Effect of Complex Inclusion Particles on the Solidification Structure of Fe-Ni-Mn-Mo Alloy
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GRAIN refinement has been investigated as an approach to improve the mechanical properties of steel welds.[1] Several research groups have attempted to refine the grain size of the heat-affected zone (HAZ) by enhancing the nucleation of acicular ferrite on the surfaces of oxide and sulfide inclusions.[2–17] Grong and Matlock reported that microstructures consisting of acicular ferrite provided weld metal and the HAZ with good mechanical properties such as strength and toughness.[2] Grong et al.[1,2,14] found that the volume fraction of acicular ferrite was strongly dependent on the chemistry of oxide particles, which was affected mainly by the [Al]/[O] ratio. Koseki and Thewlis also reported that the acicular ferrite volume fraction increased markedly at a [Al]/[O] ratio of around 1.0 in gasshielded metal arc welds, corresponding to the presence of the MnAl2O4 (galaxite) spinel.[7] Terasaki and Komizo directly observed the acicular ferrite grown from the inclusion surface using laser scanning confocal microscopy (LSCM).[15] Ito and Nakanishi[18] and Mori et al.[19] found that TiO and TiN phases in the inclusion surface layers significantly increased the acicular ferrite volume
JUN SEOK PARK, Graduate Student, and JOO HYUN PARK, Professor, are with the School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, South Korea. Contact e-mail: [email protected] CHANGHEE LEE, Professor, is with the Department of Material Science and Engineering, Hanyang University, Seoul 133-791, South Korea. Manuscript submitted: December 1, 2011. Article published online September 28, 2012. 1550—VOLUME 43B, DECEMBER 2012
fraction.[7] Yamada et al.[16] also observed that a Ti-enriched layer existed at the interface between the amorphous phase and MnAl2O4 oxide in low carbon submerged arc weld metal and considered that a Ti-enriched layer of a Ti-O system contributed to the decrease in interfacial energy between inclusions and ferrite. Later, Nishizawa proposed that TiO could facilitate grain refinement based on its metallic bond character.[20] Koseki and Thewlis stated that in low Al-content weld metals, oxygen is available for Ti-oxide formation, whereas when the Al content is high, oxygen is killed by Al and little is left for Ti-oxide formation.[7] They concluded that the most effective nucleants for acicular ferrite in weld metals are the crystalline phases of TiO and TiN, and the galaxite spinel MnAl2O4.[7] Several experimental and model-based studies have been performed to understand the fundamental phenomena underlying the heterogeneous nucleation of delta ferrite in various kinds of substrate particles during solidification processes. Bramfitt investigated the effects of various carbides and nitrides on the critical undercooling of the iron melt and proposed that lattice disregistry was an important factor affecting the efficiency of heterogeneous nucleation.[21] Similar results were obtained for several oxide inclusions by Ohashi et al.[22] Recently, Nakajima et al.[23] measured the critical undercooling o
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