Carbon Redistribution Process in Austempered Ductile Iron (ADI) During Heat Treatment—APT and Synchrotron Diffraction Study

Abstract

In-situ synchrotron diffraction and atom probe tomography (APT) have been used to study the carbon diffusion and redistribution process in austempered ductile iron (ADI) during austempering. The process of carbon content change in bainitic ferrite during different austempering temperatures has been determined quantitatively. The transformation in ADI is controlled by decarburization of supersaturated ferrite and carbide precipitation and has been found to be divided into three stages based on a model developed for bainitic steels by Takahashi and Bhadeshia. The formation, morphology and composition of carbides and carbon clusters in ferrite after austempering have been identified unequivocally by APT. Finally, the relationships of carbon content in ferrite, carbon gap values, and austempering temperatures in the ADI alloy were expressed using empirical equations.

Phase Transition Kinetics in Austempered Ductile Iron (ADI) with Regard to Mo Content

“…As described, the atomic carbon concentration of 25% is not reached. This stands in contrast to earlier work, where the existence of cementite in ADI with comparable Si amount was documented [ 35 ].…”

“…As described, the atomic carbon concentration of 25% is not reached. This stands in contrast to earlier work, where the existence of cementite in ADI with comparable Si amount was documented [35]. One possible explanation would be the absence of cementite phase due to the high silicon amount in ductile iron.…”

“…Using his model leads to values according to Table 2. However, we applied the model of Voigt to remain consistent with our previous work [10,24,34,35]. During the austempering at 300 and 350 • C , the maximum C content ω γ max was not reached within 2.5 h. The evolution of C content in austenite was of asymptotic shape.…”

“…Using his model leads to values according to Table 2 . However, we applied the model of Voigt to remain consistent with our previous work [ 10 , 24 , 34 , 35 ].…”

A Unique Quenching and Deformation Dilatometer for Combined In Situ Neutron Diffraction Analysis of Engineering Materials

“…In ADI, the austenite to ferrite transformation is affected by carbon diffusion and takes place during the austempering process between 250 and 450 C. [30,31] This kind of phase transformation is also defined as stage I reaction in ADI material. After a certain time and dependent on temperature, the carbon enriched retained austenite further decomposes into ferrite and cementite in the so-called stage II reaction.…”