Alloy and alloy pipe

Abstract

Provided are: an alloy having excellent nitriding resistance in a high-temperature ammonia environment; and an alloy pipe using such an alloy. The present invention provides an alloy containing 0.01-0.10 mass% of C, 0.30-5.00 mass% of Si, 0.01-1.00 mass% of Mn, at most 0.03 mass% of P, at most 0.03 mass% of S, 40.00-55.00 mass% of Ni, 20.00-27.00 mass% of Cr, 0.50-5.00 mass% of Al, and 2.50-3.50 mass% of Cu, with the balance consisting of Fe and unavoidable impurities, wherein the total Cr and Al content is at least 24.00 mass%.

Description

Alloys and alloy pipes

[0001] The present invention relates to alloys and alloy tubing.

[0002] Toward achieving carbon neutrality, ammonia, which is easier to transport and store than hydrogen, is attracting attention as a fuel. Furthermore, because ammonia can be decomposed to extract hydrogen, it is expected to be used as a hydrogen carrier. In facilities that burn ammonia or produce hydrogen from ammonia, the metal materials that make up the facilities are exposed to high-temperature ammonia environments for long periods of time. In such metal materials, a nitride layer is formed on the surface due to nitriding by ammonia. As the thickness of this nitride layer increases, brittle fracture and peeling of the nitride layer occur, leading to a decrease in the thickness of the metal material and resulting in deterioration. Therefore, improved nitriding resistance is required for metal materials used in high-temperature ammonia environments.

[0003] Known metal materials with high nitriding resistance include metal materials that form a chromium passive film, such as nickel-based alloys. Patent Document 1 discloses a nickel-based alloy containing 2 to 4% Al, in which a surface of the alloy is coated with an Al film having a thickness of 0.5 μm or more. ２ O ３ Patent Document 1 describes a reactor tube for ethylene production, characterized in that it is coated with an alumina barrier layer containing . In the reactor tube for ethylene production of Patent Document 1, the presence of the alumina barrier layer is said to suppress the penetration of nitrogen. Furthermore, Patent Document 2 describes a Ni-based alloy to which Si and Y are added. Patent Document 2 presumes that the mechanism by which nitriding resistance is improved is that a protective layer that prevents nitrogen diffusion is formed on the surface by Si.

[0004] JP 2016-223017 A JP 2011-252199 A

[0005] Patent Document 1 does not evaluate nitriding resistance, and Patent Document 2 only examines nitriding resistance in high-temperature air. Therefore, it is unclear whether the alloys described in Patent Documents 1 and 2 have nitriding resistance in an atmosphere in which nitriding is extremely likely, such as a high-temperature ammonia environment. As such, metal materials with excellent nitriding resistance in a high-temperature ammonia environment have not been sufficiently examined up to now.

[0006] The present invention has been made in view of the above circumstances, and aims to provide an alloy that has excellent nitriding resistance in a high-temperature ammonia environment, and an alloy pipe made of such an alloy.

[0007] The invention made to solve the above problem is an alloy containing C: 0.01% by mass to 0.10% by mass, Si: 0.30% by mass to 5.00% by mass, Mn: 0.01% by mass to 1.00% by mass, P: 0.03% by mass or less, S: 0.03% by mass or less, Ni: 40.00% by mass to 55.00% by mass, Cr: 20.00% by mass to 27.00% by mass, Al: 0.50% by mass to 5.00% by mass, and Cu: 2.50% by mass to 3.50% by mass, with the balance being Fe and unavoidable impurities, and the total content of Cr and Al being 24.00% by mass or more.

[0008] Another invention made to solve the above problem is an alloy pipe made from the above alloy.

[0009] According to the present invention, it is possible to provide an alloy that has excellent resistance to nitriding in a high-temperature ammonia environment, and an alloy pipe using such an alloy.

[0010] An alloy and an alloy pipe according to one embodiment of the present invention will be described in detail below.

