High temperature deformation and fracture assessment of similar steel welds [Elektronische Ressource] / vorgelegt von Ümit Ceyhan

Informations

• Publié par: technische_universitat_clausthal
• Publié le: 01 janvier 2007
• Nombre de lectures: 42
• Langue: English
• Poids de l'ouvrage: 15 Mo

Extrait

High Temperature Deformation and Fracture Assessment of Similar Steel Welds Dissertation zur Erlangung des Grades eines Doktors der Ingenieurwissenschaften vorgelegt von Dipl.-Ing. Ümit Ceyhan aus Ankara, Türkei genehmigt von der Fakultät für Natur- und Materialwissenschaften der Technischen Universität Clausthal Tag der mündlichen Prüfung 14.12.2006

Die vorliegende Arbeit wurde am Institut für Werkstoffforschung des GKSS- Forschungszentrums, Geesthacht und am Institut für Werkstoffkunde und Werktofftechnik der Technischen Universität Clausthal durchgeführt. Vorsitzender der Promotionskommission Prof. Dr. Wolfgang Schade Hauptberichterstatter Prof. Dr. Bilal Dogan (GKSS) Berichterstatter Prof. Dr. Lothar Wagner Berichterstatter Prof. Dr. Volker Wesling

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Acknowledgment I would like to express my sincere gratitude to Prof. B. Dogan for giving me the opportunity to conduct this work in GKSS Research Centre, Geesthacht, Germany, and for his invaluable guidance, support and supervision of this work. I am also grateful to Prof. L. Wagner for his interest in this work and for taking over the co-referee of the thesis in Technical University of Clausthal, Germany. I wish to thank to Prof. K.-H. Schwalbe for his support and invaluable discussions during the conduction of this work. I am also thankful to Dr. B. Petrovski and Prof. X.Zheng for their constructive comments and guidance during the experimental and metallographic studies. I also would like to thank to partners of the European Project CRETE and VAMAS TWA31 for fruitful discussions which have contributed a lot to the present work. Thanks are due to A.C. Cruz from ISQ, Portugal, and A. Chowdhury from ERA, UK, for providing the test material. I wish to thank to my colleagues in Materials Research Institute of GKSS Research Centre for their technical support, Ms. P. Fischer in metallography, Mr. K. Erdmann, Mr. M. Horstmann, Mr. Tek in materials testing. Finally, I wish to express my deepest gratitude to my lovely wife Zeynep and my newly born daughter Elif who gave me the strength during the entire time of conduction of this work have made my life rewarding. This work was carried out at the GKSS Research Centre, Geesthacht, Germany between 2002 and 2006. Ümit Ceyhan th Geesthacht, June, 20 2006

