JP2004268137A - Welded structure made of low thermal expansion coefficient alloy, and welding material for alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a welded structure made of a low thermal expansion coefficient alloy which has a welding metal excellent in solidification cracking resistance, reheat cracking resistance, and toughness in very low temperatures, and also to provide a welding material for a low thermal expansion coefficient alloy which has a large capacity to secure performance and an excellent manufacturability. <P>SOLUTION: A welding metal of the welded part in the welded structure includes: 30-45% Ni; 0.01-0.5% C; 0.01-0.5% Si; 0.01-1.0% Mn; and 0.0002-0.02% Al, and further includes 0.1-4% in total of at least one of Ta and Hf, and the balance Fe and impurities. The impurities include ≤0.015% S, ≤0.015% P, and ≤0.05% O(oxygen). The welding material has the same chemical composition as the welding metal except that the welding material includes >1.5% but ≤6% Ta or >0.1% but ≤6% Hf, or >0.1% but ≤6% in total of Ta and Hf (in this case, Hf exceeds 0.1%). <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to storage tanks and pipes used for storing and transporting low-temperature substances such as liquefied natural gas, so-called pipes and various kinds of attached equipment, which pipes are connected by butt welding and girth welding. A material composed of an Fe—Ni-based low thermal expansion coefficient alloy and having at least a part of which has a welded portion (in the present invention, these are collectively referred to as “low thermal expansion coefficient alloy welded structure”); The present invention relates to a welding material for an alloy having a low coefficient of thermal expansion and excellent in manufacturability suitable for use in the manufacture of a welded structure made of an alloy.

  It is widely known that an Fe-Ni alloy having a specific component ratio has a very small linear expansion coefficient as an Invar effect. Representative examples thereof include Fe-36% Ni and Fe-42% Ni. These are used in parts where expansion and contraction due to temperature change poses a problem, utilizing their low thermal expansion coefficient.

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

  (a) In addition to Ni and Fe, C: 0.05 to 0.5%, Nb: 0.5 to 5%, and optionally containing Mn, Ti, Al, Ce, Mg, etc. as necessary. The welding material which improved welding crack resistance at the time of welding by doing it (patent document 1).

  (b) Ni: 30 to 45%, C: 0.03 to 0.3%, Nb: 0.1 to 3%, P: 0.015% or less, S: 0.005% or less, Si: 0. 0.05 to 0.6%, Mn: 0.05 to 4%, Al: 0.05% or less, and O: 0.015% or less, and the relationship between Nb and C is [% Nb] × [% C ] Welding material having a solidified structure made fine by defining ≧ 0.01 so as to achieve both reheat crack resistance and toughness during multi-layer welding (Patent Document 2).

  (c) By setting the content of C and Nb to a value satisfying −0.01 [% Nb] + 0.04 ≦ [% C] ≦ −0.04 [% Nb] +0.40, the solidified structure can be refined. A welding material that prevents reheat cracking during multi-layer welding and enables circumferential welding in all positions by defining the amounts of Si, Mn, Al, O, and S within a predetermined range (Patent Document 3).

  (d) The content of Nb and / or Zr is limited in relation to the amounts of S and Si so that the carbides complicate the solidification structure and make the weld metal compatible with weld cracking resistance, toughness and SCC resistance. By limiting the content of Nb and / or Zr in relation to the amount of C and the relationship between the amounts of Si, Mn, Al, O, and S, the welded structure having the weld metal having Welding material with improved weldability and productivity (Patent Document 4).

  (e) Ti and Nb are added in the range of 0.1 to 0.5%, respectively, to generate carbides or oxides thereof to make the solidification structure fine, and to provide resistance to weld cracking during multi-layer welding, especially A welding material capable of improving heat cracking property, toughness, circumferential weldability, and the like (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 low thermal expansion coefficient Fe—Ni alloy satisfying 100 ≧ 0.5 (Patent Document 6) .

  However, when the welding materials for low-thermal-expansion alloys (a) to (f) described above are applied to the production of actual welded structures, there are the following problems.

