STRUCTURAL MATERIAL AND WELDING MATERIAL COMPOSED OF Ni-BASED ALLOY

Abstract

To provide a structural material formed of a Ni-based alloy that achieves excellent corrosion resistance and excellent SCC resistance.SOLUTION: There is provided a structural material composed of a Ni-based alloy, the alloy containing Cr and Fe as essential components, wherein the mass concentration of Cr is 26% or more and 36% or less, the mass concentration of Fe is 5% or less, and the mass concentration of Mo is 1% or less.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a structural material made of a Ni-based alloy that is used in nuclear power plants, chemical plants, etc., and to a welding material formed from this structural material. [Background technology]

[0002] Ni-based alloys, which are alloys containing Ni as the main component and also Cr and Fe, are materials with excellent mechanical properties and corrosion resistance, and are used in a wide range of applications, from general industrial to nuclear equipment structures. In particular, Ni-based alloy welding materials are used in a variety of welds.

[0003] On the other hand, the thermal history during welding causes precipitation of Cr carbides in the heat-affected zone of the weld or base metal, resulting in Cr deficiency near the grain boundaries, which is known to be one of the causes of reduced corrosion resistance at the grain boundaries and the occurrence of stress corrosion cracking (hereinafter referred to as SCC). As a countermeasure, the formation of Cr carbides can be suppressed and SCC resistance can be improved by increasing the content of the solute element Cr, which is advantageous for corrosion resistance, and by adding elements that easily combine with carbon to form carbides, i.e., elements that stabilize carbon.

[0004] As background art in this technical field, for example, Patent Document 1 and Patent Document 2 can be mentioned. Patent Document 1 discloses a Ni-based alloy as a welding material with excellent SCC resistance and weldability, which contains, in mass concentrations, the following components: 30.0 to 36.0 mass% Cr, 0.050 mass% or less C, 1.0 to 3.0 mass% Fe, 0.50 mass% or less Si, 3.0 mass% or less Nb+Ta, 0.70 mass% or less Ti, 0.10 to 3.50 mass% Mn, and 0.5 mass% or less Cu. Patent Document 2 also describes a steel containing 24% by mass or more and less than 26% by mass of Cr, and at least one of Nb, Ta, and Ti, in which the mass concentration (mass%) of Nb is [%Nb], the mass concentration (mass%) of Ta is [%Ta], and the mass concentration (mass%) of Ti is [%Ti], and the ratio [%Nb] + [%Ta] + 2 [%Ti] is 0.42 or more and 4.32 or less. The techniques of Patent Documents 1 and 2 both suppress the formation of Cr carbides and improve SCC resistance by adding Nb, Ta, and Ti, which are carbon stabilizing elements, in addition to a high Cr mass concentration.

[0005] Furthermore, with regard to Ni-based alloy welding materials, Patent Document 3 discloses a welding material comprising 26 to about 30 mass% Cr, 2 to about 4 mass% Fe, 2 to about 4 mass% Mn, 2 to about 3 mass% Nb, 1 to about 3 mass% Mo, 0.6 mass% or less Ti, 0.03 mass% or less C, 0.05 mass% or less N, 0.6 mass% or less Al, 0.5 mass% or less Si, 0.01 mass% or less Cu, 0.02 mass% or less P, 0.01 mass% or less S, and the balance being Ni and unavoidable impurities. Patent Document 4 also describes a steel sheet containing C: 0.04 mass% or less, Mn: 7 mass% or less, Fe: 1 to 12 mass%, Si: 0.75 mass% or less, Al: 0.01 to 0.7 mass%, Ti: 0.01 to 0.7 mass%, Cr: 25.0 to 31.5 mass%, Ta: 1 to 10 mass%, Mo: 1 to 6 mass%, and N: 0.1 mass% or less, and containing, as inevitable impurities, Ca + Mg: less than 0.002 mass%, P: 0.0 Disclosed is a Ni-based high Cr alloy welding wire containing 0.2 mass% or less of Cu, 0.01 mass% or less of O, 0.0015 mass% or less of S, 0.0015 mass% or less of H, 0.08 mass% or less of Cu, 0.05 mass% or less of Co, and the balance being Ni, in which the contents of S, Ta, Al, and Ti satisfy the following relational expression (1), and the contents of Ta, Mo, and N satisfy the following relational expression (2). 12000S+0.58Ta-2.6Al-2Ti≦19.3 (1) Ta+1.60Mo+187N≧5.7 (2)

