JPH10277746A - Tig welded joint and its welding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a TIG welded joint part with high toughness having an impact value of 50 J/cm<2> or over at 20 deg.C, at which a brittle fracture is said avoidabl, to the welded joint part of refining type low alloy steels manufactured by singly adding or adding after mixing W, V or Nb to 1 to 2.25 CrMo steel, and its welding method. SOLUTION: In this welding, the number of the layers of welded metal layers (average depositing quantity of one pass) is three layers or more, and the welded metal layers other than a final welded metal layer have the fine martensite structure of a grain size No.6 or over. And, the TIB welded joint whose impact value is 50 J/cm<2> or over at 20 deg.C, which is measured with a 2 mmV notch test piece, is obtained, and the welding speed of the welded metal layers on and after a second pass is made to be 7 cm/min or below. The deposition quantity per pass is adjusted to a rage of 0.7 g/cm to 2.5g/cm, or the height of a deposition layer per pass is adjusted to a range of 1.0 mm-3.6 mm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a W, V or N steel for 1-2.25CrMo steel used for heat exchangers and pressure vessels.
b of tempered ferritic steel with or without b added
The present invention relates to an IG welding (inert gas tungsten arc welding) joint and a welding method thereof.

[0002]

2. Description of the Related Art In a heat exchanger, a pressure vessel and the like, a working pressure and a working temperature are increasing in order to improve plant efficiency. As the operating temperature is increased and the operating pressure is increased, materials having excellent high-temperature strength characteristics and corrosion resistance are required in the pressure-resistant portion. In recent years, in order to meet such a demand, a material has been developed in which an element such as V or Nb is added singly or in combination with an element such as V or Nb to an austenitic steel or a ferritic steel which has been generally used so far, to increase the high-temperature strength. One of the representative examples is Mod.9Cr-1Mo steel developed by ORNL in the United States. This material is a CrMo steel that has been used in the past, and V and Nb are added in combination, normalizing and tempering heat treatment. And so on, and succeeded in dramatically increasing the creep rupture strength by so-called tempering heat treatment. This material has already been put into practical use and is used in many power generation boilers. In response to this trend, Mod.9C
Not only high Cr steel such as r-1Mo steel but also 1-2.
Even in low alloy steels such as 5CrMo steel, tempered high-strength ferritic steels to which W, V or Nb is added have been developed. W, V or N is applied to CrMo steel as described above.
b, alone or in combination, when tempered high-strength ferritic steel is applied as a structural member, use a welding material suitable for the strength of the material used, such as MIG welding, MAG welding, coated arc welding or TIG welding. By the arc welding method, so-called multi-layer welding, which includes a plurality of passes (pass: one welding operation performed in the welding direction), is performed. As a general inspection, weld joints are inspected for the presence of internal defects by X-ray inspection or the like. In the case of structures used at high pressure, such as boilers for power generation, they are usually completed or in use. During service inspection, the soundness of the weld is confirmed by a pressure test such as a water pressure test. At this time, if the toughness of the welded portion (welded metal portion and heat-affected zone) is extremely low, there is a risk that defects overlooked in X-ray inspection and brittle fracture starting from sharp irregularities on the surface may occur.
The weld must have toughness to withstand the pressure test.
As a guide for required toughness, generally 50J at 20 ° C
/ Cm ² or more ( ² mm V notch test piece standard) is required. In the case of a Mod.9Cr-1Mo steel weld, the impact value of the weld metal at 20 ° C. is typically 50 J /
It has been confirmed that a value of not less than cm ² can be obtained. However, by adding W, V or Nb alone or in combination to 1-2.25CrMo steel, the weld metal of the welded joint of the tempered high-strength ferritic steel is affected by the effect of the components added for strengthening. Therefore, its hardness is the conventional 1-2.25Cr
It is easy to be harder than the weld metal of the weld joint of Mo steel,
As a result, it has become apparent that the material becomes brittle and does not satisfy the required impact value. More specifically, the impact characteristics of the weld metal in the weld joint produced by the arc welding method differ depending on the welding method and the welding material. According to conventional knowledge, the impact characteristics of the weld metal by the TIG welding method are different from those of other welding methods. Although it is said to be superior to the law,
When a tempered low alloy steel obtained by adding W, V or Nb alone or in combination with 2.25CrMo steel is actually welded by the TIG welding method and the impact value of the weld metal is measured, it is usually 20 ° C. Is known to be 45 J / cm ² or less. In the TIG welding method, the heat input of about 20 to 30 kJ is normally suppressed to about 10 kJ, the amount of welding in one pass is reduced, and the reforming of the deposited metal by the influence of heat from the subsequent pass (the destruction of the solidified structure and A method called so-called low heat input welding method aiming at miniaturization or the like may be adopted (hereinafter, a case where welding is performed with a heat input of about 10 kJ is called a low heat input welding method). In the above-mentioned Mod.9Cr-1Mo steel, since the amount of Cr is large, it is easy to be quenched and the solidification structure is close to the martensite structure. When this steel is welded by a low heat input welding method, the martensitic structure is refined under the influence of heat from a subsequent pass, and the toughness is improved. Japanese Patent Application Laid-Open No. 1-237074 is an example of such an example, which states that toughness can be ensured by suppressing the amount of welding to 2 mm or less. However, 1
In the case of a tempered low alloy steel obtained by adding W, V, or Nb alone or in combination to ~ 2.25CrMo steel,
Most of the weld metal has a solidified structure. In addition, since the solidified structure is not easily destroyed by the influence of reinforcing elements such as W, V or Nb, it is hard to be tempered by the low heat input TIG welding method, and the impact value of the weld metal is only about 30 J / cm ² at 20 ° C. No. As another means for improving the toughness of the weld metal, there is a method of performing a heat treatment after welding for a long time on the welded joint. This is to improve the toughness by softening the hard structure of the weld metal which is almost solidified by heat treatment after welding. However, ferritic steels that have been strengthened by tempering heat treatment such as normalizing and tempering heat treatment, when the tempering conditions are severe, may reduce the base metal strength due to decomposition or aggregation of precipitated carbides. There is a limit to the method of improving toughness by heat treatment after welding.

