A Study of the Carbon Distribution in Two Carbide-Free Bainite Alloys

Two low-carbon (0.22 wt. %) carbide-free bainite (CFB) alloys were prepared by isothermal transformation at three different transformation temperatures. The distribution of carbon in bainitic ferrite (BF), retained austenite and martensite phases was studied using a combination of in-situ high-energy x-ray diffraction (HEXRD), atom probe tomography (APT) and electron energy loss spectroscopy (EELS). These complementary techniques provided consistent and unambiguous results, confirming the presence of significant excess carbon levels in BF. The carbon concentration in BF increases as the isothermal transformation temperature is decreased. Statistical analysis of the APT and EELS data indicates that the excess carbon content in bainitic laths does not follow a normal distribution. APT reconstructions suggest that only part of this excess carbon is segregated to defects or clusters. No significant evidence of bainite tetragonality was observed. The addition of vanadium, a strong carbide-forming element, did not influence the carbon distribution in BF. However, the mean dislocation density in the vanadium alloy was found to be systematically higher than the reference steel.