[0006] Furthermore, Patent Documents 5 and 6 disclose techniques relating to alloy powders. Patent Document 5 discloses a powder alloy having a composition, in mass %, of C: 0.005 to 0.50%, Si: 0.10 to 0.50%, Mn: 0.01 to 0.50%, Cr: 15 to 30%, Al: 3.5 to 6.0%, Fe: 1.0 to 5.0%, and the balance being Ni and unavoidable impurities. Patent Document 6 discloses a Ni-based powder alloy for build-up welding that is excellent in corrosion resistance, weldability, and hardness, and that contains, by mass%, C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, Cr: 20.0 to 30.0%, Mo: 4.0 to 12.0%, Nb: 0.5 to 4.0%, Fe: 0.1 to 6.0%, and one or two of Al and Ti in a total amount of more than 0.8% but not more than 2.0%, with the balance being Ni and unavoidable impurities, and that has a hardness of 280 HV or more after aging treatment under conditions of 600 to 900°C. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2020-196043 [Patent Document 2] Japanese Patent Publication No. 2020-19981 [Patent Document 3] Japanese Patent Application Laid-Open No. 2013-527805 [Patent Document 4] International Publication No. 2012 / 105452 [Patent Document 5] Patent Publication No. 2021-011629 [Patent Document 6] Japanese Patent Application Laid-Open No. 2014-111265 Summary of the Invention [Problem to be solved by the invention]

[0008] Conventional Ni-based alloys have been developed commercially with a high Cr content in order to improve corrosion resistance and SCC resistance. However, as a result of investigations by the inventors of the present invention, it has been found that if the mass concentration of Fe is high, there is a concern that the corrosion resistance and SCC resistance of the oxide film and grain boundaries may decrease. It should be noted that none of the above Patent Documents 1 to 6 discloses the effect of the Fe content on SCC resistance.

[0009] In order to solve the above-mentioned problems, an object of the present invention is to provide a structural material made of a Ni-based alloy that has excellent corrosion resistance and SCC resistance, and a welding material formed from this structural material.

[0010] The above and other objects of the present invention and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0011] The structural material of the present invention is composed of a Ni-based alloy containing Cr and Fe as essential components, with the Cr mass concentration being 26% or more and 36% or less, the Fe mass concentration being 5% or less, and the Mo mass concentration being 1% or less. The welding material of the present invention is formed from the structural material of the present invention. [Effects of the Invention]

[0012] According to the present invention, the Ni-based alloy constituting the structural material has a high Cr mass concentration and a low Fe mass concentration, so that a structural material having excellent corrosion resistance and SCC resistance can be obtained. Furthermore, because of its excellent corrosion resistance and SCC resistance, it can be used as a structural material in stress corrosion environments with high-temperature water, such as the reactor bottom of a BWR.

[0013] Problems, configurations, and effects other than those described above will become clear from the following description of the embodiments. [Brief explanation of the drawings]

[0014] [Figure 1] Schematic diagram of TR-type SCC. [Figure 2] This is a photograph showing an example of the sampling position of a CBB test piece (in the case of SUS316L). [Figure 3] FIG. 1 is a diagram showing the relationship between the crack number density in the central cross section of a test piece after a CBB test and the Fe mass concentration. [Figure 4] FIG. 1 shows the crack density on the surface of a test specimen after an SSRT test. [Figure 5] A, B Cross-sectional SEM photographs of the test specimen after the SSRT test. [Figure 6] A conceptual diagram of a boiling water reactor (BWR). DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings and the like. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various changes and modifications are possible by those skilled in the art within the scope of the technical ideas disclosed in this specification. Furthermore, in all drawings used to explain the present invention, parts having the same function are designated by the same reference numerals, and repeated explanations thereof may be omitted.

[0016] First, before describing the embodiments of the present invention, an outline of the present invention will be described.

