Fe-based alloy, and corrosive wear resistant and wear resistant fe-based alloy film

Abstract

Provided are: an alloy film having excellent environmental resistance even in an environment where corrosion and wear simultaneously occur at high temperatures; and an alloy from which the alloy film can be formed.　The Fe-based alloy is characterized by comprising, in terms of mass%, 10 ≤ Cr ≤ 40, 15 ≤ Ni ≤ 50, 0.03 ≤ C ≤ 0.9, 0.3 ≤ Si ≤ 2, and 1.5 ≤ B ≤ 3.5, with the remainder consisting of Fe and unavoidable impurities.

Description

Fe-based alloys and corrosion- and wear-resistant Fe-based alloy coatings

[0001] The present invention relates to an Fe-based alloy and an Fe-based alloy coating, and more particularly to an Fe-based alloy and an Fe-based alloy coating that can form an alloy coating having excellent environmental resistance in high-temperature environments where corrosion and corrosive wear are problematic.

[0002] Inside incinerators that burn waste, biomass, etc., the chlorine contained in the fuel creates a severe high-temperature, corrosive environment. In particular, chlorides contained in the atmosphere concentrate and accumulate on the surfaces of heat exchangers, which are colder than the ambient temperature, causing severe corrosion. Furthermore, in the case of fluidized-bed boilers, in addition to corrosion, wear caused by the fluidized medium can cause severe thinning. Protectors are installed to prevent this thinning. While installing protectors is effective, it also reduces the heat transfer efficiency of heat exchangers. Therefore, surface treatments such as thermal spraying and overlay welding are often used to prevent thinning.

[0003] Common problems with thermal spray coatings include the formation of pores in the coating and poor adhesion to the substrate. HVOF (High Velocity Oxygen Fuel) thermal spraying, which increases the particle velocity during spraying, can reduce the porosity of the coating compared to plasma spraying. However, it is not possible to completely eliminate pores, and the coating is only physically bonded to the substrate, resulting in poor adhesion. Therefore, self-fluxing alloy spraying, which remelts the coating after spraying to form a metallurgical reaction layer with the substrate and eliminates pores in the spray coating, significantly improving the properties of the spray coating, is used. Self-fluxing alloy spraying is known to impart excellent corrosion resistance because the remelting process reduces pores in the coating and inhibits the penetration of corrosive substances. However, the composition of the self-fluxing alloy powder that can be used for self-fluxing alloy spraying is limited. Self-fluxing alloys are required to have a melting start temperature of 1,000°C or less and a wide temperature range between the liquidus and solidus. If the melting point is too high, not only will it be difficult to melt, but there is also the concern that raising the temperature to the melting point may have a thermal effect on the base material. On the other hand, if the temperature range is narrow, it becomes difficult to control the temperature during the remelting process, making it difficult to produce a high-quality coating.

[0004] The most commonly used self-fluxing alloy powder is SFNi4 (2.14A NiCrCuMoBSi 69 15 3 3A), as specified in JIS H8303:2010. SFNi4 is a Ni-Cr alloy consisting of 12 wt%-17 wt% Cr, 4 wt% or less Mo, 3.5 wt%-5.0 wt% Si, 5 wt% or less Fe, 0.4 wt%-0.9 wt% C, 2.5 wt%-4.0 wt% B, 1 wt% or less Co, 4 wt% or less Cu, and the remainder Ni. It offers corrosion resistance in a wide range of environments and a high hardness of 50-60 HRC, making it an alloy with excellent corrosion and wear resistance. SFNi4 is also easy to process (remelting process), making it suitable for use in a wide range of applications. For specific applications, alloys such as improved SFNi4 have also been proposed.

[0005] For example, a Ni-based self-fluxing alloy powder that suppresses fluidity during remelting treatment and contains 10 wt% to 16.5 wt% Cr, 4.0 wt% or less Mo, 3.0 wt% to 5.0 wt% Si, 15.0 wt% or less Fe, 0.01 wt% to 0.9 wt% C, 2.0 wt% to 4.0 wt% B, 3.0 wt% or less Cu, 50 ppm to 500 ppm O, with the remainder being Ni and unavoidable impurities, and satisfies a Si / B ratio of 1.2 to 1.7, has been proposed, as well as a part having excellent corrosion resistance and / or wear resistance and having a coating formed by thermal spraying this Ni-based self-fluxing alloy powder (Patent Document 1: JP 2015-143372 A).

[0006] Also proposed is a Ni-based self-fluxing alloy powder that contains 12 wt% to 17 wt% Cr, 3 wt% to 8 wt% Mo, 3.5 wt% to 5.0 wt% Si, 5.0 wt% or less Fe, 0.4 wt% to 0.9 wt% C, 2.5 wt% to 4.0 wt% B, 4.0 wt% or less Cu, 200 ppm or less O, and the remainder being Ni and unavoidable impurities, and that satisfies 0 ppm≧−20Mo%+100 (Patent Document 2: JP 2006-265591 A).

[0007] Furthermore, a Ni-based self-fluxing alloy powder for thermal spraying has been proposed (Patent Document 3: JP 2006-161132 A) that contains 30.0 wt% to 42.0 wt% Cr, 0.5 wt% to 2.0 wt% Mo, 2.0 wt% to 4.0 wt% Si, 5.0 wt% or less Fe, 2.5 wt% to 4.5 wt% C, 1.5 wt% to 4.0 wt% B, and the remainder being Ni and unavoidable impurities. This Ni-based self-fluxing alloy powder for thermal spraying is produced by atomization, and it is disclosed that chromium carbide with a particle size of 5 μm or less is uniformly precipitated inside the particles, resulting in improved high-temperature erosion resistance.

