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Slag-Metal Reactions in the Electroslag Process

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Slag-Metal Reactions in the Electroslag Process Slag-Metal Reactions in the Electroslag Process Slag-metal reactions are fundamentally controlled by the thermodynamic driving force, the physical nature of the reaction products, and mass transport in the liquid slag and metal phases BY B. M. PATCHETT AND D. R. MILNER ABSTRACT. Slag-metal reactions in these boundary layers that controls of requirements such as arc stability, electroslag melting have been eval­ the rate of reaction. The thermody­ protection of the weld metal from uated in terms of the fundamental namics of the reaction determines the atmosphere, weld metal deoxi- controlling parameters. Simple metal the equilibrium state established at dation and alloying, ease of slag systems using pure aluminum and the interface and hence the concen­ removal, etc. With present highly iron, and binary iron alloys, have tration, or activity, gradient causing evolved industrial fluxes this results been melted in inert fluxes to which diffusion across the boundary layer. in many chemical reactions oc­ known quantities of reactive constitu­ A highly favorable reaction curring simultaneously under the ent were added. Ingots, fluxes, slags produces a high concentration complex physical conditions created and electrode tips were, analyzed gradient and hence a high rate of by the electric arc, so that the funda­ chemically and metallographically, reaction. The nature of the reaction mental analysis of the reactions and slag bath temperatures were product also influences the rate of taking place is very difficult. The measured with sheathed thermo­ reaction. Liquid miscible reaction usual approach is to assume that a couples. Aluminum was also products readily diffuse and are state of thermodynamic equilibrium melted in transparent silica crucibles, transported away. Gaseous reaction is attained, on the basis that the and motion picture films of the melt­ products formed at the interface high temperatures and high surface- ing process provided information on slow the rate of reaction by inter­ to-volume ratio counteract the short the velocity of the slag motion and fering with the diffusion of reactants time available for reactions to be 1 its influence on metal transfer and through the slag boundary layer, and completed. The chemical composi­ droplet size. Bath velocities of about solid reaction products slow the re­ tions of the reaction products in the 100 cm/sec circulate the slag past action rate by blocking off part of the metal and slag phase are then the electrode tip and detach some of area available for reaction. The rate analyzed in terms of thermodynamic the molten metal as showers of of reaction is particularly high where data extrapolated from steel-making small droplets. the reactant in the slag is soluble in and an "effective reaction tempera­ Most of the reaction occurs at the the metal and is transported into, ture" is arrived at. electrode tip. This is due to the high and reacts with elements in, the bulk The temperatures so calculated for surface area-to-volume ratio of the of the metal, e.g., the oxidation of covered electrode and submerged melting metal and the intense silicon and carbon in steel by iron arc welding vary between 1500 and stirring in the reacting slag and oxide in the slag. 2100 C.2"6 This variation in effective metal phases. The fluid motion is reaction temperature, together with caused by the action of the Lorentz the fact that different elements in forces and rapidly transfers the reac­ Introduction the same weld, e.g., silicon and phos­ tive constituents towards the re­ Slag-metal reactions are of impor­ phorus, react as though their effec­ 5 action zone at the slag-metal inter­ tance in many fusion welding proc­ tive reaction temperatures differed, face. At the interface there are esses, but comparatively little is plus the further fact that slag and boundary layers in the flux and the known about the mechanisms which metal compositions depend on weld­ 6 metal, and it is diffusion across control them. The predominant rea­ ing speed, rate and size of droplet 7 8 son for this is that slag-metal reac­ transfer and mass transfer, all sug­ tions have been associated primarily gest that equilibrium is not attained. with the arc welding of ferrous mate­ The concept of an "effective reaction B M. PA TCHETT is with the Department rials, and they are therefore only a temperature" is thus an approxima­ of Materials. Cranfield Institute of Tech­ part of the difficult problem of de­ tion to an otherwise difficult to eval­ nology, Cranfield, Bedfordshire, England, veloping practically effective fluxes uate situation, in which reactions are and D. R. MILNER is with the Department attempting to attain to an equilibrium of Industrial Metallurgy, The University, and electrode coatings. Birmingham, England. Paper presented at Over a period of 70 years of largely corresponding to some higher tem­ the AWS 53rd Annual Meeting held in empirical development, fluxes have perature and kinetic processes deter­ Detroit, during April 10-14, 1972. been devised to satisfy a multiplicity mine the extent to which they occur. WELDING RESEARCH SUPPLEMENT! 