JP4215161B2 - Welded structure made of low thermal expansion coefficient alloy and welding material for low thermal expansion coefficient alloy - Google Patents

Description

  The present invention is a storage tank and pipes used for storage and transportation of low-temperature substances such as liquefied natural gas, so-called pipes and various devices attached to the pipes, which are joined together by circumferential welding. An alloy composed of an Fe—Ni-based low thermal expansion coefficient alloy and having a welded portion at least in part (in the present invention, these are collectively referred to as “welded structure made of low thermal expansion coefficient alloy”), and this low thermal expansion coefficient The present invention relates to a welding material for a low thermal expansion coefficient alloy excellent in manufacturability suitable for use in manufacturing an alloy welded structure.

  It is widely known as the Invar effect that an Fe—Ni alloy having a specific component ratio has a very small linear expansion coefficient. Typical examples thereof include Fe-36% Ni and Fe-42% Ni. These have been used in areas where expansion and contraction due to temperature changes is a problem, taking advantage of its low thermal expansion coefficient.

  Needless to say, it is desirable to use a welding material having a thermal expansion coefficient equivalent to that of the base material when a member made of an Fe—Ni low thermal expansion coefficient alloy is welded and assembled into a welded structure. Therefore, various types of welding materials for low thermal expansion coefficient alloys as shown below have been proposed.

  (a) In addition to Ni and Fe, C: 0.05 to 0.5%, Nb: 0.5 to 5%, optionally containing Mn, Ti, Al, Ce, Mg, etc. By making it, the welding material which improved the weld cracking resistance at the time of welding (patent document 1).

  (b) Ni: 30-45%, C: 0.03-0.3%, Nb: 0.1-3%, P: 0.015% or less, S: 0.005% or less, Si: 0.00. While limiting to 05-0.6%, Mn: 0.05-4%, Al: 0.05% or less, and O: 0.015% or less, the relationship between Nb and C is [% Nb] × [% C ] A welding material in which the solidification structure is made fine by defining ≧ 0.01 so that both reheat cracking resistance and toughness during multi-layer welding can be achieved (Patent Document 2).

  (c) By setting the contents of C and Nb to a value satisfying −0.01 [% Nb] + 0.04 ≦ [% C] ≦ −0.04 [% Nb] +0.40, the solidified structure is refined. In addition, while preventing reheat cracking during multi-layer welding, welding materials that enable circumferential welding in all positions by regulating the amounts of Si, Mn, Al, O, and S within a predetermined range (Patent Documents) 3).

  (d) Limiting the content of Nb or / and Zr in relation to the amount of S and the amount of Si, making the solidified structure complicated by the carbide, and achieving both weld crack resistance, toughness and SCC resistance of the weld metal By limiting the content of Nb or / and Zr in relation to the amount of C and the relationship between the amounts of Si, Mn, Al, O, and S in all positions Welding material with improved weldability and manufacturability (Patent Document 4).

  (e) Ti and Nb are added in the range of 0.1 to 0.5%, respectively, and these carbides or oxides are formed to refine the solidification structure, and weld crack resistance during multilayer welding, A welding material that can improve thermal cracking and toughness, circumferential weldability, etc. (Patent Document 5).

  (f) Ta: 0.1 to 1.5%, Ti: 0.05 to 0.5%, and the content of C, Nb, Ta, and Ti is 20 × C / (2Nb + Ta + 4Ti) ≧ 1.0, {1.3 × (Ta / 181) + (Ti / 48) −1.2 × (Nb / 93)} × welding material for Fe—Ni alloy satisfying 100 ≧ 0.5 (Patent Document 6) .

  However, the above-mentioned welding materials for low thermal expansion alloys (a) to (f) have the following problems when applied to the production of an actual welded structure.

  In the welding material (a), of the cracks that occur during welding, solidification cracks that cause cracks in the weld metal part obtained during the welding are fully considered, but thick welded materials such as large welded structures are considered. The reheat cracking in which the weld metal part obtained by the previous welding during the multi-layer welding breaks due to the heat cycle of the subsequent welding is not sufficiently considered, and cannot be used with confidence in the multi-layer welding.

  On the other hand, the welding materials (b) to (e) are considered with respect to reheat cracking during multi-layer welding, but all of their specific means add Nb, Zr, or Ti, By forming these carbides and oxides, the solidified structure becomes a fine and complex structure, and reheat cracking during multilayer welding is prevented.

