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AUTHORS: Jürgen Schröder, Michael Ege and Andrey KirikovInstitut Dr. Foerster GmbH & Co. KG and OOO Kompaniya Nordinkraft

T raditional ultrasound inspection of steel plates useswater as a coupling medium between the piezoelectric

transducer (PET) and the test object. Because water cannotbe used as a couplant for surface temperatures of morethan 100°C, plates are usually tested cold at the end of theplate finishing operations. If plates could be tested hot thengreater control of the production process would be possible.

Non-destructive surface inspection of hot materials hasbeen in practical use for many years using eddy currents,however, internal testing has been more problematic.Fundamentally, only X-rays and ultrasound are able topenetrate deep into a metallic test object and to detectinternal defects. The use of X-rays is limited by manytechnical, economic and ecological factors, and untilrelatively recently there was no reliable and sensitivesolution available to excite and receive ultrasonic wavesin hot material.

Today, however, we have reliable and extremelysophisticated dry ultrasonic testing equipment based onEMAT. It has been in use for more than five years and sixsets are in daily use at various European plants for testingsteel plates with surface temperatures of more than100°C. Figure 1 shows transport of a hot steel plate to aSTATOSON® F hot inspection station shown in Figure 2.

ULTRASONIC TESTING OPTIONSToday there are two kinds of ultrasonic testing equipment(UTE) available for plates and strip:

` Wet with PETs` Dry with EMATs

Both types of equipment are based on the sametechnique of flaw detection, contain similar componentsand require similar design of electronics, mechanics,control system, and software. Both conform with strictmodern requirements, providing the necessary sensitivity,and meeting all the contemporary internationalstandards. The main difference is the principle of

Ultrasonic testing of hot-rolled plates up to 650°C has been made a practical reality through theuse of the Electromagnetic Acoustical Transducer (EMAT). Early identification of defects improvesprocess control and yield, saving time, energy and money. An assessment of plate mechanicalproperties is also made.

Ultrasonic testing of hot plates using EMAT

r Fig.1 Transport of a hot steel plate to the inspectionstation

q Fig.2 Front view of aSTATOSON® F testing station

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generating the ultrasonic waves.Wet ultrasonic systems use traditional PETs which need a

couplant in order to transport a longitudinal ultrasonicwave from the transducer into the test object. As water isalmost the only practicable solution for plates this restrictsthe theoretical range of allowable test object temperaturesto between 0°C and 100°C. Due to several practicalreasons the useable temperature range is even smaller.

Dry ultrasonic testing systems are based on the EMATprinciple (see Figure 3). EMAT is a device that producesand receives ultrasonic oscillations just as theconventional piezoelectric probe. However, unlike PET, itdoes not need any couplant and does not need to haveany mechanical contact with the test object; coupling isprovided by means of electromagnetic forces. EMATgenerates ultrasound directly on the surface and, as thereis no ultrasound outside the test object, in most cases,dirt, paint and scale on the product surface do notimpede testing.

FEATURES AND ADVANTAGES OF ULTRASONIC TESTING WITH EMATRapid development in recent years of dry ultrasonictesting has been possible due to major technologicalimprovements in electronics such as powerfulmicroprocessors and digital signal processing. Theseimprovements have enabled the development of modernsoftware algorithms and fast data processing which areimportant for coping with the small signals produced indry ultrasonic testing. Also, with the availability of newmaterials more powerful EMATs could be developed.

There are several fundamental characteristics of EMATwhich facilitate some additional capabilities of ultrasonictesting. The most important characteristics are:

` EMAT operates over a very wide range oftemperatures (40°C to +650°C).The upper limit isdetermined by the Curie temperature of the materialto be tested. The actual installations for hot testingare between 150°C and 650°C. There is no preferredtemperature

` EMAT generates and receives polarised shear(transverse) waves

` EMAT excites waves strictly normal to the surface ofthe material to be tested, regardless of the probeinclination. This facilitates obtaining extremely stableback-wall echoes

` The use of shear waves has advantages over use oflongitudinal waves, for instance, they can detectsurface cracks or vertical defects that do not reflectPET ultrasound because the energy is scattered

