Calliari I., Breda M., Frigo M. Pellizzari M., Ramous E. Isothermal ageing in lean duplex stainless steels

Abstract

The favourable combination of mechanical properties and corrosion resistance in Duplex Stainless Steels (DSS) is due to their biphasic microstructure, consisting of almost equal volume fractions of ferrite and austenite. However, all types of DSS are subject to secondary phases precipitation phenomena, especially in the temperature range 600–1000°C, which can occur even for very short soaking times, compromising the interesting properties of these steels.

The main secondary phase observed in DSS is the intermetallic σ-phase, which forms from ferrite via eutectoidic decomposition at high temperatures, and is generally accompanied by the formation of another intermetallic, the χ-phase. The precipitation of these phases is mainly observed in the high-alloyed DSS grades, while in the lower-alloyed Lean DSS it has not been reported, probably owing to reduced levels of chromium and molybdenum. On the contrary, in all DSS grades nitrides precipitation is usually observed.

In this work, the precipitation kinetics of secondary phases in two Lean DSS are examined, after isothermally treating the materials within the critical temperature ranges. In grade LDX 2101, only nitrides were detected, whereas a significant precipitation of σ-phase was ob-served in LDX 2404 for soaking times longer than 1 hour.

Keywords

Lean duplex stainless steels, isothermal heat treatments, σ phase, chromium nitride, precipitation kinetics.

Calliari Irene – Researcher DII, University of Padova, Padova, Italy. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..

Breda Marco – Ph.D. student, Industrial Engineering Department (DII), University of Padova, Padova, Italy. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it..

Frigo Marco – Customer Service Manager, Outokumpu S.p.A., Genova, Italy.

Massimo Pellizzari – Associate Professor Industrial Engineering Department (DII), University of Trento, Trento, Italy.

Emilio Ramous – Full Professor DII, University of Padova, Padova, Italy.

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Details: Published: 10 September 2015; Last Updated: 10 September 2015; Hits: 2415