EARLY STAGES OF DECOMPOSITION OF FERRITE IN DUPLEX STAINLESS-STEEL

The decomposition of a duplex stainless steel during aging at 600-degrees-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 gamma grain, is found only on certain gamma/alpha boundaries and its nucleation is shown to be strongly dependent on the gamma boundary plane of the gamma/alpha interface. Chromium depletion in the vicinity of gamma/alpha boundaries caused by M23C6 precipitation initiates the alpha --> gamma transformation by migration of existing gamma/alpha boundaries. Another mechanism for this bcc --> fcc phase change is observed through the growth of Widmanstatten gamma plates into alpha. Precipitation of sigma begins after M23C6 formation has stopped. The existence of M23C6 on gamma/alpha boundaries is crucial to sigma formation because the majority of sigma particles are found to nucleate on nodes in migrating gamma(new)/alpha boundaries caused by M23C6 particle pinning. Another less common nucleation site for sigma is migrating gamma(new)/alpha boundaries. The sigma particles nucleated on either of the sites have the Nenno orientation relationship with the parent gamma. In the latter stages, the sigma and new gamma consume the alpha grains via a cellular growth mechanism.