The structure of a steel changes with temperature due to phase transformations. This phase change is generally companied by a variation in the specific volume which is exhibited as a departure from the behaviour of th...The structure of a steel changes with temperature due to phase transformations. This phase change is generally companied by a variation in the specific volume which is exhibited as a departure from the behaviour of the thermal expansion or contraction, at the temperature at which the change occurs. This behaviour can be detected as a change in the dimensions of a suitable test piece, and forms the operating principle of dilatometry. The dilatometric technique may be applicable to the investigation of the phase transformation kinetics if the relation between the fractions of phases, the temperature, the compositions of phases and the dilatation can be built up. Efforts have been made to analyze the dilatation during an isothermal transformation or to calculate the dilatation during the heating of plain carbon steels using the thermodynamic function. The existing models are, however, not directly applicable to the determination of the phase transformation kinetics from a dilatation curve. A detailed analysis has been made of the length change of a hypoeutectoid steel during a continuous cooling. A model has been developed in which the transient dilatation is calculated based on the fraction of the phases present. The model takes into account the redistribution of carbon and is applicable to the determination of the phase transformation kinetics from the dilatation data during a cooling of a hypo-eutectoid steel. The model was validated by comparing the model results with the experimental results of an interstitial free steel. Experiments have been done with an interstitial-free steel and a C-Mn steel. The model has been applied to the calculation of the transformation kinetics indicated by the dilatation curves. Excellent agreements between the model and the experiments have been obtained.展开更多
文摘The structure of a steel changes with temperature due to phase transformations. This phase change is generally companied by a variation in the specific volume which is exhibited as a departure from the behaviour of the thermal expansion or contraction, at the temperature at which the change occurs. This behaviour can be detected as a change in the dimensions of a suitable test piece, and forms the operating principle of dilatometry. The dilatometric technique may be applicable to the investigation of the phase transformation kinetics if the relation between the fractions of phases, the temperature, the compositions of phases and the dilatation can be built up. Efforts have been made to analyze the dilatation during an isothermal transformation or to calculate the dilatation during the heating of plain carbon steels using the thermodynamic function. The existing models are, however, not directly applicable to the determination of the phase transformation kinetics from a dilatation curve. A detailed analysis has been made of the length change of a hypoeutectoid steel during a continuous cooling. A model has been developed in which the transient dilatation is calculated based on the fraction of the phases present. The model takes into account the redistribution of carbon and is applicable to the determination of the phase transformation kinetics from the dilatation data during a cooling of a hypo-eutectoid steel. The model was validated by comparing the model results with the experimental results of an interstitial free steel. Experiments have been done with an interstitial-free steel and a C-Mn steel. The model has been applied to the calculation of the transformation kinetics indicated by the dilatation curves. Excellent agreements between the model and the experiments have been obtained.
