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. on the tendency practical conditions finds its reason sure of developing CO which approaches to loss or gain of the elements in the only in the reduction of dissolved O 1 under atmospheric conditions. (Due weld metal and the oxides in the slag. during the phases of the welding process to surface tension effects the CO pres­ On the other hand the direction may where slag and metal are no longer sure may be substantially over 1 atm differ for the same flux and setting, de­ strongly intermixed. Interaction between during the interaction.) pending on the composition of the wire slag and metal then comes to an end. From the preceding considerations it and/or the base metal. Evaluation of In the liquid and the solid state of

WELDING RESEARCH SUPPLEMENT! 359-s the metal too, reactions strive for cor­ more strongly acid in character. Basic the weld metal is the O-activity. It also responding equilibria. This results in oxides are characterised by their tend­ follows that the O-activity is only par­ binding of a part of the dissolved O ency to split off O ions e.g. tially influenced by the basicity which under formation of secondary slag- only governs that part of the O-ac­ CaO ±5 Ca++ + O— (8) particles in the liquid and finely dis­ tivity given by Si02. persed tertiary particles in the solid Acid oxides on the contrary bind avail­ It is therefore not justified to believe weld metal by the corresponding ele­ able (free) O ions e.g. that the properties of a flux, as far as ments. composition and mechanical properties At equal contents of Mn and/or SiQ2 + 20 Si04 (9) of the weld metal are concerned, may be Si a lower dissolved O content in the fully characterised by a basicity formula, liquid weld metal consequently results The chemically neutral (amphoter) of which many exist. in a lower dissolved O content of the oxides can do both, depending on the As far as there is any need for such a solid weld metal at atmospheric tem­ composition of the slag as a whole, but formula of secondary order it should perature. to a small degree only. These chemical neither contain amphoteric or very reactions also proceed to corresponding From the metallurgical point of view weak oxides like MnO, FeO, A1203, equilibria. An oxide is now regarded it is therefore desirable that the flux Ti02 a.o. nor CaF2 which is an almost as more strongly basic the more O ions fully dissociated neutral compound in has the lowest possible MnO and Si02 content. This flux approaches ideal are split off at the same total concen­ the molten flux. metallurgical neutrality in the most tration of the oxide. As an oxide be­ complete way, which means that in­ comes more strongly acid, more O ions are bonded. In molten fluxes CaO Dissolved Oxygen Affects dependently of the composition of wire Impact Properties and base metal the change of composi­ is almost completely dissociated i.e. behaves in a very strongly basic fashion, tion of molten wire plus base metal is One can hardly omit to conclude limited to a minimum. Fluxes contain­ while SiOo behaves in a very strongly acidic fashion. from the foregoing reasoning that dis­ ing MnO and/or Si02 can approach a solved O is a very important if not the practical metallurgical neutrality, which The consequence is that as far as O decisive factor in determining impact means that none or little difference ions are available SiO*2 is almost fully properties. Obviously this theory is sus­ exists in Mn and Si content of weld metal bonded to Si04 . tained by several facts. calculated from the geometry of the Moreover Si04 ions tend on their It is a general experience that with weld only and weld metal at equilibrium turn to bind SiO-2 molecules to complex­ decreasing acidity, i.e. decreasing conditions. However, this is only pos­ ions. The consequence of the presence of O content these properties improve sible in a limited range of wire and base basic oxides therefore is that a corre­ and vice versa. Also that beyond the sponding part of the Si0 -concentration metal compositions and related settings. 2 ratio CaO/Si02 = 2, the improvement is Under all conditions where the flux is becomes metallurgically inactive. In of a lower order. not metallurgically neutral in relation other words the flux gets the properties Steel made under vacuum generally to the weld metal, the final composition of a flux of lower Si02 content and be­ has very good impact properties. This of the latter differs more or less from haves metallurgically in that way. is especially true for very pure iron mol­ that calculated from wire composition, Though CaO, as has been elucidated in ten in vacuum. Additions of strongly plate composition and dilution. This the preceding chapter, does not directly deoxidizing elements like Al, Ca, Mo and means that loss or gain of the elements take part in the metallurgical reactions, others in amounts that have still no in the weld metal occurs. On repeated it indirectly has a strong influence on appreciable alloying effects, generally welding on the preceding weld metal, these reactions through chemical bind­ improve impact properties. ing of Si0 . The binding to Si0 losses or gains accumulate asymptotic­ 2 4 It is on the other hand obvious that is nearly complete at the mol. ratio ally, i.e. in a decreasing degree to a in welding and steelmaking under CaO/Si0 =2(chemically the formula of finite content, where the flux becomes 2 oxides with high O activity (MnO, FeO, Ca SiO-i, Caorthosilicate), which differs ideally metallurgically neutral for the 2 dead soft electrodes) the impact proper­ only little from the corresponding weight weld metal. ties are extremely low. ratio. Higher ratios have therefore a Flux/wire combinations for a given It is true that one can wonder whether minor effect only, which is in accordance base metal resulting in strong losses dissolved O, which under all conditions with experience on impact behavior. or gains per run are therefore not suit­ can be present only in extremely small able for multi-run welding. Under prac­ Like CaO, BaO and the alkali oxides amounts, has such a tremendous in­ tical conditions there is nearly always are strongly basic. As far as other oxides fluence on impact properties. some loss in C and a gain in Si. are concerned MgO is usually regarded However, it should be realized that Apart from those cases where silicon as rather strongly basic, but apparently the same can be said with respect to is required as an alloying element behaves less basic than CaO. This means similar small amounts of C and N (strength in higher tensile steels) it can that a much higher MgO/SiOo ratio is which are proven to be responsible for be regarded as unwanted. Application needed to obtain a similar result. A1203 the yield phenomenon, strain aging, blue of wire having more than a trace up to is amphoteric. In the steel industry it is brittleness and elastic after-effect in 0.15% Si is therefore an exception. even regarded as weakly acid. mild steel. Though the majority of fluxes lead Due to a lack of understanding of to a gain of Mn, some types lead to the real meaning of its important metal­ References rather high losses. Consequently wires lurgical role, MnO is usually also re­ This paper is based on the following pa­ with corresponding Mn content in a garded as rather strongly basic. How­ pers in which the ion theory was applied range of 0.5 to 2.0% or even more have ever, it is, like FeO chemically ampho­ on fluxes and the dissolved O theory was to be used. teric too. This already follows from the put forward. 1. Report, 4th Orientation day, Auto­ experience that in fluxes its metallur­ matic Welding. Nov. 1963, p. 5. Chemical Properties of Fluxes in gical activity seems hardly or not be 2. Discussions, Welding Symposion, Ned. influenced, neither by CaO nor by Si0 . Ver. Lastechniek Nov. 1964, p. 26. Molten State 2 3. Dissolved Oxygen in Weld Metal, Basicity Formulas Lastechniek 11 1. 1965. p. 211. The several oxides present in the flux 4. Automatic Welding Under Flux. Het Ingenieursblad. 1966 no. 3, p. 116. may vary from more or less strongly It follows from the preceding con­ 5. I.I.W. Doc. XII-B-11-1966 and XII-A- basic via chemically neutral to less or siderations that the primary factor for 33-71.

360-s | JULY 197 2