Fundamentals of Hydrogen Solubility in Calcium-Alumino-Silicate Molten Fluxes Containing NaF
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DUCTION
RECENT abundant use of AHSS (advanced high strength steels) has been driven by the needs of the automotive industry, i.e., greater safety while ensuring a light weight, high strength, and increased formability.[1–5] AHSS typically contains high contents of Si, Al, Mn, Cr, and Mo, which have been known to simultaneously improve the tensile strength and ductility.[2–4] However, the weldability of steels containing these elements deteriorates, and the evolution of various defects has become a significant issue. One particular problem that has yet to be fully resolved is hydrogen absorption during welding and subsequent hydrogen-assisted cold cracking (HACC).[6,7] HACC has been found to occur both in the heat-affected zone (HAZ) and in the base metal, but HACC is prevalent near defects in the HAZ.[8] Although removing the hydrogen source would be effective, moisture during welding is always present, and direct prevention of moisture infiltration during welding is not economically feasible. However, an effective way to reduce the hydrogen content in the weld zone could be the utilization of an optimized
SUNG HOON CHUNG and IL SOHN are with the Materials Science and Engineering Department, Yonsei University, Seoul 120749, South Korea. Contact e-mail: [email protected] Manuscript submitted April 9, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
welding flux, which can not only inhibit direct contact of the weld zone with the atmosphere but also absorb existing hydrogen in the weld metal.[9–11] During the welding process, the welding flux can actively cover the molten weld pool, protecting the arc zone from the atmosphere, and preventing oxidation. A well-designed welding flux would minimize the amount of hydrogen dissolved in the weld metal, subsequently inhibiting HACC, and thus, an understanding of hydrogen dissolution to optimize welding fluxes is critical. Beyond welding, hydrogen issues are a constant problem persisting in the continuous casting of steels. Many of the alloys added during secondary steelmaking may be saturated with moisture due to their exposure to the atmosphere, and dissolved hydrogen can be carried over to continuous casting.[12] During solidification, the gas solubility is significantly lowered and hydrogen evolves, which can either react with the molten flux as hydroxyls or form bubbles and affect the behavior of cuspidine (Ca4Si2O7F2) crystallization.[13] With greater crystallization and bubble formation, the heat flux is reduced, and breakouts can be exacerbated. Accordingly, hydrogen solubility control in molten fluxes for continuous casting is also an important factor to ensure optimal mold flux behavior during high hydrogen content dissolved in steels.[14–16] The control of hydrogen solubility in welding and continuous casting mold fluxes is dependent on the composition.[17] Past works on hydrogen solubility in molten fluxes have shown that hydrogen solubility is dependent on the chemical composition and structure.[18–21] Hydrogen has been reported to dissolve in
molten fluxes as free- and inco
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