KR20020011662A - Method of Detecting Internal Cracks of Steel Products using Laser-Ultrasonic - Google Patents

Abstract

PURPOSE: A method for measuring faulty of metal by using laser inducing ultrasonic waves is provided to improve quality of a product by reducing faulty through exactly detecting the faulty of the metal and to reduce a producing cost. CONSTITUTION: A laser is injected to a test sample of metal and then is reflected to a mirror slantingly mounted in the middle of the test sample. Then the test sample is crashed to the laser to generate ablation effect melting the metal by crashing the laser. Then, ultrasonic waves are generated in the test sample by repulsive force of the ablation effect. An ultrasonic transducer is installed on a rear portion of the test sample without contacting to the test sample with a specific interval. Am amplifier amplifies the ultrasonic detected from the ultrasonic transducer to exactly analyze the ultrasonic wave. The amplified ultrasonic is transferred to an oscilloscope to output corrugation. A low laser generates the ultrasonic by thermo elastic effect. A high laser generates strong ultrasonic on a surface of material by the ablation effect.

Description

Method of Detecting Internal Cracks of Steel Products using Laser-Ultrasonic}

The present invention relates to a method for measuring an internal defect of a metal material using laser guided ultrasonic waves.

Metal materials are generally produced through high temperature heat treatment and then processed into various final products. However, when a defect (for example, bubbles, foreign matters, etc.) exist inside the metal material, not only can it be processed into the final product, but even if the processing is performed, the physical properties of the product are greatly reduced. Therefore, it is very important to measure the internal defects of the firstly produced metal material, but until now, in the field of producing the metal material, it was not possible to measure the internal defects of the metal material due to the high temperature in the production process. After cooling the finished product, measurement of defects was inevitably performed.

Defects that are found to be defective internally through these measurements are returned to the initial process through the destruction and remelting process.However, if the internal defects can be easily measured during the continuous production process of the metal material, such defects can be detected. Immediate removal on site will reduce metal scrap and reduce manufacturing costs.

The present invention has been made to solve such a problem, and an object of the present invention is to provide a method for non-destructively and immediately detecting defects present in a metal material produced in a continuous process.

According to the present invention, a laser is projected on a metal to be measured to generate ultrasonic waves inside the metal material, a non-contact ultrasonic probe is installed at a distance from the surface on which the metal material collides with the laser, and the ultrasonic wave passing through the metal material is described in detail. A method is provided for measuring internal defects in a metal material using laser guided ultrasound, which consists of receiving by one non-contact ultrasonic transducer and analyzing the ultrasonic wave received by the non-contact ultrasonic transducer.

In this case, analyzing the ultrasonic waves received by the non-contact ultrasonic transducer, amplify the ultrasonic waves received by the non-contact ultrasonic transducer by an amplifier. The amplified ultrasonic waves are transmitted to the oscilloscope to output a waveform, and the waveforms of the ultrasonic waves output by the oscilloscope are transmitted to a computer to evaluate the characteristics of the ultrasonic waves by a computer.

The non-contact ultrasonic transducer used in the present invention may be any one of an acoustic microphone, an air transducer, or an electromagnetic acoustic transducer.

1 is a block diagram schematically illustrating a system for measuring defects inside a metal material using the method of the present invention.

Figure 2a is a graph showing the waveform of the laser-ultrasonic signal measured in the time domain by the broadband ultrasonic transducer in Example 2 of the present invention.

FIG. 2B is a frequency spectrum of the waveform of the laser-ultrasonic signal measured in FIG. 2A. FIG.

3A to 3D are graphs showing signals received by a contact longitudinal ultrasonic transducer having resonant frequencies of 0.5 MHz, 1.0 MHz, 2.25 MHz, and 5 MHz, respectively, in Embodiment 2 of the present invention.

4A to 4D are graphs showing frequency characteristics of ultrasonic waves received when an experiment is performed using an air probe as an ultrasonic probe in Example 4 of the present invention.

FIG. 5 is a graph showing an average of results obtained by scanning a laser 104 times on a surface of a test piece and an average of 200 times by using a focusing lens in Example 6 of the present invention. FIG.

