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

[0005] , , ,

[0006]

[0001] The present disclosure relates to tubes of Fe-Cr-Al alloy. More specifically, the present disclosure relates to tubes manufactured from a specific Fe-Cr-Al powder composition.

Background Art

[0002] Fe-Cr-Al tubes have excellent heat resistance up to approximately 1450 °C, and at the same time, they also have extremely good shape stability and corrosion resistance. Such high-temperature applications of Fe-Cr-Al tubes cover a wide range from oxidation environments, sulfidation environments to carburization environments. The Fe-Cr-Al tubes provide several advantages compared to other tube materials (e.g., chromia-forming tube materials in these demanding environments). This is mainly due to the ability of Fe-Cr-Al tubes to form a dense and adherent alumina layer, which protects the tubes from corrosion and atmospheric attack.

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

[0004] Therefore, in this technical field, there still exists a need to further enhance the crack resistance of tubes of Fe-Cr-Al alloy.

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

[0006] Summary of the Invention Therefore, this disclosure provides an iron-chromium-aluminum (Fe-Cr-Al) alloy tube made from a specific powder composition, the powder composition of which is optimized to produce a tube that is less brittle at room temperature, and thus cold working is possible.

[0007] This Fe-Cr-Al tube has the following composition (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 remainder is Fe and unavoidable impurities. It is characterized by containing a powder having [a certain property], and having TiN as an inoculant.

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

[0009] Detailed explanation This disclosure relates to an Fe-Cr-Al tube, the tube having the following composition (weight %): 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 ≤ 600 ppm; The remainder is Fe and unavoidable impurities. It is characterized by containing a powder having [a certain property], and having TiN as an inoculant.

[0010] The alloying elements of the powders disclosed herein are described in more detail below. The terms "weight %" and "wt%" are used interchangeably. Furthermore, the enumeration of properties or contributions mentioned for specific elements should not be considered exhaustive.

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

[0012] Chromium (Cr) 12.0-25.0% by weight Chromium is an important element because it improves the corrosion resistance of the resulting tube and increases its tensile and yield strength. Furthermore, chromium facilitates the formation of Al2O3 in the final tube through the so-called third-element effect (i.e., the formation of chromium oxide in the transition oxidation stage). If the amount of chromium is too low, the corrosion resistance will be lost. Therefore, it is desirable that chromium be present in an amount of at least 12.0 wt%, for example, at least 15.0 wt%, or for example, at least 20.0 wt%. If there is too much chromium, it leads to α~α' decomposition and 475°C embrittlement, and also leads to an increase in the solid solution hardening effect on the ferrite structure. Therefore, the maximum chromium content is set to 25.0 wt%, for example, a maximum of 24.0 wt%, for example, a maximum of 23.50 wt%, for example, a maximum of 23.0 wt%, for example, a maximum of 22.50 wt%, for example, a maximum of 22.0 wt%, or for example, a maximum of 21.50 wt%. According to several embodiments, the chromium content is 12.0 to 25.0% by weight, for example 18.0 to 24.0% by weight, for example 20.0 to 23.50% by weight.

[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, thin Al2O3 layer on the surface of the manufactured tube (which protects the underlying surface from further oxidation). Aluminum also increases electrical resistance. Too little aluminum results in a loss of the ability to form the Al2O3 layer, thus reducing electrical resistance. Therefore, it is desirable for aluminum to be present in an amount of at least 3.50 wt%, for example, at least 4.00 wt%, for example, at least 4.50 wt%, for example, at least 4.80 wt%. Too much aluminum leads to brittleness at low temperatures and promotes the formation of undesirable brittle aluminides. Therefore, the maximum amount of aluminum is set at 6.50 wt%, for example, a maximum of 6.00 wt%, for example, a maximum of 5.50 wt%, for example, a maximum of 5.40 wt%, for example, a maximum of 5.30 wt%, for example, a maximum of 5.20 wt%. According to several embodiments of this disclosure, the aluminum content is 3.50 to 6.50% by weight, for example 4.00 to 5.50% by weight, for example 4.50 to 5.50% by weight.

[0014] Titanium (Ti) 0.20~1.10% by weight Titanium is an important element because it forms TiN with nitrogen. According to one embodiment, the Ti / N ratio (by weight %) is preferably at least 3.3, for example, at least 4.5, based on the molar weights of Ti and N.