  Regarding the welding material of (a), solidification cracking, in which cracks occur in the weld metal part obtained during welding, among cracks occurring during welding, is sufficiently considered, but for thick-walled materials such as large welded structures. At the time of multi-layer welding, reheat cracking in which the weld metal portion obtained by the previous welding is cracked due to the heat cycle of the subsequent welding is not sufficiently considered, and cannot be used with confidence in 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 the specific means is to add Nb, Zr, or Ti, By forming these carbides and oxides, the solidification structure is made into a fine and complicated structure to prevent reheat cracking during multi-layer welding.

  According to the test results performed by the present inventors, when Nb, Zr, and Ti were added, they were combined with carbon in the liquid phase during solidification to form eutectic carbides, The structure became fine and complicated, and was effective in preventing reheat cracking during multi-layer welding.

  However, the allowable range of the added amounts of Nb, Zr and Ti is narrow, and when these added 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. It was clarified that the solidification cracking susceptibility significantly increased when solidified cracks were present in the phase, and solidification cracking occurred. This means that in actual commercial production, it is necessary to strictly control the amounts of these elements to be added, and that the margin for securing performance is small.

  The welding material of (f) is characterized in that Ti and Ta are added in a complex manner, the solidified structure is made fine by these carbides, and hot cracking during welding is prevented. However, according to the study of the present inventors, Ta has a large oxidative consumption at the time of welding, and uses a U-groove necessary for securing a sound backwashing in, for example, automatic circumferential welding of a pipe in all positions. In the case of welding with a large base material dilution, or in the case of 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 JP-A-11-104885 WO 00/20160 JP 2001-179486 A JP-A-2003-19393

  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 not only sufficient weld cracking resistance, that is, solidification cracking resistance and reheat cracking resistance, but also excellent toughness at cryogenic temperatures. To provide things. Further, the second object is not only that the above-mentioned weld metal can be reliably obtained, but also that a product having a large margin for securing performance and a product having a predetermined performance can be stably obtained. An object of the present invention is to provide a welding material for an expansion coefficient alloy.

  A third object is, for example, in the automatic circumferential welding of pipes in all positions, welding with a large base material dilution using a U-groove necessary for ensuring sound backwashing, or welding with a large base material dilution such as plasma welding. Another object of the present invention is to provide a welding material capable of reliably forming a weld metal having the above-described excellent characteristics.

The gist of the present invention resides in a low thermal expansion coefficient alloy welding structure of the following (1) and a low thermal expansion alloy welding material of the following (2).
(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 coefficient of thermal expansion alloy, and the weld metal at the weld joint is 30% to 45% Ni by mass%. %, C: 0.01 to 0.5%, Si: 0.01 to 0.5%, Mn: 0.01 to 1.0%, Al: 0.0002 to 0.02%. Each contains more than 0.1% and 4% or less of Ta or Hf, or more than 0.1% and 4% or less of Ta and Hf, and the balance consists of Fe and impurities. A welded structure made of a low-thermal-expansion-coefficient alloy, wherein 0.015% or less, P is 0.015% or less, and O (oxygen) is 0.05% or less.
(2) 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 0.002 to 0.02%, and more than 1.5% to 6% Ta or 0.1% to 6% Hf, or a total of more than 0.1% to 6% The following Ta and Hf (however, when the total content is 1.5% or less, Hf is set to an amount exceeding 0.1%), and the balance consists of Fe and impurities. A welding material for a low-thermal-expansion coefficient alloy, characterized in that 0.015% or less, P is 0.015% or less, and O (oxygen) is 0.05% or less.

  The welding metal and the welding material for a low-thermal-expansion coefficient alloy constituting the welding structure made of a low-thermal-expansion coefficient alloy according to the present invention are all replaced with a part of Fe, and It may contain at least one component selected from at least one group.

First group: Co: 0.1 to 10% by mass%.
Second group: One or more of Nb, Zr and Ti in a mass% of 0.01 to 0.05% in total.
3rd group: One or more of Ca, Mg, La and Ce in total of 0. 0005-0.01%.

  4th group: B: 0.0005 to 0.008% by mass%.

  The present inventors have conducted various experiments to achieve the above object, and as a result, have found the following and completed the present invention.