6A to 6C are graphs showing transverse waves transmitted and received with electromagnetic transducers on both sides of a test specimen in Example 7 of the present invention;

FIG. 7 is a graph illustrating an enlarged result of the signal of FIG. 6C for measuring the speed of a transmitted / received signal and analyzing a signal reflected by a defect.

Hereinafter, the configuration of the present invention in more detail as follows.

1 is a schematic view of the entire system of the present invention. When the laser is projected onto a metal specimen as shown in FIG. 1, the laser is reflected on a mirror positioned obliquely between the laser and the specimen, so that one side of the metallic specimen (Bottom in the example shown), the ablation (ie, melting and evaporation of the metal surface of the part hitting the laser) occurs on the part of the test piece which hits the laser. At this time, ultrasonic waves are generated inside the test piece due to the repulsive force of the ablation phenomenon.

If the ultrasonic transducer is placed in contact with the test specimen at a distance from the opposite side of the test specimen face to the laser, the ultrasonic wave generated inside the test specimen passes through the test specimen and is received by the ultrasonic probe.

Since the received ultrasonic waves are very weak, in order to analyze them more reliably, the weak ultrasonic waves received by the ultrasonic transducer are delivered to the amplifier to amplify the ultrasonic waves, and the amplified ultrasonic waves are transmitted to the oscilloscope to output the waveform. Then, by transmitting the unique waveform of the ultrasonic wave output by the oscilloscope to the computer, the characteristic of the ultrasonic wave is evaluated to determine whether there is a defect inside the metal material.

In the present invention, in order to generate ultrasonic waves on the surface of the test piece, a high power laser pulse is used. In general, when a strong laser pulse is irradiated onto the surface of the material, ultrasonic waves are generated on the surface of the material by approximately two mechanisms. At low laser power, ultrasonic waves are generated by the thermoelastic effect, and at high laser power, strong ultrasonic waves are generated on the material surface by the ablation effect.

When the laser beam has a high power, unlike the low power, a part of the surface of the medium dissolves and evaporates, and a laser induced plasma (blue flame in the visible region) is generated.

Ultrasonic waves in all modes are generated in the ablation region by the laser. Among them, the longitudinal wave is known to be the largest. This is because the repulsive force acts most perpendicularly to the surface of the medium when the material on the surface evaporates and flows out of the medium while generating a plasma flame.

Therefore, when detecting a defect in the test piece by using the laser-guided ultrasonic wave in the present invention, when there is a defect in the path of the ultrasonic wave generated by the laser, the progress of the ultrasonic wave is interrupted by the defect, and the Since it is attenuated, it is possible to determine whether there is a defect from the size of the received ultrasonic wave. Therefore, if the path of the ultrasonic wave generated by the laser is known, it is possible to determine the reception position and the inspectable area.

Induction equations for the far-field beam directivity of longitudinal waves Ur and transverse waves U _θ generated in the ablation region by a high power laser are shown in Equations 1 and 1 below. It becomes like 2.

here Represents the heat diffusivity of the medium.

Hereinafter, an embodiment in which a defect in a metal material is measured at a production site of the metal material using the method of the present invention will be described.

<Example 1>

A semicircular steel block having a diameter of 280 mm was used as a test piece, and the propagation direction characteristic of the ultrasonic wave generated by the laser was measured using a system such as that shown in FIG. A wideband ultrasonic transducer and a 2.25 MHz contact piezoelectric ultrasonic transducer were used to measure the beam direction characteristics of the longitudinal wave, and a 2 MHz contact piezoelectric ultrasonic wave transducer was used to measure the beam direction characteristic of the shear wave. Used. Each probe described above measured the propagation direction characteristics of the ultrasonic wave while constantly moving at intervals of 10 ° based on the central axis of the test piece.

Comparing the propagation direction characteristics by theoretical calculations with the actual propagation direction characteristics, it was found that the approximate pattern is consistent. However, the longitudinal direction measured using two transducers with different frequency characteristics showed a slightly different propagation direction. From these results, it was assumed that the propagation direction characteristics of the ultrasonic wave were affected by the frequency of the generated ultrasonic wave.

