Austenitic alloy powder and uses thereof

Abstract

The present disclosure relates to an austenitic alloy powder and its use for obtaining a refractory article, the composition of the powder and its properties resulting in a material having high tensile strength and excellent creep strength at high temperatures. Furthermore, the article also has good steam oxidation resistance, good high temperature corrosion resistance and sufficient structural stability. The present disclosure also relates to a HIP process in which the powder is used.

Description

[Technical field]

[0001] The present disclosure relates to an austenitic alloy powder and its use for obtaining heat-resistant articles. The present disclosure also relates to articles manufactured from said austenitic alloy powder, more specifically to HIPed or solution annealed articles, which have high tensile strength and excellent creep strength at high temperatures due to the alloying element composition of the powder and its properties. Furthermore, the articles also have good steam oxidation resistance, good high temperature corrosion resistance, and sufficient structural stability. The present disclosure also relates to a method for manufacturing said articles. [Brief description of the drawings]

[0002] [Figure 1] FIG. 1 is an SEM image of a delivery condition article including an austenitic alloy powder. The article has been annealed, such as by solution annealing, and has an isotropic structure and a substantially uniform grain size; [Diagram 2] FIG. 2 shows the number of Z-phase precipitates per mm2 of the polished section of the delivery condition article, i.e., the article was manufactured via HIP and solution annealing; [Diagram 3] FIG. 3 shows the average cross-sectional area of ​​Z grains of abrasive specimens of delivery condition articles, i.e., articles manufactured via HIP and solution annealing; [Figure 4] FIG. 4 is an SEM image of the article having a uniform and well-dispersed distribution of Z-phase particles. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0003] [Detailed Description] The present disclosure relates to C 0.03-0.30; Si ≤ 0.80; Mn ≤ 1.0; P ≤ 0.03; S ≤ 0.03; Cr 20.0-27.0; Ni 22.0-32.0; Mo ≤ 1.0; Co 0.5-3.0; Cu 1.0-5.0; Nb 0.1-1.0; W 0.50~5.0: Ti ≤ 0.10; Al ≤ 0.05; Mg ≤ 0.05; B ≤ 0.008; N 0.10-0.50; O ≦300ppm; and the balance being iron and unavoidable impurities, and the austenitic alloy powder has a particle size distribution of more than 0 μm and not more than 600 μm.

[0004] The present disclosure also relates to an article manufactured from the powders defined above or below, which contains nano Z-phase particles (precipitates) when said article is in delivery condition, i.e. after HIP and solution annealing. These particles affect the strength, and said article has high strength at both room and high temperatures, especially when manufactured by HIP (hot isostatic pressing) method. In addition, these Z-phase particles were found to be uniformly distributed in said article. The Z-phase particles have a particle size of less than 1 μm, which means that they are nanoparticles, and are composed substantially of the elements Nb, N, Cr. All numbers and sizes of Z-phase particles are based on the apparent number and size of Z-phase particles obtained on randomly selected polished cross-sectional surfaces of the article in the final state, i.e. delivery condition, by measuring these particles on the cross-sectional surfaces with a SEM using Oxford Aztec Feature software. The article is heat resistant in the sense that it can withstand high temperatures, and also has high temperature strength and corrosion resistance.

[0005] The powders defined above and below have been consolidated into a solid article using hot isostatic pressing, referred to above and below as HIP or "HIP process". However, the powders defined above and below may be used in other techniques, such as additive manufacturing.

[0006] The elements contained in the austenitic alloy powders and austenitic alloy articles are described below. The list of properties stated for each alloying element should not be considered exhaustive, and elements may have other effects not mentioned. The ranges of alloying elements disclosed below are for both the powders and articles. Weight percentages are expressed as weight percent (wt%) or weight %.

[0007] Carbon (C) is an effective component for imparting appropriate tensile strength and creep rupture strength required for high temperature steel. However, if excessive carbon is added, toughness decreases and weldability may also deteriorate. For these reasons, the carbon content is specified to be in the range of 0.03 to 0.30 wt%. According to an embodiment, the carbon content may be 0.04 to 0.20 wt%. According to an embodiment, the carbon content may be 0.05 to 0.10 wt%.