[0011] [Alloy] An alloy according to one embodiment of the present invention contains C: 0.01% by mass to 0.10% by mass, Si: 0.30% by mass to 5.00% by mass, Mn: 0.01% by mass to 1.00% by mass, P: 0.03% by mass or less, S: 0.03% by mass or less, Ni: 40.00% by mass to 55.00% by mass, Cr: 20.00% by mass to 27.00% by mass, Al: 0.50% by mass to 5.00% by mass, and Cu: 2.50% by mass to 3.50% by mass, with the balance being Fe and unavoidable impurities, and the total content of Cr and Al being 24.00% by mass or more.

[0012] The alloy exhibits excellent nitriding resistance in a high-temperature ammonia environment. While the reason for this is unclear, the following is presumed. As described above, the thicker the nitride layer formed in a high-temperature ammonia environment, the more likely it is to suffer brittle fracture or peeling of the nitride layer. Furthermore, even if the nitride layer is thin, voids are generated along the grain boundaries within the nitride layer, which can initiate intergranular cracking and peeling. Therefore, in order to improve nitriding resistance in a high-temperature ammonia environment, it is necessary to suppress the growth of the nitride layer and the generation of voids at the grain boundaries within the nitride layer. As confirmed in the examples described below, Ni is thought to be an element that suppresses the growth of the nitride layer while promoting the formation of voids. Cu is also thought to have a similar effect to Ni. This is presumably because Ni and Cu lower the solid solubility limit of nitrogen, and therefore, if the Ni and Cu contents are high, nitrogen that has penetrated into the alloy will gasify. Therefore, it is necessary to have a component composition that can suppress both the growth of the nitride layer and the generation of voids while maintaining appropriate Ni and Cu contents. As a result of their investigations, the inventors found that a component composition that causes discontinuous precipitation of nitrides can suppress both the growth of the nitride layer and the generation of voids. Specifically, they found that by setting the Ni content to 40.00 mass% or more and 55.00 mass% or less, the Cu content to 2.50 mass% or more and 3.50 mass% or less, and setting the total content of Cr and Al to 24.00 mass% or more, discontinuous precipitation of nitrides occurs, thereby suppressing both the growth of the nitride layer and the generation of voids. While the reason why discontinuous precipitation of nitrides can suppress both the growth of the nitride layer and the generation of voids is unclear, the following is presumed. The discontinuous precipitation of nitrides is thought to involve the diffusion of substitutional solid solution elements that do not form nitrides, which slows the growth of the nitride layer. Furthermore, the discontinuous precipitation of nitrides is thought to suppress the generation of voids because grain boundaries, which are the generation sites of voids, move due to the discontinuous precipitation of nitrides. For these reasons, it is presumed that the alloy has excellent nitriding resistance in a high-temperature ammonia environment.

[0013] The range of the content of each element in the alloy and the reasons for limiting it will be explained below.

[0014] (C: 0.01% by mass or more and 0.10% by mass or less) C (carbon) is an element that acts to increase high-temperature creep rupture strength and imparts heat resistance to the alloy. Therefore, the lower limit of the C content is 0.01% by mass, preferably 0.02% by mass, and more preferably 0.03% by mass. However, if C is contained in excess, the amount of chromium carbide increases, which makes the retention of the chromium passive film unstable and affects nitriding resistance. Therefore, the upper limit of the C content is 0.10% by mass, preferably 0.08% by mass, more preferably 0.07% by mass, and even more preferably 0.06% by mass.

[0015] (Si: 0.30 mass% or more and 5.00 mass% or less) Si (silicon) is an element necessary for improving oxidation resistance and nitridation resistance. Therefore, the lower limit of the Si content is 0.30 mass%, preferably 0.50 mass%, more preferably 0.70 mass%, and even more preferably 0.80 mass%. However, if Si is contained in excess, an intermetallic compound is formed, which leads to a decrease in workability such as ductility and toughness. Therefore, the upper limit of the Si content is 5.00 mass%, preferably 3.00 mass%, more preferably 2.00 mass%, even more preferably 1.50 mass%, and even more preferably 1.00 mass%.

