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.
Highlights
Austempered ductile iron (ADI) consists of ausferrite and nodular graphite and shows advantageous mechanical properties in many industrial applications [1,2]
In our previous in-situ neutron diffraction studies on carbon diffusion in austempered ductile iron (ADI), the carbon content changes in austenite and ferrite during austempering have been systematically investigated [3,12,13]
Using the relationship between the ferrite lattice parameter and the carbon content in ferrite, the whole process of carbon diffusion between austenite and ferrite during austempering was investigated by in-situ synchrotron diffraction
Summary
Austempered ductile iron (ADI) consists of ausferrite and nodular graphite and shows advantageous mechanical properties in many industrial applications [1,2]. After systematically researching this transformation process in steels with different carbon and silicon contents (C = 0.1–1.0 wt%, Si = 0.45–2 wt%) [4,5,6,7], a transition temperature between upper and lower bainite around 350 ◦ C has been proposed for a variety of steels by Matas and Hehemann [8] This transition temperature is a function of the alloying elements and their respective content. It is unable to clarify the absence of carbides in cast iron with a large silicon content at temperatures higher than 350 ◦ C [10,11] Due to these exceptions and because previous studies were inadequate in describing the carbon redistribution process during bainitic transformation in ADI in detail, it is necessary to precisely measure that process during and after austempering. In-situ synchrotron diffraction was used to open up the possibility of investigating previously not examined diffusion processes with high temporal resolution (i.e., one complete diffraction pattern per 2 s)
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