  First, detailed observations of reheat cracks generated during multi-layer welding were performed. As a result, a portion where Si and C were significantly concentrated and a portion where S was significantly concentrated were recognized at the crystal grain boundary where reheat cracking occurred. Further, as a result of observation of the fracture surface form, a melting mark was observed in a portion where Si and C were concentrated, and a state in which the portion where S was concentrated was broken at a grain boundary was observed.

  From the above, in the reheat cracking, Si and C segregated at the crystal grain boundary of the bead (weld metal) formed earlier form a eutectic compound having low melting point with Fe of the matrix by the next pass thermal cycle. However, it is considered that the liquefaction opening or the segregation of S at the grain boundaries lowers the grain boundary fixing force, and that portion is opened by thermal stress.

  Then, it was confirmed that when Nb, Zr and Ti were added, as described in Patent Documents 2, 3, 4, and 5, the reheat cracking could be prevented. These effects are as follows: Nb, Zr and Ti combine with carbon in the liquid phase during solidification to form eutectic carbides, so that Si and C segregated at the columnar crystal boundaries have a low melting point eutectic compound with Fe and Si. In addition to suppressing the generation, the shape of the dendrite boundaries and grain boundaries, which are the segregation sites of S, is complicated with the generation of the eutectic carbide, and the segregation of S is dispersed by increasing the area thereof, It was considered that the decrease in the grain boundary fixing force was suppressed.

  However, as a result of further studies by the present inventors, as described above, Nb, Zr, and Ti are contained in amounts larger than that fixed as carbides during solidification, and exist as free elements in the liquid phase. In this case, it was found that the susceptibility to solidification cracking was significantly increased. The reason was considered as follows.

  That is, Nb, Zr, and Ti are elements that have a small solid-liquid partition coefficient, concentrate in the liquid phase at the time of solidification, and expand the solid-liquid coexistence temperature range. When carbon is sufficiently present in the liquid phase, Nb, Zr and Ti are crystallized out of the liquid phase as carbides, and the concentration of these elements is reduced, so that the expansion of the solid-liquid coexisting temperature range is suppressed. However, when Nb, Zr and Ti are present in excess in amounts necessary for fixing carbon, these elements are present in a free state in the liquid phase, which causes an increase in the solid-liquid coexistence temperature range. In addition, the susceptibility to false cracking increases. This means that the margin of control of the components of the weld metal and the weld material in actual commercial production is narrow, which is a problem in stably producing a product having a predetermined performance.

  In order to solve this problem, as a result of further studies by the inventors, it has been found that rather than utilizing Nb, Zr and Ti, Ta and Hf having an extremely large atomic weight compared to these elements are utilized. Thus, it became clear that, in addition to the prevention of reheat cracking, a sufficient margin was obtained for the prevention of solidification cracking. The reason is as follows.

  Like Nb, Zr and Ti, Ta and Hf 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 (weld metal) is heated by the welding heat cycle at the time of multi-layer welding, Si and C segregated at the boundary of the columnar crystal do not form eutectic with Fe. In addition, the eutectic carbide, like the eutectic carbide of Nb, Zr and Ti, makes the solidification structure a fine and complex structure. As a result, the boundary area of the columnar crystal which is the segregation site of S increases, the segregation amount of S per unit grain boundary area decreases, and the reduction of the grain boundary fixing force is suppressed, which is caused by the segregation of Si and C. Neither cracks nor cracks caused by S segregation occur.

  Further, since Ta and Hf have a larger atomic weight than Nb, Zr and Ti, the amount required to fix the same amount of C as a eutectic carbide increases. For this reason, the effect of the compositional supercooling is increased, and the growth of the crystal in a direction orthogonal to the main growth direction of the crystal is further promoted, so that a so-called branch crystal is easily generated. As a result, the shape of the grain boundary becomes more complicated than when Nb, Zr and Ti are utilized, and the effect of preventing reheat cracking is further improved.

  Further, Ta and Hf exceeding the amount necessary to fix C as a eutectic carbide naturally exist in a free state in the liquid phase, but even in this case, the solid-liquid state is higher than that of Nb, Zr and Ti. It has been found that the coexistence temperature range has a small expansion width and a large margin for preventing solidification cracking. In other words, when Ta and Hf are used, the latitude in controlling the components of the weld metal and the welding material having both excellent solidification cracking resistance and reheat cracking resistance in actual commercial production is expanded, and these are stabilized. It became clear that it can be manufactured.

