Precipitation of M/sub 23/C/sub 6/ type carbide on twin boundaries in austenitic stainless steels

An electron microscopy investigation was made of M/sub 23/C /sub 6/ carbide precipitation in two kinds of austenitic stainless steels, A (AISI 316L type alloy) and B (DIN 4981 type alloy). The specimens were water quenched from high annealing temperatures and aged at 800 degrees C. In particular, the precipitation process on twin boundaries was carefully examined. The M/sub 23/C/sub 6/ carbides were observed to form first on grain boundaries and on incoherent twin boundaries as small precipitates. The formation of lamellar M/sub 23/C/sub 6/ precipitates then proceeded on coherent twin boundaries and in the region adjacent to incoherent twin boundaries. The morphology of precipitates was found to depend strongly on the stress field. The growth of lamellar precipitates appears to be a consequence of planar distortion on {111} planes. The geometrical form of M/sub 23/C/sub 6/ precipitates is characterized by {111}- and {110}-type interfaces. In general, the {111} interfaces were predominant. One exceptional mode of precipitation was observed, however, in which the {110}-type lamellar precipitate formed around an undissolved residual NbC particle in Alloy B.