Commission of the European Communities

technical steel research

Measurements and analysis

SURFACE INSPECTION OF CONTINUOUSLY-CAST BILLETS

Report EUR 10158 EN

Blow-up from microfiche original

Commission of the European Communities

technical steel researcii

Measurements and analysis

SURFACE INSPECTION OF CONTINUOUSLY-CAST BILLETS

D.H. SAVIDGE

BRITISH STEEL CORPORATION 9, Albert Embankment GB-LONDON SE1 7SN

Contract No 7210-GB/802 (1.7.1980 - 30.6.1984)

FINAL REPORT

Directorate-General Science, Research and Development

1986 EUR 10158 EN

Published by the COMMISSION OF THE EUROPEAN COMMUNITIES

Directorate-General Information Market and Innovation

L-2920 LUXEMBOURG

LEGAL NOTICE

Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of

the following information

©ECSC-EEC-Euratom, Brussels· Luxembourg

Surface Inspection of Continuously-Cast Billets

F I N A L R E P O R T

Agreement 7210.GB/802

D.H. Savidge

British Steel Corporation Swinden Laboratories

EUR 10158 EN

FR 80-9 841 7210.GB/802

British Steel Corporation SURFACE INSPECTION OF CONTINUOUSLY CAST BILLETS ECSC Agreement No. 7210.GB/802

SUMMARY The objective of the research was to assess various non-destructive testing methods which could be applied to the surface inspection of continuously cast billets. The report considers the population of defects likely to be encountered and the reasons for choosing just three of the many NDT methods available. Tests which were carried out in the laboratory using a range of billets enabled a single face inspection machine for on-line works application to be designed. This machine and its installation at the Templeborough Works of BSC are described and the results obtained on a series of trials are given. A critical assessment of the results of these trials is made and leads to a general conclusion that although the optical method is the only one capable of detecting pinhole type defects it is unsuitable for on-line works application. This is mainly because of the need for a pre-painted billet. Of the other two methods examined, i.e. polarised microwave and eddy current arrays both can detect most of the other types of defects though not with equal efficiency and at this stage is is concluded that a combination of the two techniques will give a reasonable level of certainty in defect detection without too much error or spurious marking. A comparison of costs, sensitivity, coverage and resolution for the various methods is given.

FR 80-9 841 7210.GB/802

CONTENTS Page 1. INTRODUCTION 1 2. CHARACTERISTIC DEFECT TYPES IN CONCAST PRODUCTS 1 3. REVIEW OF INSPECTION TECHNIQUES 1

3.1 Eddy Current Techniques 1 3.2 Flux Leakage Techniques 2 3.3 Microwave Techniques 2 3.4 Optical Techniques 2 3.5 Thermal Techniques 2 3.6 Ultrasonic Techniques 3 3.7 Selection of Inspection Techniques 3

4. OPTICAL INSPECTION SYSTEM 3 4.1 Linescan Camera 3 4.2 Illumination 4 4.3 Paint System 4 4.4 Initial Trial Results 5

5. MICROWAVE INSPECTION 6 5.1 Microwave Head 6 5.2 Initial Trial Results 6

6. EDDY CURRENT INSPECTION 7 6.1 Eddy Current Test Head 7 6.2 Initial Trials 7

7. PLANT INSTALLATION 7 8. PLANT TRIALS 8 9. DISCUSSION OF RESULTS 8 10. CONCLUSIONS 9 11. REFERENCES 10 12. ACKNOWLEDGEMENTS 10

TABLES 11 FIGURES 14 APPENDICES 55

FR 80-9 841 7210.GB/802

LIST OF TABLES

1. Summary of Common Concast Defects 2. Comparison of Lamps and Luminaires 3. Comparison of Inspection Techniques

LIST OF APPENDICES

1. Optical Inspection System 2. Microwave Inspection System 3. Eddy Current Inspection System

FR 80-9 841 7210.GB/802

LIST OF FIGURES

1. Optical Alignment of Linescan Camera Above Billet

2. Standard M Series Camera System Mk II

3. Thorn 'Sunspot' Narrow Beam Floodlight

4. Effect of White Paint Spray on Noise and Image Definition

5. Spray Gun For Use in Pre-Paint System

6. Laboratory Test Results Using the Optical Inspection Head

7. Fibre-Optic Recording of Linescan Camera Response From Painted Billet

8. Optical Inspection System Performance on Pinhole Defects (Threshold = 9/16)

9. Optical Inspection System Performance on Defects Other Than Pinholes (Threshold = 9/16)

10. Microwave Head Assembly

11. Microwave Head - Horn Types

12. Repeated Longitudinal Scans Along Billet Containing Artificial Defects

13. Typical Microwave Response From a Production Billet Indicating Significant Surface Defects

14. Part of the Single Face Inspection Machine at Templeborough Works Showing Three Microwave Heads Set to Cover One Face

15. Billet Surface Containing Several Areas of Laps or Shell, Position 53

16. Recorder Trace From Above Area Showing Responses at Positions 53 and 54 Which Exceed the Threshold

17. Billet Area Corresponding to Position 54

18. General View of Area Containing Laps or Double Skin Civing Rise to Peaks 76-79 in Fig. 19

19. Recorder Trace Showing Seven Significant Peaks Including Those From the Area in Fig. 18

20. Billet Surface Beyond the Position of Peak 79 Which Did Not Produce Any Significant Responses

21. Billet Surface Containing a Pit 15 χ 18 mm in Size

22. Microwave Response in the Inspection Channel Which Passed Through the Pit

23. Plant Installation of Microwave Heads

24. Combined View of Microwave and Optical Inspection Units

25. Eddy Current Inspection - Sliding Shoe Probe

26. Eddy Current Inspection - Fixed Height Probe

27. Eddy Current Probe Array in Test Position

28. Eddy Current Inspection - Typical Probe Response From the Servocontrolled 6-Probe Array

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29. Block Diagram of the Concast Inspection Equipment 30. Electronic Rack Containing the Signal Processing for the Optical and

Microwave Inspection (Top Three Sub-Racks), the Eddy Current Processing (Sub-Rack 4) and the Computer System (Sub-Rack 5)

31. Calibration Billet Containing Machined Defects 32. Computer Printout From Test Billet 33. Microwave Head Responses With the Sawcut Test Billet 34. Eddy Current Responses With the Sawcut Test Billet 35. Microwave Head Responses on Test Billet With Heavy Reciprocation Marks 36. Eddy Current Responses on Test Billet With Heavy Reciprocation Marks 37. Computer Printout From a Typical Production Billet 38. Photographs of Defects in a Typical Concast Billet 39. Photographs of Defects in a Typical Concast Billet 40. Photographs of Defects in a Typical Concast Billet 41. Photograph of a Defect in a Typical Concast Billet 42. Photograph of a Defect in a Typical Concast Billet 43. Optical Inspection Performance Indices 44. Microwave Inspection Performance Indices 45. Eddy Current Inspection Performance Indices

Vili

FR 80-9 841 7210.GB/802

LIST OF APPENDICES FIGURES

Al.1 Internal Arrangement of Linescan Camera Al.2 Test Card and Oscilloscope Trace for Measuring Response of

Linescan Camera Al.3 Schematic Diagram of Electronic Signal Processing Al.4 Defect Pattern Recognition/Classifier Parameters Al.5 Illustration of Defect Classifier Searching for Transverse Defects A2.1 Directional Characteristics of a Microwave Head A2.2 Microwave Head A2.3 Microwave Head Response for Various Defect Depths A2.4 Microwave Head Response at Varying Clearances A2.5 Lift-Off Characteristics of a Microwave Head A2.6 Microwave Head Response at Various Angles A3.1 Photograph of the Eddy Current Probe Array in a Raised Position Above

a 140 mm Square Billet A3.2 Block Diagram of Eddy Current Servo System A3.3 Block Diagram of Eddy Current Signal Processing in Each Probe Channel A3.4 Use of a Calibration Roll to Optimise Eddy Current Response A3.5 Optimised Eddy Current System Responses Using Calibration Roll A3.6 Eddy Current Waveforms With Modified Calibration Roll

FR 80-9 841 7210.GB/802

British Steel Corporation

CONTROLE DE SURFACE DES BILLETTES EN COULEE CONTINUE

Accord CECA n°7210.GB/802

RESUME

L'objectif du projet de recherche était d'étudier les diverses méthodes non destructives pouvant être utilisées pour inspecter la surface de billettes en coulée continue.

Le rapport examine la série de défauts susceptibles d'être rencontrés et les raisons pour lesquelles trois seulement des nombreuses méthodes NDT ont été retenues. Les essais ont eu lieu en laboratoire sur une série de billettes et ont permis de construire une machine d'inspection une face pouvant être utilisée sur une unité de production.

Le rapport décrit cette machine et son installation à l'usine BSC de Templeborough, ainsi que les résultats d'une série d'essais.

Les résultats de ces essais ont fait l'objet d'une évaluation critique et la conclusion d'ensemble est que bien que la méthode optique soit la seule capable de détecter des défauts du genre retassure, elle ne convient pas aux unités de production et ceci essentiellement parce que les billettes doivent préalablement être enduites de peinture.

Les deux autres méthodes étudiées, à savoir les micro-ondes polarisées et le courant de Foucault peuvent toutes deux détecter la plupart des autres types de défauts, mais toutefois pas avec la même efficacité et à ce stade, la conclusion est que la combinaison des deux méthodes permet de détecter des défauts avec un degré de certitude raisonnable, sans trop d'erreurs ou de fausses marques.

Le rapport donne une comparaison des coûts, de la sensibilité, des bhamps d'application et des résolutions.

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FR 80-9 841 7210.GB/802

SOMMAIRE Page

1. INTRODUCTION 1

2. TYPES DE DEFAUTS CARACTERISTIQUES DES COULEES CONTINUES 1

3. EXAMEN DES METHODES D'INSPECTION 1

3.1 Courant de Foucault 1 3.2 Dispersion de flux 2 3.3 Micro-ondes 2 3.4 Méthodes optiques 2 3.5 Méthodes thermiques 2 3.6 Méthodes ultrasonores 3 3.7 Sélection des méthodes d'inspection 3

4. SYSTEME D'INSPECTION OPTIQUE 3

4.1 Caméra à balayage linéaire 3 4.2 Eclairage 4 4.3 Peinture 4 4.4 Résultats des essais initiaux 5

5. INSPECTION PAR MICRO-ONDES 6

5.1 Tête à micro-ondes 6 5.2 Résultats des essais initiaux 6

6. INSPECTION PAR COURANT DE FOUCAULT 7

6.1 Tête d'essai à courant de Foucault 7

6.2 Essais initiaux 7

7. INSTALLATION EN USINE 7

8. ESSAIS EN USINE 8

9. DISCUSSION DES RESULTATS 8

10. CONCLUSIONS 9

11. REFERENCES 10

12. REMERCIEMENTS 10

TABLEAUX 11

FIGURES 14

APPENDICES 55

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LISTE DES TABLEAUX

1. Résumé des défauts caractéristiques des billettes en coulée continue

2. Comparaison des lampes et des luminaires

3. Comparaison des méthodes d'inspection

LISTE DES APPENDICES

1. Système d'inspection optique

2. Système d'inspection par micro-ondes

3. Système d'inspection par courant de Foucault

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FR 80­9 841 7210.GB/802

LISTE DES FIGURES

1. Alignement optique de Ια caméra à balayage linéaire au­dessus de la

billette

2. Système standard de caméra Mkll série M

3. oProjecteur à faisceau étroit Thorn "Sunspot"

4. Incidence de la peinture blanche sur la définition du bruit et de

1'image

5. Pistolet à peinture utilisé pour l'application de la couche de peinture

préalable

6. Résultats des essais en laboratoire faits avec la tête d'inspection optique

7. Enregistrement sur fibre optique de la réaction de la caméra à

balayage linéaire à la billette peinte

8. Performance du système d'inspection optique sur les défauts en retassure

(seuil = 9/16)

