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Slag-Metal Reactions During Welding: Part II. Theory

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Slag-Metal Reactions During Welding: Part II. Theory Slag-Metal Reactions during Welding: Part II. Theory U. MITRA and T.W. EAGAR A kinetic model is developed to describe the transfer of alloying elements between the slag and the metal during flux-shielded welding. The model accounts for changes in alloy recovery based on the geometry of the resulting weld bead. It also distinguishes compositional differences be- tween single-pass and multiple-pass weld beads. It is further shown that the final weld metal oxygen content is directly related to the weld solidification time as well as the type of flux used. I. INTRODUCTION also indicates that oxygen is transferred in this region. The results presented in Table VIII of Part IL11indicate INPart I of this series,[ll the previous theories of slag- that in multiple-pass welds, the top weld contains more metal reactions during flux-shielded welding were re- oxygen than the bottom layer. This is consistent with a viewed. Experiments demonstrated that the widely held mechanism of oxygen transfer in the droplet zone. droplet reaction theory cannot explain the transfer of al- Table VIII of Part IL1l indicates that for some welds, loying elements between the slag and the metal. In this although the weld metal gains oxygen, it loses silicon paper, a new theory is presented to explain these chem- and manganese. If, in addition to manganese and silicon ical interactions. In Part 111,[411the theory is tested using transfer, the oxidation of iron is also considered, an ox- data from submerged arc welding (SAW). ygen balance indicates that the final amount of oxygen It is proposed that chemical interaction between the transferred to the weld metal during the slag-metal re- slag and the metal occurs in three zones, as indicated in actions is much lower than the amount of oxygen trans- Figure 1: ferred at an intermediate stage of the reaction. In fact, (1) the zone of droplet reactions, in many cases, an oxygen balance based solely on slag- (2) the zone of dilution and weld pool reactions, and metal reactions indicates that the weld metal should have (3) the zone of cooling and solidifying weld pool. a lower oxygen content than the electrode/baseplate used, which is contrary to experimental observation made dur- 11. ZONE OF DROPLET REACTIONS ing the course of this study as well as that reported in the literat~re.['~-~~-~~] In this region, the droplet forms at the electrode tip The oxygen present in the arc plasma which is re- and then travels through the arc column, as shown sche- sponsible for the plasma-metal reactions in this zone has matically in Figure 1. The entire process occurs in a few two possible sources: millisecond^,[^-^-^^ and the temperature of the droplets is (1) decomposition of flux constituents into suboxides and very high: in the range of 2000 OC to 2500 0C.151Due to oxygen and the high temperatures, it is thermodynamically possible (2) contamination from the atmosphere. for several chemical reactions to occur. However, results Both sources have been considered earlier by Eagar,[6'121 of some preliminary experiments presented earliefill show Chai and Eagar,L71 and more recently by L~U.[~]The de- that there is a negligible amount of alloy transfer composition of flux constituents into suboxides and ox- (Si, Mn, Cr) in this region. ygen seems to be the primary source of oxygen, since Although the alloying elements Si, Mn, and Cr are not different fluxes produce different oxygen levels in the transferred in this zone, the results of our investigation, weld metal, depending on the stability of the flux con- as well as data from several other researchers, indi- stituent~.[~-~,~~-~~] cate that oxygen is transferred to the metal in this The analysis of Chai and Eagar on binary oxide- zone.[4,6-81The strongest evidence comes from the results calcium fluoride shows that even oxides stable of Lau,f81who determined the oxygen content in the elec- under steelmaking temperatures (such as MgO) may de- trode tips, in the droplets after their flight through the arc column, and in the weld pool. He also found that compose to gaseous suboxides or vapors and oxygen in the arc plasma and lead to the transfer of considerable changing the welding parameters did not significantly in- amounts of oxygen into the weld metal. Contamination fluence the oxygen content in the droplets. The obser- by oxygen from the atmosphere plays a much smaller vation of pores and inclusions in electrode tips and droplets r0le[~-*3~*9'~1but cannot be totally neglected as a source by other researcher~[~,~]as well as in the present work of oxygen. The fact that oxygen is transferred into the droplets in this zone, whereas there is little exchange of the alloying U. MITRA, Project Leader and Senior Member, Research Staff, is elements, may not be very surprising, if the