Influence of ageing treatment temperature and duration on σ-phase precipitation and mechanical properties of UNS S32750 SDSS alloy

Elisabeta Mirela Cojocaru; Doina Raducanu; Anna Nocivin; Vasile Danut Cojocaru

doi:10.1016/j.jare.2020.11.005

Influence of ageing treatment temperature and duration on σ-phase precipitation and mechanical properties of UNS S32750 SDSS alloy

J Adv Res. 2020 Nov 14:30:53-61. doi: 10.1016/j.jare.2020.11.005. eCollection 2021 May.

Authors

Elisabeta Mirela Cojocaru¹, Doina Raducanu¹, Anna Nocivin², Vasile Danut Cojocaru¹

Affiliations

¹ Materials Science and Engineering Faculty, University POLITEHNICA of Bucharest, 060042 Bucharest, Romania.
² University OVIDIUS of Constanta, 900527 Constanta, Romania.

Abstract

Introduction: Super-Duplex Stainless Steeles (SDSS) proved an excellent potential for its use in many chemical and offshore applications due to their both high mechanical properties and a high corrosion resistance in chloride ion solutions.

Objectives: This study evaluates the influence of ageing treatment temperature and duration on σ-phase precipitation and mechanical properties of UNS S32750 SDSS alloy.

Methods: The influence of ageing treatment on microstructural features was analysed by SEM-EBSD (Scanning Electron Microscopy - Electron Backscatter Diffraction) technique, while on mechanical properties by tensile and impact testing techniques.

Results: The obtained results show that for the short duration ageing treatments the σ-phase nucleates, within the δ-phase matrix, at the δ/γ grains boundary by the δ → σ precipitation, while for long duration ageing treatments the σ-phase nucleates, within the δ-phase matrix, at the δ/δ grains boundary, or in other favourable nucleation sites, due to the eutectoid decomposition δ → σ + γ₂. Experimental data showed that at low temperatures, i.e. 780 °C, in order to induce the precipitation of σ-phase, the minimum incubation time is situated close to 20 min, and that increasing treatment temperature decreases the minimum incubation time, with at 850 °C and 920 °C the σ-phase being firstly detected at 5 min. The σ-phase precipitation shows the highest precipitation kinetics at 850 °C, when the maximum weight-fraction is obtained for each treatment duration when compared with ageing treatments performed at 780 °C and 920 °C.

Conclusion: All presented data brings valuable insight into the σ-phase precipitation phenomena and its influence on UNS S32750 SDSS alloy's exhibited mechanical properties and, also, can provide researchers and industrial steel processors a guide regarding the selection of optimal ageing treatment parameters to avoid/minimise the embrittlement induced by the precipitation of deleterious σ-phase.

Keywords: Mechanical properties; Microstructure; Super Duplex Stainless Steels (SDSS); σ-phase.