10. Ensemble de la tête à micro­ondes

11. Tête à micro­ondes ­ type à cornes

12. Balayages longitudinaux répétés le long de la billette présentant des

défauts artificiels

13. Réaction typique en micro­ondes provenant d'une billette de production

présentant des défauts de surface significatifs

14. Partie de la machine d'inspection une face de l'usine de Templeborough

montrant les trois têtes à micro­ondes réglées pour couvrir une face

15. Surface de billette présentant plusieurs zones de recouvrement ou

revêtement

16. Impression enregistrée de la zone sus­mentionnée montrant les réactions

aux positions 53 et 54 qui dépassent le seuil

17. Zone de billette correspondant à la position 54

18. Vue d'ensemble de la zone présentant des recouvrement ou une double

pellicule donnant naissance aux pointes 76­79 à la fig. 19

19. Impression enregistrée montrant sept pointes significatives, y compris

celle de la zone de la fig. 18

20. Surface de la billette au­delà de la position de la pointe 79 n'ayant

pas produit de réponses significatives

21. Surface de billette présentant une piqûre de 15 χ 18mm

22. Réponse des micro­ondes dans le canal d'inspection passant à travers

la piqûre

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FR 80V9 841 7210.GB/802

23. Installation en usine des têtes à micro-ondes

24. Vue des unités d'inspection optique et par micro-ondes combinées

25. Inspection par courant de Foucault - sonde à semelle coulissante

26. Inspection par courant de Foucault - sonde à hauteur fixe

27. Sonde à courant de Foucault en position d'essai

28. Inspection par courant de Foucault - réponse typique des sondes provenant du système de six sondes à servocommande

29tj Diagramme schématique de l'équipement d'inspection de coulée continue

30. Rack électronique contenant l'unité de traitement des signaux pour l'inspection optique et par micro-ondes (trois sous-racks supérieurs), le traitement par courant de Foucault (sous-rack 4) et le système informatique (sous-rack 5)

31 . Billette calibrée présentant des défauts usinés

32. Sortie imprimée provenant de la billette d'essai

33. Réponses de la tête à micro-ondes avec la billette d'essai sciée

34. Réponses du courant de Foucault avec la billette sciée

35. Réponses de la tête à micro-ondes sur la billette d'essai avec de fortes marques de va-et-vient

36. Réponses du courant de Foucault sur la billette d'essai avec de fortes marques de va-et-vient

37. Sortie imprimée provenant d'une billette de production typique

38. Photo de défauts d'une billette en coulée continue typique

39. Photo de défauts d'une billette en coulée continue typique

40. Photo de défauts d'une billette en coulée continue typique

41. Photo d'un défaut d'une billette en coulée continue typique

42. Photo d'un défaut d'une billette en coulée continue typique

43. Indices de performance de l'inspection optique

44. Indices de performance de l'inspection par micro-ondes

45. Indices de performance de l'inspection par courant de Foucault

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FR 80-9 841 7210.GB/802

LISTE DES FIGURES DES APPENDICES

Al.l Agencement intérieur de Ια caméra à balayage linéaire

Al.2 Carte d'essai et impression d'oscilloscope pour mesurer la réponse

de la caméra à balayage linéaire

Al.3 Diagramme schématique du traitement des signaux électroniques

Al.4 Reconnaissance de la forme des défauts/paramètres de classification

Al.5 Illustration du classeur de défauts à la recherche de défauts

transversaux

A2.1 Caractéristiques de la direction d'une tête à micro-ondes

A2.2 Tête à micro-ondes

A2.3 Réponse de la tête à micro-ondes pour diverses profondeurs de défaut

A2.4 Réponse de la tête à micro-ondes à divers jeux

A2.5 Caractéristiques de levage d'une tête à micro-ondes

A2.6 Réponse de la tête à micro-ondes à divers angles

A3.1 Photo de la sonde à courant de Foucault en position soulevée au-dessus

d'une billette carrée de 140mm

A3.2 Diagramme schématique du système à servocommande de courant de Foucault

A3.3 Diagramme schématique du traitement des signaux de courant de Foucault

dans chacun des canaux de la sonde

A3.4 Utilisation d'un rouleau de calibrage pour optimaliser la réponse du

courant de Foucault

A3.5 Réponses optimalisées du système de courant de Foucault en utilisant

un rouleau de calibrage

A3.6 Forme d'ondes du courant de Foucault avec un rouleau de calibrage modifié

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PE 80­9 841 7210.GB/802

British Steel Corporation

Oberflächeninspektion der stranggegossenen Knüppel

EGKS Vertrag Nr. 7210.GB/802

Zusammenfassung

Es war Ziel dieses Forschungsprogramms, die verschiedenen zerstörungsfreien Prüfverfahren zu bewerten, die für die Ober­flächeninspektion der stranggegossenen Knüppel eingesetzt werden könnten.

In diesem Bericht werden die Anzahl der Defekte, die man wahrscheinlich antrifft, und auch die Gründe berücksichtigt, warum gerade drei der vielen zur Verfügung stehenden zerstö­rungsfreien Prüfverfahren gewählt wurden. Prüfungen, die im Labor unter Einsatz von verschiedenen Knüppeln durchgeführt wurden, ermöglichten den Entwuf einer Inspektionsmaschine für einen Oberflächenbereich für schritthaltenden Einsatz im Stahlwerk.

Diese Maschine und die Installation im Templeborough Stahlwerk der British Steel Corporation werden beschrieben, und die aus einer Reihe von Versuchen gewonnenen Ergebnisse werden angegeben.

Die Ergebnisse dieser Versuche werden kritisch bewertet, und man kommt zu der allgemeinen Schlußfolgerung, daß, obwohl die opti­sche Methode, die einzige ist, mit der man kleine lochartige Defekte nachweisen kann, sie ungeeignet für schritthaltenden Einsatz im Stahlwerk ist. Dies liegt hauptsächlich daran, weil man einen vorher angestrichenen Knüppel braucht.

Von den anderen zwei untersuchten Verfahren, d.h. polarisierte Mikrowellen­ und Wirbelstromanordnung, können beide die meisten anderen Defekttypen nachweisen, aber nicht mit gleicher Wirksam­keit, und in diesem Stadium kommt man zu deshalb der Schluß­folgerung, daß eine Kombination der beiden Verfahren ein angemessenes Niveau an Sicherheit des Defektnachweises ohne zuviele Fehler oder falsche Markierung gegeben wird.

Ein Vergleich der Kosten, der Empfindlichkeit, der Erfassung und der Wiedergabe der verschiedenen Verfahren wird auch angegeben.

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Inhaltsverzeichnis Seite

1. Einleitung

2. Charakteristische Defekttypen in stranggegossenen Erz eugnissen

3. Überblick der Inspektionsmethoden 3.1 Wirbelstrommethoden 3.2 Flußmittelverlustmethoden 3·3 Mikrowellenmethoden 3-4 Optische Verfahren 3.5 Thermische Verfahren 3.6 Ultraschallmethoden 3.7 Auswahl der Inspektionsmethoden

4. Optisches Inspektionssystem 4.1 Linienabtastkamera 4.2 Beleuchtung 4.3 Anstrichsystem 4-4 Ergebnisse der ersten Versuche

5. Mikrowelleninspektion 5.1 Mikrowellenkopf 5.2 Ergebnisse der ersten Versuche

6. Wirbelstrominspektion 6.1 Testkopf des Wirbelstromes 6.2 Erste Versuche

7. Installation im Stahlwerk

8. Versuche im Stahlwerk

9. Diskussion der Ergebnisse

10. Schlußfolgerungen

11. Literaturnachweis

12. Danksagungen

Tabellen

Abbildungen

Anhänge

1 1 2 2 2 2 3 3

3 3 4 4 5

6 6 6

7 7 7

7

8

8

9

10

10

1 1

14

55

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FR 80-9 841 7210.GB/802

Aufstellung der Tabellen 1. Zusammenfassung der gewöhnlichen Stranggußdefekte 2. Vergleich zwischen den Lampen und Leuchten 3· Vergleich der Inspektionsmethoden

Aufstellung der Anhänge 1. Optisches Inspektionssystem 2. Mikrowelleninspektionssystem 3· Wirbelstrominspektionssystem

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FR 80-9 841 7210.GB/802

Aufstellung der Abbildungen 1. Optische Ausrichtung der Linienabtastkamera über dem Knüppel 2. Kamerasystem Mk II der genormten M Serie 3. Thorn "Sunspot" Schmalstrahlflutlicht 4· Auswirkung der Bespritzung mit weißer Farbe auf die Geräusch-

und Bildschärfe 5· Spritzpistole für Einsatz im Voranstrichsystem 6. Labortestergebnisse unter Einsatz des optischen

Inspektionskopfes 7. Faseroptikaufzeichnung der Reaktion der Linienabtastkamera

von dem angestrichenen Knüppel 8. Leistung des optischen Inspektionssystems auf die

kleinen Lochdefekte (Schwellenwert = 9/16)

9. Leistung des optischen Inspektionssystems auf Defekte, die keine kleinen Löcher sind (Schwellewert = 9/16)

10. Montage des Mikrowellenkopfes 11. Mikrowellenkopf - Signaltypen 12. Wiederholte Längsabtastungen entlang des Knüppels, der

künstliche Fehler enthält 13· Typische Mikrowellenreaktion von einem Produktionsknüppel,

Andeutung bedeutender Oberflächendefekte 14. Teil der Inspektionsmaschine für eine Oberfläche im

Templeborough Stahlwerk, gezeigt werden die drei Mikrowellenköpfe für Erfassung einer Fläche

15· Knüppeloberfläche, die mehrere Doppelungen oder Naben enthält, Position 53

16. Aufzeichnungsspur des oben angegebenen Bereiches, gezeigt werden die Reaktionen an den Positionen 53 und 54, die den Schwellenwert überschreiten

17. Der der Position 54 entsprechender Knüppelbe reich

18. Allgemeine Ansicht des Bereiches, der Überwalzungen oder Doppelhaut enthält, die den Anstieg der Höchst­werte 76-79 in Abb. 19 zur Folge haben

19· Aufzeichnungsspuren, gezeigt werden die sieben bedeutenden Höchstwerte einschließlich der des in Abb. 18 angegebenen Bereichs

20. Knüppeloberfläche jenseits der Position des Höchst­wertes 79, der keine bedeutenden Reaktionen zur Folge hatte

21. Knüppeloberfläche, die ein I5 Ï 28 mm großes Loch enthält 22. Mikrowellenreaktion im Inspektionskanal, der durch

das Loch geführt wurde 23· Betriebsinstallation der Mikrowellenköpfe 24 Kombinierte Ansicht der optischen und Mikrowellen­

inspektionseinheiten 25· Wirbelstrominspektion -Gleitkontakt sonde 26. Wirbelstrominspektion - Sonde auf fester Höhe 27· Anordnung der Wirbelstromsonde in der Testposition

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FR 80-9 941 7210.GB/802

28. Wirbelstrominspektion - typische Sondenreaktion von servogesteuerter 6-SondenanOrdnung

29· Blockbild der Instrumente für Stranggußinspektion 30. Elektronisches Gestell, das die Signalverarbeitung

für die optische und Mikrowelleninspektion (obere drei Baugruppenträger), die Wirbelstromverarbeitung (Baugruppenträger 4) und das Rechnersystem (Baugruppenträger 5) enthält

31. Eichungsknüppel, der die mit der Maschine gemachten Defekte enthält

32. Rechnerprotokoll des Testknüppels 33· Reaktionen des Mikrowellenkopfes des Testknüppels

mit Sägeschnitt 34· Wirbelstromreaktionen des Testknüppels mit Sägeschnitt 35· Reaktionen des Mikrowellenkopfes des Testknüppels

mit starken Markierungen der Hinundherbewegung 36. Wirbelstromreaktionen des Testknüppels

mit starken Markierungen der Hinundherbewegung 37· Rechnerprotokoll eines typischen Produktionsknüppels 38. Defekt aufnahme eines typischen, stranggegossenen Knüppels 39· Defekt aufnahme eines typischen, stranggegossenen Knüppels 40. Defektaufnahme eines typischen, stranggegossenen Knüppels 41. Defekt aufnähme eines typischen, stranggegossenen Knüppels 42. Defekt aufnahme eines typischen, stranggegossenen Knüppels 43· Leistungsindexe der optischen Inspektion 44· Leistungsindexe der Mikrowelleninspektion 45· Leistungsindexe der Wirbelstrominspektion

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FR 80­9 841 7210.GB/802

Aufstellung der Anhänge zu den Abbildungen

A1.1 Interne Anordnung der Linienabtastkamera A1.2 Prüfkarte und Ozsilloskopspur für Reaktionsmessung

der Linienabtastkamera A1.3 Schematisches Diagramm der elektronischen Signal­

ve rarbeitung A1.4 Parameter für Defektmustererkennung

und Klassifizierung A1.5 Darstellung der Klassifizierungssuche für Querdefekte A2.1 Charakteristische Richtungsmerkmale für einen Mikro­

wellenkopf A2.2 Mikrowellenkopf

A2.3 Mikrowellenkopfreaktion für verschiedene Defekttiefen A2.4 Mikrowellenkopfreaktion bei verschiedenen Abständen A2.5 Charakteristische Abhebemerkmale eines Mikrowellenkopfes A2.6 Mikrowellenkopfreaktion an verschiedenen Winkeln A3·1 Aufnahme der Anordnung der Wirbelstromsonde in

erhöhter Stellung über dem 140 mm2 Knüppel

A3·2 Blockbild des Wirbelstromservosystems A3.3 Blockbild der SignalVerarbeitung des Wirbelstroms

in jedem Sondenkanal Α3·4 Einsatz der Eichungswalze, um die Wirbelstromreaktion

zu optimieren A3·5 Optimierte Reaktionen des Wirbelstromsystems unter Einsatz

der Eichungswalze Α3·6 Wirbelstromwellenformen durch die modifizierte Eichungswalze

