Methods for the Determination of Bainite, Martensite and Austenite Fractions in Austempered Steel

Austempering is a hardening process based on the isothermal transformation of austenite to bainite. The transformation percentage of bainite must be quantified to determine the bainite transformation kinetics. During the austempering process if the austempering holding time is not long enough the bainitic transformation is incomplete, and the residual austenite will transform to martensite during the cooling to room temperature. The result may be microstructure containing Bainite, Martensite and Retained Austenite. In the present work, color tint-etching was used to distinguish phases in austempered AISI 5160 steel samples. After color tint-etching, the microstructural constituents display different colors under the optical microscope with polarized light to estimate the percentage of each microstructural constituent. X-ray Diffraction (XRD) analysis with Rietveld refinement was also conducted to determine the percentage of each microstructural feature. These experimental results are compared with those determined by tint etching. Both results show that with austempering time holding time increase the bainite percentage while the martensite percentage decreases. However, the martensite percentage determined by tint etching method is much higher than that determined by XRD analysis.