JPS6068176A - Butt welding method of heavy-gauge low-alloy steel - Google Patents

Abstract

PURPOSE:To perform satisfactorily fitting welding without possibility of SR cracking by build-up welding the part near the rear side of the groove of heavy- gauge low-alloy steels with a welding metal consisting of an alloy having the lower sensitivity with SR cracking than the base metals then subjecting the steel to normal butt welding without SR treatment. CONSTITUTION:A buttering weld zone N is formed only on the rear side near the rear side part of the groove of light-gauge low-alloy steels at such a width that extends to about 10mm. from the center of the regular weld line by using a welding material consisting of an alloy comps. having the lower sensitivity with SR cracking than the base metals in the stage of butt welding the light-gauge low-alloy steels. The regular butt weld zone M is then formed as it is without SR treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a butt welding method that reduces stress relief annealing (SR) cracking that is often encountered in welding low alloy steels, and in particular to butt welding for insert weld joints of thick plate materials. It is about the method.

The cause of SR cracking is thought to be that SR treatment is performed with high tensile residual stress being applied to the weld heat-affected zone or the grain coarsening region that forms in the weld metal. The susceptibility to SR cracking varies depending on the alloy composition, and SR cracking occurs particularly frequently in thick plate butt welded joints of cast steel and forged steel in which the Cr-MO=■ component system is expressed. For example, in the welded parts of the nozzles and lids of high-temperature pressure vessels consisting of component systems such as
There is a strong risk of cracking, and since it is a high-temperature pressure vessel, there is a potential for serious accidents to occur. Therefore, regarding SR cracking, 1. Theoretical studies on metallurgical phenomena, studies on alloy compositions for cracking susceptibility, and studies on welding methods have been carried out, and each has achieved results. No solution has yet been established as to what kind of construction method should be used to prevent the occurrence of cracks in such joints.

The present invention belongs to a technical achievement based on a study from the aspect of a welding method, and is a result of pursuing problems in previous studies on welding methods.

In other words, as an example of the study regarding the welding method in 1.
1) A method of buttering welding the entire surface of the butt groove with a welding material that is less susceptible to SR cracking than the main body, then applying SR treatment, and then performing main welding, and (2) a method near the toe of the butt weld. A method is known in which a reheated bead is welded to the base metal to refine the crystal grains, especially in the heat affected zone of the base metal. However, in method 1), there are problems summarized as shown in (1) to (2) below.

(2) Although it is considered to be an effective method for butt joints with relatively small plate thicknesses, the number of man-hours for battering welding itself increases significantly for plates with a thickness of 100 mm or more.

■When this method is applied to the inset welded joints of thick plates, which is one of the objects of the present invention, it is not an appropriate measure to prevent cracking for the reasons described later, although it requires a large amount of man-hours. The man-hours and costs required for battering are wasted.

■Depending on the shape of the joint, there is a risk of SR cracking occurring during the SR treatment after buttering welding.

In addition, in the method r2) above, (1) even though reheating does not affect the weld bead, the heat-affected zone of the bead has coarse grains and is likely to become a hardened structure; (2) in the vicinity of the Uranami bead; There are problems such as inapplicability.

Both of these methods (1) and (2) were considered for practical application simply from the perspective of metallurgical phenomena, that is, the relationship between cracking susceptibility and alloy composition or metal structure, but as mentioned above, the occurrence of SR cracking Since residual stress is an essential factor as a cause, construction methods that ignore residual stress or residual stress distribution in individual joints are not wasteful and foolish;
It may even be dangerous. It goes without saying that if the welding method used for general steel types is used, in which welding is performed without buttering the groove surface, there is a high risk of SR cracking occurring.

The present invention has been made with attention to the above circumstances, and
The present invention aims to provide a butt welding method that not only does not cause SR cracking but also produces a thick plate welded part with high joint strength.

However, the present invention is based on the base material! 11 After overlay welding near the back surface of the groove of a thick plate of low alloy steel using a welding material with an alloy composition that has low ISR cracking susceptibility, it can be done without SR treatment! The main point is that actual butt welding is performed.

The content of the present invention will be explained in detail below while clarifying the background of the experimental considerations.

