ISOTHERMAL DECOMPOSITION PROCESSES OF AUSTENITE IN FE-CR-C ALLOY-STEELS

The formation mechanism of an unusual structure formed at lower temper atures 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 me tallographic observations. The morphology of this structure is quite d ifferent from those of lamellar eutectoid products, but very similar t o that of bainite in optical microscopic levels. TEM observation shows that this structure was consisted of fine lamellar carbide/ferrite ag gregates, and grew preferentially in edgewise directions. Dendritic gr owth of the eutectoid decomposition products led to the morphology sim ilar to bainite. The interlamellar spacings of them were continuously changed with decreasing transformation temperature and exhibited the s ame inclination as that constructed for pearlite in plain carbon steel s. The carbide particles precipitated during the formation of this str ucture were not cementite but alloy carbides such as M(23)C(6). This s tructure, therefore, is a sort of eutectoid transformation products, a nd the formation mechanism is essentially diffusional and different fr om that of bainite where both diffusional and shear processes are thou ght to operate.