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Automatic Verification of
Induction Hardening
Using Eddy Current and
Preventive
Multi-Frequency Testing |
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By ARNOLD
HORSCH
ibg Prutcomputer GmbH, Ebermannstadt, Germany |
Increasing
quality demands, new product liability
regulations, as well as international market
networks force manufacturers to take special
measures encompassing the field of material
testing. Nowadays, specified tolerances are
extremely small. Therefore processes have to
include the conducting of100% material,
structure and hardening tests on a fast,
reliable and simple basis. The technology
applied must be the latest state of art, it must
comply with maximum safety requirements and be
very economical.
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 Eddy
current testing, especially Preventive
Multi-Frequency Testing (PMFT) is applicable for
a large range of uses. It has been successfully
introduced worldwide to test all kinds of
material, including the determining of
microstructure and hardening characteristics.
There
are numerous applications for such a system.
Today, every company carries out spot checks
during manufacturing,
 in order to guarantee the
quality of products. Within the framework of
control one of the most important problems is to
ensure repeatability of quality. Quality control
faces major problems within this area; these might
start with supply of the material or products.
Parts properties; that is structure, hardness or
case depth, which are determined by the
induction hardening process, are subject to
variation. To determine correct heat treatment,
the test method must be non-destructive; thus,
eddy current testing is applicable.
Computer-based multi-frequency eddy current test
stations with 8 to 32 testing frequencies and up
to 16 test positions has greatly increased the
efficiency of this testing method. Its field of
application ranges from structure, hardening and
case depth inspection to material mix
inspection. One of the most outstanding
advantages of modern eddy current test systems
using PMFT is their capability to detect
unexpected defects.
Table I gives a few application examples of
nondestructive material testing for verification
of induction hardening by means of eddy current
multi-frequency technology.
WHY 100%
TESTING?
Variation in product usually is in accord with
statistical analysis and can be predicted or
estimated. For this reason it is sufficient to
make a certain number of spot checks in order to
conclude the general nature of the quality from
the test data.
Problems develop if during manufacturing some
occurrences arise which are not subject to
standard statistical distribution. First, one
needs to know what may happen unexpectedly.
Table II lists possible hardening errors.
Modern process monitoring systems, which should
be available for all new machinery, allow
detection of some of these errors while they
develop. These errors may occur as isolated or
in combination with others. If several minor
deviations accumulate, the process monitoring
system does not immediately respond, while
defective parts are being produced. Simple
methods like hardness inspection cannot be used
for reliable determination of the error. Spot
checks will fail if the error occurs for a short
time between two spot checks.
 Generally, spot checks are only capable of
detecting slow changes in the process.
Unpredictable errors occurring for a short time
are unlikely to be detected by spot checks.
Thus, inspection using eddy current test method
must be used if 100% of all defective parts are
to be found.
How DOES EDDY CURRENT TESTING WORK?
Basically, the test installation consists of a
coil with a sending and a receiving winding. The
two windings are only loosely coupled. In the
empty coil, a low voltage is induced in the
receiving winding by the magnetic field of the
sending winding.
If a test part approaches the coil system, the
coupling factor between the sending and the
receiving winding changes (see Fig. 1). This
change is mainly determined by electrical and
magnetic conductivity (permeability) of the test
part. These two electromagnetic properties are
strongly influenced by the microstructure of the
test part. If the part is too hard, permeability
is different than for an annealed part. |
What Is
Permeability?
The variable of ferromagnetic materials decisive
for eddy current testing is permeability.
Permeability actually means relative
permeability, a number without any dimension,
which indicates how much better a certain
material can conduct electric flux lines in
comparison to air. Air has the value of 1. A
steel which can be magnetized has a value
between 10 to a few 1,000. The correlation
between permeability and field strength is not
at all linear. In
case of a very small field strength,
permeability is low (starting permeability).
With an increasing field strength permeability
increases to a maximum value, whereupon it
decreases subsequently to a smaller value. The
reason can be seen in the dislocation of the
Bloch wall as well as in the direction of the
"Magnetic Weiss's Domain." Every structure and
every material has a characteristic
permeability. This means that parts with
different heat
treatments and different microstructures exhibit
different permeability. Fig. 2 shows the
different permeability curves for different
materials4. If testing is done at frequencies
where eg. C45 and 100 Cr6 have the same
permeability, the two materials cannot be
separated or distinguished. This may happen if
material mix between 100 Cr6 and St70 is
expected and the eddy current instrument is
optimally set to cause this result. |
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WHY
PREVENTIVE MULTI-FREQUENCY TESTING (PMFT)?
Eddy current testing is the most economical and
most reliable method for 100% inspection. The
method is not new, but there is still some
discomfort with it due to bad experience with
old monofrequency systems which were used in the
past. Test results obtained from these
monofrequency systems can be used with
restrictions only.
Monofrequency Test
Monofrequency systems commonly use a group of OK
parts and a group of NOT GOOD parts, eg.
improper case depth, to set the instrument for
the test task. With this situation many
influencing factors are not considered.
Considering all errors listed in Table II, it
becomes clear that one cannot have master parts
with all possible defects to set the instrument.
There is not a complete set of defective parts
covering all defects possible for every part;
furthermore, artificial defects are never as
effective as real defect, and it is very
difficult to simulate improper heat treatment to
create such defects.