[0008] Furthermore, a corrosion-resistant and abrasion-resistant heat transfer tube for heat exchange has been proposed, in which a protective coating made of a Ni-based self-fluxing alloy containing 12 wt% to 17 wt% Cr, 4 wt% or less Mo, 3.5 wt% to 5.0 wt% Si, 5.0 wt% or less Fe, 0.4 wt% to 0.9 wt% C, 2.5 wt% to 4.5 wt% B, and 4.0 wt% or less Cu is formed on the outer surface of an iron-based metal tube (Patent Document 4: JP 2000-119781 A).

[0009] However, conventional Ni-based self-fluxing alloys are not said to have sufficient environmental resistance to erosion-corrosion (hereinafter sometimes abbreviated as "EC"), which is the simultaneous occurrence of corrosion and wear, and also have the drawback of being expensive because they contain large amounts of expensive Ni, making the materials expensive.

[0010] On the other hand, when inexpensive Fe is used as the main component, it is known that the melting point of the alloy rises, making remelting difficult, and there are no Fe-based self-fluxing alloys in the JIS standard, so Fe-based alloys are generally used for overlay welding. Overlay welding has a large heat input during construction, which has a large thermal impact on the base material and can cause deformation.

[0011] As an Fe-based cladding alloy, a low-carbon, high-silicon, high-chromium, boron-niobium, iron-based corrosion-resistant, wear-resistant alloy has been proposed, which contains 15 to 31 wt% Cr, 10 wt% or less Mo, 2.5 to 4.5 wt% Si, 0.5 to 2.0 wt% C, 0.5 to 3.5 wt% B, 10 wt% or less Mn, 7 wt% or less Cu, 16 wt% or less Ni, Nb + V: 8 wt% or less, the remainder being iron and inevitable impurities, and in which the compounding ratio of Cr to (Si × B) satisfies a specific relationship formula (Patent Document 5: Japanese Patent No. 4310368). This alloy is disclosed as having improved wear resistance due to increased hardness through the precipitation of carbides, while exhibiting corrosion resistance due to the Cr in the base material, resulting in excellent wear resistance and corrosion resistance, and as the Ni content is small, the material cost is lower than that of Ni-based alloys. However, Patent Document 5 evaluates the corrosion resistance in an aqueous solution, and does not provide data on corrosion resistance in a high-temperature environment containing Cl, so the corrosion resistance and wear resistance in the high-temperature, highly corrosive, and highly abrasive environment to which immersed heat transfer tubes in fluidized bed boilers are exposed are unknown.

[0012] In order to provide a metal component having excellent corrosion resistance and wear resistance, a method has been proposed in which a high-carbon iron-based alloy powder consisting of 3.0-6.0 wt% C, 5.0-15.0 wt% Cr, 5.0-8.0 wt% V, 2.0-6.0 wt% Ni, 0.5-3.0 wt% Mo, 1.0-4.0 wt% Si, and the balance Fe is sprayed by high-velocity flame spraying or plasma spraying to form a thermal spray coating having excellent corrosion resistance and wear resistance (Patent Document 6: JP 10-121221 A). However, the examples in this publication show that either wear resistance or corrosion resistance is improved compared to cast material pieces, but no alloy coating having both improved properties is disclosed.

[0013] Also, a high Cr alloy has been proposed that has excellent resistance to high-temperature erosion and corrosion, containing 0.5-1.5% C, 1.0-4.0% Si, 0.5-2.0% Mn, 35-60% Cr, the balance being Co and / or Fe and unavoidable impurities, and having a ratio of Cr to C of 35≦Cr / C≦90 (Patent Document 7: JP-A-11-80902).

[0014] A continuous casting roll to be installed in a continuous casting facility has been proposed, which contains, by mass%, up to 0.07% C, 0.2 to 1.5% Si, up to 3% Mn, 13 to 20% Cr, 0.5 to 4% Ti, and 0.1 to 0.5% N, with the balance being Fe and inevitable impurities, and has an overlay weld layer on the roll surface that satisfies 1≦Ti / N≦20, and which has excellent corrosion resistance and wear resistance (Patent Document 8: WO2010 / 047137).Patent Document 8 discloses that the uniform distribution of chromium in the metal matrix and the inclusion of titanium nitride result in excellent corrosion resistance, wear resistance, and thermal crack resistance.

[0015] The present applicants have also developed a Ni-based thermal spray alloy powder containing 15 wt% to 25 wt% of Cr, 0 wt% to 5 wt% of Mo, 0.5 wt% to less than 2.0 wt% of Si, 5 wt% or less of Fe, 0.3 wt% to 0.7 wt% of C, and 4 wt% to 7 wt% of B, with the balance being Ni and unavoidable impurities (Patent Document 9: JP 2018-131645 A), a Ni-Fe-based alloy powder containing 15 mass% to 35 mass% of Cr, 10 mass% to 50 mass% of Fe, 0 mass% to 5 mass% of Mo, 0.3 mass% to 2 mass% of Si, 0.3 mass% to 0.9 mass% of C, and 4 mass% to 7 mass% of B, with the balance being Ni and unavoidable impurities, and a Ni-Fe-based alloy powder containing the same. and a corrosion-wear-resistant and wear-resistant alloy coating (Patent Document 10: JP 2019-210499 A) that produces nodular precipitates with a coverage of 30% or more using γNi and Ni. Also proposed is a corrosion-wear-resistant and wear-resistant alloy coating (Patent Document 11: JP 2021-80524 A) that contains 10% by mass or more and 50% by mass or less of Cr, 0% by mass or more and 70% by mass or less of Ni, 0% by mass or more and 10% by mass or less of Mo, 0% by mass or more and less than 5% by mass of Si, 0.05% by mass or more and 1% by mass or less of C, 0% by mass or more and 1% by mass or less of B, with the balance being Fe and inevitable impurities, and has a surface on which a plurality of protrusions with a maximum diameter of 0.1 mm or more and 3 mm or less and a maximum height of 0.1 mm or more and 2 mm or less are present at a maximum separation distance of 0.1 mm or more and 5 mm or less. These Ni-Fe-based alloy coatings are made of γNi and Ni. ３ The alloy coating contains a matrix phase containing B eutectic and 30 vol% precipitates containing borides and carbides, and by forming unique irregularities on the surface of the alloy coating, it has improved corrosion resistance and wear resistance at high temperatures.

