Intercritically Austenitized Quenched and Tempered Ductile Iron

Ductile iron with a matrix of ferrite plus martensite was produced using a ductile iron alloyed with ∼0.7 wt% manganese and ∼0.5 wt% nickel. Three different volume percentages of martensite (16, 24 and 37 vol %) were formed by austenitizing in the intercritical region (ferrite + austenite) followed by quenching in a polymeric solution to room temperature. The materials were evaluated in the untempered condition and after tempering at 400C (752F) and 500C (932F) for one hour. This paper reports the effects of volume percent martensite and tempering conditions on microstructure and tensile properties. Semi-quantitative chemistry and microhardness of ferrite and martensite were also determined as a function of martensite volume percent. The results indicated that the carbon available to form the austenite during intercritical austenitizing came mainly from the carbon in the matrix and that partitioning of substitutional elements is limited. The results also showed that ultimate and yield strength and hardness increased and elongation decreased as the volume fraction of martensite increased. Tempering significantly increased the elongation with only a small decrease in the strengths.