University of Arkansas, Fayetteville ScholarWorks@UARK Theses and Dissertations 5-2013 Creep, Fatigue and Creep-Fatigue Interactions in Modified 9% rC - 1% Mo (P91) Steels Valliappa Kalyanasundaram University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/etd Part of the Mechanics of Materials Commons, Structural Engineering Commons, and the Structural Materials Commons Recommended Citation Kalyanasundaram, Valliappa, "Creep, Fatigue and Creep-Fatigue Interactions in Modified 9% Cr - 1% Mo (P91) Steels" (2013). Theses and Dissertations. 692. http://scholarworks.uark.edu/etd/692 This Dissertation is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of ScholarWorks@UARK. For more information, please contact [email protected], [email protected]. CREEP, FATIGUE AND CREEP-FATIGUE INTERACTIONS IN MODIFIED 9% Cr – 1% Mo (P91) STEELS CREEP, FATIGUE AND CREEP-FATIGUE INTERACTIONS IN MODIFIED 9% Cr – 1% Mo (P91) STEELS A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering By Valliappa Kalyanasundaram Madurai Kamaraj University Bachelor of Engineering in Mechanical Engineering, 2004 University of Arkansas Master of Science in Mechanical Engineering, 2008 May 2013 University of Arkansas ABSTRACT Grade P91 steel, from the class of advanced high-chrome ferritic steels, is one of the preferred materials for many elevated temperature structural components. Creep-fatigue (C-F) interactions, along with oxidation, can accelerate the kinetics of damage accumulation and consequently reduce such components’ life. Hence, reliable C-F test data is required for meticulous consideration of C-F interactions and oxidation, which in turn is vital for sound design practices. It is also imperative to develop analytical constitutive models that can simulate and predict material response under various long-term in-service conditions using experimental data from short-term laboratory experiments. Consequently, the major objectives of the proposed research are to characterize the creep, fatigue and C-F behavior of grade P91 steels at 625 C and develop robust constitutive models for simulating/predicting their microstructural response under different loading conditions. This work will utilize experimental data from 16 laboratories worldwide that conducted tests (creep, fatigue and C-F) on grade P91 steel at 625°C in a round-robin (RR) program. Along with 7 creep deformation and rupture tests, 32 pure fatigue and 46 C-F tests from the RR are considered in this work. A phenomenological constitutive model formulated in this work needs just five fitting parameters to simulate/predict the monotonic, pure fatigue and C-F behavior of grade P91 at 625 C. A modified version of an existing constitutive model is also presented for particularly simulating its isothermal creep deformation and rupture behavior. Experimental results indicate that specimen C-F lives, as measured by the 2% load drop criterion, seem to decrease with increasing strain ranges and increasing hold times at 625°C. Metallographic assessment of the tested specimens shows that the damage mode in both pure fatigue and 600 seconds hold time cyclic tests is predominantly transgranular fatigue with some presence of oxidation spikes. The damage mode in 1800 second hold time cyclic tests is an interaction of transgranular fatigue with dominant oxide spikes and creep cavitation. Other experimental results including the statistical analysis and inter- and intra-laboratory variability in the C-F lifetimes are provided in the text. Scatter factor for any of creep, monotonic, pure fatigue and C-F simulations is shown to be at a maximum of ~ 1.3, in comparison to > 5 expected for a RR. Moreover, the microstructural variability between nominally homogeneous specimens can be inherently accounted by the formulated constitutive model. This dissertation is approved for recommendation to the Graduate Council. Dissertation Director: __________________________________________ Ashok Saxena, Ph.D. Dissertation Committee: __________________________________________ Rick J. Couvillion, Ph.D., P.E. __________________________________________ Stuart R. Holdsworth, Ph.D. __________________________________________ Douglas E. Spearot, Ph.D. __________________________________________ Min Zou, Ph.D. DISSERTATION DUPLICATION RELEASE I hereby authorize the University of Arkansas Libraries to duplicate this dissertation when needed for research and/or scholarship. Agreed _______________________________________ Valliappa Kalyanasundaram Refused _______________________________________ Valliappa Kalyanasundaram ACKNOWLEDGMENTS First and foremost, I would like to express my grateful thanks to the Almighty for