·Table of Contents
Methods of Ultrasonic Testing of Rolled Steel with the Use of Electro-Magnetic Transducers (EMAT)
Andrey V. Kirikov,
Technical Director of "Nordinkraft Co.", JSC.
The topicality of development of modern ultrasonic systems with the use of electro-magnetic acoustic transducers for sheet metal testing is proved in the report. The methods of increase of sensitivity, jam protection, as well as informativity of testing procedure are suggested.
The typical feature of the known systems based on 'traditional' methods of rolled steel testing is obligatory use of coupling medium between transducer and test object surface.
Necessity to use water often conflicts with modem steel technologies. It is still hard to provide desirable test object speed, test temperature, condition of test object surface. Those conventional test systems can hardly be placed into technological lines. Water use, even in cases when water s used with inhibiters, often causes corrosion and influences the final products.
Non-contacting methods of generating ultrasonic waves by means of EMAT provide testing at high and low temperatures, stability of probing signals amplitude in spite of presence of various deposits on the test surface; the methods don't conflict with technologies that prohibit the use of coupling medium.
By means of EMAT it is easy to generate both transverse waves propagating on normal to the surface and transverse normal waves. EMAT doesn't submerge the test surface, thus it is possible to avoid reverberation processes, effects on medium verge, as well as effects in the layer of coupling or immerse liquid. The acoustic path of EMAT flaw detector is quite simple for ultrasonic waves are generated and spread only in the test object (TO) itself. Only the TO itself, imperfections it comprises are capable to modify the picture of the wave field.
We have reasons to consider transverse waves propagating on normal to the test s ace to be
more sensitive to some defects (2). They propagate twice more slowly than longitudinal waves; the fact creates favorable conditions for thickness measurement and resolution increase when echo-method is applied.
In most cases electromagnetic contact of EMAT with the test surface is much more stable than acoustic contact of piezoelectric transducers. Oscillations of probing 'bottom' signal on non-defective plots of rolled sheet rarely exceed 4-6 dB.
The acoustic axis doesn't deflect when EMAT position is modified regarding the test surface.
The use of transverse waves of definite polarization provides high sensitivity detection of cracks even in cases when their axis is parallel the direction of testing. The shortcomings of EMAT are their relatively reduced probing signal sensitivity and probing signal noise protection.
One of the possible methods to essentially increase EMA-system sensitivity is application of coherent processing of the coming signals. Noise doesn't correlate with the useful signal and has casual amplitude and phase. Its average value (according to the large number of tacts/beats) approaches to zero at a certain point of time axis.
Theoretically, coherent accumulation of the useful signal allows getting any large signal-to-noise ratio. For instance, after hundred multiple accumulation (N=100) the signals on the noise level would 20 dB exceed the latest ones!
But evaluation of the useful signal is realized according to the law N 1/2 (N is a umber of accumulations) and therefore requires time tk.
Let's take for T - the period of informative signal entrance (period of probe pulse sending), NH -is the necessary number of accumulations), SH and are, consequently, the needed and initial signal-to-noise ratios, then tk= T (SH / )2. Hence, use of accumulator allows to obtain any sensitivity needed, but its meaning is limited by the quick-action speed of defect registration system. The quick-action can be increased by time reduction T between neighboring probe pulses that is increase in their effective sending frequency.
In conventional flaw detectors probe pulses go at a definite time interval (T=const.). Their sending frequency is chosen so that reverberations caused by the previous pulse should complete their way by the next pulse.
Sometimes, probe pulse frequency is modified in order to distinguish false pulses. But there are no methods for constant variation of the value in the known systems.
Combination of coherent accumulation and variation in the interval of probe pulse sending accomplished by a definite program gives substantial increase in system quick-action (diminution of the parameter tk).
The described methodology is the basis of our Fully automatic non-contacting ultrasonic inspection systems reliably operating at Iron and Steel, Machine building and Tube Works.
Fully automatic non-contacting ultrasonic inspection systems of 'Sever' type are designed for testing sheet rolled stock. Tests can be performed according to different standards and technical recommendations.
