Slag-metal reactions during welding: Part II. Theory
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INTRODUCTION
IN Part I of this series, ~1 the previous theories of slagmetal reactions during flux-shielded welding were reviewed. Experiments demonstrated that the widely held droplet reaction theory cannot explain the transfer of alloying elements between the slag and the metal. In this paper, a new theory is presented to explain these chemical interactions. In Part III, t4~1the theory is tested using data from submerged arc welding (SAW). It is proposed that chemical interaction between the slag and the metal occurs in three zones, as indicated in Figure 1: (1) the zone of droplet reactions, (2) the zone of dilution and weld pool reactions, and (3) the zone of cooling and solidifying weld pool. II.
ZONE OF DROPLET REACTIONS
In this region, the droplet forms at the electrode tip and then travels through the arc column, as shown schematically in Figure 1. The entire process occurs in a few milliseconds, t2,3,4] and the temperature of the droplets is very high: in the range of 2000 ~ to 2500 ~ tSj Due to the high temperatures, it is thermodynamically possible for several chemical reactions to occur. However, results of some preliminary experiments presented earlie~ ~l show that there is a negligible amount of alloy transfer (Si, Mn, Cr) in this region. Although the alloying elements Si, Mn, and Cr are not transferred in this zone, the results of our investigation, as well as data from several other researchers, indicate that oxygen is transferred to the metal in this zone.IS.6-81 The strongest evidence comes from the results of Lau, tsl who determined the oxygen content in the electrode tips, in the droplets after their flight through the arc column, and in the weld pool. He also found that changing the welding parameters did not significantly influence the oxygen content in the droplets. The observation of pores and inclusions in electrode tips and droplets by other researchers I3"91 as well as in the present work
U. MITRA, Project Leader and Senior Member, Research Staff, is with the Thin Film Materials Department, Philips Laboratories, North American Philips Corporation, Briarcliff Manor, NY 10510. T.W. EAGAR, Richard P. Simmons Professor of Metallurgy, Leaders for Manufacturing Professor of Materials Engineering, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 18, 1989. METALLURGICAL TRANSACTIONS B
also indicates that oxygen is transferred in this region. The results presented in Table VIII of Part I t~j indicate that in multiple-pass welds, the top weld contains more oxygen than the bottom layer. This is consistent with a mechanism of oxygen transfer in the droplet zone. Table VIII of Part I t~] indicates that for some welds, although the weld metal gains oxygen, it loses silicon and manganese. If, in addition to manganese and silicon transfer, the oxidation of iron is also considered, an oxygen balance indicates that the final amount of oxygen transferred to the weld metal during the slag-metal reactio
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