[0017] SCC is a material degradation phenomenon that occurs primarily as a result of the interaction of three factors: material, environment, and stress. SCC mechanisms can be classified according to the material used: active path corrosion (APC) and tarnish rupture (TR) for soft metals, and hydrogen embrittlement (HE) for high-strength alloys. In the case of a thin passive oxide film with a thickness on the order of nanometers, such as that of stainless steel, anodic dissolution of the metal plays an important role, causing APC-type SCC. On the other hand, in the case of thick passive oxide films on Ni-based alloys, which are on the order of several hundred nm to mm in thickness, -3 Strain of this magnitude will cause fracture, resulting in TR-type SCC.

[0018] Here, FIG. 1 shows a schematic diagram of TR-type SCC (cited from the following documents 1 to 3). Reference 1: Masahiro Yamamoto and 12 others: Stress Corrosion Cracking (SCC) Phenomena and Their Evaluation Methods in Light Water Reactors - SCC Evaluation Methods for Predicting Aging Phenomena in Reactor Internal Structures and Piping, Technical Data Collection - (Commissioned Research), JAEA Review 2012-007, p.22(2012) Reference 2: "Introduction to Environmental Materials Science," by the Japan Corrosion Protection Association, Maruzen, p. 44 (1993) Reference 3: EN Pugh: Environment -Sensitive Mechanical Behavior, Corden and Breach, New York, P.351 (1966)

[0019] In Figures 1(a) to (f), the oxide film is indicated by the hatched layer. As shown in Figure 1(b), the oxide film cracks due to stress, and as shown in Figure 1(c), slippage occurs, causing the cracked oxide film to separate and create a gap. The metal in contact with the gap then changes to an oxide film, and as shown in Figure 1(d), the oxide film grows, protruding from the gap. Furthermore, as shown in Figure 1(e), the oxide film cracks again, and as shown in Figure 1(f), slippage occurs, causing the cracked oxide film to separate and create a gap. In Figure 1(f), a step is created on the wall surface between the original position and the protruding position of the oxide film above and below the crack. These processes are repeated until stress corrosion cracking progresses. Figure 1(g) shows a perspective view of one of the fracture surfaces when a crack occurs. As shown in Figure 1(g), steps are formed in places along the direction of the crack, as indicated by the arrow in the figure.

[0020] In TR-type SCC, only the oxide film cracks mechanically; that is, the metal itself does not crack directly, but first transforms into an oxide and then cracks (see, for example, Reference 2). Furthermore, the crack grows through the repeated process of destruction and repair of the oxide film. In other words, the properties of the oxide film have an important effect on the occurrence of TR-type SCC.

[0021] In a high-temperature water environment, NiFe is more likely to diffuse outward than Cr in a Ni-Cr-Fe ternary alloy. According to the following reference 4, Fe and Ni are dissolved from the surface during the dissolution and reprecipitation process, and an Fe- and Ni-rich oxide (Ni,Fe)Fe2O4 grows on the surface, forming an outer oxide layer of discontinuous plate-like or spinel-like oxides. Literature 4: W. Kuang, et al., The Mechanism of Oxide Film Formation on Alloy 690 In Oxygenated High Temperature Water, Corrosion Science 53, pp. 3853-3860 (2011)

[0022] However, the outer oxide layer has many microvoids, which means it cannot completely block high-temperature water, and oxygen diffuses inward quickly, so it does not provide protection to the metal matrix. When the mass concentration of Fe is high, the Fe, Ni-rich outer oxide layer becomes thicker. Note that because the oxide and the metal matrix have different physical properties such as the linear expansion coefficient and modulus of elasticity, as the oxide film becomes thicker, it becomes more susceptible to destruction under tensile stress, accelerating the repeated process of destruction and repair of the oxide film.

[0023] Meanwhile, an inner oxide layer consisting of Cr oxides is formed beneath the Ni / Fe-rich outer oxide layer (on the metal matrix side). This dense inner oxide layer of Cr oxides is highly protective and is important for the corrosion resistance of the underlying metal matrix. At oxidized grain boundaries, the mass concentration of Fe gradually decreases from the grain boundary to the matrix, while the mass concentration of Cr increases, following the same principle as in the surface oxide film. When Ni / Fe-rich oxides concentrate at the grain boundaries, the corrosion resistance of the grain boundaries decreases, and SCC occurs under tensile stress.

