Study on the characterization of surface ferrite transformation in carbon steel using in-situ thermovoltage measurement

In this paper, a self-designed in-situ thermovoltage measuring system was built to monitor thermovoltage of 0.45% carbon steel during heat treatment with precisely designed cooling rate. The results show that an abrupt change in thermovoltage curve was observed, and the transition temperature of the surface ferrite transformation was precisely obtained, which are 765 degrees C and 723 degrees C, respectively. Based on the measured austenite to ferrite transition temperature range, designed cooling rate during heat treatment of 0.45% carbon steel was performed, which realizes precise control of surface ferrite transformation. In addition, quenching at the phase transition start temperature (765 degrees C) determines the formation mechanism of the surface ferrite layer. The carbon distribution at the interrupted ferrite grain development was measured by electron probe microanalyzer (EPMA), and which corresponds to the formation mechanism of surface ferrite layer presented in the author's previous study. The results in current study confirm the practical application of the self-designed in-situ thermovoltage measuring system which provides a new method of monitoring and controlling material phase transformation during heat treatment.