IN DEGREE PROJECT MECHANICAL ENGINEERING,SECOND CYCLE, 30 CREDITS
, STOCKHOLM SWEDEN 2018
Standardization of Eddy Current Testing Calibration for Valve Spring Wire
ANNICK INGABIRE
ROBIN OLSSON
KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
KTH Industrial Engineering and Management
Standardization of Eddy Current Testing
Calibration for Valve Spring Wire
Thesis supervisor MSc students
Company: Hanna Ahl, MSc Robin Olsson
KTH: Robert Tomkowski, PhD Annick Ingabire
Examiner: Andreas Archenti, Professor
Master of Science Thesis MG213X
KTH Royal Institute of Technology
ITM / Dep Production Engineering
TPRMM-16
SE-100 44 STOCKHOLM
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Sammanfattning
Elektromagnetisk provning (ECT) har blivit en av de mest använda metoderna för att
kvalitetssäkra produkter där icke-destruktiv testning av material krävs. Vid provning av ståltråd
interagerar utrustningen med det testade materialet och upptäcker ytdefekter och, i viss
omfattning, om strukturen skiljer sig från det kalibrerade provet. Om produkten befinner sig
utanför specifikationen, skrotas den eller ombearbetas. Denna Mastersuppsats undersöker
kalibreringsförfarandet för elektromagnetisk provning som utförs av Suzuki Garphyttan, som är en
av de största tillverkarna i världen av ventil- och transmissionsfjädrar för bilindustrin. Genom de
slutsatser som framgår av denna rapport, som baseras på undersökningar gjorda i vetenskapliga
artiklar och genom att analysera den data som inhämtats från produktionen, presenteras en
standardisering av kalibreringsförfarandet. Detta är nödvändigt för att säkerställa såväl testernas
tillförlitlighet, såväl som minimering av risken för att skrota ut material på grund av felaktiga
inställningar, till exempel på grund av otillräckligt signal-brus (S/N)-förhållande. Fokus ligger på
sond-baserad, roterande testning, i denna avhandling kallad circografen, eftersom den är manuellt
kalibrerad.
Några av de konstaterade resultaten i rapporten är:
Standard Operating Procedures (SOP)-baserade instruktioner implementeras i företagets
kvalitetssystem. Detta för att minska variationer i kvalitet mellan olika operatörer och
maskiner.
Ett förslag på intervaller för värden (fasvinkel, förstärkning, korrigering av filter och så
vidare) presenteras. Detta är baserat på insamlad unik produktionsdata från operatörer
och utförda test.
Fasvinklarna som används varierar inom specifika intervaller och bestäms av materialval i
allmänhet och frekvensval i synnerhet.
Konduktivitets- och permeabilitetsvärdena för oljehärdad tråd, liksom penetrationsdjupet
för tre olika frekvenser presenteras.
Härdningsfel kan inte detekteras i roterande provning
Ökat kolinnehåll minskar den elektriska ledningsförmågan och ger ökad resistivitet, vilket
gör att fasen flyttar sig och resulterar i ett minskande gap mellan brussignal och
spricksignal.
Nyckelord: NDT, ECT, ståltråd, konstgjorda defekter, sprickor, kalibrering
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Abstract
Eddy current testing (ECT) has become one of the most extensively used methods to secure the
products and constructions when non-destructive testing is required. In typical cases of steel wire
testing, the equipment interacts with the tested material and discovers surface defects and, to a
limited extent, if the inner structure is differing from the calibration sample. If the product is
found to be outside specification, it is either scrapped or reworked. This master thesis investigates
the Eddy current testing calibration procedures performed by steel wire manufacturer Suzuki
Garphyttan, which is one of the largest producers in the world of valve and transmission spring
wire for the automotive industry. By the research shown in this thesis, based on the investigation
made in scientific papers and by analyzing data extracted from production, a standardization of
the calibration procedure is being presented. This is to secure both the testing reliability, and
minimizing the risk of scrapping material due to inaccurate settings, for example due to
insufficient signal to noise (S/N) ratio. The focus is on probe-based, rotating testing, in this thesis
called the circograph, since it is manually calibrated.
Some of the findings established in the report:
Standard Operating Procedures (SOP) based instructions is being implemented in the
company's Quality system. This is to decrease the process variations between different
operators and machines.
Suggestions of intervals for values (Phase angle, gain, filter correction and so forth) are
presented. These values are based on collected unique production data from operators and
machines, as well as performed tests.
The phase angles used are ranging between specific value intervals, and set by material
choice in general and choice of frequency in particular.
The conductivity and permeability values for oil-tempered wire, as well as penetration
depth for three different frequencies, are presented.
Hardening error cannot be detected in the circograph.
Increased carbon content is decreasing conductivity and increasing resistivity, causing the
phase to move slightly and decreasing the gap between noise signal and crack signal.
Keywords: NDT, ECT, Steel wire, artificial defect, crack, Calibration.
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Foreword
This thesis hereby acknowledges that it is not the result of one’s single effort but rather a
product of combined efforts from different people and organizations. Our gratitude is
hence extended for the advice, guidance, expertise and encouragements shared with us to
everyone who helped put together this piece of work.
Special appreciation is hereby expressed to the following professionals for their remarkable
contribution to the project:
KTH Thesis Supervisor PhD. Robert Tomkowski for his immense support and
enlightenment throughout the project.
Suzuki Garphyttan Thesis supervisor Mrs. Hanna Ahl, MSc, who helped us
through the project in every aspect of the standardization of their ECT
procedures.
KTH Professor Andreas Archenti, as the examiner of this thesis work.
Suzuki Garphyttan as a company, and their professional staff including Stefan
Kandelid (R&D engineer), Erik Elfversson (process manager), Mika Hynninen
(Eddy current technician and expert), Thomas Wrenninge (operations manager),
who has been of great value sharing their knowledge and discussing results and
theories.
Some of the operators have been of great value sharing the knowledge of
calibration, and all operators have been collecting the information of today’s best
praxis, making it possible to compile it into a full instruction. We cannot mention
all here, but we show gratitude to Therese Magnusson, Fredrik Nilsson and Per
Rickardsson and others for taking time to share their knowledge and competence.
All our KTH colleagues in the School of Industrial Technology and Management
are hereby thanked very much for their everyday support during the project period,
as well as for the 2 years of the Master. Among many, Ove Bayard, PhD, has
been of great help and support.
Our Final Thanks go to our families and friends for their constant encouragements
and support they have demonstrated throughout the duration of studies at KTH,
especially when the project was ongoing.
Robin Olsson and Annick Ingabire
Stockholm, June 2018
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NOMENCLATURE
Abbreviations
Abbreviation Significate
SG Suzuki Garphyttan
NDT Non-destructive Testing
NDE Non-destructive evaluation
EC Eddy Currents
ECT Eddy Current Testing
OT Oil-Tempered
ISO International Standards Organization
AISI American Iron and Steel Institute
ASTM American Society for testing and Materials
SIS Swedish Standards Institute
SS Svensk Standard
TS Technical Committee
SI International System of Units
SOP Standard Operating Procedure
CM Color marks
UHT Ultra High Tensile Steel
EDM Electrical Discharge machining
S/N Signal to noise
AC Alternative current
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Notations
Latin Symbols Unit Description
A [mm2] Area
D [mm] Diameter
f [KHz]or [kg/m3] Frequency
G [dB] Gain
R [Ohms] or [Ω] Resistance
L [m] Length
H [A/m] Magnetic field strength
B [T or Wb/m2] Magnetic flux density
Greek symbols Unit Description
µ [H/m] or [Wb/(A m)] Magnetic Permeability
σ [S/m] Electrical conductivity
ρ [Ohm meter] or [Ωm] Resistivity
ω [Rad/s] Angular frequency
δ [μm] Standard Penetration depth
π 3.14159 Pi
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Contents
1. INTRODUCTION .................................................................................................................... 9
1.1 TECHNOLOGY OVERVIEW ....................................................................................................................9 1.2 THESIS AIMS .................................................................................................................................... 10 1.3 PROJECT SCOPE .............................................................................................................................. 10 1.4 ABOUT SUZUKI GARPHYTTAN ........................................................................................................... 11
1.4.1 Wire production ...................................................................................................................... 11 1.4.2 ECT and the reason for inline testing. ..................................................................................... 14
1.5 THE CHALLENGE OF THIS WORK ........................................................................................................ 15 1.6 THESIS LAYOUT ............................................................................................................................... 16
2. FRAME OF REFERENCE ...................................................................................................... 17
2.1 EXISTING TECHNOLOGY ALTERNATIVES .............................................................................................. 17 2.2 EDDY CURRENT TESTING .................................................................................................................. 19
2.2.1 History .................................................................................................................................... 19 2.2.2 Generation of Eddy currents .................................................................................................... 19 2.2.3 Factors affecting Eddy Current Testing ................................................................................... 20
2.3 EC TESTING REFERENCE STANDARDS ................................................................................................. 31 2.3.1 Standards for EC testing equipment ........................................................................................ 31 2.3.2 Standards for calibration ......................................................................................................... 32 2.3.3 Standard Operating Procedure (SOP) for calibration ............................................................... 32
3. METHODOLOGY ...................................................................................................................35
3.1 QUANTITATIVE AND QUALITATIVE EVALUATION OF THE CALIBRATION PROCEDURE ................................. 35 3.1.1 Quantitative analysis ............................................................................................................... 35 3.1.2 Qualitative analysis ................................................................................................................. 35
3.2 EQUIPMENT USED ............................................................................................................................ 36 3.2.1 Circograph and the procedure of calibration ............................................................................ 36 3.2.2 Defectomat ............................................................................................................................. 44 3.2.3 Contracer and other measuring equipment ............................................................................... 44 3.2.4 Calibration samples ................................................................................................................. 45 3.2.5 Microscope and mounted samples ........................................................................................... 46 3.2.6 Generally about the methods for this thesis paper .................................................................... 46
4. RESULTS .............................................................................................................................. 47
4.1 RESULTS ANALYSIS ........................................................................................................................... 47 4.1.1 Phase angle determination ...................................................................................................... 48 4.1.2 Gain ........................................................................................................................................ 51 4.1.3 Filter correction ...................................................................................................................... 54 4.1.4 Conductivity and permeability measurement ............................................................................ 57 4.1.5 Frequency and phase ............................................................................................................... 60
4.2 OTHER OBSERVATIONS ..................................................................................................................... 62 4.2.1 How the amount of color marks seemed to trend with increased alloy content ......................... 63 4.2.2. Defect of different conductivity (lead, Pb) .............................................................................. 64 4.2.3 Artificial defect reliability ........................................................................................................ 65 4.2.4. Calibration with lead (Pb) disturbance or directly from lead ................................................... 67 4.2.5. Dirt, oxide remains and oil covering the artificial defect .......................................................... 69
4.3 DEFINITION OF IMPROVEMENT POSSIBILITIES ...................................................................................... 70 4.4 RESULT OF SOP INSTRUCTION FOR OPERATOR ................................................................................... 71
5. CONCLUSIONS ..................................................................................................................... 73
5.1 CONCLUSIONS .................................................................................................................................. 73
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5.2 DISCUSSIONS ................................................................................................................................... 74 5.3 FUTURE WORK ................................................................................................................................. 75
LIST OF TABLES ...................................................................................................................... 76
LIST OF FIGURES ..................................................................................................................... 76
REFERENCES ............................................................................................................................ 78
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1. Introduction
This Introduction section is divided into six parts: 1) Technology overview, 2) thesis aims,
3) project scope, 4) a summary about Suzuki Garphyttan where the work has been
performed, 5) the challenges of this thesis and 6) the thesis layout. The aim is to give an
overview and introduction of the thesis work.
1.1 Technology overview
It is a common phenomenon that valve spring wires, which are used in engines for the
automotive industry, can have mechanical defects or cracks causing a spring failure,
which eventually lead to a failure in the complete engine of the car or the truck. The
defects causing the breakages can be present in the raw material, as it is delivered as wire
rod from the steel mill, or introduced later on during the various stages of wire production
to its final dimension and final material properties. Depending on the context, such
defects must be clearly identified and removed before reaching the customer, since even
the smallest crack or defect can cause a breakage of the spring, and causing a failure in
the engine. This means both the wire and the spring needs to be tested, to ensure no
critical defects exist.
There are several established methods for non-destructive testing, including but not
limited to: X-rays, ultrasonic testing and eddy current testing (ECT) [2].
During valve spring wire production, every precaution is taken to avoid a damaged wire
surface. Material is removed in an initial process called shaving, where the outer oxide
layer of the wire rod from the steel mill is removed, including potential cracks. After that,
the wire is carefully protected throughout the wire production process, to avoid sharp
edges as well as protection from metal to metal contact in the different process steps.
Still, a final, non-destructive testing procedure needs to be done to ensure the quality,
and this testing consists of using ECT. ECT is suitable for testing of surfaces on
conductive materials like steel. By using ECT, surface imperfections deeper than 40µm
[3], can be detected, marked with color and subsequently removed in the spring coiling
operation.