The longitudinal wave ultrasonic waves generated by the laser in the ablation region can detect the largest ultrasonic signal at the epicenter and can be considered to have a certain propagation width. However, using a 32mm steel block to measure the area that can be inspected, the results show that the receiving ultrasonic transducer is very attenuated as it leaves the source.

It is assumed that the influence of the ultrasonic beam is different from that of the ultrasonic beam. Therefore, when detecting the laser-guided ultrasonic waves by the ultrasonic sensor, such as the ultrasonic probe, it is found that the alignment to the source is very important.

<Example 2>

Using the same system as FIG. 1, the test piece was experimented using what was produced by the thickness of 32 mm. A wideband ultrasonic transducer was used as an ultrasonic transducer for receiving ultrasonic waves generated by a laser, and a contact piezoelectric ultrasonic transducer was also used to generate ultrasonic waves with a signal received by a pulse-echo and a laser, The signals received using piezoelectric ultrasonic transducers for reception were compared.

The laser used a laser device having a pulse width of 6 ns, and the ultrasonic generation was made with a laser pulse single shot of 650 mJ.

FIG. 2A shows the waveform of the laser-ultrasound signal measured in the time domain by the broadband ultrasonic transducer in this embodiment, and FIG. 2B shows the results of frequency spectrum based on FIG. 2A. From this result, it can be seen that the maximum frequency component of the ultrasonic waves generated from the system described above is 0.78 MHz.

Next, frequency characteristics were investigated using contact longitudinal ultrasonic transducers with different resonant frequencies. To this end, the ultrasonic signals received by the pulse-echo method of the general ultrasonic flaw detector and the ultrasonic signals received by the laser-ultrasound method of the present invention were compared.

3A, 3B, 3C, and 3D show signals received by a contact longitudinal ultrasonic transducer with resonant frequencies of 0.5 MHz, 1.0 MHz, 2.25 MHz, and 5 MHz, respectively, and at the top of each figure. The frequency graph shown is an ultrasonic signal received by the laser-ultrasound method of the present invention, and the frequency graph shown below is an ultrasonic signal received by the conventional pulse-echo method.

According to the experiments described above, since the sensitivity of the ultrasonic transducers at each frequency is different from each other, it is not meaningful to compare these measured values with each other. However, the comparison of two signals at each frequency can be meaningful because the same transducer is the receiving transducer and only the source is different.

When comparing signals at each frequency, only the case of the 1 MHz ultrasonic probe showed a larger signal when the laser was used as the source than the signal received by the pulse-echo method of the general ultrasonic flaw detector. This result supports the reliability of the maximum frequency component 0.78 MHz, measured using a broadband ultrasonic transducer.

<Example 3>

A laser was used as an ultrasonic generator, an acoustic microphone was used as a receiving probe, and an experiment was performed using a 32 mm steel block as a test piece.

When the results were averaged more than about 100 times, even when the air gap (ie, the distance between the specimen and the acoustic microphone) was 20 mm, the signal generated through the specimen could be captured. This signal was formed after the time it took for the ultrasonic wave to progress through the specimen and after the ultrasonic wave had traveled through the air layer from the specimen to the acoustic microphone. Therefore, by holding a time gate around the first signal and measuring the strength of the signal, it was possible to identify an abnormal state inside the specimen. However, according to this method, when a large defect such as a pipe exists, it is possible to measure the defect, but it was judged that it cannot be used when a defect of a few mm size exists.

<Example 4>

Using the same system as in Figure 1, an air transducer (a kind of non-contact ultrasonic transducer) was used as the ultrasonic transducer.

Ultrasonic propagation in air is very attenuated and, in general, ultrasonic transducers should be physically attached to the specimen without air layers. Because of this, there are many limitations in applying ultrasonic waves to industrial sites. Therefore, the development of non-contact ultrasonic test method has many advantages for industrial applications. For example, it is possible to evaluate porous materials in the initial manufacturing state of uncured plastics, green ceramics, or powder metallurgy products, or continuously, such as plastics, rubber, and paper. The non-contact ultrasonic test method is very useful for evaluating the material to be rolled with a.

