JP5251486B2 - Machining method using ultrasonic impact treatment - Google Patents

Description

  The present invention relates to a technique for improving fatigue characteristics of, for example, a bridge girder of a bridge, parts for automobile and motorcycle undercarriages, and molded parts such as home appliances. The present invention relates to a processing method using an ultrasonic impacting device for introducing compressive stress and improving fatigue characteristics.

In general, a welded portion and its vicinity (hereinafter, also referred to as a weld toe) in a welded structure are likely to have tensile residual stress due to thermal shrinkage after welding, and at the same time have a notch shape with a steep angle. Due to the fact that it is easy, it tends to become a stress concentration part when cyclic stress is applied to the welded joint.
Similarly, a tensile residual stress is also generated in a plastic deformation region of a metal part to which a strong tensile stress is applied by cold press forming or the like.

  In particular, welded structures such as bridge girders, undercarriage parts of automobiles, molded parts such as automobile wheels, etc. are used in environments subject to repeated loads. It becomes a place where fatigue cracks occur, and is a major cause of reducing fatigue properties.

  Conventionally, shot peening, hammer peening, ultrasonic hammering, etc. are methods for introducing compressive stress from the outside to the tensile residual stress part generated at the weld toe of the welded structure and improving the fatigue characteristics. Are known.

  Shot peening is a method of introducing a compressive residual stress on the surface of the object to be processed by spraying a hard metal sphere of 1 mm or less onto the surface of the object to be processed with compressed air or the like. Hammer peening is a method of introducing a compressive residual stress to the surface of the object by mechanically hitting the surface of the object to be processed using a hard metal rod.

  On the other hand, the ultrasonic hitting process is known as a method capable of introducing compressive residual stress to a relatively deep region with less reaction and better workability than the shot peening and hammer peening. This ultrasonic striking treatment introduces compressive residual stress to the surface of the object to be treated by vibrating an ultrasonic vibration terminal made of hard metal at a high frequency using ultrasonic waves and hitting the surface of the object to be treated with the terminal. It is a method to do.

  However, shot peening, hammer peening, and ultrasonic hitting treatment are all methods of applying compressive stress to the surface of the object to be processed by hitting with a hard metal. As a result, the surface properties are deteriorated, and furthermore, the recess formed on the surface may become a stress concentration part and a fatigue crack initiation point, so there is a limit to dramatic improvement in fatigue strength. .

  This ultrasonic impact processing technique is disclosed in Patent Document 1. Moreover, as a technique which improves the fatigue strength of a welded joint using the technique, it is disclosed by patent documents 2-4, for example.

  Patent Document 2 discloses a method for defining a hitting position in order to improve fatigue strength after welding the overlapped end portions. However, since the striking position specified by this method was the position including the weld toe, when a large amount of compressive residual stress was introduced to the treated surface, the recesses formed on the striking surface were stress concentrating and fatigued. There is a possibility that it becomes a crack initiation base point, and there is a limit to dramatically improve the fatigue strength. Moreover, although compressive stress is introduced into the weld toe, the absolute value has a limit due to the shape change.

  Patent Document 3 and Patent Document 4 disclose that ultrasonic hitting is also performed on a portion that does not include the weld toe, but these weld joint shapes are corners where stress concentration at the weld toe is high. There is a problem that it is a meat welding joint and cannot be applied to a butt-welded joint having a small stress concentration at the weld toe.

US Pat. No. 6,171,415 JP 2004-130315 A JP 2004-130313 A JP 2004-130316 A

  In view of the above-described conventional state of the art, the present invention greatly increases the residual tensile stress generated at the weld toe of a butt welded joint where two metal plates are butted against each other using an ultrasonic impact treatment method. An object of the present invention is to provide an ultrasonic impact treatment method for efficiently introducing a compressive residual stress and improving the fatigue characteristics in an environment where a repeated load is applied.