his eternal blessings in all of my sensible career endeavors and successes. I am really proud to have Mrs. K. Pitchammai and Mr. C. Kalyanasundaram as my Parents, who have been strong pillars of endless love and inspirational support for me at all times. “Thank you” should be an understatement for their utmost generosity, compliant deeds and sacrifices and I will remain everlastingly indebted to them in this birth. I also acknowledge the warmth love extended to me by my sisters, brother-in-laws and to all others in my family. All of my friends and acquaintances have directly or indirectly assisted me in numerous ways during the course of this accomplishment and I thank them all for giving me the moral support and love beyond my imagination. From the bottom of my heart, I am eager to express my heartfelt gratitude and admiration to few wonderful people who have immensely helped me in my academic dissertation journey. In this regard, this work would not have seen the light of the day without the unconditional and unswerving support of my advisor, Dr. Ashok Saxena. His tremendous zeal, creative insight and problem solving skills have always inspired me to greater heights and I cherish this invaluable opportunity to learn from him. My competence as a scientific researcher has also significantly improved under his supervision, for which I am forever thankful. Besides academic themes, I am deeply touched by the love extended to me and my colleagues by him and Mrs. Madhu Saxena during social gatherings for which I am forever grateful. I would also like to sincerely thank my reading committee members, Dr. Rick J. Couvillion, Dr. Stuart R. Holdsworth, Dr. Douglas E. Spearot and Dr. Min Zou for their invaluable time, comments and suggestions during the course of this work. In particular, I take this instance to thank Dr. Holdsworth and Dr. Edoardo Mazza once again for offering me an opportunity to directly work with them for 26 weeks at EMPA, Dübendorf, Switzerland. Much appreciation should be extended to all of my former and current colleagues and peers for their consistent help during the course of this work. In this context, Sau Wee Koh, Jeff Evans, Rahul Rajgarhia and Santosh Narasimhachary at the Mechanical Properties and Research Laboratory (MPRL) deserve a special mention and I greatly enjoyed working with them on various aspects of materials science research. I also thank Rodolfo Montelongo for spending quality time assisting me in my research during his summer research program in 2010. The fruitful discussions I had with my mentors and colleagues at EMPA are also acknowledged. My special thanks go to Dr. Sunder, Girish and other staff at BiSS, India for all their help and support in maintaining the efficient functioning of testing equipments at MPRL. I also sincerely appreciate the boundless support and encouragement of a dedicated team of staff at the University of Arkansas (UA), especially at the Engineering Research Center (ENRC), without whom this endeavor would have been much more tedious. The accomplishments I have achieved during the doctoral program would not have been readily possible without the technical support of Jeff Knox and his quick assistance in short notice during critical times. Technical assistance offered by Jeff Mincy and Jeff Metz, Struers in metallographic specimen preparation and related analysis is also greatly appreciated. Timely administrative assistance offered by Kathy Jones and Kristy Fink at the College of Engineering and all the Mechanical Engineering staff at UA is also acknowledged. Special mention also goes to Mike and June Brosius, Eric denBoer and Genevie Payne for keeping ENRC a lively place to do scientific research. Finally to all those who have assisted me throughout this undertaking and not been explicitly mentioned herein, I sincerely thank each and every one of you! I gratefully acknowledge Electric Power Research Institute, Charlotte, USA for offering the test material investigated in this work. I also thank all the ASTM E2714-09 round-robin participants for contributing most of the experimental test data utilized in this dissertation work. Financial support from the Irma F. and Raymond C. Giffels’ Chair in Engineering and the Sam Walton Fellowship Foundation is also profoundly appreciated. “Families are the compass that guides us. They are the inspiration to reach great heights, and our comfort when we occasionally falter.” Brad Henry Dedicated to my Parents and Family for their boundless support and love TABLE OF CONTENTS Chapter 1: INTRODUCTION ...................................................................................................................