[0016] JP 2015-143372 A JP 2006-265591 A JP 2006-161132 A JP 2000-119781 A Japanese Patent No. 4310368 A JP 10-121221 A JP 11-80902 A WO2010 / 047137 JP 2018-131645 A JP 2019-210499 A JP 2021-80524 A

[0017] An object of the present invention is to provide an alloy coating having excellent environmental resistance even in high-temperature environments where corrosion and abrasion simultaneously occur, and an alloy on which such an alloy coating can be formed. In particular, an object of the present invention is to provide an alloy coating having excellent corrosion and abrasion resistance at high temperatures for equipment used in high-temperature, highly abrasive, and highly corrosive environments, such as an immersed heat transfer tube in an internal circulating fluidized bed boiler, and an alloy, preferably a thermal spray alloy, on which such an alloy coating can be formed.

[0018] The present invention provides an alloy coating having excellent environmental resistance even in a high-temperature environment where corrosion and wear occur simultaneously, an alloy capable of forming the alloy coating, a method for producing the alloy coating, a heat transfer tube having the alloy coating, and an incinerator or boiler equipped with the heat transfer tube.

[0019] Conventional SFNi4 has a relatively low melting point and excellent workability. ３ The corrosion resistance and wear resistance are improved by forming an alloy coating containing a matrix containing a B eutectic and 20 vol% of boride and carbide precipitates. The Ni-Fe-based alloys disclosed in the applicant's prior applications (Patent Documents 10 and 11) contain γNi and Ni. ３ The present inventors have conducted extensive research and have found that the alloy coating has improved corrosion resistance and wear resistance at high temperatures by forming a matrix containing a B eutectic and an alloy coating containing 30 vol% of boride and carbide precipitates, and forming specific irregularities on the surface of the alloy coating. ３ The present inventors have discovered that the B eutectic is inferior in corrosion resistance and wear resistance at high temperatures, that boride precipitates are superior to carbide precipitates in corrosion resistance and wear resistance at high temperatures, that inhibiting the oxidation of easily oxidized Cr prevents the penetration of Cl into the matrix of the alloy coating, thereby improving corrosion resistance, that forming an Fe-based oxide film near the surface of the alloy coating improves the wear resistance and corrosion resistance at high temperatures (hereinafter also referred to as "high-temperature wear resistance and corrosion resistance"), that controlling the Ni content in the matrix of the alloy coating can exert a spalling-inhibiting effect, and that controlling the Cr concentration in the precipitates can improve the hardness of the alloy coating, and have thus completed the present invention. Specific embodiments of the present invention are described below.

[0020] [1] An Fe-based alloy characterized by, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and inevitable impurities. [2] An Fe-based alloy characterized by, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and inevitable impurities. [3] An Fe-based alloy according to [1] or [2], containing 10% or more by mass of Fe. [4] An Fe-based alloy according to [1] or [2], containing 30% or more by mass of Fe. [5] An Fe-based alloy according to [1] or [2], characterized in that it comprises a matrix consisting of, by mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities, and boride precipitates consisting, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy. [6] In mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, the balance being unavoidable impurities, and Ni ３1. An Fe-based alloy according to claim 1, further comprising: a matrix containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, and the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy. [7] An alloy coating comprising, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, and the balance being Fe and unavoidable impurities. [8] An alloy coating characterized by comprising, in mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, and the balance being Fe and inevitable impurities. [9] The alloy coating according to [7] or [8], characterized in that it comprises: a matrix consisting of, by mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the alloy coating.

[10] In mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, the balance being unavoidable impurities, and Ni ３The alloy coating according to [7] or [8], characterized in that it comprises: a matrix phase containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, and the balance being unavoidable impurities, wherein the boride precipitates are present in the alloy coating at a ratio of 20 vol% to 40 vol% of the entire alloy coating.

[11] A heat transfer tube having an alloy coating according to any one of the following (1) to (6).(1) An alloy coating characterized by, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and inevitable impurities; (2) An alloy coating characterized by, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and inevitable impurities; (3) An alloy coating characterized by, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and inevitable impurities, (4) An alloy coating comprising, by mass%, a matrix consisting of 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, of 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy coating; (5) An alloy coating comprising, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, (5) An alloy coating comprising, by mass%, a matrix consisting of 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, of 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy coating; (6) An alloy coating comprising, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, In mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, and the remainder being unavoidable impurities, Ni. ３(6) An alloy coating comprising: a matrix containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the alloy coating at a ratio of 20 vol% to 40 vol% of the entire alloy coating; (7) An alloy coating comprising, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, In mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, and the balance being inevitable impurities, ３ An alloy coating comprising: a matrix containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, and the balance being unavoidable impurities, wherein the boride precipitates are present in the alloy coating at a ratio of 20 vol% to 40 vol% of the entire alloy coating.

[12] An incinerator having the heat transfer tube according to

[11] .

[13] A boiler having the heat transfer tube according to

[11] .

