Effect of Cr on Phase Transformation Behavior of Austenite in Fe-20Mn-9Al-1.2C-xCr Low-Density Steels During Isothermal Aging

The influence of Cr on the microstructural evolution of austenite in Fe-20Mn-9Al-1.2C-xCr (wt%, x = 0, 3 and 6) low-density steels during isothermal aging at 650 °C for various durations was systematically investigated. With the isothermal aging processed, the 0Cr and 3Cr samples underwent the divorced eutectoid transformation followed by the eutectoid transformation, while only the eutectoid transformation was observed in the 6Cr sample. Meanwhile, increasing Cr content changed the eutectoid transformation products from ferrite + κ-carbide in the 0Cr sample to ferrite + κ-carbide + M₂₃C₆ carbide in the 3Cr sample, and to ferrite + M₇C₃ carbide in the 6Cr sample. The Cr addition significantly increased the A1 temperature (655 °C) of the 0Cr sample to 712 °C of the 3Cr sample, and to 841 °C of the 6Cr sample. As a result, the temperature difference between the A1 temperature and experimental phase transformation temperature (650 °C) was enlarged, which provided a greater driving force for the eutectoid transformation and accelerated the transformation rate of eutectoid transformation. In addition, the Cr addition had a significant effect on the diffusion of constituent elements, decreased the interlayer spacing of pearlite structure from 625 ± 30 nm in the 0Cr sample to 385 ± 25 nm in the 3Cr sample, and to 150 ± 20 nm in the 6Cr sample, refining the eutectoid structure. These findings revealed the mechanism regarding the effect of Cr addition on the eutectoid transformation of austenite, offering valuable insights into the microstructure design of high-performance low-density steels.