Metallographic study of phase changes in heat treated ductile iron

Continuous cast ductile iron samples were hardened. It was found that carbide dissolved in the austenite faster than graphite and that this material requires higher austenitizing temperatures and longer austenitizing time than steels. Tempered microstructures were affected by the prior austenitizing treatment and it appeared that the amount of carbon dissolved in the austenite determined the tempered microstructure. Several attempts to determine the austenite grain size indicated that it is not as easy to determine in ductile iron as in steels. Two techniques, heat etching and an isothermal transformation method, were found to reveal prior austenite grain size. It was concluded that carbon precipitated directly as graphite from martensite in ductile iron rather than as combined carbon the way it does in steels. The reason for direct precipitation of graphite from martensite was not known. Repeated quenching and subsequent tempering might lead to the replacement of the primary graphite dispersion by a secondary graphite dispersion. This could alter properties but it did not appear to be a practical process because of the large number of quenching and tempering cycles required. A relationship between regular Rockwell C hardness and the matrix hardness was established which was in good agreement with that in the literature. An approximate partial isothermal transformation diagram is reported --Abstract, Pages ii-iii