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Solidification Behavior and Hot Cracking Susceptibility of High Manganese Steel Weld Metals

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Solidification Behavior and Hot Cracking Susceptibility of High Manganese Steel Weld Metals Solidification Behavior and Hot Cracking Susceptibility of High Manganese Steel Weld Metals THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Benjamin James Sutton Graduate Program in Welding Engineering The Ohio State University 2013 Master's Examination Committee: Professor John Lippold, Advisor Professor Sudarsanam Suresh Babu Copyright by Benjamin James Sutton 2013 Abstract Recent attention has been given to developing austenitic high-Mn steels for cryogenic service conditions. Specifically, the austenite stabilizing capacities of Mn and C are being exploited to create lower cost alternatives to other cryogenic materials (9Ni steel, Invar, 304 SS, etc.) which are commonly used during the construction of liquefied natural gas (LNG) tanks. The proposed steel alloys contain Mn levels in the range of 20 to 28 wt% and C additions on the order of 0.4 wt%. Although austenite stability is beneficial from a low temperature mechanical property standpoint, the presence of such high concentrations of austenite stabilizing elements causes concern with regard to hot cracking during welding. The solidification cracking susceptibility of a wide range of high-Mn steel weld metal compositions was assessed through cast pin tear (CPT) testing, a small-scale laboratory test which has previously been used to assess solidification cracking in other austenitic materials. A new generation of the CPT test recently developed at The Ohio State University was employed which utilizes electromagnetic levitation casting. The tested composition ranges fell within the following ranges (wt%): Mn (14-34), C (0-0.7), and Al (0-3). Impurity elements were also present in the following ranges (wt%): S (0.005-0.011) and P (0.003-0.026). A total of 12 compositions were tested. It was found that C and P controlled the solidification cracking susceptibility of these alloys. ii In addition to solidification cracking testing, solidification temperature range (STR) analysis was performed on the test matrix using single-sensor differential thermal analysis (SS-DTA) and modified Scheil solidification simulations. STR analysis has previously been related to the weld solidification cracking tendencies of austenitic alloy systems, with large STR values relating to an increased susceptibility to solidification cracking. The measured and calculated STR values in this investigation exhibited a similar relationship. Results indicate that the tested alloys all exhibit primary austenite solidification. It was determined that C and P segregation were primarily responsible for STR expansion. Optical and scanning electron microscopy (OM and SEM) were used to characterize specimens from both CPT testing and SS-DTA analysis. It was found that fractured CPT specimens exhibited dendritic fracture surface morphologies typically associated with solidification cracking. Weld metal samples demonstrated microstructures of martensite (α’or ε) and austenite in varying proportions. Increasing Mn and C contents tended to stabilize austenite, as expected. Solute segregation resulting from weld solidification tended to stabilize austenite along interdendritic regions of alloys containing martensite. Energy-dispersive X-ray spectroscopy (EDS) analysis was utilized to analyze Mn, S, and P segregation in various weld metal samples. It was found that S impurities tended to form MnS inclusions prior to terminal solidification, indicating that the presence of S is less detrimental to solidification cracking in high-Mn steels than other austenitic materials. Evidence of a P-rich eutectic constituent was found along interdendritic regions in the alloy which demonstrated the highest susceptibility to iii solidification cracking. The same alloy showed apparent interdendritic liquation cracking within the heat affected zone (HAZ) immediately adjacent to the fusion boundary (FB) in a welded sample having an as-solidified microstructure prior to welding. In general, it was determined that the solidification cracking susceptibility of the high-Mn steel weld metal compositions explored in this investigation was relatively low. This conclusion was reached based on findings from CPT results that were obtained at the upper threshold of restraint conditions of the test. iv Dedication To my loving parents, James and Rosanne Sutton, who have unquestionably supported me and provided encouragement throughout my life. v Acknowledgments First and foremost I would like to thank my advisor, Professor John Lippold. It was an enjoyable and educational experience working under your guidance. I would also like to thank Professor Suresh Babu for being a part of my examination committee, as well as being a great instructor. I really enjoyed your courses. I thank Dr. Park and Dr. Hong of POSCO for providing materials, funding, and general support on this project. I extend a special thank you to Boian Alexandrov for all of his support and direction since joining the WJMG. I could not have asked for a better mentor in my early research days as an undergraduate student. I extend many thanks to fellow graduate students and WJMG members, past and present. Specifically, I would like to thank Jeff Rodelas, Tapasvi Lolla, and Ryan Smith for sharing their abundant knowledge on all things metallurgy and characterization related. A sincere thank you goes to Tim Luskin for all of his hard work and helpfulness with the levitation casting system. I send a special thank you to Vern Kreuter and Evan O’Brien for all of their help over the past two years. Last but not least, I thank all of my family and friends for their constant love and support. vi Vita October 19, 1987 ............................................Born – Columbus, OH USA June 2011 .......................................................B.S. Welding Engineering The Ohio State University Columbus, OH USA 2011 to present ..............................................Graduate Research Associate The Ohio State University Columbus, OH USA Fields of Study Major Field: Welding Engineering vii Table of Contents Abstract ............................................................................................................................... ii Dedication ........................................................................................................................... v Acknowledgments.............................................................................................................. vi Vita .................................................................................................................................... vii List of Tables ..................................................................................................................... xi List of Figures ................................................................................................................... xii Chapter 1: Introduction and Motivation ............................................................................. 1 Chapter 2: Background ....................................................................................................... 5 2.1 LNG Tank Service Requirements ............................................................................. 5 2.2 High-Mn Steel Metallurgy ........................................................................................ 6 2.2.1 Phase Stability .................................................................................................... 6 2.2.2 Low Temperature Behavior .............................................................................. 10 2.2.3 Other High-Mn Steel Applications ................................................................... 13 2.3 Solidification Cracking ........................................................................................... 16 2.3.1 Weld Solidification ........................................................................................... 18 2.3.2 Solidification Cracking Theory ........................................................................ 24 viii 2.3.3 Weldability Tests for Solidification Cracking .................................................. 30 2.4 Welding of High-Mn Steels .................................................................................... 34 Chapter 3: Objectives ........................................................................................................ 46 Chapter 4: Materials and Experimental Procedures .......................................................... 48 4.1 Materials .................................................................................................................. 48 4.2 Cast Pin Tear Testing .............................................................................................. 52 4.3 Thermal Analysis .................................................................................................... 57 4.4 Solidification Simulations ....................................................................................... 59 4.5 Characterization ...................................................................................................... 61 4.5.1 Optical Microscopy .......................................................................................... 61 4.5.2 Scanning Electron Microscopy ......................................................................... 62 Chapter 5: Cast Pin Tear testing ......................................................................................
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