Final strip and sheet steel product is commonly inspected with great scrutiny to qualify material for high-end product requirements. Surface flaws such as slivers, cracks, laps, etc., disqualify these materials from being used in automotive and big box applications. Internal defects such as voids, cracks, laminations, porosity and segregation may remain undetectable with surface inspection methods as they have not yet manifested at the surface. These internal defects often propagate to the surface where ultimately they are detectable in the finished product stage in the form of slivers, blisters, etc., although remaining undetectable in the steel making, hot-rolling, pickling, cold rolling and subsequent finishing operations. Surface flaws are a key cause of down grading of finished product and a significant cost to the steel maker as all value added operations are complete before detection and down grading are possible. In today’s competitive environment, it is key to maximize utilization of mill assets and to avoid adding value to material which can be known early in the manufacturing process to contain deleterious defects. Using proven methods of volumetric material inspection in two separate case studies, methods have been developed to allow the steel maker to identify poor material early in the process thus avoiding the value added processes on these materials and only processing materials which with a probability of final inspection passage.

1. Detection of internal defects prior to value-added
processes to minimize final product surface defects
Jeffrey S. Monks
Sales and Marketing Director
Innerspec Technologies, Inc.
4004 Murray Place
Lynchburg, VA 24501
www.innerspec.com

2. INTRODUCTION
Final strip and sheet steel product is commonly inspected with great scrutiny to qualify material for
high-end product requirements. Surface flaws such as slivers, cracks, laps, etc., disqualify these
materials from being used in automotive and big box applications. Internal defects such as voids,
cracks, laminations, porosity and segregation may remain undetectable with surface inspection
methods as they have not yet manifested at the surface. These internal defects often propagate to
the surface where ultimately they are detectable in the finished product stage in the form of slivers,
blisters, etc., although remaining undetectable in the steel making, hot-rolling, pickling, cold rolling
and subsequent finishing operations. Surface flaws are a key cause of down grading of finished
product and a significant cost to the steel maker as all value added operations are complete before
detection and down grading are possible.
In today’s competitive environment, it is key to maximize utilization of mill assets and to avoid
adding value to material which can be known early in the manufacturing process to contain
deleterious defects. Using proven methods of volumetric material inspection in two separate case
studies, methods have been developed to allow the steel maker to identify poor material early in the
process thus avoiding the value added processes on these materials and only processing materials
which with a probability of final inspection passage.
CLAD METALS IN COIN STOCK
A recent application of strip inspection was for the inspection of clad coin stock material. In recent
years (late 1960’s) most worldwide coinage transitioned from solid precious metals (gold, silver,
copper) to a multi-layered clad material comprised mostly of a copper core and outer layers of alloy
to appear silver or gold but wear longer and obviously cost far less to produce.
Figure 1: Multi-Layered Clad Coin Stock Sample Showing area of disbond occurrence
AVAILABLE TEST METHODS:
Previous common methods of clad coin stock inspection available are, as in the steel industry,
Visual inspection (manually or automated) and ultrasonic normal beam inspection. Since visual
inspection can only detect surface discontinuities it is not suitable for disbond (internal) defect
detection. Ultrasonic Normal beam inspection relies on ultrasound being sent through a material
from the front surface to the back wall where the sound is reflected and then being seen again on
the front surface. The time required to send and receive this ultrasonic energy is directly
proportional to the material thickness. This method is referred to as “time-of-flight” as it is the time
required for the reflected sound to return to the sensor than indicates the material thickness.
When the material possesses an internal flaw, a portion of the ultrasonic energy is reflected earlier
in time than if the sound had traveled to the back wall of the material. This results in a decreased
time-of-flight and indicates either a material thickness change or an internal reflector (defect) is
present. In most strip materials (solid and clad) the thickness of the material is well controlled, so a
decreased time-of-flight is normally indicative of an internal flaw.

3. Figure 2: Normal Beam Method, and Time of Flight “A-Scan” Showing Material Back Wall and Internal Flaw Signal
While normal beam inspection is the historical method of choice using piezoelectric sensors it has a
few very important shortcomings:
 It requires very precise positioning of sensors to reflect from the backwall
 It uses a liquid couplant to transmit the sound from the transducer into the material
 Sensors have a limited beam width so a large number of sensors are required to inspect an
entire strip width which may be up 7’ in length.
With piezoelectric ultrasonic inspection, sensor alignment is critical to ensure sound will travel to
the back wall and reflect back to the sensor. This alignment is very difficult in automated
environments as strip material typically has curvature, camber and irregular surface conditions.
When a piezoelectric sensor is not perfectly aligned to a parallel surface, sound is sent into the
strip at an angle and does not reflect back to the sensor. This is because the sound is generated
in the sensor and transduced into the material under inspection via a layer of liquid couplant. The
need for perfect alignment is a well-known limitation of this technique and normally requires costly
material handling equipment to ensure the best possible alignment to the material.
The couplant used for the transmission of sound also poses important constraints. Couplant
needs to be maintained with minimum turbulence and free of bubbles and contamination to avoid
ghost reflections, which are difficult to ensure at speeds of over 1m/s.
Another drawback to this method is cost. A typical piezoelectric sensor is 1” in diameter or less.
Because the entire strip needs to be inspected, the normal beam piezoelectric method requires
dozens of sensors. Being that each sensor requires electronics to perform the inspection, the
cost of a normal beam inspection system is extremely high.
THE EMAT SOLUTION
One of the most significant UT developments of the last 20 years is the advent of non-contact
solutions that do not require coupling, such as Electro Magnetic Acoustic Transducers (EMAT),
used for ultrasonic testing of metals.

