ABSTRACT: The large grain size of hot-rolled copper plate and the hysteresis of copper itself cause the scattering and hysteresis attenuation of ultrasonic, which seriously affect the result of ultrasonic flaw detection. In this paper, through the test and application comparison of different types of ultrasonic flaw detectors at home and abroad for hot-rolled copper plates with different thickness, the better instrument and method suitable for the flaw detection of hot-rolled copper plates are explored, it also provides some useful reference data for the further development of the instrument.

Subject words: hot-rolled Copper, coarse grain, high hysteresis, ultrasonic flaw detector application

1.Introduction

Copper and copper alloy products are widely used in aerospace, aviation, nuclear power, ships, ships, seawater desalination, metallurgy and many other important industrial sectors because of their excellent conductivity, heat conduction and corrosion resistance, in these industries, the internal quality of copper products is required to be more and more high, requiring the production enterprises to carry out 100% ultrasonic inspection of copper products, qualified products can be ex-factory. In recent years, there has been a new round of investment upsurge, large steel groups at home and abroad without exception choose hot-rolled red copper plate to replace the original cast iron plate as the cooling stave of blast furnace. Compared with cast iron, red copper has more than 10 times the electrical and thermal conductivity of cast iron, and more than 3 times the service life of cast iron, so it is widely used in blast furnace cooling stave. Due to the thick columnar crystal formed during melting and casting of red copper, it is difficult to eliminate this kind of thick crystal in the final product in the subsequent hot rolling process, plus the hysteresis of red copper itself, therefore, the ultrasonic scattering and the drilling hysteresis absorption attenuation are especially large, which seriously affects the ultrasonic flaw detection results. The author has carried out many years'testing on hot-rolled copper plates of different thickness by using different types of ultrasonic flaw detectors at home and abroad, the experimental results show that the domestic and imported digital instruments and analog instruments and digital instruments have significantly different detection results for different thickness of red copper plate. In this paper, by comparing the application of different types of ultrasonic flaw detector in hot-rolled copper plate at home and abroad, the better instrument and method suitable for hot-rolled copper plate are explored, it also provides some useful reference data for the further development of the instrument.

2.Characteristics of microstructure of hot-rolled copper plate

The hot-rolled Copper Plate used for cooling stave of blast furnace is Tuz, which is usually used to cast billets with thickness of 2-295 ~ , and then directly rolled out the finished products on the hot-rolling mill.

Because of the narrow crystallization range of red copper, it is easy to form thick columnar crystals (as shown in Fig. 1) during melting and casting, which are difficult to be removed during hot rolling, and finally remain in hot-rolled red copper plate to form thick grains, some grains have a diameter of MM (as shown in Fig. 2) . This kind of coarse structure has strong scattering effect on the ultrasonic wave. In addition, copper itself has strong hysteresis and strong attenuation effect on the ultrasonic wave, because of the particularity of the tissue structure, there are many technical difficulties in ultrasonic testing.

3.Comparison of instrument application

For hot-rolled copper plates of different thickness, because of different deformation amount, the degree of fragmentation of the cast structure is different, the grain size of the structure is different, and the scattering degree of the ultrasonic wave is also different, hot-rolled copper plates of the same thickness have the same degree of hysteresis absorption. Through a lot of experiments, it is proved that when the thickness is less than 70 mm, the biggest influence on ultrasonic attenuation is the grain size of the material, the thickness is between 70 and 140 mm, mainly the influence of drilling hysteresis absorption, when the thickness is more than 140 mm, both the grain size and the hysteresis absorption affect the test results. For this reason, we have carried on the application comparison experiment to the ultrasonic flaw detector according to three thickness interval.

1. When the thickness is less than 70 ~ , the above-mentioned test instruments are used to test the normal sensitivity of hot-rolled red copper plates with thickness of 50 mm, 60 ~ and 70 ~ , respectively, with the matching frequency of 1.25 Mhz, 2.5 Mhz and 5 Mhz, the results show that both domestic and imported ultrasonic testing can accurately detect the internal defects of sheet metal, and the reproducibility is good. For hot-rolled copper plate with thickness less than 70 ~ , any type of ultrasonic flaw detector commonly used in the market can meet the requirements of plate flaw detection. 2. The thickness between 70140mrn is also tested by the above-mentioned apparatus with 1.25MHZ, . 25mhz and 5MHZ straight probes. The normal sensitivity of the hot-rolled copper plates with the thickness of 80 ~ , 100 ~ , 120 ~ and 140 ~ is tested, the results show that: (l) when the ultrasonic attenuation is serious and the sensitivity is increased to the maximum, only one bottom wave appears, and the noise amplitude is more than 80% when the detector matches the frequency of 5Mhz, therefore, the thickness of 80 ~ above hot-rolled red copper plate can not be explored. (Z) both domestic and imported analog instruments and imported digital instruments with matching frequencies of 1.25 mhz and 2.5 Mhz can accurately find the internal defects of the sheet and have good reproducibility. However, the normal sensitivity of the direct probe with the matching frequency of 2.5 MHZ can only detect the copper plate with the thickness of 80 ~ and 100 ~ , while the noise of the hot-rolled copper plate with the thickness of 120 mm and 140 ~ is too big to detect the copper plate with the thickness of 140 ~ When the matching frequency is 1.25 Mhz and the normal sensitivity is used, the hot-rolled copper plate with thickness of 140 ~ can not be detected. 3. When the thickness is more than 140 ~ , the hot-rolled copper plates with thickness of 160 mm, 180 ~ , 200 ~ and 210 mm are tested by using the above-mentioned test instruments with 1.25 Mhz, 2.5 Mhz and 5 MHZ straight probes, all the instruments can only display the bottom wave once under the normal sensitivity, the noise is too big, can not carry out the flaw detection. It is found that the analog flaw detector made in China does not match the frequency of 1 Mhz and 0.5 Mhz, and the digital flaw detector made in China does not match the frequency of-05 Mhz, the direct probe with matching frequency of IMHZ can detect hot-rolled red copper plate with thickness of 160mm, but it can not be detected with thickness of more than 160mm. Imported analog flaw detector with the same IMHz and 0. SMHz frequency does not match, but the imported digital flaw detector can match IMHz and 0. The direct probe of SMHz frequency, adopting the direct probe of 1MH rate and adjusting with normal sensitivity, can detect hot-rolled copper plate with thickness over 200 ~ . Conclusion (L) hot-rolled copper plate with thickness less than 7 omm, microstructure grain size and hysteresis of copper have little effect on ultrasonic testing results. Domestic or imported instruments, whether analog or digital, are used, all of them can be well used in flaw detection. (2) if the thickness of hot-rolled copper plate is between 70-140 ~ , it is better to adopt domestic and imported analog flaw detector or imported digital flaw detector. (3) for hot-rolled Copper Plate with thickness greater than 140 ~ , only imported digital flaw detector can be used for flaw detection at present, and the flaw detection frequency should be IMHz. (4) matching frequency IMHZ and. . 05N/l probe is not as effective as imported digital flaw detector. (5) when the sensitivity of domestic digital ultrasonic flaw detector is raised to 85% (i. e. the gain is raised to above SSDB) , the noise of domestic digital ultrasonic flaw detector is much larger than that of imported instrument. (6) how to match the lower ultrasonic frequency (the frequency is lower than 0. SMHZ) is also a problem to be solved by the instrument manufacturer and the flaw detector. 

Source: Chinanews.com, by Li Xianghai