Quantification of Retained Austenite in Low-Carbon Steels

Three steels of carbon content between 0.2 and 0.4 wt pct were directly quenched to room temperature after austenitization at 1223 K (950 °C). Attempts were made to quantify the amount of retained austenite in the full quenched condition using X-ray diffraction analysis. The direct estimation of the amount of austenite remaining untransformed after full quenching using X-ray diffraction analysis is difficult since high-intensity peaks of austenite and martensite overlap and merge together and austenite transforms to martensite during sample preparation owing to its low stability. The results indicate that the amount of retained austenite in fully quenched sample of low-carbon steel is underestimated using routine XRD. However, when the full quenched samples were subjected to a tempering treatment at 673 K (400 °C) for 300 seconds, the (111) peak of austenite and (110) peak of martensite were observed distinctly and the amount of retained austenite was estimated to be close to the value calculated using Koistinen and Marburger equation and measured by EBSD analysis of this sample. This was attributed to the carbon enrichment of austenite due to carbon depletion from martensite which results in peak separation. In addition, stability of austenite increases and it does not undergo deformation-induced martensite transformation.