Real-Time Phase Evolution in Additively Manufactured Steel

Static Tools Miss Dynamic Changes

Traditional EBSD methods are excellent for documenting structure. But only before and after thermal treatment: During heating, critical transitions unfold that can’t be captured once the system cools. In the case of martensitic steels, this means missing intermediate phases and mechanisms that control recrystallization and final grain size.

To truly understand heat-driven transformations, especially in complex materials like additively manufactured steels, researchers need a way to track microstructural change as it happens, not just infer what probably occurred.

Automated In Situ EBSD During Controlled Heating

The sample was a wire-arc additively manufactured 300M martensitic–bainitic low-alloy steel. Heating was carried out using a NewTec FurnaSEM stage, fully integrated with a Tescan CLARA UHR SEM.

Data collection followed a structured sequence: EBSD maps were acquired every 2 °C between 750 °C and 900 °C, with secondary electron images captured every 50 °C. Pre- and post-heating EBSD maps provided baseline and endpoint comparisons.

The sample was mounted in the furnace stage inside the SEM, with roughened zirconium coupons placed nearby to capture any free oxygen released during heating and protect the chamber.

All stage control, imaging, and data logging were automated via eRemora (NewTec), controlling the SEM, heating stage, and EBSD through SharkSEM and Oxford Instruments Aztec APIs, with no user intervention.

Transient Phases, Caught in the Act

Throughout the heating cycle, EBSD revealed a phase that had never been observed directly: a misoriented intermediate structure between lath martensite and fully recrystallized austenite. Caused by a memory effect, this phase displayed high kernel average misorientation (KAM), suggesting localized stress and recrystallization potential.

As the temperature increased, these high-KAM zones acted as nucleation points. The low-KAM grains that emerged gradually replaced them, leading to a fully recrystallized microstructure by 900 °C. None of this was visible in pre- or post-heating EBSD alone.

The experiment showed that dynamic grain evolution can now be studied in real time, giving metallurgists a powerful tool for understanding how temperature, structure, and time interact in AM steels and beyond.