JP2020019981A - Ni-BASED WELD METAL AND WELD STRUCTURE - Google Patents

Abstract

To provide an Ni-based weld metal excellent in SCC resistance and capable of maintaining soundness of a weld zone exposed to corrosive environment, and a weld structure using the same.SOLUTION: A Ni-based weld metal contains Cr:24 mass% to 32 mass% and one or more kind of Nb, Ta and Ti, and has [%Nb]+{%Ta]+2[%Ti] of 0.42 to 4.32, where Nb concentration (mass%) is [%Nb], Ta concentration (mass%) is [%Ta], and Ti concentration (mass%) is [%Ti]. A weld structure has a weld zone formed by the Ni-based weld metal.SELECTED DRAWING: Figure 9

Description

  The present invention relates to a Ni-based weld metal which can be used in a corrosive environment and has excellent SCC resistance, and a welded structure using the same.

  In boiling water reactors (Boiling Water Reactor: BWR) and pressurized water reactors (Pressurized Water Reactor: PWR), high temperatures and high pressures of about 300 ° C are applied to welds applied to piping, structural materials, equipment, etc. Of reactor water may come into contact with the liquid. In addition, various radicals that promote corrosion are generated in the reactor water by the action of radiation. Since a residual stress acts on a weld in a nuclear reactor in such a severe corrosive environment, measures against stress corrosion cracking (SCC) are taken from various viewpoints.

  As a countermeasure against stress corrosion cracking, a technique for mitigating a corrosive environment by a water chemistry method such as hydrogen injection or noble metal injection is known. In addition, a technique for removing a tensile residual stress by performing heat treatment, cooling treatment, peening, or the like is also known. In addition, the materials themselves have been improved, and low carbon stainless steels having SCC resistance by reducing the amount of C and Ni-based alloys having a reduced amount of solid solution C by adding Nb have been developed. .

  As a welding material, a Ni-based weld metal excellent in heat resistance and corrosion resistance is used, including a support portion of a reactor internal structure, a penetration portion of a reactor pressure vessel, a nozzle portion, and the like. Ni-based weld metals are hardly dissolved in carbon, have good toughness, and are resistant to thermal fatigue. Therefore, Ni-based weld metals are also used for welding dissimilar materials such as carbon steel, low alloy steel, and stainless steel. TIG welding, MIG welding, covered arc welding, and the like have been used for welding Ni-based alloys, but electron beam welding, laser welding, and the like have also been used.

  Conventionally, a weld metal equivalent to a 600 series alloy such as 76Ni-15Cr-8Fe has been used as a Ni-based weld metal in a welded portion in a nuclear reactor. As welding metals equivalent to 600 series alloys, there are 82 alloys for applications such as TIG welding and 182 alloys for applications such as covered arc welding, which are said to have good heat resistance and corrosion resistance. However, in recent years, in the primary cooling system of a nuclear power plant, a case has been reported in which stress corrosion cracking has also occurred in a welded portion using a weld metal equivalent to a 600 series alloy.

  Under these circumstances, in recent years, a welding metal equivalent to a 690 series alloy, such as 62Ni-29Cr-9Fe, which has been improved in SCC resistance by increasing the amount of Cr, has been used as a welding material instead of a welding metal equivalent to a 600 series alloy. Spreading. Conventionally, with respect to a Ni-base weld metal equivalent to a 690 series alloy or a Ni-base alloy equivalent to a 690 series alloy, a technique for controlling the chemical composition of corrosion resistance, SCC resistance, weld crack resistance, and the like by controlling the chemical composition has been proposed. .

  For example, Patent Document 1 discloses that, by weight%, Cr: 26 to about 30%, Fe: 2 to about 4%, Mn: 2 to about 4%, Nb: 2 to about 3%, Mo: 1 to about 3 %, Ti: 0.6% or less, C: 0.03% or less, N: 0.05% or less, Al: 0.6% or less, Si: 0.5% or less, Cu: 0.01% or less, A welding material comprising P: 0.02% or less, S: 0.01% or less, and the balance consisting of Ni and unavoidable impurities is described (see claim 16).

  Patent Document 2 discloses that, by mass%, Cr: 29 to 37%, Al: 0.001 to 1.8%, Fe: 0.10 to 7.0%, Si: 0.001 to 0.50 %, Mn: 0.005 to 2.0%, Ti: 0.00 to 1.00%, and / or Nb: 0.00 to 1.10%, Mg and / or Ca: 0.0002 to 0. 0%, respectively. 05%, C: 0.005 to 0.12%, N: 0.001 to 0.050%, P: 0.001 to 0.030%, O: 0.0001 to 0.020%, S: maximum It describes an alloy comprising 0.010%, Mo: 2.0% at maximum, W: 2.0% at maximum, and the balance being Ni and unavoidable impurities (see claim 1).

  In Patent Document 3, C: 0.04% or less, Si: 0.01 to 0.5%, Mn: 7% or less, Cr: 28 to 31.5%, Nb: 0. 5% or less, Ta: 0.005 to 3.0%, Fe: 7 to 11%, A1: 0.01 to 0.4%, Ti: 0.01 to 0.45%, V: 0.5% And P: 0.02% or less, S: 0.015% or less, 0: 0.01% or less, N: 0.002 to 0.1% as inevitable impurities, with the balance being Ni A filler having the following composition is described (see claim 1).