  Furthermore, the following facts regarding Ta have been found. That is, since Ta is oxidatively depleted during welding, if the Ta content in the welding material is small, it is not possible to secure a sufficiently effective Ta amount in the weld metal in welding with a large base metal dilution, and the effect of preventing weld cracking. Becomes smaller. Therefore, a sufficient amount of Ta must be contained in the welding material in consideration of both oxidation consumption and base metal dilution.

  Specifically, in the weld metal, the amounts of C, Si, and S, which are the direct causes of reheat cracking, are limited to 0.5% or less, 0.5% or less, and 0.015% or less, respectively. And Hf in a range of more than 0.1% and 4% or less, respectively, or a total of Ta and Hf in a range of more than 0.1% and 4% or less. It became clear that hot cracking (reheat cracking and pseudo-solid cracking) at the time can be reliably prevented.

  On the other hand, in the welding material, the amounts of C, Si and S, which are the direct causes of the reheat cracking, are limited to 0.5% or less, 0.5% or less and 0.015% or less, respectively. More than 6% Ta or more than 0.1% and 6% or less Hf, or a total of more than 0.1% and 6% or less of Ta and Hf (provided that the total content of Ta and Hf is When the Hf content is 1.5% or less, the Hf content is set to an amount exceeding 0.1%), whereby high-temperature cracking (reheat cracking and pseudo-solid cracking) during multi-layer welding can be reliably prevented. It became clear.

  In general, when the Ta content is increased, the workability when manufacturing a welding wire is reduced. However, by applying strong working in an appropriate temperature range as described in the section of Examples later, coarse Ta carbide harmful to hot working is finely dispersed to reduce the influence on hot working cracks. Thereby, a decrease in workability can be suppressed. This is also newly found by the present inventors, and as a result, a welding material containing more than 1.5% of Ta can be put to practical use.

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

Ni: 30 to 45%
Ni is a main element constituting the low thermal expansion alloy. In order to obtain a sufficiently low coefficient of linear expansion, 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: 0.01-0.5%
C is an element that stabilizes the austenite phase as a matrix of the base material, forms eutectic carbides with Ta and Hf, makes the structure of the weld metal fine and complicated, and reduces the reheat cracking susceptibility. A content of at least 01% is required. However, more than 0.5% of C reacts with Si segregated at the grain boundaries of the weld metal and Fe in the matrix to form a low melting point compound, and conversely increases the susceptibility to reheat cracking. For this reason, the C content is set to 0.01 to 0.5%. The lower limit is preferably 0.02%, and more preferably 0.05%. The upper limit is preferably 0.45%, and more preferably 0.4%.

Si: 0.01-0.5%
Although Si is added as a deoxidizing agent, no effect is obtained with a content of less than 0.01%. On the other hand, excessive Si segregates at the grain boundaries during solidification of the weld metal, reacts with C and the Fe of the matrix to form a compound having a low melting point, and causes reheat cracking during multi-layer welding. Therefore, the upper limit of the Si content is set to 0.5%. The lower limit is preferably 0.02%, and more preferably 0.05%. The upper limit is preferably 0.45%, and more preferably 0.4%.

Mn: 0.01 to 1.0%
Mn is also added as a deoxidizing agent similarly to the above-mentioned Si. However, if the content is less than 0.01%, no effect can 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 deoxidizing agent similarly to the above-mentioned Si and Mn, but if the content is less than 0.0002%, no effect is obtained. On the other hand, excessive Al increases the amount of inclusions and degrades the toughness of the weld metal. Therefore, the upper limit of the Al content is set to 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%. If the Al content in the welding material exceeds 0.008%, the welding workability decreases. Therefore, it is desirable that the upper limit of the Al content of the welding material be 0.008%.

Ta, Hf:
For the weld metal, the content is more than 0.1% and 4% or less, or the total of two kinds is more than 0.1% and 4% or less.

