particularly useful as electric heating elements or as components in high-temperature applications (applications operating between 400 and 1350°C). The tubes according to the invention are particularly useful as components in electric heating applications. The tubes according to the invention can also be used to protect other tubes against wear and corrosion at high temperatures (e.g. thermocouple protection tubes). Thus, the tubes according to the invention can be used both in electric heating and in high-temperature applications. Furthermore, the tubes according to the invention can be used as nuclear cladding tubes. The tubes according to the invention can also be used as gas tubes in heat exchangers or as gas lance tubes.

[0037] The invention is further illustrated by the following non-limiting examples. EXAMPLES

[0038] powder composition Four Fe-Cr-Al powders (see Table 1 for their respective compositions) were produced with different titanium and nitrogen contents. Powders 1 and 2 are comparative examples, while powders 3* and 4* are powders according to the invention. These powders were produced by induction melting followed by gas atomization. A metal melt with a specific composition is poured through a small melt nozzle into an atomization chamber filled with an inert atmosphere. A system of high-velocity gas nozzles breaks the melt stream into very fine droplets, which are cooled and then transformed into solid particles in an instant in the air. The particles are collected and cooled to ambient temperature in an inert atmosphere. The powders are sieved to .about.45 μm.

[0039] Table 1: Composition of Fe-Cr-Al powder (wt%) TIFF2024543067000001.tif157170

[0040] The grain refinement effect by the introduction of TiN inoculant is already achieved and visually perceptible in the solidified microstructure of the as-sprayed powder. Qualitatively, the degree of monocrystallinity and polycrystallinity can be visually perceived by the "grain contrast imaging technique" or "electron channeling contrast imaging technique", which is briefly described below. Fe-Cr-Al powder is mixed with conductive Bakelite powder and formed into a solid cylindrical puck. One of the flat surfaces of the puck is ground to a sufficient depth and then polished to a very high surface finish. Thus, when analyzed by a scanning electron microscope (SEM), the ground puck surface shows the polished portions of many powder particles. The depth to which the incident SEM electrons penetrate the crystalline metallic material to be investigated, and therefore the number of backscattered electrons reflected back, depends on the crystal orientation of the crystals to be investigated in the sample. Thus, different crystal orientations of the grains and the direction of the incident electrons result in different amounts of reflected backscattered electrons, which ultimately leads to different contrast between these investigated grains, and this effect is best perceived with a back-scatter electron detector.

[0041] The results of this qualitative analysis, carried out on powder particles ranging from 1 to 45 μm from these four powders, are shown in Figures 1a-d). The analysis showed that the powder with a combination of high titanium and high nitrogen content (powder 4) had the highest degree of polycrystallinity and the smallest average grain size. Powder particles of powder 4 also had the highest number of cubic TiN precipitates. The powder with medium levels of titanium and nitrogen content (powder 3) had the second highest degree of polycrystallinity. The powder with low titanium and low nitrogen content (powder 2) had the lowest degree of polycrystallinity. Powder 1 showed no or only limited grain refinement. It can therefore be concluded that for grain refinement by inoculants, both the titanium and nitrogen levels should be increased simultaneously to obtain TiN inoculants that promote the nucleation of ferrite grains.

Claims

1. 1. An iron-chromium-aluminum (Fe—Cr—Al) alloy tube, the Fe—Cr—Al alloy having the following composition (by weight): Cr 12.00~25.00; Al 3.50-6.50; Ti 0.20-1.10; N 0.06-0.20; Zr 0.05-0.20; Y 0.01-0.15; C ≤ 0.050; Si ≤ 0.50; Hf ≤ 0.30; Ta ≤ 0.30; Mn ≤ 0.40; Ni ≤ 0.60; O ≦600 ppm; The balance is Fe and unavoidable impurities wherein TiN is present as an inoculant.

2. The powder has the following composition (by weight): Cr 12.00~25.00; Al 3.50-6.50; Ti 0.20-1.10; N 0.06-0.20; Zr 0.05-0.20; Y 0.02-0.15; C ≤ 0.050; Si ≤ 0.50; Hf ≤ 0.30; Ta ≤ 0.30; Mn ≤ 0.40; Ni ≤ 0.60; O ≦600 ppm; The balance is Fe and unavoidable impurities 10. The tube of claim 1, wherein TiN is present as an inoculant.

3. 2. The Fe-Cr-Al alloy tube of claim 1, wherein the Cr content is 18.0 to 24.0 wt. %.

4. 2. The Fe-Cr-Al alloy tube of claim 1, wherein the Al content is 4.0-6.0 wt. %.

5. 2. The Fe-Cr-Al alloy tube of claim 1, wherein the Ti content is 0.30-1.00 wt.%.

6. 2. The Fe-Cr-Al alloy tube of claim 1, wherein the N content is 0.09-0.20 wt. %.

7. 2. The Fe-Cr-Al alloy tube of claim 1, wherein the Zr content is 0.07-0.10 wt. %.

8. 2. The Fe-Cr-Al alloy tube of claim 1, wherein Ti / N≧3.

3.

9. 9. Use of a powder having a composition according to any one of claims 1 to 8 for manufacturing a tube.

10. Use of an alloy having a composition according to any one of claims 1 to 8 for manufacturing a tube.