[0016] (Mn: 0.01 mass% or more and 1.00 mass% or less) Mn (manganese) is a deoxidizing element. Therefore, the lower limit of the Mn content is 0.01 mass%, preferably 0.10 mass%, more preferably 0.15 mass%, and even more preferably 0.30 mass%. However, if an excessive amount of Mn is contained, it bonds with S (sulfur) to form MnS, which reduces local corrosion resistance. Therefore, the upper limit of the Mn content is 1.00 mass%, preferably 0.70 mass%, more preferably 0.60 mass%, and even more preferably 0.50 mass%.

[0017] (P: 0.03 mass% or less) P (phosphorus) is an element inevitably contained in the alloy, and if contained in excess, hot workability is reduced. Therefore, the upper limit of the P content is 0.03 mass%, preferably 0.02 mass%, and more preferably 0.01 mass%. On the other hand, the lower limit of the P content may be 0 mass% or 0.0001 mass%.

[0018] (S: 0.03 mass% or less) S (sulfur) is an element inevitably contained in the alloy, and if contained in excess, hot workability, corrosion resistance, pitting corrosion resistance, etc. are reduced. Therefore, the upper limit of the S content is 0.03 mass%, preferably 0.01 mass%, and more preferably 0.005 mass%. On the other hand, the lower limit of the S content may be 0 mass% or 0.0001 mass%.

[0019] (Ni: 40.00 mass% or more and 55.00 mass% or less) Ni (nickel) is an element effective in ensuring nitriding resistance and can suppress the thickness of the nitrided layer by suppressing the penetration of nitrogen. Therefore, the lower limit of the Ni content is 40.00 mass%, preferably 43.00 mass%, more preferably 45.00 mass%, and even more preferably 47.00 mass%. This alloy may be referred to as a nickel-based alloy. On the other hand, if Ni is contained in excess, it may lead to the generation of voids in the nitrided layer, an increase in material costs, etc. Therefore, the upper limit of the Ni content is 55.00 mass%, preferably 52.00 mass%, and more preferably 50.00 mass%.

[0020] (Cr: 20.00 mass% or more and 27.00 mass% or less) Cr (chromium) is an element that not only forms a passive film and improves corrosion resistance, but also delays the onset of nitriding and suppresses the thickness of the nitrided layer. Therefore, the lower limit of the Cr content is 20.00 mass%, preferably 22.00 mass%, and more preferably 23.00 mass%. However, if Cr is contained in excess, phase stability at high temperatures decreases. Therefore, the upper limit of the Cr content is 27.00 mass%, preferably 26.00 mass%, more preferably 25.00 mass%, and even more preferably 24.80 mass%.

[0021] (Al: 0.50 mass% or more and 5.00 mass% or less) Al (aluminum) is an element necessary for improving oxidation resistance and nitridation resistance. Therefore, the lower limit of the Al content is 0.50 mass%, preferably 1.00 mass%, more preferably 2.00 mass%, and even more preferably 3.00 mass%. However, if Al is contained in excess, an intermetallic compound is formed, which leads to a decrease in workability such as ductility and toughness. Therefore, the upper limit of the Al content is 5.00 mass%, preferably 4.50 mass%, and more preferably 4.00 mass%.

[0022] (Cu: 2.50 mass% or more and 3.50 mass% or less) Cu (copper) is an element effective in ensuring nitriding resistance and is an element that can suppress the penetration of nitrogen and thereby reduce the thickness of the nitrided layer. Therefore, the lower limit of the Cu content is 2.50 mass%, and preferably 2.70 mass%. However, if Cu is contained in excess, it may cause voids in the nitrided layer, a decrease in hot workability, etc. Therefore, the upper limit of the Cu content is 3.50 mass%, and preferably 3.30 mass%.