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Summary The structural integrity assessment of welded components is of industrial interest which requires well characterised material behaviour and data obtained at service temperatures. The functionally graded microstructure of weldments in weldment zones of base metal (BM), heat affected zone (HAZ) and weld metal (WM) affects the stress and strain distribution at the tip of a crack in a component made of these materials. The ferritic P22 and bainitic-martensitic P91 steels similar welds were chosen to study the deformation and fracture behaviour in high temperature tensile and fracture mechanics tests at 550 °C and 600 °C, respectively. Micromechanisms of deformation were studied on microtensile tests on both materials, which served as basis for fracture mechanical studies of crack initiation and growth. Creep crack initiation and creep crack growth studies were carried out on specimen geometries of C(T), CS(T), SEN(B) and RNB(T) which are of industrial importance due to the similarity of loading to components. The data obtained from high temperature crack initiation and growth tests were analysed following the recently drafted Code of Practice (CoP). The data were used for two most widely used defect assessment methods, the British TDFAD of R5 and German 2CD methods. The assessment methods, developed originally for base metals, predict well the failure in weldments of the studied materials. The prospects of applicability of the assessment methods and weaknesses are reported. Sensitivity analyses were carried out to study the effects of input parameter variations on the assessment method and life predictions. Both deterministic and probabilistic sensitivity analyses were applied to determine the extent of possible error in defect assessment. The present work has contributed to the recently drafted CoP for high temperature testing and analysis of weldments, as well as the European assessment methods, extending their use into assessment of weldments. Zusammenfassung Die Bewertung von Bauteilen industrieller Anlagen auf Sicherheit und strukturelle Integrität setzt Materialkennwerte sowie Bruchmechanikdaten bei Einsatztemperaturen voraus. In dieser Arbeit wurden Daten an Grundmaterial (BM), Wärmebeinflusszone (HAZ) und Schweissgut (WM) von artgleichen Schweissverbindungen von ferritischem Stahl P22 bei 550°C sowie martensitischem Stahl P91 bei 600°C ermittelt. Die Gefüge und Abmessung der Schweissgutzone beeinflussen die Spannungs- und Dehnungsfelder an der Rissspitze eines Bauteils. Mikroskopische Untersuchungen zum Verformungsverhalten und Bruchmechanismen wurden an Zug- und Bruchmechanikversuchsproben durchgeführt mit dem Ziel die Wechselwirkung zwischen dem Gefüge und der ermittelten Materialdaten zu korrelieren. Dabei eignet sich insbesondere das neu-entwickelte Mikrozugversuchsverfahren zur Untersuchung der Vervormungs - und Bruchmechanismen in der schmalen Schweissmaterialzone. Die Hochtemperaturbruchmechanikversuche wurden an Bruchmechanikproben industrieller Bedeutung ( C(T), CS(T), SEN(B) und RNB(T)) durchgeführt. Die ermittelten Daten wurden durch Anwendung der europäischen Fehlerbewertungsmethoden, der britischen TDFAD von R5 sowie der deutschen 2CD bewertet. Die beiden Methoden, die zur Bewertung von Bauteilen aus Grundmaterial entwickelt worden sind, können zur Bewertung von Bauteilen mit Schweissverbindungen angewandt werden. Sensitivitätanalysen der Änderung der Eingangsparameter durch Anwendung der deterministischen und probabilistischen Methode wurden durchgeführt.

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CONTENTS 1.Introduction........................................................................................................................1 2.State of the Art....................................................................................................................3 2.1.Fracture Mechanics Concepts ...................................................................................... 3 2.1.1.Linear Elastic Fracture Mechanics Concepts................................................... 3 2.1.1.1.Stress Function Methods .................................................................... 3 2.1.2.Small Scale Yielding........................................................................................ 5 2.1.3.Elastic Plastic Fracture Mechanics .................................................................. 6 2.1.3.1.J Integral ............................................................................................. 6 2.1.4.Time Dependent Fracture Mechanics .............................................................. 9 2.1.4.1.Creep Crack Growth ........................................................................... 9 * 2.1.4.2.11C Integral......................................................................................... 2.1.4.3.Short Time Versus Long Time Behaviour ....................................... 13 2.1.4.4.NSW Creep Crack Growth Model.................................................... 15 2.1.4.5.Creep Crack Initiation ...................................................................... 15 2.2.High Temperature Defect Assessment of Weldments ............................................... 18 2.2.1.18Introduction .................................................................................................... 2.2.2.Creep Failure in Plant Weld Components...................................................... 19 2.2.3....................................................... 21Choice of Fracture Mechanics Parameter 2.2.4.Design and Assessment Procedures ............................................................... 22 2.2.4.1.............................................................................. 23The R5 Procedure 2.2.4.2........................................................... 23The British Standard BS 7910 2.2.4.3.The French Design Code RCC-MR – A16....................................... 24 2.2.4.4................................................................... 25The ASME III Approach 2.3.Creep Crack Initiation: Investigation and Assessment .............................................. 26 2.3.1.26Introduction .................................................................................................... 2.3.2.Determination of Creep Crack Initiation Parameters..................................... 27 2.3.2.1.Definition of creep crack initiation................................................... 27 2.3.2.2.Stress Intensity Factor K................................................................... 27 * 2.3.2.3.27C Integral......................................................................................... 2.3.2.4.Crack Tip Opening Displacement (CTOD)...................................... 28 c 2.3.2.5.K ........................................... Creep Crack Initiation Toughness 30 mat 2.3.3.Assessment Methods for Creep Crack Initiation ........................................... 31 2.3.3.1.Time Dependent Failure Assessment Diagram Method................... 31 2.3.3.2.Two-Criteria-Diagram Method......................................................... 34 2.4.Scatter Analysis of High Temperature Experimental Data........................................ 37 2.4.1.Introduction .................................................................................................... 37 2.4.2.Scatter in High Temperature Crack Growth Data.......................................... 38 2.4.2.1.Scatter due to Test Equipment.......................................................... 38 2.4.2.2.Scatter due to Testing Procedures .................................................... 39 2.4.2.3.Scatter Introduced During Assessment of Data................................ 39 2.4.3................................................ 40Presentation of Scatter in Crack Growth Data 2.4.4............................. 41Deterministic Sensitivity Analysis of Crack Growth Data 2.4.5.Probabilistic Sensitivity Analysis of Crack Growth Data.............................. 43 2.4.5.1.Monte Carlo Simulation ................................................................... 44 3.Materials...........................................................................................................................45 3.1.Introduction ................................................................................................................ 45

