This review starts with a description of the most accepted casting parameters controlling chunky graphite appearance, i.e. cooling rate, melt composition and nodule count. Since all attempts to relate this graphite degeneracy to macrosegregation have failed, numerous investigations have focused on microstructure features. It is thus well established that chunky graphite develops as eutectic cells with interconnected strings of graphite growing along the "c" direction and having nearly constant characteristic sizes. These observations suggest a strong parallel to coral graphite, with the relatively rapid growth rate of chunky graphite being due to diffusion coupling between graphite and austenite. The importance of melt chemistry on graphite shape is well known, and the effect of impurities or alloying elements on the change from spheroidal to chunky graphite at given growth conditions is shortly reviewed. The importance of cerium and the way to counteract its deleterious effect is stressed. Finally, the role of elements, low-level and trace, on the transitions in lamellar graphite irons (lamellar to undercooled to coral) is used to support a discussion of the morphological transition from spheroidal to chunky graphite.
展开▼
