Theoretical modeling and experiments for phase transformation of low carbon steel during continuous cooling

• Main Authors: Chien-Shun Yun, 雲建順
• Other Authors: Jui-Chao Kuo
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/02556070176344339393

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 97 === The thesis is focused on the establishment of a model, which predicted the phase transformation process of low carbon steel during continuous cooling, and utilized some experiments to prove the mathematic model. In this model, the progress of transformation is assumed as follows. The transformation from austenite to ferrite starts at the Ar3 temperature and ended at the Ar1 temperature followed by the transformation of pearlite . The ferrite transformation follows ‘nucleation and growth’ model at the early stage and then follows ’site saturation’ model at the later stage. Owing to the phase transformation equation consisted of nucleation and growth mechanisms, so it’s necessary to calculate the diffusivity, nucleation rate and growth rate previously. However, these calculations were functions of some thermodynamic parameters, which included the equilibrium concentration and Gibbs free energy change of phase transformation. As a result, it’s a must to establish a thermodynamic model to calculate these thermodynamic parameters. The study also used the EBSD to analyze the final microstructure, including phase fraction and grain size distribution. Because the model required the Ar3 and Ar1 temperature of different cooling rate, we experimented the thermal expansion analysis of different cooling rate to derive the Ar3 and Ar1 temperature, and then calculated the transformation fraction of pre-eutectoid ferrite within the range. Ultimately, we proceeded the EBSD analysis to observe the phase fraction of pre-eutectoid ferrite. In comparison with the experimental and simulated result, the value was much closer in the slow cooling rate than that in the quick cooling rate.