4. While the sound in piezoelectric
transducers is generated in the probe
and transmitted into the part through
the couplant, an EMAT induces
ultrasonic waves into a test object with
two interacting magnetic fields. A
relatively high frequency (RF) field
generated by electrical coils interacts
with a low frequency or static field
generated by magnets to generate a
Lorentz force in a manner similar to
an electric motor. This disturbance is
transferred to the lattice of the
material, producing an elastic wave.
In a reciprocal process, the interaction
of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil
circuit. For ferromagnetic conductors, magnetostriction produces additional stresses that
enhance the signals to much higher levels than could be obtained by the Lorentz force alone.
Various types of waves can be generated using different combinations of RF Coils and Magnets.
EMATs have all the benefits of ultrasonic testing, but because the sound is generated in the part
inspected, they enjoy some unique advantages for strip and clad inspection:
 Dry Inspection (no couplant). Not having couplant permits more reliable readings (no
couplant errors) and makes this technology easier to automate and integrate in production.
High inspection speeds and high temperatures are also a fundamental advantage of EMATs.
 Insensitive to Surface Conditions. EMATs are not sensitive to oxides, oil, water or uneven
surfaces and can inspect through thin coatings of material.
 Unique Wave Modes. Because they do not depend on liquid to transmit the sound, EMATs
can generate some guided wave modes that are not available or very difficult and impractical
to generate with piezoelectric transducers.
 Time-of-flight and amplitude inspection methods can replace dozens of channels of
piezoelectric ultrasonic equipment which would be required for Normal Beam strip inspection.
The main disadvantage of EMAT is the low efficiency of the transducer which requires high
voltages and very precise electronic designs to generate and detect the signals. These
disadvantages have become less relevant with the advent of new electronics and software that
enhance complex signal processing in real time. Additionally new proprietary Modeling software
and Finite Element Analysis tools have perfected the tedious task of EMAT sensor design.
INSPECTION WITH GUIDED WAVES
It is possible to generate any ultrasonic wave mode with EMAT inspection that is possible with
piezoelectric ultrasonic inspection. EMAT is also capable of generating unique guided waves
which are very difficult to generate with the piezoelectric method. Figure 4 compares wave
modes available for piezoelectric and EMAT ultrasonic methods. Lamb waves were determined
to be the wave mode of choice for clad inspection for their sensitivity to material thickness, where
in single layer materials, a combination of Raleigh (surface) waves, Lamb and Shear Horizontal
waves provide detection of an array of defect types, both internal and surface breaking.
Figure 3: Comparison of piezoelectric and EMAT sound generation

5. Wave Types in Solids Particle Motion Technique*
Longitudinal Parallel to wave direction Piezo, EMAT
Shear Vertical Perpendicular to wave direction Piezo, EMAT
Shear Horizontal
Perpendicular to wave direction on
a horizontal plane
EMAT
Surface – Rayleigh Elliptical orbit - symmetrical mode Piezo, EMAT
Surface – Bleustein-
Gulyaev
Piezo, EMAT
Plate Wave – Lamb
Component perpendicular to
surface (extensional wave)
EMAT
Plate Wave – Love
Parallel to plane layer,
perpendicular to wave direction
Piezo, EMAT
Stoneley (Leaky Rayleigh) Wave guided along interface Piezo, EMAT
Sezawa Antisymmetric mode Piezo, EMAT
Figure 4: Types of Waves in Piezoelectric and EMAT Ultrasonic Methods
A guided wave is one which propagates constrained by boundaries such as a surface, a plate, a
tube, rod or pipe, rail or other structure. Ultrasonic guided waves can travel from centimeters to
tens of meters in a structure. Guided waves that are constrained by top and bottom surfaces are
referred to as plate wave. Plate waves are Raleigh, Lamb or Shear Horizontally polarized waves.
Guided Wave Test Methods
The use of guided waves offers essentially 3 methods of defect detection. These are reflection
(where a wave strikes a defect and reflects back toward the sensor), time of flight measurement
(where the time a wave takes to propagate from point “A” to point “B” is monitored), and
amplitude (where the received signal is monitored for signal amplitude where the strength of the
signal determines the presence or absence of defects).
Inspection of strip material for laminations using guided waves has important benefits over the
conventional normal beam approach. Whereas piezoelectric transducers must send ultrasonic
energy from the top surface to the back surface to measure time-of-flight, an EMAT-generated
guided wave fills up the full volume of the material and permits inspection of the full strip width at
line speeds with a small number of sensors.
When Lamb waves encounter a thickness change in a structure they propagate at a different
velocity. As a material becomes thinner, the wave moves at a higher velocity through the thin
section than the thicker sections. This sensitivity to thickness characteristic of Lamb waves make
it the ideal wave mode for clad material bond testing (or lamination detection in single layer
Figure 5: Reflection and Time of flight / Amplitude Methods of Detection with Guided Waves

