Abstract
Precipitation of M23C6 and σ-phase during isothermal aging of δ/γ duplex stainless steels has been studied mainly by means of transmission electron microscopy coupled with energy dispersive X-rayanalysis. M23C6 particles form either at γ/γ or atγ/δ boundaries. The particles are crystalographically related to either one of the γ grains atγ/γ boundaries or the γ grain atγ/δ boundaries. The M23C6 particles nucleating atγ/δ interface boundaries grow into the untransformed δ-ferrite as an M23C6/newγ eutectoid structure. The habit plane is the{111} γ//{111}M23C6, which is most parallel to the edgewise growth direction. Since C atoms formingM23C6 are supplied from theγ behind the eutectoid structure, the reaction will finally stop after entire consumption of C atoms. Thenσ-phase particles nucleate on theδ/newγ interface boundary in Nenno orientation relationship with the new γ, and grow into theδ-ferrite as aσ/δ cellular structure with the (001)σ //(111)γ habit plane. In some cases, impingements between σ andγ grains nucleating separately occur, and result in a tangled eutectoid structure without any specific orientation relationship. Since the partition of alloying elements betweenγ andδ-ferrite phases accelerates precipitation of both M23C6 and σ-phase, the solution treatment in theδ-ferrite single phase region largely retards these reactions.
