Carbide formation in an austempered ductile cast iron during prolonged austempering and tensile loading at elevated temperatures

Abstract

Since carbide precipitation in austempered ductile cast iron (ADI), which is detrimental to the ductility, is still not well explored, a 3.6wt%C-2.4 wt% Si ADI was chosen as a test material in this study. The results reveal the formation of Fe2.5C and Fe6C after prolonged austempering at 643 K for 94 h. Tensile loading of the specimens austempered at 643 K for 30 min gives rise to the formation of Fe3C, Fe2C, Fe7C3 and Fe2.4–3C successively with increasing testing temperature from 513 to 873 K, where Fe3C, Fe2C and Fe2.4–3C depict a preferred orientation. This formation sequence as a function of temperature is the same as that of a bainitic steel reported in the literature, but the carbides formed at and below about 673 K cannot be formed without tensile loading. As the lath-like structure is present, both intergranular and intragranular carbide particles are commonly observed. The former, which are formed from large intergranular austenite plates, are considered to cause the elongation drop in high-temperature tensile testing. The intragranular carbides are formed from the small austenite blocks within the acicular ferrite.