By hitting a specific range in the bond part of a butt welded joint where two metal plates are butted together with an ultrasonic vibration terminal, even if a bead shape defect occurs, higher fatigue strength can be ensured than before. The present invention provides a method for improving the fatigue strength of a butt-welded joint, the gist of which is as follows.
(1) A processing method for improving the fatigue strength of a butt-welded joint by subjecting the vicinity of a weld toe of a butt-welded joint welded by butting the ends of two steel plates to each other, When forming a processing band having a maximum depth of 20 μm to 300 μm from the surface of the steel sheet by subjecting the material surface in the direction along the weld toe to ultrasonic striking, the striking position is the weld toe And the processing length per unit time at the striking processing position is T, and the end of the working band is located in the range of 0.1 mm to 7 mm from the weld toe. When the diameter of the impact treatment pin is D [mm], the impact treatment density A [sec / mm ³ ] defined by the following formula 1 is 0.0125 [sec / mm ³ ] or more. A processing method using ultrasonic striking treatment, characterized by being:
A = 1 / (T · D ² ) (Formula 1)

  According to the present invention, by optimally controlling the processing position in the processing method using ultrasonic hammering processing, out of the tensile residual stress generated at the weld toe of the butt weld joint in which two steel plates are butted together and welded In particular, large compressive residual stresses can be efficiently and effectively introduced in the direction in which repeated loads are applied, and as a result, the fatigue characteristics of welded structures and molded parts can be improved as compared with conventional ones. .

Embodiments of the present invention will be described in detail with reference to FIGS.
FIG. 1 is a diagram illustrating a processing method using an ultrasonic striking device.
In FIG. 1, the butt steel plate 1 and the steel plate 2 are butt welded by a weld metal 3. The ultrasonic hitting process is performed by hitting a steel plate in the direction along the weld metal 3 with the ultrasonic vibration terminal 4. As a result, a processed band 5 along the weld metal 3 is formed in the portion hit by the ultrasonic vibration terminal 4.

  As shown in FIG. 1, a processing band 5 is formed on the surface of the steel plate 1 along the weld toe portion 7 by an ultrasonic impact treatment, and the processing band 5 is processed so as not to include the weld toe portion 7. . At that time, the position of the work band end 6 near the weld metal 3 of the work band 5 is 0.1 mm or more and 7 mm or less from the weld toe 7 near the work band, that is, the work band end 6. It was found that the distance between the weld toe 7 and the weld toe portion 7 is preferably 0.1 mm or more and 7 mm or less.

  If compressive stress is present at a place where fatigue occurs, the fatigue life is improved. At this time, the fatigue strength improvement effect is considered to be greater as the absolute value of the compressive residual stress is larger. In particular, when the stress concentration coefficient at the weld toe is not large as in the case of butt welding, the fatigue improvement effect is great. Furthermore, the higher the strength of the steel material, the higher the residual stress of the introduced compression. Therefore, the higher the steel material strength, the greater the fatigue improvement effect.

  However, in the conventional ultrasonic hitting method for hitting the weld toe as shown in FIG. 2, when experimented, the position of the weld toe before hitting was hit. In the direction perpendicular to the processing direction of the processing band generated by processing (X direction shown in FIG. 2), a compressive stress is introduced, but the value is not sufficiently large.

  In addition, the compressive residual stress in the X direction, which is perpendicular to the working band processing direction, at a position 9 slightly away from the working band processing end 6 on the side far from the weld metal is more than the working band processing end on the side far from the weld metal. It was also found that the absolute value of the compressive stress introduced was large.

The reason is estimated as follows.
In ultrasonic striking treatment, compressive residual stress is introduced around the work zone that has been hit by the ultrasonic vibration terminal driven by ultrasonic waves. It is considered that a part of the compressive residual stress is released by the change of.
On the other hand, at the position away from the hitting location, there is no shape change, and all of the introduced compressive residual stress remains, resulting in a compressive residual stress having a higher absolute value than the hitting position, and even more than the processing band. In the distant region, it is considered that the tensile residual stress is reversed due to the stress balance.

From the above results, we assumed the following.
The value of the compressive residual stress in the direction perpendicular to the processing direction is related to the distance between the hitting position and the toe part, and the hitting process is performed away from the toe part, and the distance to be released is appropriate. Thus, the absolute value of the compressive residual stress in the direction perpendicular to the processing direction of the toe portion can be increased.

