Thermodynamics and Kinetics of Eutectic Crystallization in White Cast Iron

Thermodynamics and Kinetics of Ferritic White Iron Cast

Abstract

The thermodynamics and kinetics of ferritic white cast iron (FWCI) are considered to provide a better understanding of its formation mechanisms. Following the introduction of FWCI, the bulk of this paper discusses the thermodynamic and kinetic aspects of formation of FWCI. This paper attempts to explain that by utilising thermodynamic and kinetic principles, FWCI with desirable properties can be created.

1. Introduction

Ferritic white cast iron (FWCI) is a form of cast iron, which due to its structure, has higher wear resistance when compared to other forms of cast iron. This makes it particularly ideal for wear-resistant components in engineering applications. FWCI is formed through a process known as crushing and crystallising white cast iron (CCWC). This process involves an increase in temperature to the melting point of the metal, followed by rapid quenching. In this state, the metal becomes very brittle and can be easily honed and polished to produce a uniform surface.

Through CCWC, the iron microstructure can be changed and made more resistant to wear, erosion and oxidation. This is due to the crystal structure of FWCI, which consists of martensite, bainite and retained austenite. The presence of these three components gives the metal its desirable traits, including increased wear resistance, increased corrosion protection and increased hardness.

2. Thermodynamics and Kinetics of FWCI Formation

The thermodynamics and kinetics of FWCI formation require much consideration and attention, as they greatly affect the properties of the final product.

The process of CCWC is mainly driven by the chemical composition of the iron and the temperature of the substrate. This is due to the fact that the iron is heated above its eutectic temperature, and then quenched rapidly. At this temperature, the iron begins to cool down and the reaction between iron and carbon begins, forming a mixture of austenite and ferrite.

The heat from the reaction then causes the formation of martensite, bainite and retained austenite within the microstructure. This process is kinetically driven and occurs at temperatures below the eutectic temperature, which is generally in the range of 815-872 °C.

The thermodynamics and kinetics of the FWCI formation affects the nature of the resulting iron. The iron microstructure is predominantly determined by the chemical composition, temperature of the substrate and the rate of cooling. Therefore, by utilising thermodynamic and kinetic principles, it is possible to create FWCI with desirable properties.

3. Conclusion

The thermodynamic and kinetic principles of FWCI formation is crucial for production of FWCI that is desirable for engineering applications. By utilising thermodynamic and kinetic principles, FWCI with desirable properties can be created. Consideration should always be given to the chemical composition, temperature of the substrate and the rate of cooling when forming FWCI.

，查找以下资料 Iron based bainite-austenite duplex structures in white cast iron offer some desirable mechanical properties as they are a combination of hard bainite ferrite and tough, ductile austenite. In order to develop such a structure, the proper heat treatment cycle is necessary and a complete understanding of the effects of temperature and time on the kinetics and thermodynamics on the growth and precipitation of both bainite and austenite is required.

The white cast iron is initially a hypereutectic variant where carbon content is above the eutectic level and the primary phase is cementite and a small amount of ferrite. At lower temperatures below the A3, bainite and pearlite will appear and replace the martensite. At higher temperatures, austenite is formed with increasing cooling rate due to the carbon content.

The kinetic studies have revealed that the austenite forms at lower cooling rate than bainite and is highly depend on the alloy composition. With increasing cooling rate and carbon content, austenite start forming and crystallizing at sufficient cooling rate and M23C6 carbides start to precipitate at temperatures between 690-840 C.

The thermodynamic studies of the austenite and bainite in white cast iron have shown that at reduced temperatures and cooling rates, the austenite start forming. With increasing cooling rate and decreasing temperatures, the austenite start dissolving and the bainite appears. This suggests that the formation of bainite is driving by the thermodynamics of the alloy system.

In conclusion, the heat treatment of white cast iron to develop a bainite-austenite duplex structure is dependent on both thermodynamic and kinetic factors. The thermodynamic driving force for bainite formation is determined by the alloy composition and the carbon level, while austenite formation is controlled by cooling rate through the arrangement of the M23C6 carbides in the microstructure. A proper heat treatment cycle must be determined in order to achieve the desired structure and mechanical properties.