Carbide types in an advanced microalloyed bainitic/ferritic Cr-Mo Steel - TEM observations and thermodynamic calculations

The strength and toughness of low alloyed ferritic/bainitic steels depend on their microstructure, which evolves during thermo-mechanical treatments along the processing chain. Chromium-molybdenum steel microstructures are complex. Therefore, only a limited number of attempts have been made to fully characterize carbide populations in such steels. In the present work, analytical transmission electron microscopy is employed to study the microstructure of a low alloyed chromium-molybdenum steel, which features ferritic (F, mainly -iron and niobium-carbides) and bainitic (B, -phase, dislocation, grain/subgrain boundaries, various MxCy carbides) regions. The crystal structure and chemical nature of more than 200 carbides are determined and their distributions in the two microstructural regions are analyzed. The present work shows how particles can be identified in an effective manner and how the microstructural findings can be interpreted on the basis of thermodynamic calculations.