For manual arc welding electrodes with a thick layer covering the chemical reactions between metal arc gases and slag flow at 2100-2300 C in a time of transition drops of molten metal through the electrode and cover arc gap. Further physical and chemical processes occur in the weld pool. The main chemical reaction during welding under flux occur only under a layer of molten flux in the welding bath. Consider key reaction in the weld zone of steel, as the most popular metal that is to be welded. Oxidation. In the area of welding, oxygen is the most harmful impurity, so as to oxidize the elements that are part of the metal seam, and degrades its quality, forming chemical compounds – oxides. Oxidation of the elements, usually occurs due to oxygen contained in the slag and gas welding zone.
Oxidation to a lesser extent may be due to oxygen surface base metal oxides (rust, scale). If you accidentally drop increasing the arc length of the electrode metal can be oxidized by oxygen in ambient air. The oxidation of initially formed ferrous oxide, which in the future under certain conditions (the ratio of oxygen and iron in the weld pool, temperature) can move into mixed oxide and iron oxide. During the welding process in the oxidation of iron main importance of iron oxide, as soon as it is able to dissolve in the liquid metal. When the oxygen content in the steel reaches 0,035%, from the solution begins to stand out excess oxygen to form iron oxide and located between the grains of the metal. Chemical reactions in the welding area are not up to the end and to a certain equilibrium between the reaction products and starting materials. The equilibrium state is characterized by simultaneous presence in the reaction zone as a free metal and its oxide at the designated ratios. The equilibrium state depends primarily on the concentration of the reactants in the reaction zone pressure and temperature.
The equilibrium state is determined by the equilibrium constant. Concentrations of the reactants determine only the direction of the reaction. The possibility of this reaction is due to chemical affinity of participating agents, in this case, the affinity for oxygen. In the presence of a base metal multiple items, they begin to oxidize all the time, but those elements whose affinity for oxygen at a given temperature greater than would be oxidized intensively and thoroughly. In the first place when welding steel oxidized iron, which is the main component. Other components are oxidized more rapidly than larger chemical affinity this component with oxygen. By reducing the degree of chemical affinity with oxygen, the components can be placed in the following order: aluminum, titanium, silicon, manganese, chromium, molybdenum, iron, nickel, copper. Carbon in increasing the temperature enhances the activity of oxygen and at 1700 C exceeds its activity titanium, and at 2100 C, aluminum. With decreasing in the reaction zone, the concentration of the components that have high affinity for oxygen, the rate of oxidation decreases. Appropriately increasing the rate of oxidation of other elements with lower affinity to oxygen, which begin to burn more intensely as long as their concentration is not drops to an equilibrium and will not end the oxidation. This process of successive increases the rate of oxidation of individual elements extending to as long as the concentration of all elements will not match equilibrium, after which the oxidation of the metal in the weld pool to an end.