X-ray Diffraction Study on Cooling-Rate-Induced γ _fcc→ε _hcp Martensitic Transformation in Cast-Homogenized Fe-26Mn-0.14C Austenitic Steel

The thermal stability of a cast homogenized austenitic steel with basic composition Fe-26Mn-0.14C has been studied using the Rietveld entire X-ray diffraction (XRD) pattern fitting technique. The decomposition of austenite was observed to be very sensitive to the cooling rate. A very high proportion (49 to 70 pct) of ε-martensites was found in the differently cooled specimens as well in the as-cast specimen. The density of various planar (stacking and twin) faults was also estimated in both ε-martensites and retained austenite. The significant variation in the extent of γ _fcc→ε _hcp martensitic transformation was interpreted in terms of athermal and isothermal martensites formed during cooling and grain size of austenite. The austenite grains were found to be comprised of a high to moderate density of stacking faults (∼10⁻³ to 10⁻²) and negligible twin fault probability (∼10⁻⁵). On the other hand, in the ε-martensites, the dominant planar fault was twins (∼10⁻³). Approximate values of the dislocation density within the retained austenite were also evaluated using the crystallite size and root-mean-square (rms) strain values obtained from the “size-strain-shape” analyses.