[0024] From the above, in addition to increasing the Cr mass concentration and adding a C stabilizing element, adjusting the Fe mass concentration is also effective in improving SCC resistance. Therefore, the inventors of the present invention performed build-up welding using Ni-based alloy welding materials on various metal materials such as stainless steel, evaluated the SCC susceptibility of weld dilution zones with different Fe mass concentrations, and found an effective range for the Fe mass concentration from this evaluation.

[0025] The structural material of the present invention is composed of a Ni-based alloy, which contains Cr and Fe as essential components, with the Cr mass concentration being 26% or more and 36% or less, the Fe mass concentration being 5% or less, and the Mo mass concentration being 1% or less. The welding material of the present invention is formed from the structural material of the present invention.

[0026] The above structural material may further be configured such that the Ni-based alloy has a Cr mass concentration of 26% or more and 36% or less, and an Fe mass concentration of more than 3% and 5% or less. In this configuration, the mass concentration of Fe exceeds 3%, which makes it possible to suppress the formation of the intermetallic compound Ni2Cr in a high-temperature environment.

[0027] The above structural material may further be configured such that the Ni-based alloy has a Cr mass concentration of 26% or more and 28% or less, and an Fe mass concentration of more than 3% and 5% or less. In this configuration, the mass concentration of Fe exceeds 3%, which can suppress the formation of the intermetallic compound Ni2Cr in high-temperature environments. Also, the mass concentration of Cr is 26% or more and 28% or less, which prevents the atomic mass concentration ratio of Ni to Cr from becoming 2:1, which also suppresses the formation of the intermetallic compound Ni2Cr.

[0028] The above structural material may further be configured such that the Ni-based alloy has a Cr mass concentration of 26% or more and 28% or less, and an Fe mass concentration of less than 1%. In this structure, the mass concentration of Cr is 26% or more and 28% or less, which prevents the atomic mass concentration ratio of Ni to Cr from becoming 2:1 and suppresses the formation of the intermetallic compound NiCr. In addition, the mass concentration of Fe is less than 1%, which further improves SCC resistance.

[0029] The above structural material may further be configured such that the Ni-based alloy has a Cr mass concentration of 31.5% or more and 36% or less, and an Fe mass concentration of more than 3% and 5% or less. In this configuration, the mass concentration of Cr is 31.5% or more and 36% or less, which prevents the atomic mass concentration ratio of Ni to Cr from becoming 2:1, thereby suppressing the formation of the intermetallic compound Ni2Cr. In addition, the mass concentration of Fe exceeds 3%, which suppresses the formation of the intermetallic compound Ni2Cr in high-temperature environments.

[0030] The above structural material may further be configured such that the Ni-based alloy has a Cr mass concentration of 31.5% or more and 36% or less, and an Fe mass concentration of less than 1%. In this structure, the Cr mass concentration is 31.5% or more and 36% or less, which prevents the atomic mass concentration ratio of Ni to Cr from becoming 2:1 and suppresses the formation of the intermetallic compound NiCr. In addition, the Fe mass concentration is less than 1%, which further improves SCC resistance.

[0031] The above structural material may further be configured to be a structural material used in an underwater environment at 250°C or higher. While conventional structural materials made of Ni-based alloys are prone to SCC in underwater environments of 250°C or higher, this structure can exhibit SCC resistance even in underwater environments of 250°C or higher.

[0032] The above structural material may further be configured to have any one of the following shapes: block, plate, rod, wire, and strip.

[0033] In the present invention, the term "structural material" refers to a member that is not in powder or particulate form but is in a solid form and can be used as a structure. In the present invention, the structural material may have a variety of shapes, including a block, a plate (a plate that forms a flat or curved surface and has a certain thickness), a rod, a thin and long wire, a strip (relatively thin and can be rolled up, for example), and the like. Furthermore, the structural material is formed into a predetermined size and shape depending on the intended use.

[0034] According to the present invention, the Ni-based alloy constituting the structural material has a high Cr mass concentration and a low Fe mass concentration, so that the structural material has excellent corrosion resistance and SCC resistance. Furthermore, because of its excellent corrosion resistance and SCC resistance, it can be used as a structural material in stress corrosion environments with high-temperature water, such as the reactor bottom of a BWR.

