JP3900490B2 - Fatigue reinforcement method for girders with flange gussets - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fatigue reinforcement method for improving durability of a girder structure having a flange gusset as a fatigue weak point portion in a crane traveling girder formed by welding and joining metal materials.
[0002]
[Prior art]
The durability of metal parts is often defined by fatigue. In a metal member configured by welding and joining a plurality of metal materials, the welded portion becomes a fatigue weak point portion, which is a factor of reducing the durability of the metal member. For this reason, various measures for improving the fatigue of welds have been taken. In particular, in a girder structure having a flange gusset, fatigue cracks are likely to occur from the flange side toe of the welded portion of the flange gusset end.
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 54-56953 [Patent Document 2]
JP 62-207579 A [Patent Document 3]
JP-A-10-279273 [Non-patent Document 1]
(Surface Nanocrystallization (SN C) of metallic Materials-Presentation of the Concept behind a New Approach, J. Master. Sci. Technol. Vol. 15 No. 15).
[0004]
There are roughly two types of measures for improving the fatigue of a welded portion of a metal member. First, there are grinding, TIG dressing, etc., which reduce the stress concentration by changing the shape of the portion where fatigue is a problem. In addition, there are hammer peening, needle peening, shot peening, low temperature transformation melts, etc., which give compressive residual stress to a portion where fatigue is a problem and reduce a substantial repetitive stress range. Of these, hammer peening is said to have both the effect of reducing stress concentration and the effect of introducing compressive stress.
[0005]
[Problem to be Solved by the Invention]
Of the above-mentioned measures to improve fatigue, the effects of measures that reduce stress concentration are visibly obvious, but in fact, in places where fatigue is a problem, slight scratches or the like rather deteriorates the fatigue strength. Therefore, the grinder processing and the like not only require skill in processing but also require time for work, which is a significant cost increase factor.
[0006]
Also, regarding TIG dressing, skilled workers are required for the work, and when applying heat to the application site, when using it for repairing metal members, etc., in order to prevent hot cracking of the welding material due to stress fluctuations During work, it is necessary to stop the use of traveling girders and so on.
[0007]
On the other hand, it is a treatment measure that introduces compressive residual stress, but since compressive residual stress is invisible, it is difficult to measure the effect after treatment and it is difficult to guarantee the effect by inspection. It is not usually used in situations where engineers with the ability to judge and diagnose cannot be present from the viewpoint of quality control.
[0008]
In addition, in hammer peening, since a large plastic deformation can be given to the processing part, the trace of the processing can be enlarged and the processing can be specified after the implementation, but the surface scratches that can be made at the time of processing can change the stress concentration, The fatigue strength may be lowered and the workability is remarkably poor due to the large reaction when the plastic deformation is given, so fine control is difficult and quality control is very difficult.
[0009]
In addition, when the above-described fatigue improvement treatment method that introduces compressive residual stress is used for repair of metal members, at a small point of 1 mm or less, which is the initial stage of fatigue crack generation, a penetrant flaw detection test, magnetic particle flaw detection test, eddy current Although detection is not possible with current inspection methods such as flaw detection tests, it is not possible to stop the progress of cracks even if the above-mentioned fatigue life improvement treatment measures are applied in a state where such cracks remain. In addition, it is considered that there is almost no effect of improving fatigue life by introducing compressive residual stress.
[0010]
In addition, when using a low-temperature transformation melt for the welded part, and when introducing compressive residual stress to the toe, the effect is high in high-strength steel, but the effect is almost lost in low-strength steel. As with TIG dressing, there are parts that are difficult to use due to construction problems, as well as the effects of compressive residual stress introduced in the same way as other treatment methods. Difficult to judge.
[0011]
As described above, the fatigue improvement treatment measures that reduce stress concentration mainly have problems in construction efficiency and the skill of the installer, while the fatigue improvement treatment measures that introduce compressive stress measure the effect. Thus, it is a problem that quality control cannot be performed, and for this reason, it has been difficult to use such a fatigue life improving treatment measure for a general metal member.
[0012]
The present invention has been made in view of the above circumstances, and as a method for improving the fatigue performance of a welded portion of a metal member, particularly a girder structure having a flange gusset that is prone to fatigue cracking, the tip of the tip is ultrasonically 20 μm to 60 μm. Using a tool that vibrates at a frequency of 15 kHz to 60 kHz, ultrasonic impact treatment is performed to perform peening that strikes the surface of the welded portion of the flange gusset, improving the fatigue strength of the welded portion, and having a highly durable flange gusset It aims at obtaining the fatigue reinforcement method of a girder structure.