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FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

B r i t i s h S t e e l C o r p o r a t i o n

SURFACE INSPECTION OF CONTINUOUSLY CAST BILLETS

ECSC A g r e e m e n t No. 7 2 1 0 . G B / 8 0 2

FINAL TECHNICAL REPORT

1. INTRODUCTION The purpose of this project was to establish the suitability of certain inspection techniques for the inspection of continuously cast steel billets. The increasing use of the continuous casting process for steel has resulted in a lag in the present stage of the art of non-destructive testing to the point where only visual inspection of concast product surfaces is available as a regular form of quality control. The traditional inspection methods which have been allied to the rolled product are all conditioned to the detection of defects which have a significant length component, i.e. seams, rolling laps, tonguing, etc. Continuously cast products contain, on the other hand, a completely different population of defects comprising such effects as pinholes, corner tearing, reciprocation marks, entrapped scum, teeming arrests, double skins and laps. This population of defects creates a tremendous problem for inspection equipment designers since none of the existing inspection techniques can be readily adapted to the new population with any measure of success. Whilst the production rates of concast machines are relatively slow compared with normal rolling practices this is offset for the smaller section sizes by the use of multiple strands during casting. Consideration of applying inspection methods on the caster would thus entail significant extra costs in order to inspect each strand. The alternative approach is to inspect the product later when all the products can be directed down one line. Under these conditions the product temperature should be significantly below 500°C. This approach has been adopted in the present work and whilst the possible advantages of strand control are lost, the engineering problems and overall equipment costs are much reduced. 2. CHARACTERISTIC DEFECT TYPES IN CONCAST PRODUCTS The design of inspection equipment for concast billets can be satisfactorily undertaken only when a knowledge of all the various kinds of defects, their sizes and frequencies is obtained. A selection of concast billets was acquired and studied on the works and Table 1 summarises and categorises the types of defects encountered. On the basis of this information various inspection techniques were reviewed and their potential assessed. 3. REVIEW OF INSPECTION TECHNIQUES 3.1 Eddy Current Techniques These are well established for rolled products at normal and high temperatures. The engineering problems obviously increase at elevated temperatures because of the closeness of the detector to the test material. However, they can be overcome, as systems are commonly in use at rolling temperatures, for example. The application of eddy currents to concast billet inspection makes it necessary to consider an inspection interval or pitch of the order of 1-2 mm to cater for the pinhole type defects. A common method of applying eddy current probes in billet surface inspection relies on the use of a high speed disc which is positioned above the product surface. If we assume that six probes can be accommodated in the disc then a rotational speed of 6000-12 000 rev/min would be necessary to obtain an inspection pitch of the order of 1 mm at a billet time speed of 0.5-1.0 m/s. To obtain sufficient probe resolution small probe sizes would need to be employed and this in turn would require small operating clearances of the order of 1-2 mm. It can be seen therefore that this method has a number of technical problems and also it will be insensitive

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to defects which are transverse to the billet length. The resolution of pinhole defects would also be in doubt. An alternative approach would be to use arrays of eddy current probes housed in a block and supported above the moving billet surface. This arrangement would be sensitive to transverse face cracking, reciprocation marks, double skins, slag/scum patches and teeming arrests. It could therefore form a useful tool for the inspection of a range of concast defects but the detection of pinholes would still present a problem. A trailing eddy current probe would also be effective in identifying transverse corner cracking and to a lesser extent longitudinal corner cracking. This technology is already embodied in the MIDAS1 scheme for wrought billet inspection and will therefore not feature in this report. 3.2 Flux Leakage Techniques Again a well proven technique but only suitable for cold billets. The magnetic sensors are deployed either to scan the surface directly, as in the Rotomat2 equipment, or indirectly, as in the Magnetograph3 equipment where a contacting tape transfers the magnetic image to a separate scanning head. In either case a temperature limit has to be imposed on the billet to ensure a satisfactory test. Attempts have been made to utilise this form of inspection on concast material and the results have been very disappointing. Concast defects such as pinholes, transverse cracking and other defects with a transverse component can escape detection and in addition the level of spurious marking is very high. It is important to remember that the level of spurious marking generated by any inspection system is just as significant a feature as its detection efficiency. Obviously any magnetic particle inspection system is subject to the same limitations as those described above since the principle is the same. The scanning interval already discussed under the eddy current technique section would be equally applicable in this case since generally mechanical means are used to create a transverse scanning pattern. This approach therefore was considered to be inappropriate to concast material. 3.3 Microwave Techniques This inspection technique is fairly new in its concept and whilst it has been applied in a few very specific applications in a laboratory environment the proposed application to in-line inspection in a steelworks plant is thought to be unique. The detector can be mounted some tens of millimetres away from the billet surface and that must be of benefit since it will reduce its susceptibility to mechanical damage. The inspection area is however relatively large and therefore one should expect that it will be responsive only to large area defects or to cracks of either longitudinal or transverse orientation. Nevertheless this form of detector shows enough merit to be considered for further evaluation but since the capital cost of each detector head is high this will have to be taken into account in the final assessment. 3. 4 Optical Techniques These offer an easier solution from the engineering point of view because the detectors can be positioned well away from the surface of the material and it is possible to examine the full length of the product at normal line speeds. One major disadvantage of these systems however is their inability to discriminate on the basis of depth or severity of defects. Differentiation is on the basis of whether a black or white image is present. Signal processing of the video information is therefore necessary to establish a pattern which could be recognised as being consistent with the various defect types. It is also necessary to provide a high intensity continuous light source to achieve the requisite illumination intensity. 3.5 Thermal Techniques Considerable effort has already been expended in Japan to extend the application of thermal or infra-red cameras to the inspection of slabs at rolling temperatures. Several techniques utilising emitted radiation and/or superimposed reflected radiation in various combinations have been used and include colour synthesis. The ultimate aim is to allow control of hot

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rectification processes on-line before further processing without cooling. The results of these developments have been varied but the overriding feature has always been that the defects would have to be quite large to ensure detection and this feature makes it inappropriate. Recently a more refined thermal inspection system has become available and is marketed by Elkem under the name of Thermomatic1*. This system has been developed for billet inspection and contains a means of inducing thermal energy into the billet surface in a controlled way. The surface is then viewed with a sensitive infra-red camera and the thermal images are correlated to allow identification of longitudinal seams. The range of billet temperature is, however, strictly limited and the system is insensitive to defects with transverse orientations. This must obviously limit its application for concast billet and indeed the manufacturers do not claim to have had any success in this application.

3.6 Ultrasonic Techniques This technique appears to be poor in terms of defect resolution because of the obscuration by scattered waves from other defects. It would be necessary to consider the use of surface waves to identify surface defects and with conventional methods the provision of a suitable couplant at the normal billet speeds and temperatures could be problematical. Also the generation of surface waves would require good control over the entry angle and with the types of surface condition commonly experienced with concast material this must be in doubt. An alternative approach to overcome these coupling problems would be to resort to the use of electromagnetic methods but this would still be subject to the limitations of defect detection already discussed even though the generation problems have been largely overcome. The latter method would also require small operating clearances of 2 mm or so to obtain satisfactory coupling. 3.7 Selection of Inspection Techniques The foregoing suggested that three methods of inspection were worthy of further evaluation: optical, microwave and eddy currents. The optical system with correct illumination levels should be capable of resolving the smaller defective areas such as pinholes, which most other methods would have difficulty in identifying. It should also be capable of responding to other types of defects which show up in relief under the incident lighting conditions. The microwave system has the benefit of reasonable operating clearances (of the order of 30 mm) and will resolve large area defects and, both longitudinal and transverse cracks of the order of 10 mm or more in length. Reciprocation marks and teeming arrest marks should also generate significant responses. The eddy current probe array on the other hand would have to be positioned closer to the surface, at say 3 mm, but should be capable of better resolution in the detection of transverse cracks, teeming arrests, reciprocation marks, etc. than the microwave technique. The project has therefore been based on the evaluation of these three methods and applied to the in-line inspection of concast billet. The test site was located at the Templeborough Works of BSC Special Steels Division. 4. OPTICAL INSPECTION SYSTEM 4.1 Linescan Camera The type of camera chosen was an M Series linescan camera system manufactured by Integrated Photomatrix Ltd. This camera employs a linear array of 1024 photodiodes at a pitch of 0.025 mm. The 51 mm focal length lens supplied with the camera allowed an operating clearance of 440 mm and presented an inspection window 200 mm across in the object plant. Each linescan diode was therefore responsive to an area 1 mm long χ 0.2 mm wide on the billet surface, offering adequate resolution for the identification of most of the defects already mentioned. This resolution is obviously unsatisfactory for the detection of cracks since the crack mouth widths are generally no greater than 0.1 mm and in many cases are significantly less. The illustration in Fig. 1 shows the camera arrangement above a typical 140 mm square billet. Figure 2 shows a view of the linescan camera and associated control electronics. It was

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intended to operate at a stop setting of f8 to allow sufficient depth of field to permit camera-billet distance variations of ±15 mm.

The details of camera operation, depth of field calculations and illumination requirements are discussed in more detail in Appendix 1, Sections Al.l, Al.2 and Al.3 respectively.

4.2 Illumination

The initial laboratory trials with illumination sources were made using four 150 W PAR 38 spotlights spaced in pairs 450 and 650 mm from the billet at angles of 36 and 38° respectively from the horizontal. Each lamp gave 6400 lumens/m2 at 1 m distance.

In order to allow an increase in billet speed and therefore scan rate the PAR 38 lamps were replaced with a single 2 kW tungsten halogen lamp, Type OHS 2000, having an output of 44 000 lumens which gave a measured six fold improvement in illumination level. The main disadvantage of this lamp was that it had to be used in the horizontal plane only, if tilted the filament would slump and become damaged. Whilst this arrangement was satisfactory for laboratory trials where billets were inspected with a flat side uppermost it was unsatisfactory for plant work where billets are presented for inspection on the diamond at increased throughput speeds.

Before plant trials commenced, many attempts were made to acquire a lamp of similar power to the OHS 2000 but which could be used at any angle and apart from infra-red heater lamps, this proved impossible. Finally it was the development of the concept of painting the billets white, thus increasing the returned energy level, that allowed the use of a lower power lamp which was not angle conscious and which was adequately protected for plant use. This new lamp (Type M40, 500 W, tungsten-halogen) and luminaire (Type OSS 500) is shown in Fig. 3 and is manufactured by Thorn Lighting Ltd. Unfortunately, since the installation of this lamp the luminaire has become obsolete and before the optical system can be expanded a suitable replacement will have to be found.

The effect of painting the billets white provided a dramatic improvement in the ability of the linescan camera to respond to the presence of defects. This was simply because the contrast between good surface and blemishes was so much improved. Figure 4 shows the video signal of the diode array of a single scan of a defect area before and after paint spraying. Note that the aperture setting was reduced from f5.6 to fl6/f22 to reduce saturation effects and this is a measure of the improvement in the level of illumination achieved.

The spraying of the billet surface with white paint clearly increases the reflectivity and preliminary estimates suggest a 6-10 times increase in light levels received by the camera. This allowed in-line inspection at 0.25 m/s billet speed with the new lamp arrangement.

A comparison of lamps and luminaires has been included in Table 2.

4.3 Paint System

The paint already developed for use on the MIDAS II Í1) billet inspection equipment was selected for this application. Briefly its characteristics are as follows:-

(a) Drying time - 2 s with a film thickness of 25 urn.

(b) Thixotropic - to avoid settlement in the paint lines.

(c) Colours - white, red, green, blue, yellow.

(d) Temperature range - 0 to 175°C.

(e) Composition - neutral base with non-flammable trichloroethane solvent.

During plant trials the paint was applied by the manual operation of an air paint spray gun (Fig. 5) and two major practical drawbacks became apparent.