  According to the test results conducted by the present inventors, when Nb, Zr, and Ti are added, they are bonded to carbon in the liquid phase during solidification to produce eutectic carbides. The structure was fine and complex, and was effective in preventing reheat cracking during multi-layer welding.

  However, the allowable range of the addition amount of Nb, Zr and Ti is narrow, and when these addition amounts are excessive, in other words, the total amount of these added elements does not form eutectic carbides in the liquid phase at the time of solidification. When it was present in a free state in the phase, it became clear that the solidification cracking sensitivity was remarkably increased and solidification cracking occurred. This means that in actual commercial production, it is necessary to strictly control the addition amount of these elements, and the margin for securing performance is small.

  The welding material (f) is characterized in that Ti and Ta are added in combination, the solidified structure is refined by these carbides, and hot cracking during welding is prevented. However, according to the studies by the present inventors, Ta has a large amount of oxidation consumption during welding, and uses a U groove necessary for ensuring a sound back wave formation in, for example, automatic all-around circumferential welding of a pipe. In the case of welding with a large base metal dilution or welding with a large base material dilution such as plasma welding, the effect of preventing weld cracking is not always sufficient.

JP-A-4-231194

JP-A-8-267272 Japanese Patent Laid-Open No. 11-104885 WO00 / 20160 Publication JP 2001-179486 A JP 2003-19593 A

  A first object of the present invention is to provide a welded structure made of a low thermal expansion coefficient alloy having a weld metal having excellent weld cracking resistance, that is, solidification cracking resistance and reheat cracking resistance as well as toughness at extremely low temperatures. To provide things. In addition, the second object is not only to obtain the above-mentioned weld metal reliably, but also to provide a low heat which is excellent in manufacturability in that a product having a predetermined performance can be stably obtained with a large margin for securing performance. It is to provide a welding material for an expansion coefficient alloy.

  The third purpose is, for example, automatic welding of all postures of a pipe, welding with a large base metal dilution using a U groove necessary for ensuring sound back-wave formation, and welding with a large base material dilution such as plasma welding. However, it is providing the welding material which can form the weld metal which has the above outstanding characteristics reliably.

The gist of the present invention resides in the following (1) and (2) low thermal expansion coefficient alloy welded structures, and (3) and (4) welding materials for low thermal expansion coefficient alloys.
(1) A welded structure in which at least one of the members joined by welding is a member made of an Fe—Ni-based low thermal expansion coefficient alloy, and the weld metal of the weld joint is in mass%, Ni: 30 to 45 %, C: 0.01~0.5%, Si : 0.01~0.5%, Mn: 0.01~1.0%, Al: it comprises from 0.0002 to 0.02 percent, in further , 0 . It contains more than 1% and not more than 4% Ta, and the balance consists of Fe and impurities, S in the impurities is 0.015% or less, P is 0.015% or less, O (oxygen) is 0.05% A welded structure made of a low thermal expansion coefficient alloy, characterized by:
(2) A welded structure in which at least one of the members to be joined by welding is a member made of an Fe—Ni-based low thermal expansion coefficient alloy, and the weld metal of the weld joint is in mass%, and Ni: 30 to 45 %, C: 0.01 to 0.5%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, Al: 0.0002 to 0.02%, It contains Ta and Hf in excess of 0.1% and 4% or less in total (however, the upper limit of the Hf content is 1.75%), the balance consists of Fe and impurities, and S in the impurities is 0 A welded structure made of a low thermal expansion coefficient alloy, characterized in that it is 015% or less, P is 0.015% or less, and O (oxygen) is 0.05% or less .
(3) By mass%, Ni: 30 to 45%, C: 0.01 to 0.5%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, Al: 0 .0002-0.02%, and more than 1.5% and not more than 6% Ta , the balance being Fe and impurities, S in the impurity being 0.015% or less, P being 0 A welding material for a low thermal expansion coefficient alloy, characterized in that the content is 0.15% or less and O (oxygen) is 0.05% or less.
(4) In mass%, Ni: 30 to 45%, C: 0.01 to 0.5%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, Al: 0 .0002 to 0.02%, and more than 0.1% in total and 6% or less Ta and Hf (however, the upper limit of the Hf content is 2.56%, and the total content of Ta and Hf Is 1.5% or less, Hf is contained in an amount exceeding 0.1%), the balance is Fe and impurities, S in the impurities is 0.015% or less, and P is 0.015 %, And O (oxygen) is 0.05% or less, a welding material for a low thermal expansion coefficient alloy .