  Patent Document 4 discloses that, by mass%, Cr: 28.5 to 31.0%, Fe: 11% or less, Mn: less than 1.0%, Nb + Ta: 2.1 to 4.0%, Mo: 7.0% or less, Si: less than 0.50%, Ti: 0.35% or less, Al: 0.25% or less, Cu: less than 0.20%, W: less than 1.0%, Co: 0. Less than 12%, Zr: less than 0.10%, S: less than 0.01%, B: less than 0.01%, C: less than 0.03%, P: less than 0.02%, Mg + Ca: 0.002 to 0.002% A solubilizer containing 0.015% and a balance of Ni and unavoidable impurities is described (see claim 6).

JP-T-2013-527805 JP-T-2015-520300 International Publication No. 2005/070612 International Publication No. 2008/021650

  Conventionally, an evaluation index for evaluating the SCC resistance of a weld metal equivalent to a 600 series alloy has been established, and the correspondence between the concentration range of the chemical component and the concentration range required for the development of the SCC resistance is good. there were. On the other hand, there is a current situation in which the evaluation index of the newly spread welding metal equivalent to the 690 series alloy is not standardized and is not sufficiently established. For this reason, an appropriate chemical composition cannot be freely selected, and there arises a problem that excellent SCC resistance cannot be stably ensured and a range of selectable chemical compositions is restricted.

  Accordingly, an object of the present invention is to provide a Ni-based weld metal having excellent SCC resistance and capable of maintaining the integrity of a welded portion exposed to a corrosive environment, and a welded structure using the same.

  In order to solve the above-mentioned problems, a Ni-based weld metal according to the present invention has a Cr content of 24% by mass or more and less than 32% by mass, contains one or more of Nb, Ta, and Ti, and has a Nb concentration (% by mass). ) Is [% Nb], the concentration of Ta (% by mass) is [% Ta], and the concentration of Ti (% by mass) is [% Ti]. [% Nb] + [% Ta] +2 [% Ti] Is 4.32 or less. Further, in the welded structure according to the present invention, a welded portion is formed by the Ni-based weld metal.

  ADVANTAGE OF THE INVENTION According to this invention, the Ni-base weld metal which is excellent in SCC resistance and can maintain the integrity of the weld part exposed to a corrosive environment, and a welded structure using the same can be provided.

It is a figure which shows the relationship between the stabilization parameter (mNBar value) of Ni-base weld metal, and a grain boundary erosion rate. It is a figure which shows the relationship between the stabilization parameter (mNBar value) of Ni-base weld metal, the maximum crack depth, and Vickers hardness. It is a figure which shows the relationship between the additive element density | concentration of Ni-base weld metal, and Vickers hardness / SCC mode. It is a figure which shows the relationship between the Cr concentration of Ni-base weld metal and a crack growth rate. FIG. 3 is a ternary phase diagram of Fe—Cr—Ni at 800 ° C. FIG. 3 is a ternary phase diagram of Fe—Cr—Ni at 650 ° C. It is a figure which shows the relationship between the upper limit of Cr concentration at which an austenite single phase becomes stable, and temperature. It is a figure which shows the relationship between the Cr density | concentration of Ni-base weld metal and the addition element density | concentration. It is a figure which shows the difference between the chemical composition of the Ni-base weld metal which concerns on this invention, and the chemical composition of a general weld metal.

  Hereinafter, a Ni-based weld metal according to an embodiment of the present invention and a welded structure using the same will be described with reference to the drawings. In the present specification, a numerical range indicated by “to” means a numerical range including those numerical values as a lower limit and an upper limit.

  The Ni-based weld metal according to the present embodiment is a Ni-Cr-Fe-based alloy mainly composed of nickel, and is a metal that forms a weld by melting and solidifying by welding. This Ni-based weld metal has a form of a weld metal integrated with a part of the base metal by welding, a form of a weld metal that has migrated to a weld but is not integrated with the base metal, or a form of a weld material. Of these, any form may be used. The welding material may be any of a rod-shaped, plate-shaped, band-shaped, powdered, or other appropriate solubilizing material, a flux-containing solubilizing material, and a consumable electrode.

  The Ni-based weld metal according to the present embodiment has a chemical composition equivalent to that of a 690 series alloy, and in addition to Ni as the main component, 24-32% by mass of Cr and one of Nb, Ta, and Ti. It contains at least the above. In this Ni-based weld metal, when the concentration (% by mass) of Nb is [% Nb], the concentration (% by mass) of Ta is [% Ta], and the concentration (% by mass) of Ti is [% Ti], % Nb] + [% Ta] +2 [% Ti] is limited to 4.32 or less based on a stabilization parameter (mNBar value) described later.

  In the Ni-based weld metal according to the present embodiment, one or more of Nb, Ta, and Ti are added so as to be in a concentration range limited based on a stabilization parameter (mNBar value), so that a grain boundary is obtained. Inter-granular Stress Corrosion Cracking (IGSCC) and Trans-granular Stress Corrosion Cracking (TGSCC) are all suppressed.

  Generally, stress corrosion cracking (SCC) of a welded portion is caused by material factors that cause sensitization of the metal structure, residual tensile stress generated around the welded portion due to solidification during welding, and environmental factors such as corrosion potential. It is said to be caused by the superposition of factors. Grain boundary sensitization is a phenomenon in which Cr carbide precipitates at crystal grain boundaries and forms a Cr-deficient layer in the vicinity thereof, and is considered to be a factor that causes IGSCC to be generated and propagated.

