THE DIVORCED EUTECTOID TRANSFORMATION IN STEEL

Experiments are presented which show that the eutectoid transformation in steel can occur by two different modes for temperatures just sligh tly below A(1). In the normal mode, the transformation product is lame llar pearlite. The second mode occurs if the austenite contains cement ite particles or nuclei with a spacing on the order of a few microns o r less. In this case, the transformation product consists of spheroida l cementite particles in a ferrite matrix. This second mode is here ca lled the divorced eutectoid transformation (DET), after recent work by Sherby and co-workers. A literature survey shows that the faster kine tics of the DET over lamellar pearlite in the presence of inhomogeneou s austenite was established before 1940, but has received little atten tion. The inhomogeneities are generally small cementite particles. Exp eriments show that the DET does not occur by a shell of one phase (fer rite) forming around the other phase of the eutectoid (cementite), as is the case in divorced eutectic growth. Rather, a fairly planar auste nite/ferrite front simply advances into the austenite, with no apparen t effect on its shape being produced by the cementite particles. A fir st-order kinetic model is presented for the growth velocity as a funct ion of undercooling below A(1) and is compared to the velocity vs unde rcooling for lamellar pearlite. The simple model indicates that the ve locity of the divorced mode should be faster than the lamellar mode at low undercooling for cementite nuclei distributed in the austenite wi th spacings less than a few microns. This result is consistent with th e experimental data.