Isothermal decomposition processes of austenite in Fe-Cr-C alloy steels

Abstract

The formation mechanism of on unusual structure formed at lower temperatures in the upper C curve where eutectoid reaction occurs in a T-T-T diagram of high Cr steels has been investigated mainly by means of metallographic observations. The morphology of this structure is quite different from those of lamellar eutectoid products, but very similar to that of bainite in optical microscopic levels. TEM observation shows that this strusture was consisted of fine lamellar carbide/ferrite aggregates, and grew preferentially in edgewise directions. Dendritic growth of the eutectoid decomposition products led to the morphology similar to bainite. The interlamellar spacings of them'were continuously changed with decreasing transformation temperature and exhibited the same inclination as that constructed for pearlito in plain carbon steels. The carbide particles precipitated during the formation of this structure were not eementite but alloy carbides such as MzjCe. This structure, therefore, is a sort of eutectoid transformation products, and the formation mechanism is essentially diffusional and different from that of bainite where both diffusional and shear processes are thought to operate.