  On the other hand, it has been widely confirmed that TGSCC is generated and propagates in a hardened portion of about 300 Hv or more in a case of a primary cooling system of a nuclear power plant. It is considered that TGSCC is likely to occur in a heat-affected zone where hardening proceeds during welding, or in a strongly-worked portion where working strain is introduced after welding.

  52 alloys and 152 alloys, which are general welding metals equivalent to 690 series alloys, have a higher Cr content and better SCC resistance than 600 series alloys. However, conventionally, an evaluation index of SCC resistance has not been standardized for a weld metal equivalent to a 690 alloy. In addition, test methods appropriate for the evaluation of SCC resistance are not always unified. Therefore, at present, there is no good correspondence between the concentration range equivalent to the 690 alloy and the concentration range required for the development of excellent SCC resistance, and the appropriateness of chemical composition selection and the freedom of selection are secured. It is hard to be done.

  Therefore, in the present embodiment, a stabilization parameter (for stabilizing the concentration relationship of the chemical components (Ni-base weld metal equivalent to 690 series alloy) that defines the concentration relationship of the chemical components so that the chemical composition exhibiting excellent SCC resistance can be correctly selected. mNBar) to limit the concentrations of the three additional elements (Nb, Ta and Ti). The stabilization parameter (mNBar) is represented by the following equation (I).

  In the formula (I), [% Nb] is the Nb concentration (% by mass), [% Ta] is the Ta concentration (% by mass), [% Ti] is the Ti concentration (% by mass), and [% C] is the C concentration (%). % By mass) and [% Cr] are Cr concentrations (% by mass).

  The three additional elements of Nb, Ta and Ti form carbides in the weld metal, and therefore have an effect of suppressing sensitization due to precipitation of Cr carbides. Therefore, if the stabilization parameter (mNBar) is introduced to limit the concentration, IGSCC, which is the main SCC mode, can be prevented.

Equation (I) is obtained by correcting the ratio of the number of atoms of the three additional elements (Nb, Ta, and Ti) to the number of carbon atoms as a function of the Cr concentration. The Cr concentration term in the formula (I) is derived by subjecting nine existing Ni-based weld metals to a grain boundary corrosion test and fitting the measured results to the grain boundary erosion rate, as in FIG. 1 below. is there. First, the Cr concentration term is assumed to be a variable represented by A × (B) ⁱ , and the variable is adjusted by fitting to the measurement result.

FIG. 1 is a diagram showing a relationship between a stabilization parameter (mNBar value) of a Ni-based weld metal and a grain boundary erosion rate.
In FIG. 1, the vertical axis represents an inter-granular penetration rate (IGP) value measured by subjecting a Ni-base weld metal to an intergranular corrosion test, and the horizontal axis represents an mNBar value calculated for the Ni-base weld metal. (Formula (I) after fitting) is shown. The plots of ● show the results when each of the existing six types of weld metal (52 weld alloy, 152 weld alloy) equivalent to the 690 series alloy was used as the test material, and the plot of ○ shows the three existing types of weld metal. It is a result in the case where each of the welding metals (82 welding alloy, 182 welding alloy) equivalent to the 600 series alloy was used as the test material.

In the intergranular corrosion test, a bead-on test piece of each test material was prepared and subjected to an improved striker test conforming to the ASTM G28 standard. Since general corrosion may progress under normal standard conditions, 50 g of 50% sulfuric acid obtained by adding 280 mL of sulfuric acid (ρ ₂₀ = 1.84 g / mL) to 720 mL of distilled water is equivalent to twice the normal amount. Was added, and the test piece was immersed in the boiling solution for 24 hours. Then, instead of the corrosion weight loss measurement having a low value and low detectability, the cross section of the test piece was observed with a microscope, and the maximum grain boundary erosion depth after 24 hours was measured.

  As shown in FIG. 1, as the mNBar value increases, the IGP value decreases. When the mNBar value exceeds 8, the IGP value decreases significantly. The mNBar value shows a good correlation with the IGP value in both of the existing 600 series and 690 series, and it can be seen that the mNBar value is effective as an evaluation index of the SCC resistance of the Ni-based weld metal. It can be said that a Ni-base weld metal having an mNBar value of 8 or more has excellent SCC resistance. According to the extrapolation using the formula (I), it can be said that the range of selection of the chemical composition can be expanded.

FIG. 2 is a diagram showing the relationship between the stabilization parameter (mNBar value) of Ni-based weld metal and the maximum crack depth / Vickers hardness.
In FIG. 2, the left axis is the maximum crack depth (length) measured by subjecting the Ni-base weld metal to a constant strain bending bending test (CBB test) with a gap, and the right axis is the Ni-base weld. The Vickers hardness (Hv5) of the metal and the horizontal axis indicate the mNBar value of the Ni-based weld metal. The plot of (1) is the measurement result of Vickers hardness, the plot of (、) is the measurement result of the maximum crack depth of IGSCC, and the plot of (○) is the measurement result of the maximum crack depth of TGSCC. The test materials are all weld metals equivalent to 690 series alloys.

  In the CBB test, a bead-on test piece of each test material having a width of 2 mm, a length of 10 mm and a thickness of 0.2 mm was prepared and subjected to an accelerated corrosion test. The test piece was attached to a test jig by providing a gap with graphite wool and imparting a bending strain of 1%. It is assumed that the dissolved oxygen concentration of the reactor water is about 200 ppb, the hydrogen peroxide concentration is 100 to 1000 ppb, and the conductivity is 0.1 μS / cm. Therefore, the corrosion conditions are: dissolved oxygen concentration: about 40 ppm, hydrogen peroxide concentration: about 20 ppm, sulfate ion concentration: about 100 ppm, conductivity: 20 μS / cm or less, and high-temperature high-pressure pure water circulated under these conditions. The test piece was immersed therein for up to 4000 hours. Then, the cross section of the test piece was observed under a microscope, and the maximum crack depth was measured.