[14] An Fe-based alloy consisting of, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, comprising: a matrix consisting, by mass%, of 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, of 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, a step of forming a thermal sprayed coating by thermal spraying powder of the Fe-based alloy, the boride precipitates being present in the matrix at a ratio of 20 vol % to 40 vol % of the entire alloy; and a step of performing a remelting treatment on the thermal sprayed coating by high-frequency induction heating at a temperature between the liquidus temperature and solidus temperature of the matrix of the Fe-based alloy, thereby forming an alloy coating.

[0021] The Fe-based alloy of the present invention can form an alloy coating that enables the extension of the life of heat transfer tubes, etc., without significantly impairing the heat transfer efficiency of the heat exchanger, as in protectors, even in severely corrosive and corrosive-wear environments involving chlorides at high temperatures, such as in waste and biomass incinerators and boilers. As a result, it is possible to provide incinerators and boilers that have improved equipment operating rates by extending the life of components without reducing the heat exchange efficiency of the heat transfer tubes.

[0022] Schematic diagram of the equipment used in the corrosion resistance and wear resistance test. Photographs of cross-sectional structure observations of six types of alloys. EPMA map analysis photograph of Ni. EPMA map analysis photograph of Fe-35Ni alloy. EPMA map analysis photograph of Fe-65Ni alloy. EPMA map analysis photograph of Ni-25Cr alloy. EPMA map analysis photograph of Fe-25Cr alloy. Graph showing E-C test results for Ni-xFe-5Cr alloy. Pure Ni and Ni-92Ni ３ Graph showing the E-C test results for alloy B. (a) Pure Ni and (b) Ni-92Ni after E-C test ３ 1. SEM photograph of the surface of Alloy B. Graph showing E-C test results for boride-based precipitates and carbide-based precipitates. Graph showing high-temperature E-C test results for alloys with a high Cr concentration in the parent phase. Graph showing high-temperature E-C test results for alloys with a low Cr concentration in the parent phase. Graph showing the volume ratio of precipitates in an alloy coating and E-C test results. Graph showing E-C test results for various alloys according to the examples. Photograph of the structure of Fe-20Ni-20Cr-2.6B-1Si-0.5C (wt.%). Photograph of the structure of Fe-20Ni-15Cr-2.6B-1Si-0.5C (wt.%). Photograph of the structure of Fe-20Ni-25Cr-2.6B-1Si-0.5C (wt.%). Photographs showing the structure of Fe-25Ni-20Cr-2.6B-1Si-0.5C (wt.%). Photographs showing the state of thermal spraying and remelting treatment performed using alloy powder of Fe-15Cr-20Ni-2.6B-1Si-0.5C (wt.%). Microscope photographs of alloy coatings at each remelting treatment temperature.

[0023] First, various test conditions described in this specification will be explained.

[0024] [Corrosion Resistance and Wear Resistance Test (hereinafter sometimes abbreviated as "EC Test")] 1 mm thick test specimens were cut from the alloys prepared by arc melting. The front and back surfaces were polished with SiC abrasive paper, and finally polished with #1200 abrasive paper before being used for the experiment. As shown in Figure 1, the test specimen (sample) was fixed to the tip of a sample holder and inserted into a quartz tube containing sand containing 0.5 wt% KCl-50 mol% NaCl salt. The sand heated to 700 °C was flowed by blowing 400 °C compressed air into the bottom of the device at 25 L / min, and collided at an impact angle of 45°. To create an atmosphere similar to the actual environment and a temperature gradient between the samples, cooling water was circulated inside the sample holder, and the sample surface temperature was maintained at 330-350 °C. During the test, the sand was heated to 700 °C by blowing 400 °C compressed air into the bottom of the device at 25 L / min. The sand was then blown into the tube at 400 °C by blowing 400 °C compressed air into the tube at 25 L / min. The sand was then blown into the tube at an impact angle of 45°. To create an atmosphere similar to the actual environment and a temperature gradient between the samples, cooling water was circulated inside the sample holder, and the sample surface temperature was maintained at 330-350 °C. During the test, the sand was heated to 700 °C by blowing 400 °C compressed air into the tube at 25 L / min. The sand was then blown into the tube at an impact angle of 45°C by blowing 400 °C compressed air into the tube at an impact angle of 45°C. ２ Salt was continuously supplied into the sand using steam from the mixed salt. The sand was replenished and the salt was replaced every 50 hours, and the test was continued for up to 250 hours. The change in mass before and after the test was measured, and the corrosion wear amount (mg / cm ２ ) was calculated.

[0025] [Corrosion Test] The test specimen was polished with abrasive paper and finally polished with a diamond abrasive having an abrasive grain size of 3 μm. ２ An immersion test was conducted in sand containing ammonium nitrate. The immersion test involves placing a salt mixture in a crucible, burying the test piece at a certain depth, and then corroding the salt mixture at high temperatures without allowing it to flow (JIS Z2293 2004). The test was conducted at 480°C for up to 49 hours, and cross-sectional structure observation was performed using a SEM, and map analysis was performed using an EPMA.

[0026] Next, the Fe-based alloy of the present invention will be described. (1) Fe-based alloy The Fe-based alloy of the present invention is suitable as a thermal spray alloy for forming an alloy coating with improved corrosion resistance and wear resistance at high temperatures, and is characterized by the following mass %: 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and inevitable impurities. The content of each element will be described below.

[0027] [Fe] The Fe-based alloy of the present invention is primarily composed of Fe. It is generally known that Fe has inferior corrosion resistance compared to Ni, and that its high-temperature chloride corrosion resistance is particularly inferior to Ni. Fe alone significantly reduces corrosion resistance in high chlorine partial pressure environments, and its melting point rises, reducing workability. However, the inventors have discovered that an oxide film primarily composed of Fe has excellent corrosion and wear resistance at high temperatures, and that when the chlorine partial pressure is low, the inclusion of Fe improves corrosion resistance and significantly increases corrosion and wear resistance.