The expected result from this project is to update instructions used by the operators
during calibration of ECT, and to establish value intervals of important parameters used
during ECT calibration. By having a standardized calibration, the possibility to minimize
signal to noise level and to avoid false alarm color marked defects is enhanced. It is also a
securement that no defects will be missed in the future due to lack of competence or
machine failures.
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There are a lot of properties, both in settings of the equipment and of the material, that
influence the result, including conductivity, phase angle, gain, steel properties, artificial
defect depth, natural defect depth and so forth. The key to a reliable result is to set those
parameters right. The importance and explanations of the terms will be further examined
in the Literature review part, and connected in Result section.
1.2 Thesis aims
The main purpose of this master thesis is to;
Update current official instructions used by the operators
Investigate how different calibration settings affect the testing reliability and try to
find the best praxis of current knowledge, to set a standard for the future.
Fill the knowledge gap of used intervals for different parameters as well as material
properties and its impact on the testing.
Today there is a short instruction on calibration settings, but the description is vague and
referring mainly to operator competence, and in the increasing requirement of modern
production, a framework resulting in an instruction should be implemented, based on the
operators’ knowledge, previous production data and a scientific background.
1.3 Project Scope
The thesis aim is to update the instructions on what settings to use when calibrating the
circograph for ECT of valve steel wire produced by a specific company. The company is
one of the market leaders in the world producing valve spring wire. The instruction
includes recommended intervals of gain, phase angles and other parameters that the
operator or management use when the calibration is being executed.
Limitations for the project are:
Only oil-tempered wire between 2.0-6.55 mm used at Suzuki Garphyttan will be
investigated.
The only non-destructive testing, NDT, method examined is eddy current testing,
ECT. This is due to the fact that ECT is the only NDT-based evaluation
performed today in the company. ECT is suitable for detecting surface defects and
cracks in conductive materials, which is a main failure cause for spring breakages
in automotive engines. Other NDT-methods are presented shortly, but the main
focus is to standardize the existing technology, not to compare different
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techniques. The company executes other tests to detect inclusions and other
material failures, which is outside the scope of this thesis.
Only circograph (rotating probes) will be investigated, not defectomat (encircling
coil). The reason for this choice is that a daily calibration is being done in the
circograph, but the defectomat already have standardized values.
The results are only valid for Foerster GmbH equipment, a German producer
considered as a leading manufacturing of ECT equipment. Foerster GmbH has also
given the names “circograph” and “defectomat”, which are used in this report.
The results and the values are only valid for the specific environment, product, site
and process which the tests has been performed. The results are however
comparable to other environments, and other examinations, articles and reports
has served as a guide on how to set the experiments and values presented in this
report. The aim is to extend this field of knowledge with the material type used
for valve spring wire.
1.4 About Suzuki Garphyttan
Suzuki Garphyttan has produced wire since 1906. Since 1927 the main product is oil
tempered valve spring wire for the automotive industry. During the years, other special
wires such as stainless round wire, flat rolled and shaped wire in carbon, low allowed and
stainless material have been added to the production portfolio.
The company’s focus is spring wire for combustion engines, mainly valve and transmission
springs, piston rings, compression rings, fuel injection springs used in cars and trucks all
over the world.
Eddy current testing is carried out on material with high demands on surface quality. It is
performed with both rotating (R) and stationary (D) probe test equipment.
1.4.1 Wire production
Basically, oil tempered wire for the automotive industry has the following procedure:
The production starts with wire rod from external suppliers, commonly called “coils” in
the industry (not to be confused with eddy current coils, which in this thesis is referred to
mainly as probes). The weight of a wire-coil is around 2 tons per coil, and is delivered in
a few different standardized dimensions, like 8.00mm or 9.00mm, and then processed to
the demands of every specific customer. The goal of the wire production is as follows:
Wire with correct dimension, like 3.95 mm or egg-shape like 3.00x2.39mm2.
The surface should be defect free
The black oxide layer should be stable and cover the wire
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It has to be hardened with the right properties (tensile strength, yield strength,
fine martensitic structure and so forth).
To achieve this, the wire is processed in 5 steps as described shortly below. The steps are:
1) Shaving
2) Recrystallization
3) Drawing
4) Hardening
5) ECT
Common procedures and production flow in wire production is shown in a more
visualizing way on the following figure 1, where also the steel grain structure and the
color of the surface is visualized.
Figure 1: The production steps and a schematic simplified view of structure, grain size, oxide layer and dimension
Now follows a more detailed description of the process steps.
(i) Shaving
Shaving is the first step performed and means that the wire is machined through a
shaving die, simply removing the top layer of the wire rod (oxide scale mainly, but also
cracks and decarburization), making it looking very shiny (see figure 2). The advantage of
this procedure is that removes cracks and defects originated from the wire rod mill or
from transportation. The disadvantage of this is that a significant part of the weight is
being removed, which is costly. Other alternatives to shaving are chemical material
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removal (pickling) and shot blasting, which is mainly removing the oxide but not deep
defects or wire rod cracks. Therefore, for the toughest applications, material removal is
necessary.
Figure 2:How the wire rod looks after shaving
(ii) Recrystallization
When the shaving process is done, the surface layer of the wire becomes very smooth, but
the upper surface layer is also becoming martensitic or deformed perlite. Since the next
production step is heavy reduction in the drawing machine, the shaved layer will crack up
when drawing reduction is performed due to the tension between the martensitic top layer
and the ductile core. This would cause heavy damage to the wire. To avoid it, the wire is
being recrystallized in a furnace. The temperature is lower than in hardening furnaces, but
enough for the surface grain structure to relocate and become more ductile.
(iii) Drawing
Drawing is the heart of wire production. Here the final dimension will be set, which the
customer has required. The drawing machines consists of several drawing capstans,
typically between 3-10 capstans, which will drag the wire through drawing dies, each die
size smaller and smaller, forcing the wire to reduce in size. Since the force is high, the
drawing process is very sensitive for defects, typically scratches and mechanical defects,
which can appear when the drawing die get damaged, the cooling in the machine is
insufficient, or there is other contact between metal equipment and the wire. The
principle of drawing is shown in figure 3.
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Figure 3: wiredrawing die
After drawing it is time to set the final material properties in hardening process.
(iv) Hardening
Hardening is a process where the wire is transformed through high temperature to
austenite, then cooled quickly and quenched in oil creating a martensitic structure, which
makes the wire hard but also brittle. Then the wire is being annealed in lower
temperature, to keep the main part of the strength but gaining more flexibility and less
brittleness.
Figure 4: Hardening procedure.
The heating process is done by using a combination of high frequency induction heater
and an indirect heating furnace. The pipes usually hold inert atmosphere, to avoid
decarburization. The furnace keeps the wire’s temperature constant until immediately
before the oil-quenching, and during the time in the furnace a surface oxide film grows,
which also plays an important role as lubrication of the wire during the spring
manufacturing. The heated wires are then oil-quenched, creating a fine martensitic
structure, and finally annealed in molten lead bath to set the tensile strength and other
mechanical properties. After tempering, water cooling bath is used for cooling the wire
and rust protection is applied before enter WIP area in front of ECT. The flow is
visualized in figure 4.
1.4.2 ECT and the reason for inline testing.
ECT as a procedure will be further analyzed in the coming sections. However, the reason
for the wire to be checked inline is to discover and sort out surface defects. There are
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many types of defects, but mainly wire rod defects and so called button type defects are
the most serious. Also mechanical marks and scratches are common. The example below
(figure 5) shows how a wire rod defect can look like on the wire, as well as its
characteristic appearance, the crack is around 270µm, and the limit is 40µm. The crack is
partly covered by material. The defects are usually difficult to see by the human eye,
unless it is specified exactly what part and position of the wire that is damaged. In ECT,
the defects, also partly covered by material, are quickly detected, which is one of the
reasons for the methods’ popularity in wide field areas like nuclear plants, airplanes, pipes
and so forth.
Figure 5: Wire Rod crack, surface picture and its corresponding crack appearance in a cut
When a defect is detected it is marked by a white color, and later sorted out by the
spring producer. Therefore, no defects appear in the engines it is intended for. In the
Results part, color marks (CM) are mentioned a few times in an evaluation purpose. CM
means the number of times a fault is discovered and color marked, and can be measured
in a total amount or for comparison purposes by CM/100kg or CM/1000m or other
preferred units. The color marking is done even if the equipment is (calibrated) right or
wrong, or disturbed in another way causing false alarms. The intention is to only mark
flaws that are outside specification, for valve spring wire material 40µm or above.
1.5 The challenge of this work
The challenge in this project is consisting of mainly the following points:
1) There is a limited description within the company on how to do a calibration,
since it is considered a craftwork and something the operator learn by experience
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and peer-to-peer learning. However, in modern production, a calibration system
shall be established to ensure that measuring and test equipment are maintained
by periodic calibration.
2) It is thought within the company, that the safety margins for defects are too high,
or that the equipment, by itself or by the human factor of insufficient calibration,
for unknown reason are too sensitive and giving signal to defects that in fact are
small, so called false alarms. This is costing the company a lot, as well as giving
extra work for their customers to remove those defected material parts. Figure 6
shows a defect that according to the equipment measurement is over 40µm, when
in fact, the depth is only around 20µm.
Figure 6: Depth of the defect
Compared to the wire rod crack shown on figure 5, this defect (figure 6) is only 20µm
and should not have been sorted away, since the limit is 40µm. This is causing over-
scrapping of material.
1.6 Thesis layout
The project thesis is arranged in five sections as listed below:
Introduction
The Introduction consists of an overview of the project including the technology
overview, thesis aims, the project scope, and a summary about Suzuki Garphyttan,
the challenge of this work and thesis layout.
Frame of reference
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In this chapter, an overview of the existing technology alternatives will be
described, as well as an in-depth description of ECT and is influencing factors.
Previous research papers will be referred to, to provide the knowledge and
information about ECT. Finally, ECT reference of standards used in the project
will be described, and a description of the SOP-based instruction that will be the
base for future calibration in the company.
Methodology
In this section, the calibration procedure of the circograph is described, as well as
other equipment used in this project, their methods and purpose which help to
reach the goals for the project.
Results
The results that are obtained with the methods described in the Methodology
section are compiled and analyzed as well as the results of the updated SOP for
calibration of the circograph.
Conclusions
Finally, a discussion of the results, conclusions and future work is presented in
Conclusions. The conclusions are based from the analysis with the intention to
answer the thesis questions in 1.2 Thesis Aims.
2. Frame of reference
The aim of this section is to provide a summary of the existing knowledge and previously
performed research in pursuing the project objectives. Existing technology alternatives in
the field of nondestructive testing is presented in this chapter, some of the factors
affecting eddy current testing as well as reference standards used in ECT.
2.1 Existing technology alternatives
Non-destructive testing, NDT, techniques are used in the metal industries to evaluate the
properties of a wide variety of materials without causing damage to the specimens. Some
of the most common NDT techniques utilized for inspection of conductive materials such
as copper, aluminum or steel are electromagnetic eddy current based, ultrasonic or liquid
penetrant testing. An overview of different techniques is described in table 1.
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Table 1: Different NDT method used in industry [1]
Method Principles Application Advantages Limitation
Ultrasonic
examination
High frequency sound
pulses from a transducer
propagate through the
test material, refracting
at interfaces
Most materials can be
examined if sound
transmission and
surface finish are good
and the shape is not
complex
Provide precise, high
sensitivity results quickly.
Thickness information,
depth and type of flaw
can be obtained from one
side of component
No permanent record
(usually). Material
attenuation, surface finish
and contour. Required
couplant
Radiographic
testing
Radiographic film is
exposed when radiation
passes through the test
object. Discontinuities
affect exposure
Most materials, shape
and structures.
Examples include welds,
castings, composites
etc. As manufactured
or in service
Provides permanent
record and high
sensitivity. Most widely
used and accepted
volumetric examination
Limited thickness based
on material. Density,
orientation of planar
discontinuities is critical.
Radiation hazard
Magnetic
particle
testing
Uses reflected or
transmitted light from
test object that is
received by a light
sensing device
Applied in industries
ranging from raw
material to finished
products and in-service
inspection
Can be inexpensive and
simple with minimum
training required. Broad
scope of uses and benefits
Only surface conditions
can be evaluated.
Effective source of
illumination required.
Penetrant
liquid
A liquid containing
visible or fluorescent dye
is applied to the surface
and enters
discontinuities by
capillary action
Virtually any solid non-
absorbent material
having uncoated surface
that are not
contaminated
Relatively easy and
materials are inexpensive.
Extremely sensitive, very
versatile. Minimal training
Discontinuities open to
the surface only. Surface
condition must be
relatively smooth and free
of contaminants
Eddy current
testing
Its technique is based on
the interaction between
a magnetic field source
and the test material
which induces Eddy
currents in the test
piece
Detect the presence of
very small cracks by
monitoring changes in
the Eddy current flow
ECT is high speed
testing, it can be
automated.