analysis by with the Thin Film Materials Department, Philips Laboratories, North Richardson[I6] on the decarburization of levitated iron American Philips Corporation, Briarcliff Manor, NY 10510. T.W. droplets is examined. Richardson showed that in the early EAGAR, Richard P. Simmons Professor of Metallurgy, Leaders for anufacturing Professor of Materials Engineering, is with the stages of decarburization of levitated iron droplets, the department of Materials Science and Engineering, Massachusetts reaction Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 18, 1989. METALLURGICAL TRANSACTIONS B VOLUME 22B. FEBRUARY 1991-73 rent, the volume of the weld metal is increased or the Coolln and Electrode ratio of the width to cross-sectional area is decreased. Solidifying Weld Pool These changes in weld geometry affect the kinetics of metal transfer. Based on these observations, a quanti- tative model was formulated to predict the amount alloying elements in the weld metal for any combination of welding consumables and process parameters. A. The Kinetic Model The model considers the slag and the metal to be two immiscible stirred liquids with an alloying element M being transferred at the slag-metal interface. Then, for an interface reaction, such as Fig. 1 -The three reaction zones which control the chemical com- position of the weld metal during SAW. to proceed, three events have to take place:[l9] (1) reaction species (the relevant ions in the oxide MOx) have to move between the bulk slag and the slag-metal interface; does not proceed forward (the superscripts b and s in- dicate bulk and surface concentration, respectively). The (2) chemical Reaction [2] has to occur at the slag-metal oxygen rapidly builds up at the surface, and this surface- interface; and active oxygen prevents the carbon from reaching the (3) the alloying element M has to move from the slag- interface and reacting. A similar phenomenon may be metal interface to the bulk metal. occurring inside the arc cavity during the process of SAW, The kinetics of slag-metal interactions may therefore be with the surface-active oxygen keeping out the other ele- controlled or affected by any of these three steps. The ments during the few milliseconds in which the drops three steps may be represented schematically by an form at the electrode tip and fall through the arc cavity. activity-distance diagram (Figure 3). B . Assumptions 111. ZONE OF DILUTION The model assumes that: AND WELD POOL REACTIONS ^"-^' (1) A neutral point (NP) exists for each welding flux. Tt. In this zone, the falling droplets become "diluted" with slag and metal are at an effective equilibrium only when molten metal from the baseplate (Figure 1). The high thenominal composition of the weld (i.e., the total com- temperature and the large convective forces in this re- position due to the simple mixing of metal from the elec- gion lead to intimate mixing of the molten metal and trode and workpiece in the absence of chemical reactions) result in vigorous chemical reactions at the slag-metal is the same as the NP; that is, no transfer of the alloying interface near the arc. The results presented earlier in element takes place at the NP. Furthermore, the NP is Part I['] indicate that slag-metal reactions do occur in this not affected by variations in the process parameters for region. fluxes free of ferroalloys or other elemental additions. Previous researchers had found that increases in volt- This assumption is based on the results of Chai and age (DC and AC) or decreases in current result in an Eagar[I41and Chai[201and the experimental data of Thier.['ll increase in the amount of alloying elements transferred (2) The equation of continuity is valid for transfer of the between the slag and the metal and had suggested dif- alloying element. That is, the mass flux (J) of alloying ferent mechanisms of element transfer to explain this element M which flows from the bulk slag to the slag- phenomen~n.[~.~*~-~~-~~]The results of our earlier experi- metal interface is equal to the mass flux of M passing ment~,[~]when the welding conditions were varied, agree through the chemical reaction stage and is also equal to with the experimental observations of these researchers. that which is passing from the interface to the bulk metal; However, since the electrodes were virtually free of Mn, however, the amount of this mass flux (J)can and will Si, and Cr, the influence of voltage and current on the change with time. transfer of these elements is clearly due to the influence (3) The mass transfer coefficients of the alloying ele- of voltage and current on the kinetics of slag-metal re- ment M in the slag (k,) and the metal (km)are indepen- actions in the weld pool. Figures 2(a) through (c), which are plots of the amount of alloying element transferred dent of one another and of the activities of the reacting between the slag and the metal against the ratio of the species.
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