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Reconsidering the Basicity of a FCAW Consumable — Part 1: Solidified Slag Composition of a FCAW Consumable As a Basicity Indicator

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Reconsidering the Basicity of a FCAW Consumable — Part 1: Solidified Slag Composition of a FCAW Consumable As a Basicity Indicator WELDING RESEARCH SUPPLEMENT TO THE WELDING JOURNAL, MARCH 2000 Sponsored by the American Welding Society and the Welding Research Council Reconsidering the Basicity of a FCAW Consumable — Part 1: Solidified Slag Composition of a FCAW Consumable as a Basicity Indicator A basicity index for a flux cored electrode was developed, taking into consideration the metal sheath, fill ingredients and weld metal composition BY E. BAUNÉ, C. BONNET AND S. LIU ABSTRACT. Based on an investigation expressing the flux/slag basicity. The slag detachability, the metallurgical performed using a set of five experimen- newly defined basicity index is found to properties of the final weldment, etc. tal FCAW electrodes, an improved ver- offer superior correlation with the weld Therefore, the multitude of flux ingredi- sion of the IIW basicity index formula is metal oxygen content, demonstrating ents used in a FCAW electrode, each fea- developed. This new methodology is de- the validity of the assumptions made in turing various functions, make the work scribed in two papers, titled Part 1: So- the present investigation. of formulators rather complex. For a par- lidified Slag Composition of a FCAW ticular FCAW electrode, with proper in- Consumable as a Basicity Indicator and Introduction formation on the chemical composition Part 2: Verification of the Flux/Slag of the deposited weld metal, the compo- Analysis Methodology for Weld Metal A flux cored arc welding (FCAW) elec- sition and nature of the core flux, a sim- Oxygen Control. To accomplish this pur- trode contains multiple powdered ingre- ple compositional relationship can be pose, the partition of the various ele- dients within a metal sheath. Moreover, obtained to describe the distribution of ments contained in the formulation of the variety of the ingredients that can be each metallic element present in the one FCAW electrode is studied and mod- used in FCAW is enormous. For these rea- electrode between the covering slag and eled in Part 1. Correspondingly, the com- sons, the intrinsic nature of welding the weld metal. position of the solidified slag is predicted fluxes is rather complex. Also, due to the In the present paper, for one particu- for this particular electrode. To verify the various chemical reactions involving lar experimental FCAW electrode, based model, the prediction of the slag chemi- these ingredients in the arc environment, on a mass balance considering the metal cal composition is compared with ex- it is not a simple task to understand how sheath, the electrode fill and the weld perimental measurements. Good accor- each constituent contributes to the gen- metal chemical composition, a simple dance is found, which shows the model eral behavior of the flux with regard to approach to predicting the composition is applicable. Also, a new way of defin- the electrode performance, i.e., the metal of the solidified slag is proposed. Hence, ing the basicity of a FCAW consumable transfer stability, the slag viscosity, the a methodology to determine the basicity based on the chemical composition of index of the slag is developed. the slag is derived. In Part 2, comparison of this innovative methodology with the FCAW Using CO2 as Shielding Gas IIW formula is achieved, as well as with other means reported in the literature for KEY WORDS A flux cored electrode is a composite, tubular electrode that consists of a metal FCAW Electrode sheath containing a core of flux (Ref. 1). E. BAUNÉ is R&D Engineer and C. BONNET Flux Cored The flux is made up of a mixture of pow- is Technical Manager with Air Liquide/Centre Basic dered ingredients, both metallic and Technique des Applications du Soudage Basicity Index nonmetallic. The main function of the (CTAS), Pontoise, France. S. LIU is Professor of Slag metallic materials is to alloy the weld Metallurgy, Center for Welding, Joining and Consumable metal, together with the alloying ele- RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT Coatings Research, Department of Metallurgi- ments contained in the metal sheath, in cal and Materials Engineering, Colorado order to increase the strength of the de- School of Mines, Golden, Colo. posited metal and deoxidize it. The non- WELDING RESEARCH SUPPLEMENT | 57-s be determined as a function of ∆G0 , the free energy of decomposition of CO2, Ρ CO2 and theΡCO ratio. This reads: 2 ρCO2 −⋅2 ∆G0 aOO==ρ ⋅ exp 22 ρ RT CO (4) Furthermore, from the various plasma- metal reactions occurring between the deoxidizing elements and oxygen, oxide compounds are formed, which tend to float to the molten weld pool surface. As solidification of the weld pool proceeds, these oxide compounds become part of the covering slag. A small amount of the oxide products may also be trapped in the solidified weld metal in the form of