Discontinuities at or near
surface can be reliably
directed. Accurate
measuring of conductivity
and coating thickness
measurements. No
physical contact required,
low cost and portable
Limited penetration into
test material, several
variable simultaneously
affect output indication,
discontinuities are
qualitative not
quantitative indications,
material must be
conductive, requires skills
when many variables are
involves, false indications
can result from edge
effects and parts geometry
The focus of this thesis is to improve and standardize ECT calibration procedures in
Suzuki’s valve spring wires. Nevertheless, knowledge about other testing techniques can
improve the understanding of how Eddy current testing works and its advantages and
limitations. The thesis does not rank the different techniques. The objective is to
understand the present testing process based on eddy currents, and to improve the
stability in the process, not to compare different techniques. In general, ECT is suitable
since the wire is conductive, which is the base to use ECT. Eddy current based evaluation
is also easy to run in line with high productivity. ECT is surface based mainly, and is very
sensitive to changes in the surface made by cracks, scratches or similar.
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2.2 Eddy current testing
Eddy current testing, ECT, is the specific technology for this project and the objective of
this subsection is to describe its history and principles, and a brief discussion is made
about some of the factors that affect ECT.
2.2.1 History
The principle of ECT has its origin based on Michael Faraday's discovery of
electromagnetic induction in 1831. Later in the 1900th century scientists noticed changes
in the properties of a conductive coil when placed in contact with metals of different
conductivity and permeability. During Second World War these effects were put into
practical use in testing materials. Much work was done in the 1950’s and 60’s, in the
aircraft and nuclear industries. Friedrich Foerster, founder of Foerster GmbH, and Robert
McMaster are seen as the main pioneers within the field [28]. Today, Eddy current testing
is an accurate and well-understood inspection technique [4].
2.2.2 Generation of Eddy currents
Eddy currents are created through a process called electromagnetic induction, and to
generate Eddy currents for an inspection, a probe is used. Inside the probe, there is a
length of electrical conductive material which is formed into a coil. Alternating current
flows in the coil at a frequency chosen based on the product to be tested. A dynamic
expanding and collapsing magnetic field forms in and around the coil, as the alternating
current flows through it. When an electrically conductive test material is placed in the
coil’s dynamic magnetic field, electromagnetic induction occurs, and Eddy currents are
induced in the material [5]. Eddy currents flowing in the material will generate their own
secondary magnetic field which opposes the coil’s primary magnetic field as shown on
figure 7. This entire electromagnetic induction process to produce eddy currents may
occur from several hundred to several million times each second depending upon the
chosen frequency [21]. In this work the main study has been done on settings with 300
kHz, with complimentary test of the frequencies 1000 kHz and 100 kHz. When a flaw is
introduced to the conductive material or test piece, the Eddy currents are disrupted, and
the signal is transferred to a screen where the crack has a different phase compared to
the normal undisrupted noise based on the calibration sample.
20
Figure 7: Generating Eddy current on the test piece [25]
The phase is built up on the analysis of the impedance, which is based on the resistance,
inductance and the reactance of the material. A change within the tested material, by
mechanical defects or by a change of material type and/or structure, will result in a
change of the impedance angle, and the signal will change, which can be detected.
There are different types of ECT-equipment; the two main types are the defectomat and
the circograph, as they are named in this thesis. The names are taken, as previously
mentioned in the Introduction section, from the manufacturing company Foerster GmbH.
The circograph, which is rotating with two or more identical probes around the test
material, is mainly for longer defects, like scratches. The defectomat, with an encircling,
non-rotating coil, has better ability to detect shorter defects. In this report, the focus is
on the circograph, since it requires adjustment every time a new dimension is run, and
several recommended values are missing. For a more in-depth description of the
calibration of the circograph, see Methodology section. Both types are however affected
by the same factors which will be presented in section 2.2.3.
2.2.3 Factors affecting Eddy Current Testing
Apart from a defect, there is a variety of factors that affect the response of an Eddy
current probe. Those factors include electrical conductivity and magnetic permeability of
the test material, the operational frequency of the eddy current probe, the skin effect and
penetration depth, the proximity of the probe to the surface and geometric properties of
the test material. Also, a factor which is partly setting based, is the so called Noise to
Signal (S/N) ratio, which is described after the equipment and material factors. A
summary of those factors is given in this subsection.
21
(i) Conductivity
Conductivity of a material is a main factor to be considered when using eddy current
testing inspection. The greater the conductivity of the material, the better flow of eddy
currents is on the surface. The conductivity is inversed to the resistivity. There are
different factors affecting conductivity of a material such as material composition, heat
treatment, hardness, temperature of a material, internal residual stresses and conductive
coatings [22].
Concerning the material properties, grain size and structure type, when a material (metal
or alloy) is subjected to heat treatment, the metal properties are changed. This change in
is causing an internal change in the material which also affects the conductivity of the
material [5]. The change in conductivity can also be detected by ECT methods, but
according to the analysis in this thesis, the defectomat (encircling testing) is more likely
to notice the change in structure, caused by for example a hardening error. The encircling
coil discovers even a slight change in conductivity (like a hardening error), while a
rotating unit mainly discovers a complete change of material, for example lead residues on
the steel surface. This is visualized in the figure 8 below, which shows how the signal
appears when the material has been unsatisfactory hardened and the material is almost
perlite. The calibration sample, which the machine refers to, is hardened and annealed
martensitic. The signal does not reach the trigger level in the rotating circograph but
gives a large signal in the encircling defectomat. Since this thesis is focusing on the
circograph and not defectomat, hardening errors are only shortly discussed here, but more
focus is put on complete changes of conductivity, like when the ECT is disturbed by
another type of material, like lead or by a change of material. The behavior of the
different steel types investigated is described in the Results section.
Figure 8: Hardening error (perlite structure). A small detection signal can be seen in the circograph (rotating probe
testing), but at large signal can be seen in the defectomat
22
Figure 8 shows hardening error (perlite structure instead of hardened martensitic
structure). Not detected, meaning it is not reaching the trigger level, by circograph (left)
but by defectomat (right), since the conductivity difference for the rotating probe is too
small. Examples of lead interference are further explained in the Results section.
In general, it is difficult to measure conductivity on wire with small diameter range, since
the probes usually used in common conductivity devices are too big for the thin wire
dimension used in this research. According to the data on typical annealed steels, the
conductivity decreases by increasing carbon content. Table 2 show different AISI steels,
and the figure shows that increasing levels of alloys in general, and carbon specifically,
decreases the conductivity. This also implies that the resistivity is increasing with
increasing carbon content.
Table 2: Different AISI-steel and their alloy content and Conductivity values [11]
Figure 9: The conductivity decreases with increasing carbon content for some random chosen annealed AISI-steels
23
Since the impedance, which is the signal picked up in ECT, is consisting of the
hypotenuse of the resistance and the reactance/inductance, a change in Resistance will
impact the impedance [24].
In this project, actual values (complete year 2017’s data) of different steel grade of wire
used at Suzuki Garphyttan are being used to analyze the steel composition. Around 20
000 tons produced were analyzed. By analyzing those values, it can be shown somewhat
different values as shown in table 3:
Table 3. Average alloy values of different grade of Suzuki Garphyttan wire
In general, the alloy content is increasing according to the end number of the steel,
OTXXX. OT101 has higher alloy content than OT70 based on this logic. An exception
from this logic is 75KD, which is used for other products than valve springs and also has
different testing requirements. As a result from this analysis, figure 10 shows that the
composition of alloys is increasing, and this is in Results section correlated to the phase
angle and the conductivity.
Figure 10: Chemical composition analysis regarding average of carbon (%)
24
Although it is difficult to get conductivity values for this thin wire with the devices found
on the market, another way of receiving conductivity values is to base the conductivity on
the resistivity values. One way is to recalculate conductivity by using Poulliet’s law [26]:
Knowing this, it is also established that conductivity is the inverse of the resistivity:
By combining these two and replacing the resistivity, the following equation defines the
conductivity:
Where:
σ is Electrical conductivity [S/m]
ρ is Resistivity [Ohm meter] or [Ωm]
A is Area [m2]
R is Resistance [Ohms] or [Ω]
L is Length [m]
This equation is used to calculate the conductivity values presented in 4.1.4 (Results).
(ii) Permeability
The permeability of a material is described as the ability with which a material can be
magnetized. If the magnetic field strength at various locations varies even slightly, these
small variations have a large effect on the impedance of the coil. These changes in the
impedance of the coil are often so large that they mask all other changes such as changes
in conductivity or dimensions [8].
High magnetic permeability makes the standard penetration depth decrease. To explore
the material internally, ferromagnetic materials are inspected at lower frequencies that
non-ferromagnetic materials [5]. In the testing environment of this thesis, the material is
being demagnetized before entering the circograph, as well as the residual magnetism is
measured once every quarter for every ECT-line. The main focus for the de-magnetization
is to eliminate the artificially created magnetic increase before the defectomat testing,
where the material is magnetized according to the hysteresis curve, to give a more reliable
testing in the defectomat. The material is then put through the demagnetization unit
which gives the material is original properties.
25
Magnetic fields consist of two closely related fields, and the relation between them defines
the permeability [27]. The two fields are called B and H. B represents the magnetic flux
density, and is measured in Tesla (T) or Weber per square meter (Wb/m2). H represents
the magnetic field intensity and is measured in Ampere per meter (A/m).
The values of the magnetic field intensity (H) is measured quarterly within the company
to control that the de-magnetization works and to secure that the magnetic properties are
not too intense. If the values are too intense the wire and the springs will attract metal
particles during the spring manufacturing, shot-blasting, annealing as well as in the motor
of the end customer. To avoid this, the machines do not start if it the demagnetization
does not work. It is in general difficult to measure the magnetic intensity. The field
intensity can differ largely on the same material and possible also how the cut is made
(by saw or by other cutting tools). On the surface, which has an oxide layer on the
outside, almost no magnetization can be noticed. The biggest value found, when the
probe is moved back and forward, is the one noticed in the protocol.
The permeability is given by [27]:
Where:
H is Magnetic field strength [A/m]
B is Magnetic flux density [T or Wb/m2]
µ is Magnetic Permeability [H/m] or [Wb/(A m)]
This equation is used to calculate the permeability values presented in 4.1.4 (Results).
(iii) Frequency
The operating frequency chosen for the inspection also have a significant effect on the
eddy current probe response. Frequency selection affects both the phase relationship and
the relative signal strength of the response from different flaws. Choosing the proper
frequency is critical to acquiring optimal resolution between flaw signals and noise
contributions from the material under test [21]. The difference between noise and signal
should be as large as possible.
Frequency can be an operator-controlled variable, but it is standardized and locked in the
testing environment which was investigated in this thesis. The main use of the frequency
is controlling the depth penetration, the density and phase of induced Eddy currents. In
general terms, higher frequencies are used to detect surface breaking discontinuities, and
lower frequencies for subsurface testing [8]. Phase difference between noise and defect is
also affected by the frequency level. Depending on the type of material this can be
26
significant differently. An example is given in figure 11 showing the phase difference
between the signal of a defect and the sample’s natural noise as a function of frequency,
the difference between noise and signal is 30°, and is reached between 250-300 kHz in in
this particular case[9]. This phase angle difference is individual according to the material
tested. As can be seen in the Results section the difference is larger for the material used
for this thesis research. Still, figure 11 and the curve seen is an example of what to be
expected in the environment this research is performed in.
Figure 11: Difference between noise and defect signal [9]
For the oil tempered wires that were investigated, almost all defects are parted around
50°-90°, which gives a more reliable testing condition with better S/N-ratio. On the other
hand, a frequency of 300 kHz gives lower penetration depth, meaning that mainly the
surface is being evaluated, increasing the risk for false alarms due to rough surface or
metal particles [9].
27
Figure 12: Example of the difference between the noise and the crack for 300 kHz, around 70°-90° in this case
(iv) Skin effect and penetration depth
As mentioned above, the skin effect and the penetration depth, are affecting the
sensitivity of the testing. The penetration depth is set by a combination of parameters
consisting of frequency, permeability and conductivity. The following equation number 5
shows how to calculate the penetration depth. Standard penetration depth depends on
electrical conductivity, magnetic permeability of the test material and on Eddy current
frequency (see figure 13 for a schematic view).
or simplified:
Where:
f =frequency [kHz]or [kg/m3]
μ = permeability [H/m] or [Wb/(Am)]
σ =conductivity [S/m]
ω =angular frequency [Rad/s]
ρ = resistivity [Ωm]
28
The later equation (6) is useful when conductivity is not given and the penetration depth
is calculated on metals or materials. Standard penetration depth decreases as
conductivity, permeability or inspection frequency increase as shown on figure 13.
Equation (5) shows how frequency influences the penetration depth. When using the low
frequency, permeability or conductivity, the penetration depth increases.
Skin effect is when Eddy current flowing in the test object at any depth produce magnetic
field which oppose the primary field thus reducing net magnetic flux and causing a
decrease in current flow as depth increases. Alternatively, Eddy currents near the surface
can be viewed as shielding the coil’s magnetic field thereby weakening the magnetic field
at greater depths and reducing induced currents [8]. Skin effect is also a limiting factor of
increasing frequency as desired.