  As shown in FIG. 2, it can be seen that the larger the mNBar value, the smaller the maximum crack depth of the IGSCC. In general, if the maximum crack depth of the IGSCC is 50 μm or less, the crack does not easily propagate further, and thus it can be evaluated that the IGSCC has excellent SCC resistance. On the other hand, the maximum crack depth of the IGSCC in the figure becomes 50 μm or less when the mNBar value is 8 or more. On the other hand, it can be seen that when the mNBar value is large, the hardness increases due to precipitation hardening, and when the mNBar value is about 35, the hardness becomes about 300 Hv and TGSCC occurs.

  According to the results shown in FIG. 2, the preferable range of the mNBar value of the Ni-based weld metal is a range that satisfies the following equation (II) from the viewpoint of preventing both IGSCC and TGSCC.

FIG. 3 is a diagram showing the relationship between the additive element concentration of the Ni-based weld metal and Vickers hardness / SCC mode.
In FIG. 3, the vertical axis represents the Vickers hardness (Hv5) of the Ni-based weld metal test piece, and the horizontal axis represents the concentration (% by mass) of the three additional elements (Nb, Ta, and Ti) of the Ni-based weld metal. Are calculated (the calculated values of the partial variables of the formula (I)). The plot of ● shows the results of the test material in which the generation and progress of IGSCC was confirmed but the TGSCC did not occur among the weld metals equivalent to the 690 series alloy. The plot of ○ shows the welding of the existing 600 series alloy. As a result of the test material which was a metal and IGSCC generation and progress were confirmed but TGSCC did not occur, the plot of ◆ shows the result of the test material in which TGSCC was generated among the weld metals equivalent to 690 series alloy. It is.

  As shown in FIG. 3, when the concentration of the additional elements (Nb, Ta, and Ti) in the Ni-base weld metal increases, the hardness increases due to precipitation hardening, and [% Nb] + [% Ta] +2 [% Ti ] It was confirmed that TGSCC was generated in the 690 system with $ 5.3. On the other hand, in the case of [% Nb] + [% Ta] +2 [% Ti] ≒ 4.32 and the hardness of about 600 Hv of about 300 Hv or about 690 Hg with smaller hardness, although IGSCC occurs, It did not occur for TGSCC.

  According to the results shown in FIG. 3, the calculated value ([% Nb] + [% Ta] +2 [% Ti]) obtained by adding the concentrations of the three types of additional elements (Nb, Ta and Ti) of the Ni-base weld metal is preferable. The range satisfies the following formula (III) from the viewpoint of suppressing precipitation hardening and preventing TGSCC.

FIG. 4 is a diagram showing the relationship between the Cr concentration of the Ni-based weld metal and the crack growth rate.
In FIG. 4, the vertical axis represents the crack growth rate (CGR) value of the Ni-base weld metal (182 weld alloy, 82 weld alloy), and the horizontal axis represents the Cr concentration (% by mass) of the Ni-base weld metal. Is shown. FIG. 4 cites test results of known literature (GA Young, et al., 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, 2007, p808-831).

The plots of □ are measured under corrosion conditions of 680 ° F., dissolved hydrogen concentration: 20 scc / kg-H ₂ , stress intensity factor: 45 ksi (in) ^0.5 , test temperature: 360 ° C., and test time: 221 days. The plot of ○ indicates a stress intensity factor of 40 ksi (in) ^0.5 , a test temperature of 360 ° C., and a test time under corrosion conditions of 640 ° F., dissolved hydrogen concentration: 20 scc / kg-H _2. : Results measured for 95 days. The plot with the arrow indicates that no SCC occurred.

  As shown in FIG. 4, it is reported in the publicly known literature that as the Cr concentration increases, the crack growth rate of SCC decreases. The crack growth rate rapidly decreases when the Cr concentration exceeds 19% by mass, and becomes substantially constant when the Cr concentration exceeds 24% by mass. It can be seen that a sufficient level of SCC resistance can be obtained when the Cr concentration is 24% by mass or more even under the severe conditions shown by the plots of □.

  According to the results shown in FIG. 4, the preferable range of the Cr concentration ([% Cr]) of the Ni-based weld metal is a range satisfying the following formula (IV) from the viewpoint of suppressing the progress of SCC and increasing the corrosion resistance. is there.

FIG. 5 is a ternary phase diagram of Fe—Cr—Ni at 800 ° C. FIG. 6 is a ternary phase diagram at 650 ° C. of Fe—Cr—Ni. FIG. 7 is a graph showing the relationship between the upper limit of the Cr concentration at which the austenite single phase becomes stable and the temperature.
As shown in FIG. 5 and FIG. 6, when the concentration of Cr, which is a ferrite stabilizing element, is high, the Ni—Cr—Fe alloy becomes a mixed phase of an austenite phase (γ phase) and a ferrite phase (α phase). , Toughness and corrosion resistance decrease. Therefore, the Cr concentration of the Ni-based weld metal is preferably adjusted to be equal to or less than the upper limit at which the austenite single phase becomes stable.

