The tensile fracture behaviour of intercritically annealed and quenched + tempered ferritic ductile iron with dual matrix structure

The tensile fracture behaviour of oil quenched from intercritical annealing temperature (ICAT) range (α + γ) and then tempered ferritic ductile iron having dual matrix structure (proeutectoid ferrite + martensite) with different martensite volume fraction and its morphologies have been studied for a ductile ferritic cast iron containing 3.5 C wt%, Si wt% and 0.318 wt% Mn. In both fine and coarse dual matrix structures microvoids formed at the interface between the graphite nodules and the surrounding structure, martensite particles, inclusions and martensite/ferrite interfaces in the necked region. The martensite morphology had an influence in determining martensite cracking. Coarse and interconnected high C martensite distributed along the intercellular boundaries cracked easily. Martensite cracking was less frequent and the microvoids were smaller in the fine structure than the coarse ones. Microvoid coalescence was the dominant form of fracture in both structures. The specimens with fine structure had higher densities of voids. In these samples, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite.