Temperature-Dependent Sigma (σ) Phase Evolution and Transformation Kinetics in 24Cr–14Ni Stainless Steel Aged at 700–1000 °C

This study investigated the temperature-dependent σ phase transformation behavior of 24Cr–14Ni stainless steel subjected to aging at 700–1000 °C for 1–8 h. At 700 °C, the σ phase mainly retained the original dendritic morphology, whereas at 800 °C, the δ → σ + γ₂ eutectoid decomposition became most pronounced. The σ phase begins to decompose from the dendrite arms. XRD analysis confirmed that σ phase precipitation was most significant at 800 °C, while only weak σ peaks were detected at 900 and 1000 °C, indicating suppressed precipitation and partial σ phase dissolution at higher temperatures. EPMA/WDS analysis showed that σ phase preferentially formed in Cr-rich δ-ferrite regions, while Ni was mainly enriched in the γ-phase. At 1000 °C, the more homogeneous Ni distribution suggested that δ-ferrite dissolution and δ → γ transformation became dominant. JMAK analysis revealed two distinct kinetic regimes: diffusion-controlled σ phase precipitation at 700–800 °C, with Avrami exponents of 0.4327–0.4606, and σ phase dissolution at 900–1000 °C, with higher Avrami exponents of 0.7932–0.8671. The apparent activation energies for precipitation and dissolution were 37.18 and 122.95 kJ·mol⁻¹, respectively. These findings indicate that σ phase transformation in 24Cr–14Ni stainless steel changes from precipitation-dominated behavior at 700–800 °C to dissolution-dominated behavior at 900–1000 °C.