Figure 13:Relative Effect of frequency, conductivity and permeability on the depth of penetration for a typically single-
coil ECT probe
(v) Proximity
Lift-off and fill factor are the terms used to describe the space that occurs between the
specimen under test and the inspection coil or probe. Both have a similar effect on the
eddy current response. Lift-off and fill factor are essentially the same thing, lift-off is
applied when rotating probes are used, and fill factor is used for encircling testing. Lift-off
is the impedance change that occurs when there is a variation in the distance between the
inspection coil probe and the test piece. Those small variations in spacing can mask many
indications and create noises. Fill factor is applied in an encircling or an internal coil;
basically it is a measurement of how well the conductor (test specimen) fits the coil [5].
The rotating surface probes should be presented to the test surface at a constant angle to
the surface with constant lift-off and pressure if possible [8]. Phase angle and wire speed
29
on every eddy current testing line is taken into consideration to analyze the changes to
the signal from the rotating surface probe.
(vi) Geometry
Geometry of the test material such as thickness and edge or end-effect can also influence
the Eddy current response. Material thickness influences the measurements, especially
when material thickness is less that the effective depth of penetration. Edge effect or
End-effect is described as a phenomenon that occurs when inspecting coil is at the end of
test piece, it occurs when Eddy current testing probe approaches the edge of a specimen,
where current has no place to flow, this may result in false indication [22]. The distortion
results in a false indication that is known as edge effect [8].
(vii) Signal to noise ratio
Signal to noise (S/N) ratio is an important word in the NDT vocabulary and should be
introduced early. It is not a characteristic of the material or equipment, but is important
to get a meaningful testing in that sense that a high ratio gives a good reliability, so the
ECT equipment sorts out real defects and not general noise. Signal to noise ratio is a
parameter that defines and relates the height of the signal to the height of the noise that
comes natural from the material itself. The response of a crack should be considerably
greater than the signal amplitude of the background noise [5]. The appearance of a crack
in ECT will be further explained in Methodology part, but a schematic picture on how to
calculate the ratio can be seen in figure 14 and equation 7.
Figure 14: Relative Effect of frequency, conductivity and permeability on the depth of penetration for a typically single-
coil ECT probe
The S/N ratio is simply calculated by:
30
A ratio less than 3:1 or 300 % should be rejected, since the risk for false alarms or
missing real defects are too big. In figure 14, the testing should not be performed
according the calculation using equation (7), the signal noise ratio is equal to:
2,33 =35,00
15,00
In this report there are two ways of measuring S/N, one conventional definition of noise
and one unique of this thesis. The conventional way, from articles and from the
equipment supplier Foerster GmbH, is to simply measure noise and signal, for example by
the Vector mode, see figure 15. There the difference between the top (in the middle) and
the level on the sides (the noise) is measured. This is illustrated in figure 15, and
correlates to the figure 16. It is called longitudinal noise or just “noise”.
Figure 15: S/N between then top (crack signal) and the sides (surface roughness noise)
A possible unfavorable part of the longitudinal noise evaluation is that it does not
consider the stability, neither the length, especially if the noise is laid down as it usually is
when ECT is performed in Garphyttan. See figure 16 and 17. The noise height is then
close to 0 (figure 17), but the signal width could be very unstable. In this thesis, for some
dimensions also the latitudinal length is measured. Often, however, the noise values in
both longitudinal and latitudinal directions correlates. High longitudinal noise also means
high latitudinal noise.
Figure 16: Normal noise (here called longitudinal) and latitudinal noise
31
To further visualize this phenomenon, figure 17 is showing the concept with latitudinal
noise, showing two different scenarios. The height of both the crack and the height of the
noise is the same in both figures; therefore the conventional S/N ratio is the same. But
the width may not be equivalent in different scenarios. Therefore, also the width and
stability of the signal should be evaluated.
Figure 17: Further developing the latitudinal noise in yellow (short) and purple (long)
A calibration resulting in a high latitudinal value, causing a planar but wide signal, is
more likely to create false alarm compared to a shorter noise when running in motion,
even if both have a really good S/N ratio during non-motion calibration. This is due to
actual production, which will always include minor vibrations, disturb from dirt etcetera.
Therefore more noise appears during running compared to when the calibration was done.
2.3 EC testing reference standards
Suzuki Garphyttan has a quality management system certified according to the
international standards for quality ISO TS 16949. The work is ongoing to also achieve the
upcoming new standard in the automotive industry, the IATF 16949. Main suppliers of
wire rod have QMS certified by a third party according to ISO 9001 as a minimum and
are also regularly audited by Suzuki Garphyttan [17].
To inspect wire by ECT, different standards are considered to provide uniformity of the
testing procedure. In this subsection different standards will be used as a reference and
the SOP instruction is based on this information.
2.3.1 Standards for EC testing equipment
In this thesis, statistical studies of data, provided by operators from different lines in the
ECT department, have been conducted to analyze the variation in the results of all
measuring equipment as specified in ISO TS 16949 section 7.6.1.
32
When using ECT equipment, its electrical characteristics shall be evaluated and used
under stable conditions. The characteristics and values are only valid for the unique
testing conditions where the instruction is applied as recommended in SS-EN ISO 15548-
1:2013 section 4.2. Therefore, as previously stated, the Results and settings stated are
unique to the product and environment it has been designed and tested for.
2.3.2 Standards for calibration
As specified in ASTM E-1212 section 9.2.2 Measuring and test equipment are calibrated
and controlled to specified requirements. A calibration system is established to ensure
that measuring and test equipment are maintained by periodic calibration against certified
equipment traceable to nationally or locally recognized standards and serviced so that
equipment function properly and are within their prescribed limits.
In this project, calibration is done using reference standard calibration samples with an
artificial EDM-made defect with a known depth. If a profile shaped wire is being
calibrated, two artificial defects are used, one on each side. An inspection system has to
be stated and it is important to make functional checks and verify the capability of the
inspection system and to provide calibrations curves as specified in SS-EN ISO
15549:2011 8.4.
It is necessary that all operators must be trained to handle ECT equipment during testing
and to perform the instructed calibration instructions.
There is a possibility to avoid calibration by using a “library”, with previous and “good”
calibrations, to reuse the settings. However, it has to be done carefully and “machine by
machine”, since no equipment is exactly the same [23].
2.3.3 Standard Operating Procedure (SOP) for calibration
SOPs are a set of instructions to complete tasks that are created for the equipment
operation, calibration and maintenance, to name a few. SOPs give a visual overview of
the calibration and how it should be performed correctly at the specified company or
under certain circumstances.
The focus for this research and project is to update the existing simple instruction for
calibration of ECT equipment and convert into a SOP-instruction, which describes the
process and give instructions to follow when performing calibration activities and what
values to use.
33
In making changes to an existing SOP, any documents referenced or affected by SOP
should be considered including forms, templates, or other documents that are part of the
process related to the SOP being updated [18].
It is preferable to have a process map (or flowchart) of the tasks which gives an overview
of linked activities that must be managed to achieve the predetermined output. Focus is
to define its boundaries and make sure that the start and final points are clear, and it
should be easy to see if the action corresponding to the existing calibration is missing
[19].
The key steps of creating or updating SOPs through process mapping are described in the
following figure 18.
Figure 18: SOP development using process mapping
(i) Standard Operating Procedure (SOP) Contents
There are common SOP sections to be considered during SOP development. A typical
SOP should include at least the following sections [20]:
Purpose: a brief of statement of the intent of the SOP.
Scope: describes the boundaries for the applicability of the SOP, certain
circumstances, conditions, organizational units, and etcetera. It is important to set
the scope within the real control of the SOPs.
Policies: contains key rules and/or constraints that apply to the process.
Responsibilities: key responsibilities of the participants in the process.
Procedures: describe all activities to be done.
Not limited to the above sections, the following sections in the SOPs is helpful [20]:
Equipment and Materials
References
Definitions
Tools
Attachments/Appendices
Document History
34
(ii) Standard Operating Procedure (SOP) Template
The SOP is standardized based on the needs of each organization. SOP template should
be easy to use. Consistent sentence flow does have benefits compared to the over-
standardizing sentences which may make them unmanageable and hard to understand.
Also, using some pictures and drawings is recommended.
Aspects of the SOP layout that need to be standardized are [20]:
SOP numbering
headers and footers
pagination
section headings
section numbering
test appearance (fonts, margins, etc.)
35
3. Methodology
In this section the process methodology for the calibration of the circograph is described,
the used method and its purpose which helps to reach the goals for the project.
3.1 Quantitative and qualitative evaluation of the
calibration procedure
The present procedure of how calibration is being performed is a combination of
quantitative and qualitative evaluation. Both have to be considered important, but as can
be seen in the Results section, it is preferable to move from Qualitative to Quantitative
evaluation where there is a possibility and reason to do so. To have as much values as
possible is also important both for education of new employees, for revisions and audits
and for the possibility to standardize the production and for product safety.
3.1.1 Quantitative analysis
Example of quantitative evaluation
The depth of the artificial defect that the operator calibrates.
The height of the signal and the noise (S/N).
All values put in the system.
3.1.2 Qualitative analysis
Example of qualitative evaluation or semi-qualitative (meaning that today no values are
being extracted even if there might be a possibility to do so) evaluation:
How “stable” the signal is, e.g. if it has a lot of small peaks vibrating.
The shape of the curve or of the noise, see Figure 19. In the figure it is very
obvious that the left curve is distorted, since it has several peaks. But for an
unexperienced eye it can be difficult to determine which of the peaks the signal is,
and what peak(s) are noise peaks. To the right the signal is very stable and clean,
which is correct.
The difference between the legs holding up the crack peak.
The quality and life-length of the calibration sample, if it has a lot of dirt inside, if
the oxide layer is removed and so forth.
As well as many of the guidelines for calibration, which is mouth-to-mouth based
or experience-based.
36
Figure 19: Example of qualitative evaluation, based on the judgement of the appearance of the curve, Insufficient shape
of curve (left, red)
3.2 Equipment used
A complete ECT line consists of the following order of equipment:
1. Incoming material WIP storage area
2. Pay-off (rotating, so the wire is winded of its load carrier)
3. Defectomat
4. Demagnetization
5. Circograph
6. Automatic color station for marking defected wire parts, above the specified limit
(40µm)
7. Cameras for checking the color marks
8. Rust protection apply
9. Pay-on (rotating, so the wire is winded on its delivery carrier)
10. Packing
Describing the circograph is the focus and will be presented below in next section.
3.2.1 Circograph and the procedure of calibration
Circograph sensor system Ro 20 P shown on figure 20, is the one of nondestructive EC
testing equipment manufactured by Foester GmbH. Ro 20 P scans the surface of round
materials (wires and profiles, typically egg-shape and elliptic), with scanning probes which
allow the maximum flaw resolution for surface-exposed, rotating systems that are used to
37
detect longitudinal surface defects. Circograph sensor system Ro 20 P has a capacity to
cover the diameter range from 2mm to 20mm. The flaw detectability for bright material
surface is from flaw depth of approximately 30μm or more. The testing is today
performed from 40μm and above. The risk of testing for even less deep defects is that the
S/N-level becomes too low. Ro 20 P equipment uses testing speed of up to 3m/s and
continuous testing [12].
Figure 20: Circograph sensor system Ro 20 P with wire going through
The purpose of the calibration, which is being done for each shift of dimension, is to
adjust the settings to the specific properties of the wire. Simplified, the calibration
equipment is based on the following parts (see figure 21):
38
Head for probes.
Probes rotating with over 14000 rpm.
Guides on both sides of the rotating head to keep the wire straight.
A calibration sample with an artificial EDM created mark of around 80µm depth.
The samples come from wire that the company has produced previously, but is not
the same wire running in the current machine and in many cases the steel grade is
unknown. The artificial mark of 80µm depth is made externally in a dual-source
arrangement from two different companies.
Demagnetization box.
Computer and screen.
Figure 21: Schematic simplified view of the calibration parts changed during calibration
The calibration is made on different views in the computer system, and the two most
important are the Y mode and the XY mode, see figure 22. There is also another mode
called the V mode, Vector mode, which can be used, see figure 44. The main
requirement, according to the knowhow of the observed company and their procedure (it
is not stated yet, before this thesis was made, in any official instruction) is that the crack
after the calibration should reach the trigger level of 80µm and the phase angle should
not be pointing directly straight up, but in the same time not lean more the +/- 10° from
the Y-axis on the positive side, see figure 23 and 24. Some operators refer to the clock
+/- 10 minutes, but that gives quite a big difference compared to degrees.
