HP40Nb reformer tube exposed to 950 °C – Machine learning informed microstructure evolution and multi-scale mechanical properties

Abstract

Austenitic stainless steel tubes extensively employed in energy sector, endure extreme service temperatures over long period of time and are susceptible to creep damage. This study investigates microstructural evolution and mechanical properties of HP40Nb (25Cr35Ni1Nb) steel exposed to 950 °C for up to 1600h, with specimens retrieved at 200h intervals. Advanced multi-scale characterization methods, including scanning electron microscopy, energy dispersive spectroscopy, and micro- and nanoindentation were utilized to analyze transformation of precipitates and resulting impact on mechanical properties. Microstructural changes (precipitate evolution, transformation and coarsening) were quantified using Machine Learning based algorithm and correlated with the mechanical properties. The as-cast microstructure comprised two primary intergranular Cr- and Nb-rich carbides in austenitic matrix. Post 200h exposure at 950 °C, secondary intragranular Cr-rich pepper carbide was observed in the austenite matrix. Following this, segregation of Si around Cr-rich precipitates occurred resulting in dark phases. At 800h, Si migrated to NbC resulting in transformation of NbC to Ni-Nb Silicide phase (G-phase). Transformation of NbC to G-phase and coarsening of the primary Cr-rich carbides was observed towards end of study at 1600h. The microstructural changes resulted in microcracks owing to precipitate transformation, coarsening and thermal coefficient mismatch. It was identified that both Nb and Cr precipitates are susceptible to microcracks post 800h at 950 °C. The study offers improved understanding of microstructural changes occurring in austenitic stainless steels used as reformer tubes during exposure to elevated service temperature.