Bainite and stress-induced martensite in an AISI type 300 steel: an X-ray diffraction study of the microstructure by the Rietveld method

The present study concerns the X-ray characterization of the microstructure of an AISI type 300 stainless steel with composition Fe–17Cr–8Ni–1Mn–0.1C (wt%), undergoing stress-induced martensite and bainite transformations following γ(f.c.c.) → α(b.c.c.) transformation reactions. The methodology applied is a Rietveld whole X-ray profile fitting technique, adopting the recently developed software MAUD (Materials Analysis Using Diffraction) which incorporates the Popa model for crystallite size and microstrain [root mean square (r.m.s.) strain] and the preferred orientation of the crystallites. The analysis also considers the lattice-defect-related features of the evolved microstructures, namely stacking-fault probabilities and dislocation-density values. The study revealed a high degree of stress-induced martensitic transformation (∼94%) in the cold-worked (hand-filed powder) state of the material. However, no low-temperature martensitic transformation could be observed in this class of materials at temperatures below the martensite start temperature (M_s). On isothermal aging at 673 K for different periods of time (1, 3 and 6 h), the austenite transforms partially into bainite with continuously varying volume fractions (17, 42 and 44%) and the corresponding microhardness values are 218, 230 and 233 kg mm⁻². The size–strain–shape analysis adopting the Popa model reveals anisotropic and almost isotropic values of the crystallite sizes and r.m.s. strains in the austenite and bainite microstructures, respectively. High values of the dislocation density (∼10¹⁶ m⁻²) were observed in the bainite due to the plastic deformation of the austenite induced by the shape change of the growing bainite. The values of all the above defect parameters have been evaluated and compared in order to elucidate the structure–property relationships.