Figure 22: The screens for XY-mode (left) and Y-mode (right)
39
All conductive materials have a so called natural noise, affected by previous mentioned
properties as conductivity, permeability but also surface roughness. The noise can if
handled incorrectly disturb the equipment and the difference between actual crack and
normal surface can be difficult to distinguish. This is in general referred to as the signal
to noise (S/N) ratio. To avoid this, the natural noise of the wire inserted should be planar
or close to planar, which also can be seen in the figures below (figure 23). In the
experience perceived in this thesis, everything that is differing of the natural noise, it
might either be a change in conductivity or a defect, the signal is pointing up, not
necessarily in the range +/- 10° from the Y-axis, but still up, which is a setting of the
system.
Figure 23: Crack point up and noise down, which gives the best S/N-ratio
To the help to reach this result, the operator has quite a lot of parameters to adjust:
Phase angle
Gain
Filter correction
Gain for second-probe
Bandwidth
Different thresholds or trigger levels
Depending on machine type, in some cases there are many more options. And then there
are “hidden” system views, where it is possible to change more in depth-settings, like
40
rotating speed, testing frequency and other parameters. Usually, the operator should not
and does not change these settings, but the settings have been used in this thesis to test
different settings.
Figure 24: Visualizing of adjustment based by crack, noise, gain, and filter correction. The crack signal points up
between +/- 10°
Phase angle
Adjusting the direction of both noise and crack is crucial. The phase angle is the single
most important setting to get an optimal S/N-ratio, since it is possible to adjust the
direction of the noise. An increased phase angle value moves the signal and noise to the
left, and a decreased phase value moves the signal to the right, see figure 25.
Figure 25: Schematic adjustment of noise or crack with phase
41
Gain
Since the difference of the noise direction and the signal direction is not 90° (which has
been optimal, meaning the noise is already planar and the crack is pointing up without
the need of additional gain), the focus in calibration is to adjust the noise to be planar by
adjusting the phase angle value. The crack will then follow after the phase adjustment
and pointing obliquely. By increasing or decreasing the gain the crack will, although it is
oblique, having the value of 80% which is 80 µm, which in this case is the desired
calibration value. Figure 26 shows how the noise and crack follow each other according
to the adjustment of the phase. In the example it is likely that the gain needs to be
lowered to make sure the signal does not get values over the trigger level. The trigger
level during calibration is the depth of the artificial crack. During production the trigger
level is set to the depth the product wire specification it should be tested against. There
is a possibility to have several trigger levels, as can be seen in several of the figures; an
example of this is figure 26.
Figure 26: The adjustment of phase and gain to get an approved result. Dashed lines are trigger levels.
Filter correction
Filtering of frequencies, where a negative value moves the filter towards lower frequencies,
meaning more low frequencies are passing through. The filter correction is moving both
the direction of the crack, as well as the shape of the legs. A crack closer to the Y-axis
has less need for gain to compensate for the oblique appearance. Positive values of filter
correction compress the graph and move the signal towards left. Negative values widens
the graph and moves the signal towards right, see figure 27.
42
Figure 27: Filter correction. Positive values compress the graph and move the signal towards left. Negative values
widens the graph and moves the signal towards right
Before starting with calibration, it is important to change the precision guide according to
the wire dimension, for example to calibrate a wire of 2.9 mm a guide of 2.95 mm is
used. After fixing the guides, then the calibration of the wire can start. To get a correct
clearance/proximity, the probes are moved in direct contact with the wire, and then
removed a certain distance, which the computer then is compensating for. During this
process, it is also important to have clean equipment, a correctly made artificial defect,
probes that are not worn out and other parameters. The schematic view of the
changeable system is shown in figure 28.
The most difficult part for the operator is to find a suitable compensation between the
crack, the noise and the compensation the computer has to do between the highest and
the lowest point, see figure 28. The crack has to be of the same depth wherever on wire
it is found, even though there are different distances from probe to crack depending
which probe is closest to the crack as seen in figure 28. To achieve the same depth from
0-360°, as compensation is being made by system. The algorithm behind this
compensation is not known and not investigated in this research.
Figure 28: Different signal height depending on the distance from probes (0-360°)
43
To eliminate this distance difference, the operator twists the wire around and tries to find
the biggest signal, reports that and then tries to find the smallest signal and report that.
The computer then defines the compensation. Example of how to find the highest point
(strongest signal or closest to the probe) is given in the figure 29. In this example, the
phase angle is around 216°. Correctly done, for round wire, the lowest point should be
opposite (180°) of the highest point, in this case 216°-180°=36°. The lowest point can be
seen already when pointing out the highest point.
Figure 29: Example of highest point and lowest point of the signal during calibration. The highest point is at 216° in
this example.
If the calibration is done correctly the defect should be compensated so that the height of
the defect in Y-mode is at the same level from 0 to 360°, or +/- 10μm since it can be
hard to achieve exactly the same height.
Figure 30: Confirm calibration when the values of the defect is the same during rotation from 0-360°
44
As seen on the figure 30, the height is within +/- 10μm when the calibration sample is
rotating in different positions. When all the parameters are set, the operator can Click
OK, and line in the material in the machine and start running. The procedure of
calibration takes around 45min in average.
3.2.2 Defectomat
The encircling defectomat manufactured by Foester GmbH is one of nondestructive EC
testing types on the market, and is used for detecting transverse flaws and short flaws by
using an encircling coil. The test piece is then moved through or passed by with a
synchronous signal display. The systems are fully automated and defects can be marked,
classified and automatically discarded in exactly the same way as the circograph.
The defectomat has a test speed up to 150m/s [13] and continuous testing. The
maximum speed is never used in reality since the circograph is in the same line with a
maximum speed of 3m/s.
3.2.3 Contracer and other measuring equipment
A contour measuring machine called a contracer, manufactured by Mitutoyo, was initially
used in this project to measure the depth of artificial defect. During the measurement of
depth of the artificial defect, the wire to be examined should be well placed and it is
important to adjust the artificial defect of the wire according to the probe of the
contracer. Measured contour data can be compared with design data and shapes in terms
of actual data and shapes rather than just analysis of individual dimensions [15]. In this
project, the measured result of the artificial defect contour was not correlating to the
values certified by the supplier. A discussion with the company and indirectly with
Mitutoyo was held, since either the Mitutoyo equipment or the calibration samples were
incorrectly made. When measuring the crack in microscope by cutting it and placing it in
a mounted sample (see 3.2.5 Microscope and mounted samples), the values perceived of
the artificial defect depth were very close to the certificate values given by the
manufacturers of the calibration samples. Mitutoyo explained that this is probably due to
the measuring speed, according to the technician who held the contact. The lowest speed
is not enough to catch enough data point for such a small crack that is almost not visible
for the human eye. Due to time limit of this thesis and the cost correlated this could not
be arranged in this project. Still, the contracer works well for shape measurement
purposes. The contracer is not used for the calibration purpose in normal production.
45
Figure 31: Contracer [16]
There are other devices used in this project. Figure 32 shows one device used to measure
resistance. For permeability measurement a device to measure magnetic field intensity (H)
was used as well as magnetic flux density (B) measured in mT units, see figure 33.
Figure 32: Resistance measurement in mΩ Figure 33: B-field hall-effect Meter instrument used to
measure magnetic flux density (B)
3.2.4 Calibration samples
Before inspecting the quality of the wire, calibration of the ECT equipment is performed
using calibration samples. The calibration samples are used as reference standards or
standardized samples to teach the machine what defects to look for. Calibration samples
used at the company observed are made by two external companies, and the delivery of
the sample is within 2 weeks from the date an order is placed. The wire is made by the
company, but as previously mentioned, it is not necessarily the same steel grade even
though the dimension is the same as the material to be tested. It is preferable the test
material and the calibration sample have the same material, alloy, shape (round or
profile), temperature, diameter etcetera. Location, size and depth of the artificial defects
made to the calibration sample must be well known before calibration.
46
After adjusting the ECT equipment for a specified response based on the calibration
sample, a signal of approximately the same amplitude and phase should be obtained from
the condition of the wire with ECT equipment.
3.2.5 Microscope and mounted samples
Due to the size of defect, a microscope can be used to investigate the location and depth
of the defect. In order to view the depth of the defect in optical microscope, a sample or
part with a defect on must be dipped in Bakelite, and should be completely covered with
Bakelite and then mounted. After heating the Bakelite, the mounted sample is cooled.
Then the polishing machine using different polishing papers to make the surface more and
more smooth. Finally the sample is etched as shown on figure 34, and the depth can be
measured using optical microscope.
Figure 34: mounted sample
3.2.6 Generally about the methods for this thesis paper
The evaluation was mainly done through different methods:
Previous data found in the production system. This includes the steel composition
values and how many color marks per employee/machine type etcetera.
Unique production data was collected. This meant the operators wrote down their
production values on a special sheet created for this purpose. The total amount of
units (steel coils) with production data was over 1800 coils or over 2200 tons of
material. The data was then analyzed with pivot tables in Excel and Minitab.
Tests made by the thesis authors were made with a lot of calibrations done in the
circograph. Especially 2.95mm and 5.00mm were used, since they are “big
runners”. Also several other dimensions were used.
Regarding devices they are already described well. This includes measuring
magnetic flux density, resistivity, magnetic intensity, conductivity, contracer and
so forth.
The equipment hardware and software is made by Foerster GmbH and operating
on Windows platform.
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4. Results
In this chapter, the results will be presented that were obtained with methods described
Methodology and that are compiled, analyzed and compared with the existing knowledge
and theory presented in the Frame of reference.
A general SOP has been implemented and signed by the process manager and the quality
manager. The main purpose was to determine what parameters that influence the final
result, and to find out what parameters are used today by the operators, and what should
be recommended in the future. A summary of the majority of the parameters can be seen
in table 4.
Table 4: Values for calibration to be used in the future.
In total, unique values from the operators were collected from over 1800 coils (production
units), representing around 2200 tons of material. Added to that comes the tests made by
undersigned of the thesis report. The values are chosen to give stable and reliable values
for ECT in this specific environment and for valve spring wire.
4.1 Results analysis
The results have been shown, and in the following section several of the different values
will be further explained.
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4.1.1 Phase angle determination
At start, the operators thought the phases values were more or less random. By analyzing
the actual values of the operators used when they calibrated, typical phase angle values
could be determined. When analyzing the values, it was discovered that in fact two
machines were having phase angles 180° differently from the others. This is related to
signal output. The operators claim that the phase angle is consistent even if the
circograph or the probes were changed, but if the computer was changed, the phase could
shift. A change of probes was tested during the project and the phase angles were the
same as previously also after new probes were installed. However, the fact that the
different circographs have operated in such different values, the general opinion has been
among the operators that “everyone is doing different”. This is not entirely correct. This
thesis is showing that it’s is not random values, but there are two ECT-lines where the
output signal is opposite 180°, which is being visualized in the figure 35. There are still
large value intervals that are never used, and they should not, since the tests performed
show the S/N-ratio will be too low outside the phase value intervals suggested in this
thesis.
After evaluation, the interval is set to:
Option interval 1: 330°-+20° (most common).
Option interval 2: 150°-200°.
For Option interval 2, the curve of the crack would have been upside down if the phase
value interval from Option 1 was used. It is not examined what happens with the
reliability of the testing if an upside-down signal is used, but the supplier thinks the
testing reliability probably is unchanged. A decision was made to run production with two
intervals as presented above, so the curve appearance is unchanged.
Figure 35: Alternating current and pickup signal pickup 180° from each other, resulting in a upside down curve
49
In fact, the 8 lines investigated have the follow spectra:
Line 1 Option interval 1: (330°-20°)
Line 2 Option interval 1: (330°-20°)
Line 3 Option interval 2: (150°-200°)
Line 4 Option interval 1: (330°-20°)
Line 5 Option interval 1: (330°-20°)
Line 6 Option interval 1: (330°-20°)
Line 7 Option interval 2: (150°-200°)
Line 8 Option interval 1: (330°-20°)
This phase value interval is covering the absolute majority of all values observed, but the
interval gives an opportunity to discover values that might mean that something is wrong
with the equipment, the calibration sample or the knowledge of the operator. An example
of this could be that an incorrect frequency is used, which would completely change the
value intervals. When the values now are standardized, the intervals can probably be
tightened more in the future, but the risk is that there will be production downtime when
production cannot run because one or more values are outside the tolerated intervals.
There are obviously many parameters to use which influence each other. That might
make it more difficult to fit the received values into a tight specification of useable,
approved values.
There is a correlation between the alloy content in the material and the phase angle as
shown in figure 35 and was discussed in “Frame of reference”. The ultra-high tensile steels
(OT90/91/101), the UHT steels, have decreased conductivity (see figure 46), increased
resistivity and a changed impedance point causing a changed impedance phase angle (see
figure 37). The difference is still quite small, with around 8°, which does not have a
significant different effect on the calibration result, see figure 36. To compensate for the
reduction of impedance, the operators compensate by changing the gain or lifting up the
curve with the filter correction. The recommendation is to stop using a random steel
grade during the calibration. Instead UHT material should be calibrated with UHT
calibration samples. Figure 36 is also including the standard deviation, showing the
difference for at least ultra-high tensile steels like OT101, OT91 and OT90 compared to
the steel grades with lower alloy (carbon) content.