[0008] Silicon (Si) is effective as a deoxidizer and also helps improve oxidation resistance. However, excessive silicon is detrimental to weldability, and in order to prevent deterioration of ductility and toughness due to the formation of sigma phase after long-term exposure, the silicon content should be ≦0.80 wt%. According to an embodiment, the Si content may be ≦0.40 wt%, for example ≦0.30 wt%.

[0009] Manganese (Mn) is a deoxidizing element and is also effective in improving hot workability, but in order to prevent a decrease in creep rupture strength, ductility, and toughness, the manganese content should be ≦1.0 wt%, for example ≦0.60 wt%.

[0010] Phosphorus (P) and sulfur (S) are detrimental to weldability and can promote embrittlement, so the phosphorus and sulfur content should be ≦0.03 wt.%.

[0011] Chromium (Cr) is an effective element for improving corrosion resistance and oxidation resistance. To obtain sufficient resistance, a chromium content of at least 20.0 wt% is necessary. However, if the chromium content exceeds 27.0 wt%, the nickel content must be further increased to produce a stable austenitic structure and suppress the formation of sigma phase. From these considerations, the chromium content is limited to the range of 20.0-27.0 wt%. According to an embodiment, the Cr content is 22.0-26.0 wt%, for example, 22.0-25.0 wt%.

[0012] Nickel (Ni) is an element necessary for the purpose of ensuring a stable austenitic structure. The structural stability is essentially dependent on the relative amounts of ferrite stabilizers, such as chromium, silicon, molybdenum, aluminum, tungsten, titanium and niobium, and austenitic stabilizers, such as nickel, carbon and nitrogen. To suppress the formation of sigma phase, the nickel content should be at least 22.0 wt%. Also, at a certain chromium value, an increase in nickel content suppresses the oxide growth rate and increases the tendency to form a continuous chromium oxide layer. However, to keep the manufacturing cost reasonable, the nickel content should not exceed 32.0 wt%. In view of the above circumstances, the nickel content is limited to the range of 22.0-32.0 wt%. According to an embodiment, the Ni content is 23.0-28.0 wt%, for example, 23.0-26.0 wt%.

[0013] Tungsten (W) and Molybdenum (Mo) Tungsten is added mainly to improve high temperature strength by solid solution hardening, and at least 0.50 wt% is required to obtain this effect. However, both molybdenum and tungsten can promote the formation of sigma phases and promote corrosion. Tungsten is considered to be more effective at improving strength than molybdenum. For these reasons, the molybdenum content is kept low, such as ≦1.0 wt%, for example ≦0.50 wt%, for example ≦0.30 wt%. To obtain the effect of solid solution hardening, the tungsten content should be more than 0.50 wt%. However, to avoid the formation of unwanted intermetallic phases, the content should not exceed 5.0 wt%. Therefore, the tungsten content is in the range of 1.5 to 4.0 wt%.

[0014] Cobalt (Co) is an austenite stabilizing element. The addition of cobalt can improve high temperature strength through solid solution strengthening and suppression of sigma phase formation after long exposure at high temperatures. However, to keep production costs at a reasonable level, the cobalt content should be in the range of 0.5-3.0 wt%, for example, 1.0-2.0 wt%.

[0015] Titanium (Ti) may be added to improve creep rupture strength through the precipitation of carbonitrides, carbides and nitrides. However, too much titanium reduces weldability and workability. For these reasons, the titanium content is ≦0.1 wt.%.

[0016] Copper (Cu) is added to generate a copper-rich phase, which is finely and uniformly precipitated in the matrix and can contribute to improving creep rupture strength. However, if the amount of copper is too large, the workability decreases. In view of these considerations, the copper content is specified to be in the range of 1.0 to 5.0 wt.%. According to an embodiment, the range of Cu is in the range of 1.5 to 3.5 wt.%.

