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Development of Roll Surface Testing Technique by use of Broad Bandwidth Surface Waves Hajime Takada, Akira Torao, Masanori Kitahama, Takashi Morii, Ryouichi Sugimoto and Akio Sato Kawasaki Steel Corporation 1, Kawasaki-cho, Chuo-ku, Chiba 260-0835, Japan +81-43-262-2894 Email : h-takada@kawasaki-steel.co.jp Contact

Abstract

A heavy damped surface wave probe with high sensitivity suitable for the automatic roll surface testing has been developed. Surface wave testing is usually applied for the testing of hot finish rolling rolls made from high-speed tool steel. However, this testing with a conventional narrow bandwidth probe has problems such as deterioration of signal-to-noise ratio and false detection owing to high structural noises.
The developed probe is improved in the structure and in its material to get both short pulse duration of ultrasound and high sensitivity. It has been proved that surface flaws in rolling rolls can be detected by use of the developed probe with high signal-to-noise ratio. Moreover, the developed probe is free from false detection of structural noises.

Keywords: Surface testing, Roll, Surface waves, Heavy damped short pulse probe

1. Introduction

Surface flaws are occasionally formed in work rolls for rolling under abnormal rolling conditions. Surface flaws remaining in a rolling roll can cause problems such as roll spalling and marking on the strip. Rolls are ground by a grinder after rolling of prescribed amount of strips. Re-profiled roll surface by a grinder is tested by a surface flaw detector. If a surface flaw is detected, the surface of rolling rolls is re-ground until the surface flaw is removed.
Surface wave testing is usually applied for the testing of hot finish rolling rolls made from high-speed tool steel [1],[2] . However, this testing with a conventional narrow bandwidth probe has problems as follows:
1. Some cracks that cause roll spalling can not be detected because of high structural noises.
2. In some cases, high structural noises are falsely detected as flaws.
A new method to resolve the problems mentioned above was needed.
These problems have been resolved by the development of a broad bandwidth surface wave probe with high sensitivity. In this paper, details of the development will be described.


2. Conventional technique for surface testing and its problems


Fig 1: Roll surface testing by use of surface waves

2.1 Method and instruments for surface testing
Surface waves are transmitted in the circumferential direction of a rolling roll by use of a surface wave probe as shown in Fig.1. The surface wave probe with nominal frequency 2MHz is often used. An example of automatic surface flaw detector is shown in Fig.2. Whole surface of the rolling roll is tested by scanning the surface wave probe in the axial direction of rotating roll (in the following figures, this scanning is called "helical scan"). The probe transmits the surface wave in the direction opposite to the rotation of a roll and receives surface flaw echoes. Water-gap method is used for the coupling between the surface wave probe and the test body as shown in Fig.3 [3] , and it is said that the water gap of 0.5mm is suitable. A considerable quantity of water used as the coupling medium remains adhering to the rotating test body owing to the friction between them. A wiper removes the water remaining in the path of surface waves for the prevention of attenuation of surface waves and appearance of spurious echoes.

Fig 2: Mechanism for automatic surface wave testing

Fig 3: Gap method for surface wave testing

2.2 Problems in conventional technique and their causes
In the surface testing of a rolling roll, back-scattered waves with small amplitude are caused by the presence of coarse grains, a rough surface and a large number of minute fire cracks produced in hot rolling (these are called "collective fine reflectors" in the following). As the conventional probe transmits and receives ultrasonic pulse whose duration is several times longer than its wavelength, back-scattered waves mentioned above tend to become large in amplitude by superimposing each other. Therefore, deterioration of detectability and some false indications have happened in surface testing with the conventional probe.

3. Development of a broad bandwidth surface wave probe with high sensitivity

3.1 Reduction of structural noises by use of a broad bandwidth probe
Height P_g of echoes from collective fine reflectors in the path of ultrasound is represented as equation (1), and is proportional to the square root of pulse duration of ultrasound, [4] .

(1)

Where, x is a distance from the probe to the reflector, a₀ is the attenuation coefficient.
From equation (1), it is understood that shortening the pulse duration of ultrasound is effective to make P_g small. As reflectivity of ultrasound at a flaw is not dependent on the pulse duration of ultrasound, signal-to-noise ratio of a flaw echo can be raised by use of a broad bandwidth probe. Consequently, detectability can be raised by use of a broad bandwidth probe. As the conventional probe sends out the oscillation of piezoelectric element without damping to get high sensitivity, of ultrasound pulse is several times longer than its wavelength. Therefore, P_g has a large value in the testing with the conventional narrow bandwidth probe.