In this example, the laser was used as the ultrasonic generator, and the air probe (0.7 MHz at -6 dB, center frequency 1.8 MHz, active area 12.5 mm) was used as the ultrasonic probe. The results are shown in Figures 4a, 4b, 4c, 4d.

The upper figure of each figure is the result when a laser was irradiated once, and the lower figure is the result obtained averaged after irradiating a laser about 20 times. When the laser was irradiated once when the air gap was about 4cm, the signal reflected from the back of the specimen could be observed.If the average of 20 irradiations was averaged, the signal returned from the back of the specimen four times was observed without any difficulty. Could.

Example 5

An artificial test piece (16 cm x 16 cm x 50 cm) containing an internal defect (a drill groove having a diameter of about 1 mm) was produced and used.

When the ultrasonic wave was generated by the laser and the generated ultrasonic wave passed through the test piece having artificial cracks, the ultrasonic wave was received by the air probe, which is an ultrasonic probe, to investigate whether or not defects could be detected by non-contact.

For this purpose, general ultrasonic flaw detection was first performed on the specimen, and the position received by the contact ultrasonic probe and the position of the internal defect (drill groove) received by the non-contact ultrasonic probe were similar.

In addition, ultrasonic waves were generated by a laser on a test piece of 15 cm having a drill groove of 1 mm, and the air probe was inspected using an ultrasonic probe. As the air gap was measured, it was possible to detect a back wall echo and a drill groove having a diameter of 1 mm even when the air gap was about 26 mm.

<Example 6>

The biggest advantage of the ultrasonic generation using a laser is that the ultrasonic wave can be generated without contact with a metal material and without a contact medium. Inevitably, however, there is a case where the laser is irradiated with the metal surface constrained in some form. Therefore, in such a case, it is necessary to consider the ultrasonic generation by a laser.

The effect when the surface is changed or covered with paint, oil, etc. is very complicated. That is, optical absorption, conversion to ultrasonic energy, and ultrasonic propagation are all affected by surface changes. In this example, the case where the surface is coated with a thin solid layer such as paint, rust or surface roughness, and the case where the surface is covered with a transparent liquid layer will be described.

Surface layers, such as paint and rust, change the reflectance on the surface. Maximum reflection (minimum absorption) is when the surface is polished. Increase paint or melt hop energy. Therefore, even at low power, the local temperature rises, thereby increasing thermoelastic stress and increasing ultrasonic intensity. Increasing surface roughness has a similar effect. Coating with black paint increases the absorption by almost 100%. As such, there is a mechanism that not only increases the strength by changing the absorption, but also changes the strength and the principle of generation.

For example, when coated with black paint, the high absorption rate causes the paint to evaporate and create stresses perpendicular to the surface, such as the ablation mechanism. Subsequent laser pulses are irradiated back to the surface where the paint is peeled off, so the effect is limited to the first few pulses. In the ablation area, the blow-off force already exists, so the paint effect is greatly reduced. But when paint, rust or something like that is present on the surface, it will definitely increase the strength in the first few irradiations. When a liquid such as water or oil covers the specimen surface, also concentrate the bipolar stress in a direction perpendicular to the specimen surface. When the liquid layer is present at high laser power (ablation area), the liquid layer evaporates at low laser power to produce a strong force on the specimen surface.

In this example, the oil used as the ultrasonic contact medium was applied onto the surface of the test piece, and the non-contact ultrasonic probe was received 26 mm from the test piece. At this time, even though the laser was scanned once, the first reflected signal could be observed. Comparing this result with the result without oil, it was found that the ultrasonic generation efficiency was greatly increased. However, even after lubricating a large number of laser pulses after lubricating the surface of the specimen, the s / n ratio of the signal did not improve. The result is that the oil is blown away by the first laser shot.

FIG. 5 shows the average of the results of 104 injections of the laser on the surface of the billet using the focusing lens and the average of the results of 200 injections. As shown, more injections did not improve the s / n ratio but rather worsened. This was because as the laser pulses were irradiated onto the surface of the specimen, the surface of the specimen gradually became mirror.

<Example 7>

In this embodiment, instead of using a laser as an ultrasonic generator, an experiment was performed using an electro-magnetic acoustic transducer (EMAT) capable of transmitting and receiving ultrasonic waves in a non-contact manner.

