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Precipitation characteristics of M 23 C 6 during aging at 800 ЊC were studied by transmission electron microscopes in two austenitic stainless steels, A (type AISI 316L) and B (type DIN 4981), which had been quenched in water after their solution treatment at high temperatures (1150 ЊC and 1275 ЊC, respectively). After precipitation at grain boundaries, M 23 C 6 precipitated at incoherent and coherent boundaries of twins and inside austenite grains. Close to an incoherent twin boundary on either side of the boundary, M 23 C 6 mostly grew as elongated plates, although elsewhere in austenite matrix these grew usually as equiaxed particles. The plates lied on planes parallel to the twinning plane and were aligned unidirectionally along the axis of intersection of the twinning plane and a {110} plane perpendicular to the twinning plane. On coherent twin boundaries, similar plates of M 23 C 6 formed along with some equiaxed particles. Existing models for the mechanism of formation of these lamellar carbides fail to explain the observed features of these carbides formed around different twins. It is suggested that the residual stress localized in the vicinity of twin boundaries in quenched specimens influences the nucleation and growth of these carbides during aging, resulting in their specific morphology.

A Iow alloy steel containing 0.10C, 0.25Si, 0,87Mn, 0.56Cr, 0,47Ni, 0.21