Formation of intergranular M₂₃C₆ in sensitized type-347 stainless steel

Formation of intergranular M₂₃C₆ carbides and thereafter Cr-depletion zones in commercially available type-347 stainless steels were observed by optical microscopy, conventional transmission electron microscopy and analytical transmission electron microscopy. At the early stage of sensitization, only NbC carbides were observed both intragranularly and intergranularly. At the later stage, formation of intergranular M ₂₃C₆ carbides and intergranular corrosion were found. A model is proposed to describe the formation kinetics of intergranular precipitation of NbC and of M₂₃C₆, as well as Cr-depletion zones, thermodynamically. Evolution mechanisms of intergranular precipitates were found in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC, 3) the formation of M₂₃C₆, then 4) the coarsening of M₂₃C₆ and formation of Cr-depletion zones.