Influence of martensite volume fraction and tempering time on tensile properties of partially austenitized in the (α + γ) temperature range and quenched + tempered ferritic ductile iron

In the present study, the effect of martensite volume fraction and tempering time on the tensile properties of ferritic ductile iron with dual matrix structure was investigated. For this purpose, specimens were intercritically annealed (partially austenitized) in the two phase region (α + γ) at various temperatures of 795 and 815° C for 20 min and then quenched into oil held at 100° C to obtain different martensite volume fractions. Some specimens were also conventionally heat treated (austenitized at 900° C and then quenched + tempered) for a comparison reason. The results showed that a structure having proeutectoid ferrite plus martensite has been developed and volume fraction of proeutectoid ferrite and martensite can be controlled to influence the strength and ductility. Specimens quenched from the (α + γ) temperature range exhibited much greater ductility than conventionally heat treated specimens. The tensile strength increased and ductility decreased with increasing martensite content. By increasing the tempering time, the yield and UTS decreased and ductility increased. The specimens tempered for 3 h and having 62% martensite volume fraction (MVF) exhibited the best combination of high strength and ductility. The tensile and proof stress of this material is much higher than pearlitic grades and ductility is lower than ferritic grades. The specimen tempered for 3 h and having ∼ 25% MVF exhibited the best combination of high strength and ductility compared to ferritic grades. However its strength is slightly lower but the ductility is almost three times higher than pearlitic grades.