[0017] Aluminum (Al) and Magnesium (Mg) Aluminum and magnesium are effective in deoxidizing during manufacturing. However, excessive aluminum promotes sigma phase precipitation, and too much magnesium can reduce weldability. For these reasons, when added, the aluminum content is ≦0.05 wt%, for example 0.003-0.05 wt%, and the magnesium content is ≦0.05 wt%, for example 0.003-0.05 wt%.

[0018] Niobium (Nb) is generally accepted to contribute to improving creep rupture strength by precipitation of carbonitrides and nitrides. However, if the amount of niobium is too large, it may deteriorate weldability and workability. In consideration of these, the content of niobium is limited to the range of 0.10 to 1.0 wt.%. According to an embodiment, the content of Nb is 0.30 to 0.70 wt.%.

[0019] Boron (B) is a finely dispersed M 23 Boron contributes to improving creep rupture strength, in part through the formation of (C,B)6 and strengthening of grain boundaries. Boron can also contribute to improving hot workability. However, too much boron can deteriorate weldability. In view of these, when added, the boron content is limited to a range of ≦0.008 wt.%, for example, 0.002-0.008 wt.%.

[0020] Nitrogen (N) is known to improve high-temperature strength and creep rupture strength, and stabilize the austenite phase. However, excessive addition of nitrogen reduces toughness and ductility. For these reasons, the nitrogen content is specified to be in the range of 0.10 to 0.50% by weight. According to an embodiment, the N content is 0.20 to 0.40% by weight.

[0021] Oxygen (O) is considered a negative element because it affects not only the welding properties but also the ductility and toughness. Therefore, the maximum content of oxygen is 300 ppm, e.g. less than 150 ppm.

[0022] When the terms "up to" or "≦" are used, it will be understood by one of ordinary skill in the art that the lower limit of the range is 0% by weight, unless another numerical value is specifically stated.

[0023] The balance is iron (Fe) and normally occurring impurities as described above. The term "impurities" refers to elements that are considered to be impurities, meaning that they are allowed to be present, but only in amounts that do not affect the properties. Impurities are therefore elements or compounds that are not intentionally added, but cannot be completely avoided, for example because they normally occur as impurities in the raw materials used in the production of steel, or in additional alloying elements. Impurities are present in the range of ≦1.0 wt.%, for example 0.50 wt.%.

[0024] According to embodiments, the austenitic alloy powders and articles of the present disclosure may include the following elements in weight percent (wt%): TIFF2024534943000002.tif109170

[0025] Moreover, the powders or articles defined above or below consist of or include all of the elements described herein, in the different ranges as described herein.

[0026] The present disclosure also relates to an austenitic alloy article that can be used in high temperature applications and that is made of a powder as defined above or below, via a HIP process as defined above or below, and thus comprises or consists of elements in the ranges disclosed above or below. According to an embodiment, said austenitic alloy article is a solution annealed article.

[0027] Furthermore, a cross-section of the article defined above or below, the cross-section being obtained via the method defined above or below, has a Z-phase precipitation density (1 / mm 2 ) at least 30,000 / mm 2This precipitation density is believed to have a positive effect on creep strength. Furthermore, optionally, the article as defined above or below may have a precipitation density of 0.15 μm instead of 0. 2 It is believed that the Z-phase particles may have an average number cross-sectional area of ​​less than 100 μm, which provides even higher creep strength. The Z-phase particles are primary particles.

[0028] Powders as defined above or below may be produced using inert gas atomization, where molten metal is poured through a nozzle and the molten metal stream is broken up by the high pressure, high velocity inert gas into a spray of rapidly solidifying metal droplets. Melting may take place in a furnace chamber of a VIM (Vacuum Induction Melter) or in an open furnace. The atomizing gas may be, for example, nitrogen or argon. Due to the rapid cooling, the powder particles are free of macrosegregation. Typical powder particle size ranges for powders suitable for HIPing may range up to 600 μm.

[0029] The raw materials may consist of virgin raw materials of elements, alloys, and / or scrap metals. The raw materials may also consist of pre-alloyed raw materials produced by AOD or VIM.

