Effect of martensite volume fraction and its morphology on the tensile properties of ferritic ductile iron with dual matrix structures

In the present study, the effects of martensite volume fraction (MVF) and its morphology on the tensile properties of ferritic ductile iron (DI) with dual matrix structures (DMS) were investigated. For this purpose, specimens were intercritically annealed (partially austenitized) in two-phase region (α + γ) at various temperatures of 795, 805 and 815 °C for 30 s and then quenched into oil held at 100 °C to obtain different martensite volume fractions. Fine and coarse dual matrix structures were obtained from two different starting conditions. Some specimens were also conventionally heat treated (austenitized at 900 °C and then quenched + tempered) for comparison reason. The quenched samples were tempered at 550 °C for various times from 1 to 3 h. The results showed that the volume fraction of proeutectoid ferrite and martensite can be controlled to influence the strength and ductility. Ductile iron with dual matrix structure exhibited much greater ductility than conventionally quenched + tempered grade. The tensile strength increased and ductility decreased with increasing martensite volume fractions and tempering time. However, dual matrix structures with fine structure exhibited lower ductility and higher strength than the specimen with coarse ones quenched from the same intercritical annealing temperature (ICAT) range. The tensile and proof stress of ductile iron with dual matrix structure is much higher than ferritic and pearlitic grades and ductility is slightly lower than ferritic grade.