[0023] (Fe and Inevitable Impurities) The basic components of the composition of the alloy are as described above, with the remaining components being Fe (iron) and inevitable impurities. Inevitable impurities include elements contained in scrap and ore, which are raw materials for the alloy, elements mixed in from the manufacturing process and manufacturing environment, and other elements added as needed, and are allowed within a range that does not adversely affect the alloy of this embodiment. Examples of inevitable impurities include Mg added for deoxidation purposes, Ca added for desulfurization purposes, O, N, etc. mixed in from raw materials, and Mo, W, Sn, etc. added as needed. Furthermore, the alloy may further contain Nb, Ti, etc., as described below.

[0024] (Total content of Cr and Al: 24.00 mass% or more) Cr and Al are elements that form nitrides, and sufficient addition of Cr and Al causes discontinuous precipitation of nitrides in the nitride layer, suppressing the growth of the nitride layer and the occurrence of voids. Therefore, the lower limit of the total content of Cr and Al is 24.00 mass%, preferably 25.00 mass%, more preferably 26.00 mass%, even more preferably 27.00 mass%, and even more preferably 28.00 mass%. The upper limit of the total content of Cr and Al is preferably 30.00 mass%, and may be 29.50 mass% or 29.00 mass%.

[0025] In the alloy according to one embodiment of the present invention, it is preferable to contain predetermined amounts of Nb and Ti as optional elements.

[0026] (Nb) Nb (niobium) is an element that combines with C to form niobium carbide, thereby increasing strength and suppressing Cr deficiency due to chromium carbide precipitation. However, excessive Nb content can lead to the formation of a low-melting-point phase in the solidification segregation region and reduced workability due to the aggregation of niobium carbide. Therefore, the upper limit of the Nb content is preferably 1.00 mass%, more preferably 0.70 mass%, and even more preferably 0.50 mass%. Furthermore, by setting the content of Nb, an element with high nitride formation ability, to the above upper limit or less, nitriding resistance can be further improved. The lower limit of the Nb content may be 0.001 mass%, 0.010 mass%, 0.100 mass%, or 0.200 mass%.

[0027] (Ti) Like Nb, Ti (titanium) is an element that combines with C to form titanium carbide, thereby enhancing high-temperature creep rupture strength and suppressing Cr deficiency due to chromium carbide precipitation. However, excessive Ti content can lead to a decrease in workability, such as ductility. Therefore, the upper limit of the Ti content is preferably 1.00 mass%, more preferably 0.50 mass%, even more preferably 0.30 mass%, and even more preferably 0.10 mass%. Furthermore, by setting the content of Ti, an element with high nitride formation ability, to the above upper limit or less, nitriding resistance can be further improved. The lower limit of the Ti content may be 0.001 mass% or 0.002 mass%.

[0028] (Total Content of Nb and Ti) As described above, Nb and Ti are elements that bond with C to form carbides, and have the effect of increasing strength and suppressing Cr deficiency due to precipitation of chromium carbides. Therefore, the lower limit of the total content of Nb and Ti is preferably 0.200 mass%, more preferably 0.250 mass%. On the other hand, from the viewpoint of further improving nitriding resistance, the upper limit of the total content of Nb and Ti is preferably 1.00 mass%, more preferably 0.700 mass%, and even more preferably 0.500 mass%.

[0029] (Grain size number, etc.) The grain size number of the alloy may be, for example, 1.0 or more, preferably 3.0 or more, more preferably 4.0 or more, even more preferably 5.0 or more, and even more preferably 6.0 or more. When the grain size number of the alloy is equal to or greater than the above lower limit, the nitrogen resistance in a high-temperature ammonia environment is further improved, and cracking of the nitrided layer becomes even less likely to occur. The grain size number of the alloy may be 10.0 or less, 9.0 or less, 8.0 or less, or 7.0 or less. The grain size number of the alloy is measured according to the comparative method of JIS G 0551 (2020), and more specifically, is a value measured under the conditions described in the examples.

[0030] In this alloy, it is preferable that D, represented by the following formula (1), is 1.00 or less: D = 100 / ([Cr] + [Al]) × G) (1) (In formula (1), [Cr] is the Cr content (mass%), [Al] is the Al content (mass%), and G is the grain size number measured according to the comparison method of JIS G 0551 (2020).) When D is 1.00 or less, discontinuous precipitation of nitrides is promoted, and the growth of the nitride layer and the generation of voids are further suppressed. In other words, when D is 1.00 or less, nitrogen resistance in a high-temperature ammonia environment is further improved, and cracking of the nitride layer is further reduced. Although the reason for this is unclear, it is thought that discontinuous precipitation begins at grain boundaries. Therefore, it is presumed that by refining the crystals after adding sufficient Cr and Al, the number of precipitation sites can be increased, thereby promoting discontinuous precipitation. The upper limit of D is more preferably 0.80, further preferably 0.70, and even more preferably 0.60. The lower limit of D may be 0.30, 0.40, or 0.50.

[0031] (Production Method) The method for producing the alloy is not particularly limited. The alloy can usually be obtained by melting to have the above-mentioned component composition. Melting can be carried out using an electric furnace, a vacuum induction melting furnace, or the like. In this production method, various conventionally known processes such as hot forging and heat treatment may be performed. The melted alloy may be produced into an ingot by, for example, an ingot casting method, or into a cast piece (slab, bloom, or billet) by a continuous casting method.

[0032] (Nitride Layer Formed) The alloy exhibits excellent nitriding resistance in a high-temperature ammonia environment, because discontinuous precipitation of nitrides occurs in the nitride layer formed in the high-temperature ammonia environment, suppressing both the growth of the nitride layer and the generation of voids in the nitride layer. From the viewpoint of exhibiting such excellent nitriding resistance, the alloy preferably exhibits discontinuous precipitation in the nitride layer formed on the surface in a nitriding test in which the alloy is left in an ammonia gas atmosphere at 600°C for 50 hours. Furthermore, the alloy preferably exhibits an average thickness of the nitride layer formed on the surface of 30 μm or less, more preferably 20 μm or less, in a nitriding test in which the alloy is left in an ammonia gas atmosphere at 600°C for 50 hours. The alloy also has the advantage that the thin nitride layer formed in a high-temperature ammonia environment suppresses a decrease in thermal conductivity due to the formation of the nitride layer. The lower limit of the average thickness of the nitride layer may be 1 μm, 5 μm, or 10 μm. The average thickness of the nitride layer is defined as the average value of the thicknesses of the nitride layer measured at any five points. The nitride layer is formed during a predetermined nitriding test, and the alloy of the present invention does not necessarily have the nitride layer. The specific conditions for the nitriding test are as described in the Examples.

[0033] (Shape, etc.) The shape of the alloy is not particularly limited and may be a plate, a rod, a tube, etc., but a tube is preferable. That is, the alloy is suitably used as a tubular material.

[0034] [Alloy Pipe] An alloy pipe according to one embodiment of the present invention is a pipe formed from an alloy according to one embodiment of the present invention. The alloy pipe may have a portion formed from a member other than the alloy according to one embodiment of the present invention.

[0035] The alloy pipe can be obtained using the alloy according to one embodiment of the present invention by conventionally known manufacturing equipment and manufacturing methods. For example, it can be manufactured by extrusion or Mannesmann pipe manufacturing using a round bar as a raw material, or by welded pipe manufacturing using a plate material in which the seam is welded after shaping. The dimensions of the alloy pipe are not particularly limited and can be set appropriately depending on the purpose, equipment, etc. of use.

[0036] (Applications of Alloy and Alloy Pipe) The alloy and alloy pipe according to one embodiment of the present invention have excellent nitriding resistance in a high-temperature ammonia environment. Therefore, the alloy and alloy pipe are preferably used in a state where at least a portion of the surface is in contact with ammonia gas, and more preferably in a state where at least a portion of the surface is in contact with ammonia gas at 200°C or higher (even 300°C or higher and 1,000°C or lower). Examples of such applications include piping and other equipment installed in ammonia combustion equipment, ammonia decomposition equipment for obtaining hydrogen gas (hydrogen production equipment using ammonia as a raw material), etc. For example, in the case of an alloy pipe used as a piping for high-temperature ammonia gas, the inner surface of the alloy pipe will be in contact with high-temperature ammonia gas. The alloy and alloy pipe may also be used in environments other than a high-temperature ammonia environment. In other words, the alloy and alloy pipe can be used in various applications similar to those of conventionally known alloys and alloy pipes.

