A model for the silicon-manganese deoxidation of steel weld metals
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I.
INTRODUCTION
THEchemistry of mild and low alloy steel weld metals has been extensively investigated and reported in the literature. 1-7 It is well established that considerable interaction takes place between the liquid weld metal and its surroundings (arc atmosphere, slag) when welding is performed in the presence of oxygen. For slag-protected processes, the flux is the main source of oxygen because of the presence of easily-reduced oxides, such as iron oxide, manganese oxide, silica, or rutile.~'2 In gas-shielded metal arc (GMA) welding, oxygen is often deliberately introduced through the shielding gas to improve the arc stability and bead morphology, but at the expense of an increased oxygen content in the weld metal and intensified losses of alloying elements. 3'4~5The oxidation reactions proceed very rapidly under the prevailing conditions because the metal temperatures are high and the interfacial contact area available for interaction is large. On cooling, the weld pool oxygen concentrations established at elevated temperatures tend to readjust by reacting with deoxidants present in the weld metal. In the case of CO2-shielded welding of C-Mn steels, it has been shown that the amount of oxygen introduced into the weld pool immediately beneath the root of the arc and subsequently combined with deoxidizers during cooling amounts to approximately 5000 ppm. 4'5 Despite the fact that significant quantities of oxygen are removed from the weld pool at this stage of the process, the analytical weld metal oxygen conO. GRONG is Assistant Professor, The Norwegian Institute of Technology, Trondheim, Norway N 7034. T.A. SIEWERT is Metallurgist, National Bureau of Standards, Boulder, CO 80303. G.P. MARTINS and D.L. OLSON are Professors, Colorado School of Mines, Golden, CO 80401. Manuscript submitted April 10, 1985. METALLURGICALTRANSACTIONS A
tent exceeds by far the value predicted from thermodynamic analyses, assuming that equilibrium conditions are maintained down to the solidification temperature. 4This situation cannot be ascribed to a large deviation from chemical equilibrium between the reactants and precipitated slag, 6 but is the result of an incomplete phase separation. The precipitated slag has a lower density than the molten weld metal and is driven upward by gravity (buoyancy effect). However, the slag does not reach the top of molten region due to the limited time available for growth and flotation of the particles (typically less than 5 seconds). Consequently, to understand the extent and direction of the weld pool deoxidation reactions, consideration must be given to the kinetics. The three basic consecutive steps in steel deoxidation are shown schematically in Figure 1. Although rate phenomena in ladle refining of liquid steel have been extensively investigated, 7 few attempts have been made to include these effects when discussing deoxidation reactions in arc welding because of the very complex nature of the process.
II.
SCOPE AND EXPERIMENTAL APPROACH
The present investigation is concerned with f
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