6. materials) as an area of disbond/lamination will split the Lamb wave into two separate waves (one
on top and bottom of the disbonded area which effects the velocity of the wave propagation and
therefore the time of flight of this wave from one side of the material width to the other).
EXPERIENCE WITH SINGLE LAYER MATERIAL INSPECTION (STEEL STRIP)
The first commercial system for inspection of thin strip using guided waves was first introduced by
Innerspec Technologies in the late 1990’s. The temate®
Pi-GW (originally introduced as temate®
4000) was designed to detect sub-surface defects such as pencil pipe, laminations and porosity,
and surface defects such as cracks and slivers in steel strip before cold rolling. Using lamb
waves the system was able to inspect material from 1.5mm to 10mm at speeds of 400m/minute
and detect defects as small as 0.07mm in thickness using the reflection method.
Figure 6: First EMAT system for Guided Wave Strip Inspection for Steel Strip Inspection
In initial testing, coils were visually inspected to identify surface defects after pickling. Analysis of
the defects detected with EMAT and found visually at different down-stream process indicates the
validity that an Ultrasonic EMAT inspection system can detect the presence of defects in the early
stages of manufacturing where down-grading material will result in cost savings. As shown on the
results table(figure 8), after pickling, 18 defects were detected with EMAT where visual examination
only detected 3 surface defects, indicating that 15 of the EMAT detected defects were internal and
not open or at the surface. After cold reduction, 7 defects were detectable visually and after electro-
galvanizing, 17 surface defects were visually detectable. In this single example, the coil could have
been downgraded after pickling saving the costs of running this material through cold reduction and
finishing operations.
Figure 7: Installation of EMAT system after Pickling and side view of Sensors

7. NEW ADVANCED TOOLS DEVELOPED
Our recent project with the supplier of coin stock material for a National Mint allowed the unique
opportunity to use advanced modeling and finite element analysis tools recently developed in-
house at Innerspec Technologies to aid in the efficient design of application specific EMAT
sensors along with the experience gained in the installation detailed previously on single layer
steel materials.
Figure 8 shows a metallurgical photo of the cross section of the three layered cladding product.
The normal thickness of the three layered structure is 0.064 inch in total and 0.016’’, 0.032’’, and
0.016’’ for the brass, copper, and brass layers respectively.
Figure 8: Metallurgical photo of the cross section of the three layered cladding product.
A Laboratory system was assembled and installed in an off line re-rolling location. This system
was tested for nearly 6 months and data was studied to continuously verify the results matched
the models developed. The guided wave EMAT method of strip inspection requires the use of a
separate transmitter and receiver held at a fixed mechanical distance from each other. In a
generally homogenous material, the time of flight of the transmitted signal to the receiver and the
amplitude of the received signal is near constant. By closely monitoring the time of reception of
the EMAT generated ultrasonic energy and the amplitude of this signal, the presence of defects in
strip materials can easily be detected.
Figure 9: Trial System Installed off line for modeling verification and Signal variance at disbond
Since the installation of this system, the customer has given positive feedback about the success
in both artificial and real defect detection. Several sample scans are shown in Figures 10-12. The
first is a disbond intentionally added during manufacturing. The second is a natural fabrication
defect at the edge of the strip, with around 14’’ length and 2.5’’ wide into the strip. The third is a
natural defect detected with EMAT system. This sample section was isolated and processed
through the coin planchet punch and rimming operations. In the stamping trial, three coins (0.94”
dia) contained the disbond of approximately 0.3” wide at the same cross-web location. These
coins came from the exact location that had been previously marked based on spikes seen in
EMAT system. Other defects found from EMAT automatic system were also verified with
ultrasonic bulk wave spot checking instruments.

8. Figure 10: Artificial lamination 0.7’’ wide
Figure 11: Natural defect on the edge with 14’’ long and 2.5’’ wide into the strip
Figure 12: Natural defect about 3 inch long and 0.3’’ wide
Ultimately a production system was designed and installed to operate in-line at line speeds.
Since its installation, disbonded coin stock has been eliminated. New guided wave dispersion
curves have been developed for other types of coin stock clad alloy inspection. The operation is
fully automated and the system includes an automated paint marking system to identify
disbonded areas on the strips edge while processing.
The temate®
Pi-GW system has now been verified in operation on both single layer and multi-
layer clad materials. With the successful implementation of proprietary guided wave modeling
and finite element analysis 100% line speed inspection is available for single and multi-layer
materials. The results have been verified via destructive and off-line nondestructive methods.
EMAT generated guided waves are key to an in-line inspection of such materials as the EMAT
method is the only way to efficiently generate guided waves into a production inspection
environment.
Manufacturers can realize compelling return on investment by using this method of inspection far
upstream in the process to not add value to lower quality materials which are prone to fail final
inspection.

9. Figure 13: temate®
Pi-GW system as installed at Clad Coin Stock Factory Front and rolling direction views