Термодинамічні закономірності структуроутворення високоміцного чавуну при модифікуванні та різних умовах кристалізації розплаву

Abstract

The article presents the results of the analysis of thermodynamic regularities of structure formation of high-strength cast iron during modification and under different conditions of melt crystallization. The concept of the formation of graphite nuclei in high-strength cast iron is proposed. It has been established that during the modification of molten iron, as a result of a significant redistribution of all elements dissolved in it, impurities (sulfur, phosphorus, etc.) are removed, which makes it possible to obtain the structure of cast iron with nodular graphite with the use of modifiers. During the crystallization of cast iron with the release of austenite, direct microliquation of silicon with a distribution coefficient less than one is characteristic, manganese is more evenly distributed in the metal, and carbide stabilizing elements are treated in the liquid phase. A thermodynamic calculation of the thermal effects of the reactions of the release of graphite from carbon dissolved in iron, the transition of silicon and manganese from a liquid state to a solid state in the process of forming high-strength cast iron is presented. The physical and latent heat of crystallization of high-strength cast iron was determined by the enthalpy method. Thermodynamic calculations were performed and the thermal effects of reactions (9) in the melt of high-strength cast iron were determined, depending on their concentrations and the temperature of the melt. Using the results of thermodynamic calculations makes it possible to study the crystallization process by changing the number and composition of phases that are formed under the influence of heat release of phase transformations. It was found that the introduction of Si into pre-eutectic alloys (from 1 to 5% at.) leads to a slight decrease in the latent heat ΔН of crystallization of primary graphite (up to 3% for each atomic percentage of Si). As a result of the change in the temperature interval, its release causes a decrease in the total physical heat of crystallization Qf. It is shown that the thermodynamic analysis of the processes that occur during the crystallization of cast iron based on the data on the change in enthalpy for the system Fe-C-Xi (Xi = Si, Mn) indicates that with an increase in carbon, the change in enthalpy in the crystallization interval changes approx lyzno by 16 kJ/mol. The physical heat of crystallization has a significant effect on the change in the value of the total heat of crystallization of cast iron Qkr Fe-C-Xi (Xi = Si, Mn) in comparison with the Fe-C alloy. The latent heat of crystallization during solidification of high-strength cast iron is between 245 and 275 kJ/kg.