[0013]
[Means for Solving the Problems]
In the first aspect of the present invention, in order to solve the above-mentioned problem, in the fatigue reinforcement method of a girder structure having a flange gusset, when the flange gusset end thickness of the welded portion of the flange gusset end is set to t In the range from the flange gusset end to at least 1 t on each of the upper surface and the lower surface, ultrasonic impact treatment was performed to form grooves to improve the toe shape.
[0014]
In the second aspect of the present invention, in the fatigue reinforcement method for a girder structure having the flange gusset according to the first aspect of the present invention, a non-destructive crack inspection test is performed before the ultrasonic impact treatment to confirm that there is no crack in advance. If a crack is found, grinding is performed with a grinder until the crack clearly disappears, and then ultrasonic shock treatment is performed.
[0015]
In the fatigue reinforcement method for a girder structure having a flange gusset according to the first invention, the third invention confirms that the leg length of the turn welding of the flange gusset is 5 mm or more before performing the ultrasonic impact treatment. In the case of less than that, after welding and increasing the leg length to 5 mm or more, ultrasonic shock treatment was performed at a temperature of 100 ° C. or less between the increased end of the bead and the former bead and the increased bead. Features.
[0016]
In the fourth aspect of the present invention, in the fatigue reinforcement method for a girder structure having the flange gusset according to any one of the first to third aspects of the invention, the ultrasonic impact treatment is performed periodically after the ultrasonic impact treatment is performed once. It is characterized by that.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The occurrence of fatigue failure of metal members is greatly influenced by stress concentration and residual stress. In a metal material subjected to a load, a transition accumulates in a stress concentration portion, which accumulates slip lines and develops into a crack, and after a crack occurs, it progresses. Residual stress is usually present as tensile residual stress in welds and the like, and it is considered that the effective repeated stress range is expanded to easily generate cracks and promote the opening of the generated cracks. Therefore, in order to improve the fatigue life of the metal material, it is necessary to alleviate the stress concentration and bring the residual stress as close to the compressed state as possible.
[0018]
The welded portion of the flange and the gusset has both a surface shape sudden change portion and a tensile residual stress, and is the weakest point in terms of fatigue strength. This suddenly changing portion of the surface shape acts as a notch and becomes a stress concentration portion, so that the stress concentration portion is plastically deformed, and the surface formed by a curved surface with a large toe radius is formed. The stress concentration part will be relaxed. At this time, if plastic deformation is applied in the plate thickness direction of the flange, the plasticized flange is restrained by the surrounding metal, thereby introducing a compressive force.
[0019]
As means for enabling plastic working on the welded portion between the flange and the gusset, peening that strikes the surface of the welded portion using a tool that vibrates the tip with an ultrasonic wave with an amplitude of 20 μm to 60 μm and a frequency of 15 kHz to 60 kHz. There is a process called an ultrasonic impact process. By using this method, plastic working can be performed on the surface of the welded portion, and compressive residual stress can be introduced to a depth of about 1.5 mm.
[0020]
This method of ultrasonic impact treatment basically does not change the basic mechanism for hammer peening and fatigue strength improvement, but changes the energy of one shot at a time to a small amount, and hits more than 10,000 times per second. By giving, the same plastic deformation is realized. In addition, since the impact force at one time is small, there is almost no recoil generated in the equipment, which is very advantageous in terms of usability and workability compared to hammer peening.
[0021]
In addition, this ultrasonic impact treatment has a very large number of impacts on the metal surface, and thus has an effect not found in conventional hammer peening on the metal surface. Moreover, since the impact energy per shot is larger than the shot peening, an effect not found in the conventional shot peening is brought about.
[0022]
First, processing uniformity can be obtained by hitting the surface many times. Even with hammer peening, it is known that a certain degree of uniformity can be obtained if several passes are performed on the same line, but the number of striking cycles of ultrasonic impact treatment is 15-60 kHz, and the obtained uniformity is hammer peening. If the processing speed is about 0.5 m / min, the surface of the welded portion is almost uniformly finished and no defects are left.
[0023]
Moreover, the surface after the treatment has a remarkable smoothness. The smoothness after the treatment by the ultrasonic impact treatment is remarkably smoother than the surface of the welded portion after the grinder finish.