  The welding metal and the welding material for the low thermal expansion coefficient alloy constituting the welded structure made of the low thermal expansion coefficient alloy of the present invention are both of the following first to fourth groups instead of a part of Fe. It may contain at least one component selected from at least one group.

First group: mass%, Co: 0.1 to 10%.
Second group: mass% and one or more of Nb, Zr and Ti in total 0.01 to 0.05%.
Third group:% by mass, and one or more of Ca, Mg, La and Ce in total 0. 0005-0.01%.

  Fourth group:% by mass, B: 0.0005 to 0.008%.

  As a result of various experiments to achieve the above-mentioned problems, the present inventors have found the following and completed the present invention.

  First, detailed observation of reheat cracks that occurred during multilayer welding was performed. As a result, a portion where Si and C were significantly concentrated and a portion where S was significantly concentrated were observed at the grain boundary where reheat cracking occurred. Further, as a result of observation of the fracture surface form, it was observed that melting marks were observed in the portion where Si and C were concentrated, and that the portion where S was concentrated was broken at the grain boundary.

  From the above, reheat cracking is caused by the thermal cycle of the next pass, and Si and C segregated at the grain boundaries of the previously formed beads (welded metal) form a low-melting eutectic compound with Fe in the matrix. However, it was considered that the liquefaction opening or S segregates at the grain boundary, so that the grain boundary fixing force becomes small and the part opens due to thermal stress.

  And when Nb, Zr, and Ti were added, it was confirmed that reheat cracking can be prevented as described in Patent Documents 2, 3, 4, and 5 described above. These effects are that Nb, Zr, and Ti combine with carbon in the liquid phase during solidification to produce eutectic carbides, so that Si and C segregated at the boundaries of the columnar crystals can form Fe and a low-melting eutectic compound. In addition to suppressing generation, the formation of eutectic carbides complicates the shape of the dendrite boundary and grain boundaries, which are the segregation sites of S, and increases the area thereof, thereby dispersing the segregation of S, It was thought that this was because the decrease in grain boundary fixing force was suppressed.

  However, as a result of further investigations by the present inventors, as described above, Nb, Zr and Ti are contained in an amount more than that fixed as carbides during solidification and exist as free elements in the liquid phase. In this case, it has been found that the susceptibility to solidification cracking is remarkably increased. The reason was considered as follows.

  That is, Nb, Zr, and Ti are elements that have a small partition coefficient between the solid and the liquid, concentrate in the liquid phase at the time of suspicion, and expand the solid-liquid coexistence temperature range. When carbon is sufficiently present in the liquid phase, Nb, Zr and Ti are crystallized from the liquid phase as carbides, and the concentration of these elements decreases, so that the expansion of the solid-liquid coexistence temperature range is suppressed. However, if Nb, Zr and Ti are present in excess of the amount necessary for carbon fixation, these elements are present in a free state in the liquid phase, leading to an expansion of the solid-liquid coexistence temperature range. Susceptibility to cracking increases. This means that the margin of component management of the weld metal and welding material is narrow in actual commercial production, which is a problem in stably producing a product having a predetermined performance.

  Therefore, as a result of further investigations by the inventors to solve this problem, rather than using Nb, Zr and Ti, rather than using Ta and Hf, which have a very large atomic weight compared to these elements. Thus, in addition to preventing reheat cracking, it has been clarified that a sufficient margin can be obtained for preventing solidification cracking. The reason is as described below.

  Ta and Hf, like Nb, Zr and Ti, concentrate in the liquid phase during solidification and combine with carbon in the liquid phase to form eutectic carbides ((Ta, Hf) C). Eutectic carbides are stable up to high temperatures. For this reason, even if the previous bead (welded metal) is heated by the welding heat cycle at the time of multilayer welding, Si and C segregated at the boundaries of the columnar crystals do not form eutectic with Fe. In addition, the eutectic carbide, like the eutectic carbides of Nb, Zr and Ti, makes the solidified structure fine and complex. As a result, the boundary area of the columnar crystals that are the segregation sites of S is increased, the amount of S segregation per unit grain interfacial area is decreased, and the decrease in the intergranular adhesion force is suppressed, resulting from the segregation of Si and C. Neither cracks nor cracks due to segregation of S occur.

