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Enhancing the Corrosion Resistance of Api 5L X70 Pipeline Steel Through Thermomechanically Controlled Processing

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Enhancing the Corrosion Resistance of Api 5L X70 Pipeline Steel Through Thermomechanically Controlled Processing ENHANCING THE CORROSION RESISTANCE OF API 5L X70 PIPELINE STEEL THROUGH THERMOMECHANICALLY CONTROLLED PROCESSING A Thesis Submitted to the College of Graduate and Postdoctoral Studies In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy In the Department of Mechanical Engineering University of Saskatchewan Saskatoon By Enyinnaya George Ohaeri © Copyright Enyinnaya George Ohaeri, April 2020. All rights reserved. PERMISSION TO USE In presenting this thesis, in partial fulfillment of the requirements for a degree of Doctor of Philosophy from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for copying this thesis in any manner, in whole or in part, for scholarly purposes may be granted by Professor Jerzy Szpunar, who supervised my thesis work or, in his absence, by the Head of the Department or the Dean of the College in which my thesis work was done. It is understood that any copying or publication or use of this thesis, or parts thereof, for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made of any material in my thesis. Requests for permission to copy or to make other use of material in this thesis in whole or in part should be addressed to: Head of the Department of Mechanical Engineering University of Saskatchewan 57 Campus Drive Saskatoon, Saskatchewan S7N 5A9 Canada OR Dean College of Graduate and Postdoctoral Studies University of Saskatchewan 116 Thorvaldson Building, 110 Science Place Saskatoon, Saskatchewan S7N 5C9 Canada i ABSTRACT Pipelines are widely used for transportation of oil and gas because they can carry large volume of these products at lower cost compared to rail cars and trucks. However, they are prone to environmentally assisted degradation. Different methods of optimizing pipeline steels such as micro-alloying, desulfurization, microstructure design, and inclusion morphology control have been used to improve their performance in different service environments. This study presents the use microstructure and texture control to improve the reliability of API 5L X70 pipeline steel. The properties of API 5L X70 steel was manipulated through thermomechanical processing. Pipeline steel plates were produced using different processing schedules. The final microstructure and texture of processed steels were determined. Also, electrochemical corrosion studies were performed on selected steels in hydrogen producing and non-hydrogen producing electrolyte solutions. In addition, samples from each steel was investigated for hydrogen induced cracking (HIC) with and without the application of tensile stress. Further annealing heat treatments were conducted on the steel with improved HIC behavior before assessing hydrogen embrittlement characteristics. All the tests were performed at room temperature. Generally, the lowest corrosion rate was measured in the hydrogen producing electrolyte due to the rapid formation of a corrosion protective film on the pipeline substrate. It was found that variations in the processing conditions affects corrosion and cracking behavior of steels. The least corrosion resistant steels experienced more intense surface deterioration after polarization. Subsequently, such steels were damaged by hydrogen attack. The steel with improved corrosion resistance displayed no visible cracking after probing in corrosive mediums and charging with hydrogen. Despite weak texture noticed in all the steels, (111) crystal planes showed better electrochemical corrosion resistance compared to (110) and (100). Moreover, microstructural features such as non-metallic inclusions/precipitates, grain characteristics, phase distribution, and local average misorientations were analyzed in relation crack initiation and propagation. Evidence suggests that cracking occurred mainly through deformed regions in the mid-thickness section. Early onset of crack formations was characterized by stepwise discontinuities. It was demonstrated that during in-situ tensile deformation and hydrogen charging, sudden failure takes place in the elastic zone. The obtained results indicate that cracks propagated through segregations of iron ii carbide around high angle grain boundaries. Also, multi-component inclusion particles comprising of angular (Ti-Nb) N precipitates, spherical Al-Mg-Ca oxides, and some traces of Mo-Mn-S influenced susceptibility to HIC. It was observed that hydrogen embrittlement susceptibility was lowered after annealing in two consecutive cycles compared to single annealing treatment. The double step heat treatment resulted in a dual-phase (ferrite and tempered martensite) microstructure with greater ductility than in original steels. Finally, the pipeline steel that was hot rolled (880 – 820 °C) and rapidly cooled (755 – 615 °C) at the rate of 25 °C/s has shown improved resistance to corrosion. iii ACKNOWLEDGEMENTS My sincere appreciation goes to the University of Saskatchewan for this great opportunity. I am endlessly grateful to Prof. Jerzy A. Szpunar for his astounding supervision. Your guidance, understanding, and fatherly advice during my studies remained resolute. I always marvel at the amount of confidence you have in my abilities. You considered me worthy of an opportunity many would have denied me. Your good will towards me has made me perpetually thankful to you. The words of advice from Prof. Ikechukwuka N. Oguocha and Prof. Akindele G. Odeshi contributed greatly towards the success of my studies. I would never forget other members of my advisory committee, Prof. Duncan Cree and Prof. Richard Evitts. I would not have been where I am today without the support from each of you. The generosity of Natural Sciences and Engineering Research Council of Canada (NSERC Strategic Grant 470033) towards funding this project is recognized. Special thanks to the University of Saskatchewan for offering me the devolved scholarship. I also appreciate the support of EVRAZ North America, CANMET Natural Resources, Ontario, Canada, and the Saskatchewan Structural Sciences Center (SSSC). The comradeship shown to me by all members of the Advanced Materials and Renewable Energy (AMRE) research group at the University of Saskatchewan will never be forgotten. My exceptional thanks go to Dr. Ubong Eduok, and Dr. Ahmed Tiamiyu for their helpful suggestions and immense encouragement. Many thanks to Garry & Eleanor Mckay, and Gerry & Shirley Falk for always being there to pray for me. To my family and friends, I cannot acknowledge you enough for patiently standing by me throughout this period. It would have been impossible to attain this height without the early teachings of my late father Mr. Ajaraonye C. Ohaeri. You planted a tree, but never sat under its shade. Instead, you left an indelible mark for humanity. My eternal respect goes to my mother Mrs Ngozi V. Ohaeri and all my siblings for always believing in me. I must say you all are the best family anyone can have. I thank you all for understanding and tolerating me through these years. I cannot thank Godswill Enyinnaya-Ohaeri enough for ensuring that my sanity remained intact during this program. Finally, I am nothing without the grace of the almighty God in whom I trust. iv DEDICATION To many who had lofty dreams, but never got the opportunity to actualize them. v TABLE OF CONTENTS PERMISSION TO USE ................................................................................................................. i ABSTRACT ................................................................................................................................... ii ACKNOWLEDGEMENTS ........................................................................................................ iv DEDICATION............................................................................................................................... v LIST OF TABLES ..................................................................................................................... xiii LIST OF FIGURES .................................................................................................................... xv LIST OF ABBREVIATIONS ................................................................................................. xxiii CHAPTER 1 : INTRODUCTION ............................................................................................... 1 1.1 Overview .......................................................................................................................... 1 1.2 Background ...................................................................................................................... 1 1.3 Historical perspective on pipeline steel development ...................................................... 3 1.4 Thermomechanically controlled processing (i.e. hot rolling) .......................................... 5 1.5 Motivation ........................................................................................................................ 9 1.6 Research objectives ........................................................................................................ 10 1.7 Research contributions ................................................................................................... 11 1.8 Thesis outline ................................................................................................................
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