The Post Weld Heat Treatment Response in the Heat Affected Zone of 2.25Cr-1Mo Steel THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By David K. Hodgson Graduate Program in Welding Engineering The Ohio State University 2013 Master‟s Thesis Committee: Professor John Lippold – Advisor Professor Sudarsanam Babu Copyright by David K. Hodgson 2013 Abstract F22 (2.25Cr-1Mo) steel is used in a variety of applications in the oil and gas industry. One of the primary uses is for subsea well equipment where exposure to elevated temperatures and sour service conditions is common. In order to be acceptable for sour service conditions, the National Association of Corrosion Engineers (NACE MR0175) stipulates maximum permissible hardness of 22 HRC (250 VHN). In welded components made of steel such as F22, this is particularly problematic due to the formation of hard martensite in the heat affected zone (HAZ). To meet these requirements a post weld heat treatment (PWHT) is used. In practice, it has been observed that materials with similar compositions and similar welding parameters will temper differently during PWHT. To investigate the cause of this discrepancy, 16 heats of different composition within the specification limits for F22 were investigated. As a basis of comparison, each heat was subjected to an austenization treatment followed by a water quench with the goal of producing a fully hardened martensitic microstructure. When comparing the hardness of these samples with previous research it was found that the heats with higher carbon content were softer than expected. Upon further investigation with optical microscopy, it was found that second phase particles (which are presumed to be alloy carbides) are still ii present in the microstructure. The presence of these carbides (along with observed autotempering) occupies carbon and contributes to a softer microstructure than as predicted from the carbon content alone. To study the temper response, autogenous spot welds were made using parameters designed to replicate the HAZ observed in a typical weld used for cladding with a corrosion resistant layer. After welding, the welds were tempered for a range of Hollomon-Jaffe parameters which was inclusive of the post weld heat treatment (PWHT) used in practice. The average hardness in the HAZ was used to calculate the average tempering rates for each heat. In the as-welded condition, two heats of the same composition displayed radically different behavior in the HAZ, with one being hardest in the coarse grained heat affected zone (CGHAZ) and the other being hardest in the fine grained heat affected zone (FGHAZ). In examination of these regions using scanning electron microscopy, there was a qualitative difference in the size and distribution of precipitates, but resolution using this technique was not sufficient to fully characterize the microstructure. Upon tempering, several different heats exhibited secondary hardening behavior as a result of specific heat treatments. When compared to the HAZ, the tempering response in the fusion zone typically displayed similar tempering response with the exception of the overall hardness being higher and the secondary hardening reactions were greater in the fusion zone. iii To explore the impact of the welding thermal cycle on the variety of carbides found to be present in F22, thermodynamic and kinetic simulations were conducted for the temperature range found throughout the HAZ. These simulations indicated that for carbides of identical size, those typically found later in the precipitation sequence were more stable and dissolved slower. iv To my wife Rachel, your love and support provide constant encouragement to be my best v Acknowledgments I would like to thank my advisor, Dr. John Lippold for providing a tremendous resource in both technical knowledge and professional guidance throughout the research and graduate school process. I‟d also like to thank Dr. Suresh Babu for participating on my examination committee, providing assistance in learning and troubleshooting simulation tools as well as providing a great foundation and introduction for my continued learning of steel metallurgy. Additionally, his efforts as Director of the National Science Foundation supported Center for Integrated Materials Joining for Energy Applications (CIMJSEA) allowed for a great opportunity to interact with industry members. Also, thanks to all of the other faculty and members of the welding engineering department at the Ohio State University for providing a great environment to learn. Special thanks go to Dean Hannam, Feng Lu and John Bartos at Cameron International for supporting this research and supplying valuable industrial experience and knowledge, which proved to be indispensable throughout this project. vi Thank you to Paul Mason and Kaisheng Wu of ThermoCalc for their assistance in developing the appropriate code and methodology to solve modeling challenges used in this research. Thanks to David Tung and Tapasvi Lolla for taking time out of busy schedules to help with SEM work, to Jeff Rodelas, Adam Hope and Ben Sutton and the other members of the Welding and Joining Metallurgy Group for always being willing to help and providing advice when welding research was foreign to me. Also, thank you to all the great friends I‟ve made during my time in Columbus for support. vii Vita August 1984 ...................................................Born - Cleveland, Ohio December 2006 ..............................................B.S. Mechanical Engineering, University of Toledo, Toledo, Ohio March 2011 to Present ...................................Welding and Joining Metallurgy Research Group, The Ohio State University, Columbus, Ohio Fields of Study Major Field: Welding Engineering viii Table of Contents Abstract ............................................................................................................................... ii Acknowledgments.............................................................................................................. vi Vita ................................................................................................................................... viii List of Tables ................................................................................................................... xiii List of Figures .................................................................................................................. xiv Chapter 1: Introduction ....................................................................................................... 1 Chapter 2: Background ....................................................................................................... 9 2.1 Heat Treatment in 2.25Cr-1Mo Steels ...................................................................... 9 2.1.1 Processing of F22 ............................................................................................... 9 2.2 Hardenability ........................................................................................................... 12 2.2.1 Effect of Alloying Additions On Hardenability ............................................... 14 2.2.2 Effect of Prior Austenite Grain Size ................................................................. 17 2.3 Martensite ................................................................................................................ 18 2.3.1 Strengthening Mechanisms in Martensite ........................................................ 18 2.3.2 As Quenched Hardness ..................................................................................... 22 ix