  All of these elements form a eutectic carbide to fix C, complicate the solidification structure, and are most important to the present invention, which has the effect of improving the reheat cracking resistance of the weld metal during multi-layer welding. And one or two of them are combined. In the case of one kind, it is necessary to contain more than 0.1% and 4% or less, respectively, and in the case of two kinds, it is necessary to contain more than 0.1% and 4% or less in total. That is, if the content of each or the total content is 0.1% or less, the above-mentioned effects cannot be obtained. If the content exceeds 4%, the content is concentrated in the liquid phase to increase the solid-liquid coexistence temperature range, and the solidification cracking susceptibility is increased. Increase. The lower limit is preferably more than 0.5%, and more preferably more than 0.6%. The upper limit is preferably 3%, and more preferably 2.5%.

  The welding material contains more than 1.5% up to 6% Ta or more than 0.1% up to 6% Hf, or a total more than 0.1% up to 6% Ta and Hf Need to be done. In the case where 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 as in the welding using the U-groove or the plasma welding. In welding, a sufficiently effective Ta amount cannot be secured in the weld metal, and the effect of preventing welding cracks is reduced. On the other hand, Hf is less oxidatively consumed during welding than Ta, but when it is 0.1% or less, the effect of preventing cracking is insufficient. If the respective contents of Ta and Hf exceed 6%, they are concentrated in the liquid phase during the welding process, leading to solidification cracking and a decrease in toughness.

  When both Ta and Hf are contained, the total content is set to more than 0.1% and up to 6%. The reasons for limiting the lower limit and the upper limit are as described above. When the total content is in the range of more than 0.1% to 1.5%, it is necessary to make the amount of Hf in the total content exceed 0.1%. For example, when the total content is 1.0%, if Hf is 0.05% and Ta is 0.95%, the oxidation consumption of Ta is large as described above. This is because the amounts of Ta and Hf are insufficient.

P: 0.015% or less P is an impurity, and excessive P increases the solidification cracking susceptibility at the time of welding. It is preferably at most 0.012%, more preferably at most 0.010%. The smaller the P content, the better.

S: 0.015% or less S is an impurity like P, and forms a low-melting eutectic during solidification of the weld metal, causing solidification cracking. In addition, segregation at the grain boundaries lowers the bonding strength of the grain boundaries and causes reheat cracking. Therefore, the S content is set to 0.015% or less. It is preferably at most 0.012%, more preferably at most 0.010%. The lower the S content, the better.

O (oxygen): 0.05% or less O is an impurity element contained in steel, and if the content of the weld metal exceeds 0.05%, the cleanliness is significantly deteriorated, resulting in embrittlement. Therefore, the O content is set to 0.05% or less. It is preferably at most 0.04%, more preferably at most 0.03%. However, if the O content in the welding material exceeds 0.008%, the alloy becomes brittle. Therefore, the O content of the welding material is desirably 0.008% or less. The lower the O content of both the weld metal and the welding material, the better.

  One of the welding metal and the welding material for a low-thermal-expansion coefficient alloy constituting the welding structure made of the low-thermal-expansion coefficient alloy of the present invention is substantially Fe from the above components, in other words, Fe and impurities other than the above. It becomes.

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

First group (Co)
Co is an element effective for obtaining a low coefficient of linear expansion, like Ni. For this reason, it is added as needed, but if the content is less than 0.1%, a sufficient effect cannot be obtained. On the other hand, Co is much more expensive than Ni, and excessive addition causes an increase in cost. Therefore, the content of Co when added is preferably 0.1 to 10%.

Second group (Nb, Zr and Ti)
All of these elements are effective elements for preventing reheat cracking of the weld metal during multi-layer welding. For this reason, one or more kinds may be added as necessary for the purpose of supplementing the reheat crack preventing effect of Ta and Hf, but if the total content is less than 0.01%, a sufficient effect cannot be obtained. On the other hand, if the total amount exceeds 0.05%, the amount of free elements present in the liquid phase becomes too large, and the solid-liquid coexistence temperature range is expanded to cause an increase in solidification cracking susceptibility. Therefore, the content of these elements when added is preferably 0.01 to 0.05% in total of one or more kinds.

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

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

  A test tube having the chemical composition shown in Table 1 and having the U-groove shown in FIG. 1 prepared by machining at the end of a tube having an outer diameter of 300 mm and a thickness of 9.5 mm was prepared. Circumferential welding with a heat input of 10 kJ / cm was carried out with an automatic TIG welding machine using the welding material having the above.