These characteristics lead to new features and benefits inplate testing:

` Improving the production process` Improving internal logistics` Improving the test quality of steel plates` Information about mechanical properties

ADVANTAGES OF TESTING STEEL PLATES EARLY IN THE PRODUCTION FLOWTesting of plates early in the production process enablesthe operator to identify problems in the manufacturingprocess directly after hot rolling of the plate, enablingcorrective actions such as changes to productionparameters or diverting slabs to alternative orders beforerolling to be initiated. In the case of plates with localdefects, decisions about further processing can be made

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a

q Fig.3Generation of ultrasonicwaves by theEMAT principle

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while the plates are still on the cooling bed, as their statusis already known. This can increase the output of the line.If the plates are no longer useful due to defects, they canbe scrapped immediately after rolling. Therefore, EMATtesting of hot material becomes a tool for process control,saving yield, time, energy and money.

IMPROVING THE TEST QUALITY OF STEEL PLATESWhichever type of transducer is applied, scale, especiallythick and loose, is able to create untested areas and/orgenerate noise, reducing reliability and sensitivity ofultrasonic testing. For PETs scale can influence the shapeand direction of an ultrasonic beam, and the ability ofultrasound to penetrate into the material to be tested.Hot materials normally do not have scale with suchcharacteristics, and certainly for plates of above 300°C noinfluence on the ultrasonic inspection process has everbeen noticed using EMAT. Therefore, the risk of havinguntested zones due to scale is low.

A common problem for all ultrasonic testing equipment,regardless of the type of applied transducers, is thedifficulty of avoiding untested zones at the edges of theplate. Because the quality of the edges is much moresignificant for the further use of the plate than the qualityof the plate body, ideally it is desirable to test platesbefore cropping. With an EMAT system the testing

process can already be accomplished before the platereaches the cropping station.

Figure 4 shows the A-scan of a defect-free area of a20mm thick plate. The y-axis shows the signal amplitudein dB, while the x-axis is the timescale in microseconds.The red bar in the graph represents the position of thegate AD between the first and second back-wall echo.Attention should be paid to the amplitude differencebetween the second (A2, blue bar) and first (A1,magenta bar) back-wall echo which is –0.9dB. For thesame plate, Figure 5 shows the A-scan for an area withan artificial surface crack of 0.3mm depth. Theamplitude difference between the second and first back-wall echoes decreases to -4.4dB, a result which wouldnot have been obtainable with longitudinal waves. Theresult does not determine the depth of the flaw but is aclear indication of its presence.

Another example is given in Figures 6 and 7. Apolarised shear wave interacts with vertical cracks muchmore strongly than a longitudinal one excited by apiezoelectric probe. It occurs if the crack is located mainlyperpendicular to the polarisation vector of the shearwave. Figure 6 shows the A-scan from a defect-free areaof a plate with a thickness of 16mm. Again the differencein amplitude between the second and the first back-wallecho is important. In this case it is –0.7dB. Figure 7shows the A-scan for the same plate with a natural

r Clockwise from top left: Fig.4 A-scan of a defect free area of a 20mm plate; Fig.5 A-scan with an artificialscratch 0.3mm deep in the same 20mm plate; Fig.6 A-scan from a defect free area of a 16mm plate; Fig.7 A-scan from the area with a natural vertical surface crack in the same 16mm plate

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vertical defect. The depth of this crack is approximately1mm. The difference in amplitude for the second to thefirst back-wall echo is now –16.8dB and is the result ofscattering of the wave due to the presence of the defect.It does not allow precise determination of the crackdepth. Nevertheless, both examples draw the attentionof the operator to non-defect free areas, which would nothave been obtainable by conventional ultrasonic testing.

MECHANICAL PROPERTIES ASSESSMENTThe use of shear waves allows variations in themechanical properties of the plate to be determined. Thepolarisation of the shear wave can be either parallel orperpendicular to the direction of rolling. The generationof one, or alternatively two, modes simultaneouslydepends on the angle of the active zone of the EMATelement relative to the rolling direction of the plate.When the active zone of the EMAT element has an angleof 45° relative to the rolling direction, both shear wavemodes will be excited with equal amplitudes.