[0028] Corrosion tests were carried out on Fe only, Ni only, Fe-35Ni, Fe-65Ni, Fe-25Cr, and Ni-25Cr (all alloys are expressed in wt %), and cross-sectional structure observation and EPMA map analysis were carried out.

[0029] The cross-sectional structure observation results of the six alloys are shown in Figure 2 (2000x magnification). It can be seen that corrosion progressed throughout the Fe-only alloy (Fe x 2000), corrosion progressed only on the surface of the Ni-only alloy (Ni x 2000), corrosion was less severe in the Fe-35Ni alloy (Fe-35Ni x 2000) than in the Fe-only or Ni alloys, with slight corrosion progressing in the surface layer, and corrosion progression was suppressed more in the Fe-65Ni alloy (Fe-65Ni x 2000) than in the Fe-35Ni alloy. In the Ni-25Cr alloy (Ni-25Cr x 2000) and the Fe-25Cr alloy (Fe-25Cr x 2000), no Cl concentration, which promotes corrosion, was observed at the alloy / coating interface, confirming that the addition of Cr suppresses Cl concentration.

[0030] An EPMA map analysis photograph of Ni is shown in Figure 3. When only Ni is used, a nickel oxide layer is formed on the surface of the nickel layer, and it is observed that Cl is concentrated between the nickel oxide layer and the nickel layer.

[0031] An EPMA map analysis photograph of the Fe-35Ni alloy is shown in Figure 4, and an EPMA map analysis photograph of the Fe-65Ni alloy is shown in Figure 5. Figures 4 and 5 show that in the Fe-Ni alloy, a nickel layer is laminated on an iron layer, and iron oxide is formed between the iron layer and the nickel layer and on the surface of the nickel layer. Comparing Figures 4 and 5, no Cl penetration is observed in the Fe-65Ni (Figure 5), while slight Cl penetration is observed in the iron oxide layer and nickel layer in the Fe-35Ni (Figure 4). However, compared with Ni (Figure 3), the degree of Cl concentration is low, and it can be said that in the Fe-Ni alloy, the iron oxide layer formed on the surface can prevent Cl penetration.

[0032] For reference, EPMA map analysis photographs of the Ni-25Cr alloy are shown in Figure 6, and EPMA map analysis photographs of the Fe-25Cr alloy are shown in Figure 7. No Cl concentration is observed in Figures 6 and 7. In the Ni-25Cr alloy (Figure 6), Ni and Cr coexist throughout, and the formation of an oxide layer of Ni and Cr is observed on the surface. In the Fe-25Cr alloy (Figure 7), Fe and Cr coexist throughout, and the formation of iron oxide and chromium oxide is observed on the surface. The surface defects in Figures 6 and 7 are due to polishing during the preparation of the observation specimen. Figures 2 to 7 indicate that the use of an Fe-based alloy results in the formation of an iron oxide layer on the surface, improving corrosion resistance and wear resistance compared to conventionally known corrosion-resistant alloys. Furthermore, Figures 2 to 7 indicate that alloys containing Fe and Ni do not exhibit defects in the surface oxide layer, indicating a spalling-inhibiting effect.

[0033] The Fe content x of Ni-xFe-5Cr (expressed in wt%) alloy was varied to 5%, 10%, 30%, 60 wt%, and 80% by mass. Sand was added and salt was replaced after 50 hours. The results of the E-C test were shown in Figure 8. Figure 8 shows that the greater the Fe content, the less metal loss there was. Ni-Fe-Cr alloys with Fe contents of 10% by mass, preferably 30% by mass or more, showed significantly reduced metal loss and were therefore superior in corrosion resistance and wear resistance. Therefore, it is desirable for the Fe content of the Ni-Fe-Cr alloy to be 10% by mass or more, preferably 30% by mass or more.

[0034] [Ni] The Fe-based alloy of the present invention contains 15% by mass or more and 50% by mass or less, preferably 40% by mass or less, and more preferably 20% by mass or more and 25% by mass or less of Ni. Ni is known to have excellent corrosion resistance, particularly excellent high-temperature chloride corrosion properties, and it is generally believed that the higher the Ni content, the better the material properties. On the other hand, since Ni is expensive, it is desirable to reduce the amount added from a cost perspective.

[0035] Pure Ni and Ni-92Ni ３ The results of the E-C test for 50 hours using the B (vol%) alloy are shown in Figure 9, and the SEM observation photograph of the alloy surface after the E-C test is shown in Figure 10. Figure 9(a) shows the comparison of the corrosion wear amount of the alloy, and Figure 9(b) shows the comparison of the metal loss amount. ３ It can be seen that the Ni alloy containing B has a larger reduction in both thickness and thickness. In FIG. 10(b), the large white bright dots are the oxidized B. ２ O ３ It is believed that this is borate glass formed by the reaction of boric acid (boric acid) with silica sand and sodium oxide. Since borate glass has a low melting point of about 450°C, it becomes semi-sintered during the E-C test, and is thought to have evaporated or mobilized, making it prone to thinning. ３ It was confirmed that B reduces corrosion resistance and wear resistance. ３ It is desirable to set the content so that the balance between Ni and B can be controlled so as to prevent the formation of B eutectic as much as possible.

[0036] [Cr] The Fe-based alloy of the present invention contains 10% by mass or more and 40% by mass or less, preferably 30% by mass or less, and more preferably 15% by mass or more and 25% by mass or less of Cr. Cr is an essential element for maintaining corrosion resistance at high temperatures, but excessive Cr content increases the melting point, leading to deterioration of coating application properties. As will be described later, it has been confirmed that Cr forms precipitates (Cr borides and Cr carbides) with B and C, increasing the hardness of the alloy and improving its wear resistance. Therefore, it is desirable to set the content so that the balance of Cr, B, and C can be regulated so that precipitates containing Cr borides and Cr carbides can be formed.

