Advanced Characterization and Calphad in Design and Development of Advanced High Strength Steels

Abstract

AbstractMicrostructure plays an important role in achieving the desired set of mechanical properties. Any design and development of new grades of steels or improvement in properties of the existing steels necessitates the understanding of the structure–property correlations through extensive microstructural characterization. Once the microstructure at different length scales has been understood, microstructural engineering through modification of the phases and phase fractions, size and distribution of the second phase via modification of the composition and processing windows may be applied to achieve the desired properties. This article describes our efforts in advanced characterization of ultra-high strength (UHS) steels using electron microscopy and 3D atom probe supported by thermodynamic calculations in the Calphad framework. TEM and APT characterization of UHS steel developed earlier at DMRL revealed the presence of atomic scale clusters of both carbon rich and carbon lean clusters. It is believed that these clusters can significantly influence both strength and toughness along with other strengthening mechanisms. These carbon lean clusters have been used as the base to tailor the microstructure for enhanced mechanical properties. ThermoCalc has been employed extensively to design a new set of compositions for experimental validation and optimization of the processing windows. Experimental results on the designed alloys reveal significant fraction of undissolved precipitates and more than 50 percent austenite when the steel containing 5wt. % Cr and Mo austenitized at 1150 °C. Atom probe results further suggest the formation of clusters and their composition approaches toward M2C with increase in tempering temperature. The volume fraction and Cr/Mo ratio of the M2C type clusters broadly correlates well with the ThermoCalc predictions.KeywordsCalphadTEM3D atom probe and clusters