Episode 33 - Dilatometry A Tool to Identify and Quantify Nonequilibrium Phase Transf. in AHSS

Описание: Presenter: Gaurav Kumar Bansal, Scientist, Materials Engineering Division, CSIR – National Metallurgical Laboratory, Jamshedpur, India

Abstract: The third-generation advanced high strength steels (AHSS) with nonequilibrium microstructure (mainly bainite, martensite and retained austenite) can achieve superior strength-ductility-toughness combination for weight reduction of structural components. The microscopic techniques generally pose difficulties for the distinction between martensite and bainite due to similarities in their morphology, carbon supersaturation, and crystallographic features.

The X-ray diffraction (XRD) measurements are also not helpful for distinguishing and quantifying their content, mostly at low carbon contents, due to similarities in their crystal structures. These phases are formed during different stages (temperature, cooling rate, time, etc.) of the processing through various modes/mechanisms of phase transformation, i.e. displacive or diffusional. Therefore, dilatometry technique has been employed to detect these phase transformations at each stage of the heat-treatment cycle. Thereafter, combinations of established empirical relations have been utilized to methodically extract the amount and composition of different phases. The use of dilatometry to study the effect of alloying elements during continuous cooling transformation will also be briefly discussed. To summarize, dilatometry has been found to be of great help to zero in on the best alloying combinations and optimized processing conditions for translation to large-scale experiments.