JPH0615461A - Method for detecting welding fault - Google Patents

Abstract

PURPOSE:To provide the method which can precisely and simply detect a welding fault. CONSTITUTION:A high frequency voltage of a high frequency power source 1 is detected by a detector 2, measured by a frequency counter 3 and stored in a digital memory 4. Then, the variation time of this frequency is obtained by a variation time arithmetic unit 5, and the displacement distance in the variation time is obtained by multiplying this data by a velocity data of a velocity meter 7 by a short circuit distance arithmetic unit 6. This value is compared with reference value by a comparing unit 8, if it exceeds a reference value, the marking is executed at the fault generation position by a marking device 10 based on the command of a follow-up device 9, and a recorder 11 records the generation of fault.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting welding defects in high frequency electric resistance welding.

[0002]

2. Description of the Related Art In the high frequency electric resistance welding method, a metal strip material is formed by rolls so that both end surfaces in the width direction thereof form a V shape, and the both end surfaces are heated and melted by applying a high frequency current, and then, The both ends are abutted and welded by a roll to obtain a welded pipe. This method is widely adopted as a very efficient process.

By the way, welding defects which occur in such a high frequency electric resistance welding method include a penetrator and a cold welding defect. The penetrator is a welding defect caused by the oxide generated by the high-temperature oxidation of the end surface to be welded remaining on the welding surface, and often occurs when the welding heat input is excessive. On the other hand, cold welding defects occur when the welding heat input is insufficient, and the joint strength is weak. Needless to say, these do not extremely deteriorate the processing performance in the processing such as pipe expansion and bending, and also deteriorate the toughness and corrosion resistance of the welded portion, which is a practical problem.

In order to prevent the above-mentioned welding defects, the welding operator always carries out the welding operation while monitoring the welding condition such as the color of the bead, and recently, the welding operator automatically monitors the welding condition such as the welding temperature. Welding heat input control is implemented.
However, it is difficult to prevent accidental welding defects caused by foreign matter being caught in the welding point from the outside by the method described above. Therefore, in practice, it is common to perform visual inspection and non-destructive inspection to identify and remove the defect occurrence site, but it may not be detectable depending on the shape and size of the defect.

It is generally known that when foreign matter is caught, the high frequency current path does not pass through the apex of the V-shape and is short-circuited by the foreign matter. FIG. 3 is a schematic diagram showing a current path when foreign matter is caught in a workpiece. V
W located at the apex of the character is a welding point, and there are feeding points P ₁ and P _{2 on} both sides of the spread portion. The original current path is P ₁ -W-P ₂ via the welding point W, but the feeding points P ₁ , P ₂
A current path when the foreign object E is present until the welding point W is the P ₁ -S ₁ -S ₂ -P ₂ via the short-circuit point S _1, S ₂ from (Figure 3 (a)).

[0006] Then the welding progresses (FIG. 3 (b)) short-circuit point S _1, S ₂ is short-circuit point is to reach the welding points W S _1, S
_No current flows from ₂ to the welding point W, and as a result, heat input is insufficient in this portion, and a local cold welding defect occurs.

Japanese Patent Publication No. 59-10871 discloses a method of detecting a welding abnormality by measuring a high frequency characteristic value of a high frequency power supply circuit and comparing it with a preset control value of the welding characteristic value. . Further, Japanese Patent Publication No. 3-23085 discloses a method of detecting a change rate of an oscillation frequency of a high frequency power source and detecting a time series frequency at which the change rate is a set value or more to detect a cold welding defect.

[0008]

However, since these conventional methods detect high frequency characteristic values such as the oscillating frequency of the high frequency voltage or high frequency current and detect the welding defect from the variation amount and the variation rate, no defect is generated. Even the things were detected, and the yield was reduced. For example, when the position where the foreign matter is caught is close to the welding point W, a welding defect does not occur because the time during which no current flows to the welding point W is short. This is because the total amount of electric power supplied during the time of moving from the power feeding points P ₁ and P ₂ to the welding point W as welding progresses acts as welding heat input, so that the total current is supplied even for a very short time. This is because the welding heat input does not decrease significantly.

However, since the high frequency characteristic value including the oscillation frequency of the high frequency voltage fluctuates, it is detected as a defect in the conventional method. Further, even if foreign matter is caught in the welded portion, the short-circuited state may be immediately resolved by the scattering of the foreign matter after the short circuit occurs once. FIG. 4 schematically shows a current path in this case.
(a) shows a short circuit, and Fig. 4 (b) shows a short circuit. Figure 4
The high-frequency characteristic value when a short circuit occurs shown in (a) fluctuates because the current path is significantly changed, and the conventional method shows the same detection result as when there is no subsequent short circuit elimination.

