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Carbide-free bainitic advanced high-strength steels (AHSS) exhibits multiphase microstructures made of fine ferrite and retained austenite. The carbon partitions between these constituting phases during the bainitic transformation were studied above and below the martensite start temperature (MS) by in-situ high-energy X-ray diffraction experiments. The time-resolved analysis of the phase fraction, austenite lattice parameter, and ferrite tetragonality offer the possibility of establishing phase-scaled consistent and precise carbon balances. In the studied steel, the carbon content in ferrite ranges between 0.1 and 0.15 wt.% after isothermal treatments, meaning that the available carbon for austenite stabilization is reduced. The stasis conditions are close to the T0′ criterion below 400°C but are not reached at higher temperatures. Our multi-step experiments also prove that the transformation below MS is very sensitive to the carbon content in austenite and auto-catalytic nucleation processes inherited from prior transformations (martensite and bainite at higher temperature). The bainitic transformation thus does not follow additivity rules. [ABSTRACT FROM AUTHOR]

We report the first ultra-fast time-resolved quantitative information on the quenching and partitioning process of conventional high-strength steel by an in situ high-energy X-ray diffraction (HEXRD) experiment. The time and temperature evolutions of phase fractions, their carbon content, and internal stresses were determined and discussed for different process parameters. It is shown that the austenite-to-martensite transformation below the martensite start temperature Ms is followed by a stage of fast carbon enrichment in austenite during isothermal holding at both 400 and 450 °C. The analysis proposed supports the concurrent bainite transformation and carbon diffusion from martensite to austenite as the main mechanisms of this enrichment. Furthermore, we give evidence that high hydrostatic tensile stresses in austenite are produced during the final quenching, and must be taken into account for the estimation of the carbon content in austenite. Finally, a large amount of carbon is shown to be trapped in the microstructure. [ABSTRACT FROM AUTHOR]

The influence of V additions on isothermally formed bainite in medium carbon steels containing retained austenite has been investigated using in-situ high energy X-ray diffraction (HEXRD) and ex-situ electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis (EDX) techniques in the transmission electron microscope (TEM). No significant impact of V in solid solution on the bainite transformation rate, final phase fractions or on the width of bainite laths was seen for transformations in the range 375–430 °C. No strong influence on the dislocation density could be detected, although quantitative analysis was impeded by ferrite tetragonality. A reduction in the carbon content of retained austenite Cγ that is not believed to be due to competition with VC or cementite precipitation was observed. No influence of V on the carbon supersaturation in bainitic ferrite Cb could be directly measured, although carbon mass balance calculations suggest Cb slightly increases. A beneficial refinement of blocky MA and a corresponding size effect induced enhancement in austenite stability were found at the lowest transformation temperature. Overall, V additions result in a slight increase in strength levels. [ABSTRACT FROM AUTHOR]

The Influence of Vanadium Additions on Isothermally Formed Bainite Microstructures in Medium Carbon Steels Containing Retained Austenite. The phrase, "Unfortunately, the W-H values for dislocation densities were B not physically b high for both alloys, almost certainly because tetragonality-induced peak splitting was present" should read, "Unfortunately, the W-H values for dislocation densities were B unphysically b high for both alloys, almost certainly because tetragonality-induced peak splitting was present". References 1 Pushkareva I., Shalchi B., Allain S., Geandier G., Fazeli F., Sztanko M., Scott C. The influence of Vanadium Additions on Isothermally Formed Bainite Microstructures in Medium Carbon Steels Containing Retained Austenite. [Extracted from the article]

The influence of V additions on isothermally formed bainite in medium carbon steels containing retained austenite has been investigated using in-situ high energy X-ray diffraction (HEXRD) and ex-situ electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis (EDX) techniques in the transmission electron microscope (TEM). No significant impact of V in solid solution on the bainite transformation rate, final phase fractions or on the width of bainite laths was seen for transformations in the range 375–430 °C. No strong influence on the dislocation density could be detected, although quantitative analysis was impeded by ferrite tetragonality. A reduction in the carbon content of retained austenite Cγ that is not believed to be due to competition with VC or cementite precipitation was observed. No influence of V on the carbon supersaturation in bainitic ferrite Cb could be directly measured, although carbon mass balance calculations suggest Cb slightly increases. A beneficial refinement of blocky MA and a corresponding size effect induced enhancement in austenite stability were found at the lowest transformation temperature. Overall, V additions result in a slight increase in strength levels. [ABSTRACT FROM AUTHOR]