On the strength of grain and phase boundaries in ferritic-martensitic dual-phase steels

Ultramicrohardness of ferrite was thoroughly studied in a low-alloyed ferritic-martensitic dual-phase steel. A series of targeted indents in coarse ferrite grains (in the as-quenched state) enabled the establishment of relationships between the hardness and the distance of the indent from grain or phase boundaries. As expected, the type and the number of boundaries in the vicinity of the indent play a catalytic role to the measured hardness. The increase in the Vickers hardness with appoaching the grain boundaries was attributed to the impediment of dislocation slip due to the boundary obstacle(s). The presence of ferrite/martensite phase boundaries introduces an additional and even more critical effect, i.e. the work-hardened ferrite grains due to the geometrically necessary dislocations associated with the martensitic transformation taking place during the production process. Transmission electron microscopy and electron backscatter diffraction studies on ferrite grains helped to identify the latter effect and provided valuable information about the misorientation and strain state of the grains before and after indentation.