FIG. 5 is a graph showing an example of the result of detection by the conventional method, in which the parameter for defect determination is the variation of the oscillation period (1 / frequency) of the high frequency power supply. The horizontal axis represents the fluctuation amount of this oscillation period, and the vertical axis represents the defect length. The size of ○ is φ267.4 × t6.
6 shows the case of the pipe, and △ shows the case of the pipe of size φ406.4 × t7.1. In the former, the fluctuation amount of the oscillation cycle is 20 nsec.
In the above case, the defect occurs, and in the latter case, it occurs at 30 nsec. Or more, and it can be seen that it is necessary to change the reference value due to the difference in size.

In addition, there are some cases in which welding defects do not occur even if such a reference value is exceeded. This is presumed to be due to the cause such as the elimination of the short circuit due to the scattering of the foreign matter as described above. As described above, even if the fluctuation amount of the oscillation period and the defect length do not show a one-to-one correspondence even with the same pipe size, and the correspondence differs when the pipe size is different, it is difficult to detect welding defects in character. Therefore, the conventional method inevitably suffers from a decrease in production efficiency and yield.

Further, since the impedance of the high frequency power source varies depending on the conditions such as the size of the material to be welded, the shape of the end face to be welded, the characteristics of the welding machine, etc., a huge amount of control is required in the conventional method using the amount of electrical fluctuation as a control value. A value table and conditional expressions are required, and setting them requires a huge number of man-hours. Further, since these are also affected by conditions such as the shape of the material to be welded, the roll setting position, and the feeding point position that are subtly different for each manufacturing, it is necessary to change the reference value each time and the reproducibility is poor.

As a result of repeated research by the inventor of the present invention, it is not the value such as the variation of the high frequency characteristic value that corresponds well with the length and shape of the welding defect, but from the time when the current path is short-circuited until it returns to the steady state. It was found that the welding speed was multiplied by the distance traveled by the short-circuited portion, that is, the change time from the occurrence of short-circuiting of the welding current due to the entrapment of foreign matter to the return to the steady state.

The present invention has been made on the basis of such knowledge, and by accurately detecting a welding defect based on the distance traveled by the object to be welded within the time when the high frequency current is not supplied to the welding point, It is an object of the present invention to provide a method capable of simply detecting a welding defect.

[0015]

A welding defect detecting method according to the present invention detects a variation time of a characteristic value of a high frequency power supply circuit in high frequency electric resistance welding, and detects a moving distance of an object to be welded during the variation time. It is characterized in that the welding defect is obtained and the welding defect is detected based on the moving distance.

[0016]

According to the present invention, a short circuit of the high frequency current due to foreign matter biting is detected by the variation of the characteristic value of the high frequency power supply circuit, the variation time is obtained, and the high frequency current is supplied to the welding point within this variation time. The welding defect can be accurately detected by detecting the welding defect on the basis of the distance that the workpiece is moved within the non-removed time. Further, the distance is less likely to be affected by various conditions such as the size of the material to be welded, so the frequency of changing the judgment reference value is greatly reduced, and the man-hours for changing the reference value are also greatly reduced. It is reduced and detection can be done easily.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings showing its embodiments. FIG. 1 is a block diagram showing the configuration of a detection device used for carrying out the welding defect detection method according to the present invention.
In the figure, 1 is a high frequency power source used for high frequency resistance welding, and this high frequency power source 1 supplies a high frequency current to the open pipe OP through contact tips C ₁ and C ₂ arranged in the widened portion of the welded portion. A detector 2 is provided to detect the high frequency voltage of the high frequency power source 1, and is connected to provide the detected high frequency voltage signal to the frequency counter 3. The frequency counter 3 measures the frequency every 1 msec. And supplies the frequency signal to the digital memory 4.

Since the storage capacity of the digital memory 4 is limited, the frequency counter 3 detects this frequency signal when the frequency becomes larger than the steady value by a certain value or more, and a trigger signal is sent to the digital memory 4 at this detection time. The digital memory 4 is sent out and starts storing only when it receives this trigger signal. Further, the trigger setting level is preferably as small as possible with the amount of fluctuation of the oscillation frequency in the steady state caused by the commercial frequency rectification ripple of the DC input of the power supply as the lower limit, and in this embodiment, it is set to 1 kHz.

