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The Environmental Effect on Corrosion Fatigue Behavior of Austenitic Stainless Steels

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The Environmental Effect on Corrosion Fatigue Behavior of Austenitic Stainless Steels The Environmental Effect on Corrosion Fatigue Behavior of Austenitic Stainless Steels by Lun Yu B.S. Nuclear Engineering (2013) Shanghai Jiao Tong University SUBMITTED TO THE DEPARTMENT OF NUCLEAR SCIENCE AND ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN NUCLEAR SCIENCE AND ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY September, 2017 © 2017 Massachusetts Institute of Technology. All rights reserved. Signature of Author: _____________________________________________________________ Lun Yu Department of Nuclear Science and Engineering August 25, 2017 Certified by: __________________________________________________________________ Ronald G. Ballinger Professor of Nuclear Science and Engineering, Materials Science and Engineering Thesis Supervisor Certified by: ___________________________________________________________________ Michael P. Short Assistant Professor of Nuclear Science and Engineering Thesis Reader Accepted by: ___________________________________________________________________ Ju Li Battelle Energy Alliance Professor of Nuclear Science and Engineering, Professor of Materials Science and Engineering Chair, Department Committee on Graduate Students This page was left blank intentionally The Environmental Effect on Corrosion Fatigue Behavior of Austenitic Stainless Steels by Lun Yu Submitted to the Department of Nuclear Science and Engineering on September 1, 2017 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Nuclear Science and Engineering ABSTRACT Corrosion fatigue is a multivariate challenge that threatens the lifetime of service of nuclear power plant materials, especially austenitic stainless steels. Both enhancement and retardation of crack growth have been observed in laboratory tests. This thesis work performs high temperature autoclave testing, post-test characterization and mechanistic modeling to understand the corrosion fatigue behavior of austenitic stainless steels in simulated light water reactor (LWR) environments. Crack growth rate (CGR) data were generated from the autoclave testing on low (0.001 wt.%) and high (0.03 wt.%) sulfur content heat 1T compact tension (CT) specimens. Tests were controlled under constant K (22-35 MPa√m) with load ratio of 0.7 and sawtooth waveform (85% rise vs. 15% fall), and at pH =10 and 288 ˚C with system pressure of 9.54 MPa. Crack enhancement was observed in low sulfur content heat specimens, and the CGR increases as the loading rise time increases. The fracture surfaces of low sulfur content heat specimens showed transgranular features with facets (“river pattern”) and few oxide particles. Crack retardation was observed in high sulfur content heat specimens, and the CGR decreases as the loading rise time increases. The fracture surfaces of high sulfur content heat specimens showed distinct features at different rise time step. Transgranular features (“river pattern”) were observed at short rise time step, while non-descript surfaces with fine octahedral oxide particles were observed at long rise time step. Additionally, tests in deuterium water were performed to enable measurements on hydrogen/deuterium concentrations in specimens using ToF-SIMS and hot vacuum extraction techniques. Deuterium pick-up from the testing environment was observed, and the enrichment of deuterium/hydrogen ahead of crack tip was also observed. Controlled experiments were also conducted, where specimens were baked prior to the autoclave testing to remove the residual internal hydrogen. Such heat treatment removing the internal hydrogen was found to not affect the crack growth behavior. Dissolved gases, hydrogen and argon respectively, were bubbled into system during the autoclave tests, and they resulted in similar crack growth behaviors. Modeling indicates that there exists an enhancement mechanism other than corrosion mass removal driving the crack growth in simulated LWR environments. Possibly it comes from the effect of corrosion-generated hydrogen. Retardation behavior and experimental observations could be understood and explained by concept and modeling of corrosion blunting. The results suggest excess conservatism of current ASME standards N-809 for high sulfur content austenitic stainless steels. Thesis Supervisor: Ronald G. Ballinger Title: Professor of Nuclear Science and Engineering, Material Science and Engineering Acknowledgement First and foremost, I would like to express my deepest gratitude towards my thesis advisor, Professor Ronald G. Ballinger. Professor Ballinger’s mentorship and expertise in materials science have helped me immensely throughout my Ph.D. study. He has supported me not only academically