[0003]

SUMMARY OF THE INVENTION 1-2.25CrMo
It is virtually impossible to improve the toughness of a weld metal of a welded joint between tempered low-alloy steels in which W, V, or Nb is added alone or in combination with steel, even when a so-called low heat input welding method is used. Nearly impossible, if there is a defect in the weld, brittle fracture may occur in the weld joint during the pressure test.

An object of the present invention is to prevent brittle fracture of a welded joint between tempered low alloy steels obtained by adding W, V or Nb alone or in combination to 1-2.25CrMo steel. Another object of the present invention is to provide a welding method capable of obtaining a high toughness TIG welded joint having an impact value of 50 J / cm ² or more at 20 ° C.

[0005]

Means for Solving the Problems In order to achieve the object of the present invention, the present invention is configured as described in the claims. That is, as described in claim 1, the present invention is a welded joint for welding tempered ferritic steels to which only W, V or Nb has been added alone or in combination to 1-2.25CrMo steel, The number of passes is at least three times, that is, the number of weld metal layers is three or more, and the weld metal layers other than the final weld metal layer have a fine martensite structure with a crystal grain size of 6 or more, and 2
This is a TIG welded joint having an impact value of not less than 50 J / cm ² at 20 ° C. measured with a mmV notch impact test piece. Also, the present invention provides a method for welding a TIG welded joint according to claim 1, wherein the welding speed of the weld metal layer after the second pass is 7 cm / min or less, and This is a TIG welding method in which welding is performed by adjusting the amount of deposition per pass from 0.7 g / cm to 2.5 g / cm. Also, the present invention provides a method for welding a TIG welded joint according to claim 1, wherein the welding speed of the weld metal layer after the second pass is 7 cm / min or less, and This is a TIG welding method in which the height of the welded layer per pass is adjusted to 1.0 mm or more and 3.6 mm or less to perform welding. The TIG welded joint according to the present invention has the following features.
A TIG welded joint of a tempered low alloy steel in which W, V or Nb is added alone or in combination to 2.25CrMo steel, the number of passes is three or more, that is, the number of weld metal layers is three. As described above, by setting the weld metal layers other than the final weld metal layer to have a fine martensite structure having a crystal grain size number of 6 or more, the impact value measured with a ² mm V notch impact test piece at 50 ° C. at 20 ° C. is 50 J / cm ^2. The high-toughness weld metal having the above properties can be easily obtained, and for example, it is possible to prevent brittle fracture of a structure such as a boiler, which has the effect of improving the safety and reliability of the structure. Also,
According to the TIG welding method of the present invention,
The welding speed is 7 cm / min or less, and the welding amount is 0.7
g / cm or more and 2.5 g / cm or less, or a welding speed of 7 g / cm or less.
cm / min or less, the effect of obtaining a high toughness weld metal is obtained only by adjusting the welding conditions so that the lamination height of one pass is in the range of 1 mm to 3.6 mm.