  On the basis of these ideas, the toe when a processing band is formed parallel to the weld line by changing the distance between the processing position and the weld line using an ultrasonic vibration terminal having a diameter D = 2 mm, 4 mm, and 8 mm. We examined how the residual stress in the vertical direction of the process changes and the fatigue characteristics change.

(experimental method)
Ultrasonic impact (UIT) device (Applied Uitrasonic): Power supply 1kw, frequency 27kHz
Ultrasonic vibration terminal diameter: 2mm, 4mm, 8mm
Output power: Depends on device and device settings. This UIT device is attached to the device
Although it can be adjusted from 1 to 9 with the rotary knob, the absolute value was unknown.
Further, when forming a processing band by performing ultrasonic hitting processing, the hitting processing speed (processing length per unit time at the hitting processing position) is T [mm / sec], and the diameter of the hitting processing pin is D [mm]. Then, the experiment was conducted by changing the impact treatment density A [sec / mm ³ ] defined by the following formula 1.
A = 1 / (T · D ² ) (Formula 1)

(Measurement of residual stress)
The residual stress was measured by X-ray diffraction using the sin2ψ-2θ method. The apparatus used for the measurement is MSF-2M manufactured by Rigaku Corporation, the X-ray tube is Cr, the detector is a scintillation measuring instrument, the voltage is 30 kv, the current is 10 mA, and the method for measuring diffraction lines is tilted. Is used, and the X-ray incidence method is the ψ constant method, and the incident angle ψ is 3 sec / in the range of 151 ° to 161 ° with respect to four points of 0 °, 15 °, 30 °, and 45 °. Measurement was performed by step operation at a step of 0.25 degrees, and the half-width method was used to determine the peak.

In the stress measurement, the [211] diffraction surface of ferrite was used, and an absorption coefficient of 850.04, a Young's modulus of 21000 kgf / mm ² , a Poisson's ratio of 0.28, and a stress constant of −32.44 were used as physical constants. The measurement area was 0.5 mm (direction perpendicular to the processing direction) × 6 mm (processing direction).

FIG. 3 is a diagram showing the relationship between the distance X from the end of the work band near the weld toe to the weld toe near the work band and the residual stress at the weld toe near the work band. It is.
In FIG. 3, there is a correlation between the distance X on the horizontal axis and the residual stress. If the distance is 7 mm or less, the residual stress of compression is higher than that in the case where the weld toe portion is ultrasonically processed. I understand that it is expensive.

From this, when the distance X between the work band end near the weld metal and the weld toe close to the work band is 7 mm or less, the work band is formed at a position including the weld toe. In comparison, the residual stress at the weld toe near the working band can be increased.
At that time, if the work band end near the weld metal is as close as possible to the weld toe, the residual stress at the weld toe near the work band can be increased, but it is too close. In this case, there is a limit because the weld toe is processed, and it is actually difficult to bring the weld toe closer to within 0.1 mm while leaving an untreated region at the weld toe.

If the residual stress at the ultrasonic hitting position can be increased as described above, the fatigue strength can be improved.
FIG. 4 shows the distance X from the work band near the weld toe to the weld toe near the work band and the fatigue strength of the weld toe near the work band (breaking life is 2 million times). It is shown that if the distance X is 7 mm or less, the fatigue strength is also improved.

  Furthermore, when the size of the ultrasonic impact terminal (pin size) was changed, the impact density per unit area (energy given to the unit area per unit time = 1 / processing length per unit time / pin length) When the area was the same, the distribution of residual stress was almost the same, and the fatigue strength was the same.

Therefore, as a result of examining the magnitude of the impact treatment density, it was found that a higher compressive residual stress can be imparted by setting the value to 0.0125 [sec / mm ³ ] or more.
That is, when the processing time per unit length at the impact processing position is T [mm / sec] and the diameter of the impact processing pin is D [mm], the impact processing density A [sec / mm ^3] is such that 0.0125 [sec / mm ^3] or more, and selected processing time per unit length in accordance with the diameter of the striking processing pin may be carried ultrasonic striking treatment.
A = 1 / (T · D ² ) (Formula 1)

The upper limit of the value of the hitting processing density is not particularly limited, but is 37.5 [sec / mm ³ ] at present due to a practical limitation of processing time.
Moreover, in order to obtain these effects more remarkably, it is desirable to strike the processed band so as to form an indentation having a depth of about 20 μm to a depth of about 300 μm on the surface before processing.

