Potential and limitations of direct austenite grain growth measurement by means of HT-LSCM

Abstract

High-temperature laser scanning confocal microscopy (HT-LSCM) employs the possibility of direct austenite grain growth observations. To ensure the results obtained are interpreted correctly, several influencing factors on the investigation outcome have to be taken into account. The present paper gives an overview of the basic experimental setup for in-situ grain growth observations and critically assesses the requirements concerning grain size measurement, materials and operational details. The extensive studies presented in this work indicate that the grain growth as seen on the sample surface is representative for the bulk material and allows the determination of an average grain size value and a full grain size distribution for every desired time step. The investigated material significantly influences the experimental outcome, which is why origin and thermal history of the sample always have to be taken into account for an interpretation of the results. Concerning the details of operation, a careful temperature referencing was proven to be a prerequisite to meet the desired temperatures within the sample. Temperature differences between set temperature and sample surface were shown to be ±30 °C following a non-linear behavior in relation to the absolute temperature. Oxidation of the sample surface can be prevented by Ar purging; however, evaporation of Mn was demonstrated to occur under standard experimental conditions. While the Mn loss did not impact the grain growth observations in this study, it is an important finding that should attract interest when using the HT-LSCM for the evaluation of other microstructural changes. Finally, some selected in-situ grain growth results are presented that demonstrate the unique potential of the HT-LSCM in determining the effect of specific alloying elements (Mo, Mn and Ni) on the grain growth kinetics as well as the impact of AlN precipitates. The achieved results feature a strong basis for grain growth modelling and the critical validation of simulation results, emphasizing the HT-LSCM as an efficient and reliable tool for various applications within steel research.