  Further, since Ta and Hf have a larger atomic weight than Nb, Zr and Ti, an amount necessary for fixing the same amount of C as eutectic carbide is increased. For this reason, the effect of compositional supercooling is increased, the crystal growth in the direction orthogonal to the main growth direction of the crystal is further promoted, and so-called branch crystals are easily formed. As a result, the shape of the grain boundary becomes more complicated than when Nb, Zr and Ti are used, and the reheat cracking prevention effect is further improved.

  Further, Ta and Hf exceeding the amount necessary for fixing C as eutectic carbide are naturally present in a free state in the liquid phase, but even in this case, compared with Nb, Zr and Ti, It was found that the expansion range of the coexisting temperature range was small and the margin for preventing solidification cracking was large. In other words, when Ta and Hf are used, the margin of component management of weld metal and welding material, which is excellent in both solidification cracking resistance and reheat cracking resistance in actual commercial production, is expanded and stabilized. It became clear that it can be manufactured.

  Furthermore, the following facts regarding Ta have been found. In other words, since Ta consumes a large amount of oxidation during welding, if the Ta content in the welding material is low, welding with a large dilution of the base metal cannot secure a sufficiently effective amount of Ta in the weld metal, and is effective in preventing weld cracking. Becomes smaller. Therefore, it is necessary that the welding material contains a sufficient amount of Ta in consideration of both oxidation consumption and base material dilution.

Specifically, in the weld metal, C is a direct cause of reheat cracking, 0.5% Si and S content respectively less, while limited to 0.5% or less and 0.015% or less, Ta In the range of more than 0.1% to 4% or less, or Ta and Hf in total exceeding 0.1% to 4% or less (however, the upper limit of the Hf content is 1.75%) It has been clarified that the high temperature cracking (reheat cracking and suspicion cracking) during multi-layer welding can be surely prevented by containing in the above.

On the other hand, in the welding material, the amount of C, Si and S, which are direct causes of reheat cracking, is limited to 0.5% or less, 0.5% or less and 0.015% or less, respectively, and 1.5% beyond 6% or less of Ta or the Ta of 6% or less exceeds 0.1% in total, and Hf (where the upper limit of the Hf content is 2.56%, and the total content of Ta and Hf In the case of 1.5% or less, by containing Hf content exceeding 0.1%, high temperature cracking (reheat cracking and suspicious cracking) during multi-layer welding can be surely prevented It became clear.

  In general, when the Ta content is increased, the workability when manufacturing a welding wire is lowered. However, by applying strong processing in an appropriate temperature range as described later in the Examples section, coarse Ta carbide harmful to hot working is finely dispersed to reduce the effect on hot working cracks. Thus, it is possible to suppress a decrease in workability. This is also a new finding by the present inventors, which has made it possible to put to practical use a welding material containing more than 1.5% Ta.

  Hereinafter, the reason why the welding metal and the welding material for the low thermal expansion coefficient alloy constituting the low thermal expansion coefficient alloy welded structure of the present invention are determined as described above will be described in detail. In the following, “%” means “mass%” unless otherwise specified. The matters common to the weld metal and the weld material constituting the welded structure made of a low thermal expansion coefficient alloy will be described without particular distinction.

Ni: 30-45%
Ni is a main element constituting the low thermal expansion alloy. In order to obtain a sufficiently low linear expansion coefficient, the content needs to be 30 to 45%. The lower limit is preferably 32%, and more preferably 34%. The upper limit is preferably 44%, and more preferably 43%.

C: 0.01 to 0.5%
C is an element that stabilizes the austenite phase that is the matrix of the base material, forms eutectic carbides with Ta and Hf, makes the weld metal microstructure finer and more complex, and lowers reheat cracking susceptibility. A content of 01% or more is necessary. However, C exceeding 0.5% reacts with Si segregated at the grain boundaries of the weld metal and Fe of the matrix to form a low melting point compound, and conversely increases reheat cracking susceptibility. For this reason, C content shall be 0.01 to 0.5%. The lower limit is preferably 0.02%, and more preferably 0.05%. Further, the upper limit is preferably 0.45%, and more preferably 0.4%.

Si: 0.01 to 0.5%
Si is added as a deoxidizer, but if the content is less than 0.01%, no effect is obtained. On the other hand, excess Si segregates at the grain boundaries during solidification of the weld metal, reacts with C and the Fe of the matrix to form a low melting point compound, and causes reheat cracking during multilayer welding. For this reason, the upper limit of Si content is 0.5%. The lower limit is preferably 0.02%, and more preferably 0.05%. Further, the upper limit is preferably 0.45%, and more preferably 0.4%.