  A common weld metal equivalent to a 690 series alloy has an Fe concentration in the range of 2 to 17% by mass. Considering the lower limit of 2% by mass of the Fe concentration, the upper limit of the Cr concentration when the austenite single-phase structure is formed stably can be determined from the ternary phase diagram. At 800 ° C. shown in FIG. 5, as shown by the solid line, when the Fe concentration is 2% by mass, the upper limit value of the Cr concentration is 35.4 from the intersection with the phase boundary between the α ′ + γ region and the γ region. % By weight is derived. At 650 ° C. shown in FIG. 6, 34.4 mass% is similarly derived as the upper limit of the Cr concentration.

  When the relationship between the upper limit of the Cr concentration and the temperature is linearly approximated, a regression line is obtained as shown in FIG. Since the maximum temperature of the reactor water is assumed to be about 300 ° C., when interpolated into the regression line, about 32% by mass is derived as the upper limit value of the Cr concentration necessary for stabilizing the austenite single phase.

  According to the results shown in FIG. 7, the preferable range of the Cr concentration ([% Cr]) of the Ni-based weld metal is a range that satisfies the following equation (V) from the viewpoint of obtaining a single-phase structure of an austenite phase.

  Based on the above formulas (II), (III), (IV) and (V), the Ni-base weld metal has a Cr concentration of 24 to 24 from the viewpoint of having excellent SCC resistance and a single-phase structure of austenite phase. 32% by mass, and the calculated value ([% Nb] + [% Ta] +2 [% Ti]) of the sum of the concentrations of the three types of additional elements (Nb, Ta and Ti) is limited to 4.32 or less. It can be said that it is preferable.

  The Ni-based weld metal according to the present embodiment may include one or more of Nb, Ta, and Ti as additional elements. Further, other elements may be contained in addition to Ni, Cr, Fe and three kinds of additive elements (Nb, Ta, and Ti). Hereinafter, the chemical components that can be added to the Ni-based weld metal and the chemical components that are allowed to be contained in the Ni-based weld metal will be specifically described.

(Carbon: C)
C stabilizes the austenite phase and contributes to improvement in hardness, strength and the like. However, if the amount of C is too large, SCC resistance and toughness decrease due to excessive precipitation of carbides. Therefore, the amount of C is preferably 0.01% by mass or more. Further, the content is preferably 0.15% by mass or less, more preferably 0.10% by mass or less, further preferably 0.06% by mass or less, and further preferably 0.04% by mass or less. When C is added, the amount of C is 0.02% by mass or more, preferably 0.10% by mass or more, more preferably 0.15% by mass or more and 0.3% by mass or less, preferably 0.25% by mass or less. Mass% or less, more preferably 0.20 mass% or less.

(Manganese: Mn)
Mn acts as a deoxidizing agent, stabilizes the austenite phase, and contributes to improvement in hot cracking resistance and the like. However, if the amount of Mn is too large, ductility and toughness decrease. Therefore, the Mn content is preferably 4.0% by mass or less, more preferably 3.0% by mass or less, still more preferably 2.0% by mass or less, further preferably 1.5% by mass or less, and 1.0% by mass. The following are more preferred. When Mn is added, the amount of Mn is 1.0% by mass or more, preferably 2.0% by mass or more, more preferably 3.0% by mass or more, and 5.0% by mass or less, preferably 4.0% by mass or less. % Or less.

(Iron: Fe)
Fe stabilizes the austenite phase and contributes to improvement in hardness, strength and the like. However, if the amount of Fe is too large, a σ phase, a Laves phase, etc. are precipitated, and ductility and toughness are reduced, and crack sensitivity is increased. Therefore, when adding another alloy element, the amount of Fe is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and still more preferably 2.5% by mass or more. Moreover, 7.0 mass% or less is preferable, 5.0 mass% or less is more preferable, and 3.0 mass% or less is still more preferable. When increasing the strength with Fe, the amount of Fe is preferably 7.0% by mass or more, more preferably 8.0% by mass or more, further preferably 9.0% by mass or more, and further preferably 10.0% by mass or more. preferable. Moreover, 17.0 mass% or less is preferable, 14.0 mass% or less is more preferable, and 11.0 mass% or less is still more preferable.

(Phosphorus: P)
P is an inevitable impurity and increases the susceptibility to hot cracking including solidification cracking, liquefaction cracking and the like. Therefore, the amount of P is preferably 0.04% by mass or less, more preferably 0.03% by mass or less, and still more preferably 0.02% by mass or less.

(Sulfur: S)
S is an inevitable impurity and increases the susceptibility to hot cracking including solidification cracking, liquefaction cracking and the like. Therefore, the S content is preferably equal to or less than 0.02% by mass, more preferably equal to or less than 0.015% by mass, and still more preferably equal to or less than 0.01% by mass.

(Silicon: Si)
Si acts as a deoxidizing agent and contributes to improvements in oxidation resistance, wettability, and the like. However, if the amount of Si is too large, toughness, corrosion resistance, and weldability are reduced, and crack susceptibility is increased. Therefore, the amount of Si is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and still more preferably 0.5% by mass or less. When Si is added, the amount of Si is 1.0% by mass or more, preferably 1.5% by mass or more, more preferably 2.0% by mass or more, and 5.0% by mass or less, preferably 4.0% by mass or less. Mass% or less, more preferably 3.0 mass% or less.

(Copper: Cu)
Cu stabilizes the austenite phase and contributes to improvements in hardness, strength, and the like. However, if the amount of Cu is too large, ductility, corrosion resistance, and weldability deteriorate. Therefore, the amount of Cu is preferably equal to or less than 1.0% by mass, more preferably equal to or less than 0.5% by mass, and still more preferably equal to or less than 0.3% by mass. When adding Cu, the amount of Cu is 0.5% by mass or more, preferably 1.0% by mass or more, more preferably 1.5% by mass or more, and 3.0% by mass or less, preferably 2.0% by mass or less. % Or less.

