TEM study of carbide precipitation in low alloyed austempered ductile iron
Conference poster, 2011
The austempering process is neither new nor novel and has been utilized since the 1930's on cast and wrought steel; quite early the technique was also tried on cast ductile iron. Nearly 50 years after the first trials, austempered ductile iron (ADI) is still widely regarded as a "new material". Kovacs (a pioneer in this field) introduced the name ausferrite for the matrix microstructure, as it contained ferrite and high carbon austenite as main constituents in the matrix. Owing to the high silicon content in the alloy, carbides are scarcely seen at least in work presented in the literature. Due to the fact that the base material in ADI is a cast component, primary carbides can appear and these carbides do no dissolve during the austenitization treatment and they still appear in the matrix as primary carbides surrounded by secondary carbides after heat treatment. In the central part of the eutectic cells, also smaller amounts of carbides (Fe3C) appear in spite of the high silicon level. In this work different techniques have been used to comprehend the origin and the types of the carbides as well as the evolution of the ausferritc microstructure.
An ADI alloy with the chemical composition 3.65 C, 2.13 Si, 0.82 Cu, 0.35 Ni, 0.27 Mo, and 0.34 Mn (wt. % and bal. Fe), was austenitized at 900°C and austempered at 400°C for 1h. Carbide precipitation in this low-alloyed ADI was characterised using conventional TEM-EDS and selected area electron diffraction (SAED). A two-stage extraction replica method was employed for TEM specimen preparation. The overall microstructure was examined with SEM in a deeply etched sample. The carbide precipitates were observed inside acicular ferrite grains. Coarsening of the carbide precipitates was not evident, indicating a typical lower bainitic structure of ADI. The precipitates in ferrite grains were identified as Fe3C by SAED; and some transition -carbides were found as well inside ferrite grains. In addition, M7C3 carbides were occasionally found along ferrite/austenite boundaries. Mo-rich eutectic precipitates of M6C, Fe3C and Fe7C3 containing small amounts of V and Si were recognized, which showed the segregation constituents. Mn-carbides and Mn-precipitates at intercellular boundaries were not observed, indicated that Mn segregation was not severe in this alloy. It can be concluded that the ADI material with the present composition is comparable with an unalloyed ADI austempered at 300°C with a longer time.