[0035] Next, the present invention will be described in detail, divided into chemical components and practical applications.

[0036] <1. Chemical Composition> (1.1 Cr mass concentration) Cr is an element necessary for maintaining corrosion resistance, and from the viewpoint of corrosion resistance, the Cr mass concentration must be 26% or more. However, considering hot workability during manufacturing, it is preferable to keep it 36% or less. For these reasons, the Cr mass concentration range is set to 26 to 36%.

[0037] (1.2 Fe mass concentration) To determine the mass concentration range of Fe, multi-layer build-up weld specimens were fabricated using austenitic stainless steel SUS316L, low-alloy steel ASME SA-508 Cl. 3, Ni-based alloy base metal NCF600, and Ni-based 82 alloy weld metal, using VDM Metals FM 52i welding wire (diameter 1.2 mm). Three test pieces measuring 50mm x 10mm x 2mm thick were then prepared for each material, taken from each multi-layer build-up welded test piece in a direction oblique to the fusion boundary. Figure 2 is a photograph showing an example of the location where a CBB test piece was taken for SUS316L. As shown in Figure 2, the test pieces were taken in a direction oblique to the fusion boundary.

[0038] Creviced bent beam (CBB) tests were performed on these specimens to evaluate their susceptibility to SCC. Graphite fiber wool was attached to the surface of the specimen to form a gap. The specimens were immersed in high-temperature water in an autoclave while applying a strain of approximately 1% to their outer surfaces using a bending jig with a curvature of 100°C. The dissolved oxygen concentration of the high-temperature water (at the inlet of the autoclave) was set to 16 ppm, the test temperature to 288°C, the pressure to 8.2 MPa, and the electrical conductivity (at the inlet of the autoclave) to 0.1 μS / cm or less. The immersion time was 3,000 hours.

[0039] After the CBB test, the longitudinal central cross section of the test specimens was observed. As a result, cracks were observed in the first weld layer of some test specimens, but no cracks were observed in the second or subsequent weld layers of any of the test specimens. Therefore, the element mass concentration of the first layer was analyzed using energy dispersive X-ray (EDX). The Cr mass concentration of all specimens was approximately the same, ranging from 22 to 24%. On the other hand, the Fe mass concentration of the first layer of each specimen was in a wide range, from 1 to 65%.

[0040] Figure 3 shows the relationship between the crack density at the center cross section of the test piece after the CBB test and the Fe mass concentration. Specifically, Figure 3 shows the relationship between the crack density observed within the length range of the first weld layer and the Fe mass concentration. These results indicate that SCC occurs when the mass concentration of Fe exceeds 5%. Naturally, the solute elements other than Cr and Fe also differ in the first weld layer of each welded test specimen, but because the differences are all less than 1%, it is believed that the effect of differences in the mass concentrations of these elements can be ignored. From the above, it was discovered that keeping the mass concentration of Fe below 5% improves SCC resistance.

[0041] On the other hand, when used in a high-temperature water environment exceeding 250°C for a long period of time, and the atomic mass concentration ratio of Ni to Cr is close to 2:1, the intermetallic compound Ni2Cr is formed due to thermal aging, which can reduce the SCC resistance of the material (see Reference 5 below). Reference 5: GA Young, David S. Morton, Nathan Lewis, Robert Morris, Joseph Pyle, Leland Barnard and Reza Najafabadi, Effect of Long Range Order on the Stress Corrosion Susceptibility of a Nickel-33 at.% Chromium Alloy, Corrosion 72-11, pp.1433-1437(2016)

[0042] However, from the viewpoint of activation energy, it has been reported that high Fe concentrations have an inhibitory effect on the intermetallic compound Ni2Cr (see Reference 6 below). Reference 6: George A. Young, Robert A. Etien, Micah J. Hackett, Julie D. Tucker, and Thomas E. Capobianco, Physical Metallurgy, Weldability, and In-Service Performance of Nickel-Chromium Filler Metals Used in Nuclear Power Systems Proceedings of the 15th International Conference on Environmental Degradation of Materials in Nuclear Power Systems - Water Reactors, ISBN . The Minerals, Metals, and Materials Society, pp. 2431-2441 (2011)

