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Effect of Electrode Coating Type on the Physico-Chemical Properties of Slag and Welding Technique

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Effect of Electrode Coating Type on the Physico-Chemical Properties of Slag and Welding Technique Ilona Jastrzębska, Jacek Szczerba, Paweł Stoch, Ryszard Prorok, Edyta Śnieżek Effect of Electrode Coating Type on the Physico-chemical Properties of Slag and Welding Technique Abstract: The article presents the results of the physico-chemical tests concern- ing the properties of various slags formed after welding with low-hydrogen and rutile electrodes as well as flux-cored wires. The slags tested were analysed for their phase compositions using X-ray diffractometry (XRD) X-ray fluorescence (XRF). The behaviour of slags at temperatures elevated to 1400°C was examined using a high-temperature microscope. The analysis of the high-temperature tests enabled the determination of slag characteristic temperatures, i.e. softening and melting points. The results obtained demonstrated various properties of slags, with the low- est characteristic temperatures for low-hydrogen slags and the highest characteristic temperatures for flux-cored wire slags. In addition, on the basis of different char- acteristic temperatures slags were classified in relation to their solidification rates. Keywords: low-hydrogen electrodes, rutile electrodes, flux-cored wires, slag properities Introduction Welding using covered electrodes and flux-cored wires is an important and indispensable tech- nique while performing welding processes in construction site conditions. Slag formed dur- ing welding by means of these methods pro- tects a weld pool against oxidation, stabilises arc burning, ensures the proper chemical com- position of a weld and provides the insulation Fig. 1. Weld in cross-sectional view protection thereof (Fig. 1). The type and chem- ical composition of coatings significantly affect The production of covered electrodes is a mul- the mechanical strength of a weld. In addition, ti-stage process (see Fig. 2). the physical properties of solid-state and liquid The first and most important issue in the pro- state coatings, such as grain size, viscosity, sur- duction of covered electrodes is the chemical face tension, thermal expansion coefficient and composition of the coating mixture. The var- heat capacity also influence the obtainment of ious behaviours of mixtures and fluxes avail- a good quality weld in a required shape [1-6]. able on the market depend on their chemical mgr inż. Ilona Jastrzębska (MSc Eng.); dr hab inż. Jacek Szczerba (PhD (DSc) habilitated Eng.); dr inż, Paweł Stoch, Professor Extraordinary (PhD (DSc) Eng.); mgr inż. Ryszard Prorok (MSc Eng.); mgr inż. Edyta Śnieżek (MSc Eng.) - AGH University of Science and Technology, Kraków No. 1/2015 BIULETYN INSTYTUTU SPAWALNICTWA 37 Initial mixture of fine The reaction of carbonate decarbonisation raw materials is endothermic in nature. In this reaction, re- leased CO₂ makes up 44% of the total calcite Homogenisation of the dry mass. The transport of CO₂ takes place as a re- mixture of raw materials sult of Knudsen diffusion through voids and cracks between phases and through spaces Homogenisation of the wet formed due to the separation of CaO crystals mixture of raw materials (raw materials + filler metals) from CaCO₃ not yet decomposed [8]. J.D. Ples- sis and the co-authors [9] examined the effect of calcium carbonate content in the phase compo- Forming – pressing sition of a low-hydrogen coating mixture on the content of hydrogen diffusing in a weld metal, with calcite content in the mixture varying be- “Brushing” tween 10% and 24%. Tests revealed a decreas- ing amount of hydrogen in a weld for mixtures containing up to 18% of CaCO₃. Greater CaCO₃ Drying amounts produced the opposite effect. While preparing coating mixtures it is also possible to use a number of other carbonates Visual inspection (Table 1), yet they are used to a lesser degree. As can be seen in Table 1, the mineral releasing Fig. 2. Simplified production scheme the greatest amount of CO₂ is magnesite, whilst of covered electrode production iron carbonate (siderite) decomposes releasing composition, the geological origin of the raw the smallest amount of the above mentioned materials used for their production and the gaseous compound. It should be emphasized manufacturing process itself [1]. The selection that among all basic oxides it is calcium oxide of raw materials for the preparation of a coating that has the greatest ability to absorb and fix wa- depends on the required phase composition of a ter in the structure, hence its excellent capabili- dry mixture. Having the foregoing in view, such ty of reducing hydrogen and oxygen contents in a dry mixture should contain minerals, which a molten metal area. The ability of magnesium during welding, release gases stabilising an arc and protecting the liquid weld pool. The most Table 1. Minerals containing the carbonate group (CO₃2-), commonly used minerals providing gaseous the products of their thermal decomposition