The electromagnetic acoustic transducer used for the measurement was for shear waves and the frequency was 1.3 MHz.

6A, 6B and 6C show the results of transmitting and receiving two electromagnetic acoustic transducers on both sides of the test piece. FIG. 6A shows a result of no signal averaging, which shows a relatively high s / n, and FIG. 6B shows a signal averaged 20 times, indicating that the s / n ratio is very high. The reason why the s / n ratio is higher than when the ultrasonic wave generated by the laser is transmitted to the electromagnetic acoustic transducer is determined that the transmitting and receiving electromagnetic acoustic transducer is tuned to the same frequency and the vibration directions of the transverse waves coincide. Fig. 6C shows the measurement result at the processed defect site, and it can be seen that the signal from the defect is detected between the signals repeated between the surface of the test piece.

FIG. 7 illustrates the result of enlarging the signal of FIG. 6C for measuring the velocity of a transmitted and received signal and analyzing a signal reflected by a defect. In FIG. 100 μs, so the speed is about 3200 m / s. And although the diameter of the artificially processed defect is very small (1 mm), the enlarged picture shows that the signal reflected from the defect is detected at a relatively high s / n ratio.

The above results are obtained under optimum conditions because the results obtained by measuring the lift-off between the electromagnetic acoustic transducer and the test specimen at room temperature in the laboratory at zero. In the actual billet production site, the surface of the billet will be rougher than the test piece used in this example, the lift-off must be maintained, and the billet temperature will be high, so the result may be different from the present embodiment.

In addition, the cooling of the transducer should be considered when designing the electromagnetic acoustic transducer for use at high temperatures. Nevertheless, it could be concluded that the method of the present invention would be sufficient for ultrasound examination at high temperature billet production lines.

As described above, the ultrasonic generation in each embodiment was received using a laser, and the ultrasonic transducer was received using various devices, that is, an acoustic microphone, an electromagnetic acoustic transducer, and a non-contact ultrasonic transducer. As a result, in the case of using an acoustic microphone, when the average of the results of laser irradiation about 100 times was obtained, it was possible to catch an ultrasonic signal passing through the test piece (32 mm steel) with an air gap of 20 mm, but it was difficult to apply to detect a fine defect.

Electromagnetic acoustic transducers (there is a limit of the fundamental lift-off) and air coupled transducers were able to detect approximately 1 mm of defects in 15 cm steel specimens.

As described above, the present invention solves the problem that internal defects in the conventional high temperature metal material cannot be measured by generating and receiving ultrasonic waves in a non-contact manner with respect to the high temperature metal material. Therefore, the method of the present invention can be applied to industrial sites. In this case, since internal defects existing in metal materials continuously produced at high temperature can be accurately measured, quality control can be improved by inducing a decrease in product defect rate, and the cost incurred from defective products can be reduced, thereby reducing product cost. This is an excellent effect.

Claims (3)

By projecting a laser onto the metal to be measured, ultrasonic waves are generated inside the metal, Install a non-contact ultrasonic transducer at a distance from a surface opposite to the surface where the metal material collides with the laser, The ultrasonic wave passing through the metal material is received by the non-contact ultrasonic probe, Analyzing the ultrasonic waves received by the non-contact ultrasonic transducer A method for measuring an internal defect of a metal material using laser guided ultrasonic waves, the method comprising: The method of claim 1, wherein analyzing the ultrasonic waves received by the non-contact ultrasonic transducer, Amplifying the ultrasonic waves received by the non-contact ultrasonic transducer by an amplifier. Transmitting the amplified ultrasound to an oscilloscope to output a waveform, Transmit the waveform of the ultrasonic wave output by the oscilloscope to a computer, To evaluate the characteristics of the ultrasonic waves by the computer A method for measuring an internal defect of a metal material using laser guided ultrasonic waves, the method comprising: The metal material using laser induced ultrasonic waves according to claim 1, wherein the non-contact ultrasonic transducer is any one of an acoustic microphone, an air transducer, and an electromagnetic acoustic transducer. How to measure internal defects