 Monofrequency
test instruments are usually not very new and
use 50Hz for excitation of eddy currents (see
Fig. 3). With some of them, one can switch from
one frequency to another, but in all cases, only
one single frequency is used for testing.
Evaluation of test results is done in different
ways, but usually it is only unidimensional.
Some test instruments provide multidimensional
evaluation of test results. Test speed for the
monofrequency method is usually not very high
because electronics used for evaluation are
rather slow. |
Preventive
Multi-Frequency Testing
 Modern
eddy current test instruments based on the
Preventive Multi-Frequency Test Method, Fig. 4,
operate in a completely different manner. Based
on the experience that different
defects cause different signals in eddy current
test instruments, a large number of test
frequencies is used now. Only OK parts are used
to set the test instrument.
 Apart
from the large number of frequencies, it is
important that a broad frequency range is
covered, ie. the ratio between the lowest and
the highest test frequency should be 1:1000 or
higher to guarantee reliable testing. Due to use
of new electronic components it was possible to
educe test time considerably. From a time point
of view, it does not matter whether two or
eight frequencies are used for testing (because
testing is done faster), and it is now possible
to really test preventively. This means that all
information contained in a material can be
'read.'
Another advantage is the multidimensional
evaluation of modern test systems. A separate
tolerance field is generated for every test
frequency (see figure 5). Only when all
tolerance fields are satisfied can one assume
that the part is OK. If a part is not OK in only
one tolerance field, the part is classified
NOT GOOD. Any change in the low, middle upper
frequency range displayed clearly. |
 ECONOMIC
SIGNIFICANCE
Apart from the aforementioned criteria for test
reliability, cost for a test system influences
the decision of which method will be used.
Considered are these factors: expenditure for
test station, test instrument, auxiliaries,
etc.; expenditure for staff necessary for
testing and evaluation; expenditure for various
nonproductive times, eg. interruption of
manufacturing for verification; scrap due to
delayed adjustments in the manufacturing process
and customer complaint.
Cost for equipment and staff can easily be
estimated. Cost for organization and scrap are
very difficult to calculate. The eddy current
test instrument (eddyliner®P) and preventive
multi-frequency
testing, make it possible to lower the
 costs of
serial tests significantly. Destructive tests
are essentially reduced to machine
readjustments, destructive inspections and
testing NOT GOOD parts. With respect to series
testing, reduction of test scrap and time saving
are dramatic (see Fig. 7).
APPLICATION EXAMPLES
Two standard examples illustrate the potential
provided by eddy current testing by Preventive
Multifrequency testing (PMFT) with an eddy
current test instrument (eddyliner®P).
Induction-Hardened Water Pump Shafts
Water pump shafts are induction-hardened by a
coil in a continuous process. The shafts are
pushed through the inductor by means of a
driving wheel. This process is very reliable.
Nevertheless, there had been complaints from
time to time because some shafts were not
properly hardened in the running surface (see
Fig. 8). Furthermore, some shafts had soft spots
 of
approx. 1/2 inch in diameter. When the process
was analyzed, it turned out that these parts
were not properly hardened because of
dimensional tolerances at certain positions as
well as malpositioning which could not be
specified. These soft spots were detected only
after 100% inspection had been introduced.
To remedy these problems would have been
possible by extensive and cost-intensive changes
in the process. Since this did not promise 100%
success, 100% eddy current testing was the ideal
solution. After one year of operation, it showed
that the error rate was less than 0.1%.
Complaints sue to improper hardening are no
longer an issue. Due to the eddy current test
system, only one test station is needed for
several induction hardeners. The multiposition
test instrument (eddyliner®P3) was used (see
Fig. 9). Total test time for all 3 relevant test
positions is less than one second. Cycle rate of
the system is more than 3 parts/sec. |
Automatic
Verification of Induction Hardening on Steering
Racks
 The
eddy current test system for control in
induction hardening of areas of power steering
racks is integrated in a two-lane machine (HWG
Inductoheat, Fig. 10). In an interlinked
manufacturing process, parts have to be tested
automatically for correct hardness and hardening
depth in the tooth and the shaft area after
induction hardening and annealing. Two test
parts are tested separately at two positions
each, one in the shaft and one in the tooth area
for hardening pattern, hardening depth, hardness
and material mix (see Figs. 11 and 12). At every
test position, testing is done with eight test
frequencies ranging from 25Hz to 25kHz. If, for
example, on lane 1 a defect occurs, this part is
sorted out. The eddy current test instrument is
interlinked with the system control. This allows
stopping of the system if several NOT GOOD parts
are sorted out in a certain period of time so
that scrap is reduced to a minimum. Cycle time
of the test instrument is much less than cycle
time of the hardening machine. The concept of
integration of this entire system for this
application is an optimum, economical solution. |
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Summary
Especially when considering product liability,
every industrial company has to meet the demand
of testing all hardened components by means of a
reliable, fast and inexpensive 100% test.
Preventive Multi-Frequency Testing (PMFT)
especially with modern test systems (eddyliner®
P) integrated with induction hardening, is a
real alternative to destructive systems. It is
possible to operate these systems as inline
testing machines or as automatic monitoring
systems with 100% reliability and without any
problems. A single-frequency test must be
considered as being unreliable. Only a
multi-frequency test, which adheres to
the described marginal conditions, resembles a
reliable method within the field of eddy current
testing. |
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