[0037] [B] The Fe-based alloy of the present invention contains B in an amount of 1.5% by mass or more and 3.5% by mass or less, preferably 3% by mass or less, and more preferably 2% by mass or more and 3% by mass or less. B is an element essential for workability (remeltability), and also precipitates as Cr borides to improve hardness. However, if there is too much B, the amount of Cr consumed as borides increases, which reduces the corrosion resistance of the matrix. Furthermore, if there is too much B, the matrix becomes too hard and brittle, so it is desirable to set the content so that it can regulate the appropriate balance of Cr and B for the matrix and precipitates. As mentioned above, the presence of Ni in the matrix ３ It has been confirmed that the presence of B eutectic leads to deterioration of corrosion resistance and wear resistance. ３ It is preferable not to form a B eutectic, and it is desirable to set the content so as to be able to regulate the balance of Ni and B in the parent phase. Furthermore, as will be described later, it is preferable to set the precipitate content to 20 vol% or more and 40 vol% or less, and in order to form such precipitates, the B content in the Fe-based alloy is preferably 1.5 mass% or more and 3.5 mass% or less.

[0038] [C] The Fe-based alloy of the present invention contains 0.03% by mass or more and 0.9% by mass or less of C, preferably 0.1% by mass or more and 0.9% by mass or less. C forms hard Cr carbides and improves hardness. However, if the C content is too high, too much Cr in the matrix is ​​consumed as carbides, resulting in a deterioration in corrosion resistance. Therefore, it is desirable to set the content so that the balance between Cr and C can be appropriately controlled in the matrix and precipitates. As will be described later, it has been confirmed that borides improve corrosion resistance and wear resistance more than carbides in precipitates. Therefore, it is desirable to set the content so that the balance between C and B can be appropriately controlled, which can promote the formation of borides and suppress the formation of carbides.

[0039] The results of an E-C test performed on alloys prepared so that the ratios of carbide and boride precipitates were as shown in Table 1 are shown in FIG.

[0040]

[0041] From FIG. 11, it can be seen that when comparing the amounts of metal loss, the carbide-based alloy (carbide-based) has slightly more loss than the boride-based alloy (boride-based), and it can be confirmed that Cr boride has better corrosion resistance and wear resistance than Cr carbide.

[0042] [Si] The Fe-based alloy of the present invention contains Si in an amount of 0.3% by mass or more and 2% by mass or less, preferably 1.5% by mass or less, more preferably 0.5% by mass or more and 1.5% by mass or less. Si is an element that improves oxidation resistance and contributes to improving corrosion resistance. However, if the amount is too large, corrosion resistance and wear resistance are reduced, and corrosion resistance is also reduced in environments containing trace amounts of chlorine. Also, if the amount is too small, workability (remelting treatment) is poor, the alloy is not sufficiently remelted, and a sufficiently dense coating cannot be formed.

[0043] The Fe-based alloy of the present invention further comprises a matrix consisting of, by mass%, 50≦Fe≦65, 0≦Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, and it is preferable that the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol%.

[0044] [Matrix] The matrix of the Fe-based alloy consists of, in mass %, 50≦Fe≦65, 0≦Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, and the balance being inevitable impurities.

[0045] [Fe] The matrix of the Fe-based alloy of the present invention contains 50% by mass to 65% by mass, preferably 55% by mass to 65% by mass, of Fe. An oxide film of Fe, which has excellent corrosion resistance and wear resistance at high temperatures, is formed in a region close to the surface of the matrix of the alloy.

[0046] [Cr] The matrix phase of the Fe-based alloy of the present invention contains 0 to 16% by mass of Cr, preferably 5 to 16% by mass of Cr. Cr exists as precipitates in the form of Cr borides and Cr carbides rather than in the matrix phase of the alloy, thereby improving corrosion resistance and wear resistance.

[0047] The E-C test was performed on two alloys with the compositions shown in Table 2, one with a high Cr concentration in the matrix and the other with a low Cr concentration, with the B content changed to 0 mass%, approximately 2 mass%, and approximately 5 mass%, respectively. The results are shown in Figures 12 and 13. In the E-C test, the amount of change in mass was measured when the sand was replaced every 50 hours. The composition of the precipitates and matrix was analyzed by observing multiple locations with an EPMA and calculating the average value.

[0048]

[0049] The amount of metal loss after 150 hours of E-C testing is also shown in Table 2. Table 2 and Figures 12 and 13 show that when the matrix does not contain B, the alloy with a higher Cr concentration in the matrix has less metal loss, but when the matrix contains approximately the same amount of B, the alloy with a lower Cr concentration in the matrix has less metal loss.

[0050] [B] The matrix phase of the Fe-based alloy of the present invention contains B in an amount of 0 mass % or more and 0.5 mass % or less, preferably 0.1 mass % or more and 0.4 mass % or less. ３ The presence of B eutectic deteriorates corrosion resistance and wear resistance, but the presence of B in the precipitates as Cr borides can improve corrosion resistance and wear resistance. Therefore, it is preferable that the amount of B present in the matrix is ​​small.

[0051] [Ni] The matrix of the Fe-based alloy of the present invention contains Ni in an amount of 20% by mass to 35% by mass, preferably 25% by mass to 35% by mass. As described above, Ni has an effect of suppressing spalling. ３ It has been confirmed that the presence of B eutectic leads to deterioration of corrosion resistance and wear resistance. ３ It is preferred that it be present as Ni rather than as a B eutectic.