Mn: 0.01 to 1.0%
Mn is also added as a deoxidizer in the same manner as Si. However, if the content is less than 0.01%, the effect cannot be obtained. On the other hand, excessive Mn lowers the toughness and corrosion resistance of the weld metal, so the upper limit is made 1.0%. The lower limit is preferably 0.02%, and more preferably 0.05%. The upper limit is preferably 0.8% and more preferably 0.6%.

Al: 0.0002 to 0.02%
Al is also added as a deoxidizer in the same manner as Si and Mn described above, but if the content is less than 0.0002%, the effect cannot be obtained. On the other hand, excess Al increases the amount of inclusions and degrades the toughness of the weld metal. Therefore, the upper limit of the Al content is 0.02%. The lower limit is preferably 0.0005%, and more preferably 0.001%. The upper limit is preferably 0.015% and more preferably 0.01%. In addition, when the Al content in the welding material exceeds 0.008%, welding workability is deteriorated. For this reason, it is desirable that the upper limit of the Al content of the welding material is 0.008%.

Ta, Hf:
In weld metal, Ta exceeds 0.1% and 4% or less, or the total of Ta and Hf exceeds 0.1% and 4% or less (however, the upper limit of Hf content is 1.75%) To do.

All of these elements are most important for the present invention, which has the action of forming eutectic carbide and fixing C, complicating the solidification structure, and improving the reheat cracking resistance of the weld metal during multilayer welding. One of these elements, Ta alone or in combination of Ta and Hf . In the case of Ta alone , it is necessary to contain more than 0.1% to 4% or less, and in the case of two kinds, it is necessary to contain more than 0.1% to 4% or less in total. That is, if the content of Ta or the total content of the two types is 0.1% or less, the above effect cannot be obtained, and if it exceeds 4%, it concentrates in the liquid phase and expands the solid-liquid coexistence temperature range, Increases the sensitivity to solidification cracking. The lower limit is preferably more than 0.5%, more preferably more than 0.6%. The upper limit is preferably 3%, more preferably 2.5%. In addition, when two types of Ta and Hf are contained together, the upper limit of the Hf content is desirably 1.75% .

The welding material should contain more than 1.5% to 6% Ta, or more than 0.1% to 6% Ta and Hf in total. When Ta is added alone, if the content is 1.5% or less, oxidation consumption during welding is large, so that the base material dilution is large, such as welding using the above-mentioned U groove or plasma welding. the welding can not be secured sufficiently effective amount of Ta in the weld metal, the effect of weld cracking prevention may turn reduced. When the content of Ta exceeds 6%, these concentrate in the liquid phase during the welding process, leading to solidification cracking and toughness reduction.

When both Ta and Hf are contained, the total content exceeds 0.1% and reaches 6% (however, the upper limit of the Hf content is 2.56%) . The reason for limiting the lower limit value and the upper limit value is the same as in the case of containing Ta alone . When the total content exceeds 0.1% and reaches 1.5%, the amount of Hf in the total content needs to exceed 0.1%. For example, when the total content is 1.0%, if the ratio of Hf is 0.05% and Ta is 0.95%, the oxidation consumption of Ta is large as described above. This is because the amount of Ta and Hf is insufficient.

P: 0.015% or less P is an impurity, and excessive P increases the susceptibility to solidification cracking during welding. It is preferably 0.012% or less, more preferably 0.010% or less. The lower the P content, the better.

S: 0.015% or less S is an impurity like P described above, and forms a eutectic having a low melting point during solidification of the weld metal, thereby causing solidification cracking. Moreover, it segregates at the grain boundary and lowers the grain boundary fixing force, causing reheat cracking. For this reason, S content shall be 0.015% or less. It is preferably 0.012% or less, more preferably 0.010% or less. The smaller the S content, the better.

O (oxygen): 0.05% or less O is an impurity element contained in steel. When the content of the weld metal exceeds 0.05%, the cleanliness is significantly deteriorated and brittleness is caused. Therefore, the O content is 0.05% or less. Preferable is 0.04% or less, and more preferable is 0.03% or less. However, if the O content in the welding material exceeds 0.008%, the alloy becomes brittle. For this reason, it is desirable that the O content of the welding material be 0.008% or less. Note that the lower the O content, the better for the weld metal and the weld material.

