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EFFECTS OF NON-METALLIC INCLUSIONS ON FATIGUE lib.tkk.fi/Diss/2004/isbn951227423X/  · PDF fileEffects of Non-metallic Inclusions on Fatigue Properties of Calcium Treated Steels

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  • Helsinki University of TechnologyDepartment of Mechanical Engineering

    Laboratory of Engineering Materials

    EFFECTS OF NON-METALLIC INCLUSIONSON

    FATIGUE PROPERTIESOF

    CALCIUM TREATED STEELS

    Pekko Juvonen

    Dissertation for the degree of Doctor of Science in Technology to be presented with duepermission for public examination and debate in Micronova (Large seminar room) atHelsinki University of Technology (Espoo, Finland) on the 10th of December, 2004, at12 oclock noon.

    Espoo 2004

  • Distribution:

    Helsinki University of Technology

    Laboratory of Engineering Materials

    P.O.Box 4200

    FIN-02015 HUT

    ISBN 951-22-7422-1 (print)

    ISBN 951-22-7423-X (pdf, available at http://lib.hut.fi/Diss/2004/isbn951227423X)

    ISSN 1456-3576

    Otamedia Oy

    Espoo 2004

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    JUVONEN, Pekko. Effects of Non-metallic Inclusions on Fatigue Properties ofCalcium Treated Steels. Espoo 2004, Helsinki University of Technology.

    Keywords: fatigue strength, nonmetallic inclusions, calcium treated steel, statisticalanalysis, statistics of extremes, ultrasonic testing, machinability

    ABSTRACT

    Fatigue behaviour of 22 industrial test charges of AISI 8620 carburizing steel withtwo different calcium treatment levels was studied. The research work consistedmainly of rotating bending fatigue tests, residual stress and surface roughnessmeasurements, electron microscopy, different steel cleanliness level and statisticalinclusion size estimation methods.

    There were no significant differences between the w/Rm ratios of the casts with thelarge amount of calcium injection and the casts with the small amount of calciuminjection. In the casts with the large amount of calcium injection, the fatigue cracksinitiated mostly from the surface and interior inclusions. In the casts with the smallamount of calcium injection, the fatigue cracks initiated mostly from the surfacediscontinuities. The inclusions responsible for fatigue crack initiation were in themost cases calcium aluminates encapsulated in calcium sulfide containing smallamounts of magnesia and/or silica. The fatigue crack initiation from cracked and non-cracked inclusions resulted in similar fatigue life on the same K level. The fatiguestrength scatter was larger in the casts with the large amount of calcium injection. Inrotating bending fatigue the w/Rm ratio was almost independent of inclusion size inthe average fatigue crack initiating inclusion size region smaller than 70-90 m.

    The results of DIN 50 602 and SFS-ENV 10247 inclusion rating methods andultrasonic tests in immersion did not correlate with the inclusions that wereresponsible for fatigue failure in these steels. The results may, however, suggestguidelines for the fatigue properties and the machinability of these steels when thecontents of certain alloying elements are taken into account. Ultrasonic tests inimmersion provide more relevant information about the fatigue properties andmachinability of these steels than the conventional inclusion rating methods do, butits resolution capability still needs improvement.

    In most casts the maximum inclusion sizes predicted by the statistics of extremevalue method were much smaller than the size of the inclusions found at the fatiguecrack initiation sites of the fatigue specimens. The studied steels seemed to have twodifferent inclusion size distributions, i.e., the inclusions detected at the polishedmicrosections and the inclusions at the fatigue crack initiation sites. Bothdistributions had similar morphology and chemical composition, which was contraryto the earlier findings of the bilinear nature of inclusion distribution in some steels.The successful application of the Murakami-Endo model with these steels requiresquite a large inspection area, approximately 8400 mm2 at least, to enable thedetection of the population of the largest inclusions, which are responsible for fatiguefailure.

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    PREFACE

    The research work presented in this thesis was accomplished during The GraduateSchool of Metallurgy financed by the Finnish Academy. The financial support ofImatra Steel Oy Ab, Research Foundation of Helsinki University of Technologyand the Walter Ahlstrm Foundation are also greatfully acknowledged.

    I wish to express my gratitude to the supervisor of my thesis, professor HannuHnninen, for his patient guidance and advice. Dr. Tech. Vesa Ollilainen fromImatra Steel Oy Ab I would like to thank for providing me motivation to finish thisthesis. I would also like to thank Mr. Ari Anonen from Imatra Steel Oy Ab forproviding me necessary information concerning the experimental part of this thesisand Mrs. Lotta Ruottinen from Imatra Steel Oy Ab for the ultrasonic testing inimmersion. I am also grateful for everyone at Imatra steel plant whose workcontribution assisted me in this thesis, as well as for the whole personnel of theLaboratory of Engineering Materials for offering help in this work when needed.

    I especially want to thank my dear Anissa for her patience and support.

    Otaniemi, February 2004

    Pekko Juvonen

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    CONTENTS

    ABSTRACT 2

    PREFACE 3

    CONTENTS 4

    LIST OF SYMBOLS 6

    ORIGINAL FEATURES 9

    1 INTRODUCTION ......................................................................................... 101.1 Inclusions in steels .................................................................................... 101.2 Calcium treated steels ............................................................................... 111.3 Steel cleanliness level estimation methods ............................................... 121.4 Inclusion properties affecting fatigue properties ...................................... 121.5 Fatigue crack initiation and crack growth................................................. 171.6 Stress concentration and fatigue notch effect ........................................... 191.7 Non-propagating cracks ............................................................................ 241.8 Small crack growth ................................................................................... 261.9 Size effect.................................................................................................. 291.10 Models describing the effects of defects on fatigue strength.................... 301.11 Murakami-Endo model ............................................................................. 34

    2 AIMS OF THE STUDY ................................................................................ 393 EXPERIMENTAL METHODS ................................................................... 40

    3.1 Materials.................................................................................................... 403.2 Fatigue tests............................................................................................... 423.3 Residual stress measurements................................................................... 443.4 Surface roughness measurements ............................................................. 453.5 Inclusion analyses ..................................................................................... 463.6 Ultrasonic testing in immersion ................................................................ 473.7 SEM and EDS investigations.................................................................... 473.8 Statistics of extreme value evaluation and application of the

    Murakami-Endo model ............................................................................. 484 RESULTS ....................................................................................................... 49

    4.1 Fatigue tests............................................................................................... 494.2 Residual stress measurements................................................................... 504.3 Surface roughness measurements ............................................................. 514.4 Inclusion analyses ..................................................................................... 534.5 Ultrasonic testing in immersion ................................................................ 544.6 SEM and EDS investigations.................................................................... 564.7 Statistics of extreme value evaluation and application of the

    Murakami-Endo model ............................................................................. 624.8 Multiple linear regression ......................................................................... 67

    5 DISCUSSION................................................................................................. 705.1 Fatigue tests............................................................................................... 70

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    5.2 Residual stress measurements................................................................... 725.3 Surface roughness measurements ............................................................. 735.4 Inclusion analyses ..................................................................................... 735.5 Ultrasonic testing in immersion ................................................................ 745.6 SEM and EDS investigations.................................................................... 765.7 Statistics of extreme value evaluation and application of the

    Murakami-Endo model ............................................................................. 816 CONCLUSIONS............................................................................................ 82

    REFERENCES 84

    APPENDIX 1. Fatigue test graphs 91

    APPENDIX 2. Fatigue crack initiating inclusion data