Technical Field

The present disclosure relates to a tube of a Fe-Cr-Al alloy. More specifically, the present disclosure relates to a tube which has been manufactured from a specific Fe-Cr-Al powder composition.

Background

Fe-Cr-Al tubes provide excellent heat resistance up to approximately 1450°C while at the same time providing an extraordinarily good form stability as well as resistance against corrosion. High temperature applications of such Fe-Cr-Al tubes range from oxidizing, sulphidation to carbonaceous environments. Fe-Cr-Al tubes offer several advantages to other tube materials, such as chromia forming tube materials in these demanding environments. This is mostly due to the capability of the Fe-Cr-Al tubes to form a dense and adherent alumina layer that protects the tube against corrosion and atmospheric attack.

CN110004367 discloses that Fe-Cr-Al tubes with the composition (in percentage by weight, wt%) of 14-22 wt% of Cr, 3-5 wt% of Al, 0.15-0.5 wt% ofY and the balance of Fe can be manufactured by preparing a powder, hot isostatic pressing the powder to a billet, forging, heat treatment, piercing and followed by reducing the diameter and/or the wall thickness of the tube by cold working steps at room temperature with sequential heat treatment steps. However, the Fe-Cr-Al tubes mentioned therein have problems with crack formation during operation.

Consequently, there still exist a need in this technical field for a tube of a Fe-Cr-Al alloy being more crack resistant.

The present disclosure aims at solving or at least reducing the above-mentioned problem.

Summary

The present disclosure therefore provides a tube of an iron-chromium-aluminum (Fe-Cr-Al) alloy which has been made from a specific a powder composition which powder composition has been optimized for providing a less brittle tube at room temperature and thus can be cold worked. The Fe-Cr-Al tube is characterized in that the tube comprises a powder with the following composition (in weight%)

Cr 12.00 to 25.00;

Al 3.50 to 6.50;

Ti 0.20 to 1.10;

N 0.06 to 0.20;

Zr 0.05 to 0.20;

Y 0.01 to 0.15;

C < 0.050;

Si < 0.50;

Hf < 0.30;

Ta < 0.30;

Mn < 0.40;

Ni < 0.60;

O < 600 ppm; balance Fe and unavoidable impurities; wherein TiN is present as inoculant.

In the present disclosure, TiN is present as an inoculant in the Fe-Cr-Al powder. It has been shown that the inoculant will provide advantages for the tube and during the manufacturing process of the tube. In particular, the TiN inoculants will introduce grain refinement. A tube made from the powder as defined hereinabove or hereinafter will not crack easily when being cold worked or when being exposed to stresses or thermal shock.

Detailed description

The present disclosure relates to a Fe-Cr-Al tube is characterized in that the tube comprises a powder with the following composition (in weight%)

Cr 12.0 to 25.0;

Al 3.50 to 6.50;

Ti 0.20 to 1.10;

N 0.06 to 0.20;

Zr 0.05 to 0.20;

Y 0.01 to 0.15; C < 0.050;

Si < 0.50;

Hf < 0.30;

Ta < 0.30;

Mn < 0.40;

Ni < 0.60;

O < 600 ppm; balance is Fe and unavoidable impurities; wherein TiN is present as inoculant.

The alloying elements of the powder 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.

Iron (Fe)

The main function for iron in the Fe-Cr-Al powder is to balance the powder composition or the composition of alloying elements of the tube.

Chromium (Cr) 12.0 to 25.0 wt%

Chromium is an important element since it will improve the corrosion resistance of the obtained tube and increase its tensile and yield strength. Further, chromium facilitates the formation of the AI2O3 layer on the final tube through the so-called third element effect, i.e. by formation of chromium oxide in the transient oxidation stage. Too low amount of chromium will result in loss of corrosion resistance. Thus, chromium shall be present an amount of at least 12.0 wt%, such as at least 15.0 wt%, such as at least 20.0 wt%. Too much chromium will enable a to a' decomposition and 475°C embrittlement and will also lead to an increased solid solutioning hardening effect on the ferritic structure. Thus, the maximum content of chromium is set to 25.0 wt%, such as maximum 24.0 wt%, such as maximum 23.50 wt%, such as maximum 23.0 wt%, such as maximum 22.50 wt%, such as maximum 22.0 wt %, such as maximum 21.50 wt%. According to embodiments, the content of chromium is from 12.0 to 25.0 wt%, such as from 18.0 0 to 24.0 wt%, such as from 20.0 to 23.50 wt%. Aluminum (Al) 3.50 to 6.50 wt%