[0024]
Further, it is known that the structure of the surface of the welded portion after the treatment becomes extremely fine by repeating plastic processing many times using ultrasonic waves. (See Non-Patent Document 1)
[0025]
Actually, as a result of using ultrasonic impact treatment for a metal material for the purpose of improving fatigue, the metal material structure is greatly changed before and after the treatment. Such an effect of making the metal material structure finer is particularly noticeable in the HAZ portion in the vicinity of the weld where the metal material structure becomes coarser, and the particle size of the HAZ that usually becomes coarser to 100 μm is an ultrasonic impact treatment. After the treatment, the particle size is so small that the particle size cannot be observed, and a unique metal material structure is achieved by the ultrasonic impact treatment.
[0026]
Further, the hardness increases as the microstructure of the metal material on the surface of the metal material becomes finer by the ultrasonic impact treatment. Regarding the hardness distribution of the base metal part, the molten metal part, and the HAZ part after the ultrasonic impact treatment, the hardness is increased by 20% or more particularly for high-strength steel often used as a weld metal material. In addition, depending on the material and processing time, the hardness may increase up to twice as much as before processing, but this does not become hard but brittle martensi, mainly due to the effect of fine graining Since it is work hardening by accumulation of transition, it is not an increase in the kind of hardness that causes weld cracking.
[0027]
Fatigue fracture of a metal material is composed of crack initiation and development. The sum of the crack initiation life and the crack growth life is the total life leading to fatigue cracks. In many cases, cracks are generated from places where stress concentration or residual stress is severe, and the generated cracks continue to further progress to finally break the member. In order to improve the life of fatigue fracture of a metal material, it is necessary to suppress the occurrence of fatigue cracks and the progress of fatigue cracks.
[0028]
However, normally, once a crack occurs in a metal material, the stress concentration at the tip of the crack is extremely large, and it is extremely difficult to stop this progress. For example, even if a stop hole is made at the tip and the hole is tightened with a high-strength bolt, if a crack tip is left, a crack may develop inside the bolt and even cut.
[0029]
When observing the initial fatigue crack, the initial fatigue crack, which is the state of the crack when the occurrence was detected, was measured by strain measurement during the fatigue test of the rotary welded specimen. About 10% of the fatigue life has elapsed, and the remaining 90% of the life is the progress life of this crack, which is almost determined unless the crack is removed.
[0030]
However, the crack in this state cannot be detected by a normal penetrant test or a magnetic particle test. If hammer peening or shot peening, which is a conventional fatigue life improving method, is performed in this state, the processing is performed with the cracks left, and apparently plastic deformation occurs on the treated surface. Since the progress of cracks cannot be stopped, the improvement effect is limited only to the reduction of stress concentration due to the shape improvement, and the life is hardly extended.
[0031]
However, when ultrasonic shock treatment is performed even in this state, the crack can be crushed and the crack tip can be prevented from opening to introduce compressive stress due to plastic deformation to a depth of about 1.5 mm. Of course, the depth at which compressive stress can be introduced can be the same level or higher even with hammer peening, but hammer peening has uneven processing effects, and it is thought that there are many parts to leave without hitting cracks, On the other hand, since the ultrasonic impact treatment has a remarkably large number of impacts as described above, it is possible to uniformly suppress the opening of cracks.
[0032]
Therefore, in order to effectively obtain the fatigue life improving effect, it is fundamental that the weld metal material is subjected to the ultrasonic impact treatment on the weld metal portion and the HAZ portion centering on the toe portion of the weld portion. The interface between the weld metal and HAZ, which is the most fatigued weak point, is strengthened against fatigue, and the adverse effect of hot cracking that occurs on the surface of the weld metal part can be remarkably mitigated. However, it should be noted that low temperature hydrogen cracking is considered to have little effect.
[0033]
In the treatment of the welded portion, the number of treatments in one treatment line is sufficient even in one pass, but in order to improve uniformity, improve controllability, and prevent excessive plastic deformation, When it is desired to suppress the input power per process, a more reliable fatigue life improvement effect can be obtained by performing two or more processes on the same line.
[0034]
An embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an overhead crane traveling girder 1 formed by welding and joining a metal material such as a shape steel as an example of a metal member, and shows a welded portion where fatigue cracks may occur in the metal member. It is.
[0035]
FIG. 2 shows a welded portion 5 between the flange 3 and the flange gusset 4 of the H-shaped metal material 2 that is likely to cause fatigue cracks in the overhead crane traveling beam 1. The weld 5 has a sudden change in surface shape and a tensile residual stress. The sudden change in surface shape acts as a notch and becomes a stress concentrated portion, so that a fatigue crack 6 is generated. It is considered that the stress range is expanded to facilitate the generation of fatigue cracks and the progress of the generated fatigue cracks is promoted. As a countermeasure against this, according to the first aspect of the present invention, as shown in FIG. To form a gently curved groove and improve the shape of the toe to a stress-concentrated shape. The range of the plate thickness t or more means that the range of the toe portion where the stress is concentrated is approximately the plate thickness, and therefore the range of the shape improvement of the toe portion is set to the plate thickness t or more. Further, by performing ultrasonic impact treatment on each of the upper surface and the lower surface of the welded portion 5, plastic deformation is given in the thickness direction, and the plasticized metal material is restrained by the surrounding metal material, and compressive residual stress is introduced. , Relieve tensile residual stress and suppress the occurrence and development of fatigue cracks.
