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 fro m 775 to 870 degrees C and quenched in oil to produce dual-phase steel micr ostructure. Tensile testing of these samples gave a series of strengths and ductilities. The tensile strength increased with the increased annealing t emperatures and the martensite percentage increased with a reduction in duc tility. Microvoids were formed near the fracture surfaces. The morphology o f the microvoids changed with the martensite percentage from decohesion of the martensite particles to the intergranular and transgranular cracks, whi ch defined the ultimate fracture mode of the specimens. The change in the m orphology of microvoids may be due to a high percentage of carbon in the st eel, which produced stresses in the matrix (ferrite) during phase transform ation.