Effect of Compositional Variation in TiO 2 -Based Flux-Cored Arc Welding Fluxes on the Thermo-physical Properties and Me
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s been significant demand in consumer needs for advanced high-strength steels (AHSS) containing significant Mn and Al, including transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels, for high-rise construction, ultra-large vessels, automotive needs, and others. Research in the development of these grades of steel and applications has seen significant strides, where the high volume of deformation twinning increases the tensile strain, showing a larger strain hardening capacity than FBDP (ferrite and bainite dual-phase) and TRIP steel.[1,2] However, the greater strength and formability of these steels have been known to make these materials highly susceptible to hydrogen-delayed cracking in flux-cored arc-welded (FCAW) joints, where the hydrogen originates from the moisture in the atmosphere or
J.B. KIM, and I. SOHN are with the Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemungu, Seoul 03722, Republic of Korea. Contact e-mail: [email protected] T.H. LEE is with the Mechanical Design Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea. Manuscript submitted November 27, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the welding fluxes. In particular, the microstructure of the heat-affected zone (HAZ), which is controlled by the heat flux during welding, seems to greatly affect the sensitivity of the cracking behavior. Furthermore, the high Mn and Al contents in the AHSS can easily oxidize, according to Kou,[3] and these oxidized inclusions in the weld zone can act as initiation points for cracking, resulting in a lower strength. Thus, to ensure optimal performance of the weld joint, it is imperative to develop welding fluxes that provide both sufficient coverage and appropriate heat fluxes to minimize welding defects. FCAW is widely used for carbon steels, low alloy steels, stainless steels, and especially high Mn-containing steels since the flux prevents contamination from air by providing adequate surface coverage during welding. Gomes et al.[4] also indicated that FCAW can provide high deposition rates, minimal waste of electrodes, greater process flexibility, high weld quality, and excellent control of the weld pool. However, Spear[5] indicated that FCAW generates a substantial amount of fumes, including ozone, nitric oxide, and carbon dioxides, due to the high electrical current and flux-cored electrode. Previous authors have indicated the importance of shielding gases such as Ar and CO2 during arc welding,[6] and the majority of manufacturers use CO2 during the
arc welding of TRIP and TWIP steels. Although 100 pct CO2 shielding gas results in significant spattering along with excessive melt downs and fusing, the use of CO2 can lead to deeper bead penetration as the surface tension of the molten pool increases due to the oxygen partial pressure of CO2. Furthermore, CO2 can dissociate at temperatures above 1873 K (1600 C) and promote reaction between the molten metal and CO in the arc column and weld pool.[
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