(Cobalt: Co)
Co stabilizes the austenite phase and contributes to improvement in weldability and the like. However, if the amount of Co is too large, the σ phase is precipitated and the ductility and toughness are reduced, or the material is activated in a radiation environment. Therefore, the amount of Co is preferably 5.0% by mass or less, more preferably 3.0% by mass or less, and still more preferably 1.0% by mass or less. When Co is added, the amount of Co can be 27.0% by mass or more, preferably 28.0% by mass or more, and 32.0% by mass or less, preferably 31.0% by mass or less.

(Aluminum: Sol / Al)
Al acts as a deoxidizing agent and contributes to improvement of oxidation resistance and the like. However, if the amount of Al is too large, toughness and weldability decrease. Therefore, the amount of Al is preferably 1.5% by mass or less, more preferably 1.3% by mass or less, still more preferably 1.1% by mass or less, further preferably 0.8% by mass or less, and 0.6% by mass. The following are more preferred. When Al is added, the amount of Al is 1.0% by mass or more, preferably 1.5% by mass or more, more preferably 2.0% by mass or more, and 5.0% by mass or less, preferably 4.0% by mass or less. Mass% or less, more preferably 3.0 mass% or less.

(Titanium: Ti)
Ti precipitates carbides to prevent sensitization, acts as a deoxidizing agent, and contributes to improvement of hot cracking resistance and the like. However, if the amount of Ti is too large, excessive carbides are precipitated, and the toughness and corrosion resistance are reduced, the weldability is reduced, and the crack sensitivity is increased. Therefore, when adding Ti, the amount of Ti is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, still more preferably 0.4% by mass or more, and still more preferably 0.6% by mass or more. , 1.0% by mass or more is more preferable. Further, the content is preferably 2.16% by mass or less, more preferably 2.0% by mass or less, and still more preferably 1.5% by mass or less.

(Niobium, tantalum: Nb, Ta)
Nb and Ta precipitate carbides to prevent sensitization and also contribute to improvement of hot cracking resistance and the like. However, if the amount of Nb or Ta is too large, excessive carbides and intermetallic compounds are precipitated, and the toughness and corrosion resistance are reduced, and the crack sensitivity is increased. Therefore, the total of the Nb amount and the Ta amount is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, still more preferably 0.5% by mass or more, and still more preferably 1.0% by mass or more, It can be 1.5% by mass or more, 2.0% by mass or more. The total of the Nb amount and the Ta amount is preferably 4.32% by mass or less, more preferably 4.0% by mass or less, still more preferably 3.5% by mass or less, and 3.0% by mass or less, 2.5% by mass or less. % Or less.

(Molybdenum: Mo)
Mo contributes to improvements in hardness, strength, oxidation resistance, corrosion resistance, and the like. However, if the Mo content is too large, segregation may occur, resulting in reduced toughness and corrosion resistance and increased cracking susceptibility. Therefore, the Mo amount is preferably 2.0% by mass or less, more preferably 1.0% by mass or less, further preferably 0.7% by mass or less, and still more preferably 0.5% by mass or less. When Mo is added, the amount of Mo is 1.0% by mass or more, preferably 2.0% by mass or more, more preferably 3.0% by mass or more, still more preferably 4.0% by mass or more, and 7.0% by mass. It can be 0% by mass or less, preferably 6.0% by mass or less, more preferably 5.0% by mass or less.

(Other added elements)
Other additive elements may be added to the Ni-base weld metal depending on the purpose in addition to the above elements. Examples of the other additive elements include W, Mg, Ca, V, Zr, B, and N. It is preferable that the total concentration of the other additive elements is 0.50% by mass or less. The concentrations of B and N are each preferably 0.05% by mass or less, more preferably 0.03% by mass or less, and still more preferably 0.01% by mass or less.

(Inevitable impurities)
The remainder of the Ni-based weld metal excluding the intentionally added elements consists of Ni and unavoidable impurities. The unavoidable impurities include impurities mixed in the raw material of the Ni-base weld metal, impurities mixed in the manufacturing process of the Ni-base weld metal, and impurities entering from the material to be welded. In addition to the above-mentioned elements which are not added, Sn, Pb, O and the like can be mentioned. Inevitable impurities are permitted to be contained if the concentration of each element is 0.05% by mass or less and the total concentration is 0.50% by mass or less.

  Next, the difference between the chemical composition of the Ni-based weld metal according to the present embodiment and the chemical composition of a general weld metal equivalent to a 690 alloy will be described.

FIG. 8 is a diagram showing the relationship between the Cr concentration of the Ni-based weld metal and the concentration of the added element.
In FIG. 8, the vertical axis represents the calculated value (calculated value of the partial variables in the formula (I)) obtained by adding the concentrations (% by mass) of the three types of additional elements (Nb, Ta, and Ti) of the Ni-based weld metal. The axis indicates the Cr concentration (% by mass) of the Ni-based weld metal. The dashed line is obtained by substituting [% C] = 0.01 and mNBar value = 8 into the formula (I), and the solid line is obtained by substituting [% C] = 0.01 and mNBar value = 35 into the formula (I). As a result, the one-dot chain line is obtained by substituting [% C] = 0.555 and the mNBar value = 8 into the mathematical formula (I). As a result, the two-dot chain line is [% C] = 0.55 and the mNBar value into the mathematical formula (I). = 35.

