JP2006183082A - WELD JOINT OF HIGH-Cr STEEL AND WELDING MATERIAL THEREFOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a weld joint of which the weld metal has superior stress-corrosion cracking resistance, and to provide a welding material suitable for use in producing it. <P>SOLUTION: The weld joint is made from an austenitic weld metal which comprises, by mass%, 0.03% or less C, 1.0% or less Si, 0.1-2.0% Mn, 22-28% Cr, more than 12% but 22% or less Ni, 0-3% Mo, 0.001-0.15% N and the balance Fe with impurities in which P is 0.04% or less and S is 0.03%, while further controlling a value FP shown by the following expression (1): FP=Ni+30C+20N+0.5 Mn-1.1Cr-1.32Mo-1.65Si+9, (wherein symbols of elements show contents (mass%) of each element), into a range of -3 to 0. The weld joint is suitable for use in a high-temperature pure-water environment of a nuclear power plant. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a welded joint such as a pipe used in a high-temperature pure water environment such as a light water reactor for nuclear power generation, and a welded joint whose weld metal is excellent in resistance to stress corrosion cracking, and the welded joint. The present invention relates to a welding material suitable for manufacturing.

  In order to prevent hot cracking during welding in austenitic stainless steel, it is known to increase the ratio of δ ferrite formed in the weld metal. However, when δ ferrite is generated, there is a concern that the low temperature toughness due to the δ ferrite itself, the distribution of component elements, the deterioration of corrosion resistance due to segregation, and the like.

  On the other hand, Patent Document 1 (Japanese Patent Laid-Open No. 7-314178) discloses a weld metal in which high temperature cracking is suppressed without generating δ ferrite in the weld metal, and low temperature toughness and high temperature embrittlement are improved. The resulting welding material for austenitic stainless steel is disclosed. This welding material obtains the above effect by limiting the sum of the Ni equivalent and Cr equivalent of the weld metal and containing an appropriate amount of Mn.

An invention relating to improvement in resistance to stress corrosion cracking (hereinafter referred to as SCC) of weld metal of austenitic stainless steel is disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 05-65530). In the invention of this patent document 2, the surface modification that can improve the stress corrosion cracking resistance of the weld heat affected zone in the light water reactor plant, in particular, withstand the corrosion due to low temperature sensitization after the reforming treatment and prevent the stress corrosion cracking during the operation period. It aims to provide quality processing technology. Then, the surface of the Fe-based or Ni-based alloy material is irradiated with a laser beam having an irradiation energy density of 1.0 to 100 J / mm, and cooled at a cooling rate of 10 ³ to 10 ⁷ ° C / s. The effect of improving the SCC resistance is obtained by forming a melt-solidified layer having a cell structure in the range of ˜3.0 μm.

  However, the invention of Patent Document 2 is directed to SCC caused by grain boundary sensitization accompanying Cr carbide precipitation, as disclosed in Patent Document 2 itself. Therefore, for example, the technique of Patent Document 2 is not a measure for improving SCC resistance against SCC caused by non-sensitization that may occur in a high-temperature pure water environment such as a pipe for nuclear power generation. This is because the mechanism of SCC generation is different.

  Furthermore, Patent Document 3 (Japanese Patent Laid-Open No. 9-137255) discloses a highly corrosion-resistant austenitic stainless steel welding material excellent in welding workability such as piping for chemical industry equipment and nuclear fuel reprocessing equipment. However, the welding material is intended to increase the uniformity of the weld bead and the ability to form the back bead, and is not intended to improve the SCC resistance of the weld in a high-temperature pure water environment.

JP 7-314178 A JP-A-5-65530 Japanese Patent Laid-Open No. 9-137255

  The object of the present invention is to improve stress corrosion cracking resistance, which is a characteristic required for welded structures such as pipes used in high-temperature pure water environments such as light water reactors for nuclear power generation, in austenitic weld metals. Is to prevent. Specifically, it is an object of the present invention to provide a welded joint in which the weld metal is excellent in resistance to stress corrosion cracking, and a welding material suitable for use in producing the joint.

  First, knowledge that is the basis of the present invention will be described. Hereinafter, “%” regarding the content of the alloy component means “% by mass”.