Here, the basis for determining the welding condition range in the TIG welded joint and the method for welding the same according to the present invention will be described. In general, the weld metal shows a solidified structure immediately after melting, and the solidified structure is destroyed in a region heated to a temperature higher than the austenitizing temperature due to the influence of heat from a subsequent pass. Start to show site organization. Further, under the influence of heat from the subsequent pass, the region of the martensite structure heated just below the austenitizing temperature is refined. Normal 1-2.
The toughness of the weld metal of the 25CrMo steel weld joint is improved because the solidification structure is easily refined by the influence of heat from the rear pass.
A tempered low alloy steel to which W, V or Nb is added alone or in combination is not easily tempered due to the effect of elements such as V added for strengthening, so that the solidified structure is hard to be refined. From the above, it is possible to improve the toughness by adjusting the welding conditions so as to effectively utilize the welding heat input and making the solidification structure finer. First, the reason why the welding speed is limited will be described. FIG. 1 shows the influence of the welding speed on the impact value and the average welding amount of each pass. 7cm / min welding speed
In the following, since the moving speed of the heat source is slow, the heating time from the rear pass becomes longer, and the structure of the front pass is sufficiently tempered. In this case, at 20 ° C., which is the required impact value, it satisfies 50 J / cm ² or more, although it depends on the amount of welding. However, when the speed is higher than 7 cm / min, the heating time is short, so that the structure of the previous pass is not sufficiently tempered. Therefore, the required impact value is not satisfied regardless of the welding amount.
From the above, the welding speed needs to be 7 cm / min or less. Next, the reason why the welding amount is limited will be described. Since the weld metal immediately after welding is also considered to be one heat source, if the amount of welding per pass is small, the heat capacity of the weld metal that has been welded becomes small, so that it is easy to cool, and the weld metal of the previous pass is sufficiently tempered. Does not reach. For this purpose, a certain amount of welding is required, but according to the results shown in FIG. 1, the lower limit of the welding amount is that the welding amount is 0.7 g / c.
If m or more, the required impact value can be satisfied. For this reason, in the present invention, the lower limit of the amount of welding per pass is set to 0.7 g / cm. When welding is performed within the range of the present invention, the welding operation time is equivalent to the conventional condition because the welding amount is sufficiently large even if the welding speed is low. However, when the amount of welding per pass increases, the temperature difference between the bottom and the surface of the weld metal that has been welded increases, and hot cracking may occur. According to the results shown in FIG. 1, when the amount of welding exceeds 2.5 g / cm, hot cracking is likely to occur. Therefore, in the present invention, the upper limit of the amount of welding per pass is set to 2.5 g / cm or less. Next, the reason why a limit is imposed on the stack height of one pass will be described. One of the factors that determine toughness is microstructure.
In the case where a tempered low alloy steel obtained by adding W, V, or Nb alone or in combination with the steel is welded by a low heat input welding method, a large solidified structure remains in the structure of the weld metal. 1-2.
A tempered low alloy steel obtained by adding W, V, or Nb alone or in combination to 25CrMo steel has a composition that is not easily tempered by the influence of additional elements such as V. The welding material of this component system has a low heat input. In the welding method, the weld metal of the previous pass cannot be sufficiently tempered.

[0007]

[Table 1]

[0008]

[Table 2]

As shown in the welding conditions of the comparative example in Table 2, a lamination height (pass height) of less than 1.0 mm does not satisfy the required impact value. The lower limit of the height was set to 1 mm or more. On the other hand, if the lamination height in one pass is too high, the region that is not affected by the heat from the previous pass and remains as a solidified structure increases, so that the toughness cannot be improved. As shown in Table 1, when the lamination height of one pass is 3.6 mm or less, the required impact value is satisfied. Therefore, in the present invention, the upper limit of the lamination height of one pass is 3.6.
mm. In the present invention, by using a hot wire TIG welding method capable of independently controlling the welding speed and the welding amount, a larger heat input can be obtained than in normal TIG welding, and the toughness can be further improved. Becomes

[0010]

Next, an example of an embodiment of the present invention will be described in more detail. FIG. 2 schematically shows a cross section of the welded joint. The groove of the base material 1 is butted, and the pass 3 is formed by the welding material 2. Pass 3 is
Under the influence of heat from the subsequent pass 3 ', a partially tempered structure 4 is formed. Then, the welded joint 5 is manufactured by the plurality of passes 3. In addition, h shows the lamination | stacking height of 1 pass. Table 1 above shows welding conditions, impact values, and path heights h of the welding method described in the present embodiment. Table 2 above shows welding conditions as a comparative example. The chemical components of the test material as the base material 1 shown in Tables 1 and 2 are summarized in Tables 3 and 5.

