JP7652176B2 - Box welded joint, box welding method, and method for manufacturing welded joint - Google Patents

Description

The present invention relates to a technology for box welding between main plates and gussets that is widely used when constructing steel structures, and more specifically to a box welded joint, box welding method, and welded joint manufacturing method that are suitable for steel structures that require excellent fatigue properties, such as steel bridges and ships.

In general, in steel structures, there are many box welded joints in which the periphery of a gusset 2 is welded to the main plate 1 (so-called box welding) as shown in Figure 2. In a box welded joint, a weld bead 3 surrounds the gusset 2, and stress concentration occurs at the toe of the weld bead 3. As a result, the welding residual stress caused by the box welding and the repeated stress caused by external forces are superimposed, causing fatigue cracks, which then propagate and cause fatigue failure. Note that an external force is a load that repeatedly acts on a steel structure from the outside. For example, if the steel structure is a steel bridge, it is a load that repeatedly occurs due to natural weather conditions (such as wind) or vehicle traffic. If the steel structure is a ship, it is a load that repeatedly occurs due to vibrations caused by power or wind and waves.

In recent years, as steel structures age, there have been an increasing number of reports of fatigue-related damage. To prevent such damage, it is necessary to regularly inspect steel structures, monitor the progress of the damage, and take measures according to the progression of the damage. In particular, when fatigue-related damage occurs in steel bridges, it is possible to reduce the external forces acting on the steel bridge by restricting vehicle traffic, but this causes traffic congestion and delays in logistics, thus having a significant negative impact on social activity. Therefore, various technologies have been proposed to improve the fatigue properties of box welded joints in steel structures.

For example, Patent Document 1 describes how fatigue life can be improved by forming extension beads on the upper surface of the main plate from both longitudinal ends of the gusset.

In addition, in the technology disclosed in Patent Document 2, a first weld bead is formed along the short side of the rectangular abutment surface where the gusset abuts against the main plate. The first weld bead is then formed by extending from both sides of the short side of the rectangular abutment surface onto the main plate. Next, a second weld bead and a third weld bead are formed along the long side of the rectangular abutment surface. The second weld bead and the third weld bead are then formed by covering the first weld bead and extending beyond the first weld bead onto the main plate.

JP 2014-233747 A JP 2018-158380 A

Satoshi Iki et al., "High-performance steel for shipbuilding - JFE Steel's life cycle cost reduction technology", JFE Technical Report, No. 5, August 2004, pp. 13-18 Satoshi Iki et al., "Effect of Microstructure on Fatigue Crack Propagation Rate", JFE Technical Report, No. 33, February 2014, pp. 55-61

However, the conventional technology has the following problems.
The technique of Patent Document 1 requires the formation of an extended bead in a shape that covers the welded turn, which results in a long welding time and poor workability. The technique described in Patent Document 2 also requires a long welding time and poor workability.

The present invention was made in consideration of the above circumstances, and aims to provide a turn welded joint, a turn welding method, and a method for manufacturing a welded joint that can dramatically improve fatigue strength using simple means.

The box welded joint of the present invention, which advantageously solves the above-mentioned problems, is a box welded joint obtained by box welding a gusset to a main plate, and is characterized in that one or both sides of an end face of the gusset perpendicular to the main plate are chamfered, and the length (t) of the short side of the abutment surface of the gusset that abuts against the main plate is in the range of 0.2 to 0.8 times the plate thickness ( _t0 ) of the gusset.

In addition, the box welded joint according to the present invention is
(a) the chamfered range of the end face of the gusset is such that the length in a direction perpendicular to the main plate is equal to or greater than the height of the weld bead in the direction perpendicular to the main plate;
(b) the main plate is made of fatigue-resistant steel;
This may be a more preferable solution.

The box welding method of the present invention, which advantageously solves the above-mentioned problems, is a box welding method for joining a gusset to a main plate by box welding, characterized in that one or both sides of an end face of the gusset perpendicular to the main plate are chamfered, and when the gusset is abutted against the main plate, the length (t) of the short side of the abutting surface is in the range of 0.2 to 0.8 times the plate thickness ( _t0 ) of the gusset.

The turning welding method according to the present invention is as follows:
(a) the range of chamfering the end face of the gusset is set to a length, in a direction perpendicular to the main plate, equal to or greater than the height of the weld bead in the direction perpendicular to the main plate;
(b) using fatigue-resistant steel for the main plate;
This may be a more preferable solution.

The method for manufacturing a welded joint according to the present invention, which advantageously solves the above-mentioned problems, is a method for manufacturing a welded joint obtained by joining a gusset to a main plate by turn welding, and is characterized in that one or both sides of an end face of the gusset perpendicular to the main plate are chamfered, and when the gusset abuts against the main plate, the length (t) of the short side of the abutting surface is in the range of 0.2 to 0.8 times the plate thickness ( _t0 ) of the gusset.