[0037] [Other Embodiments] The present invention is not limited to the above-described embodiment, and can be implemented in various other forms, including modifications and improvements, in addition to the above-described forms.

[0038] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0039] [Examples and Comparative Examples] Nos. 1, 11-16, 22-27: Using a vacuum induction melting furnace (VIF), 20 kg cylindrical ingots were prepared with the chemical compositions listed in Tables 1 and 5 (the remainder consisting of Fe and unavoidable impurities). The ingots were heat-treated at 1150°C for 24 hours, hot-forged in the temperature range of 1150°C to 950°C, and then cut into sizes of 80 mm wide, 20 mm thick, and 250 mm long. Plate-shaped materials were then obtained by heat-treating at 1150°C for 5 minutes and water-quenching. For Nos. 23, 25, and 27, plate-shaped materials were obtained by further cold-rolling Nos. 22, 24, and 26, respectively, by 50% reduction, heat-treating at 1060°C for 5 minutes, and water-quenching, respectively, to adjust the grain size. Test pieces measuring 10 mm wide, 2 mm thick, and 10 mm long were prepared from the obtained plate-shaped materials for nitriding tests. In Tables 1 and 5, "-" indicates that the element was not intentionally added. Nos. 2-3, 7-10, and 17-21: Button ingots were prepared using a button melting furnace, each having the composition shown in Tables 1 and 3 (the balance being Fe and unavoidable impurities). The ingots were subjected to homogeneous heat treatment at 1200°C for 24 hours and then water-quenched. Test pieces measuring 10 mm wide, 2 mm thick, and 10 mm long were prepared from the ingots for nitriding tests. In Tables 1 and 3, "-" indicates that the element was not intentionally added. Nos. 4-6: Test pieces measuring 10 mm wide, 2 mm thick, and 10 mm long were prepared from commercially available ingots having the composition shown in Table 1 (the balance being Fe and unavoidable impurities).

[0040] [Nitriding Test] The entire surface of the prepared test specimen was wet-polished to a grit size of 600 and ultrasonically cleaned in alcohol for degreasing. The test specimen was then inserted into a quartz tubular furnace (inner diameter 86 mm, outer diameter 92 mm, length 800 mm). After purging with nitrogen, ammonia was passed through the quartz tubular furnace at a flow rate of 1 L / min. The temperature was raised to 600°C and maintained for 50 hours to conduct an ammonia nitriding test. After the nitriding test, the test specimen was embedded in resin for cross-sectional observation, and a 10 mm x 2 mm cross section was mirror-polished. (Presence or Absence of Discontinuous Precipitation in the Nitrided Layer) The 10 mm x 2 mm cross section was observed using a scanning electron microscope (SEM) to confirm the presence or absence of discontinuous precipitation in the nitrided layer. (Average Thickness of the Nitrided Layer) The 10 mm x 2 mm cross section was observed using an optical microscope, and the distance from the surface of the test specimen to the boundary of the matrix (thickness of the nitrided layer) was measured at five points. The average value was calculated as the average thickness of the nitrided layer. (Crack Number Density in Nitrided Layer) A 10 mm x 2 mm cross section was observed using an optical microscope, and the number of cracks that occurred in the nitrided layer was divided by the observed length of the test piece to determine the crack number density. (Grain Size Number) The test piece for the nitriding test was embedded in resin, and a 10 mm x 2 mm cross section was mirror-polished. The 10 mm x 2 mm cross section was subjected to electrolytic etching using 65% nitric acid, and the exposed grain boundaries were observed, and the grain size number was measured using the comparative method described in JIS G 0551 (2020). The results are shown in Tables 2, 4, and 6. Note that the average thickness and crack number density of the nitrided layer were not determined for test pieces No. 17 to 21.

