Investigations of thermophysical properties of submerged arc welding slag using a rutile-acidic flux system

Abstract

For TiO₂–SiO₂–MgO and SiO₂–MgO–Al₂O₃flux system, the thermophysical and physicochemical behaviour of submerged arc slags studied. Slag plays an essential role in shielding the weld joint from atmospheric contamination. The mass percentage of each flux ingredient evaluated by the X-ray fluorescence technique. Twenty-one slags developed based on the mixture design approach. Thermogravimetric, DSC, and Hot-disc techniques used to analyze submerged arc slags. Thermogravimetric & DSC analysis was performed from 25 ºC to 900 ºC to find the thermal properties. The density measurement of slag performed at ambient temperature. Bond-behaviour was studied by Fourier transform infrared spectroscopy (FTIR) technique. Regression models for different slag properties developed. The multi-objective technique utilized to optimize different slags.

Introduction

To improve the arc stability, fluidity, slag detachability, bead appearance, and to refine the weld metal agglomerated fluxes are frequently used in submerged arc welding process. Mechanical as well as metallurgical properties of weld metal depend upon the ingredients of the welding flux used. The physical as well as chemical properties of fluxes get affected by transfer of these ingredients during submerged arc welding process [1]. To understand the interaction between weld metal and flux it is necessary to correlate the properties with the composition of the weld. Because transfer extent from slag to weld and weld to slag depends upon these slag metal interactions occurring in arc plasma region. It is very difficult to understand the mechanism of element transfer in submerged arc welding due to the complex slag metal as well as gas metal reactions. Weld joint composition after solidification in SAW process depends upon the dilution, parent metal composition and slag metal reactions. Since slag metal reactions plays an important role in deciding the final weld joint composition so the proper understanding of element transfer from flux to slag and vice versa is required [2], [3]. Due to high temperature involved and very fast occurrence of slag metal reactions in arc region, it is not necessary to achieve stable equilibrium. The kinetics as well as other chemical factors is equally responsible which needs to be understood for determining the final weld metal composition during element transfer in SAW process. An instantaneous equilibrium is established at the interface when a slag is placed in contact with an iron alloy. The diffusion coefficients, equilibrium constants and weld metal composition are widely dependent upon the chemical potential of flux ingredients [4], [5]. For improving the strength, various micro constituents such as Mn, Si, and B added in the steel, which provides a solid solution with iron and gives appropriate strength to ferrite. By refining the ferrite grains, the yield strength increased. The electrochemical reactions can be more influential in metallic additions to the weld from the slag. Flux constituents play an important role in deciding the inclusions content in the weld metal or slag. In submerged arc welding, molten droplet passes through high temperature plasma after detaching from the filler wire and combines with various ions present in the arc region. The extent of reactivity of various ions with that of molten droplets depends upon the parent metal & wire composition, and flux composition. There is an intimate contact of slag with that of molten metal takes place due to stirring action in the weld pool during transfer of arc in high temperature region. These interactions in the weld region depend upon the kinetic considerations [6], [7], [8]. Beidokhti and Pouriamanesh investigated the influence of filler metal content on the mechanical behaviour and microstructure of SAW welded API X65 steel. Acicular-ferrite microstructure developed exhibit excellent mechanical properties. For the sample having higher levels of titanium and boron content, tensile strength and impact toughness was higher. As the amount of acicular ferrite expands, the ductile-to-brittle transition temperature reduced. For the microstructure containing 80% of acicular ferrite, the impact energy observed about 70 J at −600C. The optimum size of oxide inclusions/particles (up to 2 μm) acted as nucleation sites for acicular-ferrite microstructure. But the mixing of the diffused fine oxide inclusions favours the nucleation of acicular ferrite, which is beneficial for improving impact properties [9]. Beidokhti et al. reported that the addition of titanium in the range of 0.02−0.08% in the weld metal promotes acicular ferrite. Further increase in titanium or manganese hardenability increased. The impact behaviour of the fused metal enhanced by mixing of Ti, but a more significant increase in the level of Ti percentage results in the development of quasi-cleavage fracture mode. Burck et al. investigated the influence of CaO, calcium fluoride, and iron oxide mixing for the SiO₂–MnO–CaF₂ flux system on weld metal chemistry. He quantitatively explained the effects of these constituents on the element displacement from the slag to weld metal and vice versa using thermodynamic data. For the manganese-silicate flux system (SiO₂–MnO–CaF₂), the carbon/oxygen appears to be controlled by a CO reaction in the 1010 steel welds while the manganese level remains constant. It noticed that when welds processed by the SiO₂–MnO–FeO flux system on the AISI 4340 and 1020 steel plates despite increasing the oxygen level shows consistent carbon contents. Davis and Coe studied the role of flux content and weld metal inclusions during the SAW welding of C-Mn steels. They observed complex changes from the composition of elevated temperature interactions in arc plasma. The transferring metallic elements are comprised of various chemical blends such as fluorides, oxides, and carbonates and during welding, these elements promote phase transformation. Different elements and oxygen disintegrate in the weld pool, due to the contribution of all oxides from the fluxes. Due to these interactios the weld mechanical as well as microstructural properties improved [10]. Chai and Eagar observed the stability of binary CaF2 oxides during the SAW process. Results indicate that the strength of metal oxides during welding was not exactly similar to their thermodynamic stability. Some fluxes in arc plasma, which is chemically supportive, may dissociate into suboxides. Such oxides produce a high level of oxygen in the weld metal than chemically stable oxide and result in reduced impact toughness of the weld metal. It observed that CaF₂ in the weld metal reduced the amount of oxygen by the dilution effect (dilution of metal oxides). In the SAW process, fluxes addition of FeO (less than 10%), MnO, and SiO₂ are the primary sources of oxygen contamination [11]. Kanjilal et al. observed the behaviour of various flux ingredients by forming a statistical model for different flux components. At suitable welding current and voltage SAW fluxes processed as per the extreme vertices approach of mixture experiments in rutile basic flux system. It observed that the single flux component is most compelling. It has a synergistic influence on weld metal transfer from silicon, oxygen, manganese, and carbon contents, and displacement of oxygen influenced by various premises of flux constituents such as thermodynamic stability, viscosity and oxygen potential [12]. In the present paper, for TiO₂-SiO₂-MgO and SiO₂-MgO-Al₂O₃flux system, the thermophysical behaviour of submerged arc slags was studied.