[0043] Based on this finding, when using in a high-temperature environment for a long period of time, it is necessary to increase the Fe mass concentration slightly, and when the Cr mass concentration is in the range of 26% to 36%, it is desirable to adjust the Fe mass concentration to more than 3% and not more than 5%. Furthermore, taking into consideration the need to avoid an atomic mass concentration ratio of Ni to Cr of 2:1, the SCC resistance effect can be further enhanced by dividing the Cr mass concentration into two ranges: 26% to 28% and 31.5% to 36%. Within these two Cr mass concentration ranges, and provided that the period of use in a high-temperature water environment is not long enough to cause thermal aging, the SCC resistance improvement effect is greater if the Fe mass concentration is less than 1%.

[0044] (1.3 Mo mass concentration) Mo may form a ternary intermetallic compound with Fe and Cr, which may embrittle the material, so it is desirable to keep the mass concentration of Mo at 1% or less.

[0045] (1.4 Mass concentration of impurities) Impurities cause a decrease in the corrosion resistance of the material, so the upper limits of the mass concentrations are preferably set to C≦0.05%, Si≦0.5%, P≦0.03%, and S≦0.02%.

[0046] (2. Practicality) A Ni-based alloy containing Cr and Fe as essential components, in which the mass concentration of Cr is adjusted to 26% or more and 36% or less, the mass concentration of Fe is adjusted to 5% or less, and the mass concentration of Mo is adjusted to 1% or less, can be suitably used in severe corrosive environments, and is suitable for use in, for example, the welds of the shroud of a BWR. [Example]

[0047] Next, the present invention will be described in more detail with reference to specific examples.

[0048] Example 1 Ingots of model alloys (Ni-Cr-Fe) were produced by induction melting. The induction melting was carried out in an Ar atmosphere at 1 atm. Block materials (particle size: approximately 10 mm) of each element were melted in an alumina crucible and poured into a copper mold to cast 300 g of prismatic ingots.

[0049] The above-described prismatic ingots were produced for each of the inventive material within the mass concentration range of the present invention and the comparative material outside the mass concentration range of the present invention. The chemical compositions of the material according to the present invention and the comparative material are shown in Table 1. Wet elemental analysis showed that the mass concentrations of the material according to the present invention were 27.1% Cr and 3.1% Fe, while those of the comparative material were 30.0% Cr and 7.9% Fe.

[0050] [Table 1]

[0051] As a method for evaluating SCC resistance, tensile test specimens were prepared from the above two types of materials and slow strain rate testing (SSRT) was carried out. The SSRT test was performed using an autoclave with a loading shaft. The engineering strain rate of the SSRT test load stroke was 4.5 × 10 -8 The dissolved oxygen concentration of the high-temperature water (at the inlet of the autoclave) was set to 16 ppm, the test temperature to 288°C, the pressure to 8.2 MPa, and the conductivity (at the inlet of the autoclave) to 0.1 μS / cm or less. The test was stopped when the engineering strain reached approximately 8%.

[0052] The surfaces of both test pieces were then observed using a scanning electron microscope (SEM), and the number of cracks was counted. The statistical crack densities on the surfaces of both test pieces are shown in Figure 4. As can be seen from FIG. 4, although the material to which the present invention is applied has a lower mass concentration of Cr than the comparative material, the crack density is one-tenth or less of that of the comparative material, confirming good SCC resistance.

[0053] The central cross section was then cut out and mirror-finished, after which the cross section near the surface was observed using an SEM. SEM photographs of the cross sections of both materials are shown in Figures 5A and 5B. Figure 5A is a photograph of the material to which the present invention is applied, and Figure 5B is a photograph of the comparative material.

[0054] 5A and 5B, the thickness of the surface oxide layer of the material to which the present invention is applied is 380 nm, which is thinner than the thickness of the surface oxide layer of the comparative material, 641 nm. This is because the Fe- and Ni-rich outer oxide layer grows slowly in the material according to the present invention, which has a low Fe mass concentration. Also, while significant cracks were observed in the oxide film of the comparative material, no cracks were observed in the oxide film of the present invention.

