Early stages of decomposition of ferrite in duplex stainless steel

Abstract

The decomposition of a duplex stainless steel during aging at 600°C has been investigated using analytical transmission electron microscopy with the intention of elucidating the nucleation sites of the new phases arising from ferrite decomposition. The first transformation product to be formed is M23C6. This carbide, having a cube–cube orientation relationship with the parent γ grain, is found only on certain γ/α boundaries and its nucleation is shown to be strongly dependent on the γ boundary plane of the γ/α interface. Chromium depletion in the vicinity of γ/α boundaries caused by M23C6 precipitation initiates the α→γ transformation by migration of existing γ/α boundaries. Another mechanism for this bcc→fcc phase change is observed through the growth of Widmanstätten γ plates into α. Precipitation of σ begins after M23C6 formation has stopped. The existence of M23C6 on γ/α boundaries is crucial to σ formation because the majority of σ particles are found to nucleate on nodes in migrating γnew/α boundaries caused by M23C6 particle pinning. Another less common nucleation site for σ is migrating γnew/α boundaries. The σ particles nucleated on either of the sites have the Nenno orientation relationship with the parent γ. In the latter stages, the σ and new γ consume the α grains via a cellular growth mechanism.MST/1905