How Fluxes Determine the Metallurgical Properties of Submerged Arc Welds

Technical Note How Fluxes Determine the Metallurgical Properties of Submerged Arc Welds Dissolved oxygen, the deciding factor in determining impact properties—in particular the transition temperature- is controlled by the molten slag and its active components BY ir. J. H. PALM ABSTRACT. From metallurgical considera Acid, Neutral or Basic stability, speed capacity, shape and tions it is concluded that the dissolved In papers and lectures on welding appearance of the weld and slag de- O-content (and not the total O-content) tachability. This means that the concen of the weld metal very likely is the fluxes one nearly always finds the opin ion that the main factor governing the tration of the constituents must be decisive factor determining its impact restricted between certain limits. properties, especially its transition tem mechanical properties of weld metal, perature. especially its impact behavior, is ba In general, the flux constituents are The dissolved O-content depends on sicity. However, in general any accept oxides like MnO, SiO,, CaO, MgO, the O-potential of the surrounding active able explanation is missing. AI>03, Ti02 and Na-jO and fluorides medium. In submerged arc welding this Moreover, the authors fail in dis like CaF-2 and Na3AlF6. In the case of is the molten slag and its active com tinguishing between metallurgical and sintered or agglomerated fluxes they ponents; as far as O is concerned, this is chemical factors and in a certain sense a may also contain deoxidizers and alloy practically restricted to SiO;*. MnO and lack of understanding in this matter ing elements. FeO. seems obvious. Basic components can only chemically As we shall elucidate later on, SiO> This is particularly true as far as the is seldom required from a metallurgical bind the acid oxide Si02, but not the apparently amphoteric oxides MnO and meaning of the word neutral is con point of view; however, it must be added FeO. Basicity therefore only governs cerned. It therefore seems justified to to obtain the necessary viscosity and the part played in the O-activity by SiOj. go somewhat closer into this and related current capacity. A high CaO content Though very important in this respect matters. is required for chemical and, indirectly, it fundamentally is (contrary to general metallurgical reasons. CaO also im opinion) a factor of secondary order on Properties of Fluxes in Molten proves arc stability. However, beyond ly. This means that the metallurgical State certain limits also determined by the quality of a flux can at most partly be other constituents, it makes the viscosity represented by basicity formula. In the several phases of the welding process the molten flux must possess of the slag too low. certain physical properties which are It also makes the flux more sensitive decisive for their application. For ex to moisture pick-up which is liable to ample, these properties concern the cause porosity. MgO has a similar The author is associated with the Hilarius melting and solidication range, the effect and ALO3 also increases the sen Wire Industry, Haarlem, Holland. viscosity range, current capacity, arc sitivity to moisture. MnO favors high 358-s I JULY 1 972 welding speed and deep penetration and follows that via the corresponding re gain or loss of Mn and Si for a given flux decreases the sensitivity to rust and actions the tendency is to arrive at the as such is therefore rather meaningless. porosity. On the other hand it decreases equilibria It can be done only for the flux in com the current capacity. bination with a given wire, base metal [CaO] [MgO] and quantitatively only in combination Kf K '•:» Metallurgical Properties of Molten [CA] [Mg] with a certain setting of machine param Flux [ALO, [MnO] eters. KA 2 3 Kj It now follows from (7) that, under The preceding arguments set limits ' [Al] ' ' [Mn] 1 2 otherwise constant conditions and as to the metallurgical properties, which [SiO,] ' KFC* [FeO] = suming all mentioned components are concern the interactions between molten Ks 2 ' [Si]" present in the molten phases; slag and molten weld metal, the latter being made up from a mixture of molten Kc [O] (7a) 1. Increase of MnO in the flux leads wire and molten base metal. |y to a higher final Si02 and FeO concen tration in the slag and a higher Mn and In general this interaction can be repre It is true that our assumption that sented by O and a lower Si and C content of the the constants are only dependent on weld metal (and vice versa). the temperature of the slag-metal melt 2. Increase of