Generally, SR cracking often occurs near the bead toe on the front or back surface of a butt weld. Therefore, in order to study the prevention of SR cracking, the present inventors determined the welding method in particular in relation to the residual stress distribution on the front and back surfaces of the joint, the SR cracking susceptibility of each part of the joint, and geometric factors such as the presence or absence of notches. Considering the importance of this, we conducted repeated experiments to clarify the relationship. Figure 1 shows the direction perpendicular to the weld line on the front and back surfaces of the weld when the welded joint of a thick plate of Cr-Mo-V steel as shown in Figure 2 is welded after initial layer welding. This figure shows the results of measuring residual stress distribution. In the figure, S is a thick plate cylindrical body, C is a thick plate disc fitted into the body S, W is a main welding part, and the unit of dimension is mm. The results in Figure 1 show that the residual stress in the direction perpendicular to the weld line is compressive stress in the weld metal on both the front and back surfaces and in the vicinity of the bead toe, which is considered to be most susceptible to SR cracking, while the peak of tensile stress is at the toe. Approximately 20 to 2
It was found that it appeared on the base metal side 5 mm away.

Next, Figure 3 shows a welded joint made of a thick plate of Cr-Mo-V steel similar to that shown in Figure 2, after buttering has been applied to only the front and back surfaces of the thick plate (SR treatment is not performed after buttering welding), and the actual welding is performed. This shows the residual stress distribution in the direction perpendicular to the weld line on the front and back surfaces of the weld. As is clear from Fig. 3, even when backling welding is performed in advance, the stress distribution form is almost the same as that shown in Fig. 1, and the base metal is located 20 to 25 mm away from the bead toe on both the front and back surfaces. A peak of tensile stress appears on the side. However, in this case, since the toe of the buttering weld almost coincides with the peak of the tensile stress, it can be seen that this is an extremely dangerous situation.

From the above, when viewed from the residual stress distribution in the direction perpendicular to the weld line, it can be said that not buttering is more advantageous in terms of preventing cracking.

Next, FIGS. 5 and 6 show residual stress distributions in the weld line direction on the front and back surfaces of each weld, that is, in each of the welds shown in FIGS. 2 and 4. According to these results, the surface side generally has tensile stress, but the toe during actual welding has a much lower value.

On the other hand, on the back side, both the weld metal part and the toe at the time of main welding have a considerably high tensile stress.

As mentioned above, it is difficult to make the back bead surface smooth because Uranami welding is applied to this welded joint. Therefore, considering the case where the Uranami weld bead shape or the upper end shape becomes irregular, this part should be
It can be said that it is extremely dangerous to use a composition that is highly susceptible to cracking.

Moreover, FIG. 7 shows the residual stress distribution in the plate thickness direction at the center of the weld line. According to the distribution diagram, the residual stress in the direction perpendicular to the weld line becomes tensile stress at 20 mm inside from the front and back surfaces, but the value itself is 10 kg/mm2.
The following is not enough to raise the issue. In addition, regarding the residual stress in the weld line direction, there is no part of the plate thickness that is particularly higher in stress than the front and back surfaces, so it can be understood that the stress on the front and back surfaces described above should be taken into consideration to prevent SR cracking.

Next, the results of a study on the required amount of buttering on the back side will be explained. As shown in FIG. 3, when the buttering width on the back surface is increased to about 20 mm to 25 mm, the toe of buttering welding coincides with the tensile stress peak in the direction perpendicular to the weld line. Therefore, the upper end of buttering welding should be at a position where the residual stress in the direction perpendicular to the welding line and in the direction of the welding line are both low, that is, the toe of buttering welding should be approximately 10 mm from the center of the welding line of the main weld. It can be said that it is appropriate to butter the width to a certain extent. FIG. 8(a) is a schematic cross-sectional view of the back side welded part subjected to buttering welding in this way, and FIG. 8(1)) is a residual stress distribution diagram in the welding line direction and in the direction perpendicular to the welding line in the welded part. The main welded portion in the figure has a high residual stress in the direction of the weld line, as described above. However, since this part is composed of buttering weld metal, the SR cracking sensitivity is low, similar to the M part, that is, the main weld and the -do toe part.