  In the present invention, the welding method for forming a joint is not particularly limited as long as it is a welding method applicable to the formation of a butt-welded joint for metal plates. In addition to high-energy beam welding such as laser welding, plasma welding, and electron beam welding, the present invention can also be applied to joints formed by generally used arc welding.

Example 1
Two steel plates having a plate thickness of 1.2 mm and a strength of 490 MPa were butted together and butt welded by laser welding, and then the impact treatment of the example of the present invention was performed using ultrasonic vibration terminals having diameters of 2 mm, 4 mm, and 8 mm. The joint size was 40 mm (width) × 200 mm (length). A YAG laser was used for laser welding, the processing point output was 3.0 kW, the welding speed was 7.5 m / min, and the focal beam diameter was 0.4 mm. The shield used was a center shield torch and nitrogen was used as the gas. The focal position of the beam was the steel plate surface.

  The ultrasonic vibration device has a power supply of 1 kW and a frequency of 27 kW, the amplitude of the ultrasonic vibration terminal is 30 μm to 40 μm, and the impact treatment speed (treatment length per unit time) varies depending on the size of the pin. Was fixed at 20 mm / sec for the 2 mm pin, 5 mm / sec for the 4 mm pin, and 5 mm / 4 sec for the 8 mm pin.

For the case where the shape of the pin and the hitting processing speed are changed as described above, the fatigue characteristics after performing the hitting process with the ultrasonic vibration terminal are compared with those in which the hitting process is processed directly above the weld toe, Those whose fatigue strength was improved by 10% or more were OK, and those with lower fatigue strength were NG.
Fatigue test conditions were one-way tension with a load ratio (minimum load / maximum load) = 0.1 and a repetition rate = 10 kHz.

These test conditions and evaluation results are shown in Tables 1-3.
As shown in Tables 1 to 3, ultrasonic impact treatment within the specified range of the present invention (blow treatment position: within a range of 0.1 to 7 mm from the weld toe, impact treatment density: 0.0125 sec / mm ³ or more) Inventive Examples 1 to 27 in which the joints were subjected to the treatment were shown to be able to improve the fatigue strength of the joint as compared with Comparative Examples 1 to 20 in which the treatment was performed outside the specified range of the present invention or the treatment was not performed.

(Example 2)
Two steel plates having a plate thickness of 1.2 mm and a strength of 780 MPa were butted together and butt welded by laser welding, and then the impact treatment of the example of the present invention was performed using an ultrasonic vibration terminal having a diameter of 2 mm. The joint size was 40 mm (width) × 200 mm (length). A YAG laser was used for laser welding, the processing point output was 3.0 kW, the welding speed was 7.5 m / min, and the focal beam diameter was 0.4 mm. The shield used was a center shield torch and nitrogen was used as the gas. The focal position of the beam was the steel plate surface.

The ultrasonic vibration device was a power source of 1 kW, a frequency of 27 kW, an ultrasonic vibration terminal having an amplitude of 30 μm to 40 μm, and an impact treatment speed of 10 mm / sec (impact treatment density: 0.025 sec / mm ³ ).

  The fatigue characteristics after performing the hammering treatment with the ultrasonic vibration terminal are compared with those in which the hammering treatment is processed immediately above the weld toe. NG. Fatigue test conditions were one-way tension with a load ratio (minimum load / maximum load) = 0.1 and a repetition rate = 10 kHz.

These test conditions and evaluation results are shown in Table 4.
As shown in Table 4, Invention Examples 28 to 36 in which ultrasonic impact treatment was performed within the specified range of the present invention (within a range of 0.1 to 7 mm from the weld toe) deviated from the specified range of the present invention. It is shown that the fatigue strength of the joint can be improved as compared with Comparative Examples 21 to 26 where the treatment was performed and Comparative Example 27 where the hitting treatment was not performed.

