Three-dimensional observations of proeutectoid cementite precipitates at short isothermal transformation times

Three-dimensional (3D) analysis techniques were used to examine a model Fe–1.34% C–13.0% Mn alloy to reveal connectivities of proeutectoid cementite precipitates whose growth was arrested at an early stage. The present results indicate that grain boundary cementite precipitates nucleate at austenite grain boundary edges and corners, then grow and spread on the grain boundary faces. No cementite precipitates were found to connect solely to austenite twin boundaries, which appear to act as barriers to precipitate growth rather than as potential nucleation sites. Cementite precipitates were all connected to austenite grain boundaries or cementite grain boundary precipitates, confirming 3D observations made earlier on a specimen transformed at a longer isothermal transformation time. Widmanstätten lath precipitates appear to emanate only from grain boundary cementite precipitates. While edges of several Widmanstätten plate precipitates were observed to intersect with areas of ‘clean’ austenite grain boundary, they may or may not nucleate directly on austenite grain boundaries.