[0011]

[Table 3]

[0012]

[Table 4]

[0013]

[Table 5]

[0014]

[Table 6]

Table 3 shows 2.25Cr-Mo-WVN.
Welding was performed using a test wire which is a chemical component of the base material 1 of the b-based ferritic steel and is a welding material 2 for the 2.25Cr-Mo-W-V-Nb-based ferritic steel shown in Table 4. Table 5
Is a chemical composition of the base material 1 of the 1Cr-Mo-V ferritic steel, and was welded using a test wire which is a welding material 2 of the 1Cr-Mo-V ferritic steel shown in Table 6. A JIS No. 4 impact test piece was collected from the center of the plate thickness of the produced welded joint. The notch direction was the side, and the notch position was the center of the weld metal. Using these test pieces, J
An impact test was performed based on IS Z2242. The temperature of the impact test was 20 ° C., and three impact tests were performed under each test condition. Embodiments and 1Cr-Mo-
V-based ferritic steel, embodiments, and 2.
It is a TIG welded joint of 25Cr-Mo-V-Nb-based ferritic steel. Among them, the embodiment and the case where the hot wire TIG welding method is used. All use automatic TIG welding equipment, welding speed is 7c
m / min or less, and the welding amount is 0.7 g / cm.
Adjusted to the range of ~ 2.5 g / cm. As a result of observing the microstructure, a region more than half of the plate thickness showed a fine martensite structure having a crystal grain size of 6 or more. As is clear from Table 1, the average value of the impact values of Embodiments 1 to 20 is 20
It was 50 J / cm ² or more at ° C. In the present embodiment,
Although an automatic TIG welding device was used because the welding speed and the amount of welding can be easily controlled, welding can also be performed manually. Comparative Examples A and B shown in Table 2 have a welding speed of 7.0 cm / min or less, but a welding amount of 0.1 cm / min.
It is less than 7 g / cm. In Comparative Examples A and B, 20 ° C.
Average value of the impact value was 50 J / cm ² or less. Comparative Examples C and D show the case where the welding speed is 7.0 cm / min or less, but the welding amount is 2.5 g / cm or more.
In each of these, hot cracking occurred. Comparative Examples E, F, G, H, I and J are all cases where the welding speed is 7 cm / min or more. Comparative Example E
Is an example of a conventional low heat input welding method. In these comparative examples, the impact value at 20 ° C. was 50 J regardless of the welding amount.
/ Cm ² or less, and does not satisfy the required impact value. Further, in Comparative Examples E and H, as in Comparative Examples A and B, the pass height was less than 1 mm, and as a result of microstructure observation, a region having a half or more of the plate thickness remained a solidified structure. Was.

[0016]

According to the present invention, 1-2.
Even in the weld metal of tempered ferritic steel in which W, V or Nb is added alone or in combination with steel, a high toughness weld metal can be obtained, and brittle fracture of structures such as boilers can be reliably prevented. Therefore, there is a great effect in improving the safety and reliability of the structure.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the welding speed and the average amount of deposition in each pass on the impact value of a weld metal of a TIG welded joint of the present invention.

FIG. 2 is a schematic view showing an example of a cross-sectional structure of a welded joint exemplified in the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Base material 2 ... Welding material 3 ... Pass 3 '... Subsequent pass 4 ... Tempered structure 5 ... Weld joint h ... Stack height of 1 pass

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/00 301 C22C 38/00 301B // B23K 101: 14

Claims (3)

[Claims] A welded joint for welding tempered ferritic steels obtained by adding W, V or Nb alone or in combination to 1-2.25CrMo steel, wherein the number of passes is at least three or more. The number of weld metal layers is 3 or more, and the weld metal layers other than the final weld metal layer have a fine martensite structure with a crystal grain size number of 6 or more, and 2
A TIG welded joint having an impact value of not less than 50 J / cm ² at 20 ° C. measured with a mmV notch impact test piece. 2. The method for welding a TIG welded joint according to claim 1, wherein the welding speed of the weld metal layer after the second pass is 7 cm / min or less, and the amount of welding per pass is 0. A TIG welding method, wherein welding is performed by adjusting the range from 0.7 g / cm to 2.5 g / cm. 3. The method for welding a TIG welded joint according to claim 1, wherein the welding speed of the weld metal layer after the second pass is set to 7 cm / min or less, and the height of the welded layer per pass. The welding method is characterized in that the welding is performed by adjusting the thickness to a range of 1.0 mm or more and 3.6 mm or less.