Mechanism of the Development of Marangoni Flow in the Metal Pool Heated by Plasma Arc.

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Experiments were conducted to clarify the melting and flow behaviors of Fe-O and Fe-0.5mass%C-S specimens heated with a plasma arc flame in a gas shielded chamber.The flow behaviors of the molten pool were observed by use of ZrO_2 particles as a marker. The observation indicated that the outward flow occurred at the surface of the molten pool for the case using the base metal with low oxygen concentration and the inward flow did for the high oxygen concentration.The temperature coefficient of surface tension (dgamma/dT) is negative for the lower oxygen concentration range than 0.007 mass%, but it is positive for the higher oxygen level.Flow behaviors of the molten pool can be regarded as a surface tension driven flow (Marangoni flow) from the relation between the temperature coefficient of surface tension and the oxygen concentration.The depth (D) of the molten pool increases and the width (W) decreases with the increase of the oxygen and sulfur concentration in the base metal.The linear relation were found between dgamma/dT and the ratio D/W of Fe-O and Fe-C-S specimens. Neither oxygen concentration nor sulfur concentration in the metal influences the shape of the plasma arc flame.Conclusively, it could be deduced that Marangoni flow was a dominant factor in determining the molten pool geometry.