Substructure of deformation zones in austempered ductile iron finish-turning chips

A series of dry finish-turning tests of a structural austempered ductile iron with PcBN cutting-inserts were run with a depth of cut of 200 μm, a feed-rate of 50 μm rev−1 and a cutting length of ∼100 m, and using machining speeds stretching from 50 to 800 m min−1. Substructure developed within chips' deformation zones was studied by transmission electron microscopy. At machining speeds equal or beyond 150 m min−1, shear-induced unlocking of incomplete austempering transformation and continuous-cooling austempering of residual and retained austenite would plausibly stand as phase transformations behind the overwhelming volume fraction of ferrite within the chips' deformation zones and related regions. Formation (and coalescence) of ferrite sub-grains within these zones, possibly concurrently with their parent shear-platelets, would be the result of rotational dynamic recrystallization. Substructure refinement in the deformation zones of chips formed under machining speeds equal or beyond 150 m min−1 could heighten the diffusion rates of cutting-insert components therein.