The evolution of the relative fraction of high-carbon austenite with austempering time and temperature was analyzed in a compacted graphite (CG) cast iron (average composition, in wt pct: 3.40C, 2.8Si, 0.8Mn, 0.04Cu, 0.01P, and 0.02S) at five different austempering temperatures between 573 and 673 K. Samples were characterized by Mössbauer spectroscopy, hardness measurements, and optical microscopy. During the first stage of transformation, the kinetics parameters were determined using the Johnson-Mehl's equation, and their dependence with temperature in the range from 573 to 673 K indicates that the transformation is governed by nucleation and growth processes. The balance between growth-rate kinetics and nucleation kinetics causes the kinetics parameter (k) to have a maximum at Ϸ623 K of 3.9 ϫ 10 Ϫ3 (s Ϫ1 ). The evolution of the C content in the high-carbon austenite was found to be controlled by the volume diffusion of carbon atoms from the ferrite/austenite interface into austenite, with a dependence of t 0.40Ϯ0.05 on the austempering time (t).
Mössbauer spectroscopy has been used to monitor the kinetics of austempering transformation in two compacted graphite (CG) cast irons alloyed with 0.11 and 0.58 wt pct of Mn, respectively. The phase relations were analyzed in terms of the Johnson-Mehl's equation, determining the kinetics parameters n (time exponent) and k (constant rate of the transformation). The values obtained were n ϭ 1.4 and k ϭ 7.47 ϫ 10 Ϫ3 s Ϫ1 for the low-Mn alloy, and n ϭ 2.2 and k ϭ 3.9 ϫ 10 Ϫ3 s Ϫ1 for the high-Mn alloy. These results, which reveal a faster kinetics for the low-Mn alloy, are coherent with metallographic observations, and the driving force obtained through the determination of the austenite carbon concentration that was determined from the Mössbauer data using the Génins model for the Fe-C configurations in the fcc lattice. The kinetics parameters are further compared to those obtained in austempered ductile iron (ADI), by analyzing the graphite morphology influence on the austempering transformation.
In this work, the phases involved in the transformation austenite ferrite ϩ high-carbon austenite in cast irons were assessed by means of Mössbauer spectroscopy and hardness measurements for samples austempered at five different temperatures between 573 and 673 K with two Mn contents. The C content in the high-carbon austenite was found to have a dependence of t 0.40Ϯ0.05 on the austempering time t, which evidences that diffusion of carbon has taken place. The kinetic parameter k determined using the Johnson-Mehl-Avrami equation has a maximum of 3.9 ϫ 10 Ϫ3 (s Ϫ1 ) at Ϸ623 K and corroborates that Mn slows the transformation rate. →
Tensile strength, fracture toughness and impact properties were evaluated in compacted graphite (CG) cast iron with ferritic, pearlitic and ausferritic microstructures. Ultimate tensile strengths for the ferritic and pearlitic samples were 337 and 632 MPa respectively. The austempered samples showed a significant increment in the strength and recording values between 675 and 943 MPa. The fracture toughness test revealed that the stress-intensity factor K IC was 34?0 MPa m 1/2 for the ferritic CG iron, 39?7 MPa m 1/2 for the pearlitic and between 51?0 and 58?0 MPa m 1/2 for the austempered irons. On the other hand, CG iron with ferritic matrix exhibited the best impact properties with absorbed energy of 33?3 J. The absorbed energy of the pearlitic CG iron was the lowest, 14?3 J, while the austempered samples showed values between 17?2 and 28?4 J. Complementing these results, the critical crack size was also analysed.
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