[0052] [C] As mentioned above, since the corrosion resistance and wear resistance are improved when the amount of Cr carbide in the precipitates is small, it is preferable that C is present in the matrix rather than precipitated. The matrix of the Fe-based alloy of the present invention contains more than 0 mass % and not more than 0.5 mass %, preferably 0.05 mass % or more and not more than 0.5 mass % of C.

[0053] [Si] Si is present almost entirely in the matrix. The matrix of the Fe-based alloy of the present invention contains 0.5% by mass to 2% by mass, preferably 0.5% by mass to 1.5% by mass of Si.

[0054] [Precipitates] The Fe-based alloy of the present invention has precipitates dispersed in the matrix phase, and the precipitates preferably account for 20 vol % to 40 vol %, and more preferably 25 vol % to 35 vol %, of the alloy.

[0055] Figure 14 shows the relationship between the precipitate percentage and the metal loss after 150 hours of E-C testing for each Fe-based alloy with the composition shown in Table 2. As shown in Table 2 and Figure 14, when the precipitate percentage is less than 20 vol% or exceeds 40 vol%, the metal loss increases and corrosion cannot be sufficiently suppressed. Furthermore, alloys with a large amount of precipitates are very brittle and difficult to work with.

[0056] The precipitates contain, in mass %, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the remainder being Cr borides consisting of inevitable impurities. The precipitates are mainly Cr borides, but a small amount of unavoidably produced Cr carbides may also be present. As described above, it has been confirmed that Cr borides improve corrosion resistance and wear resistance more than Cr carbides.

[0057] [Fe] The precipitated Cr boride contains 30% by mass or more and 55% by mass or less of Fe, preferably 35% by mass or more and 55% by mass or less.

[0058] [Cr] The Cr boride precipitate contains 30% by mass to 60% by mass, preferably 35% by mass to 60% by mass of Cr. Cr can exist as a carbide or a boride, but in the present invention, it is preferred that Cr exists as a Cr boride to improve corrosion resistance and wear resistance.

[0059] [Ni] The Cr boride precipitate contains Ni in an amount of 0% by mass or more and 4% by mass or less, and preferably 1% by mass or more and 3.5% by mass or less.

[0060] [B] In order to improve corrosion resistance and wear resistance, B is preferably present in the precipitate as Cr boride rather than in the matrix. The Cr boride precipitate contains B in an amount of 6% by mass to 10% by mass, preferably 6.5% by mass to 9.5% by mass.

[0061] [C] C is preferably present in the matrix rather than in the precipitate as Cr carbide. The precipitate may contain 0% by mass or more and 0.1% by mass or less, preferably 0% by mass or more and 0.05% by mass or less of C. The lower the C content in the precipitate, the better.

[0062] [Si] Preferably, Si is present in the matrix without being precipitated. The precipitate contains Si in an amount of 0% by mass to 0.1% by mass, preferably 0% by mass to 0.05% by mass.

[0063] The Fe-based alloy of the present invention can be used to form an alloy coating on a substrate that has excellent corrosion resistance and wear resistance at high temperatures. The alloy coating of the present invention will be described below.

[0064] (2) Alloy Coating The alloy coating of the present invention is characterized by comprising, in mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, and the balance being Fe and inevitable impurities. The details of each element are as described above for the Fe-based alloy.

[0065] The alloy coating of the present invention is characterized by containing, in mass %, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, and the balance being Fe and inevitable impurities. The details of each element are as described above for the Fe-based alloy.

[0066] Furthermore, the alloy coating of the present invention preferably comprises a matrix consisting of, by mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities, and boride precipitates consisting, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the alloy coating. Details of the elements, matrix, and precipitates are as described above for the Fe-based alloy.

[0067] Furthermore, the surface of the alloy coating of the present invention does not have the nodular precipitates of the corrosion-resistant and wear-resistant alloy coating of Patent Document 10 or the specific shaped irregularities of the corrosion-resistant and wear-resistant alloy coating of Patent Document 11.

[0068] The alloy coating of the present invention can be formed by thermal spraying the Fe-based alloy powder of the present invention and then remelting it by high-frequency induction heating. The remelting temperature is preferably between the liquidus temperature and solidus temperature of the parent phase of the Fe-based alloy, for example, 1100°C to 1250°C, preferably 1150°C to 1200°C. When using an Fe-based alloy powder having a composition of Fe-20Ni-15Cr-2.6B-1Si-0.5C, the remelting temperature is preferably 1160°C to 1180°C.

[0069] By forming an alloy coating on a substrate using the Fe-based alloy of the present invention, corrosion resistance and wear resistance in a high-temperature corrosive environment can be improved. Therefore, the present invention also provides a heat transfer tube having the alloy coating of the present invention, and a heat exchanger, incinerator, and boiler each having the heat transfer tube.

[0070] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0071] The composition of alloys having the four compositions shown in Table 3 and a comparative example, a Ni-30Fe-20Cr-5.5B-1Si-0.5C thermal spray material, were analyzed by EPMA, and the above-mentioned E-C test was carried out for 100 hours. The structure after the E-C test was observed by SEM. The E-C test results are shown in Figure 15, and photographs of the structure before the E-C test are shown in Figures 16 to 19. For reference, Figure 15 also shows the E-C test results for the Ni-30Fe-20Cr-5.5B-1Si-0.5C arc spray material.

[0072] As shown in Figure 15, the four Fe-based alloys of the present invention (Fe-20Ni-20Cr-2.6B-1Si-0.5C, Fe-20Ni-15Cr-2.6B-1Si-0.5C, Fe-25Ni-20Cr-2.6B-1Si-0.5C, and Fe-20Ni-25Cr-2.6B-1Si-0.5C) all exhibit reduced metal loss and are superior in corrosion resistance and wear resistance compared to the conventional alloy (Ni-30Fe-20Cr-5.5B-1Si-0.5C thermal spray material).