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3.2.2.25Cr1Mo Steel (P22) .............................................................................................. 47 3.3.Mod - 9Cr1Mo Steel (P91) ........................................................................................ 48 4.Experimental Procedure..................................................................................................51 4.1.Welding and Heat Treatment of Test Materials ......................................................... 51 4.2.Microtensile Tests ...................................................................................................... 52 4.3............................................................. 54High Temperature Fracture Mechanics Tests 4.3.1.Test Specimens .............................................................................................. 54 4.3.2.Testing Equipment ......................................................................................... 55 4.3.3.Specimen Preparation .................................................................................... 56 4.3.4.Crack Length Monitoring............................................................................... 58 4.3.5.Test Procedure................................................................................................ 59 4.3.6............................ 62Post Test Measurements and Metallographic Examination 4.3.7.Determination of Creep Crack Growth Correlation Parameters .................... 63 4.3.7.1.Stress Intensity Factor, K.................................................................. 63 * 4.3.7.2.63C Integral......................................................................................... 5.Results...............................................................................................................................66 5.1.Damage and Fracture in Microtensile Tests .............................................................. 66 5.1.1.Microtensile Test Results............................................................................... 66 5.1.1.1..................................................................... 66P22 Similar Weldments 5.1.1.2.P91 Similar Weldments .................................................................... 68 5.1.2.Metallography ................................................................................................ 72 5.1.2.1.P22 Similar Weldments .................................................................... 72 5.1.2.2.P91 Similar Weldments .................................................................... 75 5.1.2.3..................................................... 77Post-Test Micro-hardness Testing 5.2.High Temperature Fracture Mechanics Test Results ................................................. 78 5.2.1.Introduction .................................................................................................... 78 5.2.2.Assessment of High Temperature Crack Growth Data – An Example.......... 78 5.2.3..................................................................................................... 85Test Results 5.2.4.Creep Crack Initiation Results ....................................................................... 87 5.2.4.1.Creep Crack Initiation Behaviour of P22 Steel Weldment............... 89 5.2.4.2.Creep Crack Initiation Behaviour of P91 Steel Weldment............... 92 5.2.5.Creep Crack Initiation and Growth Tests Data Analyses .............................. 95 5.2.5.1.................. 96Creep Crack Growth Behaviour of P22 Steel Weldment 5.2.5.2................ 105Creep Crack Growth Behaviour of P91 Steel Weldment 5.2.6.Creep Master Curve Concept Applied to CCG Data  of P22 and P91 Weldments .......................................................................... 113 5.2.7................................................. 115Failure Assessment Using TDFAD Method 5.2.7.1.Application of the TDFAD to P22 Steel Weldment....................... 115 5.2.7.2.Application of the TDFAD to P91 Steel Weldment....................... 118 5.2.8....................................................... 119Failure Assessment Using 2CD Method 5.2.8.1.Application of the 2CD to P22 Steel Weldment............................. 120 5.2.8.2.Application of the 2CD to P91 Steel Weldment............................. 121 5.3.Sensitivity Analysis of Crack Growth Data ............................................................. 123 5.3.1.Introduction .................................................................................................. 123 5.3.2.Deterministic Sensitivity Analysis............................................................... 124 5.3.2.1.Deterministic Sensitivity Analysis of Effect of Material  Properties on Crack Growth Rate Correlations.............................. 124 5.3.2.2.Deterministic Sensitivity Analysis of Effect of Geometrical  Factors on Crack Growth Rate Correlations .................................. 125