The present invention provides a turn welded joint, a turn welding method, and a method for manufacturing a welded joint that dramatically improves the fatigue strength of the weld bead in the turn welding of a gusset using simple means. At the same time, it is possible to avoid problems and deterioration of the performance of steel structures due to fatigue fracture from the welded portion.

1A is a schematic diagram showing a boxing welded joint according to one embodiment of the present invention, in which (a) is a perspective view, (b) is a top view, and (c) is a cross-sectional view showing a fatigue fracture surface of a main plate. 1A is a schematic diagram showing a boxing welded joint according to a conventional technique, in which (a) is a perspective view, (b) is a top view, and (c) is a cross-sectional view showing a fatigue fracture surface of a main plate.

The following is a detailed description of the embodiments of the present invention. Note that the drawings are schematic and may differ from the actual product. The following embodiments are intended to illustrate the equipment and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to that described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

The inventors considered that it is important to suppress both the initiation and propagation of fatigue cracks in order to increase the fatigue strength of boxing welded joints. As a result of examining the mechanism by which fatigue cracks occur in boxing welded joints, it was clarified that fatigue cracks occur by the following mechanism. Figure 2 shows a conventional boxing welded joint. Figure 2(a) shows a perspective view of a boxing welded joint in which a gusset 2 is box-welded to a main plate 1a of conventional steel with a weld bead 3. Figure 2(b) shows a top view. Figure 2(c) is a cross-sectional view showing a fatigue fracture surface 4a of the main plate 1a near the weld bead toe 31 of the gusset end surface 23. As shown in Figure 2(c), on the short side 23a side of the approximately rectangular abutment surface where the gusset 2 abuts against the main plate 1a, the toe of the weld bead 3 becomes a stress concentration source, and fatigue cracks occur from there. In many cases, multiple small cracks occur at the toe of the weld bead 3. It was found that these microcracks grow and merge to form a fatigue fracture surface 4a, which then forms a large fatigue crack. This is because the shape of the weld bead toe 31 is inevitably not uniform, and the degree of local stress concentration varies. In addition, after a fatigue crack initiates from the weld bead toe 31, the crack propagates to the main plate, causing fatigue failure. In this case, the length b of the region where cracks can initiate is approximately equal to the gusset plate thickness _t0 .

An example of a boxing welded joint developed based on the above-mentioned study is shown in FIG. 1. FIG. 1(a) shows a perspective view of a boxing welded joint in which a gusset 2 is box-welded to a main plate 1 made of fatigue-resistant steel with a weld bead 3. FIG. 1(b) shows a top view. FIG. 1(c) is a cross-sectional view showing a fatigue fracture surface 4 of the main plate 1 near a weld bead toe 31 of a gusset end surface 22. As shown in FIG. 1, by making the length of the short side 22a of the contact surface of the gusset that contacts the main plate smaller than the plate thickness t ₀ of the gusset, the range in which microcracks can occur before coalescence can be limited and the number of microcracks before coalescence can be reduced. Therefore, it was found that it is possible to prevent cracks from growing suddenly large due to coalescence. If the length t of the short side 22a of the contact surface of the gusset 2 that contacts the main plate 1 is larger than 0.8 times the plate thickness t ₀ of the gusset, the effect of suppressing crack coalescence cannot be obtained. For this reason, the length t of the short side 22a of the abutment surface of the gusset 2 that abuts against the main plate 1 is set to 0.8 times or less the plate thickness _t0 of the gusset. More preferably, the length t of the short side 22a of the abutment surface of the gusset 2 that abuts against the main plate 1 is set to 0.7 times or less the plate thickness _t0 of the gusset. Here, the short side 22a of the abutment surface of the gusset 2 refers to a surface that is perpendicular to the wide surface of the surfaces perpendicular to the main plate surface of the gusset 2 that is erected perpendicular to the main plate 1. In the example of Fig. 1, it refers to a surface that is chamfered from both sides of the gusset end surface 23 in Fig. 2 and has a narrower width in the plate thickness direction.

On the other hand, if the length t of the short side 22a of the contact surface of the gusset 2 that contacts the main plate 1 is made too short, stress concentration increases. It has also been found that although crack coalescence is suppressed, the formation of fatigue cracks may be accelerated due to the action of excessive stress. If the length t of the short side 22a of the contact surface of the gusset 2 that contacts the main plate 1 is smaller than 0.2 times the plate thickness t ₀ of the gusset 2, the fatigue life reduction due to the large stress concentration may exceed the fatigue life extension effect due to the crack coalescence suppression. Therefore, the overall fatigue life extension effect cannot be obtained. Therefore, the length t of the short side 22a of the contact surface of the gusset 2 that contacts the main plate 1 is set to 0.2 times or more the plate thickness t ₀ of the gusset 2. More preferably, the length t of the short side 22a of the contact surface of the gusset 2 that contacts the main plate 1 is set to 0.3 times or more the plate thickness t ₀ of the gusset 2.