Aluminum is an important element since aluminum, when exposed to oxygen at high temperatures, will form a dense and thin AI2O3 layer on the surface of the manufactured tube, which will protect the underlying surface from further oxidation. Further, aluminum increases the electrical resistivity. At too low amounts of aluminum, there will be a loss of the ability for the formation of the AI2O3 layer and thereby will the electrical resistivity be reduced. Thus, aluminum shall be present in an amount of at least 3.50 wt%, such as at least 4.00 wt%, such as at least 4.50 wt%, such as at least 4.80 wt%. Too high content of aluminum will cause brittleness at low temperatures and will also enhance the formation of unwanted brittle aluminides. Thus, the maximum aluminum is set to 6.50 wt%, such as maximum 6.00 wt%, such as maximum 5.50 wt%, such as maximum 5.40 wt%, such as maximum 5.30 wt%, such as maximum 5.20 wt %. According to embodiments of the present disclosure, the content of aluminum is from 3.50 to 6.50 wt%, such as from 4.00 to 5.50 wt%, such as from 4.50 to 5.50 wt%.

Titanium (Ti) 0.20 to 1.10 wt%

Titanium is an important element since titanium will together with nitrogen form TiN. According to an embodiment, due to the molar weights of Ti and N, the ratio of Ti /N in weight-% should be at least 3.3, such as at least 4.5.

Additionally, titanium may also reduce the activity of carbon by the formation of TiC and may furthermore improve high temperature creep strength. A too low amount of Ti will result in that not enough TiN inoculates is present in the present powder for nucleation of ferrite crystals during solidification in the additive manufacturing process. Further, at too low content of Ti, there will be a high risk of the formation of unwanted chromium carbides and/or brittle aluminum nitrides. Hence, titanium shall be 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, a too high content of titanium may have a negative effect on the formation of AI2O3 as TiO2 may be formed. For these reasons, the maximum content of Ti is set to 1.10 wt%, such as maximum 1.00 wt%, such as maximum 0.90 wt%, such as maximum 0.8 wt%. According to embodiments of the present disclosure, the content of Ti is from 0.20 to 0.80 wt% such as from 0.20 to 0.70 wt%, such as from 0.24 to 0.60 wt%. Nitrogen (N) 0.06 to 0.20 wt%

Nitrogen is an important element since nitrogen will together with titanium form a TiN particle. In the present disclosure, TiN will function as an inoculant and is therefore a desired particle. According to embodiments, due to the molar weights of Ti and N, the ratio of Ti /N in weight-% should be at least 3.3, such as at least 4.5.

Nitrogen is also an important element as it will enable precipitation of other metallic nitrides, such as ZrN. ZrN will improve the high temperature creep resistance. However, too low amounts of nitrides will be formed if the nitrogen content is too low. Accordingly, the nitrogen shall be 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%. Further, if the nitrogen content is too high in relation to the titanium content, there may be a risk that AIN will be formed, which will have a negative impact on the oxidation resistance. For these reasons, the maximum content of N is set to 0.20 wt%, such as maximum 0.15 wt%, such as maximum 0.10 wt%. According to embodiments of the present disclosure, the content of N is from 0.060 to 0.20 wt% such as from 0.07 to 0.15 wt% such as from 0.07 to 0.12 wt%.

TiN inoculants

The tube comprising the Fe-Cr-Al alloy made from the powder as as defined hereinabove or hereinafter will have homogenously distributed TiN inoculants. TiN is a desired inoculant which will introduce grain refinement in the tube. The resulting grain structure of the obtained tube has a significantly reduced average grain size compared to atypical conventional Fe-Cr- A1 tubes without these TiN inoculants.

Hence, the homogenous and finely dispersed TiN inoculants in the present Fe-Cr-Al powder will provide for a tube which will have a more fine-grained Fe-Cr-Al alloy. This will provide for reduced cracking behavior during and/or after cold working of present tube. This will also provide that the presentl tube will be more resistant to strain.

Zirconium (Zr) 0.05 to 0.20 wt%

Zirconium is an important element in the present powder composition as zirconium will reduce the activity of C and N by the formation of ZrC or ZrN precipitates. Zirconium can also improve the high temperature creep strength of a manufactured tube. Too low amount of Zr will increase the risk of the formation of unwanted chromium carbides and/or aluminum nitrides. Accordingly, zirconium shall be present in an amount of at least 0.05 wt%, such as at least 0.07 wt%, such as at least 0.10 wt%. On the other hand, a too high content of zirconium may have a negative impact on the formation of AI2O3. For these reasons, the maximum content of zirconium is set to 0.20 wt%, such as maximum 0. 15 wt%. According to embodiments of the present disclosure, the content of zirconium is from 0.05 to 0.20 wt%, such as from 0.07 to 0.20 wt%, such as from 0.070 to 0.10 wt%.