Si0 in the flux leads MxOv + yFe t=i XM + yFeO (1) is not quantitatively correct. In fact 2 to a higher final MnO and FeO con this is only true on application of the in which MxOy may be any metal (or centration in the slag and a higher Si thermodynamic activities instead of other oxide in the slag) and M is the and O and a lower Mn and C content in the concentrations of the constituents. corresponding element dissolved in the the weld metal. These quantities differ more or less molten metal. The reactions in any case 3. Increase of MnO and SiO- simul due to mutual physical and chemical 2 must thermodynamically endeavour to taneously will, depending on their ratio, effects of the constituents, but for a reach equilibrium conditions and equi generally lead to an increase of Mn and qualitative approach to the direction of librium will therefore be more or less Si in the weld metal simultaneously, the reactions, its application seems to be approached. though beyond or below extreme ratios sufficiently justified. This is particularly The summarized equilibrium conditions Si or Mn may decrease. O always in valid for a given flux where during the of (1) are: creases, while C always decreases. welding process the changes of concen [MxOyP'r [FeO] trations in the flux and the absolute It also follows from (7) that :(1) in K KF [MJx/ y [Fe] concentrations of the elements in the crease of Mn in the wire leads also to a = K [FeO] (2) weld metal are small. higher Si and C and a lower O content Fe of the weld metal. The MnO content of In 7a the constants Kca, KMK, KU the slag increases the SiO. and FeO In which MxOy and FeO are the con (and also, KLI, Ksr, Ki!a, KT;, a.o. centrations of these oxides in the slag if necessary to consider) are so ex content decreases; increase of Si in the and M and Fe the concentrations of those tremely high that the corresponding re wire also leads to a higher Mn and C elements dissolved in the weld metal at actions can be left completely out of and a lower O content of the weld the moment that equilibrium is establish consideration. This means that prac metal. The Si02 content of the slag in ed, i.e. the reactions have come to their tically no trace of Ca, Mg, Al, etc. is creases, the MnO and FeO content de theoretical end. Km is a constant depend transferred from the slag to the metal. creases. ing on the temperature only. Since the In other words only the reactions in Summarizing, we find that increase weld metal comprises of iron with at which Mn, Si, C and Fe take part are of of MnO and/or Si02 always contribute the most a few percent of other ele practical importance and decisive for to increase of Mn + Si and at the same ments, Fe may be regarded as equal to the eventual composition of the weld time to a higlier dissolved O content of one. metal. the weld metal. Also the reaction: Hence (7a) may be simplified to: On the other hand increase of Mn and/or Si in the wire also always con FeO — Fe + O (3) [MnO] [SiO,]1 tribute to increase of Mn + Si, how KM,, — — rvSi 1 /2 takes place and tends towards equi [MO] [Si] ever, at the same time to a lower dis librium >1 solved O conteni of the weld metal. Kp [FeO] =K (7) t C [C] Equal Mn and Si content in the weld [ ] KFe -pg- = KF. [FeO] = [O] (4) metal can therefore be obtained at The absolute values arrived at equilib different O levels. in which [O] is the concentration of rium not only depend on the concentra It is generally accepted that Si as an dissolved atomic O in the weld metal tions at the start of welding, but also alloying element in the weld metal does at equilibrium. on the ratio of slag and molten metal. not favor impact properties, particu Taking the presence of C in the weld The latter is in its turn given by the larly the transition temperature. The metal into account one has: setting of the welding machine (current, same is also very probably true for Mn. voltage and speed). This means that Dissolved O definitely seems to have a O + C t=i CO (5) even for the same flux, wire and wire disastrous influence on impact strength. with the corresponding equilibrium: diameter, base metal, thickness of base This a.o. is confirmed by the high im metal and shape of the seam, the final pact values and low transition tempera [CO] composition may differ more or less ture of extremely pure iron, made under K, K ro] (6) [C] widely. The settings of the machine, vacuum. The apparently favorable role °lf= however, have no influence on the direc of limited amounts of Mn and Si under CO may be the concentration or pres tion of the reactions, i.e.

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