  In the austenitic steel weld metal typified by so-called 18-8 series stainless steel, carbide precipitates at the grain boundary due to the thermal cycle during welding, so a so-called Cr-deficient region with a low Cr concentration is present near the grain boundary. This results in increased SCC sensitivity. Therefore, conventionally, measures have been taken to reduce carbon that causes carbides to 0.03% or less, but SCC may occur even in weld metals in which Cr carbides are not formed, that is, no Cr-deficient region is generated.

  As a result of research, the present inventors have found that SCC also occurs in the weld metal due to segregation of impurity elements, particularly P and S, during solidification, and causes the occurrence of SCC by precise control of the solidification process described below. It was found that solidification segregation can be mitigated, and as a result, SCC resistance is greatly improved.

  (1) In the solidification process of the weld metal, P and S are usually more easily concentrated in the liquid phase than in the solid phase, and the concentration is superimposed as the solidification progresses. In the liquid phase at the end of the solidification process, the weld metal The density is on the order of 100 to 1000 times the average density. After completion of solidification, a weld metal that is used without applying processing such as rolling or heat treatment is used in a state where high-concentration impurities exist in the solidified grain boundary.

  (2) Grain boundaries having a high impurity concentration are easily dissolved in a corrosive environment, resulting in an increased stress corrosion cracking sensitivity.

  (3) Concentration of impurities in the liquid phase is relatively reduced when the solid phase is an austenite phase, but is still concentrated, even when the solid phase is a ferrite phase.

  (4) However, by first solidifying the ferrite phase to suppress the concentration of impurities in the liquid phase, the resulting concentration is still austenite crystallized from the liquid phase at a temperature higher by 10 ° C. than the solidification end temperature. The concentration of impurities at the solidified grain boundary can be significantly suppressed by physically changing the concentration position of impurities by moving the solid-liquid interface. Even if the austenite phase crystallizes in the middle of the solidification process, if the difference between the temperature at that time and the solidification completion temperature is less than 10 ° C, in other words, just before completion of solidification, the solid-liquid interface does not move. Since it is sufficient and the effect of suppressing the concentration of impurities is small, it is important to control the austenite phase crystallization timing during the solidification process.

  As a concrete means for realizing the above technical knowledge, adjustment of alloy elements is most effective. As a result of repeated experimental studies, the present inventors have confirmed that the value of FP represented by the following formula (1) may be within an appropriate range, and completed the present invention.

  The gist of the present invention is the following weld joint 1 and the following weld material 2.

  1. Weld metal in mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.1-2.0%, Cr: 22-28%, Ni: more than 12% to 22%, Mo: 0-3 % And N: 0.001 to 0.15%, the balance is Fe and impurities, P in the impurities is 0.04% or less, S is 0.03%, and the value of FP represented by the following formula (1) is A welded joint characterized by being an austenitic weld metal in the range of -3 to 0.

FP = Ni + 30C + 20N + 0.5Mn−1.1Cr−1.32Mo−1.65Si + 9 (1)
In addition, the element symbol in said (1) Formula shows content (mass%) of each element.

  This welded joint is suitable as a joint for a structure used in a high-temperature pure water environment of a nuclear power generation facility.

  2. In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.1-2.0%, Cr: 22-28%, Ni: more than 12% to 22%, Mo: 0-3% and N: 0.001 to 0.15% is contained, the balance is made of Fe and impurities, P in the impurities is 0.04% or less, S is 0.03%, and the value of FP represented by the following formula (1) is -3 to 0 An austenitic welding material characterized by being in a range up to.

FP = Ni + 30C + 20N + 0.5Mn−1.1Cr−1.32Mo−1.65Si + 9 (1)
In addition, the element symbol in said (1) Formula shows content (mass%) of each element.

  This welding material is suitable for producing the weld joint described in 1 above.

  First, the FP value represented by the above equation (1) will be described.

  When the value of FP exceeds 0, solidification of the weld metal starts in the austenite phase. On the other hand, when the value of the formula (1) is less than −3, solidification starts in the ferrite phase, but crystallization of the austenite phase in a state higher by 10 ° C. or more than the end temperature of solidification does not occur. Therefore, the present inventors are a new finding, "segregation of impurities at the solidified grain boundary by starting solidification in the ferrite phase and crystallizing the austenite phase at a temperature higher than the solidification end temperature by 10 ° C or higher. In order to realize a technique of “to avoid SCC and improve SCC resistance”, it is essential that the value of FP is −3 or more and 0 or less. A more preferred FP value is a value in the range of not less than −2.5 and not more than −0.5.

