Superbainite transformation in ultrahigh carbon steel and its microstructure and properties

The austempering microstructure of ultrahigh carbon steel with 1.26wt% carbon content and its effect on tensile properties were studied. The results show that a duplex structure of superbainite and residual carbide in ultrahigh carbon steel is obtained after austempering. The formation of superbainite is due to the high carbon content and addition of aluminum. Because of the grain refinement of prior austenite, the nucleation rate of superbainite increases and its growth path shortens, which is helpful to accelerate the transformation rate. Morphological observation shows that the thickness of the laminar bainite ferrite (BF) and retained austenite film decrease with the decrease of austempering temperature. Austenitizing temperature plays little role on the thickness of BF, but the superbainite size enlarges with the increase of austenitizing temperature. The tensile test results show that the tensile strength increases when the austempering time is prolonged, while the elongation first increases and then decreases. Increasing austempering temperature or austenitizing temperature will lead to decrease of tensile strength and increase of elongation.