Yttrium (Y) 0.01 to 0.15 wt%

The addition of yttrium improves the oxidation resistance of a manufactured tube. Too little amount of added yttrium will result in reduced oxidation resistance. For this reason, yttrium must be added in the amount of at least 0.01 wt%, such as at least 0.02 wt%, such as at least 0.04 wt%, such as 0.05 wt%, such as 0.06 wt%. However, if too much yttrium is added, this will cause hot embrittlement. As a result, the maximum content of yttrium content is set to 0.15 wt%, such as 0.10 wt%, such as 0.08 wt%.

Carbon (C) < 0.050 wt%

Carbon is an element which is not added on purpose but is an unavoidable element due to powder handling. This element may cause reduction in hot ductility and formation of metallic carbides. Thus, in order to limit the presence of too many metallic carbide precipitates, the carbon content must be <0.050 wt%, such as < 0.040 wt%, such as <0.030 wt%.

Silicon (Si) < 0.50 wt%

Silicon may be present in levels of up to 0.50 wt% in order to increase electrical resistivity and to increase hot corrosion resistance. However, above this level, the hardness will increase and also there will be brittleness at low temperatures.

Tantalum (T a) < 0.30 wt%

Tantalum may optionally be added and if added, tantalum will improves the high temperature creep strength. Tantalum may also reduce the carbon activity by the formation of TaC precipitates and therefore the maximum tantalum content is set to 0.30 wt%.

Hafnium (Hf) < 0.30 wt%

Hafnium may optionally be added. The addition of hafnium will improve the high temperature creep strength. However, hafnium may reduce the carbon activity by the formation of HfC precipitates. Therefore, the maximum hafnium content is set to < 0.30 wt%. Manganese (Mn) < 0.40 wt%

Manganese may be present as an impurity. Manganese may disturb the formation of the AI2O3 layer and thus have a negative impact on the oxidation resistance. Thus, the maximum content of manganese is < 0.40 wt%, such as < 0.20 wt%.

Nickel (Ni) < 0.60 wt%

Nickel may be present as an impurity. Nickel may however increase the hardness and brittleness at low temperatures. Thus, the maximum content of nickel is therefore < 0.60 wt%, such as <0.5 wt%.

Oxygen (O) < 600 ppm

Oxygen may be present in the form of oxides. The maximum content allowed i is < 600 ppm.

According to embodiments, the powder may also include minor fractions of one or more of the following impurity elements such as but not limited to; Magnesium (Mg), Cerium (Ce), Calcium (Ca), Phosphorus (P), Tungsten (W), Cobalt (Co), Sulphur (S), Molybdenum (Mo), Niobium (Nb), Vanadium (V) and Copper (Cu) and in an amount up to 0.2 wt%.

Additionally, the Fe-Cr-Al powder as defined hereinabove or hereinafter may comprise the alloying elements mentioned herein in any of the ranges mentioned herein. According to one embodiment, the present tube of Fe-Cr-Al consists of all the alloying elements mentioned herein, in any of the ranges mentioned herein.

The Fe-Cr-Al powder as defined hereinabove or hereinafter may be manufactured through different methods. For example, but not limiting to: directly by gas atomization;

- heating a powder comprising all the alloying element in the ranges mentioned hereinabove or hereinafter but with a low nitrogen content in a nitrogen rich atmosphere, i.e. nitriding the powder;

- mixing a powder comprising all the alloying element in the ranges mentioned hereinabove or hereinafter but with a low nitrogen content with a powder containing fine particle of less stable nitride;

- mixing fine/small particles of TiN with a Fe-Cr-Al powder so that the obtained powder will have the same alloying element composition as defined hereinabove or hereinafter. According to an embodiment, the tube of an Fe-Cr-Al alloy as defined hereinabove or hereinafter may be manufactured by a method comprising the following steps: i) preparing a Fe-Cr-Al powder with a composition as defined hereinabove or hereinafter. ii) hot isostatic pressing (HIP) the Fe-Cr-Al powder to a billet. iii) machining of the HIP:ed billet into an extrusion billet. iv) extrusion of the extrusion billet to a tube blank. v) cold working the tube blank to final dimensions.

According to another embodiment, the tube of the Fe-Cr-Al alloy as defined hereinabove or hereinafter may be manufactured by a method comprising the following steps: i) preparing a Fe-Cr-Al powder with a composition as defined hereinabove or hereinafter. ii) hot isostatic pressing (HIP) the Fe-Cr-Al powder to a bloom. iii) hot working the HIP:ed bloom by hot rolling and/or hot forging to a round bar. iv) machining of the round bar into extrusion billet. v) extrusion of the extrusion billet to a tube blank. vi) cold working the tube blank to final dimensions.