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Firstly, because the drying time of the paint is so fast, the spray gun was subject to blockages during operation. An attempt to dislodge the paint blockages by movement of the needle in the jet of the gun by opening and closing the operate solenoid at high speed proved to be unsuccessful. By controlled dilution of the paint with solvent and by using a larger nozzle, paint gun blockages have been significantly reduced, although blockages can still occur occasionally. Secondly, the painting of billets introduced a defect dressing problem in that de-seaming lances would not penetrate the paint layer, thus preventing the rectification of defects. After some experiments it was found that the paint constituent causing this problem was the binder. Removal of the binder eliminated the dressing problem but allowed the paint pigment to separate from the solvent and so a small amount of binder had to be retained in the paint formula. The result is that whilst settlement and de-seaming problems have been overcome, the burning of paint does produce some fumes and extraction may have to be considered. The painting system would have to produce an even opaque coating of paint on the billet surface and at the same time minimise the effects of overspray so that contamination of the optical system is avoided. The spray gun illustrated in Fig. 5 contains the atomised paint in a cone of compressed air and trials confirmed a minimum of overspray. The final pre-paint system incorporated this gun and was equipped with a purpose designed fume extraction system. 4.4 Initial Trial Results The preliminary laboratory trial was conducted on a 1 m length of billet which had a representative range of defect types. The results are shown in Fig. .6 where it can be seen that the majority of significant defects have been identified but the oscillation mark failed to produce a significant response. This is attributed to the fact that the oscillation mark itself was not severe and the angled pre-paint system is designed to minimise the response from casting ripples unless these are severe. Additional spurious signals were obtained down the edges of the billet at various locations and these were examined in detail. It was found that in all cases these spurious marks could be attributed to either poor paint coverage or the presence of oscillation marks on the billet corners. The provision of the correct threshold levels rather than a fixed single threshold at the billet edges should overcome this effect. This was achieved on the plant equipment by incorporating a system for measuring the average video height at any point across the billet width and generating a threshold level as a fixed proportion of this average level, see Appendix 1, Section Al.5. Figure 7 is a fibre optic recording of the linescan camera response from a painted billet. A series of works trials have also been carried out on a single face of seven concast billets with known defect content. The emphasis at the start of the trials was to determine the best sensitivity that could be achieved for the detection of pinholes and to minimise any totally spurious indications. The histogram in Fig. 8 is typical of the performance obtained on one works trial. The most significant feature of the trial results was the absence of totally spurious indications. All indications related to surface blemishes of one kind or another. Some of the blemishes assessed by the works quality control personnel were not considered to be harmful in relation to subsequent product processing and in consequence this proportion of the defect population has been classified in an overmarked category. The test billet contained a population of 19 pinhole defects in the 1-2 mm size range and all but two defects were identified by the optical system. There were eight pinholes of less than 1 mm diameter and four of these were missed. This performance is not surprising since the projected size of each linescan diode element covers an area 1 mm in length and 0.2 mm in width and therefore the scanning pattern in relation to the small pinholes can be critical. No pinholes of greater than 2 mm diameter were missed. The judgement made by the quality control staff in relation to insignificant defects was based on the depth of the pinhole. The optical system is unable to resolve defect depth and therefore it is not possible to modify its response to improve this overmarking characteristic.

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The test billet also contained a population of 22 other forms of surface blemishes which were all detected by the optical system but yet again a significant level of overmarking was present as shown in Fig. 9.

5. MICROWAVE INSPECTION

5.1 Microwave Head

The microwave head illustrated in Fig. 10 measures 100 χ 100 χ 170 mm long and is fitted with a square section flared horn because this was least sensitive to tilt and at the same time afforded the largest beam width. The technical details are contained in the technical Appendix 2 where parameters such as beam width, beam profile, defect depth responses and variations in both operating clearance and incidence angle are more fully discussed and illustrated. Figure 11 shows the horn styles that were evaluated before the square section was adopted.

The objectives in the design of this type of detector head were:-

(a) To obtain a consistent and low background noise level.

(b) To provide a single well defined response from a defect.

(c) It should operate without direct coupled interference from the microwave source since this makes the defect response very unpredictable and creates multiple side lobes to the response.

(d) It should operate at sensible operating clearances, say 20-40 mm.

(e) The influences of lift-off interaction should be minimised.

Virtually all of these objectives have been met but this has not been without difficulty. Extreme care has to be taken with the selection, manufacture and assembly of all the internal microwave components to give repeatable and reliable characteristics.

5.2 Initial Trial Results

Several methods of introducing a calibration standard have been tried including various gauges of wire affixed to the surface of a billet but the most predictable and reliable results were obtained from machined slots introduced in the conventional manner.

To test the response of this type of detector on production material a short trial was conducted at the laboratories on a concast billet. Figure 12 indicates the performance on a range of artificial defects, one longitudinal, one transverse and an area containing a multiple defect. Two consecutive runs have been included on the same trace with the start positions identified and it should be noted that the results are quite repeatable. The trace in Fig. 13 is the result of a single scan along the length of a typical concast billet with a defective surface and significant responses were produced coinciding with the defective areas.

These trial results were sufficiently encouraging to warrant installation of multiple heads in the plant. The initial installation of three microwave heads, see Fig. 14, included a pneumatically retractable mounting plate which was caused to operate if the billet surface approached too near to the microwave horns. A proximity device mounted on the plate was used to sense this movement. A sequence of trials was conducted and the photographs of billet surface and associated output responses have been included in Figs. 15 to 22 inclusive. The detectors were positioned 30 mm above the surface and were distributed evenly across the surface. Even with this limited coverage it was possible to show that there was a marked correlation between signal size and the defect severity as judged by visual methods. This work indicated that further trials with better surface coverage would be required. Three more detector heads were purchased and the final installation as shown in Figs. 23 and 24 indicates the overall size of the installation. In Fig. 23 one of the microwave heads is shown in an alternative position to investigate the possibility of corner inspection. The laboratory trials with the microwave

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head in this position proved very disappointing and this latter mode of operation was not pursued further. 6. EDDY CURRENT INSPECTION 6.1 Eddy Current Test Head The details of the eddy current inspection system are described in Appendix 3. The six eddy current test coils in a light alloy carrier are centred at 20 mm centres across the billet and are designed to provide a reasonable sampling of the billet surface. A further five probes can be interposed to provide a 10 mm probe pitch if this is found to be necessary. The design of the support arms allows the probe block to be positioned parallel to and at 3 mm from the billet surface by means of two high speed servomotors. This arrangement of static probes was considered to be more appropriate to the detection of defects in concast billets where the occurrence of surface defects tend to be transverse rather than longitudinal in orientation. Photocells mounted on each side of the test head assembly control the application and retraction of the mechanism. 6.2 Initial Trials Trials in the laboratory were initially conducted to determine the best manner in which to support the probes above the test surface. One test involved placing the coil in a sliding shoe arrangement which was trailed down the billet length. Figure 25 shows the responses obtained and it is quite apparent that the prime signals produced by the probe show little evidence of defect content until further detailed processing is applied. The shoe rides over the casting ripples and the latter does not therefore have a strong influence over the output signals. This is in strict contrast to the second test where the probe was positioned at a fixed distance above the surface and under these conditions pronounced signals were produced from the casting ripples, see Fig. 26. These oscillation mark responses are observable in both probe channels and after subsequent processing the final output shows little evidence of this possible interfering effect illustrating the importance of signal processing techniques. The design of a non-contacting servocontrolled probe was initiated and the illustration in Fig. 27 shows the final assembly positioned over a concast billet at 3 mm clearance. The final trials at the laboratory produced the traces shown in Fig. 28 which represents a section of a total trace obtained by passing a concast billet under the servocontrolled probe array. This billet contained artificial defects and these are clearly observable on the output traces. 7. PLANT INSTALLATION The arrangement of the test installation is shown in Fig. 29 and this indicates that the combined inspection head assembly was mounted on a bogie which could be removed into an off-line position for maintenance and access. The power rectifier system was used to power the light source and provided a relatively smooth voltage source to avoid undue variations in illumination at supply frequency. The eddy current inspection head required signal buffering on the bogie to maintain good signal to noise ratios and this buffering was housed in the same enclosure as the servocontrol system. The microwave heads were mounted on a plate which could be retracted to prevent billet damage should this possibility be sensed. The illumination for the optical detector was arranged to be at 45° to the billet surface and directed at the roofs of casting ripples to minimise the marking of relatively insignificant oscillation marks. The electronic equipment rack and the electrical cabinet were housed in an annex building attached to the main inspection bay. The electronic rack contained the optical processing, microwave processing, eddy current processing and a microprocessor package to collect all the defect data from the three inspection systems, see Fig. 30. The inspection results for each billet were printed out in such a way that the position along the billet and across the billet could be readily identified for all defects registered. The pseudo length increment clock was derived from the camera timing clock and therefore it was necessary to synchronise this clock with the actual billet speed to

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achieve the correct length registration on the printout. Restrictions on space at this particular site and financial constraints prevented the inclusion of purpose designed encoder stations before and after the inspection equipment. 8. PLANT TRIALS It was obviously necessary to confirm the correct operation of all three inspection systems before any specific trial could be undertaken. The provision of a suitable test billet as illustrated in Fig. 31 allowed a performance check to be made and a characteristic printout to be obtained. This test run produced a printout as shown in Fig. 32 which has been suitably annotated to identify the defective areas. The stagger in relative positions arose from small differences between the actual billet speed and expected billet speed - an error of only 1% will reflect a final displacement error of 0.1 m in a billet 12 m in length. The test billet was created from a good production billet but even so this still contained the odd defect which introduced extra marking. The printout of the microwave signals confirmed that the defect size must exceed 0.6 λ (6 mm) to be detectable - all the holes up to 4 mm in diameter escaped detection. It can also be seen that because the beam width of the microwave heads was large, signals were produced over several length increments. The pre-painting of the billet corners was patchy and this was evident by the frequent occurrence of Channel 1 and Channel 7 defects along the whole length of the billet in respect of the optical results. The coverage of the optical channel was reduced to prevent these corner problems confusing the analysis of true face defects. The signals produced by both the microwave and eddy current systems for the above test billet have been included in Figs. 33 and 34. The stagger in the physical position of the six microwave heads was responsible for the misalignment of the microwave traces. The performance of the eddy current and microwave heads on rough billet surfaces is also of importance and Figs. 35 and 36 show the final outputs from both inspection systems respectively when a billet with heavy reciprocation marks was passed through the inspection area. There was a marked increase in background noise from the eddy current head with larger indications where severe reciprocation marks were present. Following on from these initial plant trials a sequence of production trials was undertaken and the computer printout of Fig. 37 is typical of the results from such trials and the photographs of the specified defective areas are included in Figs. 38 to 42. There would be little benefit to the reader in presenting all the subsequent trial information in the same format and therefore the performance indices for the three types of inspection system have been included in graphical form, Figs. 43 to 45. 9. DISCUSSION OF RESULTS The results for each technique have been expressed in the form of three inspection indices. Firstly the detection efficiency is expressed as the ratio 100 χ LD/(LD + LM) where LD is the length of detected defects and LM is the length of missed defects. The second index relates to overmarking and is defined as 100 χ LS/LB where LS is the length of overmarked defects plus totally spurious indications. This index is best expressed this way because it is not necessarily related to defect content at all. The third index relates to the product risk and is calculated from the ratio 100 χ LM/LB. This index gives the percentage of the billet length containing harmful defects. It should be appreciated that all these results must be somewhat subjective as they rely on the visual interpretation of defective areas by the normal quality control personnel. The performance of the optical inspection system, see Fig. 43, was somewhat disappointing in some respects, particularly the high levels of overmarking that were evident. Unfortunately it is not possible to reduce this level of overmarking by decreasing test sensitivity because the detection of pinholes would be greatly impaired and this would defeat one of the prime reasons for the inclusion of this type of detector in the first place. The frequency of occurrence of pinhole defects was also low such that on occasion drilled holes were deliberately introduced to confirm the continued detection capability of

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the system. The quality of the painted billet surface in terms of its opacity is a significant problem and accidental marking of the painted surface prior to inspection is also a problem and calls for careful billet handling. The inability to resolve cracks of either orientation restricts its use to topographical features such as pinholes, bleed marks, reciprocation marks, teeming arrests, etc.

The microwave system on the other hand has a much coarser resolution and only starts to respond on larger area defects such as bleed marks, reciprocation marks, etc. It has, however, the ability to resolve both transverse and longitudinal defects which is of decided benefit when defects of this type are a production problem. The low levels of overmarking in conjunction with its detection performance suggests a useful tool in the inspection of concast surfaces, see Fig. 44. It also has the benefit of a sensible lift-off clearance at 30 mm.