  Welding materials were prepared by applying a reduction of forging ratio 5 to a material having a chemical composition shown in Table 2 in a temperature range of 1150 to 1250 ° C., and then applying a reduction of forging ratio 10 in a temperature range of 1000 to 1200 ° C. to remove coarse Ta carbide. The wire was finely divided and dispersed, and this wire was repeatedly subjected to cold working and softening annealing at 1100 ° C. to obtain a wound wire having an outer diameter of 1.6 mm.

  A side bending test piece having a thickness of 2 mm, a width of 9.5 mm, and a length of 120 mm is sampled from the welded joint obtained by the above-described circumferential welding so that the direction perpendicular to the welding line is the longitudinal direction, and the bent surface is measured. After mirror-polishing, a bending test was performed under the condition of a bending radius of 4 mm, and the bending surface was observed at a magnification of 500 to investigate whether or not cracks occurred.

  Table 3 shows the chemical composition of the weld metal and the results of the above investigation. As is evident from the table, no cracks were observed in the weld metals X1 to X10 obtained using the weld materials WX1 to WX10. On the other hand, cracks occurred in the weld metals Y1 to Y3 obtained using the welding materials WY1 to WY3 having no chemical composition defined in the present invention.

  As is clear from the results of the examples, the welded structure having the weld metal having the chemical composition specified in the present invention has excellent crack resistance of the welded portion and high safety. Further, according to the welding material of the present invention, the above-described welded structure can be reliably manufactured. Furthermore, since the welding material of the present invention uses Ta and Hf as eutectic carbide forming elements that make the structure of the weld metal fine and complicated, the performance securing margin is large and the predetermined performance can be improved. Can be manufactured stably. According to the welding material of the present invention, particularly in the automatic circumferential welding of all positions of the pipe, a large base material dilution such as plasma welding or a large base material dilution using a U-groove necessary for ensuring a sound backwash is formed. 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 reference numerals

1, 2: Butted portion of test tube

Claims (10)

  At least one of the members joined by welding is a welded structure in which a member made of an Fe—Ni-based low coefficient of thermal expansion alloy is used. : 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 more than 1% and 4% or less of Ta or Hf, or more than 0.1% and 4% or less of Ta and Hf, and the balance consists of Fe and impurities. % Or less, P is 0.015% or less, and O (oxygen) is 0.05% or less.   The welded structure made of a low-thermal-expansion-coefficient alloy according to claim 1, containing 0.1 to 10% by mass of Co instead of part of Fe.   The welding made of a low thermal expansion coefficient alloy according to claim 1 or 2, wherein one part or more of Nb, Zr and Ti is contained in a total of 0.01 to 0.05% by mass% instead of part of Fe. Structure.   The method according to any one of claims 1 to 3, wherein one or more of Ca, Mg, La and Ce are contained in a total of 0.0005 to 0.01% by mass% instead of part of Fe. Welded structure made of low thermal expansion coefficient alloy.   The welded structure made of a low-thermal-expansion coefficient alloy according to any one of claims 1 to 4, which contains 0.0005 to 0.008% by mass of B instead of part of Fe.   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 1.5% and up to 6% Ta or 0.1% and up to 6% Hf, or a total of more than 0.1% and up to 6% Ta And Hf (however, when the total content is 1.5% or less, the amount of Hf exceeds 0.1%), and the balance consists of Fe and impurities, and S in the impurities is 0.015%. % Or less, P is 0.015% or less, and O (oxygen) is 0.05% or less.   The welding material for a low thermal expansion coefficient alloy according to claim 6, further comprising 0.1 to 10% by mass of Co instead of part of Fe.   8. The low thermal expansion coefficient alloy welding according to claim 6, wherein one or more of Nb, Zr and Ti are contained in a total of 0.01 to 0.05% by mass% instead of part of Fe. material.   9. The method according to claim 6, wherein one or more of Ca, Mg, La and Ce are contained in a total of 0.0005 to 0.01% by mass% instead of part of Fe. Welding material for low thermal expansion coefficient alloys. The welding material for a low thermal expansion coefficient alloy according to any one of claims 6 to 9, further comprising 0.0005 to 0.008% by mass of B instead of part of Fe.

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