A Phase-field Simulation Of Austenite To Ferrite Transformation In Low Carbon Steels

Since coming into steel age, steel has always to be important materials needed by human society. Recently steel has been challenged by ceramics, high polymer and nonferrous metals, but it still plays an important part in material fields and is widely used to various fields in our national economy. In material fields, steel holds the dominant position, which will exist for a long time, and it's also very important to study the property of steel materials.Research in phase transformation which influences the physical and chemical properties of materials has an important effect on steel properties' study. To study the thermodynamics and kinetics of austenite-ferrite transformation, establish the quantitative relationship between processing technology and microstructure, has important significance for designing material composition and making processing technology to obtain the needed material properties.In recent years, as one of the methods of simulating materials' microstructure, phase-field simulation which becoming more and more important is an ideal method to simulate phase transformation at present. This paper successfully simulates the isothermal transformation and massive transformation of austenite-ferrite by establishing the phase-field simulation model.In process of isothermal transformation, the ferrite nuclear grows up gradually. The supersaturate carbon atoms in ferrite phase are rejected to austenite continuously so that they are obvious stacking in the interface of austenite and ferrite where the carbon concentration keeps rising, then it makes a gradient of carbon concentrate in austenite. Under the concentrate gradient drive, the carbon atoms in interface transfer to the inside of austenite so that the carbon concentration of austenite where is far away from the interface increase high continuously with the phase transformation developing.Temperature has some effect on the isothermal transformation of single ferrite nuclear. At the same time, the carbon concentration gradient in austenite decreases with the temperature rising. At the range of low temperature the growth speed of the ferrite nuclear accelerate with the temperature increasing, but contrary at the high temperature.In process of massive transformation, carbon accumulates in the interface of austenite-ferrite and after the interface moving the carbon concentration of newly formed ferrite does not show rapid decrease, but almost equal to mother austenite phase. It shows that shot-range diffusion of carbon happens in the interface during transformation which is massive transformation.The phase-field model which we build in this paper and the application program show a better physical mechanism, and the result of simulation also reveals that the change law of carbon concentration and the growth law of ferrite nuclear accord with the theory of phase transformation. Our research lays the foundation for the further study in simulation of austenite-ferrite transformation using phase-field method.