50
Figure 36: Phase comparison with standard deviation, values over 360° means that the value in machine is slightly over
0°. Values come from collected production data.
Figure 37: The impedance angle, the difference between noise and crack, is slightly smaller for the UHT-steels. The
angle for OT70 (α) is larger than for OT91 (β)
The defectomat is not investigated in this report, but since it is reacting heavily on
hardening structure errors, see figure 8, it is likely that the change in conductivity has a
larger impact in the defectomat than in the circograph, which is also seen in the figure
52, where the color marks are increasing with increasing alloy content.
51
4.1.2 Gain
Gain is an important parameter, and is used to decrease or increase a signal. The
operator has two-three gain parameters to calibrate, but the focus has been on the main
gain, which is giving the highest values, and is based on probe number one. The second
gain values are for probe number two, and is usually following probe number one but
have lower values.
Figure 38: Gain and Phase angle comparison for 5,00 mm OT70 wire. Filter correction is kept at 0
All dimensions, grades and machines have an optimal value range. In figure 38, the
optimal gain and phase is shown for 5.00 mm. Optimal in this case means as low gain as
possible, since the gain is not only increasing the crack but also the noise. For the given
example, as well as the majority of the wire, a phase angle around 340 to 370° (+10) also
gives the lowest values of gain, but it can be observed that the curve is quite flat on the
bottom. This is correlated to that the gain is still minimized if the phase is moved and
adjusted on the other side of the Y-axis, which can be seen in figure 39.
52
Figure 39: Different crack curves. A curve with a phase angle far from the trigger level has to use more gain, since it
has a longer distance from the trigger calibration level.
It is always possible to lift up a crack signal by increasing the gain. It is hard to define
when the limit for the gain is. The system investigated has a gain upper limit of 63.4dB.
This means that theoretically it is possible to lay the crack phase down as oblique as it is
still possible to reach the trigger level with a total gain of 63.4dB. “Possible” does not
mean “recommended”. Using over 60dB will give a much distorted signal.
The operators tell, regarding the direction of the crack, that a crack direction from -10 to
+ 10° from the Y-axis is acceptable. Some say “ten minutes to or over 12 o’clock”. None
of them is completely wrong or right. However, since the signal vs noise often is around
70°-90°from each other, it is quite natural that the crack is standing up when the noise is
lying down. This also means that less gain needed, and to use as less gain as possible to
reach the trigger line is preferred. Figure 40 is showing when the “clock”-rule is used. The
gain is in this case increased, the phase moved 45° and the noise is almost -45° from the
Y-axis line. Meaning: 10 to or 10 over, if referring to the clock. This setting makes it
more difficult for the machine to get the right settings, since the noise is standing up and
consequently the S/N-ratio will be insufficient.
53
Figure 40: Phase is changed so it is laid down 45°. This is lifting up the noise direction and gives a disturbed signal.
It is recommended that degrees should be used, and not the clock. This gives a
requirement that the phase should differ from +10° to -10° from the Y axis. It is not
extremely important to keep this interval values, but using the clock as some operators
refer to leads to a more unstable curve so too much gain is used.
Figure 41: Showing correct interpretation of a spectrum giving the lowest gain and the lowest noise
A factor that has high correlation with the gain is the lift off, or clearance as it is called
by the program made by Foerster GmbH. A larger distance from the probe to the surface
requires more gain to compensate for a weakening signal, see figure 42. This is also
increasing the noise level. However, it is hard to know what the optimal distance is. The
risk is that if the wire and the probe are too close, so called kinks (knots in the wire)
touches the probe giving a high signal or in worst case break the probes completely,
which is creating a high cost when replacing. But a high gain value can indicate that the
probes are not mounted correctly so the lift off distance is larger than suitable. A possible
recommendation is that the thinnest dimension, from 2-3 mm, could have a decreased
54
clearance, especially since the operators quite often complain the finer sizes are harder to
calibrate than the thicker dimensions.
Figure 42: Increase of gain necessary when lift off or clearance is increased
Worth mentioning, the clearance was not affecting the phase angle according to the tests
performed in this thesis. Regarding gain for different frequencies, this is introduced in the
next section.
4.1.3 Filter correction
Filter correction, to tune in and out different frequencies, is mainly used to correct the
legs during calibration and to make them “straight”. Filter correction also has the ability
to lift the crack signal, see Methodology part and especially figure 27. The main part of
all operators use the filter correction -1, which means the filter accepts more signals with
less frequency. See figure 43. Using -1, and negative values in general, stabilizes the curve
which is usually good for the testing. According to our experience the value of 0 is usually
as good as -1 and could be used more often. Positive values increase the signal to noise
ratio. A value of +3, which has never been used by the operators during this work, is
actually giving the best S/N-values, when used in calibration vector mode. But, the signal
is then very unstable, which can be seen on all the peaks seen in the vector mode (see
figure 44). It is almost impossible to run on +3 in real production.
Also, noise in S/N calculation is based on the vector mode. But considering the
latitudinal noise, which was introduced in Methodology, it is clearly seen in the figure that
55
the latitudinal noise level increases (which is negative for testing) when using a filter
correction of +2 and above.
Figure 43: Pie chart of the values used in production. -1 is the most common value used.
Using -1 or -2 is giving the possibility to decrease the gain and get straight legs. However,
at the same time the latitudinal noise is increasing. The majority of the employees put
the phase down and does not care about the signal being very unstable. By using value of
0 or +1, it can be estimated that the total S/N-level reaches the lowest point, and this
thesis claims that seeking the optimal point is decreasing the risk for unwanted false
alarms. -2 could increase the straightness of the legs, but the S/N ratio should according
to this thesis be more important than the signal appearance. In fact, the collected data
from the operators also shows the operators have the same view.
Figure 44: Filter correction +3 has the lowest S/N-ratio, but has a high latitudinal noise and the very unstable. The
two small figures to the right show filter correction value 0 and +3. +3 has best S/N ratio has, but the noise is full of
sharp peaks.
56
A lot of tests have been made on how different frequencies are correlated to filter
correction settings. No big changes, at least not significant, have been observed. Some
observations can be stated:
The higher frequency used, the filter correction tends to be more negative, since
the higher frequency means a more unstable signal. A value of 1000 kHz requires
more stabilization than 100 kHz, therefore negative values are more relevant to
use. This can also be seen in the figure 45 below. For 1000 kHz, negative filtering
values are recommended. If positive filtering values are used, the gain rises rapidly.
The gain for 300 kHz and 1000 kHz is following each other on the negative
filtering values. However, when using positive filtering values, the green curve
representing the 1000 kHz needs more and more gain compared to 300 kHz and
100 kHz.
The gain for 100 kHz has consequently higher gain than the other two frequencies
when using negative filtering values, meaning that it is not optimal to use for this
material. Even though the penetration depth is larger, which could stabilize the
testing, but increased gain is not preferable.
Finally, it is important to conclude that the gain is depending on the filter
correction, see figure 45. This is true on all frequencies and the most obvious
observation.
Figure 45: Gain is correlating heavily on the filter correction. Values average based on dimension 2,6 mm, 2,95 mm,
2,90 mm, 3,75 mm, 3,95 mm, 5,00 mm and 5,50 mm.
57
4.1.4 Conductivity and permeability measurement
In this subsection, analyses of different factors that can affect conductivity and
permeability have been made.
(i) Values in production
The equation (3) that was used to calculate the conductivity was:
The results of these calculations are being shown in figure 46 and table 5. The results
follow well the values received in the “Frame of Reference” - section when annealed AISI-
steels were compared regarding conductivity. When the carbon content increases, the
conductivity decreases. See figure 46, which is based on over 30 measured values. Since
resistivity is the reverse of conductivity as given in the equation (2),
resistivity increases with UHT-steel.
Figure 46: The different conductivity values for selected materials. Carbon content is increasing from the left to the
right on steel grade
The average values of conductivity of different steel grade are also shown in the following
table:
58
Table 5: Steel grades and their conductivity value in oil tempered structure
Permeability values are more difficult to achieve, especially since magnetic properties are
variating a lot depending on where (and how) the wire is cut and where the measuring
probe is placed. As explained in Frame of Reference, it is necessary to calculate the
magnetic flux density (B) and the magnetic intensity (H) of the wire, to receive the
permeability. This has been done and the values are as follows:
Figure 47 shows the average calculating of the values got from measurement of
Magnetic flux density (B) = 1.1mT
And figure 48 shows the average calculation of values got from measurement of
the Magnetic intensity (H) = 6.40 A/cm.
Therefore, Magnetic permeability = 1.7*10-6Wb/Am (valid for all steel grades of oil-
tempered wire produced in Garphyttan). The values are not split among the different
steel grades, since the unsatisfactory variation of the values may lead to unwanted
conclusions. Totally around 40 samples measured in two ends, and the highest value of
each sample represents the wire result. An enhanced study of more values and during
longer time period may lead to a differentiation of the steel grades based on the
permeability. Instead, an average is being presented, which can be seen in the figures
showing the standard deviation for B and H. The permeability value, 1.7*10-6Wb/Am, is
used for the penetration depth calculations.
59
Figure 47: Magnetic flux density on oil tempered hardened samples (mixed steel grades)
Figure 48: Magnetic density on oil tempered hardened samples (mixed steel grades)
The penetration depth for different steel grades is presented in Table 6. The equation
used is:
60
where the permeability is kept constant (1.7*10-6 Wb/Am). In reality, this is not correct,
but could serve as an understanding of the penetration depth. The conclusion is:
The penetration depth is around 900μm for 100 kHz, 500μm for 300 kHz and
300μm for 1000 kHz.
UHT-steel should in general have a bigger penetration depth.
Table 6: The penetration depth measured in μm for different steel grades and frequencies. The permeability is
considered constant for all grades
Wire rod cracks have been found the latest years in production with a maximum level of
around 500μm. The frequency 300 kHz is in general sufficient for this material. To go
deeper in the material will make the ECT less sensitive for false alarms, but the phase
angle difference between noise and crack will shrink and the S/N ratio will decrease,
causing more color marks. The tradeoff for less sensitive testing is a higher S/N ratio,
and this is not worth the change (for this type of material). 300 kHz is probably not the
most efficient frequency, but it is close. To get the perfect frequency, it is necessary to do
the evaluation made in the figure presented in Frame of Reference, see figure 11, and to
do it for every steel grade.
4.1.5 Frequency and phase
Throughout the thesis, the frequency has been a central part of the research. It has been
concluded:
The penetration depth is affected by the frequency.
Trends have been noticed that higher frequency tends to increase the need for a
negative filter. Also, gain is higher for 100 kHz for negative filter values.
61
The largest difference in daily work for the operator is the correlation between phase
angles and the frequency, if the frequency can be changed. All other recommended values
presented would be within the current intervals if the frequency is changed, but not the
phase interval. This can be visualized in figure 49 and figure 50. Figure 49 show typical
values for a random diameter examined which a values around:
300°for 100 kHz
335° for 300 kHz
60° (360+60=420° adjusted value)
This is a typical example; the real interval needs to be set through a lot of collected
values. Some dimensions have been investigated, which can be seen in figure 50. The
general conclusion is that the larger the frequency from 100-1000 kHz is, the more
increased phase angle values are obtained. In figure 49 the adjusted value is shown, in
reality it is 60° (360+60=420° adjusted value), the adjusted value makes it to easier show
the trend within a diagram and not in a unit circle.
Figure 49: Phase correlation with frequency for 100 kHz, 300 kHz and 1000 kHz.
Discussion has been held about the impact on dimensions regarding the phase. As can be
seen in figure 50, it has been difficult to prove or disprove. No significant difference can
be seen regarding this when changing the dimension on the same frequency.
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Figure 50: Phase angle for planar noise of different dimensions of wire used for frequency 100 kHz, 300 kHz and 1000
kHz
The conclusion is that all phase angle values presented in the SOP instruction have to be
revised if the phase frequency changes. If increased to 1000 kHz, the range will be around
+60° according to observations.
4.2 Other observations
During the work several notes has been taken about other parameters affecting the
reliability of the testing. Some are explained, some of the notes are in the SOP, and some
needs more evaluation. The subjects presented here are:
How the amount of color marks seemed to increase with the steel grade and an
increased alloy amount.
“Defects” consisting of residues of a material with a completely different
conductivity, in the case lead (Pb).
False calibration on lead mark.
Dirt, oxide remains and oil covering the artificial defect
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4.2.1 How the amount of color marks seemed to trend with increased
alloy content
Initial information achieved pointed in a direction that an increasing alloy content created
more color marks (more defects), see figure 51. A possible reason for this trend is that
UHT materials are more difficult to process, and therefore more damages appear and
more defects are found. But when evaluating the real defects, there could not be
confirmed that the defects found in for example an UHT high-alloyed steel like OT90,
was worse (deeper depth and more rough surface) than for defects found in a steel grade
like low-alloyed OT70. Still, the color marks per weight or length increased, both in
defectomat (encircled testing) and circograph (rotating testing), by increased alloy
content. This can be seen in figure 51.
