Chemical Reaction At Slag/Metal Interface During Quasi-steady Welding Based On Non-equilibrium Thermodynamics

Chemical reactions occurring in slag/metal/gas interface under welding condition play an important role in original determination of weld metal for its numerous physical-chemical performances, the theory correlated with such problem is a main constituent in welding metallurgy. However, for its characteristic of open and far from chemical equilibrium, scholars at home and abroad have yet not make a thorough investigation effectively for this special metallurgical reaction. As an initial introduction of non-equilibrium dissipative structure theory to welding chemical reaction which is open and far from equilibrium, the submerged arc welding with intensive slag/metal interaction was used to investigate its chemical reaction during quasi-steady state using MgO-CaO-SiO₂ and CaO-Al₂O₃-SiO₂ two typical slag systems in iron and steel making. The method of non-equilibrium thermodynamics was adopted to investigate its chemical metallurgical behavior quantitatively so as to explore some inherent non-equilibrium chemical reaction regularities occurring in slag/metal interface during quasi-steady state.Based on agglomerated fluxes made with two typical slag systems, the test has been designed for samples of liquid droplet metal and weld pool metal to be taken out with non-oxidation method in each stage during quasi-steady welding. Some slag/metal chemical reaction information occurring in quasi-steady state could be obtained with the help of physical-chemical measurement and microstructure analysis. Combining with established geometric model for droplet reaction zone and weld pool reaction zone, it could be used as experimental evidence to study direction and extent of chemical reaction quantitatively. The experimental result confirmed the feasibility of this program and achieved the desired purpose.To resolve the effective concentration of substances existed in molten slag and actually taking part in slag/metal chemical reaction during quasi-steady period, as a consideration of both ionic theory and molecular theory in metallurgy field, the coexistence theory which had been put forward in recent years was applied to calculate concentrations of structure unites in molten slag with non-linear numerical simulation method. The numerical calculation was carried out according to dynamic equilibrium relationship between free ions and molecules combining ternary phase diagrams of two slag systems at high temperature. The result of calculating shows that the concentrations of free ionic, simple molecular compounds and composite compounds vary with the basic index of slag and appear certain regularity. The concentration of free oxygen ion in molten slag increases with the increase of basic index value. It has shown a good application prospect in study of welding metallurgy for thermodynamics in quantity applying coexistence theory to calculate effective concentrations of structural unit in molten slag.For a constant substance and energy exchange between reaction system of slag-metal interface and external environment during quasi-steady welding, one method which can be used for quantitative description of slag/metal chemical reaction with the concept of non-equilibrium stationary state was introduced in this paper. According to dynamic equilibrium relationship between non-zero thermodynamic flux resulted from constant thermodynamic driving force existed in non-equilibrium chemical reaction and fluxes of reactants or products, combining analysis of samples taken out during quasi-steady welding, the direction and extent for chemical reaction could be analyzed quantitatively. This makes it possible to overcome shortcoming resulted from seriously in conflict with actual situation using traditional equilibrium thermodynamic method to study welding chemical reaction. In addition, by analyzing with Lyapunov stability theory, it also shows that this non-equilibrium stationary state has the stability against the disturbance of boundary conditions under certain welding technology condition.With concept of non-equilibrium stationary state in submerged arc welding during quasi-steady state, the non-equilibrium metallurgical characteristic of SiO₂ /Fe at slag/metal interface in droplet reaction zone and weld pool reaction zone for slag system was calculated with non-equilibrium thermodynamic method, and the quantitave calculating result for thermodynamics shows that the direction of chemical reaction occurring in droplet stage is in oxidation direction resulted from SiO₂ in molten slag with liquid metal Fe, the reaction is in non-linear area of non-equilibrium thermodynamics with large thermodynamic driving force; while in weld pool stage is in inverse de-oxidation direction with departure from equilibrium in non-linear area also, but the distance and chemical driving force are smaller than that in droplet stage.To counter some foreign scholar's viewpoint by which there exists electrochemical reaction at slag/metal interface under direct current welding condition, experiments and kinetics of oxygen pick-up resulted from electrochemical reaction in droplet reaction zone was investigated for two slag systems. It shows that electrochemical reaction exerts liquid metal an obvious effect on its oxygen pick-up in the case of high basic index value. For slag system used in experiment, oxygen pick-up from electrochemical reaction can be almost as high as that from thermo-chemical reaction. Therefore it is necessary to fully consider of this effect for developing high basic index welding consumables or formulating direct current welding technology.