3.2 Development of a broad bandwidth surface wave probe with high sensitivity
The oscillation of a piezoelectric element is damped by a damping block to get short pulse in case of a general type of broad bandwidth probe. Consequently, both transmitting and receiving sensitivity of a broad bandwidth probe are much lower than those of the conventional probe. Gain of a receiving amplifier needs to be set a high value to compensate the decline in sensitivity, so miss-detection of electrical noises coming over the air is liable to happen. The developed probe is improved in the structure and in its material to get high sensitivity to prevent miss-detection of electrical noises coming over the air.

1. Selection of materials (piezoelectric element, resin wedge)
   To get high sensitivity and to lower the amplitude of spurious echo, a piezoelectric element with high electro-mechanic coupling factor in the thickness direction and low electro-mechanic coupling factor in the radius direction is used as the oscillator of ultrasound. The resin with low ultrasound attenuation and high acoustic impedance is used as the wedge material.

2. Modification of incident angle
   Incident angle q_i from the wedge to roll surface is determined on the basis of surface wave velocity in high-speed tool steel to satisfy the Snell's law defined as equation (2).

   (2)

   Where, C_W is the ultrasound velocity in the resin wedge, C_R is the surface wave velocity in the test body, and q_i is the refraction angle (90° .in the case of the surface wave) .

3.3 Performance and usefulness of the developed probe
Photo.1 shows the waveform of surface wave transmitted by the developed probe as contrasted with the waveform of surface wave transmitted by a conventional surface wave probe. Sensitivity of the developed probe with the center frequency of 2MHz and the pulse duration of 1.5 wavelength is equal to that of the conventional probe.

(a) developed probe	(b) conventional probe
Photo 1: Waveforms of echoes from side wall

Photo.2 shows some test results using an artificial surface flaw with 0.2mm in depth and 0.5mm in width. The height of structural noises observed by use of the developed probe is lower by approximately 6dB than that observed by the use of the conventional probe. As a result, surface flaws in rolling rolls can be detected by use of the developed probe with high signal-to-noise ratio. Sallow small surface flaws can be also detected with high signal-to-noise ratio. Test results using artificial surface flaws with the developed probe have proved that surface flaws with a cross section above 0.06mm² can be detected with signal-to-noise ratio beyond 10dB. As for the conventional probe, it is said that the artificial flaw with a cross section 0.2mm² is critical for the detection. The flaw, which has a cross section 1/3 times smaller than the conventional critical cross section, can be detected by use of the developed probe.

MB:Main Bang F:Flaw Echo B:Edge Echo SE:Spurious Echo in Wedge ME:Mode Converted Echo Photo 2: Test results of artificial surface flaws detection

Fig.4 shows the test results of a work roll with a crack formed during rolling by use of the developed probe and the conventional probe installed in an automatic roll surface flaw detector. Gain of ultrasonic receiver was set the same value in both tests. While height of the crack echo with the developed probe is as high as with the conventional probe, height of the noise with the developed probe is much lower than that with the conventional probe. As a result, cracks can be detected with high signal-to-noise ratio by use of the developed probe.
Fig.5 shows the test results of a ground work roll by use of the developed probe and the conventional probe installed in an automatic roll surface flaw detector. High indications are observed with the conventional probe. These are falsely detected collective minute harmless fire cracks remaining in ground surface. On the contrary, no false indication can be observed with the developed probe. The developed probe is effective to prevention of the false detection of collective minute harmless fire cracks. As a result, excessive grinding of roll surface owing to false detection of collective minute harmless fire cracks could be prevented, and roll consumption could be reduced more than 20% while actual operations of roll grinding in Kawasaki Steel Corporation.

Fig 4: Comparison of the test results of roll with an actual surface flaw

Fig 5: Comparison of the test results of roll with harmless small fire cracks in its surface

4. Conclusions

A broad bandwidth surface wave probe with high sensitivity has been developed. Detectability of surface flaws can be raised and excessive grinding of roll surface owing to false detection of collective minute harmless fire cracks can be prevented by use of this probe. Consequently, roll consumption could be reduced more than 20% while actual operations of roll grinding in Kawasaki Steel Corporation.
The developed probe can be applied to the surface testing of work rolls for cold rolling and backup rolls as well.

References

1. Y.Ando, H.Sorano, Y.Sano: Surface Crack Detector for Rolling Mill Rolls by Surface Ultrasonic Wave, CAMP-ISIJ, Vol.5, No.2(1992), 515(in Japanese)
2. Y.Tsuchiya, Y.Ohnishi, A.Koido, K.Shiraishi, K.Masuda, K.Okimoto: Application of Surface Crack Detector for Work Rolls of Hot Strip Mill, CAMP-ISIJ, Vol.6, No.2(1993), 505(in Japanese)
3. Japanese Unexamined Patent Publication No. 7-294493
4. I .N. Ermolov : NDT International DECEMBER(1976), 275-280

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