According to yet another embodiment, the tube of the Fe-Cr-Al alloy as defined hereinabove or hereinafter may be manufactured by a method comprising the following steps: i) preparing a Fe-Cr-Al powder with a composition as defined hereinabove or hereinafter. ii) Filling a hollow capsule with the Fe-Cr-Al powder iii) Cold isostatic pressing (CIP) the capsule comprising the Fe-Cr-Al powder. iv) Heating the capsule comprising the Fe-Cr-Al powder. v) extrusion of the capsule comprising the Fe-Cr-Al powder to a tube blank. vi) Removing the remains of the capsule by pickling, providing a tube blank. vii) cold working the tube blank to final dimensions. The tube of the Fe-Cr-Al alloy as defined hereinabove or hereinafter may be more easily welded without addition of filler material. This is a result of the grain refining effect caused by the present TiN inoculants.

The tube of the Fe-Cr-Al alloy as defined hereinabove or hereinafter will operate well in temperatures up to 1350°C. Furthermore, the present tube of the Fe-Cr-Al alloy will have a significant high-temperature corrosion resistance and a high resistance against oxidation, sulphidation and carburization. Additionally, the present tube will have significant high- temperature creep strength, form stability and high electrical resistivity. The present tube is especially useful as an electrical heating element or as a component in high temperature applications (in applications operating between 400 to 1350 °C). The present tube is also especially useful as a component in electrical heating applications. The present tube may also be used for protecting another tube against high temperature wear and corrosion such as thermocouple protection tubes. Hence, the present tube may be used in both electrical heating and high temperature applications. Further, the present tube can be used as nuclear cladding tube. The present tube can also be used as gas tubes in heat exchangers or as gas lance tubes.

The invention is further described by the following non-limiting examples.

Examples

Powder compositions

Four Fe-Cr-Al powders (see Table 1 for their composition) were produced with varied titanium and nitrogen content. Powder 1 and 2 are comparative example and Powder 3* and 4* are inventive powders. The powders were produced by induction melting and subsequent gas atomization. A metallic melt with the specified composition is poured through a small melt nozzle into an atomizing chamber filled with inert atmosphere. With a system of high velocity gas nozzles, the melt stream was disintegrated into very fine droplets which were cooled down and then transferred to solidified particles in-flight a fraction of a second. The particles were collected and cooled to ambient temperature within the inert atmosphere. The powders were sieved to -45 pm. Table 1. Composition of the Fe-Cr-Al powders in weight%

The grain refining effect through the introduction of TiN inoculants can be obtained and visually perceived already in the solidification microstructure of the as-atomized powders. Qualitatively, the degree of mono-crystallinity vis-a-vis poly crystallinity can be visually perceived through the “grain contrast imaging technique” or “electron channeling contrast imaging technique”, briefly described as follows. The Fe-Cr-Al powder is mixed with electrically conductive Bakelite powder and molded into a solid cylindrical puck. One of the flat surfaces of the puck is ground to sufficient depth and then polished to very high surface finish. Thereby, polished sections of a number of powder particles will be visible on this polished puck surface when analyzed by scanning electron microscope (SEM). The depth to which incoming SEM electrons penetrate the studied crystalline metal material and thereby also the number of back-scattered electrons that are reflected back depend on the crystal orientation of the studied crystals in the sample. Thus, grains of different crystal orientation vis-a-vis the direction of the incoming electrons will result in different amounts of reflected back-scatter electrons and thus ultimately to a difference in contrast between these studied grains. Consequently, this effect is best perceived with the back-scatter electron detector.

The results of this qualitative analysis performed on powder particles in the particle size range 1 to 45 pm from the four powders can be found in Figure la-d). The results of this analysis are that the powder having a combination of a high titanium content and a high nitrogen content (Powder 4) had resulted in the highest degree of poly crystallinity and the smallest average grain size. The powder particles of Powder 4 also displayed the highest number of cubic TiN precipitates. The second highest degree of poly crystallinity was displayed by the powder having a medium level of titanium and nitrogen content (Powder 3). The powder with both low titanium content and low nitrogen content (Powder 2) is displaying the lowest degree of poly crystallinity. Powder 1 displayed none or only limited grain refinement. Thus, it is concluded that, in order to obtain the inoculant-imposed grain refinement, both the titanium level and the nitrogen level should be elevated simultaneously to obtain the TiN inoculants that promote the ferrite grain nucleation.