Effects of austenitizing temperature on the kinetics of bainite reaction at constant austenite grain size in FeC and FeNiC alloys

The influence of austenitizing temperature on a subsequent diffusional transformation has been investigated. The overall bainite transformation rate in a slightly hypo-eutectoid Fe-7.6Ni-0.48C alloy is increased by a double austenitizing treatment. Optical microscope observations have revealed that this acceleration of bainite transformation is caused mostly by the acceleration of nucleation rate at austenite grain boundaries. Intergranular fracture surface observations have shown that a second austenitizing treatment enhances cementite precipitation on grain boundaries. It is considered that the enrichment of sulfur at grain boundaries by a second austenitizing treatment enhances the precipitation of cementite during the subsequent cooling to the transformation temperature. When the second austenitizing temperature is 750°C or below, disc shaped Fe-rich FeS precipitates are formed together with the cementites. The initial stage of bainite transformation has shown that the cementites and Fe-rich FeS precipitates act as preferential nucleation sites for bainite. The precipitation of cementite and Fe-rich FeS by a double austenitizing treatment is also observed in a Ni free Fe-0.61C alloy.