In addition, the N part, which is about iomm away from the center of the weld line, that is, the upper end part during buttering welding (base metal heat affected zone), is a part that is highly susceptible to SR cracking, but the residual stress in the direction perpendicular to the weld line is zero or compressive stress. The stress in the weld line direction is also lower than that of the actual weld. Furthermore, it is advantageous to be able to smoothen the surface of the N part by machining or the like after buttering welding in order to prevent SR cracking. Therefore, as mentioned above, when carrying out the present invention in which buttering welding is performed only on the back side with a width of about 10 mm from the center of the weld line of the main weld, and then main welding is performed without performing SR treatment, SR cracks It has become clear that a plate joint with extremely low risk of occurrence can be obtained.

It should be noted that the buttering thickness from the back side in the plate thickness direction does not need to be too large from the perspective of residual stress, and is to the extent that the SR cracking susceptibility of the buttering part does not noticeably increase due to the dilution of the base material main components. (for example, the component dilution rate is less than 10%).

Next, examples of the present invention will be shown.

Inset welded joints of thick plates of low-alloy heat-resistant steel with the components shown in Table 1 (the joint shape is the same as in Figure 2) were welded using various construction methods, and the presence or absence of SR cracking was comparatively investigated. Table 2 shows the construction conditions.

That is, in the table, A and B are methods without buttering and full-face buttering, respectively, which are cited as conventional methods, C is the above-mentioned front and back buttering method, which does not belong to the conventional method, and D is a method according to the present invention. As the welding material, a commercially available coated arc welding rod for heat-resistant steel was used for both buttering and main welding. Further, SR treatment after bagling was not performed other than method B.

The SR treatment after main welding was performed at 690°C for 10 hours.

A penetrant test was conducted on the welded portion after the SR treatment to check for cracks. The results are shown in Table 3, which also lists the required welding man-hours for each construction method.

(%) Table 3 The results in Table 3 can be summarized as follows.

In method A, small horizontal cracks occur on the backside weld bead surface,
In method B, cracks were observed at the upper end of the buttering on the fuselage side during the SR treatment after bagling welding, so actual welding was not performed. In method C, cracks mainly occurred near the battering weld toe on the plate surface side. In contrast, D of the present invention
No cracks were observed on either the front or back surfaces using the method.

On the other hand, looking at the required man-hours, method A requires 60 hr (main welding only), method B requires 60 hr for battering only, and method C requires 50 hr for battering + 60 hr for main welding, totaling 110 hr, while method D of the present invention requires battering Sh. ten welding 6
It can be seen that the number of man-hours required for buttering is extremely small, with a total of 65 hours in 0 hours.

2.Although no experiments have been conducted, it is conceivable that cracks would not have occurred if the SR treatment after battering had not been performed in method B. However, the number of man-hours required for battering welding in this case is 60 hours, which is more than 10 times that of the method of the present invention, so it is extremely wasteful, and even if SR treatment is not performed, it is not a method worth considering. ) is not available.

Since the present invention is constructed as described above, butt welding, especially fit welding, of thick low alloy plates can be performed satisfactorily without fear of SR cracking.

[Brief explanation of drawings]

Figures 1 and 3 are explanatory diagrams of the distribution of residual stress in the direction perpendicular to the weld line on the front and back surfaces of a thick plate inset welded joint, Figures 2 and 4 are explanatory diagrams of the shape of a thick plate inset welded joint, and Figures 85 and 3 are Figure 6 is an explanatory diagram of the residual stress distribution state in the weld line direction on the front and back surfaces of a thick plate inset welded joint, and Figure 7 is an explanatory diagram of the residual stress distribution state in the plate thickness direction at the center of the weld line of a thick plate inset welded joint. Figure 8(a) is a schematic cross-sectional view of the back side weld of a thick plate inset weld joint, Figure 8(b) is an explanatory diagram of the residual stress distribution state in the weld line direction and the direction perpendicular to the weld line in the back side weld. Figures 9 to 12 are explanatory diagrams of the construction conditions of the example. C...Thick plate disk S...Thick plate cylindrical body W...Main welding part B Buttering Department Applicant: Kobe Steel, Ltd. Figure 1 Figure 2 Figure 4 1111-φ600-- --- Figure 3 Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] In a method of butt welding thick low alloy steel plates, SR treatment is performed after overlay welding near the back surface of the groove of the thick copper plate using a welding material made of an alloy composition with lower SR cracking susceptibility than the base metal. A method for butt welding thick plates of low-alloy steel, which is characterized in that actual butt welding is performed as is.