Thermodynamics and Kinetics of Crystallization of Primary Cementite (Fe3C) in White Cast Iron

Background and Introduction of Iron-Carbon System

Iron-carbon compounds or compounds with iron and carbon elements is one of the most important family in metal material science. The Fe-C system is the most important alloy system in metal technology and engineering materials, because steel and cast iron are the main components of iron and carbon, which are usually quenched and tempered. High-carbon steel and low-carbon steel, high-grade cast iron and low-grade cast iron are the main members of the system. All these materials are widely used in industry, thus making the Fe-C system play an irreplaceable role.

The biggest difference between iron and carbon is that when different amounts of carbon enter into the iron lattice, different kinds of crystals are formed, and Fe-C compounds will appear in four distinct forms: ferrite, cementite, austenite and carbide. Among them, ferrite and cementite are known as proeutectic, austenite and carbide is known as eutectic, and the solubility of carbon in ferrite and austenite is known as eutectoid.

These distinct crystalline forms or so-called microstructures is the underlying factor for the mechanical properties of steel and cast iron. The microstructures, also known as microstructure matrix, are the result of thermodynamic and kinetic studies of the Fe-C system. The overall Fe-C system consists of various components and can be divided into solid solution system, eutectic system, eutectoid system and peritectic system. Its thermodynamic and kinetic properties are closely related to the components and structures of the Fe-C system.

Thermodynamics and Kinetics of White Cast Iron

White cast iron is a kind of cast iron formed by the initial precipitation of Fe3C. In this system, carbon will enter into the iron lattice as a solid solution and form austenite. When the carbon content exceeds the eutectoid point, the austenite will begin to transform into Fe3C and form a white appearance. This phenomenon is called the eutectoid reaction.

From the thermodynamic point of view, the eutectoid reaction determines the heat treatment temperature of white cast iron. At the beginning, cooling forms hyper-eutectoid and hyper-eutectoid white cast iron, but with increasing reaction temperature, the eutectoid reaction will occur in stages and the fraction of components in the reaction products will change. In addition, under certain conditions, the Fe-C system will form carbides and base metal liquid, and this phenomenon is called the peritectic reaction.

The solidification process of white cast iron is a result of the joint action of thermodynamic and kinetic factors. The cooling rate and temperature range of solidification determine the microstructure. During the solidification process, as solidification progresses, two-phase structures, such as proeutectoid and eutectoid, gradually form. In addition, the type and quantity of compounds formed also depends on the cooling rate.

Process Model of White Cast Iron

When studying the thermal and dynamic properties of white cast iron, the traditional phase model cannot provide enough accuracy and accuracy. In this case, a three-dimensional cellular automata (3dCA) is used for simulation. The 3dCA is a tool for simulating and studying periodic and mixed processes in phase transitions related to Fe-C systems. By simulating the casting structure with 3dCA, the influence of microstructural parameters such as solid/liquid interface velocity and lattice site selection on the final structure of white cast iron can be studied qualitatively.

Due to the difference in atomic size, the 3dCA automatically arranges the electronic environments of lattice sites to form a stable continuous energy environment. The energy system, combined with kinetic factors, can accurately reproduce the actual solidification process of white cast iron. In addition, the 3dCA preliminarily precisely implements the mechanism of carbon diffusion process in the molten iron.

Simulated results show that 3dCA can reproduce the development patterns of white cast iron microstructures, taking into account the effects of temperature and shell temperature control, solidification rate on microstructure. The simulated results also showed the beneficial effect of uniform pressure on the solidification of white cast iron and the impact of convection during solidification.

Conclusion

The Fe-C system is very important in terms of structure and properties and has great significance in industry. White cast iron formed by the initial precipitation of Fe3C has high hardness and wear resistance, which is widely used in industry. In this paper, thermodynamic and kinetic studies on white cast iron were discussed. The thermodynamic and kinetic properties of white cast iron are determined by the components and structures of the Fe-C system. The 3dCA is used to simulate the process of white cast iron, which can qualitatively study the influence of microstructural parameters on the final structure of white cast iron. In addition, the simulated results also showed the beneficial effects of uniform pressure on solidification of white cast iron and the impact of convection during solidification.

Thermodynamics and Kinetics of Formation of Primary Fe3C in White Cast Iron

White cast iron is an alloy of iron with carbon and usually contains 2.4 to 4.5% of carbon and about 1% of silicon. The primary constituents of white cast iron are ferrite, cementite and a carbide of the elements present. Cementite (Fe3C) is the main constituent chosen for this study. When heated, white cast iron undergoes a carbon diffusion-controlled diffusion process which leads to the formation of cementite from the ferrite matrix.

The thermodynamics of formation of Fe3C in white cast iron is controlled by the Gibbs free energy and thermodynamic equilibrium constants. At atmospheric pressure, the Gibbs free energy for the following reaction between Maltese and carbon is: Fe + C → Fe3C. At higher temperatures, the entropy favors the formation of Fe3C, meaning that the driving force for the reaction is mainly entropic and that the reaction proceeds spontaneously.

The kinetics of the formation of Fe3C in white cast iron is complex and involves several factors such as the rate of carbon diffusion, the difference in size of primary carbon and iron particles, and the presence of impurities. The rate of carbon diffusion into the iron matrix is estimated by the diffusion coefficient, which is inversely proportional to temperature and the activation energy required for the reaction. The activation energy for the reaction is about 83 kJ/mol and is strongly influenced by impurities.

The formation of Fe3C can also affect the physical properties of the white cast iron, such as hardness and fracture toughness. As the amount of Fe3C increases, the hardness and fracture toughness of the component increase, leading to an improvement in wear resistance.

To summarize, the thermodynamics and kinetics of formation of primary Fe3C in white cast iron is an important factor for understanding its behaviour in terms of structure and properties. The rate of formation of Fe3C is influenced by several factors, including the Gibbs free energy, the diffusion coefficient, the activation energy and the presence of impurities. The resulting improvement to the wear resistance of the component can be beneficial for applications that require high levels of wear resistance.