'Sever-1' is designed to satisfy unique scientific and technological requirements; it is the newest technological advance in iron and steel industry. 'Sever-1' is in non-stop operation at the JSC 'SEVERSTAL Iron and Steel Works' in Cherepovets, Russia since 1997.
The Installation uses electromagnetic-acoustic transducers. With these transducers ultrasonic testing can be done without touching the surface and needs no coupling medium. The new technology provides high-test speed, high performance and comfort, testing at a wide range of temperatures.
Decisive importance of the installation is application of 'air cushion' between the probing transducer and test surface (compared to conventional inspection systems that use liquid medium). It gives minimal but reliable maintenance of a stable gap. This constant air clearance protects transducers from mechanical abuse, friction an~ heat influence radiated by test surface. It herewith enhances transducers longevity and reliability.
The measuring module consists of 96 EMA transducers that provide the customer with thorough testing.
The 'Sever-1' Inspection System is controlled from a PC based command and control console which is connected to the main system computer. The main system computer coordinates the whole testing process, controls all the system. Data Acquisition, Analysis on Test Object Condition and Memory Retention are completely synchronous with testing.
The 'Sever-1' Inspection System is easy to install, operate or service, and can be easily placed into existing production line.
The 'Sever-2' Inspection System is designed for continuous testing of steel plates and strips (sequentially, test objects - TO) with thickness of 6-16 mm.
The installation comprises two relatively independent systems:
- "S" - system is used for continuous testing of longitudinal edges by means of shift ultrasonic waves propagating on normal to the surface;
- "L" - system is used for testing TO section by means of normal SH-waves.
Measuring modules of the systems are disposed on a sliding platform of technologic roll conveyer.
Electronic units of the system (except for the primary ones) and System of Inspection Management are located in the Management post premise.
Sensors set of automatics equipment coordinates test temperature, velocity/speed, stripe geometry, angles and mechanisms condition.
New technology of the installation 'Sever-5-32' provides detection of stratifications, non-metallic insertions, holes, declines, friable zones, and other discontinuities in strips band or sheet.
Every line comprises two functionally independent systems of testing:
- System of testing edges (STE);
- System of testing base metal (STBM).
Both systems (STE and STBM) are connected to the managing computer complex, it co-ordinates their operation, gives powerful data acquisition, process control, and information management capabilities.
The 'Sever-6-08' Inspection System is the best system in the world intended for automated ultrasonic non-contacting testing of rolled sheet. It is in reliable operation at Magnitogorsk Integrated Iron and Steel Works, JSC. The new technology of the system provides high-test speed, testing at a wide range of temperatures. Width and thickness of the test object are not limited. High-sensitivity and high-quality testing is provided by the installation even in cases when the test surface is impure or covered with dross.
The 'Volga' Inspection System is intended for fully automatic non-contacting ultrasonic testing of helical (spiral) and longitudinal welded pipes. It is in non-stop operation at Industrial Association Volzhsky Tube Works, JSC since 1999.
The installation embodies two independent testing systems. One for testing weld seams and heat effected zones for longitudinal and radial defects, the other for testing tube ends for laminations.
Acoustic-electromagnetic non-contacting installation "Volna" is designed for fully automatic ultrasonic testing and sorting of rods. It is in non-stop operation at the JSC 'SEVERSTAL Iron and Steel Works' in Cherepovets, Russia since 1997 and at Kursk Bearing Plant since 1994.
Longitudinal surface cracks are the most frequent and dangerous flaws by manufacturing long product of round section. SH-ultrasonic waves raised and accepted by electromagnetic-acoustic transducers (EMATs) are used in the installation for the products quality inspection.
Thus, we face great changes in the sphere of ultrasonic inspection; the so-called 'dry' method of generating ultrasonic waves is the up-to-date method of rolled steel testing. We face new epoch, and the breakthrough of the new Millennium are non-contacting ultrasonic systems of 'Nordinkraft Co.', JSC.
Rods are sorted by testing results adjusted to the chosen inspection criteria.
The installation also provides testing of pipes of similar geometric dimensions. The installation is designed as a functionally completed automated line. It comprises mechanical equipment, a pneumo- and hydrosystem, equipment of automatics, block of electromagnetic-acoustic transducers, block of electronic equipment and managing computer complex (MCC).