  One of the weld metal and the welding material for the low thermal expansion coefficient alloy constituting the low thermal expansion coefficient alloy welded structure of the present invention is substantially composed of Fe, in other words, Fe and impurities other than the above. It will be.

  Another of the weld metal and the welding material for the low thermal expansion coefficient alloy constituting the low thermal expansion coefficient alloy welded structure of the present invention is at least one of the first group to the fourth group, instead of a part of Fe. A weld metal and a weld material containing at least one component selected from the group. Hereinafter, these components will be described.

First group (Co)
Co is an element effective for obtaining a low linear expansion coefficient like Ni described above. For this reason, although it adds as needed, sufficient effect is not acquired with content less than 0.1%. On the other hand, Co is very expensive compared to Ni, and excessive addition causes an increase in cost. Therefore, the Co content when added is preferably 0.1 to 10%.

Second group (Nb, Zr and Ti)
These elements are all effective elements for preventing reheat cracking of the weld metal during multilayer welding. For this reason, in order to supplement the effect of preventing reheat cracking due to Ta and Hf, one or more kinds may be added as necessary. However, if the total content is less than 0.01%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.05% in total, the amount existing as a free element in the liquid phase becomes too large, and the solid-liquid coexistence temperature range is expanded to cause an increase in solidification cracking sensitivity. Therefore, the content of these elements when added is preferably 0.01 to 0.05% in total of one or more.

Third group (Ca, Mg, La and Ce)
All of these elements combine with S to form sulfides that are stable at high temperatures, and have the effect of contributing to prevention of reheat cracking. For this reason, in order to obtain this effect, one or more kinds may be added. However, if the total content is less than 0.0005%, a sufficient effect cannot be obtained. On the other hand, if the total content exceeds 0.01%, the cleanliness is deteriorated and the workability and toughness are deteriorated. Accordingly, the content of these elements when added is preferably 0.0005 to 0.01% in total of one or more.

4th group (B)
B is an element that is easily segregated at the grain boundary, and has an effect of contributing to prevention of reheat cracking by segregating at the grain boundary prior to S during the reheating process to reduce grain boundary segregation of S. For this reason, when it is desired to obtain this effect, it may be added, but if its content is less than 0.0005%, a sufficient effect cannot be obtained. On the other hand, when it exceeds 0.008%, the solidification cracking sensitivity is increased. Therefore, the B content when added is preferably 0.0005 to 0.008%.

  A test tube having a U groove shown in FIG. 1 was prepared by machining at the end of a tube having an outer diameter of 300 mm and a wall thickness of 9.5 mm having the chemical composition shown in Table 1, and the chemical composition shown in Table 2 was obtained. Using the welding material having, circumferential welding with a heat input of 10 kJ / cm was performed with an automatic TIG welding machine.

  The welding material is a material having a chemical composition shown in Table 2, and a forging ratio of 5 is applied in a temperature range of 1150 to 1250 ° C., and then a forging ratio of 10 is applied in a temperature range of 1000 to 1200 ° C. The element wire was subdivided and dispersed, and the element wire was repeatedly subjected to cold working and soft annealing at 1100 ° C. to obtain a wound wire having an outer diameter of 1.6 mm.

  A side bend specimen having a thickness of 2 mm, a width of 9.5 mm, and a length of 120 mm is taken from the welded joint obtained by the above circumferential welding so that the direction perpendicular to the weld line is the longitudinal direction, and the bending surface is After mirror polishing, a bending test was performed under the condition of a bending radius of 4 mm, the bending surface was observed at a magnification of 500, and the occurrence of cracks was investigated.

Table 3 shows the chemical composition of the weld metal and the results of the above investigation. As is clear from the table, no cracks were observed in the weld metals X1 to X9 obtained using the weld materials WX1 to WX9 . On the other hand, cracks occurred in the weld metals Y1 to Y3 obtained using the weld materials WY1 to WY3 having no chemical composition defined in the present invention.

  As is clear from the results of Examples, a welded structure having a weld metal having a chemical composition defined in the present invention is excellent in crack resistance of a welded portion and has high safety. Moreover, according to the welding material of this invention, the said welded structure can be manufactured reliably. Furthermore, since the welding material of the present invention uses Ta and Hf as the eutectic carbide forming elements that make the microstructure of the weld metal fine and complex, the margin for securing the performance is large, and the predetermined performance A product having the above can be manufactured stably. According to the welding material of the present invention, especially in all-round automatic circumferential welding of pipes, large dilution of the base metal such as welding of the base metal using a U groove necessary for ensuring sound back-wave formation and plasma welding is large. Also in welding, it is possible to obtain a welded structure having a weld metal having excellent crack resistance and high safety.

It is a figure which shows the groove shape of the test tube used in the Example.

Explanation of symbols

1, 2: Test tube butt

Claims (12)

At least one of the members to be joined by welding is a welded structure that is a member made of an Fe—Ni-based low thermal expansion coefficient alloy, and the weld metal in the weld joint is in mass%, Ni: 30 to 45%, C : 0.01~0.5%, Si: 0.01~0.5% , Mn: 0.01~1.0%, Al: comprises from 0.0002 to 0.02%, in addition, 0. It contains more than 1% and not more than 4% Ta, and the balance consists of Fe and impurities, S in the impurities is 0.015% or less, P is 0.015% or less, O (oxygen) is 0.05% A welded structure made of a low thermal expansion coefficient alloy, characterized by: At least one of the members to be joined by welding is a welded structure that is a member made of an Fe—Ni-based low thermal expansion coefficient alloy, and the weld metal in the weld joint is in mass%, Ni: 30 to 45%, C : 0.01 to 0.5%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, Al: 0.0002 to 0.02%, and 0 in total 0.1% to 4% or less of Ta and Hf (the upper limit of the Hf content is 1.75%), with the balance being Fe and impurities, S in the impurity being 0.015% Hereinafter, P is 0.015% or less, and O (oxygen) is 0.05% or less . A welded structure made of a low thermal expansion coefficient alloy . The welding structure made of a low thermal expansion coefficient alloy according to claim 1 or 2 , wherein the weld metal contains 0.1 to 10% of Co by mass% instead of a part of Fe. The weld metal according to any one of claims 1 to 3 , wherein the weld metal contains 0.01 to 0.05% in total of at least one of Nb, Zr and Ti in mass% instead of part of Fe. The welded structure made of a low thermal expansion coefficient alloy as described. The weld metal according to any one of claims 1 to 4 , wherein the weld metal contains 0.0005 to 0.01% in total of at least one of Ca, Mg, La and Ce in mass% instead of a part of Fe. A welded structure made of a low thermal expansion coefficient alloy according to claim 1. The weld structure according to any one of claims 1 to 5 , wherein the weld metal contains 0.0005 to 0.008% B in mass% instead of part of Fe. In mass%, Ni: 30-45%, C: 0.01-0.5%, Si: 0.01-0.5%, Mn: 0.01-1.0%, Al: 0.0002- It contains 0.02%, further contains more than 1.5% and 6% or less of Ta , the balance is made of Fe and impurities, S in the impurities is 0.015% or less, and P is 0.015% Hereinafter, a welding material for a low thermal expansion coefficient alloy, characterized in that O (oxygen) is 0.05% or less. In mass%, Ni: 30-45%, C: 0.01-0.5%, Si: 0.01-0.5%, Mn: 0.01-1.0%, Al: 0.0002- Further, Ta and Hf including 0.02% and exceeding 0.1% in total and 6% or less (however, the upper limit of Hf content is 2.56%, and the total content of Ta and Hf is 1. In the case of 5% or less, Hf is an amount exceeding 0.1%), the balance is made of Fe and impurities, S in the impurities is 0.015% or less, P is 0.015% or less, A welding material for low thermal expansion coefficient alloys, characterized in that O (oxygen) is 0.05% or less . The welding material for a low thermal expansion coefficient alloy according to claim 7 or 8, which contains 0.1 to 10% of Co by mass% instead of part of Fe. The low thermal expansion according to any one of claims 7 to 9 , wherein, in place of a part of Fe, at least 1% of Nb, Zr, and Ti is contained in a total of 0.01 to 0.05% by mass%. Welding material for coefficient alloys. Instead of a part of Fe, by mass%, Ca, Mg, claim 7 containing 0.0005 to 0.01 percent in total of one or more of La and Ce as set forth in any one of up to 10 Welding material for low thermal expansion coefficient alloys. The welding material for a low thermal expansion coefficient alloy according to any one of claims 7 to 11 , which contains 0.0005% to 0.008% B in mass% instead of a part of Fe.

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