[0073] 16 to 19, the precipitate ratio was approximately 30 vol% (32.8 vol% to 33.4 vol%), achieving the target ratio. Although there was some variation in the structure due to the alloy being produced using arc melting, composition analysis using EPMA confirmed that the precipitates were borides.

[0074]

[0075] The precipitate content was confirmed for alloys having the four compositions shown in Table 4. Examples 1 and 2, which are Fe-based alloys of the present invention, had precipitates in the range of 20 vol % to 40 vol %, but Comparative Examples 1 and 2, which had a B content exceeding 3.5 wt %, had precipitates exceeding 40 vol %.

[0076]

[0077] An alloy coating was formed by thermal spraying using an alloy powder of Fe-15Cr-20Ni-2.6B-1Si-0.5C (wt.%) and then remelting by high-frequency induction heating. The remelting temperature was changed in 20°C increments from 1120°C to 1200°C, as shown in Figure 20. Microscope photographs of the cross sections of the alloy coatings after remelting at each temperature are shown in Figure 21.

[0078] At remelting temperatures of 1120°C and 1140°C, the alloy powder was not completely melted, and some of the sprayed powder particles remained. At remelting temperatures of 1160°C and 1180°C, the sprayed powder particles were completely melted. At a remelting temperature of 1200°C, the surface became wavy, and a uniform alloy coating could not be formed. Therefore, when using an alloy powder of Fe-15Cr-20Ni-2.6B-1Si-0.5C (wt.%), the remelting temperature is higher than 1140°C and lower than 1200°C, preferably 1150°C or higher and 1190°C or lower, and more preferably 1160°C or higher and 1180°C or lower.

[0079] The remelting temperature must be a temperature at which the sprayed alloy powder can be completely remelted to form a uniform surface, and is preferably a temperature between the solidus temperature and liquidus temperature of the matrix of the alloy powder.

Claims

1. An Fe-based alloy characterized by the following contents, in mass percent: 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities.

2. An Fe-based alloy characterized by the following contents, in mass percent: 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities.

3. An Fe-based alloy according to claim 1 or 2, characterized in that it contains 10 mass % or more of Fe.

4. An Fe-based alloy according to claim 1 or 2, characterized in that it contains 30 mass % or more of Fe.

5. An Fe-based alloy according to claim 1 or 2, characterized in that it comprises: a matrix consisting of, by mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy.

6. By mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, the balance consisting of inevitable impurities, Ni ３ 3. The Fe-based alloy according to claim 1, characterized in that it comprises: a matrix phase containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, and the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix phase at a ratio of 20 vol% to 40 vol% of the entire alloy.

7. An alloy coating characterized by the following components, in mass percent: 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities.

8. An alloy coating characterized by the following components, in mass percent: 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the remainder being Fe and unavoidable impurities.

9. An alloy coating according to claim 7 or 8, characterized in that it comprises: a matrix consisting of, by mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the alloy coating.

10. By mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, the balance being unavoidable impurities, and Ni ３ 9. The alloy film according to claim 7, comprising: a matrix phase containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, and the remainder being unavoidable impurities, wherein the boride precipitates are present in the alloy film at a ratio of 20 vol% to 40 vol% of the entire alloy film.

11. A heat transfer tube having an alloy coating according to any one of the following (1) to (6): (1) an alloy coating characterized by, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, (2) an alloy coating characterized by, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, (3) an alloy coating characterized by, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, (4) An alloy coating comprising, by mass%, a matrix consisting of 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, of 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy coating; (5) An alloy coating comprising, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, (5) An alloy coating comprising, by mass%, a matrix consisting of 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, of 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the matrix at a ratio of 20 vol% to 40 vol% of the entire alloy coating; (6) An alloy coating comprising, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, In mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, and the balance being inevitable impurities, ３ (6) An alloy coating comprising: a matrix containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, wherein the boride precipitates are present in the alloy coating at a ratio of 20 vol% to 40 vol% of the entire alloy coating; (7) An alloy coating comprising, by mass%, 10≦Cr≦30, 15≦Ni≦40, 0.1≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, In mass%, 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, and the balance being inevitable impurities, ３ 1. An alloy coating comprising: a matrix phase containing no B; and boride precipitates consisting of, by mass%, 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, and the balance being unavoidable impurities, wherein the boride precipitates are present in the alloy coating at a ratio of 20 vol% to 40 vol% of the entire alloy coating.

12. An incinerator having the heat transfer tube according to claim 11.

13. A boiler having the heat transfer tube according to claim 11.

14. An Fe-based alloy consisting of, by mass%, 10≦Cr≦40, 15≦Ni≦50, 0.03≦C≦0.9, 0.3≦Si≦2, 1.5≦B≦3.5, with the balance being Fe and unavoidable impurities, comprising: a matrix consisting, by mass%, of 50≦Fe≦65, 0<Cr≦16, 20≦Ni≦35, 0≦B≦0.5, 0<C≦0.5, 0.5≦Si≦2, with the balance being unavoidable impurities; and boride precipitates consisting, by mass%, of 30≦Fe≦55, 30≦Cr≦60, 0≦Ni≦4, 6≦B≦10, 0≦C≦0.1, 0≦Si≦0.1, with the balance being unavoidable impurities, a thermal spray coating formed by thermal spraying powder of an Fe-based alloy, the boride precipitates being present in the matrix at a ratio of 20 vol % to 40 vol % of the entire Fe-based alloy; and a remelting treatment of the thermal spray coating by high-frequency induction heating at a temperature between the liquidus temperature and solidus temperature of the matrix of the Fe-based alloy, to form an alloy coating.

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