This phenomenon can be explained as follows. Thespeed of sound in steel depends on its physicalproperties (elastic constants). These physical propertiestypically are different in the three main directions of theplate after rolling. Therefore, it is expected that thevelocity of the two modes of a shear wave will bedifferent and hence the time to receive the back-wallechoes after exciting the wave will be different for thetwo modes.

Figure 8 shows an A-scan obtained with a 45° turnedEMAT from a defect-free area of a 16 mm plate. It can beclearly seen that the sequence of reflections of ultrasoundfrom the walls of a plate consists of two peaks. Thesereflections are the result of the two modes of shear wavespropagating with different velocities, v⊥ and vIIrepresenting vertical and parallel to the rolling directon.The time difference increases when higher orders of backwall echoes are used and also depends on the thicknessof the plate.

The ratio of the shear wave velocities v⊥/vII definesacoustical anisotropy that correlates directly withdifferent mechanical properties (MP) of the material. The

idea is not to determine the MPs themselves, but to payattention to all plates where the ratio v⊥/vII is beyondthe range of typical values. The MP assessment can beperformed by a multi-channel UT-system simultaneouslywith the flaw detection process. In this case activeelements of every second probe should be turned by 90°relative to the previous one. Figure 9 shows a C-scan of asteel plate indicating areas with abnormal mechanicalproperties. Areas in red have a value higher than theaverage while the green marked areas are those where thevalue is lower than the average.

EMAT ULTRASONIC TESTING EQUIPMENTAlthough international standards and requirements fromsheet metal users are becoming more uniform, the actualrequirements of sheet metal manufactures concerningtesting equipment are still extremely individual. TheSTATOSON® F is highly modular and can be tailored toany kind of dimension of the plates to be tested. Table 1shows the most common specifications.

Attribute ValueWidth mm 1,000 – 5,000 Length mm 4,000 – 30,000 Thickness mm 4 – 200 Temperature °C -40 – +650Surface After hot rollingTest speed m/s up to 2

Figure 10 shows an alternative set up for an ultrasonictesting station. In this arrangement the carriage of theEMAT transducers can be moved sideways out of theproduction line. This is extremely useful for servicing.

As the quality of the edges of the plates is crucial,international standards require far more sophisticatedtesting for the edges than for the plate body. Therefore,the ultrasonic testing equipment is divided into twodifferent parts:

` Plate body testing station` Edge testing stations

Originally international standards required a test of theplate without 100% coverage. This was mainly because

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r Fig.9 C-scan of a steel plate indicating areas with abnormal mechanical properties

r Fig.8 A-scan obtained with the 45° turned EMATfrom a defect-free area of a 16mm plate

a

r Table1 Common specifications for plate testingmachines

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automatic testing equipment was not available. As themanual testing of the plates was time-consuming theplate was only tested grid-like. Automatic testingsystems of today allow for grid-like testing, testing with100% coverage and testing with overlap, therefore,transducers are mounted in a single, double, or even inthree rows. Overall the STATOSON® F testing station iscapable of controlling up to 600 EMATs. Figure 11shows the arrangement in two rows.

SUMMARYFor more than five years, six sets of dry ultrasonictesting equipment based on EMAT have been in daily use at various European plants for testing steel

plates with surface temperatures of more than 100°C.The systems are used both for defect detection,assessing mechanical properties of plates and asproduction control tools.

Because the requirements from steel plants are veryspecific the equipment is of modular design. MS

Jürgen Schröder is Division Manager and Michael Ege is a Physicist, both at Institut Dr. Foerster GmbH & Co KG,Reutlingen, Germany. Andrey Kirikov is Head of NDT-Department at OOOKompaniya Nordinkraft, Cherepovets, Russia

CONTACT: schroeder.juergen@foerstergroup.de

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q Fig.10MoveableSTATOSON® Ftesting station

r Fig.11 EMATarrangementwith two rows

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