Oxide Formation Mechanisms in High Manganese Steel Welds
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INTRODUCTION
IN the last several decades, high Mn TWIP/TRIP (twin-induced plasticity/transformation-induced plasticity) has been investigated by a number of researchers. Idrissi et al.[1] reported that the stacking fault energy (SFE) is an important factor for determining mechanical property and deformation character. As a result, there have been several attempts to control deformation behavior by adding high SFE alloying elements such as Mn and Al.[2] Although these elements can change the deformation behavior of TWIP steel from brittle to ductile,[3] they can also allow easy creation of oxide inclusions between the high affinity elements and oxygen. As was reported by Liu et al. and Sato et al.,[4,5] even a single oxide precipitate can act as a primary or secondary void nucleation site in the specimen. Park et al.[6] recently reported that oxides in high Mn- and Al-alloyed steels can have several different types of morphologies, depending on the exact chemical composition of the specimen. The formation mechanisms of oxide inclusions in steel welds are critically important because the morphology and structure of oxides have an important role in their DOOYOUNG KIM, Master Course, KYUTAE HAN, Ph.D. Candidate, and CHANGHEE LEE, Professor, are with the Department of Materials Science & Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Contact e-mail: [email protected] BONGKEUN LEE, Research Engineer, and ILWOOK HAN, Principal Engineer, are with the POSCO Technical Research Laboratories, 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 January 13, 2013. Article published online December 7, 2013 2046—VOLUME 45A, APRIL 2014
mechanical properties. In high-Mn steel welding processes, the formation of MnS is generally unavoidable because of the strong Mn-S interactions. Wakoh et al.[7] reported that MnS can crystallize or precipitate in alloys, depending primarily on the S content. Although the MnS phase can have various morphologies in the alloys,[8,9] its precipitation behavior in steel welds has not yet been discussed exhaustively. The morphology and volume fraction of precipitated phases could affect the performance of the steel.[8] It should be noted that the interfacial energy between the MnS inclusion and the c-matrix which affects the mechanical properties are lower than that of other nonmetallic inclusions.[10] In the high manganese steel weldment, almost all of oxides contain MnS in the outside due to the high manganese contents. In case of the strain with low strain rate, i.e., crack tip opening displacement (CTOD), the effect of low interfacial energy between the matrix and inclusions on mechanical values of the weldment could be stronger in the view of the decohesion between the matrix and inclusions than rapid strain rate cases, i.e., impact test. Therefore, the formation/wrapping type of the MnS on the oxide could be the important factor on mechanical properties
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