The frequency signal stored in the digital memory 4 is read by the fluctuation time calculation device 5, and the time from the start of the frequency fluctuation to the return to the steady state is calculated, and the fluctuation time data is sent to the short circuit distance calculation device 6. Is given. On the other hand, a speedometer 7 is provided in the middle of the transfer path of the pipe making to obtain the welding speed by rotating the pipe in contact with the pipe, and the speed data is given to the short-circuit distance calculating device 6. The short-circuit distance calculating device 6 multiplies the speed data by the fluctuation time data to obtain the moving distance of the workpiece within the fluctuation time. The comparing device 8 compares the moving distance with the reference value, and when the moving distance exceeds the reference value, it outputs a reject signal to the tracking device 9.

The follow-up device 9 is capable of moving the pipe in the transfer direction, and controls the marking device 10 provided for marking the pipe and recording by the recorder 11 provided for recording the welding defect. It is provided to do this. Further, the follow-up device 9 is provided with the speed data of the speedometer 7, and when receiving the reject signal from the comparison device 8, the marking device 10 outputs a position signal indicating the position of the welding defect according to the speed data.
And call to recorder 11. Upon receiving this position signal, the marking device 10 follows the defect occurrence position, performs spray marking on the defect occurrence position of the pipe, and the recorder 11 records the defect occurrence.

FIG. 2 is a graph showing an example of the result when the method of the present invention is carried out using the detection equipment shown in FIG.
The horizontal axis represents the moving distance (mm), and the vertical axis represents the defect length (mm). In addition, ∘ indicates the case where a pipe having a size of φ267.4 x t6.6 was manufactured, and Δ indicates a case where a pipe having a size of φ406.4 × t7.1 was manufactured.
As is clear from FIG. 5, the defect length corresponding to the moving distance is the same even if the tube size is different, and in this embodiment, a defect is generated when the moving distance is 5 mm or more. Even if a short circuit occurs, no welding defect will occur (defect length = 0 m
It is understood that m) is when the movement distance is 5 mm or less,
In the case of this embodiment, the reference value may be set to 5 mm.

As described above, according to the method of the present invention, it is possible to detect defects with one reference value (5 mm in this embodiment) regardless of the pipe size and welding speed, and no excessive detection as in the conventional method is recognized. . The short circuit of the high frequency current path affects the effective value of the high frequency voltage or high frequency current of the high frequency power supply circuit, the high frequency characteristic value such as the oscillation frequency or the phase difference.
Therefore, not only the oscillation frequency of the high frequency voltage used in the present embodiment but also the high frequency characteristic value capable of detecting the short circuit is used to detect the welding defect according to the present invention, and the same effect can be obtained.

Also in the high frequency electric resistance welding using the induction heating coil, when the high frequency current path is short-circuited, the characteristic value of the high frequency power supply circuit as described above varies. Therefore, the method of the present invention can be applied not only to the resistance heating method by direct energization shown in this embodiment but also to the induction heating method.

[0024]

As described above, in the method of the present invention, a welding defect is detected based on the distance traveled by the object to be welded during the short circuit time, that is, the time when the high frequency current is not supplied to the welding point. The present invention has an excellent effect such that it is not necessary to change the judgment reference value depending on the welding conditions such as the pipe size and the welding speed and the excess detection is not performed, and the yield is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a detection device used for carrying out a welding defect detection method according to the present invention.

FIG. 2 is a graph showing an example of test results of the method of the present invention.

FIG. 3 is a schematic diagram showing a current path when foreign matter is caught.

FIG. 4 is a schematic diagram showing a current path when foreign matter is caught.

FIG. 5 is a graph showing an example of test results of a conventional method.

[Explanation of symbols]

1 High frequency power supply 2 Detector 3 Frequency counter 4 Digital memory 5 Fluctuation time calculation device 6 Short circuit distance calculation device 7 Speedometer 8 Comparison device 9 Tracking device 10 Marking device 11 Recorder OP Open pipe W Welding point P ₁ , P ₂ Feed point E Foreign matter S ₁ , S ₂ Short-circuit point C ₁ , C ₂ Contact tip

Claims (1)

[Claims] 1. In high frequency electric resistance welding, it is possible to detect a variation time of a characteristic value of a high frequency power supply circuit, obtain a moving distance of an object to be welded at the variation time, and detect a welding defect based on the moving distance. Characteristic welding defect detection method.