but also emotionally through the rough road to finish my Ph.D. degree. I would also like to thank my research sponsor, Naval Nuclear Laboratory, for funding this research project. I am very appreciative to Dr. Denise J. Paraventi and Lindsay B. O’Brien at Naval Nuclear Laboratory. They have been great mentors and an amazing source of support throughout this research project. I would like to express my appreciation to my thesis committee, Professor Ju Li and Professor Michael P. Short. They have given tremendous inputs, feedback and suggestions to my thesis work. Their wisdom, expertise in materials science and engineering, and careful scrutiny of my research work have been instrumental to my success at MIT. It was also an unforgettable and inspirational memory studying with them. A special acknowledgement goes to my lab-mate, Xuejun (Tony) Huang. I was very honored to work with a talented researcher like him. He has generated many excellent inputs into this thesis work and has helped me develop solid understanding of material failure. Besides, I had chances to work with many extraordinary researchers both inside and outside MIT. I am appreciative to Kevin B. Fisher at University of Michigan for his tremendous help in Atom Probe Tomography analyses. I am also appreciative to Dr. Vincent. S. Smentkowski at GE Global Research Center for his considerable assistance in ToF-SIMS measurements. It is my pleasure to work with and learn from those talents. I would also like to thank Pete W. Stahle for his help in autoclave testing and other laboratory work. He was also a proxy for me to understand American culture. Last but not least, I would like to acknowledge my friends and family. It might not be professional to address this here, but they have been instrumental to my success. They have kept inspiring me, dragging me out of the darkness, pushing me out of my comfort zone, and encouraging me to move forward. I would not have completed my Ph.D. study without their unconditional love and support. They are precious gifts given by God to me. I would like to take this chance to express my sincerest gratitude and love to them: Zhuoxuan (Fanny) Li, Rongtao (Anthony) Ma, Cong Su, Akira Sone, Yue (Liam) Shui. I thank everyone I encountered at MIT for sharing this life journey with me. Our path ahead is paved in love. TABLE OF CONTENT CHAPTER 1: INTRODUCTION & BACKGROUND ------------------ 16 1.1 INTRODUCTION -------------------------------------------------------------------- 16 1.2 ENVIRONMENTAL DEGRADATION IN NUCLEAR SYSTEMS --------------------- 17 1.3 EMPIRICAL APPROACH TO CF ---------------------------------------------------- 20 1.4 CRACK GROWTH IN AUSTENITIC STAINLESS STEELS ------------------------- 25 1.4.1 Crack enhancement in austenitic stainless steels ---------------------- 26 1.4.2 Crack retardation in austenitic stainless steels ------------------------ 29 1.5 OVERVIEW OF THE THESIS -------------------------------------------------------- 32 CHAPTER 2: CRACK GROWTH MECHANISM ---------------------- 33 2.1 FATIGUE CRACK GROWTH -------------------------------------------------------- 33 2.2 CRACK ENHANCEMENT MECHANISM -------------------------------------------- 35 2.2.1 Slip dissolution mechanism ----------------------------------------------- 35 2.2.2 Hydrogen based mechanism ---------------------------------------------- 37 2.3 CRACK RETARDATION MECHANISM --------------------------------------------- 40 2.3.1 Oxide induced closure mechanism --------------------------------------- 40 2.3.2 Creep induced retardation mechanism ---------------------------------- 42 2.4 SUMMARY REMARKS -------------------------------------------------------------- 44 CHAPTER 3: EXPERIMENTAL PROGRAM --------------------------- 45 3.1 OVERALL PROGRAM DESCRIPTION ---------------------------------------------- 45 3.2 MATERIALS AND TEST SYSTEM -------------------------------------------------- 45 3.2.1 Materials --------------------------------------------------------------------- 45 3.2.2 Test system ------------------------------------------------------------------- 47 3.3 TEST CONDITIONS ----------------------------------------------------------------- 50 3.4 POST-TEST CHARACTERIZATION ------------------------------------------------- 52 3.5 TESTS IN D2O WATER ------------------------------------------------------------- 56 3.5.1 Motivation ------------------------------------------------------------------- 56 3.5.2 Test procedures ------------------------------------------------------------- 58 3.5.3 Hydrogen/deuterium loss -------------------------------------------------- 59 3.6 CONTROLLED EXPERIMENT ------------------------------------------------------ 60 3.6.1 Motivation ------------------------------------------------------------------- 60 3.6.2 Test procedure --------------------------------------------------------------
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