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5.3.3.Probabilistic Sensitivity Analysis ................................................................ 126 5.3.3.1.Probabilistic Sensitivity Analysis of Crack Growth Rate  Correlations .................................................................................... 126 5.3.3.2.Probabilistic Sensitivity Analysis of Estimation of Crack  Initiation Time with TDFAD ......................................................... 129 5.4.Metallographic Studies ............................................................................................ 130 5.4.1.Deformation and Micromechanics of Cracks in P22 and P91 Steels .......... 130 5.4.2.................................. 136Metallography of Crack Initiation and Crack Growth 6.Discussion........................................................................................................................139 6.1.Damage and Fracture Analysis by Microtensile Tests............................................. 139 6.2.High Temperature Fracture Behaviour .................................................................... 140 6.2.1.High Temperature Fracture Mechanics Testing........................................... 140 6.2.2.................................. 141Determination of Displacement Rates and CCG Rate 6.2.3.Test Results .................................................................................................. 143 6.3.Metallographic Studies ............................................................................................ 145 6.4.Failure Assessment Using Defect Assessment Methods ......................................... 146 6.5.Sensitivity Analysis.................................................................................................. 147 7.Conclusions.....................................................................................................................148 References.............................................................................................................................151 Nomenclature........................................................................................................................158 Appendix A...........................................................................................................................162 Appendix B............................................................................................................................169 Appendix C...........................................................................................................................182 Lebenslauf.............................................................................................................................186

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Chapter 1 – Introduction

1. INTRODUCTION Failures of high temperature components in power generation and petrochemical plants account a substantial portion of the direct costs of operation along with environmental damage and even costing human lives. Historically, the manufacturer’s experience has been the basis for the safe design of critical engineering components. However, in recent times crack initiation, growth and failure analyses based on the assumption of the existence of defects in the structure have become more accepted for design and as remaining life prediction methodology. The operational and plant assessment experience indicates that in the majority of cases where failure occurs in components, defects predominate in the vicinity of weldments. Therefore, time dependent failure at high temperatures by creep crack initiation (CCI) and creep crack growth (CCG) in structural joints imposes a limit on component service life in plants. Although the concepts used for time dependent fracture analysis of homogeneous bodies are commonly applied for defect assessment of weldments, complex structure of weldments having various weldment zones which exhibit particular interactions, requires novel aspects in testing and defect assessment. Recent collaborative efforts of European Creep Collaborative Committee (ECCC) [1] and Creep Group of European Thematic Network FITNET [2] indicate the need for novel methods in defect assessment of weldments and harmonise the existing know-how in the industry and academia for a unified defect assessment method for weldments.

The most widely used standard for creep crack growth testing of metallic materials, ASTM E 1457-00 [3], is mainly addressing testing homogeneous materials in compact tension, C(T), type specimens. However, complexity of stress conditions due to geometrical factors and heterogeneous microstructure in welded industrial components requires harmonised testing and assessment methods. Therefore, the outstanding need for high temperature characterisation of CCI and CCG behaviour of weldments in alternative industrial type specimens has been the subject of collaborative efforts of ESIS TC11 [4] and European project CRETE [5], which had the objective of harmonising testing procedures in order to obtain data for use in defect assessment of weldments. A European Code of Practice (CoP) [6] has become the output of these efforts which provides guidelines for specimen selection, testing and data analysis for weldments that include novel aspects such as testing of industrial type specimen geometries for creep crack initiation and growth testing.

The main purpose of this study is to contribute to the current knowledge and to developing methodology in high temperature defect assessment of weldments. This includes improvement of current testing methods by introducing novel aspects of industrial specimens and the peculiarities of weldments. It is commonly accepted that improvement of the reliability of defect assessment procedures requires profound testing and data assessment methods in which microstructural and geometrical aspects are of primary importance. Therefore, present work aims at contributing to improve a) testing methods, b) data assessment methods, c) understanding of damage and fracture (CCI and CCG) behaviour, d) utilisation of test data in defect assessment methods, e) treatment of data scatter in defect assessment of weldments at high temperatures.

In this thesis, high temperature damage and fracture behaviour of similar weldments of most commonly used low alloy ferritic 2.25CrMo (P22) and newly developed high strength martensitic Mod-9CrMo (P91) steels are characterised at 550°C and 600°C, respectively. Firstly, microtensile (MT) tests are conducted on specimens machined out of different weldment zones of P22 and P91 steels. MT tests facilitate the determination of local tensile and damage properties of each weldment zone without any constraint effect of adjacent zones. The finding of MT test aids understanding local material behaviour and deformation

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