From the above, the fatigue life of the box-welded structure can be improved by controlling the length t of the short side 22a of the contact surface of the gusset 2 that contacts the main plate 1 within an appropriate range, such as 0.2 to 0.8 times the plate thickness t0 of the gusset ₂ .

In order to make the length t of the short side 22a of the abutment surface of the gusset 2 that abuts against the main plate 1 smaller than the plate thickness t ₀ of the gusset, the edge of the end surface 23 of the gusset 2 perpendicular to the main plate 1 can be chamfered as a simple method. The chamfered surface 21 may be one side of the end surface of the gusset 2, or both sides of the end surface of the gusset 2. The chamfered shape may be either flat or curved. It is sufficient as long as it has a shape that can reduce the crack initiation area length a. In addition, it is preferable that the chamfered surface 21 has a length from the end that abuts against the main plate 1 in a direction perpendicular to the main plate, which is equal to or greater than the height h of the weld bead in a direction perpendicular to the main plate. In addition, the angle that the chamfered edge 21a makes with the short side 22a of the abutment surface at the surface where the gusset 2 abuts against the main plate 1 is preferably in the range of 15° to 75°, more preferably in the range of 30° to 60°.

In this embodiment, the effect can be achieved regardless of the material of the main plate 1 and gusset 2. In particular, since the size of the crack front on the main plate 1 side at the early stage of fatigue crack initiation can be limited, a further longer life can be expected by applying a main plate 1 with a low fatigue crack propagation rate (fatigue cracks are less likely to propagate). It is preferable to use fatigue-resistant steel for the main plate 1. Fatigue-resistant steel is a steel material that improves the fatigue crack propagation resistance of the base material, or improves the fatigue crack initiation resistance of the welded joint, or improves both properties, thereby extending the fatigue life of a welded steel structure. In general, a slab with appropriate chemical composition is manufactured through an optimal cooling and rolling process. For example, a steel material with a fatigue crack propagation speed about half that of conventional materials, as described in Non-Patent Document 1 and Non-Patent Document 2, can be used.

Steel types having the composition shown in Table 1 were used for the main plate and gusset, and boxing welded joints were fabricated. The repeated stresses shown in Table 2 were applied, and the life until fracture, that is, the number of repeated stresses, was evaluated. The plate thickness _t0 of the main plate and the gusset was the same. The chamfering of the gusset end surface was a 45° flat surface. Steel types A to C are conventional steels. Steel type D is a fatigue-resistant steel.

Test No. 5 in Table 2 is a conventionally welded joint shown in Figure 2, and a joint with a longer fracture life (more repeated stresses) was considered to be preferable. All of the inventive examples had a preferable fracture life.

The turn welded joint, turn welding method, and welded joint manufacturing method of the present invention are industrially useful because they can avoid performance degradation and problems in steel structures caused by fatigue fractures at the welded parts.

1. Main plate (fatigue-resistant steel)
1a Main plate (conventional steel)
2 Gusset 21 Chamfered surface 21a Chamfered edge 22 Gusset end surface 22a Contact surface short side 23 Gusset end surface 23a Contact surface short side 3 Weld bead 31 Area where cracks may occur (toe of weld bead)
4 Fatigue fracture surface 4a Fatigue fracture surface (crack coalescence)
t: Length of short side of contact surface t0 _: Gusset plate thickness a, b: Length of area where cracks can occur h: Height of weld bead

Claims (7)

A box weld joint obtained by box welding a gusset to a main plate,
One or both sides of the end face of the gusset perpendicular to the main plate are chamfered,
A boxing welded joint, wherein a length (t) of a short side of a contact surface of the gusset that contacts the main plate is in a range of 0.2 to 0.8 times a plate thickness (t ₀ ) of the gusset. The turn welded joint of claim 1, in which the chamfered area of the end face of the gusset is perpendicular to the main plate and has a length equal to or greater than the height of the weld bead perpendicular to the main plate. A welded joint as described in claim 1 or 2, in which the main plate is made of fatigue-resistant steel. A method of joining a gusset to a main plate by turn welding, comprising the steps of:
One or both sides of the end face of the gusset perpendicular to the main plate are chamfered;
The box welding method comprises: when the gusset is brought into contact with the main plate, a length (t) of a short side of the contact surface is set in a range of 0.2 to 0.8 times the plate thickness (t ₀ ) of the gusset. The turn welding method according to claim 4, in which the chamfering range of the end face of the gusset is set to a length in a direction perpendicular to the main plate that is equal to or greater than the height of the weld bead in a direction perpendicular to the main plate. The turn welding method according to claim 4 or 5, in which fatigue-resistant steel is used for the main plate. A method for manufacturing a boxing welded joint obtained by boxing welding a gusset to a main plate, comprising:
One or both sides of the end face of the gusset perpendicular to the main plate are chamfered;
The method for manufacturing a welded joint, wherein when the gusset abuts against the main plate, the length (t) of the short side of the abutting surface is in the range of 0.2 to 0.8 times the plate thickness (t ₀ ) of the gusset.

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