protection include calcite, supplied with lime- and the amounts of CO₂ released in stone, having a chemical formula of CaCO₃. The a decarburisation reaction [3,4]. thermal decomposition of CaCO₃ takes place Thermal Amount of at 950°C, leading to the formation of CaO and Mineral decomposition released CO₂, products % by weight CO₂ (1), which, in turn decomposes (in arc plas- calcite – CaO, CO₂ 44.0 ma) into carbon monoxide and atomic oxygen CaCO₃ in accordance with the reactions (2, 3) [7]: magnesite – MgO, CO₂ 52.2 CaCO₃→CaO + CO₂↑+ 178 kJ/mol (1) [7,8] MgCO₃ siderite – FeO, CO₂ 38.0 2CO₂↑→2CO↑+ O₂↑ (2) [9] FeCO₃ dolomite – CO↑→C +½O₂↑ (3) [9] MgO, CaO, CO₂ 47.7 CaMg(CO₃)₂ 38 BIULETYN INSTYTUTU SPAWALNICTWA No. 1/2015 oxide to react with water is significantly infe- commonly used in the production of coating rior if compared with that of CaO, which is mixtures and is provided in the form of fluorite additionally responsible for the predominant (melting point 1418°C). As the low-hydrogen popularity of calcium carbonate [10-16]. (basic) components of mixtures are character- The thermal decomposition of magnesite and ised by a greater ability to fix water, CaF₂ is that of siderite are similar, in accordance with commonly used in the production of low-hy- the reaction (1). In turn, dolomite decomposes drogen (basic) electrodes. Tests conducted by in a two-stage reaction. During the first stage, at C.S. Chai and co-authors [18] revealed that flu- a temperature of 700-750°C, reaction products orine, F₂, reacts with hydrogen molecules form- are CaCO₃, MgO and CO₂. The second stage ing hydrogen fluoride, HF, in accordance with takes place at a temperature of 900-1000°C and the reaction (4): leads to the decomposition of CaCO₃ into CaO F₂+H₂→2HF↑ (4) [9] and CO₂ [16]. It should be mentioned that the greater the carbonate fraction, the higher the ba- Hydrogen fluoride does not dissolve in liquid sicity of slag and the lower the hydrogen content iron and evaporates from an arc area [9]. Sadly, in a weld [9]. This is due to a greater tendency calcium fluoride is not sufficiently active and to fix water in a structure through the absorp- its part enters slag. Therefore, it seems advisa- tion of hydrogen from arc plasma, which pre- ble to investigate the behaviour of other fluo- vents the hydrogen from entering a liquid metal. rides. It has been found that calcium fluoride Other raw materials used in the production in the presence of silicon dioxide, SiO₂, reduc- of coatings include clays containing alumin- es its oxidising ability forming silicon fluoride, ium silicates e.g. kaolinite – Al₄Si₄O₁₀(OH)₈, SiF₄, as can be seen in the reaction (5): alkaline feldspars containing aluminosili- 2CaF₂+SiO₂→SiF₄↑+2CaO (5) [18] cates such as orthoclase (KAlSi₃O₈) and albite (NaAlSi₃O₈), quartzites enriched in silicon di- During submerged arc welding it is possible to oxides (96-98 % mol.) and high-purity tita- observe the formation of a small amount of sil- nium dioxide containing over 95% TiO₂. The icon fluoride which decreases the partial pres- third group is composed of ferroalloys sup- sure of hydrogen. In addition, it has been found plied to a mixture in order to deoxidise a liq- that the formation of SiF₄ does not reduce the uid metal in a weld. Ferromanganese (FeMn), content of silicon in a weld [18]. T. Lau and ferrotitanium (FeTi) and ferrosilicon (FeSi) co-authors [19] noticed the reaction between belong to the group of deoxidising raw mate- calcium fluoride and aluminium oxide in slag, rials most commonly used for the preparation in accordance with the equation (6): of coating mixtures and fluxes. The addition of 3CaF₂+Al₂O₃→2AlF₃↑+3CaO (6) [19] fine iron (Fe) enables faster melting of the elec- trode, thus increasing welding process efficien- Fluorite is entirely included in the slag ratio, cy. Welding without the aforesaid component expressed by a basic component – acidic com- requires greater power consumption, is respon- ponent ratio in accordance with the equation sible for a droplet-type liquid metal transport (7), describing the metallurgical behaviour of mode (more difficult to control) and leads to a a coating mixture and having a significant ef- greater number of spatters [18]. fect on the toughness of a weld [12]. The high- The appropriate fluidity of molten slag and er the basicity, the lower the hydrogen content the reduction of diffusive hydrogen content are in a weld and the greater the toughness of a obtained by adding components containing flu- weld [12,20]. oride ions [9]. Calcium fluoride, CaF₂, is most No. 1/2015 BIULETYN INSTYTUTU SPAWALNICTWA 39 BI=(CaO+MgO+BaO+SrO+Na₂O+K₂O+ prevents the excessively fast drying of a mix- Li₂O+CaF₂+0,5∙(MnO+FeO))/ ture, which could trigger the formation of cracks (SiO₂+0,5∙(MnO+FeO)) (7) [20] on a coating surface. Plasticisers of graining H. Terashima and co-authors [21] have observed below 0.3 mm are used in small amounts (ap- a decrease in the content of hydrogen diffus- proximately 0.5-1%) as additions to mixtures ing in a weld from 12 to 2ml per 100g of molten [24].
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