Effect of microvoid formation on the tensile properties of dual-phase steel

A steel containing 0.32 wt. % C, 0.88 wt. % Mn, 0.99 wt. % Si, 0.9 wt,% Ni, and 0.9 wt.% Cr was intercritically annealed at different temperatures from 775 to 870 degrees C and quenched in oil to produce dual-phase steel microstructure. Tensile testing of these samples gave a series of strengths and ductilities. The tensile strength increased with the increased annealing temperatures and the martensite percentage increased with a reduction in ductility. Microvoids were formed near the fracture surfaces. The morphology of the microvoids changed with the martensite percentage from decohesion of the martensite particles to the intergranular and transgranular cracks, which defined the ultimate fracture mode of the specimens. The change in the morphology of microvoids may be due to a high percentage of carbon in the steel, which produced stresses in the matrix (ferrite) during phase transformation.