  Next, the effect of the component which comprises a weld metal and the welding material for producing it, and the reason for limitation of content are demonstrated.

C: 0.03% or less C contributes to stabilization of the austenite phase as an austenite forming element. However, when the content is excessive, carbides are formed and the corrosion resistance is deteriorated. In particular, the weld metal is highly susceptible to intergranular corrosion due to the precipitation of Cr carbides, that is, it is sensitized and the corrosion resistance is greatly deteriorated. Therefore, the C content is 0.03% or less. In order to prevent sensitization, it is effective to make the C content as low as possible, and the more desirable C content is less than 0.02%.

Si: 1.0% or less
Si is used as a deoxidizer, but if Si is excessively contained, the hot cracking susceptibility during welding increases, so the upper limit of its content is 1.0%. More desirable is 0.60% or less. In order to obtain the effect as a deoxidizer, the lower limit is preferably 0.1%.

Mn: 0.1-2.0%
Mn is added as a deoxidizer and contributes to the stability of the austenite phase. Moreover, it contributes also to the improvement of hot workability at the time of processing a welding material into a wire. Furthermore, it is an element that plays a major role in preventing hot cracking by fixing S during welding. In order to obtain these effects, the lower limit is made 0.1%. However, if it is excessively contained, sulfides preferentially concentrate on the surface of the weld metal, thereby lowering the corrosion resistance of the steel material and causing problems such as deterioration in welding workability and generation of fumes. Therefore, the upper limit of the Mn content is 2.0%. A more desirable upper limit is 1.5%.

Cr: 22-28%
Cr is an essential element for ensuring the corrosion resistance of the weld. When the Cr content is less than 22%, this effect cannot be sufficiently obtained in a severe corrosive environment. On the other hand, if the Cr content exceeds 28%, hot working when producing a welding material for obtaining a weld metal becomes difficult. Therefore, the appropriate range of Cr content is 22-28%. More preferred is 23 to 28%, and most preferred is 24 to 27%.

Ni: Over 12% to 22%
Ni is an important element that has a great influence on the morphology of the weld metal when it solidifies as an austenite phase forming element. As described above, steel containing 22% or more of Cr needs to contain more than 12% of Ni in order to stabilize the austenite phase. More preferred is 14% or more. The upper limit of the Ni content is determined by keeping the FP value expressed by the above formula (1) within the range of −3 to 0, which is mainly caused by the correlation with the Cr content, etc. From the viewpoint of crystallization, the upper limit is 22%.

Mo: 0-3%
Mo may not be contained. Mo is mainly effective for improving the pitting corrosion resistance, and contributes to the improvement of the SCC resistance by suppressing the pitting corrosion that can be the starting point of SCC. In order to obtain such an effect, the content is preferably 0.2% or more. However, since an intermetallic compound that causes a decrease in toughness precipitates when excessively contained, the upper limit was made 3%. A preferred range is 0.5-2.4%.

N: 0.001 to 0.15%
N is an element effective for stabilizing the austenite phase of the weld metal and at the same time increasing its strength. However, when it is excessively contained, the back bead forming ability is deteriorated. Furthermore, it combines with Cr in the steel to form Cr nitride, reducing the grain boundary corrosion resistance. Therefore, the N content is 0.001 to 0.15%. More desirable is 0.005 to 0.10%.
The weld metal constituting the welded joint of the present invention is composed of Fe and impurities in the balance other than the above components. Among impurities, it is necessary to suppress the upper limit of P and S in particular.

P: 0.04% or less
When P is present in a large amount, it causes an increase in weld cracking susceptibility and segregation at the solidified grain boundaries promotes SCC susceptibility. Therefore, the content is 0.04% or less.

S: 0.03% or less
S is an element useful for increasing the penetration depth of the molten pool and improving the back bead forming ability. However, when the content is excessive, the formed sulfide deteriorates the corrosion resistance, and causes a scratch when the welding material is processed into a wire. Furthermore, the hot cracking sensitivity at the time of welding becomes high. Therefore, the content is 0.03% or less. Desirable is 0.01% or less.

