EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Duplex stainless steels (DSSs), containing bcc and fcc phase, are one of the strategically important structural materials in critical components of e.g. chemical industry, gas sector, nuclear power plants, etc. due to their excellent corrosion resistance and mechanical properties. However, the application of DSSs is limited when they are exposed to 250-500 ºC due to the separation of the original bcc phase into Fe-rich (𝛼) and Cr-rich (𝛼′) bcc phases, which occurs when these steels are thermally treated inside the Fe-Cr miscibility gap. This phase separation (PS) is the main reason which is responsible for the low-temperature embrittlement of DSSs, changing mechanical properties such as hardness of ferrite and impact toughness significantly.
Due to the importance of DSSs, significant efforts have been made to investigate the PS in the bcc single-phase model alloys and fcc+bcc dual-phase alloys. However, a full understanding of the key mechanisms governing PS and subsequently embrittlement is still lacking. One limitation is that the investigations have been almost solely performed by ex-situ characterization of the nanostructure that develops during thermal treatment. In-situ characterization could provide a significant contribution, especially on the early stages of the process. Effects of alloying content, cooling rate, externally applied magnetic field, etc. on the PS are investigated. Quantitative information about the spinodal decomposition such as wavelength, amplitude have been calculated by our developed method to calibrate the current theoretical model. Mechanical testing, e.g. micro-hardness and impact toughness, will complement the neutron scattering in order to obtain a comprehensive understanding of the correlation of microstructure-property evolution. The gained knowledge can be used for the further design and optimization of the new grades of DSSs. These would pave the way for controlling the notorious ‘475 °C embrittlement’ in technical applications, and to promote the development of more sustainable DSSs by increasing their lifetime.