[0055] Furthermore, the average Cr mass concentration in the oxide film of both materials was determined by EDX analysis. It is assumed that in order to maintain good corrosion resistance, it is necessary to maintain the Cr mass concentration at 18% or more. In the materials to which the present invention was applied, the mass concentration of Cr in the oxide film was maintained at a high level of 25% or more. In contrast, the mass concentration of Cr in the oxide film of the comparative material was only about 16%, which was below 18%. In other words, although the comparative material has a high Cr concentration of 30% in the metal matrix, Fe and Ni form oxides preferentially over Cr, which significantly reduces the Cr concentration in the oxide film, resulting in a decrease in the protective properties of the oxide film. The above examples demonstrate the principle and effect of the structural material made of the Ni-based alloy of the present invention.

[0056] (Application example) A structural material made of the Ni-based alloy of the present invention can be applied to, for example, equipment inside a nuclear reactor. Here, as an example of application of the structural material of the present invention, a case where it is applied to a boiling water reactor (BWR) will be described.

[0057] Figure 6 is a conceptual diagram of a boiling water reactor (BWR). As shown in Fig. 6, a boiling water reactor (BWR) 10 has fuel assemblies 12 arranged inside a cylindrical core shroud 11. Control rods 13 that control fuel combustion are arranged between the fuel assemblies 12, and these control rods 13 are driven up and down by a control rod drive system 14 below. A steam separator 15 is arranged above the fuel assemblies 12, and a steam dryer 16 is arranged above the steam separator 15. Reactor cooling water introduced into the core shroud 11 boils and turns into steam through heat exchange with the fuel assemblies 12, and the moisture-containing steam is stripped off in the steam separator and turned into dry steam in the steam dryer 16, which is then guided from the core shroud 11 to an external turbine (not shown).

[0058] The structural material made of the Ni-based alloy of the present invention is expected to be applied to the welds of the core shroud 11 in a boiling water reactor (BWR) 10, as shown in Figure 6, which is used in a high-temperature water environment at a temperature of 288°C.

[0059] It should be noted that the present invention is not limited to the above-described embodiments and examples, and includes various modifications. For example, the above-described embodiments and examples have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those having all of the described configurations. [Industrial Applicability]

[0060] Structural materials made of the Ni-based alloy of the present invention are expected to have a wide range of applications, including as structural members used in stress-corrosion environments in nuclear reactors, nuclear power plants, chemical plants, and the like. [Explanation of symbols]

[0061] 10 Boiling water reactor (BWR), 11 Core shroud, 12 Fuel assembly, 13 Control rod, 14 Control rod drive system, 15 Steam separator, 16 Steam dryer

Claims

1. A structural material made of a Ni-based alloy containing Cr and Fe as essential components, with the Cr mass concentration being 26% or more and 36% or less, the Fe mass concentration being 5% or less, and the Mo mass concentration being 1% or less.

2. 2. The structural material according to claim 1, wherein the Ni-based alloy has a Cr mass concentration of 26% or more and 36% or less, and an Fe mass concentration of more than 3% and 5% or less.

3. 2. The structural material according to claim 1, wherein the Ni-based alloy has a Cr mass concentration of 26% or more and 28% or less, and an Fe mass concentration of more than 3% and 5% or less.

4. 2. The structural material according to claim 1, wherein the Ni-based alloy has a Cr mass concentration of 26% or more and 28% or less, and an Fe mass concentration of less than 1%.

5. 2. The structural material according to claim 1, wherein the Ni-based alloy has a Cr mass concentration of 31.5% or more and 36% or less, and an Fe mass concentration of more than 3% and 5% or less.

6. 2. The structural material according to claim 1, wherein the Ni-based alloy has a Cr mass concentration of 31.5% to 36% and an Fe mass concentration of less than 1%.

7. 2. The structural material according to claim 1, which is a structural material used in an underwater environment at 250°C or higher.

8. 2. The structural material according to claim 1, which has any one of the following shapes: block, plate, rod, wire, and strip.

9. A welding material formed from a structural material made of a Ni-based alloy containing Cr and Fe as essential components, with the Cr concentration being 26% or more and 36% or less, the Fe concentration being 5% or less, and the Mo concentration being 1% or less.

10. The welding material according to claim 9, wherein the structural material is a structural material used in an underwater environment at 250°C or higher.

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