The eddy current system also performed reasonably well as illustrated in Fig. 45 and the servo positioning system also performed well by providing a smooth contact free ride down the billet surface. Its performance is not unlike that of the microwave head except that it responds to the smaller area defects which the microwave head is unable to resolve. The overmarking is conditioned by the test sensitivity employed and it should be noted that the optimum lift-off cancellation was degraded somewhat to allow the detection of the larger drilled holes. It is now felt that if this lift-off cancellation were restored to its original level the degree of overmarking could be reduced still further. Additionally the convexity of the billet surface caused enhanced sensitivity with the central probes and the provision of gain compensation will also serve to reduce this contribution to overmarking.

10. CONCLUSIONS

Table 3 has been included to summarise the inspection performance of the three inspection systems and it will be seen that the judgement on a specific technique becomes a difficult task when confronted by so many variable factors.

The initial judgement was to reject the optical inspection method as a practical · inspection proposition on the basis that the only advantageous feature is its unique ability to respond to pinhole type defects. There are however so many disadvantages that these outweigh this singular advantage. They are:-

(a) The need for pre-painting the billet surface which in itself produces a significant operating cost of approximately £3/t. Pre-painting is also likely to produce significant maintenance problems in normal production operation.

(b) The need to include a fume extraction system in the initial equipment supply with associated costs.

(c) There could also be a need to consider fume extraction facilities in the torch dressing areas if the fumes prove to be obnoxious.

(d) The levels of overmarking are high and would cause significant bottlenecks in rectification areas with the appropriate loss in throughput.

(e) Line speed is also restricted to 0.25 m/s and this is only half the current production line speed.

(f) The increased line length required to accommodate the pre-painting station for a full inspection system, particularly when it is realised that further support rollers cannot be tolerated until the painted billet has passed the camera/light source station. This in itself will necessitate large gaps between support rollers and will limit the minimum billet length that can be handled.

The judgement on the two remaining techniques is not so easy to formulate. The individual cost of a microwave head is high (of the order of £5000) but the coverage means that about six heads are required to inspect a billet face

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producing an overall cost per billet face of the order of £30 000 plus any additional mechanical support framework, size change mechanism and signal buffering. The eddy current test head assembly with say 10 probe coils (assuming concast defects have a transverse component of at least 10 mm) would cost approximately £5000 on the same basis including servomotors probe coils and test head carrier. Comparing costs of detector heads shows a marked advantage to the eddy current system. This latter advantage together with improved resolution for the eddy current probe array has to be traded against other factors such as inspection end loss, reduced operating clearance (3 mm) and more extensive signal processing in the main electronic equipment racks. One further feature that needs to be taken into consideration is the fact that no one system responds to all defect types and the combination of both systems could well be necessary to give the detection assurances that most plants require. The detection of corner cracks on concast products is much easier to resolve since only the eddy current method offers a satisfactory means of effecting corner inspection. With all three techniques, scale removal by shot blasting is absolutely necessary to ensure a satisfactory test. The final recommendation is that the combined effect of the microwave and eddy current techniques is necessary to achieve a satisfactory inspection standard on the production line at the Templeborough Works. With further development of either technique it may be possible to switch the emphasis to only one type of detector head but at this point unnecessary product risk would be involved. The satisfactory detection of pinholes has yet to be resolved but optical methods using the linescan camera are unsuitable for works use. 11. REFERENCES 1. Johnson, W., Savidge, D.H. and Tompkin, Β., MIDAS II - Seven Years

of Development and Operational Experience. Proceedings of the 1st International Conference on Surface Conditioning and Detection of Surface Defects, MEFOS, S-951 28, Lulea, Sweden, 1984, ρ 1:1.

2. Rotomat Bar or Bloom Inspection Equipment, Manufactured by Institut Dr. Forster, D7410, Reutlingen, Grathwohlstrasse 4, West Germany.

3. Magnetograph - Square Billet Inspection Machine, Manufactured by Institut Dr. Forster, D7410, Reutlingen, Grathwohlstrasse 4, West Germany.

4. Storset, M., Therm-O-matic Billet Inspection System, ibid ρ 2:1. 12. ACKNOWLEDGEMENTS The author wishes to acknowledge the contributions made by his colleagues in the completion of this contract and to thank all works personnel for their valued cooperation.

AP

10

TABLE 1 SUMMARY OF COMMON CONCAST DEFECTS

Type

Pinhole defects

Slag/scum defects

Transverse cracking on faces

Teeming arrests

Longitudinal corner cracks

Transverse corner cracks

Laps and double skins

Reciprocation marks

Description

Symmetrical circular holes up to 2 mm in diameter. Can occur individually or in clusters. Larger irregular shaped depressions of the order of >5 mm in diameter. May be porous in appearance but can be filled with slag/powder. Normally associated with a lack of hot ductility on the caster strand as the product straightens out. Usually located on the upper surface. A deep circumferential groove or depression extending around the full billet surface. Of the order of 1-2 mm in depth and usually sited on the corner apex. Usually caused by uneven shrinkage in the mould which in turn induces thermal stresses and tearing. Introduced in the mould and are conditioned by the type and amount of added lubricant. Folds in the billet surface resulting from the rise of liquid metal over an already soldified layer. Overlaps of 5 to 10 mm are not uncommon but the depth is usually restricted to 1 mm. Generated by the reciprocation of the mould.

00 o

00

to

(Π M S 00 o

TABLE 2 COMPARISON OF LAMPS AND LUMINAIRES

*d oo o

oo

Lamp Reference

PAR 38 single ended

2 kW double ended

M40 500 W single ended

Lamp Type

Tungsten filament

Tungsten halogen

Tungsten halogen

Lamp Output (Lumens)

1 740

44 000

8 500

Luminaire Reference

Integrated

OHS 2000

OSS 500

Peak Intensity (Cd/Lumen)

3.70

1.84

12.3

Peak Illumination

(Cd)

6 400

81 000

105 000

^1 to

tn oo o to

TI Sd oo o

TABLE 3 COMPARISON OF INSPECTION TECHNIQUES oo

Test Parameters

Face detection - pinholes - small area defects - large area defects - longitudinal cracks - transverse cracks

Corner detection - longitudinal cracks - transverse cracks

Overmarking - insignificant and spurious

Operating clearance, mm

Inspection end loss, mm

Line speed limitations, m/s

Surface scale removal

Other surface treatment

Detector head costs

Signal processing costs

Optical

Yes Yes Yes No No No No High

450 10 0.25

Yes Pre-paint (3)

Low High

Microwave

No No Yes Yes Yes No No Low (1)

30 10 None

Yes None

High

Low

Eddy Current

No Yes Yes Yes Yes Yes Yes Medium (1)

3 100 (2)

None

Yes None

Low High

(1) Conditioned by surface roughness.

(2) Varies with line speed.

(3) Approximately £3/t allowing for overspray and solvent losses. Paint film thickness assumed to be 0.1 mm

IO

n tn oo o to

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

1024 Diodes @ 0.025 mm pi tch Ar ray

f = 51 mm

> Object

OPTICAL ALIGNMENT OF LINESCAN CAMERA ABOVE BILLET FIG. 1 (Rl /4127)

14

» CO O I

STANDARD M SERIES CAMERA SYSTEM MK II FIG. 2

td co o IO

FR 80-9 841 7210.GB/802

THORN 'SUNSPOT' NARROW BEAM FLOODLIGHT FIG. 3

16

FR 80-9 841 7210.GB/802

Optical scan across a shot blasted (a) billet. Aperture setting <f/5.6

Same area as in (a) but after (b) spraying. Aperture setting between

f/16 and f/22

EFFECT OF WHITE PAINT SPRAY JN NOISE AND IMAGE DEFINITION FIG. 4

17

FR 80-9 841 7210.GB/802

SPRAY GUN FOR USE IN PRE-PAINT SYSTEM FIG. 5

18

O-I 0-2 0-3 0-4 0-5 0-6 0-7 0-8 0-9 1-0

►π sa

co o I

co Λ.

DS = Double skin

OM = Oscillation mark

PH = Pinhole

PT = Pit

■ Correctly identified defects

□ Missed defects

& Spurious signals

LABORATORY TEST RESULTS USING THE OPTICAL INSPECTION HEAD FIG. 6 to

t»

co o IO

FR 80-9 841 7210.GB/802

Threshold 0.20 V (a) Threshold 0.30 V (b) FIBRE-OPTIC RECORDING OF LINESCAN CAMERA RESPONSE FIG. 7

FROM PAINTED BILLET

20

FR 80-9 841 7210.GB/802

Missed defects

Number of defects 20

18 -

16 -

14 -

12 _

10 -

6 i

Overmarked defects

Correctly marked defe

OPTICAL INSPECTION SYSTEM PERFORMANCE ON PINHOLE DEFECTS (THRESHOLD = 9/16)

FIG. 8 (R1/8978)

21

FR 80-9 841 7210.GB/802

K3 Overmarked defects Correctly marked defects

Number of defects

10

8

6

4

2

0

-

OPTICAL INSPECTION SYSTEM PERFORMANCE ON DEFECTS FIG. 9 OTHER THAN PINHOLES (THRESHOLD = 9/16) (R1/8979)

22

FR 80-9 841 7210.GB/802

MICROWAVE HEAD ASSEMBLY FIG. 10

23

FR 80-9 841 7210.GB/802

MICROWAVE HEAD - HORN TYPES FIG. 11

24

FR 80-9 841 7210.GB/802

F r Start

I Start

Γ δ r

Γ

ί ί F

Longitudinal

Multiple

Transverse

Start

REPEATED LONGITUDINAL SCANS ALONG BILLET CONTAINING FIG. 12 ARTIFICIAL DEFECTS

25

FR 80-9 841 7210.GB/802

TYPICAL MICROWAVE RESPONSE FROM A PRODUCTION BILLET INDICATING SIGNIFICANT SURFACE DEFECTS

FIG. 13

26

FR 80-9 841 7210.GB/802

PART OF THE SINGLE FACE INSPECTION MACHINE AT TEMPLEBOROUGH WORKS SHOWING THREE MICROWAVE HEADS

SET TO COVER ONE FACE FIG. 14

27

FR 8 0 ­ 9 8 4 1 7 2 1 0 . G B / 8 0 2

B I L L E T SURFACE CONTAINING SEVERAL AREAS OF

LAPS OR S H E L L , P O S I T I O N 5 3

F I G . 15

ÏÏS31 Λ ■.■'­,.:

HBËnWKMWjfe|LU4llHl . L j m i m ij ! PU.ÍI

yj $Λ

'.. v . . ; , . tt*%*rø»KfeMMlMNMNWM

T h r e s h o l d

S i g n a l

IWÎiWI|)i1Wfl]illlii»iiltliiÎlB^MM|M|MÉ|ÉliB S i g n i f i c a n t

d e f e c t s

RECORDER TRACE FROM ABOVE AREA SHOWING RESPONSES

AT P O S I T I O N S 5 3 AND 54 WHICH EXCEED THE THRESHOLD

F I G . 16

FR 80-9 841 7210.GB/802

BILLET AREA CORRESPONDING TO POSITION 54 FIG. 17

GENERAL VIEW OF AREA CONTAINING LAPS OR DOUBLE

SKIN GIVING RISE TO PEAKS 76-79 IN FTG. 19 ■

FIG. 1

2 9

FR 8 0 - 9 841 7210 .GB/802

aSAfcafc

T í ìkr'-ii 7tn7% ?4

RECORDER TRACE SHOWING SEVEN SIGNIFICANT PEAKS FIG. 19 INCLUDING THOSE FROM THE AREA IN FIG. 18

BILLET SURFACE BEYOND THE POSITION OF PEAK 7 9 WHICH FIG. 20 DID NOT PRODUCE ANY SIGNIFICANT RESPONSES

30

FR 80­9 841 7210.GB/802

BILLET SURFACE CONTAINING A PIT 15 χ 18 mm IN SIZE F I G . 21

tmamiuumiktmI'tmiwuitiiiMii'in ι ^,i/t^\'^mnmmitinmmmw*m^»if»^msiÎm»mti\m^AuiwiugHMMj^ Thresho ld

S i g n a l

^ffmm&itw.;*M.ri''

r:Kr~

rT~,

i~'""~~~*—~

r'~ " ι" Ί " " - Τ 4 " " " — ~ ~ » — . ^ ^ ^ ^ - ρ » » » ^ ^ ^

Significant defects

MICROWAVE RESPONSE IN THE INSPECTION CHANNEL WHICH PASSED THROUGH THE PIT

FIG. 22

31

FR 80-9 841 7210.GB/802

PLANT INSTALLATION OF MICROWAVE HEADS FIG. 23

COMBINED VIEW OF MICROWAVE AND OPTICAL INSPECTION UNITS""

FIG. 24

32

UI

Billet speed 115 mm/s

A Lift-off signal -

Β Defect signal

C

= A + Β —

D dC dt π

ΫΝΛ-

f ^

—/'■Ά

^ \

^

I I 1

^\r\^^s/\/^f^^^r^\p^rr*

Λ^Ι

vM I

V

A»!

