In the present study, an unalloyed ductile iron containing Fe-3 . 50C-2 . 63Si-0 . 318Mn-0 . 047Mg (wt-%) were intercritically austenitised (partially austenitised) in two phase region azc at various temperatures of 795, 805, 815 and 830uC for 20 min and then quenched into salt bath held at austempering temperature of 365uC for various times to obtain different ausferrite volume fractions (AFVFs). Results showed that dual matrix structure containing proeutectoid ferrite, new ferrite (also called epitaxial ferrite) and ausferrite (bainitic ferritezhigh carbon austenite, which is retained or stabilised austenite) has been developed. Within each of the austempered series in azc temperature range, new ferrite volume fraction increased with increasing intercritical austenitising temperature (ICAT). Although, transforming percentage of new ferrite from parent austenite present at ICAT increased with decreasing ICAT. Some specimens were also conventionally austempered from 900uC for comparison. The new ferrite was absent in these samples. The volume fraction of proeutectoid ferrite, new ferrite and ausferrite can be controlled to determine the strength and ductility. Austempered specimens in azc temperature range exhibited much greater ductility than conventionally austempered ones. The tensile strength increased while ductility decreased with increasing AFVF. On the other hand, the ductility increased with increasing proeutectoid ferrite and new ferrite volume fractions at the expense of strength. The specimen with ,47 . 2%AFVF exhibited the best combination of high strength and ductility. The strength and ductility of this material is much higher than that of ferritic grades. Its strength is at the same level as while ductility almost more than four times higher than that of pearlitic grades. Meanwhile, the specimen with ,75%AFVF exhibited the best combination of high strength and ductility compared with those of pearlitic grades. The strength of this material is much higher and its ductility is almost more than two times higher than that of pearlitic grades yet slightly lower than that of ferritic grades. This material also meets the requirements for the strength of quenched and tempered grades and its ductility is higher than that of this grade.

In the present study, an unalloyed ductile iron containing 3?50%C, 2?63%Si, 0?318%Mn and 0?047%Mg was intercritically austenitised (partially austenitised) in the two phase region (azc) at temperatures of 795 and 815uC for 20 min, and then was quenched into a salt bath held at an austempering temperature of 365uC for various times to obtain various ausferrite volume fractions. Fine and coarse dual matrix structures were obtained from the two different starting conditions. Some specimens were also conventionally austempered from 900uC for comparison. Results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferritezhigh carbon retained or stabilised austenite) was developed. The phase previously described as new ferrite (also called epitaxial ferrite) was observed to form following heat treatment only in specimens with coarse austenite dispersion after austempering from the (azc) temperature range. It was observed that the parent austenite dispersion present at the intercritical austenitising temperature has an effect on the volume fraction of high carbon austenite following austempering. Finer austenite dispersions produced more stabilised high carbon austenite than coarse ones for a given austempering time. The volume fractions of proeutectoid ferrite, new ferrite, ausferrite can be controlled to influence the strength and ductility. Specimens austempered from the (azc) temperature range exhibited much greater ductility than conventionally austempered samples. Within each of the series austempered from the (azc) temperature range, the tensile strength increased and ductility decreased with increasing ausferrite volume fraction and decreasing proeutectoid ferrite volume fraction. However, specimens having a fine ausferritic structure without new ferrite exhibited lower ductility and higher strength than specimens with a coarse ausferritic structure under the same austempering conditions. The specimen with ,60% AFVF (coarse structure) and ,65% AFVF (fine structure) exhibited the best combination of high strength and ductility compared to pearlitic grades, but their ductility was slightly lower than ferritic grades. These materials satisfy the requirements for the strength of quenched and tempered grades and their ductility superior to that of this grade.

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