[0015] In addition, titanium can reduce the activity of carbon by forming TiC and further improve the high-temperature creep strength. If the amount of Ti is too small, 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 Ti content is too low, the risk of forming undesired chromium carbide and / or brittle aluminum nitride increases. Therefore, it is desirable for titanium to be present in an amount of at least 0.20 wt%, for example at least 0.25 wt%, for example at least 0.30 wt%. On the other hand, if the titanium content is too high, TiO2 can be formed, which may have a negative impact on the formation of Al2O3. For these reasons, the maximum Ti content is set to 1.10 wt%, for example a maximum of 1.00 wt%, for example a maximum of 0.90 wt%, for example a maximum of 0.8 wt%. According to multiple embodiments of the present disclosure, the Ti content is 0.20 - 0.80 wt%, for example 0.20 - 0.70 wt%, for example 0.24 - 0.60 wt%.

[0016] Nitrogen (N) 0.06 - 0.20 wt% Nitrogen is an important element because it forms TiN particles together with titanium. In the present disclosure, since TiN functions as an inoculant, this is the desired particle. According to multiple embodiments, it is desirable for the Ti / N ratio (wt%) to be at least 3.3, for example at least 4.5, based on the molar weights of Ti and N.

[0017] Nitrogen is also an important element to enable the precipitation of other metal nitrides (e.g., ZrN). ZrN improves high-temperature creep resistance. However, if the nitrogen content is too low, the amount of nitrides formed will be too small. Therefore, it is desirable for nitrogen to be present in an amount of at least 0.06 wt%, for example at least 0.07 wt%, for example at least 0.08 wt%, for example at least 0.09 wt%. Furthermore, if the nitrogen content is too high relative to the titanium content, there may be a risk of forming AlN (which has an adverse effect on oxidation resistance). For these reasons, the maximum content of N is set to 0.20 wt%, for example a maximum of 0.15 wt%, for example a maximum of 0.10 wt%. According to multiple embodiments of the present disclosure, the content of N is 0.06 to 0.20 wt%, for example 0.07 to 0.15 wt%, for example 0.07 to 0.12 wt%.

[0018] TiN inoculant The tube containing the powder-made Fe-Cr-Al alloy defined above or below has a uniformly distributed TiN inoculant. TiN is the desired inoculant that brings about grain refinement in the tube. The grain structure produced in the resulting tube has a significantly smaller average grain size compared to a conventional typical Fe-Cr-Al tube without these TiN inoculants.

[0019] Therefore, the homogeneously and finely distributed TiN inoculant in the Fe-Cr-Al powder according to the present invention results in a tube having a Fe-Cr-Al alloy with finer grains. Thereby, during the cold working of the tube according to the present invention and after such cold working, the crack behavior is reduced. Thereby also, the tube according to the present invention has higher strain resistance.

[0020] Zirconium (Zr) 0.05 to 0.20 wt% Zirconium is an important element in this powder composition because it reduces the activity of C and N by forming ZrC or ZrN precipitates. Zirconium can also improve the high-temperature creep strength of the manufactured pipes. If the amount of Zr is too low, the risk of unwanted chromium carbide and / or aluminum nitride formation increases. Therefore, it is desirable that zirconium be present in an amount of at least 0.05% by weight, for example, at least 0.07% by weight, for example, at least 0.10% by weight. On the other hand, if the zirconium content is too high, it may adversely affect the formation of Al2O3. For these reasons, the maximum zirconium content is set to 0.20% by weight, for example, a maximum of 0.15% by weight. According to several embodiments of this disclosure, the zirconium content is 0.05 to 0.20% by weight, for example, 0.07 to 0.20% by weight, for example, 0.070 to 0.10% by weight.

[0021] Yttrium (Y) 0.01-0.15% by weight The addition of yttrium improves the oxidation resistance of the manufactured pipe. Too little yttrium will reduce oxidation resistance. Therefore, yttrium must be added in an amount of at least 0.01% by weight, for example, at least 0.02% by weight, for example, at least 0.04% by weight, for example, 0.05% by weight, or for example, 0.06% by weight. However, too much yttrium can lead to hot embrittlement. Consequently, the maximum yttrium content is set at 0.15% by weight, for example, 0.10% by weight, or for example, 0.08% by weight.

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

[0023] Silicon (Si) ≤ 0.50 wt% Silicon can be present at levels up to 0.50% by weight to increase electrical resistance and improve high-temperature corrosion resistance. However, beyond this level, hardness increases and the material becomes brittle at low temperatures.