[0030] The present disclosure relates to a method in which powder is consolidated into a solid dense material using hot isostatic pressing (HIP), the method comprising: a) providing a form that defines at least a portion of the shape of the article; providing an austenitic alloy powder as defined above or below; b) filling at least a portion of said foam with said powder; c) hot isostatically pressing the foam at a predetermined temperature, at a predetermined isostatic pressure, and for a predetermined time, such that the powder particles metallurgically bond to one another and all interparticle voids are closed, thereby forming a solid; d) annealing the solid body at a predetermined temperature for a predetermined time; according to an embodiment, the annealing is solution annealing; e) The subsequent process of quenching the annealed body. Includes.

[0031] The form, which may also be called a mold or capsule, may be formed, for example, from low carbon steel, into which the powder is poured. The air between the powder particles in the powder-filled mold may be evacuated. The mold is then sealed, usually by welding, and the powder-filled, evacuated capsule is placed into a HIP vessel.

[0032] The form is subjected to hot isostatic pressing at a predetermined temperature, a predetermined isostatic pressure, and a predetermined holding time. In this process, the powder solidifies into a solid part through the combined effect of heat and external pressure acting on the capsule. The external pressure is applied by an inert gas pressure, such as argon gas pressure, introduced into the container, which is further increased by increasing the temperature in the container. The predetermined holding time can be from 10 minutes to 3 hours. The predetermined pressure can be 900-1500 bar and the predetermined temperature can be 1100-1270°C, for example 1100-1200°C. In this way, the temperature and pressure in the HIP container as well as the holding time are selected so that the gaps between the powder particles are closed and solid diffusion bonding between the particles occurs.

[0033] The solidification process may take place entirely in the solid state, ie the temperature of the process is set such that no apparent liquefaction occurs within the powder volume.

[0034] To dissolve unwanted phases that may form during cooling after HIP, the solidified solid is annealed, e.g., solution annealed, followed by quenching in a liquid such as water or oil. The annealing method may be carried out at a temperature of 1100-1250°C for 10-60 minutes.

[0035] The resulting article has homogeneous composition and isotropy throughout due to the rapid cooling of the powder particles in the atomization process and the consolidated solid obtained by the HIP process.

[0036] The present disclosure is further illustrated by the following non-limiting examples. EXAMPLES

[0037] The capsule used for HIPing the powders had approximate outer dimensions of 178×69×49 mm. The three different HIP temperatures were 1150° C., 1200° C., and 1250° C. The HIP pressure was about 100 MPa and the duration was about 2-4 hours. Table 1 shows the compositions of the powders used in the examples.

[0038] The solidified powder body was subsequently solution annealed and then quenched, having approximate dimensions of 150 x 63 x 23 mm. Solution annealing was performed at temperatures ranging from 1100 to 1250 °C for 30 to 50 minutes, followed by water quenching.

[0039] The material pieces were polished to an OPS surface finish (0.025 μm oxide polished surface finish) and microstructural documentation was performed using both optical microscopy (Leica Reichert MEF4M, 500X) and SEM (Zeiss Sigma VP).Furthermore, quantification of the number, size and morphology distribution of Z-phase precipitates was performed using an Oxford Aztec Feature.

[0040] Characterization was performed on a Zeiss VP FEG-SEM using software from Oxford Instruments. The number, size and morphology (backscatter detector mode) of bright contrast precipitates were measured over a fixed area of ​​the polished section.

[0041] The calculations were performed as described below: Number density = number of observed Z-phase particles divided by the total analyzed area where actual feature counts were performed Particle size = apparent cross-sectional area of ​​identified Z-phase particles

[0042] FIG. 1 discloses an SEM image of the material in which it can be seen to have an isotropic structure and an essentially uniform grain size.

[0043] FIG. 4 discloses an SEM image of a material with a uniform and finely dispersed distribution of Z-phase particles.

[0044] The number density of Z-phase precipitates is shown in FIG.

[0045] The average particle size of the Z-phase particles is shown in FIG.

