Analysis of Sigma Formation Mechanisms in Aged Hyper Duplex Stainless Steel




Abstract: The evolution of σ-phase in a hyper duplex stainless steel is studied after isothermal ageing at elevated temperatures. EBSD is used to evaluate misorientation angles distribution and crystallographic orientation relationships (ORs) in coarser and finer parts of the microstructure, reflecting nucleation of σ-phase at the ferrite boundaries and austenite-ferrite interphases, respectively. The growth of σ-phase takes place as eutectoid transformation of ferrite into σ-phase and secondary austenite (γ2). The σ-phase corresponds to a high transformation temperature at interface boundaries, associated with rapid diffusion, whereas coral shaped microstructure with pronounced σ / γ2 lamellae reflect a slower transformation at lower temperatures. The K-S OR, with (111)γ1ǁ(001)σ, (110)αǁ(001)σ, and (111)γ1ǁ(110)α, was identified after high temperature aging, while (111)γ1,2ǁ(001)σ was found at low temperature condition. No preferential ORs are found for the finer coral shaped σ-phase at low temperature. Differences in misorientation angles between the different subregions are discussed.

Authors: Sanjeev Kumar, S. Krisam, A. Keplinger, and E. Povoden-Karadeniz

Keywords: σ-phase, EBSD, hyper DSS steel, ageing heat treatment

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7 pages


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