Figure 51: CM (color marks) is increasing with increasing alloy content, by meter units.
By analyzing the figures, the increasing trend due to increasing alloy content is only
significantly relevant for the defectomat. The increased amount of defects was increased
for all dimensions and shapes in the defectomat. The circograph, which is the focus in
this report, is not affected by the steel grade to the same extent. This is due to that the
phase is laid down as planar as possible by the operator, which is being described
previously in this report. Therefore, the increase of amount of color marks in circograph is
not related to the grade, but the fact that an increased part of the UHT material is based
on profile shape. This can be very well seen in figure 52, the left figure. The values for
different steel grades are consistent regarding color marks, but are separated by shape, if
it is round or profile. Profile wire has almost twice as many color marks compared to
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round, indicating the compensation of the dimension ratio is not working optimal. It is
not known if and how much this can be improved.
Figure 52: Round and profile, color marked defects in circograph and defectomat.
Figure 52 shows that by separating the round and profile, there is no longer an increase in
CM in circograph by steel grade. As results from this analysis, the increased amount of
CM in circograph is because of UHT-steels (OT90,91,101) mainly consists of profiles,
which is more difficult to calibrate than round. This is correlating with the results shown
in this report, that there is a slight change in conductivity, affecting the defectomat more
than the circograph.
4.2.2. Defect of different conductivity (lead, Pb)
An issue in ECT for wire that has been annealed in lead (see figure 53 and 54), is the
residues that can be found on the wire occasionally, often resulting in a reaction in the
circograph, misinterpreting the lead for a defect, since the magnetic field is changed.
Since it is possible to sort out certain frequencies and/or phases, it is tempting to sort
out the signal from lead, if it differs from other mechanical defects or scratches. This is
possible to do in the equipment. However, as can be seen on the figure 53, the signal that
comes from lead marks is actually in the similar phase as normal defects, except that the
form is straighter with no clear “legs” visible.
65
Figure 53: Lead signal in XY-mode noticed in production. Note the similarity between normal crack’s appearance
Figure 54 is showing how a lead (Pb) mark can look like. This is enough for the
circograph to give a false alarm.
Figure 54: A lead mark sticks to the steel wire
4.2.3 Artificial defect reliability
Tests have been made with different types of artificially made defects, both made by saw
and by EDM. An example of this comparison can be seen in figure 55. In general, the
defect types look very much the same. An issue was that the Contracer, as described in
3.2.3., did not show reliable values, and due to cost limitations, the testing of this did not
continue. The standard in the SOP is still the EDM marked artificial defects.
66
Figure 55 EDM artificial defect compared to saw manufactured.
(i) Having the right defect on the right place
According to Foerster GmbH, the supplier of the testing equipment, the defect should be
placed on the longest and the shortest distance from the probes. For round wire the
clearance or proximity is the same wherever the artificial defect is placed. For profile
shaped wires, which have two calibration marks instead of one, the two defects should be
set in a certain angle, to make sure the longest and the shortest distance from the probes
are achieved.
Figure 56: Placement of artificial defects on Oval and Profiles wires. Picture from Foerster and from Garphyttan wire
(right)
However, in this thesis, the angle is always 90° between the two defects for profiles,
which is some cases, might not be correct. But the egg shaped is usually designed to have
the shortest and longest position very close to the nominal values, see Figure 56. It has
not been proved any bigger difference with this wrong placement of the defect. It is easier
for the manufacturer of the calibration samples to place the defect 90° from each other,
as shown in figure 57.
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Figure 57 Egg- shaped wire an incorrect placement of an artificial crack.
A more correct placement would be as in the figure 56 (to the right), where the angle
made in green would represent where the defect for this specific profile would be placed
on the flat side. It could not be stated through research how much this disorientation of
the defect actually influenced noise levels and compensation.
4.2.4. Calibration with lead (Pb) - disturbance or directly from lead
This thesis work shows that sometimes there are lead residues also on top of the
calibration samples, both in direct connection to the artificial defect, or on the other side,
or nearby the crack. This increases heavily the risk that the operator calibrates with lead
disturbance or completely on a lead defect. This is something the thesis work suggests
the company to examine before using the sample in production, and this is instructed in
the SOP.
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Figure 58: Lead is present on a calibration sample; it is direct connection to the artificial crack (to the right)
Figure 58 shows how it can look like when calibrating on lead and not on the defect.
Typically, the values suggested by the machine are strange, for example a gain of 57dB
(that is why it is so important with value intervals to detect easily when the calibration
probably is not correctly executed). However, still, it can be quite similar in shape
compared to a calibration on an artificial defect, see figure 59. The stability of noise is
very poor when calibrating based on a Pb mark.
Figure 59: A calibration made on the calibration sample with a lead (upper). When putting in the real defect (bottom),
supposed to be 80µm, it shows much higher values.
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As mentioned, although the lead-based calibration is very unstable, the signal is quite
similar to a calibration from a conventional artificial crack. However, when calibrating
first on lead, then putting in the real crack, the picture and noise level is extremely high,
as seen in the figure. This will be the same in actual production, the number of “defects”
will be very high, since the testing will be very sensitive, see figure 59 for an explanation.
4.2.5. Dirt, oxide remains and oil covering the artificial defect
In several cases, when the calibration samples were investigated, remains of oil, dirt or
oxide were covering the artificial defect, see figure 60. It is of high importance for the
operator to make sure no lead, dirt or anything else is covering the defect or the probes.
Figure 60: Two examples when the depth of the calibration sample is covered by dirt
Before calibration process begins, it is necessary to clean the calibration sample especially
in the artificial defect or in the calibration mark. Figure 61 shows one calibration sample
with the calibration mark from figure 60 when it has been cleaned.
Figure 61: Cleaned artificial defect on the calibration sample
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4.3 Definition of improvement possibilities
By taking out defects that have been previously considered over 40µm by the ECT and
measure their real values, a potential of improvement can be estimated. An example of
this is a defect seen in figure 61. When the defect is cut into two pieces and checked in a
microscope, the defect can be measured. In the example in figure 61, at least 53% of the
defect depth is actually not a real depth. The example is calculated as follows:
Depth of the defect was 18.49µm.
Nominal value is 40.00µm (at least)
“Potential” = 40.00-18.49 = 21.11 µm.
21.11/40.00= 53%
In many cases, when color marks of so called defects were examined, no defects could be
found at all on the surface. This means it is likely some of the defects are only false alarm
defects.
Figure 62: Depth of the defect was 18.49µm. Potential= 40µm-18.49=21.11 µm
Since cutting defects and check the real depth in a microscope is a very time consuming
measurement method, and no 3D microscope was available, another way to evaluate the
variations in the process is by checking the amount of color marks per employee over a
long time. Or in other words, how many times the equipment judges the defects to be
over the trigger level 40µm. The result shows a big spread in values from the “best”
operator (or the machine she/he works on) with the lowest amount of color marks, where
the best has less than half of the color marks compared to the operator with the “worst”
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number has, see table 7. Still, the almost the same wires are being processed in the
machines, so the outcome should be similar.
Table 7: Big spread of the amount of color marks (defects) detected by different operators during 1 year and 4 months
in the circograph. This indicates larges variance and an unstable process.
The potential with the standardization is that these the color marks per meter per
employee should show less variance in the future, resulting in a more correctly performed
ECT.
4.4 Result of SOP instruction for operator
The main thesis aim was to create an instruction which is easy for all operators to read,
understand and apply. The instruction is a summary of all the parameters investigated in
this report, which is important to apply to get an accurate result. The instruction gives an
overview of calibration activities to follow when calibrating the ECT equipment
circograph. This SOP instruction is designed according to ISO standards and best known
praxis, as shown in the first page of this SOP instructions. Previous research on the
SOPs for calibration has been used, as shown in the chapter “Frame of reference”. The
following figure, see figure 63, shows the first page of the SOP instructions done in this
project. The rest is belonging to the company and is a part of SG know-how.
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Figure 63: Part of the confidential SOP instruction for operators working with ECT.
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5. Conclusions
A discussion of the results, conclusions and future work is presented in this chapter. The
conclusions are based on the analysis with the intention to answer the formulation of the
thesis questions that were presented in the first chapter Introduction.
5.1 Conclusions
Several findings have been established in this thesis work. Suitable settings of phase, gain
and filter correction has been established. The values were obtained from production
data, from around 1800 “coils” or production units, whereby the operators filled in new,
unique information about these values, which had never been collected before. The values
have then been confirmed by tests and analysis done, and regarding the big amount of
data, the conclusions could be made with larger certainty.
Also, scientific articles have confirmed the general trends and conclusions, even if there is
a slight difference in the materials from the wire studies in this thesis work compared to
the research articles. ISO-standards of calibration have been referred to, to make sure the
instructions on how to make standard operating calibration by the operators is correctly
done.
Some of the conclusions confirmed through this thesis are:
There are certain phase angles intervals that are suitable for testing. Since rotating
testing is made by alternate currents, the signal can sometimes be picked up 180°
from the normal interval range, according to the sinus curve appearance. This
creates a primary and secondary phase angle interval, with the difference of 180°.
The gain is adjusted according to how much the signal must be lifted up. To use
low gain is preferable.
Filter correction is filtering the frequency, and is increasing or decreasing the need
to use gain. Filter correction is also increasing or decreasing the S/N ratio, and
make the curve more or less stable. Negative filter values give a more stable curve
but decreased S/N ratio.
Frequency is the main contributor to set the phase angle interval. Three different
frequencies have been examined, where the middle alternative of 300 kHz shows
the best properties.
The penetration depth has been calculated.
Conductivity and permeability values have been obtained.
74
Steel with higher carbon content has higher resistivity, lower conductivity and a
change of the impedance point occurs. This means the phase angles increase for
UHT steels.
It is important to check so no lead (Pb) or dirt is present during testing.
S/N ratio has been discussed and how to set phase, gain and filter correction to
get the best values.
Putting the noise phase planar is of major importance to get a sufficient S/N ratio
(3:1). Therefore, it is important to find the biggest angle difference between the
noise and the crack, so the crack stands up and the noise is longitudinal. This can
be done by trying more frequencies.
Instructions are important to avoid running on values that differs from normal
production, and an SOP makes it easier to follow for engineers, operators and
technicians. By having value intervals, abnormalities will easier be detected and
eliminated.
The purpose of this thesis work has been fulfilled:
1) Updated instructions used by the operators
2) It has been explained how different calibration settings affect the testing reliability
3) Filled the knowledge gap of used intervals for different parameters as well as
material properties and its impact on the testing.
5.2 Discussions
There are several notes to be made during this thesis work. Certain dimension of high
runner has been chosen. However, there are a lot of individual parameters to set, and the
equipment (circograph) has its own algorithms to calculate compensation, as well as the
incoming material has slightly different properties and the calibration artificial defects
may also have a variation in their shape. Therefore, the results may differ slightly when
repeated, but the general trends are the same.
The SOPs instruction for operator consists of how to calibrate ECT equipment
(circograph), and will not only help the operators in daily basis but will be used for new
operators as introduction training, which will save time and cost for new operators.
By implementing this SOPs instruction, calibration setting parameters will be used by the
employees. The objective is to decrease the variation between machines and operators,
which hopefully will decrease the over-scrapping of material. It will also ensure customers
that the same testing is performed every time.
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From practical tests done during this project and also by using several research papers
and ISO standards, the contribution of this project on the overall global knowledge of
ECT consists of gathering together all required quantitative values used in calibration of
ECT equipment used in this specific environment. It is based on a combination of real
production data from inline ECT, as well as tests performed when the machine is not
moving, which makes the results reliable.
5.3 Future work
It was not easy to get the permeability values throughout this thesis. As said in the frame
of reference part, the values of magnetic field intensity (H) measured in A/m, have been
measured quarterly within the company to control that the de-magnetization works and
to secure the magnetic properties are not too strong.
To get values of magnetic flux density (B) measured in Tesla (T) or Weber per square
meter (Wb/m2) because of limited time and unavailability of instrument, an instrument
called B-field hall-effect Meter was used. Since the variation was quite big, an average of
all steel grades was applied. If more values were obtained, individual values of
permeability for each steel grade could be obtained. A future study could increase the
sample size and the reliability of the equipment to find differences between steel grades
and how magnetic permeability of test material affects Eddy current testing.
A key for better reliability in testing is to find the optimal testing frequency, where the
angle between signal and noise is as close to 90° as possible. It was only possible to
measure three frequencies in this report due to machine settings. This study can be
extended in the future.
Also, the finding that the profile shape is heavily affecting the output for defects could be
of further interest, as well as the placement of a artificial defect.
Finally, the expectation is that the SOP instruction will be a living document where
technicians, operators and engineers together find the best praxis continuously. Although
eddy current is well understood and evaluated concept, every situation is unique, and
small changes may lead to a revise in the standards.
