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 M 23 C 6 This carbide, having a cube-cube orientation relationship with the parent y grain, isfound only on certain y/a boundaries and its nucleation is shown to be strongly dependent on the y boundary plane of the y/a interface. Chromium depletion in the vicinity of γ/α boundaries caused by M 23C 6 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 Widmanstatten y plates into α. Precipitation of σ begins after M 23 C 6 formation has stopped. The existence of M 23C 6 on γ/α boundaries is crucial to σ formation because the majority of σ particles are found to nucleate on nodes in migrating Y new/α boundaries caused by M 23 C 6 particle pinning. Another less common nucleation site for σ is migrating Y new/α boundaries. The σ particles nucleated on either of the sites have the Nenno orientation relationship with the parent y. In the latter stages, the σ and new γ consume the a grains via a cellular growth mechanism. © 1994 The Institute of Materials.