Mössbauer Analysis of Low-Temperature Bainite

Low-temperature bainite, obtained by the transformation of austenite at temperatures as low as 200oC for times as large as several days, has been reported to have extraordinary mechanical properties including the highest reported hardness of any bainitic steel. The unusual properties are a consequence of the fine scale of the microstructure, which contains bainite plates with thickness in the range 20-40 nm. The microstructure also contains carbon-enriched retained austenite which contributes to the properties via a number of mechanisms. In this work, the microstructure of a high carbon bainitic steel with Si to avoid cementite precipitation and Co to accelerate the transformation has been studied using Mössbauer spectroscopy for a series of samples transformed isothermally at 200oC for time periods of 26, 34 and 96 hours. The total austenite content is almost identical (~13 wt%) for these samples although the carbon concentrations of the phases differ as a function of transformation time. The austenite increases its carbon content from 5.4 atomic % after 26 h transformation to 6.3 at.% after 96 h, while the final bainitic phase retains about 2.2 at.% of C. These results are consistent with data obtained using atom probe tomography for samples transformed isothermally for 12 days. INTRODUCTION High-strength steels obtained by transforming austenite at a low-temperature result in extraordinarily thin plates of bainitic ferrite, in the range 20-40 nm depending on the transformation temperature [1-3]. The low transformation temperature is achieved by increasing the carbon and, to some extent, adding substitutional solutes such as manganese. Cementite precipitation is prevented by appropriately alloying with silicon, to result in a simple microstructure which is a mixture of bainitic ferrite and carbon-enriched austenite. X-ray diffraction shows that the carbon concentration in austenite is close to the T0 boundary, defined as the locus of the points at which the Gibbs free energies of austenite and ferrite of identical composition are equal. It has also been reported that the bainitic ferrite is significantly supersaturated with carbon [1-3]. We deal here with the evolution of the microstructure and phase chemistries using Mössbauer spectroscopy. EXPERIMENTAL The chemical composition of the steel used is given in Table 1. Samples were prepared, sealed in an Ar atmosphere and submitted to the thermal treatment sketched in Figure 1. The Vickers hardness was measured, as also shown in Figure 1. There is a decrease in hardness after 25 h of transformation; the final value of the hardness is achieved after about 80 h of transformation. This behaviour is related to the development of the bainitic phase, as shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) [3]. Samples obtained after 26, 34 and 96 h of transformation are representative of the different microstructures developed. The 26 h sample is a mixture of retained austenite, martensite and a very small amount of bainitic ferrite. After 34 h, bainite forms in Industrial Applications of the Mössbauer Effect, eds M. Garcia, J. F. Marco, F. Plazaola, American Insitute of Physics, 2005, pp. 338-343 339 larger quantities, thus suppressing martensitic transformation; the 96 h sample contains only bainitic ferrite (87%) and retained austenite. In this last case, it was estimated from XRD that the C content was about 8.5 at% in the austenite phase and about 1 at% in the ferrite phase. TABLE 1. Composition of the studied steel C Si Mn Mo Cr Co P S Fe Atomic % 3.56 3.03 1.96 0.13 1.03 1.37 0.00 0.00 88.91 1000