Linking up of HT-LSCM and DSC measurements to characterize phase diagrams of steels

Abstract

The phase transformation sequence during the solidification of carbon steels strongly influences their behavior in the casting process. Therefore, most exact knowledge of the dependence of the transformation characteristics on the steel composition is of highest relevance for process and quality optimization. The influence of alloying elements like C, Mn or Si on phase transformation is well understood as far as their content is rather low. New steel grades, like high-alloyed TRIP- or TWIP-steels contain almost up to 10 wt.-% of Si and Al and 30 wt.-%Mn. The present work focuses on first results of the parallel investigation into phase transformation of Fe-Al-C alloys by means of Differential Scanning Calorimetry (DSC) and Thermo-Optical Analysis (TOA) with a High-Temperature Laser-Scanning-Confocal-Microscope (HT-LSCM). DSC is a well established method for the accurate measurement of all phase transformation temperatures accompanied by significant enthalpy changes. Due to small enthalpy changes, DSC results are limited with respect to the γ/δ-transformation. Besides dilatometry and X-ray diffraction, the optical in-situ observation of phase transformation by HT-LSCM proved to be a comprehensive method. After a short description of the methods, results for the Fe-0.4%Al-0.22%C and Fe-1.5%Al-0.22%C systems will be discussed in detail and finally compared with results from computational thermodynamics.