  The above is the chemical composition of the weld metal and the weld material constituting the weld joint of the present invention. However, it is important to adjust so that the FP value defined by the above formula (1) falls within the range from -3 to 0 within the range of each alloy component. This is because the excellent SCC resistance of the weld metal is ensured.

  The steel material (base material) constituting the welded joint of the present invention is, for example, austenitic stainless steel such as JIS SUS 304, SUS 304L, SUS 309, SUS 310, SUS 316 and SUS 316L, and their equivalent steel types. .

  As a welding method for forming the weld metal of the present invention, arc welding (covered arc welding, TIG, MIG, submerged arc welding, etc.) is preferable.

  After melting stainless steels of various chemical compositions in a vacuum high-frequency furnace, they are forged and rolled into 2 mm diameter wire rods, which are then used for buttering welding on the SUS 310 plate so that the influence of dilution of the base metal can be ignored. Then, TIG welding was performed thereon. In TIG welding, a weld metal was produced by laminating and welding with a heat input of 15 kJ / cm. The thickness of all weld metal parts was 25 layers and about 20 mm. The chemical composition of all the weld metal parts is shown in Table 1. In Table 1, the FP value defined by the equation (1) is also shown.

  An SCC test piece having a thickness of 2 mm, a width of 10 mm, and a length of 50 mm was sampled from the upper layer portion of the obtained weld metal by machining, and as shown in FIG. In order to form a gap as an SCC acceleration factor between the jig 2 and the graphite fiber wool 3, the bolt is sandwiched between steel mold jigs with a radius of curvature of 100 mm and pressed with a pressure press. The sample was clamped with 4 and fixed, given a gap and constant strain, immersed in an autoclave and subjected to the SCC test. The corrosive environment of the autoclave was pure water to which 30 ppb sulfate ions were added at 300 ° C., and the test time was 2000 hours.

  The surface of the test piece after the test was observed with a 10-fold stereo microscope to observe the presence or absence of cracks. The results are summarized in Table 1. A circle indicates that no SCC has occurred, and a cross indicates that an SCC has occurred.

  As shown in Table 1, no SCC was recognized from the signs A1 to A5, in which the weld metal satisfies the requirements defined in the present invention. On the other hand, SCC occurred in the code A6 having an FP value of less than −3 and the codes A7 and A8 having an FP value exceeding 0.

  According to the present invention, a weld joint in which the weld metal is excellent in SCC resistance in high-temperature pure water can be obtained. This weld joint can be produced using the welding material of the present invention. The present invention is particularly effective for the construction of nuclear power generation facilities.

It is a figure which shows the setting of the test piece in a stress corrosion cracking test, (a) is a front view, (b) is a side view.

Explanation of symbols

1: Test piece, 2: Jig, 3: Graphite fiber wool, 4: Bolt

Claims (4)

Weld metal in mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.1-2.0%, Cr: 22-28%, Ni: more than 12% to 22%, Mo: 0-3 % And N: 0.001 to 0.15%, the balance is Fe and impurities, P in the impurities is 0.04% or less, S is 0.03%, and the value of FP represented by the following formula (1) is A welded joint characterized by being an austenitic weld metal in the range of -3 to 0.
FP = Ni + 30C + 20N + 0.5Mn−1.1Cr−1.32Mo−1.65Si + 9 (1)
The element symbol in the above formula (1) indicates the content (% by mass) of each element.   The welded joint according to claim 1, which is for a welded structure used in a high-temperature pure water environment of a nuclear power generation facility. In mass%, C: 0.03% or less, Si: 1.0% or less, Mn: 0.1-2.0%, Cr: 22-28%, Ni: more than 12% to 22%, Mo: 0-3% and N: 0.001 to 0.15% is contained, the balance is Fe and impurities, P in the impurities is 0.04% or less, S is 0.03%, and the FP value represented by the following formula (1) is -3 to 0 An austenitic welding material characterized by being in the range up to.
FP = Ni + 30C + 20N + 0.5Mn−1.1Cr−1.32Mo−1.65Si + 9 (1)
The element symbol in the above formula (1) indicates the content (% by mass) of each element. The welding material of Claim 3 used for producing the welded joint of Claim 1 or Claim 2.

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