ι I

I I

fs I I

I ί V

^ΛΛ

wV

^v

J+J\f

■A^—S*m

'WV

Av

N ^

I I

I

I

I

Κ I I

ft

L I' I

~v

Nv

Λν*

Vw

1

I

1

vy 1 1

YJ I 1

\A

KV

^

W

Sensitivity

200 mV/cm

_ 200 mV/cm

— 50 mV/cm

_ 200 mV/cm

Chisel cut Saw cut Chisel cut

TI So

co o I kO

00

Λ.

EDDY CURRENT INSPECTION - SLIDING SHOE PROBE FIG. 25 (R1/8985)

-j

to

Cd v. co o to

TI SÖ

co o

co

B i l l e t speed 115 mm/s

A

B .

Defect s i g n a l _

C

= A + B

D

dC d t

/ ■ >

y

- v

X

ΛγΛ

*v

y

\ \

A '

w

Ά λ b

f d

ι/Α A ~~Ί

Ih

Ar

Λ« » y "*M¡

^

Ί

Λ J

Λ^ ι

j V

*Λ

Α \

V,

LÍ V

^

W/<

Ι ι

/ r̂t /ι \— Λ «Γ r V ^

ν ^

Λ­ν^

i>Wv

W j ι

<"VT ' L

ι

Ε^\ι^ f ι

ν ν^

^^^^W^f 1

ι ι ι ι

\Mf^*V^-AM ι

\ Ν

JtAytfV

S e n s i t i v i t y

_ 20 0 mV/cm

­ 20 0 mV/cm

_ 50 mV/cm

_ 200 mV/cm

C h i s e l c u t Saw c u t C h i s e l c u t

EDDY CURRENT INSPECTION ­ FIXED HEIGHT PROBE FIG. 26 ( R 1 / 8 9 8 6 )

to

o

Ω td

■ \

co o to

FR 80-9 841 7210.GB/802

EDDY CURRENT PROBE ARRAY IN TEST POSITION FIG. 27

35

Billet speed 250 mm/s Chart speed 5 div./s

LO

σι

Lift off signal r

I

Defect signal ­· v*./"

Combined signal

(Α + Β)

D

Signal C

clamped to zero

dC/dt + full wave

rectification

tJO^

u¿

Λ / ^

A ^

V ^ ­Aw»

n/vV\

/ Y ^

^ ¿ ^ t e

'UCÄ: è

A

ΑΛ/S

,N y \ - ^ ΛΛ ^W^-w^·^

r^^K^f

'V. .Λ/Ά, Λ,

vi—V^Kv,

? Vv

ν1

V U

2 ί Δ ^

>vM/W

IHTO

J L

■AwV ί ν^ ,Η ι ¡V-WM-v ν ^ ^

V^^V

Aj.

"Ί

AV

az. ■νν̂ ν ν

ν- - Α Λ Λ ' ;

7Τ 2 V/div.

Λ/1 Α

■V

2~v7d i v .

Λ vV\-

.. 5 V/d iv .

Λ V Γ ^

5 V/div.

Λ Λ Λ / W

V.-n'-V '

Λ̂ / ,

5 V/div.

^ V ' i A A V A V /L

Chisel cut Saw cut Chisel cut

pä

co o I ^D

00

ro

Ci Cö

CO o ho

EDDY CURRENT INSPECTION

TYPICAL PROBE RESPONSE FROM THE SBRV0C0NTR0LLED 6­PROBE ARRAY

FIG. 28

Bogie un i t

^ ^

ΖΓΛ

π I

I

α

Eddy current inspec­tion unit

Z£

5Z

Servo control

£

Optical inspec­tion unit

Micro wave inspec­tion UHI

o <z o Electronic and computer processing

è

C

Visual inspection cabin

^ ^

^T Prepaint station

Electrical system c =

Power recti­fier system

Û

xzz 7ZT

Û Photocells

τι Sd co o I IO

co Oi

BLOCK DIAGRAM OF THE CONCAST INSPECTION EQUIPMENT FIG. 29 (R2/20) en

Co \ CO

o IO

FR 80-9 841 7210.GB/802

ELECTRONIC RACK CONTAINING THE SIGNAL PROCESSING FOR THE ΠΡΦΤΓΑΤ. AND MICROWAVE INSPECTION (TOP THREE SUB-RACKS),

THE EDDY CURRENT PROCESSING (SUB-RACK 4) AND THE COMPUTER SYSTEM (SUB-RACK 5)

FIG. 30

OJ

to

1500

C h a n n e l

No.

6000

_ 1 2 500

50 ,50 . 100 ι DU ι DU χ IUU , . DU 1 DU l

t î t t t t

2000

50 χ 5 0

1 hole 2 mm diameter

1 hole 3 mm diameter

5 holes 4 mm diameter

• · ^ . *­

I ­ I

3 holes 8 mm diameter

5 holes 4 mm diameter

2 slots χ 2.5 mm deep­

τι Sd co o ι

CALIBRATION BILLET CONTAINING MACHINED DEFECTS FIG. 31

(R2/1204) to

Ω co \ oo o to

FR 80­9 841 7210.GB/802

CONCASΤ B I L L E T I N S P E C T I O N AT TEMPLEBOROUGH

DATES T I M E :

Ι Ν S F' E C Τ Ι Ο Ν S Y S 'ï' E M F' A Ε A M Ε Τ E R S s

MANUALLY SET.. PLEASE RECORD THEM..

Β 11... ι... Ε τ s ι;;: α υ Ε Ν C Ι:;: Ν Ο Μ Β E R ;: ο ο ο ο

D ι s τ Α Ν c F: < · D ι;:: F' Ε C: Τ S

PROM NOSE MICROWAME O P T I C A L

O,. Ö 6

O,. O 7

O „ 0 8

0 „ :i. s

1,. 4 5 .1. 3 4 5

1 ., 4 6 1 2 3 4 5 6

1 „ 4 7 :!. 2 3 4 5 ó

:l... 4 8 :í. 2 A 1

1„49 4 5

I . , 5 0 1 2 3 4

1 . 8 3

2.. 12

2 „ 5 :l.

2 . , 7 6

1 „ 2 '"■

.... ...., y

"X "> Λ ·../ .1 Λ.. ' 'T

4 „ 6 2

4 „ 9 2

4 „ 9 3

5 . .74

5 ,. 7 9

5 . , 8 1

5 „ 8 2

5 „84

5„ 8 7

5 „ 8 9

5 „ 9:1.

5 „ 9 4

5 „ 9 5

5 „96

Ci „ '-.Ι Κ,'

ï íiDYCUERENT

PROBE HEAD UF

1 2 3 4 3 Sawcut

.1. Λ.-. ·.:!■ >4 ,.) o

4 ram

d i a .

4 mm

d i a .

8 mm

d i a .

COMPUTER PRINTOUT FROM TEST BILLET FIG. 32

( C o n t i n u e d . . . )

40

FR 8 0 - 9 841 7210.GB/802

7, 7. 7, 7, 8, 9 , 9, 9, 9. 9 , 9, 9, 9 „

I O . I O . 1 0 . 1 0 . 10., 10., 10., 1 0 . 10. . 1 o „ 11 . ,

46 4 7

, 14 0 6

, 0 7 , 14

59· 6 0 69 38 39 40 4 1 4 2 4 3 4 4 4 5 7 7 78 74

1 1 . . 7 5 1 1 . 8 1 11 11

Λ. t...

94 95 07 34 35 38 40

1 2 1 2 1 2 1 2

3 3 3

4 4 4 4

4 4 4

5 6 5 6 5 6 I " -

•.J

Sawcut

B i l l e t end

5 6

BILLET LENGTH- 12.. 43

FIG. 32

41

FR 80-9 841 7210.GB/802

MICROWAVE HEAD RESPONSES WITH THE SAWCUT TEST BILLET FIG. 33 (R2/1206)

42

F R 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

l ,j

WiMwMuw^^tø^

I

EDDY CURRENT RESPONSES WITH THE SAWCUT TEST BILLET FIG. 34 (R2/1207)

43

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

JUv^^^AwVV /V

Λ\*Ι<Γ vJVw^^vW v W ^ ^

MICROWAVE HEAD RESPONSES ON TEST BILLET WITH FIG. 35 HEAVY RECIPROCATION MARKS (R2/1208)

44

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

m%iJì\i

WWUAAX*M*^^ W>AW

EDDY CURRENT RESPONSES ON TEST BILLET WITH FIG. 36 HEAVY RECIPROCATION MARKS (R2/1209)

45

FR 80-9 841 7210.GB/802

CONCAST B I L L E T INSPECTION AT TEMPLEBÖROUGH

DATES TIMES

Ι N S Ρ E C Τ IO Ν S Y S Τ E M F' ARAM Ε Τ E R S :

MANUALLY SET,. PLEASE RECORD THEM,.

B I L L E T SEQUENCE NUMBER:: OOOO

D E PECI· S OM NOSE

ο,. o :i. 0., 0 6

0., 0 7

MIC RO WA Æ OPTICAL

1

EDDYCURR

1 2 3 4

PROBE HE

ENT

5 6

\1\ ϋ Ρ 0 . 3 4 1 2 .. 2 'J 2 „31 ¿. „ V „·:.

3, . 7 5

3„ 8 8

4 „ 0 1

4 ,. 16

4 „ 4 1

4 . ,77

5 , . 4 5

6 ., 4 0

6.. 5 8

6 . . 5 9

6 „ 6 0

6 „ 6 1

6 „ 6 2

7 „ 0 0

7 „ 6 8

8 . 4 3 1

3

4

4

3

1 I

.1.

3 4

1 2

4

4

4

j 4 ■·>■

1

7 7

/

7

6

6

3 4

1 2 3 4

1 2 3

1 2 3

6

6

6

8 . 4 4 1 2

8 . 5 6 ' 4

9 „ 5 9 1

11 „ 0 6

11 . .07 1 1 . , 0 8

1 1 . . 9 8

11 . , 9 9

12.. 0 0

1 2 . . 0 1

12 „ 0 2

1.2..23 .1.2 „ 4 2

12. . 5 4

i. 1

1 2 6 7

Λ."!

0

■ " >

3 3

"> "Χ Λ~ :.Ι

'■■' 3

'"',

ó

6

6

FIGURE

3 8 ( a )

38 (b)

3 9 ( a )

39(h)

4 0 ( a )

41

40 (hi

42

BILLET LENGTH:: 12 „ 5 7

COMPUTER PRINTOUT FROM A

TYPICAL PRODUCTION BILLET

FIG. 37

46

FR 80-9 841 7210.GB/802

(a)

(b) PHOTOGRAPHS OF DEFECTS IN A TYPICAL CONCAST BILLET FIG. 38

47

FR 80-9 841 7210.GB/802

(a)

PHOTOGRAPHS OF DEFECTS IN A TYPICAL CONCAST BILLET FIG. 39

48

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

N s-* :

isSBsSSfiaS

(a)

PHOTOGRAPHS OF DEFECTS IN A TYPICAL CONCAST BILLET FIG. 4 0

49

FR 80-9 841 7210.GB/802

PHOTOGRAPH OF A DEFECT IN A TYPICAL CONCAST BILLET FIG. 41

50

FR 80­9 841 7210.GB/802

':S¿a3$i. !

­ ν ­,«<,·ίΓ,;^'-ν .

' * < ■ : · .