[0036]
In the configuration of the second invention, cracks that can be removed by ultrasonic impact treatment are cracks that cannot be detected by nondestructive crack testing, and cracks larger than that cannot be removed by ultrasonic impact treatment. It is to be removed with a grinder before the sonic impact treatment.
[0037]
In the configuration of the third invention, as described above, the shape improvement of the toe portion of the welded portion and the range of eliminating stress concentration are at least the plate thickness or more. Since it is not enough, it is increased to 5 mm or more and subjected to ultrasonic impact treatment. Further, the ultrasonic shock treatment at 100 ° C. or less is because introduction of the compressive residual stress is more efficient when the temperature is lowered than when the temperature is hot immediately after welding.
[0038]
In the fourth aspect of the invention, the ultrasonic fatigue treatment is performed periodically even after the ultrasonic impact treatment, so that the generated fatigue cracks are removed at a small stage that can be removed by the ultrasonic impact treatment. It means improving durability.
[0039]
【The invention's effect】
The ultrasonic impact treatment in the fatigue reinforcement method of the present invention gives 10,000 or more hits per second, and the hitting force is small once, so the usability and workability are superior compared to other peening. Impact processing becomes possible, and the shape of the toe portion can be improved in a range that is more than the thickness of the welded portion of the flange gusset, which suppresses stress concentration and provides a fatigue reinforcement method for a girder structure with excellent flange gusset.
[0040]
Since the ultrasonic impact treatment of the present invention hits the metal surface many times, the uniformity of the treatment is obtained, the smoothness of the surface after the treatment is obtained, and the shape improving property of the toe portion of the welded portion is excellent. In addition, this is a fatigue reinforcement method for a girder structure having a flange gusset that can efficiently introduce compressive residual stress.
[0041]
The ultrasonic impact treatment of the present invention can remarkably refine the metal surface structure after repeated plastic working by numerous impacts, can suppress the occurrence of fatigue cracks, and has an effective flange gusset. It becomes a structural fatigue reinforcement method.
[0042]
Since the ultrasonic impact treatment of the present invention can be removed at a stage where fatigue cracks do not become large by performing treatment periodically after performing treatment once, the fatigue reinforcement method of the girder structure having an effective flange gusset and Become.
[Brief description of the drawings]
FIG. 1 is a perspective view showing an embodiment of a traveling girder as a girder structure to which a fatigue reinforcing method is applied.
FIG. 2 is a perspective view of a welded portion between a flange gusset and a flange.
FIGS. 3A and 3B are perspective views showing an ultrasonic impact treatment range of a welded portion. FIGS.
[Explanation of symbols]
1: overhead crane traveling girder 2: H-shaped metal material 3: lower flange 4: flange gusset 5: weld 6: fatigue crack

Claims (4)

For the flange side toe of the welded part of the flange gusset end of the girder structure with flange gusset, when the thickness of the flange gusset is t, the range from the flange gusset end to at least 1t each of the upper and lower surfaces, Fatigue reinforcement method for girder structures with flange gussets, characterized by the formation of grooves by applying ultrasonic impact treatment and improved toe shape. Before performing the ultrasonic shock treatment, a nondestructive crack detection test is performed to confirm that there are no cracks in advance. If a crack is found, grinding is performed with a grinder until the indication of the crack disappears, and then the ultrasonic shock treatment is performed. The fatigue reinforcement method for a girder structure having a flange gusset according to claim 1. Before performing the ultrasonic impact treatment, confirm that the length of the flange gusset turning weld is 5 mm or more, and if it is less than that, increase the welding to increase the leg length to 5 mm or more. The fatigue reinforcement method for a girder structure with flange gussets according to claim 1, wherein an ultrasonic shock treatment is performed at a temperature of 100 ° C. or less between the toe portion of the bead and the former bead and the additional bead. The fatigue reinforcement method for a girder structure having a flange gusset according to any one of claims 1 to 3, wherein the ultrasonic impact treatment is periodically performed after the ultrasonic impact treatment is performed once.

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