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List of Tables TABLE 1: DIFFERENT NDT METHOD USED IN INDUSTRY [1] ................................................................................... 18 TABLE 2: DIFFERENT AISI-STEEL AND THEIR ALLOY CONTENT AND CONDUCTIVITY VALUES [11] ................................ 22 TABLE 3. AVERAGE ALLOY VALUES OF DIFFERENT GRADE OF SUZUKI GARPHYTTAN WIRE .......................................... 23 TABLE 4: VALUES FOR CALIBRATION TO BE USED IN THE FUTURE. ........................................................................... 47 TABLE 5: STEEL GRADES AND THEIR CONDUCTIVITY VALUE IN OIL TEMPERED STRUCTURE ......................................... 58 TABLE 6: THE PENETRATION DEPTH MEASURED IN ΜM FOR DIFFERENT STEEL GRADES AND FREQUENCIES. THE
PERMEABILITY IS CONSIDERED CONSTANT FOR ALL GRADES ........................................................................... 60 TABLE 7: BIG SPREAD OF THE AMOUNT OF COLOR MARKS (DEFECTS) DETECTED BY DIFFERENT OPERATORS DURING 1
YEAR AND 4 MONTHS IN THE CIRCOGRAPH. THIS INDICATES LARGES VARIANCE AND AN UNSTABLE PROCESS. ....... 71
List of Figures FIGURE 1: THE PRODUCTION STEPS AND A SCHEMATIC SIMPLIFIED VIEW OF STRUCTURE, GRAIN SIZE, OXIDE LAYER AND
DIMENSION .............................................................................................................................................. 12 FIGURE 2:HOW THE WIRE ROD LOOKS AFTER SHAVING ........................................................................................... 13 FIGURE 3: WIREDRAWING DIE .............................................................................................................................. 14 FIGURE 4: HARDENING PROCEDURE. .................................................................................................................... 14 FIGURE 5: WIRE ROD CRACK, SURFACE PICTURE AND ITS CORRESPONDING CRACK APPEARANCE IN A CUT .................... 15 FIGURE 6: DEPTH OF THE DEFECT ....................................................................................................................... 16 FIGURE 7: GENERATING EDDY CURRENT ON THE TEST PIECE [25] ........................................................................... 20 FIGURE 8: HARDENING ERROR (PERLITE STRUCTURE). A SMALL DETECTION SIGNAL CAN BE SEEN IN THE CIRCOGRAPH
(ROTATING PROBE TESTING), BUT AT LARGE SIGNAL CAN BE SEEN IN THE DEFECTOMAT ................................... 21 FIGURE 9: THE CONDUCTIVITY DECREASES WITH INCREASING CARBON CONTENT FOR SOME RANDOM CHOSEN ANNEALED
AISI-STEELS ............................................................................................................................................ 22 FIGURE 10: CHEMICAL COMPOSITION ANALYSIS REGARDING AVERAGE OF CARBON (%) .............................................. 23 FIGURE 11: DIFFERENCE BETWEEN NOISE AND DEFECT SIGNAL [9] ........................................................................... 26 FIGURE 12: EXAMPLE OF THE DIFFERENCE BETWEEN THE NOISE AND THE CRACK FOR 300 KHZ, AROUND 70°-90° IN THIS
CASE ....................................................................................................................................................... 27 FIGURE 13:RELATIVE EFFECT OF FREQUENCY, CONDUCTIVITY AND PERMEABILITY ON THE DEPTH OF PENETRATION FOR A
TYPICALLY SINGLE-COIL ECT PROBE........................................................................................................... 28 FIGURE 14: RELATIVE EFFECT OF FREQUENCY, CONDUCTIVITY AND PERMEABILITY ON THE DEPTH OF PENETRATION FOR
A TYPICALLY SINGLE-COIL ECT PROBE ........................................................................................................ 29 FIGURE 15: S/N BETWEEN THEN TOP (CRACK SIGNAL) AND THE SIDES (SURFACE ROUGHNESS NOISE) ......................... 30 FIGURE 16: NORMAL NOISE (HERE CALLED LONGITUDINAL) AND LATITUDINAL NOISE ................................................. 30 FIGURE 17: FURTHER DEVELOPING THE LATITUDINAL NOISE IN YELLOW (SHORT) AND PURPLE (LONG) ........................ 31 FIGURE 18: SOP DEVELOPMENT USING PROCESS MAPPING ..................................................................................... 33 FIGURE 19: EXAMPLE OF QUALITATIVE EVALUATION, BASED ON THE JUDGEMENT OF THE APPEARANCE OF THE CURVE,
INSUFFICIENT SHAPE OF CURVE (LEFT, RED) ................................................................................................ 36 FIGURE 20: CIRCOGRAPH SENSOR SYSTEM RO 20 P WITH WIRE GOING THROUGH ...................................................... 37 FIGURE 21: SCHEMATIC SIMPLIFIED VIEW OF THE CALIBRATION PARTS CHANGED DURING CALIBRATION ........................ 38 FIGURE 22: THE SCREENS FOR XY-MODE (LEFT) AND Y-MODE (RIGHT) .................................................................. 38 FIGURE 23: CRACK POINT UP AND NOISE DOWN, WHICH GIVES THE BEST S/N-RATIO ................................................ 39 FIGURE 24: VISUALIZING OF ADJUSTMENT BASED BY CRACK, NOISE, GAIN, AND FILTER CORRECTION. THE CRACK SIGNAL
POINTS UP BETWEEN +/- 10° .................................................................................................................... 40 FIGURE 25: SCHEMATIC ADJUSTMENT OF NOISE OR CRACK WITH PHASE ................................................................... 40 FIGURE 26: THE ADJUSTMENT OF PHASE AND GAIN TO GET AN APPROVED RESULT. DASHED LINES ARE TRIGGER LEVELS.
.............................................................................................................................................................. 41 FIGURE 27: FILTER CORRECTION. POSITIVE VALUES COMPRESS THE GRAPH AND MOVE THE SIGNAL TOWARDS LEFT.
NEGATIVE VALUES WIDENS THE GRAPH AND MOVES THE SIGNAL TOWARDS RIGHT ............................................ 42 FIGURE 28: DIFFERENT SIGNAL HEIGHT DEPENDING ON THE DISTANCE FROM PROBES (0-360°) ................................... 42
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FIGURE 29: EXAMPLE OF HIGHEST POINT AND LOWEST POINT OF THE SIGNAL DURING CALIBRATION. THE HIGHEST POINT
IS AT 216° IN THIS EXAMPLE. ..................................................................................................................... 43 FIGURE 30: CONFIRM CALIBRATION WHEN THE VALUES OF THE DEFECT IS THE SAME DURING ROTATION FROM 0-360° .. 43 FIGURE 31: CONTRACER [16] .............................................................................................................................. 45 FIGURE 32: RESISTANCE MEASUREMENT IN MΩ FIGURE 33: B-FIELD HALL-EFFECT METER INSTRUMENT USED TO
MEASURE MAGNETIC FLUX DENSITY (B) 45 FIGURE 34: MOUNTED SAMPLE ............................................................................................................................ 46 FIGURE 35: ALTERNATING CURRENT AND PICKUP SIGNAL PICKUP 180° FROM EACH OTHER, RESULTING IN A UPSIDE DOWN
CURVE .................................................................................................................................................... 48 FIGURE 36: PHASE COMPARISON WITH STANDARD DEVIATION, VALUES OVER 360° MEANS THAT THE VALUE IN MACHINE IS
SLIGHTLY OVER 0°. VALUES COME FROM COLLECTED PRODUCTION DATA. ....................................................... 50 FIGURE 37: THE IMPEDANCE ANGLE, THE DIFFERENCE BETWEEN NOISE AND CRACK, IS SLIGHTLY SMALLER FOR THE UHT-
STEELS. THE ANGLE FOR OT70 (Α) IS LARGER THAN FOR OT91 (Β) .............................................................. 50 FIGURE 38: GAIN AND PHASE ANGLE COMPARISON FOR 5,00 MM OT70 WIRE. FILTER CORRECTION IS KEPT AT 0 ......... 51 FIGURE 39: DIFFERENT CRACK CURVES. A CURVE WITH A PHASE ANGLE FAR FROM THE TRIGGER LEVEL HAS TO USE MORE
GAIN, SINCE IT HAS A LONGER DISTANCE FROM THE TRIGGER CALIBRATION LEVEL. ........................................... 52 FIGURE 40: PHASE IS CHANGED SO IT IS LAID DOWN 45°. THIS IS LIFTING UP THE NOISE DIRECTION AND GIVES A
DISTURBED SIGNAL. .................................................................................................................................. 53 FIGURE 41: SHOWING CORRECT INTERPRETATION OF A SPECTRUM GIVING THE LOWEST GAIN AND THE LOWEST NOISE .. 53 FIGURE 42: INCREASE OF GAIN NECESSARY WHEN LIFT OFF OR CLEARANCE IS INCREASED ........................................... 54 FIGURE 43: PIE CHART OF THE VALUES USED IN PRODUCTION. -1 IS THE MOST COMMON VALUE USED. ........................ 55 FIGURE 44: FILTER CORRECTION +3 HAS THE LOWEST S/N-RATIO, BUT HAS A HIGH LATITUDINAL NOISE AND THE VERY
UNSTABLE. THE TWO SMALL FIGURES TO THE RIGHT SHOW FILTER CORRECTION VALUE 0 AND +3. +3 HAS BEST
S/N RATIO HAS, BUT THE NOISE IS FULL OF SHARP PEAKS. ........................................................................... 55 FIGURE 45: GAIN IS CORRELATING HEAVILY ON THE FILTER CORRECTION. VALUES AVERAGE BASED ON DIMENSION 2,6 MM,
2,95 MM, 2,90 MM, 3,75 MM, 3,95 MM, 5,00 MM AND 5,50 MM. ................................................................... 56 FIGURE 46: THE DIFFERENT CONDUCTIVITY VALUES FOR SELECTED MATERIALS. CARBON CONTENT IS INCREASING FROM
THE LEFT TO THE RIGHT ON STEEL GRADE ................................................................................................... 57 FIGURE 47: MAGNETIC FLUX DENSITY ON OIL TEMPERED HARDENED SAMPLES (MIXED STEEL GRADES) ........................ 59 FIGURE 48: MAGNETIC DENSITY ON OIL TEMPERED HARDENED SAMPLES (MIXED STEEL GRADES) ................................ 59 FIGURE 49: PHASE CORRELATION WITH FREQUENCY FOR 100 KHZ, 300 KHZ AND 1000 KHZ. .................................... 61 FIGURE 50: PHASE ANGLE FOR PLANAR NOISE OF DIFFERENT DIMENSIONS OF WIRE USED FOR FREQUENCY 100 KHZ, 300
KHZ AND 1000 KHZ ................................................................................................................................. 62 FIGURE 51: CM (COLOR MARKS) IS INCREASING WITH INCREASING ALLOY CONTENT, BY METER UNITS. ........................ 63 FIGURE 52: ROUND AND PROFILE, COLOR MARKED DEFECTS IN CIRCOGRAPH AND DEFECTOMAT. ................................ 64 FIGURE 53: LEAD SIGNAL IN XY-MODE NOTICED IN PRODUCTION. NOTE THE SIMILARITY BETWEEN NORMAL CRACK’S
APPEARANCE ........................................................................................................................................... 65 FIGURE 54: A LEAD MARK STICKS TO THE STEEL WIRE ........................................................................................... 65 FIGURE 55 EDM ARTIFICIAL DEFECT COMPARED TO SAW MANUFACTURED. .............................................................. 66 FIGURE 56: PLACEMENT OF ARTIFICIAL DEFECTS ON OVAL AND PROFILES WIRES. PICTURE FROM FOERSTER AND FROM
GARPHYTTAN WIRE (RIGHT) ...................................................................................................................... 66 FIGURE 57 EGG- SHAPED WIRE AN INCORRECT PLACEMENT OF AN ARTIFICIAL CRACK. ................................................ 67 FIGURE 58: LEAD IS PRESENT ON A CALIBRATION SAMPLE; IT IS DIRECT CONNECTION TO THE ARTIFICIAL CRACK (TO THE
RIGHT) .................................................................................................................................................... 68 FIGURE 59: A CALIBRATION MADE ON THE CALIBRATION SAMPLE WITH A LEAD (UPPER). WHEN PUTTING IN THE REAL
DEFECT (BOTTOM), SUPPOSED TO BE 80µM, IT SHOWS MUCH HIGHER VALUES. ................................................ 68 FIGURE 60: TWO EXAMPLES WHEN THE DEPTH OF THE CALIBRATION SAMPLE IS COVERED BY DIRT ............................. 69 FIGURE 61: CLEANED ARTIFICIAL DEFECT ON THE CALIBRATION SAMPLE .................................................................. 69 FIGURE 62: DEPTH OF THE DEFECT WAS 18.49µM. POTENTIAL= 40µM-18.49=21.11 µM .......................................... 70 FIGURE 63: PART OF THE CONFIDENTIAL SOP INSTRUCTION FOR OPERATORS WORKING WITH ECT. .......................... 72
78
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