'■"TV-- ' ; . :

PHOTOGRAPH OF A DEFECT IN A TYPICAL CONCAST BILLET FIG. 42

51

FR 80-9 841 7210.GB/802

Detection index, %

100

80

60

Overmarking index φ and

risk index, % ^

Low Medium High (0.44) (0.50*

Detection threshold setting

2.5

2.0

1.5

1 .0

0.5

OPTICAL INSPECTION PERFORMANCE INDICES FIG. 43 (R2/1925)

52

FR 80-9 841 7210.GB/802

Detection index, % O

i

100

80

60

40

20

n

k

Overmarking index φ and

risk index, % " 4

Low Medium High (1.13) (1.38) (1.50)

Detection threshold setting

MICROWAVE INSPECTION PERFORMANCE INDICES FIG. 44 (R2/1926)

53

FR 80-9 841 7210.GB/802

Detection index, % Q

100 -

80

60

40

20

Overmarking index and

risk index, % Δ

J_ J_ Low Medium High

(2.40) (3.50) (4.00) Defect threshold settings

2.0

2.5

1 .5

1 .0

0.5

EDDY CURRENT INSPECTION PERFORMANCE INDICES FIG. 45 (R2/1927)

54

FR 80-9 841 7210.GB/802

APPENDIX 1 OPTICAL INSPECTION SYSTEM Al.1 Operating Principle of the Linescan Camera The M series camera system manufactured by Integrated Photomatrix Ltd. houses a linear self scanned photodiode array of 1024 diodes. The diodes are at 0.025 mm centres and are each 0.125 mm long. It contains all the necessary logic and video processing circuits to operate the array and to output a video signal for viewing on an oscilloscope or for further signal processing. The camera houses a 51 mm focal length lens with variable aperture settings. The camera operation is based on the 'bucket brigade' principle in which two shift registers are connected to the photodiode array (Fig. Al.l) and is arranged to cater for odd and even diodes as shown. The MOS transistor switches allow the output of the appropriate diode to be directed to the odd or even video line in synchronism with the clock trains φ̂ and Φ2. By connecting these video lines in parallel a serial stream of video information is obtained at the output. The photodiodes in the array operate under reverse bias and with exposure to light the capacitor discharges according to the level of illumination and when the diode is subsequently sampled the amount of energy required to restore the charge on the capacitor is then a measure of the integral of the light energy falling on the diode element between these sampling periods. Al.2 Depth of Field Calculations The camera-billet operating clearance of 440 mm was determined from the need to provide an inspection width of 200 mm, to allow some latitude (±30 mm) on billet sideways movement during test. The lens supplied with the camera has a focal length of 51 mm and it is necessary to determine the depth of field that can be used in testing to minimise variations in focus caused by product movement. The first step in the calculation is to determine the hyperfocal distance, which is the distance between the object plane and the first focal point of the lens, and then the depth of field can be calculated. An aperture setting of f8 was chosen as an initial start point.

Hyperfocal distance, h = li C Ν

h = 13 m

where f = focal length = 51 mm c = circle of least confusion = 25.4 pm Ν = aperture size = 8

The circle of least confusion is equal to the width of one diode element in this case. The object-lens distance (u) was measured as 453 mm and with the above value of h the depth of field can be calculated:-

Near limit

Far limit

—

=

hu h + u hu

—

=

437.7 mm

469.3 mm h - u

This produces a depth of field = near limit - far limit = 31.6 mm

From this calculation it is clear than with a stop value of f8 the camera to billet distance could vary by ±15.8 mm without sensibly affecting the focus on the array. The inspected surface area associated with each diode is 0.2 mm wide χ 1.0 mm long.

55

FR 80-9 841 7210.GB/802

Al.3 Illumination of the Billet Surface Initial trials were performed with 150 W PAR 38 spotlights spaced in pairs 450 and 650 mm away from the surface and at 36 and 56° to the horizontal respectively. Each lamp produced 6400 lumens/m2 at a distance of 1 m and this resulted in an irradiance of 110 W/cm2 on the diode elements assuming a test surface reflectivity of 0.1 and an 85% transmission factor through the lens stopped down to f8. The linescan camera has a response of 30 V per mJ/m2 which, with an integration time of 10 ms, results in a calculated output of 3.3 V. In practice the measured value was 3 V showing reasonable agreement with estimated value. The typical billet speeds at the inspection position can be of the order of 0.25 to 1.00 ms and to ensure 100% coverage with the linescan camera we would need to create an inspection scan every 1 mm of billet movement. The time for the billet to move 1 mm will be of the order of 1 to 4 ms necessitating a scan rate of 250-1000 Hz. This implies that, at the fastest billet speeds, each diode will need to be sampled every 1.0 us. As a consequence of this billet speed factor, the available light needs to be increased in proportion to billet speed to offset the reduction in time available for each diode to respond to the integral of incident light. The PAR 38 spotlights were therefore replaced with a 2 kW tungsten halogen lamp type OHS 2000 which has a horizontal filament and provides a higher illumination intensity in the field of view with a more even illumination across the billet width. The total output from the lamp was 44 000 lumens giving a measured sixfold improvement in illumination level. Some problems were experienced with this form of light source due to thermal slump of the linear tungsten element in any position other than truly horizontal. Since the application on plant requires that the source be mounted at various angles an alternative source was selected and this was not subject to this limitation - a Type M40 50.0 W tungsten halogen lamp with a Type OSS 500 luminaire fitting and manufactured by Thorn Lighting Ltd. Al.4 Detection Characteristics Since one of the major objectives of this form of inspection was to detect the presence of pinholes in the concast surface it is essential that its performance in this area is carefully assessed. To enable this assessment to be carried out a test card was created which consisted of a series of black lines of differing widths on a white background. The test card was positioned under the camera and the video output waveform was photographed (see Fig. Al.2). This test confirmed that provided a full diode is obscured a good individual response will be obtained and since each diode corresponds to a billet surface area 0.2 mm wide χ 1.0 mm long it should be possible to respond to pinholes 1 mm in diameter. The scanning pattern will influence this detection since the diode may only be partly obscured on two consecutive scans. This implies that 1 mm diameter pinholes will have a low detection confidence and 2 mm diameter pinholes should have a high detection confidence. This detection performance matches fairly well with the levels of discrimination currently employed in standard visual inspection. Al.5 Signal Processing The signal processing circuits have to perform a number of functions aimed at reducing the data rates and making the identification of defects more certain. The processing includes provision for dealing with variations in light level across the billet and from billet to billet. The video responses are judged on their severity and then classified according to the defect pattern produced. Figure Al.3 illustrates the overall processing for the optical data. The light intensity from the billet is converted into video signals at the output of the camera and associated processor. The output from each individual optical diode is converted into an 8 bit digital signal and with the scan rates referred to in Section Al.3 this will have to be achieved in 1 or 2 MS. It will be appreciated that system clocks of the order of 10 MHz will be required to process this information. From the measurement of individual responses of each

56

FR 80­9 841 7210.GB/802

optical diode a running average for each diode is produced over approximately ten consecutive scans. This running average is then compared with each subsequent scan to determine which diode element responses are significantly different from the previous average surface signal. This is achieved in a fast binary comparator whose output is then directed to storage for further signal processing. The pattern of defective information thus obtained is then examined to recognise the presence of characteristic defect patterns. The defect storage grid at this point is in 0.2 mm channels across the billet width, i.e. 1024 channels maximum, and 16 scans along the length of the billet, equivalent to 16 mm of billet movement. This storage grid is continuously updated with new data and the storage area is accessed by various defect pattern recognition circuits to identify defective regions on the billet. The output from the various pattern recognition circuits is grouped into one composite output signal which corresponds to a 1 cm grid pattern on the billet surface. The initial trials were conducted with an oscillographic recorder as an output device but this was subsequently replaced with a small microprocessor which integrates the responses from the three inspections into one printed record. The generation of pulses to represent billet movement are derived for convenience from the camera timing circuits and when a test is being conducted the conveyor speed and camera clock are synchronised.

The recognition of defect patterns is achieved by inspecting the defect storage grid 16 (1 mm) channels long by 1024 (0.2 mm) channels wide. The recognition circuits identify the number of defective pixels defined within a specified area. The table shown in Fig. Al.4 shows the options available for the various recognition circuits according to defect type. A specific illustration of a defect classifier for the identification of transverse defects is included in Fig. Al.5 where three consecutive length scans (and up to 64 width channels) contribute to the detection process.

Al.6 Manufacturers of Proprietary Items

Linescan camera: Integrated Photomatrix Ltd. The Grove Trading Estate Dorchester Dorset DTI 1SY ENGLAND Telephone 0305 63673 Telex 41166

Lamp and luminaire: Thorn Lighting Ltd. Commercial House Lawrence Road London N15 4EG ENGLAND Telephone 01 802 3151 Telex 893024

Marking paint: Mebon Ltd., Blackwell Road, Hathwaite, Sutton­in­Ashfield, Notts., England.

Telephone 0623 511000 Telex 37448

Spray gun, Type 9AU Gray Campling Ltd. Magnalux House Southcote Road Bournemouth Β 1 3SN ENGLAND Telephone 0202 29182Í Telex 418241

57

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

Scan Ι . Ρ

V Ref

Scan I .Ρ

End of Scan A

Odd video

S u b s t r a t e

Even video

End of Scan Β

INTERNAL ARRANGEMENT OF LINESCAN CAMERA F I G . A1.1 (R1 /4126)

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

Χ LT5

(a) Oscilloscope Trace

ΕΞ ÏWidth, mm 3.62

_ 3.16

1.77 1.58 0.67 0.54 0.34 0.36

(b) Test Card

TEST CARD AND OSCILLOSCOPE TRACE FOR MEASURING F I G . A 1 . 2 RESPONSE OF LINESCAN CAMERA (Rl /4128)

5 9

00

o I

Light from billet

σι o

Camera and

processor

A + D

converter

Billet de tec tor

Nose and t aü

detector

State e g is ter ι clock

pulse gen.

Control s ignals

Edge position

ave rage r

Clock pulse gen .

Aver ­ager

Compar­ator

Delay and

s tore

Pinhole defect

r ecogn .

T r a n s ­v e r s e gen .

Other defect

r ecogn .

4 Types

Other defect recogn.

Defect collation

Charac­t e r

gene r ­a tor

Output c i rcui ts

Recor­der

Paper record

SCHEMATIC DIAGRAM OF ELECTRONIC SIGNAL PROCESSING FIG. A 1 . 3 (R1 /5249)

a

CO

o M

FR 80-9 841 7210.GB/802

Defect Type

Pinholes Transverse Longitudinal Slag/Scum Cold Shut

Examination Window

Rows

1 - 4 1 - 4 1 - 1 7 1 - 7 1-16

Columns

1 - 8 14 - 64 1 - 4

14 - 64 All (1024)

Pixel Logic AND OR OR --

Threshold Count in Window

0 - 7 0 - 6 3 0 - 1 6 0 - 444 0 -4080

Rows are across the billet at 1 mm spacing Columns are along the billet at 0.2 mm spacing

DEFECT PATTERN RECOGNITION/CLASSIFIER PARAMETERS FIG. A1.4

61

FR 80-9 841 7210.GB/802

0.2 mm

Column i

Max. Number of Columns 1024

Row

X XX X

X

X

Λ

•

X

X

Χ χ

χ

χ

>

χ

χ

χ

χ

χ

χ

χ

χ

χ

χ

χ

χ

χ

χ

Χ >

)

( χ >

> χ

κ

ι

t χ χ

t χ

χ

χ χ

χ χ

χ

Χ Χ

χ χ

χ χ

Χ χ

xx

χ

χ

χ

χ *

χ

χ

χ

J

κ

>

< χ

<

χ

χ

χ

χ

χ

χ

χ

χ

χ χ

t mm

Position A

Position B

ILLUSTRATION OF DEFECT CLASSIFIER SEARCHING FOR TRANSVERSE DEFECTS

FIG. A1.5

62

FR 80-9 841 7210.GB/802

APPENDIX 2 MICROWAVE INSPECTION SYSTEM A2.1 Operating Principle of the Microwave Detection Head With this method microwaves are directed at the billet surface in a polarised form. The presence of defects in the surface causes the re-radiated energy to have a component at right angles to the plane of incidence. A detector is mounted internal to the head and this responds only to this component. By this means the presence of defects can be identified. The unit contains a Gunn oscillator operating at 33 GHz and a means for polarising these microwaves before they are directed through a flared horn. Experiments have been performed with a variety of horn styles, as discussed in the main text, but the square aperture flared horn was selected because it afforded the largest beam width and the least dependency on tilt. The surface area irradiated by a horn 30 mm above the surface is 15-20 mm in diameter and the final detected output can contain unwanted components which arise from constructive and destructive interference patterns produced when a small amount of the polarised transmitted energy leaks back into the sensitive microwave detector diode. The design and construction of this type of detector head is in consequence extremely critical of the mechanical dimensions of individual parts and the assembly accuracies of all the internal microwave elements and waveguides. It will also be appreciated that the use of robust microwave components at 33 GHz gives significant problems in the supply of components since most of these items are earmarked for military use and are not readily available on the commercial market. It is necessary to operate at these test frequencies to ensure adequate test resolution. The best theoretical resolution is approximately 0.6λ which in this case is 6 mm and therefore ideally it would be preferable to work in the region of 50 to 100 GHz but this technology is not yet available on a commercial basis. A2.2 Directional Aspects of the Detector Head The microwave sensing head exhibits planes of maximum and minimum responses as illustrated in Fig. A2.1. This end-on view of the microwave horn indicates two maximum axes through the corners of the horn such that if a defect passes along one of these planes a maximum response will be registered. The reverse is true if the defect passes along the dotted lines which represent the minimum axes. This directional feature could be useful for minimising unwanted signals from the output response simply by arranging the orientation appropriately. It also serves to make the device equally sensitive to both longitudinal and transverse crack defects in the case of concast defect inspection. A2.3 Beam Profile Figure A2.2(b) illustrates the dimensions of the beam pattern associated with the microwave horn. The beam profile at various distances from the tip of the horn is reminiscent of ultrasonic transducers in that the characteristic exhibits both 'near zone' and 'far zone' regions. The defect response illustrated in Fig. A2.2(a) was obtained by passing a rectilinear notch under a static head. This response is smooth and regular and indicates a 3 dB bandwidth of approximately 10 mm at an operating clearance of 30 mm. A2.4 Detector Head Characteristics The defect depth characteristic was obtained by moving a test head across a series of standard machined notches at a fixed probe height as illustrated in Fig. A2.3. These results have an interesting property in that a detection null occurs at a defect depth of 5 mm which represents half a wavelength at the operating frequency of 33 GHz and is believed to function as a short circuit. This peculiarity is not considered to be a practical limitation since it is unlikely that a concast defect will exhibit a constancy in depth at this value and thereby escape detection - concast defects are, by their very nature, variable in form. The magnitude of the output signal should therefore reflect defect severity.

