Phase transformation during in situ heating of low-activation austenitic steel

The peculiarities of phase transformations in dispersion-strengthened low-activation chromium-manganese austenitic steel under high-temperature heating and cooling are studied using in situ X‑ray diffraction (XRD) and transmission electron microscopy (TEM). The presence of ε‑martensite plates in this steel after ageing (700 °C, 100 h) is shown. This phase is formed under local internal stresses arising from intensive growth of dispersed particles of M₂₃C₆ carbides (M – Cr, Mn, Fe) during the steel cooling from the aging to room temperature. The comparatively low stacking fault energy (25 mJ/m²) of steel and the low stresses of ε‑martensite formation favor the γ→ε transformation. The in situ studies show that under subsequent high-temperature heating up to 500–590 °C, the ε‑phase completely transforms into austenite. During cooling from 590 °C to room temperature, this phase appears again. The formation of ε‑martensite does not affect the structural-phase stability of austenite near the temperature range of anticipated operating conditions.