Factors Affecting the Inclusion Potency for Acicular Ferrite Nucleation in High-Strength Steel Welds
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IN recent years, development of steel plates with excellent combinations of strength and toughness has accelerated; these plates are used in pipelines, buildings, and bridges, as well as in shipbuilding and offshore constructions. To support the use of such high-performance steel, its weld metal must be mechanically comparable to the base steel. However, as the weld metal is made stronger, it tends to become less tough and more susceptible to cold cracking. Acicular ferrite microstructure, which is the product of intragranular nucleation on inclusions, has been reported to improve resistance to cold cracking[1] and to be beneficial for achieving high toughness[2–4] by forming an interlocking structure that reduces the effective grain size. It was also reported that a significant decrease in the ductile–brittle transition temperature of highstrength weld metal with tensile strength above 1 GPa can be achieved by interspersing acicular ferrite of about 5 vol pct in the matrix.[5] Thus, the development of acicular ferrite is recognized as a key technology to enhance the toughness of high-strength weld metal and thereby to ensure the stability of final welded structures. However, considering transformation kinetics, the formation of acicular ferrite becomes less likely with YONGJOON KANG and SEONGHOON JEONG, Ph.D. Candidates, and CHANGHEE LEE, Professor, are with the Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea. Contact e-mail: [email protected] JOO-HEE KANG, Senior Researcher, is with the Materials Modeling and Characterization Department, Korea Institute of Materials Science, Changwon 51508, Republic of Korea. Manuscript submitted October 12, 2015. Article published online March 23, 2016 2842—VOLUME 47A, JUNE 2016
decreasing transformation temperature in more highly alloyed weld metals due to the prevailing formation of low-temperature transformation products such as bainite and martensite.[3,6,7] Therefore, especially in high-strength weld metals, there is a clear need for potent inclusions that reduce the activation energy barrier for intragranular nucleation of acicular ferrite. Many researchers have demonstrated that several inclusion phases can act as effective nucleation sites for acicular ferrite, and have proposed several mechanisms to reduce the activation energy for nucleation of acicular ferrite: (1) reducing lattice misfit between the inclusion and the ferrite to ensure their low interfacial energy,[3,4,8–14] (2) increasing the chemical driving force arising from local depletion of Mn near the inclusion,[3,4,15–22] and (3) increasing the elastic strain energy around the inclusion arising from the difference in the thermal expansion coefficient between the inclusion and matrix.[3,4,23] These three factors are not mutually exclusive and depend substantially on the composition and structure of the inclusion. Thus, the nucleation potency of the inclusion can be maximized if the inclusion phase is carefully chosen to make all these factors relevant; however, the
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