[0024] Tantalum (Ta) ≤ 0.30% by weight Tantalum can be added selectively, and when added, it improves the high-temperature creep strength. Tantalum can also reduce the activity of carbon by forming TaC precipitates, and for this reason, the maximum tantalum content is set at 0.30% by weight.

[0025] Hafnium ≤ 0.30% by weight Hafnium can be added selectively. Adding hafnium improves the high-temperature creep strength. However, hafnium can reduce carbon activity due to the formation of 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. Since manganese can hinder the formation of the Al2O3 layer, it can have an unfavorable effect on oxidation resistance. Therefore, the maximum manganese content is ≤0.40% by weight, for example, ≤0.20% by weight.

[0027] Nickel (Ni) ≤ 0.6% by weight Nickel may be present as an impurity. However, nickel can improve hardness and brittleness at low temperatures. Therefore, the maximum nickel content is ≤0.60 wt%, for example, ≤0.5 wt%.

[0028] Oxygen (O) ≤ 600 ppm Oxygen can exist in the form of an oxide. The maximum permissible content is ≤600 ppm.

[0029] According to several embodiments, the powder may also contain trace fractions of one or more of the following impurity elements (this is merely an example) in amounts up to 0.2% by weight: magnesium (Mg), cerium (Ce), calcium (Ca), phosphorus (P), tungsten (W), cobalt (Co), sulfur (S), molybdenum (Mo), niobium (Nb), vanadium (V), and copper (Cu).

[0030] Furthermore, the Fe-Cr-Al powders specified above or below may contain the alloying elements mentioned herein within the range of those mentioned herein. According to one embodiment, the Fe-Cr-Al tube according to the present invention consists of all the alloying elements mentioned herein within the range of those mentioned herein.

[0031] The Fe-Cr-Al powder specified above or below can be manufactured by various methods, for example, • Directly by gas atomization, • A powder containing any alloying element (but with a low nitrogen content) within the range mentioned above or below is heated in a nitrogen-rich atmosphere, i.e., the powder is nitrided. • A powder containing any alloying element (but with a low nitrogen content) within the range mentioned above or below is mixed with a powder containing fine particles of a less stable nitride. • Fine / small particles of TiN are mixed with Fe-Cr-Al powder so that the resulting powder has the same alloying elemental composition as previously defined or below. It can be manufactured by, but is not limited to, these methods.

[0032] According to one embodiment, the Fe-Cr-Al alloy tubes specified above or below can be manufactured by a method comprising the following steps: i) Prepare Fe-Cr-Al powder having the composition specified above or below. ii) The Fe-Cr-Al powder is subjected to hot isotropic pressure (HIP treatment) to form a billet. iii) The HIP-treated billet is machined to produce an extruded billet. iv) Extrude the billet to form a blank tube. v) The blank tubes are cold-worked to achieve the final dimensions.

[0033] According to another embodiment, the Fe-Cr-Al alloy tubes specified above or below can be manufactured by a method comprising the following steps: i) Prepare Fe-Cr-Al powder having the composition specified above or below. ii) Apply hot isotropic pressure (HIP treatment) to the Fe-Cr-Al powder to create a bloom. iii) The HIP-treated bloom is hot-worked by hot rolling and / or hot forging to form a round bar. iv) Machine a round bar to create an extruded billet. v) Extrude the billet to form a blank tube. vi) Cold work the blank tubes to achieve the final dimensions.

[0034] According to yet another embodiment, the Fe-Cr-Al alloy tubes specified above or below can be manufactured by a method comprising the following steps: i) Prepare Fe-Cr-Al powder having the composition specified above or below. ii) Fill the hollow capsule with Fe-Cr-Al powder. iii) Capsules containing Fe-Cr-Al powder are subjected to cold isotropic pressurization (CIP treatment). iv) Heat the capsule containing Fe-Cr-Al powder. v) Extrude capsules containing Fe-Cr-Al powder to form blank tubes. vi) Remove capsule residue by acid pickling to obtain a blank tube. vii) Cold work the blank tubes to achieve the final dimensions.

[0035] The Fe-Cr-Al alloy tubes specified above or below can be welded more easily without the addition of filler material. This is a result of the grain refinement effect brought about by the TiN inoculant according to the present invention.

