Variation in the Chemical Driving Force for Intragranular Nucleation in the Multi-pass Weld Metal of Ti-Containing High-
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INTRODUCTION
A high density of inclusions, consisting mainly of oxides, inevitably develops in welding processes because oxygen dissolution occurs actively into the liquid weld pool. These inclusions play an important role in determining the mechanical properties of the weld metal. They can have a detrimental effect on toughness by providing crack initiation sites during cleavage fracture[1] or void initiation sites during dimple fracture[2]; however, they can simultaneously lead to enhanced toughness by providing heterogeneous sites for acicular ferrite nucleation. The interlocking structure of acicular ferrite can act as an obstacle for crack propagation thereby improving the toughness, especially at low temperatures.[3,4] Since acicular ferrite is nucleated at inclusions as a result of competition with prior austenite grain boundaries, the amount of acicular ferrite is controlled by both the relative quantity (i.e., the number of nucleation sites) and the relative quality (i.e., the nucleation potency) between the prior austenite grain boundaries and the inclusions. In a quantitative respect, when nucleation sites are assumed to be inert substrates, acicular ferrite development is favored when the prior austenite grains YONGJOON KANG, Ph.D. Candidate, and CHANGHEE LEE, Professor, are with the Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Contact e-mail: [email protected] KYUTAE HAN, Senior Researcher, formerly with the Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea, is now with the Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea. JOO HYUN PARK, Professor, is with the Department of Materials Engineering, Hanyang University, Ansan 426-791, Republic of Korea. Manuscript submitted November 25, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
are larger and when the inclusion density is higher.[5–9] However, a qualitative ranking of the various nucleation sites was also confirmed to significantly affect the nucleation of acicular ferrite. The potency of the prior austenite grain boundaries can be reduced by adding an alloying agent, such as boron, which segregates strongly to the grain boundaries.[10,11] Importantly, many researchers have identified the factors that improve the inclusion potency for acicular ferrite nucleation. These include (1) lattice coherency between the inclusion and ferrite,[12–15] (2) an increase of the chemical driving force due to the depletion of austenite-stabilizing elements (such as Mn) in the austenite adjacent to the inclusion,[12,16–23] and (3) strain energy due to the difference in the thermal contractions between the inclusion and austenite.[24,25] In early studies, acicular ferrite was defined as intragranularly nucleated Widmansta¨tten ferrite,[5] but it was described as intragranularly nucleated bainite by later researchers.[26–28] In contrast, Thewlis et al.[29] reported that acicular ferrite was not a single transformation product but was formed by
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