Study of the austenite reversion in a cold-rolled duplex stainless steel

Abstract

In this work, the UNS S32304 lean duplex stainless steel will be investigated. Due to the combination of good mechanical properties and corrosion resistance, this steel has applications in both chemical and petrochemical industries. Strain-induced martensite (SIM) forms upon cold rolling in this steel. The main focus of this project is the study of austenite reversion after cold rolling and further annealing. Starting from the material with 80% thickness reduction, it will be subjected to continuous annealing on a dilatometer, varying the maximum temperature reached up to 1000°C. Isothermal annealing will also be performed at 400°C, with annealing time varying from 5 to 120 min. These last heat treatments will be performed to investigate the anomalous increase of magnetization observed at this temperature, before the beginning of austenite reversion (observed in a previous work). The samples from dilatometry experiments and isothermal annealing will be characterized by means of magnetization measurements, X-ray diffraction and hardness Vickers testing. The microstructural characterization will be performed using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), the latter being linked to the techniques of EBSD (Electron Backscatter Diffraction) and ECCI (Electron Channeling Contrast Imaging). Representative samples will be also characterized by APT (Atom Probe Tomography). Thermodynamic calculations will be performed using Thermo-Calc© software. An analysis of EBSD maps for isothermally annealed samples up to 800°C (obtained in a previous work) will be performed aiming to distinguish among ferrite, alpha'-martensite, reverted austenite and untransformed austenite as well. (AU)