Abstract
Spheroidal graphite cast irons containing 0.06 % Mn and 0.45 % Mn were quenched during austempering at 523 K to 698 K and the variations of microstructures and carbon content of retained austenite were examined to clarify the influence of manganese on the transformation process. Bainitic ferrite nucleated at grain boudary of austenite and grew in the form of plates in each grain, while the bainite transformation was inclined to proceed from graphite nodule to eutectic cellular boudary because of normal segregation of manganese. The coaseness of the bainitic ferrite was evaluated as the spacing and width of plate-like bainitic ferrite. They decreased with decreasing transformation temperature, while manganese little affected the coaseness. Manganese retarded the transformation, and so called γ-pools remained at cellular boundary of 0.45 % Mn iron where manganese increased to more than double the alloy content during solidification. The bainite transformation proceeded more homogeneously at a higher rate, and the enrichment of carbon into austenite and also the complete decomposition of austenite occured faster in 0.06 % Mn iron.