491-s By contrast the electroslag process offers the possibility of the use of Table 1 — Chemical Composition of Slags and Ingots Melted in Inert Fluxes much simpler fluxes under condi­ Slag Electrode tions in which slag-metal reactions composition, composition Ingot Composition can be more readily evaluated, since Base metal Inert flux % Al % Na % K % Na % K it utilizes resistance heating in the slag and does not involve problems Al 20% NaF-KCI 0.70 < .001 < .001 .055 .002 of arc stability and the other com­ % Fe 0 C % Si % c % Si % Ba MgF 1.08 02 .02 plexities of arc welding encountered Fe 2 <*01 <.01 — Fe BaF 1.07 02 <01 .02 <01 when using simple slags.9 Most 2 — Fe-C BaF2 1.10 20 <01 .19 <-01 .001 work on slag-metal reactions in the Fe-Si MgF2 0.91 0.85 electroslag process has been related — — — — Fe-Si BaF2 1.00 — 0.91 — 0.84 — to the use of the technique for the re­ fining of high quality steels where some evaluation has been made of the reactions giving rise to contam­ ination and inclusions.10 Loss of omits the electromagnetic forces eters. Then in the third stage sim­ essential elements and the presence which arise from current flow and plified industrially significant re­ of inclusions is related to the use of ox­ which are liable to be of consid­ actions are evaluated in terms of ides in the flux, and it has been found erable significance. (In arc welding the principles established in the first that the order of reactivity of various such forces have been shown to ex­ two stages. oxides is that which would be pre­ ert a very marked effect on mass dicted from their thermodynamic sta­ transport and therefore on the extent Experimental bility.10 of reaction.12) In electroslag welding the composition of the fluxes used In typical industrial systems involv­ The processes which determine ing multi-component metals and has been strongly influenced by the extent to which reactions take fluxes several competing reactions covered electrode and submerged place have been investigated for a are usually taking place simultane­ arc experience, although it has also model system using low melting ously. These bring about small but been realized that with less complex point organic and inorganic materi­ significant changes in the composi­ 11 conditions simpler principles of flux als by Crimes. In these experi­ tion of the deposited metal, but the design are possible.13 ments the reaction between drop­ fact that only limited changes occur lets and the fluid through which they The objective of the present work makes chemical, metallographic and fall has been examined. It was found has been to clarify the reaction proc­ theoretical analyses difficult. Thus that the reaction is controlled by dif­ esses in electroslag welding by ap­ for this research simple systems fusion through a boundary layer at proaching the problem in three have been selected involving the the reacting interface, with the rate stages. First, the physical conditions minimum of constituents in the flux controlling step in the boundary of melting, fragmentation and trans­ and metal, while the first reactions layer of either the simulated metal or fer of electrode material, slag motion considered have involved massive the slag phase, depending on the cir­ and temperature are considered. In and therefore readily detectable cumstances. The bulk of reaction oc­ the second stage simple (non-indus­ changes in composition. curred during droplet transfer rather trial) systems involving massive reac­ Two metal systems, based on than during droplet formation or at tions have been analyzed to deter­ aluminum and iron, have been ex­ the "ingot". Such a model, however, mine the major controlling param- amined by melting in an inert flux to which a reactive constituent has been added. With the lower melting point aluminum system it was pos­ sible to observe the metal transfer ELECTRODE and slag motion by melting in trans­ parent silica crucibles. A massive reaction with iron was examined by putting nickel oxide in the flux to give WATER transfer of nickel into the iron. With rr±±r aluminum two reactions involving massive transfer of an alloying element from the slag to the metal were investigated by putting copper chloride or copper oxide in the flux as the active constituent. In all three V systems the element freed from the slag is completely miscible in the metal and forms readily analyzed al­ COPPER loys. CRUCIBLE The industrial type reactions investigated were based on iron and WATER JACKET BASE PLATE steel. They were of two types: first, reactions involving transfer of silicon or manganese into the molten metal from Si02 or Mn02containing slags; i second, the removal of silicon and carbon from electrode wires by slags containing various oxides.
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