A general weld metal equivalent to a 690 series alloy has a C concentration in the range of 0.25% by mass or less, and is often suppressed to a low C amount of the order of 10 ^-2 % by mass. On the other hand, a preferable range of the mNBar value of the Ni-based weld metal is a range of 8 to 35 as shown in Expression (II). By substituting these numerical values into Equation (I), the respective curves shown in FIG. 8 are obtained. It can be said that these curves show the allowable concentration ranges that can be employed for securing the SCC resistance with respect to the Cr concentration and the additive element concentration.

  For example, in a Ni-based weld metal having a C concentration of 0.01% by mass or more and an mNBar value of a minimum value of 8 or more, the concentration in a region above the broken line in FIG. 8 is allowed. On the other hand, in the case of a Ni-based weld metal having a C concentration of 0.055% by mass or less and an mNBar value of 35 or less, the concentration in the region below the two-dot chain line is allowed. In the case of a Ni-based weld metal having an mNBar value of 35 or less and a higher C concentration, the concentration in a region above the two-dot chain line is allowed.

  As a general weld metal equivalent to a 690 series alloy, a weld metal equivalent to a 690 series alloy is defined in American Society of Mechanical Engineers (ASME) standards. Patent Literatures 1 to 4 disclose a weld metal equivalent to a 690-based alloy and a 690-based alloy. Table 1 shows these specifications and the standard values of chemical compositions and literature values shown in the literature.

FIG. 9 is a diagram showing the difference between the chemical composition of the Ni-based weld metal according to the present invention and the chemical composition of a general weld metal.
In FIG. 9, the vertical axis represents the calculated value (calculated value of the partial variable of the formula (I)) obtained by adding the concentrations (% by mass) of the three types of additional elements (Nb, Ta, and Ti), as in FIG. 8. The horizontal axis shows the Cr concentration (% by mass). The bold frame in the drawing indicates a region that satisfies the formulas (II), (III), (IV), and (V) in the region above the broken line in FIG. The hatched area corresponds to the standard value of a general weld metal equivalent to the 690 series alloy, and the shaded area corresponds to the literature value of a general weld metal equivalent to the 690 series alloy. (See Table 1).

  In addition, with respect to these common weld metals corresponding to the 690 series alloy (see Table 1), a calculated value obtained by adding the concentrations (% by mass) of three kinds of additional elements (Nb, Ta and Ti) and a Cr concentration (% by mass) ) And the codes of the regions in FIG. 9 are shown in Table 2.

  In FIG. 9, among the regions above the broken line in FIG. 8, the regions satisfying the formulas (II), (III), (IV), and (V) (the thick frame in FIG. 9) have excellent SCC resistance. It can be a Ni-based weld metal having an austenitic single phase structure. Among these regions, a region that does not overlap with existing standard values and literature values can be said to be a region in which the degree of freedom in selecting a chemical composition is extended more than before.

  That is, as shown in the thick line frame of FIG. 9, as one preferred form of the Ni-based weld metal according to the present embodiment, Cr: 24% by mass or more and less than 26% by mass, and [% Nb] + [% Ni-based weld metal in which Ta] +2 [% Ti] is 0.42 or more and 4.32 or less.

  Further, as another preferable embodiment of the Ni-based weld metal according to the present embodiment, Cr is not less than 26% by mass and less than 27% by mass, and [% Nb] + [% Ta] +2 [% Ti] is 1.%. A Ni-based weld metal having a value of more than 5 and less than 2.0 is exemplified. In addition, Cr: not less than 26% by mass and less than 27% by mass, and Ni-based welding in which [% Nb] + [% Ta] +2 [% Ti] exceeds 3.6 excluding the standard value and is 4.32 or less. Examples include metals and Ni-based weld metals that exceed 4.2 and not more than 4.32 excluding the standard value and the literature value.

  Further, as another preferable embodiment of the Ni-based weld metal according to the present embodiment, Cr is more than 31% by mass and not more than 31.5% by mass, and [% Nb] + [% Ta] +2 [% Ti] is And a Ni-based weld metal exceeding 3.0 and not more than 4.32 excluding only the standard value.

  Further, as another preferable embodiment of the Ni-based weld metal according to the present embodiment, Cr is more than 31.5% by mass and 32% by mass or less, and [% Nb] + [% Ta] +2 [% Ti] is And a Ni-based weld metal of 4.32 or less excluding the standard value, and a Ni-based weld metal of more than 3.1 and 4.32 or less excluding the standard value and the literature value.

  When the Ni-based weld metal according to the present embodiment described above is in the form of a weld metal or a weld metal, the entire welded portion only needs to satisfy the above chemical composition. In the case of the form of a welding material such as a solubilized material, the welding material itself, the core material of the welding material coated with a flux or the like, or the entire welded portion formed using the welding material has the chemical composition described above. What is necessary is just to satisfy.

  The welding method used for the Ni-based weld metal according to the present embodiment is not particularly limited. As a welding method, for example, various welding methods such as TIG welding, MIG welding, covered arc welding, submerged arc welding, electroslag welding, electron beam welding, and laser welding can be used. The joint type, groove shape, heat input, welding speed, welding position, bead shape, and the like can be appropriately adjusted according to the application, chemical composition, and the like.