inclusions. In the present study, several assump- tions are made to simulate the various re- actions involving oxygen in welding ex- periments carried out using an experimental basic-type flux cored wire under 100% CO2 shielding. A descrip- tion of the reactions taking place in the Fig. 1 — X-ray diffraction pattern of slag sample collected after welding with the experimental arc column is then conceived. As a result, electrode. the partition of the various elements pre- sent in the constitution of the FCAW elec- trode between the deposited weld metal metallic ingredients are often slag form- dioxide can then further decompose into and the solidified slag can be deter- ers and arc stabilizers, i.e., helping to carbon and oxygen (O2), as indicated by mined, as well as the composition of the produce a smoother arc. They provide a the following reactions: solidified slag. From this knowledge, a secondary shielding action and help pu- 1 methodology to calculate the basicity CO (g) → CO (g) + ⁄2 O (g) (1) rify the weld metal. The nonmetallic in- 2 2 index of the FCAW electrode is derived. gredients also help in reducing weld 1 CO (g) → C (s) + ⁄2 O (g) (2) spatter and in controlling the melting 2 Basicity Index of a Flux/Slag System characteristics of the electrode. Proper selection of the core elements As a result, a certain amount of oxy- The concept of basicity was first is of prime importance for any welding gen is available in the arc atmosphere to adopted to answer the needs of the steel application of acceptable quality. When react with the various elements present in making industry. It was used to evaluate choosing the ingredients for a new flux the molten metal (metal droplets and the sulfur refinement ability of a slag in design, formulators need to carefully weld pool). Therefore, when CO2 is used steel ladle refining practices (Ref. 3). study the physical and chemical charac- as the shielding gas and dissociates dur- Later, the use of basicity index was teristics of the raw materials to utilize, ing welding, a strong oxidizing effect broadened to approximately measure the e.g., their particle shape and size distrib- takes place. That is partly why deoxidiz- flux oxidation capacity, and the same ution, their chemical composition and ers have to be added to the fill ingredients principles were applied to the welding purity, their hygroscopic tendency and of the electrode to compensate for the technology. Consequently, it has been their chemical stability. Also, the fill ratio various oxidation sources that include general practice to use the basicity index of the electrode, i.e., the ratio of the flux the oxidizing effects of CO2 and the O2- to characterize a welding flux with regard weight over the total electrode weight, containing constituents of the electrode. to its physical and chemical properties. has to be closely controlled in order to Also, in the presence of carbon in the The slag viscosity and the weldment obtain reproducible weld quality along molten weld pool, CO2 can react to form quality, as represented by the amount of the weld bead. CO according to the reaction below — oxygen pickup in the weld metal and the Carbon dioxide is certainly the most Boudouard equation (Ref. 2). weld metal notch toughness, can both be widely used shielding gas for FCAW steel estimated as a function of the basicity electrodes that require auxiliary gas CO2 (g) + C (s) → 2 CO (g) (3) index. Thus, a high basicity slag is con- shielding. Its selection is mainly due to sidered to be one that exhibits a high the possibility of high welding speeds, The simultaneous presence of CO and concentration (or a high activity) of free Ρ 2– good weld penetration and, most of all, CO2 oxide ions O . It would also tend to ex- hibit an increased breakdown of the low cost. At room temperature, carbon CO determines theΡCO ratio, which dioxide is a relatively inactive gas. When 2 three-dimensional silicate network struc- also controls Ρ , the partial pressure of RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT heated to high temperatures in the weld- O2 ture (Refs. 3, 4). Such a slag would there- ing arc, however, part of the CO2 disso- oxygen of the arc environment, as gov- fore be composed of a large quantity of ciates to form carbon monoxide (CO), erned by Equations 1 and 2. Thus, the ac- dispersed cations in the broken silicate which is more stable than CO2. Carbon tivity (or partial pressure) of oxygen can network, contributing to a more fluid slag 58-s | MARCH 2000 in the molten state. Defining the concept of basicity of slags by means of the O2– activity, however, is virtually not feasible, since activities or concentrations of free oxide ions are impossible to measure ex- perimentally. In practice, weld metal mechanical properties and weld metal oxygen con- tent are related to the welding fluxes by means of the basicity index, which is usu- ally defined as the following: Σ Basic Oxides Basicity Index = Σ Acidic Oxides (5) Equation 6 is the most widely used ba- sicity index formula and is recognized by the International Institute of Welding Fig.
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