[0036] The Fe-Cr-Al alloy tubes specified above or below operate well at temperatures up to 1350°C. Furthermore, the Fe-Cr-Al alloy tubes according to the present invention have excellent high-temperature corrosion resistance and high resistance to oxidation, sulfidation, and carburizing. In addition, the tubes according to the present invention have extremely high high-temperature creep strength and dimensional stability, and high electrical resistance. The tubes according to the present 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 present invention are particularly useful as components in electric heating applications. The tubes according to the present invention can also be used to protect other tubes from wear and corrosion at high temperatures (e.g., as protective tubes for thermocouples). Therefore, the tubes according to the present invention can be used in both electric heating and high-temperature applications. Furthermore, the tubes according to the present invention can be used as nuclear cladding tubes. The tubes according to the present invention can also be used as gas tubes in heat exchangers or as gas lance tubes.

[0037] The present invention will be further described by the following non-limiting embodiments. [Examples]

[0038] powder composition Four Fe-Cr-Al powders (see Table 1 for their respective compositions) were prepared with different titanium and nitrogen content. Powders 1 and 2 are comparative examples, while powders 3* and 4* are the powders according to the present invention. These powders were prepared by induction melting followed by gas atomization. A metal molten material having a specific composition was poured through a small melting nozzle into a spray chamber filled with an inert atmosphere. A high-speed gas nozzle system broke the molten material into very fine droplets, which were cooled and then instantaneously converted into solid particles in the air. The particles were collected and cooled to ambient temperature in an inert atmosphere. These powders were sieved to ~45 μm.

[0039] Table 1: Composition of Fe-Cr-Al powder (weight %) TIFF0007912065000001.tif157170

[0040] The grain refinement effect of introducing a TiN inoculant is already visible in the solidified microstructure of the sprayed powder and can be visually perceived. Qualitatively, the degree of single crystallinity and polycrystallineity can be visually perceived using "grain contrast imaging technique" or "electron channeling contrast imaging technique," which are briefly explained below. Fe-Cr-Al powder is mixed with conductive bakelite powder and formed into a solid cylindrical pack. One of the flat surfaces of the pack is polished to a sufficient depth, and then polished to a very high level of surface finish. As a result, when analyzed with a scanning electron microscope (SEM), numerous polished areas of powder particles become visible on the polished pack surface. The depth to which incident SEM electrons penetrate the crystalline metallic material to be investigated, and the number of backscattered electrons reflected back, depend on the crystal orientation of the crystal to be investigated in the sample. Therefore, the amount of reflected backscattered electrons differs depending on the crystal orientation of the crystal grains and the direction of the incident electrons, ultimately resulting in differences in the contrast between these investigated crystal grains. This effect is therefore best perceived by a backscatter electron detector.

[0041] The results of this qualitative analysis, performed on powder particles in the particle size range of 1 to 45 μm from these four powders, are shown in Figures 1a to d). The results of this analysis showed that powder 4 (high titanium content and high nitrogen content) exhibited the highest degree of polycrystallineity and the smallest average grain size. Powder 4 also had the highest number of cubic TiN precipitates. The second highest degree of polycrystallineity was observed in powder 3 (medium levels of titanium and nitrogen content). Powder 2 (low titanium and nitrogen content) exhibited the lowest degree of polycrystallineity. Powder 1 showed little to no grain refinement. Therefore, it can be concluded that, in order to refine the grains using an inoculant, both the titanium and nitrogen levels should be increased simultaneously to obtain a TiN inoculant that promotes ferrite grain nucleation.

Claims

1. A tube made of iron-chromium-aluminum (Fe-Cr-Al) alloy, wherein the Fe-Cr-Al alloy has the following composition (by weight %): Cr 12.00~25.00; Al 3.50-6.50; Ti 0.30-1.10; 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 remainder is Fe and unavoidable impurities A tube manufactured from a powder containing [a certain substance], with TiN present as an inoculant and Ti / N ≥ 3.

3.

2. A tube made of Fe-Cr-Al alloy according to claim 1, wherein the Cr content is 18.0 to 24.0% by weight.

3. A tube made of Fe-Cr-Al alloy according to claim 1 or 2, wherein the Al content is 4.0 to 6.0% by weight.

4. A tube made of Fe-Cr-Al alloy according to claim 1 or 2, wherein the Ti content is 0.30 to 1.00% by weight.

5. A tube made of Fe-Cr-Al alloy according to claim 1 or 2, wherein the N content is 0.09 to 0.20% by weight.

6. A tube made of Fe-Cr-Al alloy according to claim 1 or 2, wherein the Zr content is 0.07 to 0.10% by weight.

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