  The material to be welded by the Ni-based weld metal according to the present embodiment is not particularly limited. Examples of the material to be welded include a nickel-based alloy, a low-carbon steel, a low-alloy steel, and a stainless steel. When the Ni-base weld metal is used for joining the materials to be welded, the materials to be welded may be welded to each other with the same material or different materials.

  According to the above-described Ni-based weld metal according to the present embodiment, it contains 24 to 32% by mass of Cr, and [% Nb] + [% Ta] +2 [% Ti] is limited to 4.32 or less. A single-phase structure of an austenite phase having good toughness and ductility is obtained, and both IGSCC and TGSCC are suppressed. Further, since the chemical composition is based on mNBar, which is effective as an evaluation index of SCC resistance, a chemical composition having excellent SCC resistance can be more accurately selected. In addition, when selecting a chemical composition, the degree of freedom in freely selecting the concentration of each component is expanded. Therefore, a Ni-based weld metal having excellent SCC resistance and capable of stably maintaining the soundness of a welded portion exposed to a corrosive environment over a long period of time is obtained.

  Next, a welded structure using the Ni-based weld metal according to the present embodiment will be described.

  The welded structure according to the present embodiment has a welded portion for joining the materials to be welded, and at least a part of the welded portion is formed of the Ni-based weld metal. The welded portion of the welded structure is formed of a Ni-based weld metal in the form of a weld metal or a weld metal, or a Ni-based weld metal in the form of a weld material, with 24 to 32% by mass of Cr and Nb, Ta, and Ti. And a chemical composition in which [% Nb] + [% Ta] +2 [% Ti] is limited to 4.32 or less.

  In the welded structure according to the present embodiment, the welded portion formed of the Ni-based weld metal may be subjected to machining or grinding to remove the hardened layer on the surface. Further, the welded portion may be subjected to a stabilizing heat treatment after being subjected to machining or grinding. Further, the welded portion may be subjected to a peening process or a polishing process after being subjected to a mechanical process or a grinding process. Examples of the peening processing include shot peening, water jet peening, and laser peening.

  The welded structure according to the present embodiment is suitably used as a structural material for nuclear facilities. In particular, it is suitable as an internal structure of a nuclear reactor, and is preferably used for a portion where high-temperature and high-pressure reactor water comes into contact with liquid. The welded portion using the Ni-based weld metal may be a joined portion for joining materials to be welded, or may be a build-up welded portion formed for the purpose of imparting functionality or repairing.

  Specific examples of the welded portion using the Ni-based weld metal include a bottom portion of a reactor pressure vessel through which a neutron instrumentation detection tube, a control rod guide tube, and the like penetrate, a lower mirror portion of the reactor pressure vessel, and a lower mirror of the reactor pressure vessel. Core, shroud, shroud support cylinder, shroud support leg, shroud support plate, etc., core shroud support, other nozzles for measurement, nozzle at feed water inlet, recirculated water For example, a nozzle portion at the entrance may be used.

  According to the above-described welded structure of the present embodiment, a single-phase structure of an austenitic phase having good toughness and ductility is formed, and a welded portion is formed of a Ni-based weld metal in which both IGSCC and TGSCC are suppressed. Therefore, it is possible to maintain the soundness of the piping, structural materials, equipment, and the like of the nuclear reactor equipment exposed to a high radiation dose for a long period of time. A welded portion in which stress corrosion cracking is stably prevented is obtained even in a portion where high-temperature and high-pressure reactor water of about 300 ° C. comes into contact with liquid.

  As described above, the embodiments of the Ni-based weld metal according to the present invention and the welded structure using the same have been described. However, the present invention is not limited to the above-described embodiments, and does not depart from the technical scope. Various modifications are included. For example, the above embodiments are not necessarily limited to those having all the configurations described above. The composition, execution method, application, etc. of the Ni-base weld metal other than the main elements are not necessarily limited to the above embodiments.

Claims (6)

  Cr: not less than 24% by mass and less than 26% by mass, containing one or more of Nb, Ta and Ti, the concentration of Nb (% by mass) is [% Nb], and the concentration of Ta (% by mass) is [% A Ni-based weld metal in which [% Nb] + [% Ta] +2 [% Ti] is 0.42 or more and 4.32 or less when the concentration (mass%) of Ta] and Ti is [% Ti].   Cr: not less than 26% by mass and less than 27% by mass, contains one or more of Nb, Ta and Ti, and has a Nb concentration (% by mass) of [% Nb] and a Ta concentration (% by mass) of [% When the concentration (% by mass) of [Ta] and Ti is [% Ti], [% Nb] + [% Ta] +2 [% Ti] is more than 1.5 and less than 2.0. .   Cr: not less than 26% by mass and less than 27% by mass, contains one or more of Nb, Ta and Ti, and has a Nb concentration (% by mass) of [% Nb] and a Ta concentration (% by mass) of [% When the concentration (% by mass) of [Ta] and Ti is [% Ti], [% Nb] + [% Ta] +2 [% Ti] is a Ni-based weld metal exceeding 4.2 and not more than 4.32. .   Cr: more than 31.5% by mass and not more than 32% by mass, containing one or more of Nb, Ta and Ti, the concentration (% by mass) of Nb is [% Nb], and the concentration of Ta (% by mass). Where [% Nb] + [% Ta] +2 [% Ti] is greater than 3.1 and less than or equal to 4.32, where [% Ta] and Ti concentration (% by mass) are [% Ti]. Base weld metal.   A welded structure in which a weld is formed with the Ni-based weld metal according to any one of claims 1 to 4.   The welded structure according to claim 5, which is an internal structure of a nuclear reactor.

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