[0041]

[0042]

[0043]

[0044]

[0045]

[0046]

[0047] In the test specimens of Examples Nos. 1 to 3 and 22 to 27, the average thickness of the nitrided layer was small and the crack density was low, resulting in excellent nitriding resistance in a high-temperature ammonia environment. On the other hand, in the test specimens of Comparative Examples Nos. 4 to 16, the crack density of the nitrided layer was high, resulting in insufficient nitriding resistance in a high-temperature ammonia environment. Specifically, for example, test specimen No. 6 had a high Ni content and a thin average thickness of the nitrided layer, but voids occurred along the grain boundaries within the nitrided layer, and cracks originated from the voids, resulting in an insufficiently low crack density. Furthermore, the results of the test specimens of the other Comparative Examples confirmed that a low Ni content tends to increase the average thickness of the nitrided layer, and a low total Cr and Al content does not result in a low crack density. In contrast, it was confirmed that by setting the Ni and other contents within an appropriate range and setting the total Cr and Al content to 24.00 mass% or more, discontinuous precipitation occurs in the nitrided layer, suppressing the generation of voids and resulting in a thin nitrided layer with a low crack density. It should be noted that discontinuous precipitation occurred in each of the test specimens of Examples Nos. 1 to 3 and 22 to 27, which had a small average thickness of the nitrided layer, a low crack number density, and excellent nitriding resistance in a high-temperature ammonia environment, whereas discontinuous precipitation did not occur in each of the test specimens of Comparative Examples Nos. 4 to 16, which had insufficient nitriding resistance in a high-temperature ammonia environment. From this tendency, it is presumed that each of the test specimens of Examples Nos. 17 to 21, in which discontinuous precipitation occurred, also had excellent nitriding resistance in a high-temperature ammonia environment. Furthermore, among the test specimens of the Examples, each of Test Specimens Nos. 1 to 3, 23, 25, and 27, in which D, expressed by the above formula (1), was 1.00 or less, had a crack number density of 4 cm －１ or less, and the nitriding resistance in a high-temperature ammonia environment was particularly excellent.

[0048] The alloy of the present invention can be suitably used as a material for piping and the like in an environment where it comes into contact with high-temperature ammonia gas.

Claims

1. An alloy containing C: 0.01% or more by mass and 0.10% or less by mass, Si: 0.30% or more by mass and 5.00% or less by mass, Mn: 0.01% or more by mass and 1.00% or less by mass, P: 0.03% or less by mass, S: 0.03% or less by mass, Ni: 40.00% or more by mass and 55.00% or less by mass, Cr: 20.00% or more by mass and 27.00% or less by mass, Al: 0.50% or more by mass and 5.00% or less by mass, and Cu: 2.50% or more by mass and 3.50% or less by mass, the remainder being Fe and unavoidable impurities, and the total content of Cr and Al being 24.00% or more by mass.

2. The alloy according to claim 1, further comprising: Nb: more than 0.000% by mass and not more than 1.00% by mass; and Ti: more than 0.000% by mass and not more than 1.00% by mass; wherein the total content of Nb and Ti is 0.200% by mass or more and 1.00% by mass or less.

3. An alloy as claimed in claim 1 or 2, which, in a nitriding test in which it is left in an ammonia gas atmosphere at 600°C for 50 hours, exhibits discontinuous precipitation in the nitrided layer formed on the surface.

4. The alloy according to claim 1 or 2, which is used in a state where at least a part of its surface is in contact with ammonia gas at 200°C or higher.

5. The alloy according to claim 1 or 2, wherein D, as represented by the following formula (1), is 1.00 or less: D = 100 / ([Cr] + [Al]) x G) ... (1) (In formula (1), [Cr] is the Cr content (mass%), [Al] is the Al content (mass%), and G is the grain size number measured according to the comparison method of JIS G 0551 (2020).) 6. An alloy tube formed from the alloy according to claim 1 or 2.