63

FR 80-9 841 7210.GB/802

The beam width variation with operating clearance is shown in Fig. A2.4. The beam width at 30 mm operating clearance is of the order of 12 mm increasing to 20 mm at operating clearances of 50 mm. These characteristics conform to the beam profile plots referred to in A2.3 and illustrated in Fig. A2.2. The excellent defect free lift-off characteristic of a typical microwave head is illustrated in Fig. A2.5 Trace B. Only two minor disturbances can be seen as deviations from the ideal and in the case of some heads even this minor deviation is absent. The defect response varies with lift-off in the manner shown in Trace A where it can be seen that the decrement in defect output signal with operating clearance has an oscillatory content. These oscillations are thought to be the result of interference effects generated by the vertical movement of the head on the recovered defect signal. The distance between the peaks or troughs is approximately 5 mm and allowing for the incident and reflected path lengths this equates to the wavelength of the microwave source. This effect has been minimised during manufacture and it is considered that this residual modulation will not seriously impede the detection performance of this device since much larger variations result from concast defects of similar outward appearance and severity. The characteristics illustrated in Fig. A2.6 show the effect of incidence angle on the output signal when traversing a defective area of a sample plate. At normal incidence (0° angle) the response is similar to that already depicted in Fig. A2.2(a). However as the angle departs from normal incidence additional modulations are produced on either side of the main response. This behaviour will not have a significant effect on the usefulness of the device since these side lobes are only produced in the case of a defect and are always minor in comparison to the main response. A2.5 Manufacturers of Proprietary Items Microwave detector head: Dr. J.C. Jackson

Department of Applied Mathematics and Theoretical Physics

University of Cambridge Silver Street Cambridge CB3 9EW ENGLAND Telephone 0223 351645 Ext. 37

64

FR 80-9 841 7210.GB/802

Max.

Max. Max.

Max.

DIRECTIONAL CHARACTERISTICS OF A MICROWAVE HEAD FIG. A2.1 (R2/1928)

65

FR 8 0 - 9 841 7 2 1 0 . G B / 8 0 2

BEAMWIDTH

(a) Defect Response at 30 mm Clearance

8mm

NEAR ZONE

FAR ZONE

25mm

ι ι

25mm

18r

(b) Beam Profile

MICROWAVE HEAD FIG. A2.2

66

'T) w 00 o I

IO

co

Output voltage

0 -

0 -

0 -

0 _

JL 20 40

Distance across plate, mm 60

MICROWAVE HEAD RESPONSE FOR VARIOUS DEFECT DEPTHS

Flaw depth, mm

6.35

5.08

2.54

1 .27

0.64

80

FIG. A2.3 (R1/8983)

to

O ω CO o to

Output voltage

0 -

0 -

0 -

20 40 60

Distance across plate, mm MICROWAVE HEAD RESPONSE AT VARYING CLEARANCES

Height above plate, mm 30

36

42

48

54

F I G . A 2 . 4 ( R 1 / 8 9 8 2 )

TI to­co o

I kD

CO 0^

- J to

a ta 00 o to

Output vol tage

12

00 o I *X>

CO

υο

Β -Λ.

Α - Defect response as a function of lift-off for a 2.5 mm deep defect

Β - Output response as a function of lift-off over a defect free surface

20 I

40 60

Lift-off, mm

LIFT-OFF CHARACTERISTICS OF A MICROWAVE HEAD FIG. A2.5 (R1/8981)

to

Ω ro \ co o to

TI oo o

oo

Output voltage

-j o

20 40 60 80 Distance across plate, mm

MICROWAVE HEAD RESPONSE AT VARIOUS ANGLES FIG. A2.6 (R1/8984) to

a \ 00 o to

FR 80-9 841 7210.GB/802

APPENDIX 3 EDDY CURRENT INSPECTION SYSTEM

A3.1 Operating Principle of the Eddy Current Test Head

The eddy current test head comprises six eddy current transducers mounted in line across the width of the billet at 20 mm centres as depicted in Fig. A3.1. The probes are potted into an aluminium alloy carrier which is supported by pantograph arms designed to maintain the probe faces parallel to the billet at all times. Allowance for billet twist was made by incorporating a pivot at each end of the probe carrier where the pantograph arms are attached. This arrangement of static probes was considered more appropriate to the detection of defects in concast billets where the occurrence of surface cracks tend to be transverse rather than longitudinal in orientation. Whilst the chosen probe array does not give 100% coverage it should give an adequate amount of data with which to assess the technique and to allow comparisons to be made with other alternative techniques. Provision is made for additional probes should this prove to be necessary.

The probe faces are maintained at the correct operating clearance by the use of a servocontrolled positioning system. The signal required to control the operating clearance is derived from the eddy current transducers and provision is made to servo each side of the probe carrier individually so as to maintain the carrier parallel to the billet face at all times thus making it tolerant of any billet twist which may be present.

The servocontrol system is intended to:-

(a) Remove the need for gain compensation with varying operating clearances.

(b) Prevent contact with the product during test so that probe wear and vibration will be minimised. Wear facings have been provided for accidental contact.

(c) Improve the lift-off cancellation features by restricting the range over which movement is possible.

Two signal channels are generated for each probe channel. One signal is responsive to the combined effects of defect content and lift-off variations and the other channel is predominantly responsive to lift-off effects only. By suitable combination of these two signals lift-off cancellation can be achieved.

A3.2 Eddy Current Servo System

The block diagram of the servo system is illustrated in Fig. A3.2. Two ironless rotor servomotors attached to the two pantograph arms which support the probe array derive their control signals from the outer probe pairs (Ρχ P2 and P5 Pg). The lift-off signal representing the closest approach of any probe pair is then linearised and forms the input to a three term controller with the normal proportional, integral and derivative terms. The resultant signal is used to drive the motor which causes the probe array to position itself 3 mm above the billet surface. Where, as in this case, the control is to be effective after the mechanism is applied it becomes necessary to gate the integral control term until the mechanism approaches the target clearance. The application and retraction is achieved by feeding an appropriate overriding control signal into the servo drive amplifiers.

A3.3 Eddy Current Signal Processing

The cancellation of surface irregularities is achieved by mixing the defect and lift-off channels as illustrated in Fig. A3.3 and then processing the resultant information in the manner shown. One processing channel relies on the accentuation of signals with high rates of change by using derivative processing and then using full wave rectification to make all signals unidirectional. The other processing channel uses a bandpass filter to filter out the unacceptable frequency components from the signal. The signals from both of these channels are fed in a controlled ratio to comparators with a variable threshold. The threshold can be selected by manual means or set by computer control. A data valid signal is generated to signify that the system

71

FR 80-9 841 7210.GB/802

is producing legitimate data and takes into account the application and settlement times of the test head. The application and retraction of the test head is initiated by photocells mounted on each side of the inspection head assembly. A3.4 Eddy Current Calibration Procedures The calibration of the eddy current channels to minimise true lift-off variations and yet be responsive to other surface irregularities presented a problem and various means were employed to effect a solution. The procedure adopted was to use a calibration roll such as that depicted in Fig. A3.4. A flat has been machined on the roll surface and a machined defect introduced into the centre of this area. The roll is then rotated under each eddy current probe in turn until specific waveforms are produced. Examination of Fig. A3.4 shows how the shape of the resultant signal varies with the magnitude of the lift-off component used in the compensation technique. The optimum mix was determined from trials on concast material and the resultant waveforms from the calibration roll at this setting were duly noted. It should be noted that at this setting there is still an obvious step created by the lift-off excursion as the flattened area passes under the probe emphasising the point that whilst true lift-off effects may be cancelled the compensation does not negate all surface contour changes. All channels were calibrated to be equal in this manner and Fig. A3.5 shows the typical signals produced at the various stages of processing which are referred to in A3.3. During further trials, changes were made to the calibration roll as indicated in Fig. A3.6 and the associated responses have also been included. A3.5 Manufacturers of Proprietary Items Industrial geared dc servomotor McLennan Servo Supplies Ltd. Type S60-78E-G09: Doman Road

Camberley Surrey ENGLAND Telephone 0276 26146 Telex 847156

Servo amplifier Address as above Type PM121:

72

FR 80-9 841 7210.GB/802

PHOTOGRAPH OF THE EDDY CURRENT PROBE ARRAY IN A FIG. A3.1 RAISED POSITION ABOVE A 140 mm SQUARE BILLET

73

Lift Off proces­sing

Linéar­iser

Propor­" tional

term

Derivative term

3 mm offset

Lift off gated integral term

Linéar­iser

Propor­tional term

Derivative term

Lift off gated integral term

Σ Servo drive

Σ Servo drive

τι w

oo o I ko

00

BLOCK DIAGRAM OF EDDY CURRENT SERVO SYSTEM FIG. A3. 2 (R2/17)

to

η ro

00

o to

­J Ui

Defect »

Lift Off m-

nffcpf .

y 1

1 Gain

"| compe ι satio

1

1

n­< '

1 j

Photocell ¿*Π <^fj

Photocell x­T~L_

<±J ~

&

d/dt

Band pass filter

Full wave recti­fier

Time delay

STL·

Compar­ator

•

1

ι

Compar­ator

&

Eddy current system fault

Defect output gate

&

m Defect dat

»­ Data valid

BLOCK DIAGRAM OF EDDY CURRENT SIGNAL PROCESSING IN EACH PROBE CHANNEL FIG. A3.3

(R2/18)

τι S)

oo

o I

<Ω

d

ω CO

o to

FR 80­9 841 7210.GB/802

10 d i v . / s

5 V / d i v .

ΛΓ

A ^:

•t JK

Λ V

=W

JLÅJKJL3

t

χ

No mixing of lift off component

Increasing lift off component

Optimum setting determined by trials

Rectified derivative signal

Defect response

Eddy current probe

USE OF A CALIBRATION ROLL TO OPTIMISE EDDY CURRENT RESPONSE

FIG. A3.4 (R2/19)

76

FR 80­9 841 7210.GB/802

Calibration roll speed 250 mm/s Chart speed 5 div/s

A

Lift off signal

Β

Defect signal

C

Combined signal

(A + B)

D

Signal C

clamped to zero

E

dC/dt + full wave

rectification

Λ

I

r

i λ!

4 A

j

Δ

Λ

il

ν

w

-ΛΑ

x̂ivU

Γ -y-ΛΓ

k

1 ■ν

U

JL­V

L

Γ

χ j

A,

il ■

η

iL

vr

w'

,ΛΝ

UÀ­,

η ^ 'J

Λ/

ν .

w

i^AÍ

k/* Λί

2 V/div.

2 V/div.

5 V/div.

5 V/div.

5 V/div.

OPTIMISED EDDY CURRENT SYSTEM RESPONSES USING CALIBRATION ROLL

FIG. A3.5

77

FR 80-9 841 7210.GB/802

Defect - 1 mm deep

Step - 1 mm

Defect - 1 mm deep

Defect signal

Lift-off signal

Mixed signal

~f~ Filtered signal

Rectified signal

EDDY CURRENT WAVEFORMS WITH MODIFIED CALIBRATION ROLL FIG. A3.6 (R2/1205)

78

CDNA10158ENC