(Example 3)
Two steel plates having a thickness of 1.2 mm and a strength of 490 MPa were butted together and butt welded by arc welding, and then the impact treatment of the example of the present invention was performed using an ultrasonic vibration terminal having a diameter of 2 mm. The joint size was 40 mm (width) × 200 mm (length).

  The ultrasonic vibration device was a power source of 1 kW, a frequency of 27 kW, an ultrasonic vibration terminal amplitude of 30 μm to 40 μm, and an impact speed of 20 mm / sec.

  The fatigue characteristics after performing the hammering treatment with the ultrasonic vibration terminal are compared with those in which the hammering treatment is processed immediately above the weld toe. NG. Fatigue test conditions were one-way tension with a load ratio (minimum load / maximum load) = 0.1 and a repetition rate = 10 kHz.

These test conditions and evaluation results are shown in Table 5.
As shown in Table 5, Invention Examples 37 to 45 in which ultrasonic impact treatment was performed within the specified range of the present invention (within a range of 0.1 to 7 mm from the weld toe) deviated from the specified range of the present invention. It is shown that the fatigue strength of the joint can be improved as compared with Comparative Examples 28 to 33 in which the treatment was performed and Comparative Example 34 in which the batting treatment was not performed.

(Comparative Example 1)
Two steel plates having a plate thickness of 1.2 mm and a strength of 490 MPa were butted together and butt-welded by laser welding, and then subjected to an impact treatment using an ultrasonic vibration terminal having a diameter of 2 mm and 4 mm. The joint size was 40 mm (width) × 200 mm (length). A YAG laser was used for laser welding, the processing point output was 3.0 kW, the welding speed was 7.5 m / min, and the focal beam diameter was 0.4 mm. The shield used was a center shield torch and nitrogen was used as the gas. The focal position of the beam was the steel plate surface.

The ultrasonic vibration device has a power supply of 1 kW and a frequency of 27 kW, the amplitude of the ultrasonic vibration terminal is 30 μm to 40 μm, and the striking speed is 30 mm / sec for the 2 mm pin and 10 mm / sec for the 4 mm pin so that the striking density changes. .
These test conditions and evaluation results are shown in Tables 6 and 7. After changing the shape of the pin and the hitting speed and changing the hitting density as described above, the hitting process using the ultrasonic vibration terminal was performed. The fatigue characteristics were compared with those obtained by performing the striking treatment immediately above the weld toe, and in all cases, the improvement rate of the fatigue strength was less than 10%.
Fatigue test conditions were one-way tension with a load ratio (minimum load / maximum load) = 0.1 and a repetition rate = 10 kHz.

It is a figure which shows the ultrasonic hit processing method in this invention. It is a figure which shows the conventional ultrasonic impact processing method. It is a figure which shows the relationship between the distance from the welding toe part to the position of an ultrasonic treatment, and the residual stress of a welding toe part. It is a figure which shows the relationship between the distance from the welding toe part to the position of an ultrasonic treatment, and the fatigue strength of a welded joint (stress range where a fracture life becomes 2 million times).

Explanation of symbols

1, 2 Metal plate 3 Weld metal 4 Ultrasonic vibration terminal 5 Machining band 6 Machining band edge 7 Weld toe X Distance from weld toe to machining band edge D Diameter of ultrasonic vibration terminal

Claims (1)

A processing method for improving the fatigue strength of a butt-welded joint by subjecting the vicinity of the weld toe of a butt-welded joint welded by butt-welding two steel plates to each other, and improving the fatigue strength of the butt-welded joint. When forming a processing band having a maximum depth of 20 μm to 300 μm from the steel sheet surface by applying ultrasonic hitting on the material surface in the direction along the part, the hitting position does not include the weld toe It is a steel plate portion, and the end of the work band is located within a range of 0.1 mm to 7 mm from the weld toe , and the processing length per unit time at the striking processing position is T [mm / sec], the impact treatment density A [sec / mm ³ ] defined by the following formula 1 is 0.0125 [sec / mm ³ ] or more when the diameter of the impact treatment pin is D [mm]. A processing method using ultrasonic striking treatment characterized by the above.
A = 1 / (T · D ² ) (Formula 1)

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