[0046] The composition used is the balance Fe and unavoidable impurities. TIFF2024534943000003.tif28170

[0047] Tests performed on the obtained items Table 2 shows the test results of the mechanical testing in the laboratory.

[0048] Tensile test results at room temperature TIFF2024534943000004.tif17170

[0049] Creep test results TIFF2024534943000005.tif71170

[0050] Creep tests were performed on HIPed and quench annealed material at a material temperature of 700°C. Tensile stress levels were selected based on creep test results of seamless tube material of similar composition, with desired service lives of a minimum of 1000 h at 220 MPa and 3000 h at 185 MPa. Creep test results are shown in Table 3. The creep rupture lives of most samples exceeded the desired service lives.

Claims

1. C 0.03 - 0.30; Si ≤ 0.80; Mn ≤ 1.0; P ≤ 0.03; S ≤ 0.03; Cr 20 - 27; Ni 22 - 32; Mo ≤ 1.0; Co 0.5 - 3.0; Cu 1.0 - 5.0; Nb 0.1 - 1.0; W 0.50 - 5.0: Ti ≤ 0.10; Al ≤ 0.05; Mg ≤ 0.05; B ≤ 0.008; N 0.10 - 0.50; O ≤ 300 ppm; An austenitic alloy powder having the following elemental composition (in weight percent), with the balance being iron and unavoidable impurities, and having a particle size distribution greater than 0 μm and less than or equal to 600 μm.

2. The austenitic alloy powder according to Claim 1, wherein the content of C is in the range of 0.04 - 0.20 wt%, for example, 0.05 - 0.10 wt%.

3. The austenitic alloy powder according to Claim 1, wherein the content of Si is less than 0.40 wt%, for example, less than 0.30 wt%.

4. The austenitic alloy powder according to Claim 1, wherein the content of Mn is less than 1 wt%, for example, less than 0.60 wt%.

5. The austenitic alloy powder according to Claim 1, wherein the content of Cr is in the range of 22.0 - 26.0 wt%, for example, 22.0 - 25 wt%.

6. The austenitic alloy powder according to Claim 1, wherein the content of Ni is in the range of 23.0 - 28.0 wt%, for example, 23.0 - 26.0 wt%.

7. The austenitic alloy powder according to Claim 1, wherein the content of Co is in the range of 1.0 - 2.0 wt%.

8. The austenitic alloy powder according to Claim 1, wherein the content of Cu is in the range of 1.5 - 3.5 wt%.

9. The austenitic alloy powder according to Claim 1, wherein the content of Nb is in the range of 0.30 - 0.70 wt%.

10. The austenitic alloy powder according to Claim 1, wherein the content of W is in the range of 1.5 - 4.0 wt%.

11. The austenitic alloy powder according to Claim 1, wherein the content of N is in the range of 0.20 - 0.40 wt%.

12. An article of an austenitic alloy having an elemental composition according to any one of claims 1 to 10, having a Z-phase precipitation number density of at least 30,000 (1 / mm ２ ). An article of an austenitic alloy.

13. The Z-phase particles have a number average cross-sectional area of less than 0.15 μm but not zero, the article of austenitic alloy according to claim 12. ２ The article of austenitic alloy according to claim 12, wherein the Z-phase particles have a number average cross-sectional area of less than 0.15 μm but not zero.

14. The article of austenitic alloy according to Claim 12, wherein the article is a HIP article.

15. The article of austenitic alloy according to Claim 12, wherein the article is an annealed article, for example, a solution annealed article.

16. A method for manufacturing an article of austenitic alloy, comprising a) providing a form that defines at least a part of the shape of the article; providing the powder according to any one of claims 1 to 11; b) filling at least a part of the form with the powder; evacuating the form to seal the form from the outside air; c) hot isostatic pressing the form at a predetermined temperature, a predetermined isostatic pressure, and for a predetermined time such that the gaps between all particles are closed and a solid high-density body is formed by solid diffusion bonding of the powder particles; d) annealing the solid body at a predetermined temperature within a predetermined time, for example, solution annealing the solid body; e) subsequently quenching the annealed body, A method comprising the steps.

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